# The Radio & Plasma Wave Investigation (RPWI) for the JUpiter ICy moons Explorer (JUICE)

J.-E. Wahlund, J. Bergman, L. Åhlén, W. Puccio, B. Cecconi, Y. Kasaba, I.

Müller-Wodarg, H. Rothkaehl, M. Morawski, O. Santolik, et al.

► To cite this version:

J.-E. Wahlund, J. Bergman, L. Åhlén, W. Puccio, B. Cecconi, et al. The Radio & Plasma Wave Investigation (RPWI) for the JUpiter ICy moons Explorer (JUICE). Space Science Reviews, 2024, 221 (1), pp.1. 10.1007/s11214-024-01110-0. insu-04868196

HAL Id: insu-04868196

https://insu.hal.science/insu-04868196v1

Submitted on 6 Jan 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# The Radio & Plasma Wave Investigation (RPWI) for the JUpiter ICy moons Explorer (JUICE)

J.-E. Wahlund · J.E.S. Bergman · L. Åhlén · W. Puccio · B. Cecconi · Y. Kasaba et al. [full author details at the end of the article]

Received: 4 October 2023 / Accepted: 10 September 2024
© The Author(s) 2024

## Abstract

The Radio & Plasma Wave Investigation (RPWI) onboard the ESA JUpiter ICy moons Explorer (JUICE) is described in detail. The RPWI provides an elaborate set of state-of-the-art electromagnetic fields and cold plasma instrumentation, including active sounding with the mutual impedance and Langmuir probe sweep techniques, where several different types of sensors will sample the thermal plasma properties, including electron and ion densities, electron temperature, plasma drift speed, the near DC electric fields, and electric and magnetic signals from various types of phenomena, e.g., radio and plasma waves, electrostatic acceleration structures, induction fields etc. A full wave vector, waveform, polarization, and Poynting flux determination will be achieved. RPWI will enable characterization of the Jovian radio emissions (including goniopolarimetry) up to 45 MHz, has the capability to carry out passive radio sounding of the ionospheric densities of icy moons and employ passive sub-surface radar measurements of the icy crust of these moons. RPWI can also detect micrometeorite impacts, estimate dust charging, monitor the spacecraft potential as well as the integrated EUV flux. The sensors consist of four 10 cm diameter Langmuir probes each mounted on the tip of 3 m long booms, a triaxial search coil magnetometer and a triaxial radio antenna system both mounted on the 10.6 m long MAG boom, each with radiation resistant pre-amplifiers near the sensors. There are three receiver boards, two Digital Processing Units (DPU) and two Low Voltage Power Supply (LVPS) boards in a box within a radiation vault at the centre of the JUICE spacecraft. Together, the integrated RPWI system can carry out an ambitious planetary science investigation in and around the Galilean icy moons and the Jovian space environment. Some of the most important science objectives and instrument capabilities are described here. RPWI focuses, apart from cold plasma studies, on the understanding of how, through electrodynamic and electromagnetic coupling, the momentum and energy transfer occur with the icy Galilean moons, their surfaces and salty conductive sub-surface oceans. The RPWI instrument is planned to be operational during most of the JUICE mission, during the cruise phase, in the Jovian magnetosphere, during the icy moon flybys, and in particular Ganymede orbit, and may deliver data from the near surface during the final crash orbit.

**Keywords** JUICE · RPWI · Ganymede · Europa · Callisto · Jupiter
# 1 Introduction & Overall Description

The Radio & Plasma Wave Investigation (RPWI) consist of three groups of instruments that provides a highly versatile set of electric and magnetic measurements associated with ambitious observational capabilities (cf. Sect. 2), with the aim to address numerous science objectives regarding foremost the Jovian space environment but also the interior and surface of the large icy moons and their interactions with the space environment (cf. Sect. 3). The instrument is described in Sect. 4 and its related software/firmware in Sect. 5. The RPWI planned operations and data products are described in Sects. 6 and 7. This paper give substantial information for using the RPWI data for science, although it is recommended that a contact is made with the PI and/or expert Co-I's institutes to understand possible caveats/features detected during flight. The RPWI instrumentation is a result of a long-term plan to investigate the Jovian system, which started at the beginning of the new millennium (Blomberg et al. 2005; Wahlund et al. 2005a,b; Cecconi et al. 2010). ESA selected the RPWI instrument package for implementation in 2013.

## 1.1 The RPWI Basic Performance

The RPWI instrument measures the electromagnetic fields and cold plasma (Table 1). From these parameters it is possible to derive several other physical characteristics within and around the icy moons and within the magnetospheres of Ganymede and Jupiter (cf. Sect. 2). We partially include some of the latest in-flight raw data in this paper, although the in-flight calibration results are not ready at the time of the manuscript submission, that show how the in-flight data and performance are compliant with predictions.

## 1.2 The RPWI Team

The RPWI team consists of hardware and software/firmware providers, ground segment and data archiving experts, and of course scientists skilled in analysis of data products, but also theory modellers that can put the observations into context, and public outreach specialists. The team has 50 science Co-Investigators, engineers, and associated scientists from 9 countries, as well as ESA and Airbus Defence and Space (ADS) supporting engineers/managers and independent companies used for manufacturing of instrument parts. A summary of design developers is given in Table 2.

<table><caption>Table 1 Basic measurement output from RPWI. Samples per second (sps)</caption><thead><tr><th>RPWI Measured Quantity</th><th>Range</th><th>Max time resolution</th></tr></thead><tbody><tr><td>Electric field vector, δE(f)</td><td>DC – 45 MHz</td><td>763 sps continuously<br>Snapshots or frequency sweeps every 30 s</td></tr><tr><td>Magnetic field vector, δB(f)</td><td>0.1 Hz – 20 kHz</td><td>763 sps continuously<br>49ksps snapshots every 30 s</td></tr><tr><td>Cold plasma parameters</td><td>Ne: 10<sup>-4</sup> – 10<sup>5</sup> cm<sup>-3</sup><br>Ni: 1 – 10<sup>5</sup> cm<sup>-3</sup><br>Te: 0.01 – 20 eV<br>Vdi: 0.1 – 200 km/s</td><td>Nominally every 30 s, or 763 sps<br>Nominally every 30 s, or 763 sps<br>Nominally every 30 s<br>Nominally every 30 s, or 763 sps</td></tr><tr><td>Density fluctuations (δn/n)</td><td>DC – 10 kHz</td><td>49 ksps snapshots every 30 s</td></tr><tr><td>Spacecraft potential (Usc)</td><td>±80 V</td><td>763 sps continuously</td></tr></tbody></table>
<table><caption>**Table 2** Providers involved in the development of RPWI design of hardware, software, calibration, theory support, archiving, operations, and public outreach. In addition, several companies have been involved in the manufacturing of the flight hardware (not included in this list)</caption><thead><tr><th>Institute/institution/company</th><th>Roles</th><th>Country</th></tr></thead><tbody><tr><td>Swedish Institute of Space Physics (IRF), Uppsala</td><td>PI institute</td><td></td></tr><tr><td>Royal Institute of Technology (KTH), Stockholm</td><td>CoI institute</td><td></td></tr><tr><td>Uppsala University, Uppsala</td><td>H/W developer</td><td></td></tr><tr><td>Linköping University, Linköping</td><td>H/W developer</td><td></td></tr><tr><td>Laboratoire d'Etudes Spatiales et Instrumentation en Astrophysique (LESIA), Paris</td><td>Co-PI institute</td><td></td></tr><tr><td>Laboratoire de Physique des Plasmas (LPP), Paris</td><td>CoI institute</td><td></td></tr><tr><td>Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E), Orléans</td><td>CoI institute</td><td></td></tr><tr><td>Institut de Recherche en Astrophysique et Planétologie (IRAP), Toulouse</td><td>CoI institute</td><td></td></tr><tr><td>Université de Versailles Saint-Quentin (LATMOS), Paris</td><td>CoI institute</td><td></td></tr><tr><td>Office National d'Etudes at de Reserches Aérospatiales (ONERA), Toulouse</td><td>CoI institute</td><td></td></tr><tr><td>Laboratoire Plasma et Conversion d'Energie (LAPLACE), Toulouse</td><td>CoI institute</td><td></td></tr><tr><td>Space Research Centre of the Polish Academy of Sciences (SRC-PAS), Warsaw</td><td>Co-PI institute</td><td></td></tr><tr><td>Astronika, LTD., Warsaw</td><td>H/W developer</td><td></td></tr><tr><td>Institute of Atmospheric Physics (IAP), Prague</td><td>Co-PI institute</td><td></td></tr><tr><td>Astronomical Institute (AsI), Prague</td><td>CoI institute</td><td></td></tr><tr><td>Imperial College London, Department of Physics</td><td>Co-PI institute</td><td></td></tr><tr><td>University of Sheffield, Space instrumentation group, Sheffield</td><td>CoI institute</td><td></td></tr><tr><td>Space Research Institute (SRI), Austrian Academy of Sciences, Graz</td><td>CoI institute</td><td></td></tr><tr><td>University of Cologne, Cologne</td><td>CoI institute</td><td></td></tr><tr><td>Dublin Institute for Advanced Studies (DIAS), Dublin</td><td>CoI institute</td><td></td></tr><tr><td>Tohoku University, Sendai</td><td>Co-PI institute</td><td></td></tr><tr><td>Tokyo University of Science, Tokyo</td><td>CoI institute</td><td></td></tr><tr><td>Kyoto University, Kyoto</td><td>CoI institute</td><td></td></tr><tr><td>Kanazawa University, Kanazawa</td><td>CoI institute</td><td></td></tr><tr><td>Nagoya University, Nagoya</td><td>CoI institute</td><td></td></tr><tr><td>Meisei Electric Co., Ltd., Isesaki</td><td>H/W developer</td><td></td></tr></tbody></table>
## 1.3 RPWI Novel Capabilities

The RPWI focuses, apart from cold plasma studies, on the understanding of how, through electrodynamic and electromagnetic coupling, the momentum and energy transfer occur in the surrounding space environments and with the icy Galilean moons. In Ganymede orbit, in-situ measurements by RPWI of the electric coupling between any sub-surface ocean, the ionosphere and magnetosphere will provide constraints on the physical characteristics of this ocean.

The RPWI instrument has outstanding new or enhanced capabilities that have not been available to previous missions to the outer planets and that are crucial to answer several fundamental science objectives of the JUICE project. Among them are:

* The capability of four Langmuir probes to measure the vector electric field from near DC to 1.5 MHz, as well as plasma convection (E × B drift, ion flux), thereby allowing the investigation of plasma electrodynamics in the Jovian system and how the sub-surface oceans and ionospheres of the icy Galilean moons couple electrodynamically to the highly variable Jovian magnetosphere.

* The capability of a tri-axial search coil, together with the Langmuir probes' ability to measure the vector magnetic and electric fields simultaneously below 20 kHz to identify Alfvén and Whistler waves, filamentary currents, flux ropes and electrostatic structures involved in the momentum and energy transfer between different particle populations involved in the interaction between the Jovian magnetosphere and the conducting ionised exospheres around the icy Galilean moons.

* The significantly enhanced capability of using both passive and semi-active methods to infer the cold (<100 eV) plasma characteristics. The implemented methods also include the capability to infer the bulk ion drift speed.

* The capability of three electric dipole antennae to fully characterize radio emissions at Jupiter (Carozzi et al. 2001; Bergman and Carozzi 2007), making it possible to determine source locations and polarization of radio emissions from the aurora, Jupiter's magnetosphere, Ganymede's magnetosphere, and characterize their variability with time and response to external forcing.

* The capability to monitor electrically charged dust properties and identify any dust-plasma interactions. This type of study on open versus closed magnetic field lines at Ganymede, and the direct observations of the electric field vector accelerating these particles toward the surface, will give insight to surface sputtering processes and their effect on the icy surfaces and atmospheres.

* The capability to directly measure in-situ the thermal and dusty plasma and electromagnetism within the partly ionized gas exhaust of water-rich plumes above active surface regions on the icy moons.

* The capability to use the Jovian radio emissions to carry out occultation of the icy moon ionospheres and passive sub-surface radar (PSSR) of the icy crust of the moons. The former gives the altitude density profiles from the radio cut-offs. The latter potentially determines the conductivity and the subsurface features of the icy crust.

## 1.4 RPWI Instrument Parts & Science Experiments

The top-level block diagram (Fig. 1) illustrates the overall RPWI baseline configuration. The RPWI instrument consists of 10 sensors, grouped in three sets, and connected to 3 receivers. The RPWI instrument have four Langmuir probes (LP-PWI) for thermal plasma and electric field measurements, three axes search coil magnetometer (SCM) for magnetic
<figure><img src="image_2.png" /><figcaption>**Fig. 1** JUICE/RPWI instrument configuration</figcaption></figure>

fields measurement, and three dipole antennas (RWI) for radio measurements. The RPWI system is built up around a main Electronics Box (EBOX), which houses dual redundant Low Voltage Power Supplies (LVPS A&B) and Digital Processing Units (DPU A&B) that serve the Langmuir Probe & Mutual IMpedance Experiment (LP-MIME) transceiver, the Low Frequency (LF) receiver, and the High Frequency (HF) receiver. The LVPS and DPU units are hardwired to each other, such that LVPS A is connected to DPU A and LVPS B is connected to DPU B. A backplane inside the EBOX distributes the analogue and digital signals, the different voltages, and a clock signal, which is generated by dual redundant crystal oscillators on the LVPS board. The LP-MIME transceiver is connected internally to the LF receiver, for which it provides single-ended and differential signals from the Langmuir probes. It also contains driver circuits for the MIME transceiver. The MIME drivers can furthermore be synchronized to the mixer frequency of the HF receiver, used for mutual impedance measurements at high frequencies. External harnesses connect the LP-PWI, SCM, and RWI sensors to the circuit boards inside the EBOX.
The EBOX is housed in the payload vault in the centre of spacecraft. Each LP-PWI sensor has a 3 m long deployable boom with the 10 cm diameter Langmuir probe at its end and with the preamplifier mounted separately near the boom root. The SCM is mounted on the MAG boom (Arce and Rodriguez 2019) and has its preamplifiers mounted inside the mechanical structure, which supports the sensors. The RWI antenna subsystem, also mounted on the MAG boom, consist of six BeCu antenna rods with the length of 1.25 m and the diameter of 1 cm, attached on a cubic mechanical chassis which stores the preamplifier electronics inside. Those six antenna rods form three dipole antennas with the tip-to-tip length of 2.5 m.

The RPWI instrumentation can alternatively be divided into experiments (GAN-DALF, MIME, SAMWISE, JENRAGE) for easier reference to science topics. GANDALF (GANymede plasma Density And Low-Frequency wave experiment) uses the four Langmuir probes and the LP receiver part. MIME makes use of both the four Langmuir probes and the RWI antenna together with the LP-MIME transceiver. SAMWISE (Search-coil and Analyzer Module for Waves and Instabilities Science Experiment) makes use of both the four Langmuir probes and the SCM together with the LF receiver. Finally, JENRAGE (Jupiter ENvironment Radio Astronomy and Ganymede Exploration) uses the RWI antenna and the HF receiver.

## 1.5 RPWI Theory Support

The RPWI investigation will benefit from tight integration of instrument data analysis with state-of-the-art numerical modelling of atmospheres and space environment (example shown in Fig. 2). The RPWI institutes access together the capability of simulating all environments relevant for RPWI investigations. The models generate parameters including plasma densities and composition, temperatures, and velocities as well as electric and magnetic fields on 3-D grids as a function of time. Simulations are driven by external input parameters describing solar wind and radiation forcing as well as neutral atmosphere properties, but otherwise respond to basic physical laws. RPWI furthermore provide simulation tools to calculate radio wave propagation (direction, polarization, intensity) through the relevant environments.

By solving well known coupled non-linear equations of continuity, energy and momentum for plasma and neutral gas distributions as well as Maxwell's equations for electric and magnetic fields, our models complement the observations, which considerably widen the scientific gain. As an example, models of Ganymede's ionosphere are driven in part by the assumed neutral atmosphere and by the magnetospheric environment. By driving such models using observed boundary conditions (magnetic fields, plasma energy distributions, etc.) and matching simulations to observed quantities at locations along the spacecraft trajectory, the local (along-track) observations are placed into a global context (e.g., Galand et al. 2025). Furthermore, the models provide key information on the origins of observed behaviour, of the physical drivers behind it, thereby enabling a considerably deeper scientific analysis of behaviour than otherwise possible. Radio emission modelling (Louis et al. 2019a) will allow us to predict the occurrence of the observable radio emissions as well as their polarization, and thereby will be used to optimize the instrument observation planning (Cecconi et al. 2021).

The in-flight calibration of the RPWI observations will take advantage of the Spacecraft Plasma Interaction Software (SPIS) simulations (cf. Sect. 2.7, Sarrailh et al. 2015) to predict the behaviour of the plasma close to the spacecraft. RPWI plan to use full 3D spacecraft rolls in the Solar Wind together with SPIS model predictions to account for the asymmetric spacecraft electric potential pattern when measuring near DC electric fields.
<figure><img src="image_3.png" /></figure>

**Fig. 2** Three-dimensional view of Ganymede's global magnetosphere (top panel, from a hybrid model), viewed from the upstream flank side with Jupiter in the background. The model simulates the interaction between Ganymede's and Jupiter's magnetospheres (Modolo et al. 2018). Plasma populations from Ganymede's ionosphere and Jupiter's magnetosphere are included and the model computes the time-dependent electromagnetic field. Colour code indicates the electron density for different plane cuts where yellow indicates high density and blue low density. The white line shows a segment of a JUICE orbit trajectory. Vectors along the JUICE s/c indicates the simulated electric field values while the reddish structure represents the Alfvén wings emphasized with an iso-surface based on a velocity value. The bottom panel gives the electric field components along the JUICE trajectory. It illustrates the RPWI team capability of simulating RPWI observations by tracing physical parameters along the spacecraft trajectories (e.g., using the 3dview tool, http://3dview.cdpp.eu, Génot et al. 2018, of the French plasma physics datacentre CDPP).

# 2 The RPWI Experiment Capabilities

## 2.1 Design Drivers

The RPWI science objectives are numerous, and the most important ones are listed in Sect. 3. To fulfil them the RPWI instrument needs to measure the complete electric and magnetic field vectors, as well as the thermal plasma properties in a wide frequency range and/or with high temporal/spatial resolution. Special ElectroMagnetic Compatibility (EMC) mitigating methods have also been implemented throughout the instrument and spacecraft development to make the RPWI measurements as free as possible from potential electromagnetic noise sources. The full assessment of remaining EMC interferences has been investigated
extensively during on-ground tests and will be further verified in-flight. For instance, the RPWI near DC electric field measurements will be compared to the $v_{ion, PEP} \times B_{J-MAG}$. Here, the Particle Environment Package (PEP) estimates the ion moments to derive the ion speed, and the JUICE MAGnetometer (J-MAG) estimates the near DC magnetic field components. Special periods of in-flight calibrations, involving full spacecraft rolls in reasonably known conditions, in the solar wind and near Earth, will characterize the influence of the electric and plasma environment around a large asymmetric spacecraft on the RPWI measurements before JUICE enters the Jovian system. The RPWI measurements and data analysis will also be supported by SPIS spacecraft environment interaction simulations (See Sect. 2.7, Sarrailh et al. 2015).

A special focus was put on the low frequency measurements below approximately 100 Hz, which the Galileo/PWS did not cover well. For instance, RPWI provides the first ever measurements of the near DC electric field vector, as well as providing detailed cold plasma measurements with several techniques (including active methods like mutual impedance and Langmuir probe sweep techniques), in the Jovian system. RPWI also provides very sensitive monitoring of the medium-frequency magnetic and electric fields. This approach also provides a very good complement to the PEP hot plasma (Barabash et al. 2025, this collection) and J-MAG magnetic field (Dougherty et al. 2025, this collection) measurements, and a focus on the planetary objectives related to the interaction of the icy moons with the magnetosphere of Jupiter. The characteristic spatial and temporal scales encountered by JUICE and the RPWI parameter coverage is displayed in Fig. 3. RPWI will be able to do science investigations in all parts of the Jovian system traversed by JUICE, and the intention is to carry out basically continuous measurements starting 3 months before Jupiter Orbit Insertion (JOI).

Another design driver has been to make omnidirectional or full vector measurements, and not be dependent on spacecraft attitude or field-of-view (FoV). This makes operation planning much easier, being only dependent on restrictions in telemetry (TM) volume and power from the spacecraft. Nevertheless, particularly the low frequency and near DC measurements by the LP sensors will depend on their exposition to ram flow and sunlight. At times, one or more LPs will be in shadow or wake. To minimize the scientific impact of this, we have designed the operation modes, so the use of probes is often redundant and/or used for different purposes.

The radio measurement design drivers are the remote sensing capabilities of the Jovian system in the spectral range up to 45 MHz. Three axis electric field antenna measurements constitute a 'goniopolarimetric' (Cecconi 2013) system, i.e., with direction finding and polarization capabilities, enabling studies of the Jovian radio sources, which are widely distributed in the Jovian magnetosphere (Cecconi and Zarka 2005). Passive sounding of natural radio emissions will further enable ionospheric radio occultations of the icy moons and enable studies of the reflected signals from the surface and sub-surfaces of these moons. Passive sub-surface radar (PSSR) measurements are planned to sound the icy crusts of Galilean satellites by using the reflections of Jovian radio emissions (HOM/DAM). Here, there exist also the capability to sample the full waveform below a few MHz in a broad frequency window.

With these capabilities, RPWI will survey the harsh space environment around Jupiter, and its interaction with icy moons, as well as monitor these moons' surface and subsurface characteristics.
<figure><img src="image_4.png" /><figcaption>**Fig. 3** The RPWI measurements will cover all expected spatial and temporal scales to be encountered by JUICE, except for magnetic variations below 0.1 Hz that will instead be covered by the J-MAG payload. The electric field and cold plasma parameters, at the lowest frequencies below about 100 Hz, have never been measured accurately before in the Jovian system. The radio waves are used for remote sensing and the spatial scale indicated refers here to their wavelengths, not to local scales in the plasma as the case is for other parameters to be measured by RPWI. See Sect. 3 for references on typical encountered plasma conditions. The following natural frequencies are ion cyclotron ($f_{ci}$), Lower Hybrid ($f_{LH}$), ion plasma ($f_{pi}$), electron cyclotron ($f_{ce}$), and the Upper Hybrid ($f_{uh}$)</figcaption></figure>

## 2.2 GANDALF

This experiment makes use of the LP-PWI sensors together with the LP-MIME receiver part of the electronics. The LP-PWI sensors sample electric voltages or currents, which is the basis of the inferred physical parameters. The probes can therefore be operated in two different modes: current sampling mode (used as Langmuir probes) for plasma diagnostics (e.g., $N_e$, $N_i$, $T_e$, $V_{di}$) or voltage sampling mode (used as electric field probes) for measurement of the electric field (E), where two of the primary tasks are to sample the near DC electric field vector and to diagnose plasma waves. The spacecraft potential ($U_{SC}$) is always measured independent of operational mode. The spacecraft potential can be used as an electron number density proxy down to very low densities ($10^{-4} \text{ cm}^{-3}$), a method that has been conducted successfully by related instruments with booms of similar size in a variety of solar system environments including the Saturn magnetosphere (Morooka et al. 2009), the solar wind (Khotyaintsev et al. 2021; Mozer et al. 2022), and the coma of comet 67P (Johansson et al. 2021).

### 2.2.1 Electric Field Mode

When used as electric field probes, the electric field components are obtained from the measurements of the potential (voltage) difference between probe pairs, while feeding these
<figure><img src="image_5.png" /><figcaption>**Fig. 4** PFM LP-MIME transfer functions for all electric field DC differential modes. Results from the on-ground calibration for different ADCs and temperatures</figcaption></figure>

probes with a constant bias current. Most of the time, RPWI will sample a selection of three
probe potential differences, and one single ended (for monitoring the spacecraft potential).
The LP-PWI probes will therefore allow estimates of the full vector electric field from near
DC up to 1.5 MHz. GANDALF will carry out continuous sampling at 763 samples per
second (lowpass), and snapshots of various sample length (usually 16 ksamples) at higher
frequencies (now set at 49 ksps or 694 ksps) to ensure relevant wave electric emissions are
monitored. The snapshots are either time-tagged or triggered within a sequence cycle (usu-
ally every 30 s but can be longer). There also exists a spectrum sweep sampling mode up
to 1.5 MHz. It is possible to use interferometer measurements between pair of probes to
estimate the phase speed of electric field fluctuations.

Several processes affect in-flight measurement performance with Langmuir probes: photoelectrons (and secondary electrons) emitted from the spacecraft, differential charging of large insulating surfaces, asymmetric structures affecting the potential pattern around the S/C, and the plasma environment itself. It has therefore been important to make the exterior of the S/C and solar panels conducting (and electrically connected) to limit the disturbances and simplify their characterization. The near DC electric field, together with the J-MAG magnetic field measurements, will enable us to continuously give values of the **E** × **B** connection (i.e., wherever JUICE is traversing in the Jovian system).

The instrument itself is very accurate in DC E-field mode. The resolution is of the order
of 5 µV/6.2 m (double probe minimum distance) ≈0.9 µV/m for DC (see Fig. 4) and better
in AC (Fig. 5). The asymmetric spacecraft (with variable solar panel positions and plasma
ram flow directions) itself will be the largest error source, and we can only determine this
source during in-flight calibration with S/C roll manoeuvres in known plasma environments
(e.g., solar wind conditions). However, a detailed EMC programme was run during the de-
<figure><img src="image_6.png" /><figcaption>**Fig. 5** PFM LP-MIME transfer functions for all electric field AC differential modes. Results from the on-ground calibration for different ADCs and temperatures</figcaption></figure>

sign and implementation of the JUICE spacecraft, which made sure most of the surfaces were conductive, thus mitigating differential charging of the surface and resulting influence of the potential pattern around the spacecraft. Our experience with near DC electric field measurements on Rosetta and Solar Orbiter with shorter booms indicate that a rather accurate DC electric field is achievable. On JUICE, in-flight calibrations of the RPWI DC E-field with the $v \times B$ electric field are planned together with J-MAG and PEP in the solar wind. Likewise, full spacecraft rolls campaigns are planned (also together with J-MAG and PEP) to map in detail the potential disturbance of the S/C on the DC electric field measurements.

Another error source is the temperature stability of the preamplifiers and the receiver, which therefore will be continuously measured in-flight for potential electric field offset corrections for each probe. The temperature stability shown in Figs. 4 and 5 can in this regard be noted. Moreover, GANDALF will intermittently measure the electric field over a known internal resistor for stability calibration purposes, detecting any ageing of the instrument. SPIS simulation (Sarrailh et al. 2015) results will also help us understand the potential error sources and give better reduction of the sampled data around the spacecraft. Similar considerations have successfully been carried out on several space physics related spacecraft before (e.g., Cluster and Solar Orbiter). Examples of in-flight data in the solar wind near Earth show a good quality of the data (Figs. 6 and 7). We therefore believe that a true DC electric field vector measurement is possible with specified accuracy over a longer time. The reference to a "near DC" capability notifies the above-mentioned difficulties to achieve the true DC electric field vector at all times of the mission.
<figure><img src="image_7.png" /><figcaption>**Fig. 6** In-flight electric waveforms in the solar wind near Earth sampled at 763 samples-per-second (low pass at 350 Hz). Voltage differentials between probe pairs 1–2 (blue) and 3–4 (black) are shown. The probe inter-distance is about 6.3 m and 9.6 m respectively. Probes 3 and 4 has the JUICE spacecraft in between, while P1-2 do not. The DC offset is not yet corrected for, so only the relative differences can be compared here. Even so, the almost exact match is promising for the RPWI capability to measure the near DC electric field. In-flight calibrations remain to be carried out</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 7** In-flight AC electric field spectrum sampled at 694 kspcs in the solar wind near Earth. The differential signals (P12, P23, P34) are very similar and indicates the noise floor. A nice correspondence between the power spectral densities versus frequency exist. Interference lines at the higher frequency end is partly due to the S/C power system but such lines appearing in the Langmuir probe data can easily be removed on board at a later stage. A single ended (P2) signal measures versus the S/C and is used for S/C potential estimations</figcaption></figure>

## 2.2.2 Density Mode

When used as Langmuir probes, the probes are biased to a specific voltage (positive or negative) or stepped in bias voltage (a so-called Langmuir sweep, see an example in Fig. 8),
<figure><img src="image_9.png" /><figcaption>Fig. 8 Example of a Langmuir probe voltage sweep from the Cassini RPWS/LP in the magnetosphere of Saturn (Gustafsson and Wahlund 2010). Panel a, b and c are respectively linear, logarithmic and the derivative of the same data. Blue dots = sweep data, red line = total fitted current based on Sheath Limited (SL) theory, red dashed = ion current, various green lines = electron currents from spacecraft photoelectrons and ambient electrons, black dashed = photoelectron current from the probe</figcaption></figure>

and the probe current is measured (Mott-Smith and Langmuir 1926). When the probe is biased negatively, the current is dominated by ions attracted to the probe and/or photoelectrons emitted from a sunlit probe. When the probe is positively biased, attracted electrons dominate the probe current. RPWI can operate the probes in the bias voltage range ±80 V, which allows for plasma measurements in a wide range of different plasma environments. The sweeps can be sampled at 3 mV resolution and with a rate of 3 ksps. RPWI has different sweep bias tables that can be used. During cruise we make use of 128 bias steps with 20 samples each (at 3 ksps). For instance, the bias voltage range of ±80 V and instrument sensitivity will be good enough to derive the bulk drift speed ($V_{di}$) directly from the sweep response (see Fig. 10) in most encountered regions. The bulk ion drift speed can be derived from the sweep characteristic for negative bias (ion sampling) and for a drifting Maxwellian ion distribution as (e.g., Whipple 1965; Fahleson et al. 1974; Holmberg et al. 2012):

$$I_i \approx q_i n_i A_{LP} \sqrt{V_{di}^2/16 + k_B T_i / 2\pi m_i} \left(1 - \frac{U_{bias} + U_{float}}{W_i}\right). \quad (1)$$

where $T_i$, $m_i$, $q_i$ and $W_i$ is the temperature, mass, charge, and energy of the ions respectively, and $A_{LP}$ is the surface area of the probe. Normally the ion thermal energy is ≪ ion kinetic ram energy, and the $k_B T_i / 2\pi m_i$ term is neglected. This puts an upper constraint to the ion
temperature. Therefore, in most cases it represents the ram ion flux to the probe. Having the plasma density, gives the ion drift speed relative the spacecraft motion. For Jupiter's magnetosphere, RPWI will use PEP ion thermal energy estimates as input to the analysis.

The voltage-biased probes can be sampled continuously up to 763 sps, and in snapshots up to 49 ksps, allowing thermal plasma electron density and ion flux to be monitored at high rates. It also allows for performing a frequency spectrum of the density variations (δn/n) of plasma waves or structures. In addition, measurements from several probes can be combined (in current sampling mode) allowing for interferometric measurements to estimate the velocity of plasma structures. It is possible to discern the propagation direction and velocity in 3D of plasma inhomogeneities (e.g., LaBelle et al. 1986; Kintner et al. 1987; Holmgren and Kintner 1990; Wahlund et al. 1998, 2009; and references therein). Except for the need to have density inhomogeneities with enough amplitude in the plasma, the used probes should preferably not be in the wake of the spacecraft yet have a reasonable alignment with the plasma flow direction, and the speed need be slow enough to give a measurable time difference (0.1–1 ms), which gives a speed limit of about 10–100 km/s. Furthermore, some probes may operate at positive bias voltage, while others operate at negative bias potential simultaneously. This allows for directly comparing electron and ion inhomogeneities as well as detect regions with significant amounts of charged dust (e.g., in plumes, see Wahlund et al. 2009). Now, electric field (voltage, bias current) and number density (current, bias voltage) measurements cannot be made simultaneously with the same probe. Likewise, the electron density (positive voltage bias) and ion density (negative voltage bias) cannot be made simultaneously with the same probe. However, RPWI has four probes. We have therefore different operation modes that need be selected and planned for when to operate (cf. Sect. 6.3). Nominally the electric field components are measured with all four probes, and any density (δn/n) mode need be specially selected. However, RPWI always carry out Langmuir probe and MIME sweeps (takes about 3 s) each measurement cycle (usually 30 s long, see Table 18).

The accuracy of the Langmuir probes to measure the ambient electron population (electron sampling side, positive bias) is to a large degree dependent on the number of photoelectrons and secondaries emitted from the S/C, which in turn will be affected by the spacecraft potential and geometry of the spacecraft. The method works best for denser thermal plasma conditions and negative spacecraft potential where the photoelectrons escape radially away from the S/C and pose a much less of a problem (see top panel in Fig. 10).

The capability of the Langmuir probes to detect the ion (usually ram) currents in thin plasma depends largely on the LP-PWI preamplifier noise level, the leakage current level, the large size of probes (10 cm) and bias voltage range (e.g., ±80 V). The photoelectron emission from the probes is relatively constant with time (variation on time durations of the solar intensity, days) and can be corrected for. An example of a JUICE RPWI Langmuir probe sweeps just after launch is shown in Fig. 9, where the noise level can be accounted for in an Operational Domain diagram (Fig. 10), where the expected encountered plasma domains can be superposed. The upcoming Earth flyby in August 2024 is expected to give an increased understanding for the potential limitations of the Langmuir probe sweeps carried out by the four probes.

The instrument leak currents of a few pA (see Table 3 and Fig. 11) were measured on the 4xFM and FS LP-PWI booms + Langmuir probe sensors before delivery to the spacecraft. The probe to inner shield is the important one for science measurements. The pre-amplifiers have lower or similar leak currents (for ~TOhm resistance). The LP-MIME DC current receiver responses are shown in Fig. 11. These values can be compared to the Cassini LP noise limit of ≈100 pA (e.g., Wahlund et al. 2009).
<figure><img src="image_10.png" /><figcaption>**Fig. 9** An in-flight Langmuir probe sweep in the solar wind. Instrument saturation occurs at 6 µA for this high-gain sweep, which results in a flat response after +60 V. Twenty samples are sampled for each bias step to see the impedance response (which flattens out in time after about 3 ms). A sweep takes less than 2 seconds. In the solar wind the sweep is dominated by the photoelectrons emitted from the probe and the spacecraft. This sweep is uncalibrated. Further analysis will reveal if solar wind ion flux and electron temperature can be extracted. However, already from this sweep it is possible to infer a spacecraft surface potential of about +5 V, as the U-I-characteristic photo-electron exponential drop is shifted roughly –5 V compared to $U_{bias} = 0$ V and knowing $U = U_{float} + U_{bias}$</figcaption></figure>

Although the frequency dependence of the transfer functions is not very dependent on the preamplifier and receiver temperatures (as seen in Fig. 11), the DC offset is. Therefore, RPWI employs regularly in-flight calibration sweeps over two known resistors to correct for this effect. The transfer functions for AC current measurements (Fig. 12 and 13) gives the sensitivity and phase stability for the $\delta n/n$ measurements.

## 2.3 MIME

This RPWI/MIME experiment is the mutual impedance probe of JUICE. Mutual impedance probes (Storey et al. 1969) are electrical instruments designed to measure certain characteristic parameters of plasmas, such as electron density and temperature (Chasseraux et al. 1972). The measurement method is based on the electrical coupling between a pair of electrical antennae (transmitting and receiving) and the plasma in which the antennae are immersed (Grard 1969). In practice, an oscillating electrical signal produced by a current generator at a given frequency is injected into the plasma to be diagnosed by a first, transmitting electrical antenna (monopolar or dipolar). Simultaneously, the electrical signal propagating through the plasma is measured by means of the electrical potential difference produced on the second, receiving antenna (generally dipolar) at the frequency of the transmission signal. The ratio between the receiving voltage and the transmitting current is called the mutual impedance. This operation, consisting of simultaneous transmission and reception at a given frequency, is repeated over several discrete frequencies, step by step, in a predefined frequency range of interest, selected within the range 3 kHz–3 MHz in the case of the RPWI/MIME experiment. The mutual impedance is then calculated successively for each
<figure><img src="image_11.png" /><figcaption>**Fig. 10** The operational domains for the JUICE/RPWI Langmuir probes. Top panel: Electron current determination domain. A worst-case, with a positive S/C potential in excess of +2 V, would give a dominating photoelectron cloud near the spacecraft (red). When the probes are inside the S/C Debye sheath (blue-dashed line limiter) a less accurate parameter determination may result. The bias Voltage limit (green) is about 1/3 of the maximum sweep range. Bottom panel: Ion current determination domain. The large sensor, sensitive receiver electronics, and the +/−80 V bias voltage sweep range makes it possible to measure the ion current in large parts of Jupiter's magnetosphere, and determine, e.g., the ion drift speed. The electron density and plasma convection can also be determined in several complementary ways by RPWI. See Sect. 3 for references on typical encountered plasma conditions</figcaption></figure>

of these frequencies in order to construct the mutual impedance spectrum which constitutes
the observable of the MIME experiment.

To perform such a measurement, a mutual impedance experiment is therefore composed
of (i) several electric sensors to transmit and receive the electric signal, (ii) electronics boards
to generate the transmitted signal and treat the received signal, and (iii) an on-board software
to define the transmitted signal and perform the on-board analysis of the received signal.
<table><caption>**Table 3** Measured leakage currents from the 4 FM booms + sensors and the FS. The original requirement was <12pA</caption><thead><tr><td>FM boom/sensor</td><td>Probe to inner shield</td><td>Outer shield to inner shield</td></tr></thead><tbody><tr><td>LP1 (blue)</td><td>1.95 ± 0.1 pA</td><td>2.0 ± 0.2 pA</td></tr><tr><td>LP2 (yellow)</td><td>1.9 ± 0.2 pA</td><td>2.4 ± 0.3 pA</td></tr><tr><td>LP3 (orange)</td><td>0.7 ± 0.1 pA</td><td>1.0 ± 0.2 pA</td></tr><tr><td>LP4 (green)</td><td>0.9 ± 0.2 pA</td><td>2.0 ± 0.15 pA</td></tr><tr><td>LP spare (pink)</td><td>1.2 ± 0.2 pA</td><td>0.5 ± 0.3 pA</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>**Fig. 11** PFM LP-MIME transfer function for density (i.e., current) single-probe modes, with both high (HI) and low (LO) gain levels. Results from the on-ground calibration showing a good stability versus temperature</figcaption></figure>

Within RPWI, the MIME experiment makes use of (i) both the LP-PWI and RWI sensors, with (ii) the LP-MIME electronic board and (iii) a specific MIME sub-software within the RPWI onboard software.

Practically, for science purposes, MIME transmission and reception can be configured to select one or two Langmuir Probes as transmitter(s); and either another pair of Langmuir Probes or the RWI antennas as receiver. The emitted signal voltage is kept small enough to avoid spurious nonlinear plasma responses (Bucciantini et al. 2022). Reception on the Langmuir probes is performed in the nominal MIME frequency range (low frequency mode: 3 kHz–1.5 MHz), that can be extended to higher frequency by performing the reception on RWI antennas (high frequency mode: 80 kHz–3 MHz). However, the MIME transmitted signal can in principle be received by any other electric or magnetic field sensor, which makes it also possible to use MIME for calibrations and cross calibrations. The active measure-
<figure><img src="image_13.png" /><figcaption>**Fig. 12** PFM LP-MIME phase difference between two single-probe in density (i.e., current) measurement mode, for both high (HI) and low (LO) gain levels as a function of frequency. Results from the on-ground calibration showing a good stability versus temperature and a phase difference <1° below 10 kHz</figcaption></figure>

ments performed by MIME are near-field measurements, which is a standard technique for
antenna calibration.

The functionality of the MIME subsystem in LP-MIME is that of a laboratory network analyser, which is an advanced Ohm-meter used to measure the frequency dependent complex impedance of an unknown device or medium, namely in this case the space plasma to be diagnosed. Details on the implementation of the LP-MIME electronics are described in Sect. 4.2.5.

MIME is a highly versatile experiment whose onboard acquisition modes can be selected by telecommand from ground to adapt to different environments encountered and to different scientific objectives. Several operation modes have been defined to prioritize either the time or frequency resolution, all within a constant telemetry envelope. Moreover, MIME has the capability to simultaneously process signals from several receiving sensors among the Langmuir Probes (when in low frequency mode) or RWI antennas (when in high frequency mode), leading to the simultaneous acquisition of two or three mutual impedance spectra from different receiving paths, useful to increase the robustness of the measurement and the subsequent plasma diagnostic as well as useful for internal calibration. This results in 96 transmission/reception configuration possibilities and in 9 nominal science modes. Calibration campaigns during the cruise phase and in the early mission phase will give insight for identifying the most appropriate subset of MIME configurations that will be used in nominal science operations. In addition to these science modes, 2 technological modes have been developed to offer the capability to:

* track the plasma dynamics with a higher time resolution. Within 1 sec, the associated operating mode performs 6 successive acquisitions over a reduced bandwidth centred on
<figure><img src="image_14.png" /><figcaption>**Fig. 13** Example of PFM LP-MIME noise level for density (i.e., current) measurement, here in high gain (HI) and measured on ADC C. Similar measurements were performed during the on-ground calibration for all four ADC (A, B, C & D) and for each gain level (HI & LO). These measurements give the equivalent noise level for $\delta n/n$ measurements</figcaption></figure>

a spectral characteristic associated to the plasma frequency previously determined on-board.

* assess the value of the magnetic field magnitude. The associated mode performs 3 synchronous high-resolution acquisitions in 3 reduced frequency ranges associated to the electron gyrofrequency and its two first harmonics (previously determined on-board), hence allowing to precisely derive the magnetic field magnitude.

MIME can produce electric field spectra either while transmitting an electric signal into the plasma to produce a mutual impedance spectrum (*active* spectrum) or without transmitting any electric signal into the plasma to produce an electric field spectrum (*passive* spectrum). The passive spectrum is used to identify any electric fluctuation present in the plasma independently of the MIME transmitted signal. In science mode, MIME outputs one passive electric spectrum for reference and one or several mutual impedance spectra that will allow ground-based analysis of the local plasma.

Physically, MIME measures the plasma transfer function, in other words, MIME is used to obtain the dielectric permittivity of the plasma. The mutual impedance spectrum is in fact expressed as an integral function of the inverse of the plasma dielectric which determines what kind of wave modes (i.e. eigenmodes) the plasma supports. The plasma's eigenmodes correspond to the frequencies at which the dielectric cancels out, so the mutual impedance spectrum contains resonances at frequencies close to the plasma's eigenmodes.

For instance, the electron plasma frequency depends only on the electron density. In an unmagnetized plasma, this resonance appears as a sharp peak or a cut-off, depending on the electron temperature, in the MIME frequency mutual impedance spectrum. It is this frequency that is sought with a mutual impedance probe to obtain an absolute measurement of
the electron density. The plasma eigenmode known as the Langmuir wave, found at frequencies equal or slightly higher than the plasma frequency, has dispersion properties that depend directly on the temperature of the electrons. In a plasma characterized by a weak magnetic field, it is this natural mode that makes it possible to identify the electron temperature.

If the plasma is magnetized, the upper hybrid and gyro-harmonic resonance frequencies (i.e. the electron Bernstein modes) appear as resonances in the mutual impedance spectra, and enable the retrieval of both the magnetic field strength and the plasma density.

In practice, it is essential that the plasma frequency, or the upper hybrid frequency, is contained within the frequency range of the instrument, in order to measure the electron density and, under certain conditions, the electron temperature. MIME measurements are therefore based on a frequency measurement, which is very well-controlled experimentally.

On the one hand, the MIME onboard products are electric spectra that are either mutual impedance spectra (also referred as *active* spectra) and electric field spectra (*passive* spectra). On the other hand, the final physical products of MIME are the electron density and temperature. They are derived from the telemetered MIME mutual impedance spectra with a ground calculation that is based on a detailed numerical modeling of the MIME experiment to produce synthetic mutual impedance spectra. This MIME experimental model uses a two-step approach.

First, we compute the electric potential emitted by a point-like charge embedded in the plasma. At this stage a hypothesis is made on the actual electron velocity distribution function, usually considered as Maxwellian, bi-maxwellian (Gilet et al. 2017) or kappa (Gilet et al. 2019).

Second, we compute the electric potential generated by MIME at the receiving sensor's location, that is influenced by the spacecraft conductive surfaces, and potentially also by the effect of the inhomogeneous plasma sheath surrounding the JUICE spacecraft on the propagation of the MIME signal (Wattieaux et al. 2019; Bucciantini et al. 2023). For this purpose, we take into account the shape of the different electric sensors as well as their interaction with the conducting spacecraft body using the so-called Discrete Surface Charge Model (hereafter DSCD). This model, first developed in Béghin and Kolesnikova (1998) was proved successful in the context of the Rosetta RPC-MIP mutual impedance experiment (Gilet et al. 2020; Wattieaux et al. 2020). This model enables us to take into account the influence of the different conductive surfaces, including the large JUICE solar panels, on the mutual impedance measurement.

In this computation, we have considered the following electric coupling between the different parts of the spacecraft and the different parts of the MIME experiment: a 4.7 nF link capacitance between the transmitter and the LP sensors used as emitting sensors, 1.92 Ω resistance and 480 pF capacitance for the harness connecting the LPs sensors to the receivers electronics, and 25 pF for the capacitance of the receiver RWI amplifiers.

We present in Fig. 14 different synthetic mutual impedance spectra expected in an unmagnetized plasma composed of a Maxwellian electron population, with electron density of 200 cm⁻³ and temperature of 1 eV, as expected at Ganymede at an altitude of about 500 km from the surface (Eviatar et al. 2001). By unmagnetized, we mean that the electron cyclotron frequency is considered negligible compared to the electron plasma frequency. To illustrate the versatility of the MIME experiment, we consider in this computation different sets of electric sensors, both in emission and reception, corresponding to three cases presented in Figs. 44 and 45 (namely cases A, B, and C). The different colors correspond to measurements on different reception antennas. We consider the following antenna configurations: dipole emission on Langmuir Probes LP1-LP4 with dipole reception on Langmuir Probes LP2-LP3 for mode A (left panel), monopole emission on Langmuir Probe LP4 with
<figure><img src="image_15.png" /><figcaption>**Fig. 14** Synthetic mutual impedance spectra of the RPWI/MIME experiment, using different instrumental modes and antenna configuration. Cases A, B, and C correspond to the transmission-reception RPWI sensor configurations described in Figs. 44 and 45. The plasma is modeled as composed of a Maxwellian electron population of density $200 \text{ cm}^{-3}$ and temperature of 1 eV</figcaption></figure>

dipole reception on both Langmuir Probes LP2-LP3 and Langmuir Probes LP2-LP1 for mode B (middle panel), dipole emission on Langmuir Probes LP2-LP4 with dipole reception on the RWI triad antenna for mode C (right panel). Note that the frequency range of operation chosen here is the entire available frequency range for MIME in LF mode (left and middle panels) and HF mode (right panel). The amplitude and phase of the complex mutual impedance spectra are shown in the top and bottom panels, respectively. The mutual impedance spectra expected in the plasma have been normalized to the mutual impedance spectra expected in vacuum. In this example, each synthetic mutual impedance spectrum, whatever the chosen electric sensors configuration, exhibits a clear resonance at the plasma frequency (~130 kHz, indicated by a vertical dashed line) that would be used to retrieve the absolute plasma density. Note that a significant phase rotation is also expected at the plasma frequency, which can also be used to diagnose the plasma density. The electron temperature controls the actual shape of the mutual impedance spectrum, and would be retrieved experimentally from a best fit procedure between the measured and the modeled mutual impedance spectra.

## 2.4 SAMWISE

By integrating several closely related subsystems, SAMWISE (Search-coil and Analyzer Module for Waves and Instabilities Science Experiment) aims at measurements of electromagnetic waves and instabilities, which can interact with different particle populations in the surrounding plasma medium. It primarily makes use of the three axial Search Coil Magnetometer antenna (SCM, see Sect. 4.2.11) for vector (3D) measurements of the fluctuating magnetic field. SAMWISE also uses the measurements of the vector (3D) electric field fluctuations, and variations of the plasma density ($\delta n/n$) by the four Langmuir Probe sensors (LP-PWI, see Sect. 4.2.8–4.2.10). All seven analogue input signals from these sensors are conditioned, sampled, and analysed by the Low Frequency Receiver (LFR, see Sect. 4.2.6),
<figure><img src="image_16.png" /><figcaption>**Fig. 15** SCM in-flight noise level for the X component as a function of frequency, obtained in the near-Earth solar wind superposing spectrum from high-frequency snapshots (48.8 ksp; blue line) and from continuous waveforms (763 Hz; black line). Spectrum from high-frequency snapshots is plotted between 150 Hz and 20 kHz. Spectrum from continuous waveforms is plotted from 1 Hz to 280 Hz. Noise level measured on ground in red. The behaviour of Y and Z components is similar to that of the X-component. Note that preliminary calibrations have been applied</figcaption></figure>

which also controls power for the SCM preamplifier. Higher-level data analysis procedures are implemented in the LFR flight software executed by the RPWI DPU (see Sect. 4.2.4, and 5.1), which also makes use of real time J-MAG magnetic field data sent on-board to RPWI.

The SAMWISE frequency range is defined by the SCM working frequency band, which covers a broad interval from 0.1 Hz to 20 kHz. Figure 15 (black and blue lines) shows the in-flight noise level which corresponds to the SCM sensitivity. This noise level is obtained combining the spectrum obtained from high-frequency snapshot waveforms (48.8 ksp) and the spectrum obtained from continuous waveforms (763 sps). The measurements have been obtained in the near-Earth solar wind during the first JUICE in-flight interference campaign after commissioning phase (July 2024). The red line shows the noise level measured on ground during SAMWISE calibrations. The agreement between these first in-flight data and on-ground measurements is overall very good, confirming that the performance of the SCM sensor satisfies RPWI science requirements. The lowest value of the noise level corresponds to the best sensitivity of about 10 fT Hz⁻¹/² close to 1 kHz. The in-flight SAMWISE data also indicate that the interferences from the JUICE platform are rather limited below 20 kHz, mostly because SCM has been accommodated on the MAG boom at about 8.2 m from the platform. Nevertheless, a few interferences have been observed. The detailed analysis of such interferences is ongoing, and they will be reported elsewhere (Retino et al., in preparation), together with the full description of the SCM sensor and performance. The two narrowband emissions above 9 kHz in Fig. 15 are related to the drive frequency of the MAG-IBS instrument. They were expected from ground measurements, are stable and can be removed by post-processing. Narrowband peaks above the noise level around 200 Hz are likely due to emissions from the reaction wheels. Such emissions can also be removed with postprocessing. Although calibrations of the SAMWISE data are preliminary, these first in-flight data already show that the performance of the SAMWISE experiment is adequate to properly resolve key plasma waves, especially Alfvén-, magnetosonic- and Whistler mode waves as well as radio waves (especially the QP and b-KOM), in the Jupiter system.

Beside the full multidimensional waveforms sampled at variable rates, which served for first in-flight measurements shown in Fig. 15, SAMWISE will also run on-board analysis procedures to reduce the telemetry volume by science-based data compression. These data
products include time and frequency averaged multidimensional spectral matrices involving the full vector of magnetic field fluctuations, the full vector of electric field fluctuations, and fluctuations of plasma density. The spectral matrices contain information on amplitudes, mutual phases, and coherences of all these signals. High level on-board processing in the LFR software result in the very compressed set of wave polarization and propagation parameters (with a heritage from the Van Allen probes missions), which will be used as preview data set for selective download of full waveform data. Another high-level data set of on-board processed data (with a heritage from the Solar Orbiter mission) is based on automatic recognition of dust impacts on the spacecraft and wave-like features from the waveforms. Spectral matrices, wave polarization and propagation data, or data products based on waveforms will be also alternatively used to trigger the full resolution waveform captures.

## 2.5 JENRAGE

JENRAGE (Jovian ENvironment Radio Astronomy and Ganymede Exploration) processes the electric field signals sampled by the three-dipole antenna (Radio Wave Instrument, RWI, see Sect. 4.2.12) and the High Frequency Receiver (HFR, see Sect. 4.2.7). Within RPWI, JENRAGE covers the high frequency range from 80 kHz up to 45 MHz. The RWI is mounted on the MAG-boom at 8.6 m away from the spacecraft body. Radio signals detected by three RWI dipole antenna are picked up by sensitive wide-band RWI preamplifiers (cf. Sect. 4.2.13), and are transferred to the HFR board in the RPWI-EBOX via the long RWI-HF harness with the length of 17 m. After a low path filter at the entrance of HFR, 3 channels are sampled with the rate of 90 Msps and digitally filtered into ~300 kHz bands. The DPU in EBOX converts those digital signals to the power and cross spectra.

### 2.5.1 Planned Operation Modes

Each dipole antenna element of the RWI has no intrinsic directivity. However, the combination of the three-axis electric field components enables detailed radio observations with the goniopolarimetric capabilities. It can provide the direction of arrival, the flux density and the polarization state and degree of observed radio waves, as a full-sky radio imager orbiting Jupiter and Ganymede. Such capabilities are applicable in the short antenna regime, i.e., when the effective electric lengths of the antennas are much shorter than the observed wavelength. The power of the three-axis radio wave detection has been proved in the Kronian system studies by the RPWS/HFR instrument aboard the Cassini spacecraft up to 2 MHz with 3 monopole antennas (10 m physical length) (Gurnett et al. 2004; Cecconi and Zarka 2005). The experiment will employ goniopolarimetry in the Jovian system, where simulations have shown that an accuracy of 2 degrees can be achieved below 1.5 MHz (Fischer et al. 2021). The in-flight RWI calibrations will provide the effective electrical length and direction of each of the three RWI antenna. JENRAGE's telemetry modes are summarized in Sect. 5.8 and 6.3. There are at least three primary modes of operation:

* The most basic mode is 'Radio-Full (nominal-sweep)', which is used during the Jovian orbiting phase, when the JENRAGE aims to monitor the Jovian radio emissions continuously. In this mode, spectra are taken once per sequence cycle (usually every 30 s but can be longer) in the frequency range 80 kHz – 45 MHz with semi-logarithmic 256 frequency channels, after averaging in frequency.

* The 'Radio-burst' mode is primary used for occultations of intense Jovian radio emission by icy moons and their ionospheres. In this mode, spectra are taken once per 0.2, 0.5, 1 or 2 seconds in 80 kHz – 2.096 MHz with linear-spacing 72 frequency channels after averaging in frequency.
* The ‘passive sub-surface radar (PSSR)’ modes target the electric characteristics of the crust and sub-surfaces of Europa, Ganymede, and Callisto, using the reflections of penetrated Jovian radio emissions (HOM/DAM) (e.g., Kumamoto et al. 2018). JENRAGE have three PSSR modes:

* ‘PSSR1’ mode takes a spectrum matrix with 4320 frequency channels (rich) or a power spectrum of 540-frequency channels (survey) once per 30 seconds. It can potentially show an ‘interferometry pattern’ appeared in a frequency-time spectrogram, which is caused by the differences of the resonance between the waves directly from Jupiter and the waves reflected from icy moons in different wavelengths. This mode is for coherent and continuous Jovian radiation during the waiting time of the arrival of reflected waves from icy moons.

* ‘PSSR2’ mode takes the down-converted waveforms with selected 16 frequency channels (rich) or their autocorrelation in measured 45 frequency channels (survey) once per 30 seconds. It is for incoherent and short Jovian radiation like ‘s-burst’. Io-DAM s-burst with dispersive fine spectral structures is the useful incident for PSSR, because its short duration of several 10 s msec can be potentially used like Chirp radar signals.

* ‘PSSR3’ is the simplest, which obtains 11.2 msec down-converted waveforms at a single frequency. The signals are transmitted to the Earth without any data processing in DPU, and all correlation analyses will be made on the ground.

### 2.5.2 Sensitivity Characteristics

Figure 16 shows the geometry of the three dipole antenna elements of the RWI on the MAG-boom. Six antenna elements are BeCu antenna rods with the length of 1.25 m and the diameter of 1 cm, attached on the cubic chassis which stores the preamplifier electronics inside. The measured components are called *U*, *V* and *W* with right-handed coordinates.

Because it was impossible to deploy S/C appendices (such as RWI antenna, MAG-boom, and solar panels) in any final ground tests, numerical antenna simulations were performed by Fischer et al. (2021). Moreover, the actual performance of the effective antenna lengths and directions can only be measured after the launch. Planned in-flight calibrations for direction finding and polarization are associated with the Earth flyby operations, using the radio signals from known source locations and wave polarization. Suitable sources are Auroral Kilometric Radiation (AKR) from the Earth in 100–800 kHz. When JUICE is not too far from Earth solar type-III bursts in wider frequency range can also be used for the flux calibration with simultaneous detections by Earth-orbiting spacecraft like Wind WAVES (Bougeret et al. 1995) and Arase PWE (Kasahara et al. 2018) up to 10 MHz. JUICE will make a flyby of Earth in August-2024, and two more in 2026 and 2029 respectively doing simultaneous measurements with other spacecraft (see also Sect. 6.4).

Just after the in-flight RWI antenna deployment operation it was detected that JENRAGE suffers multiple harmonic lines of noise from 200 kHz (not shown) and upward with a wide-band enhancement in the 3–10 MHz range (see Fig. 17). RPWI suffers no science loss due to the strong Jovian radiation, but it may impact the objectives requiring weak signal detections in the frequency range 3–10 MHz, like PSSR. The upcoming Earth flyby in August 2024 will characterise this interference and mitigation methods will be applied like the line noise elimination logic in the onboard HF software in the DPU. The real performance of JENRAGE and its suitable operation with the revised telemetry definitions will be carefully established during the long cruise.
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 16** The Radio Wave Instrument (RWI), a tri-dipole antenna system which consist of 3-axis electric field antenna with 2.5 m tip-to-tip length. (above) The location of the RWI on the MAG-Boom deployed from the spacecraft. (below) The axis configuration of the RWI just before launch. Side note, compared to Fig. 2 in Fischer et al. (2021) the V and W antennas became interchanged during integration</figcaption></figure>

### 2.5.3 Numerical Antenna Calibration for RWI

Antenna pattern is critically important for the JENRAGE science for the direction finding and polarization analyses. The numerical analysis presented by Fischer et al. (2021) describes the expected reception properties of the RWI antennas by calculating the so-called effective antennas. Table 4 gives the spherical coordinates of the physical RWI antennas in comparison to the effective antennas for a capacitive load of 25pF. The angle $\gamma$ in the last column gives the angular deviation of the effective antenna from the physical one, which
<figure><img src="image_19.png" /><figcaption>**Fig. 17** Detection capability of the electric field by JENRAGE is due to the combination of RWI and HF. RWI preamplifier temperature is assumed to be -140 °C. Real antenna calibration is not executed yet. The noise floor expected before launch is in pink dashed line. Green and red lines show the noise level seen after the launch suffered by the radiation of the spacecraft system. Expected levels of Jovian radiation at Ganymede (peak and typical) and the galactic background radiation are also shown</figcaption></figure>

is caused by the induced currents on the conductive surface of the spacecraft. The deviations lead to U-V, V-W, W-U angles between the effective dipoles of 72.7°, 82.1°, and 92.6°, respectively.

As described in Sect. 2.5.2, Table 4 shows that the effective length of the W antenna is around twice that compared to the U and V antenna. The reason for this is most likely that the currents on the MAG boom induce larger currents in the V-dipole, since the angle between the MAG boom and the V dipole is just 28°, whereas the angles between the U and W-dipole and the MAG boom are 74° and 68°, respectively.

The results in Table 4 were calculated for a frequency of 300 kHz, which is in the so-called quasi-static frequency range where the wavelength is still large in comparison to the dimensions of the spacecraft. Fischer et al. (2021) found that the effective angles deviate by less than 2° up to 1.5 MHz, so reasonable direction-finding and polarization measurements can be done up to this frequency. At higher frequencies, the effective antennas become complex quantities depending on frequency and incoming wave direction. The gain pattern of each dipole changes from a torus-like shape at quasi-static frequencies to a pattern with multiple lobes at higher frequencies as Fig. 18 shows. Actual performance will be confirmed by in-flight calibration using the received signals of known source locations and wave polarization, for example Auroral Kilometric Radiation (AKR) from the Earth.

## 2.6 Positions and Alignment of RPWI Sensors

The locations of the RPWI sensors on the JUICE spacecraft are shown in Fig. 19. The origin of the spacecraft coordinate system is defined to be located at the separation plane between the S/C and the launch adapter, in the centre of the launch adapter, i.e., slightly outside the spacecraft body at the bottom of Fig. 19.

The LP-PWI probe mechanical positions are given in Table 5, where also the LP-to-LP separation vectors, based on the LP SPICE positions, and the hardware limited simultaneous sensor combinations of LP-PWI are given. The hardware constrains the combinations
<table><caption>**Table 4** Spherical coordinates of physical and effective RWI antennas with loaded ports (25pF). The letter h denotes the effective length, the angle θ is the colatitude, and φ is the azimuth in the JUICE spacecraft coordinate. The angle γ gives the deviation of the effective antennas from the physical ones. Table adapted from Fischer et al. (2021)</caption><thead><tr><th rowspan="2">Antenna name</th><th colspan="3">Physical antennas</th><th colspan="4">Effective antennas loaded ports (25 pF)</th></tr><tr><th>h [m]</th><th>θ [deg]</th><th>φ [deg]</th><th>h [m]</th><th>θ [deg]</th><th>φ [deg]</th><th>γ [deg]</th></tr></thead><tbody><tr><td>U+U-</td><td>2.5</td><td>69.6</td><td>-22.1</td><td>0.26</td><td>45.1</td><td>-25.9</td><td>24.7</td></tr><tr><td>V+V-</td><td>2.5</td><td>22.1</td><td>134.4</td><td>0.48</td><td>27.7</td><td>155.9</td><td>10.6</td></tr><tr><td>W+W-</td><td>2.5</td><td>98.1</td><td>64.9</td><td>0.23</td><td>84.8</td><td>73.1</td><td>15.6</td></tr></tbody></table>

<figure><img src="image_20.png" /><figcaption>**Fig. 18** Far-field gain pattern of U-antenna for 1 MHz (left), 18 MHz (middle), and 45 MHz (right). The colour scales go from 0 (dark blue) to 2 (red) and denote the total linear gain</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Fig. 19** RPWI (and other) sensor locations on the JUICE spacecraft: LP-PWI 1 through 4, RWI, and SCM. In this representation the launch adapter is on the side facing below and not visible</figcaption></figure>

to only four possible differential modes, while allowing all four single-ended modes. The single ended modes can of course be further combined to all 6 differential components by on-ground analysis.
<table><caption>**Table 5** Langmuir probes URF (SPICE) mechanical positions</caption><thead><tr><th rowspan="2">LP-PWI</th><th colspan="3">Spacecraft coordinates [mm]</th><th rowspan="2">Separation [mm]</th></tr><tr><th>X</th><th>Y</th><th>Z</th></tr></thead><tbody><tr><td>LP-PWI 1 (LP1)</td><td>2484</td><td>2895</td><td>4377</td><td></td></tr><tr><td>LP-PWI 2 (LP2)</td><td>1455</td><td>-3238</td><td>5303</td><td></td></tr><tr><td>LP-PWI 3 (LP3)</td><td>1455</td><td>-3238</td><td>-1849</td><td></td></tr><tr><td>LP-PWI 4 (LP4)</td><td>-2768</td><td>2686</td><td>4432</td><td></td></tr><tr><td>LP1-LP2</td><td>1029</td><td>6133</td><td>-926</td><td>6287.3</td></tr><tr><td>LP1-LP3</td><td>1029</td><td>6133</td><td>6226</td><td>8799.8</td></tr><tr><td>LP1-LP4</td><td>5251</td><td>209</td><td>-55</td><td>5256.4</td></tr><tr><td>LP2-LP3</td><td>0.00</td><td>0.00</td><td>7153</td><td>7152.0</td></tr><tr><td>LP2-LP4</td><td>4223</td><td>-5924</td><td>872</td><td>7327.1</td></tr><tr><td>LP3-LP4</td><td>4223</td><td>-5924</td><td>-6281</td><td>96114</td></tr><tr><td colspan="5"><strong>Possible AC & DC differential modes</strong></td></tr><tr><td></td><td>ΔU1</td><td>ΔU2</td><td>ΔU3</td><td></td></tr><tr><td>Mode 0 [3D] (default)</td><td>LP1-LP2</td><td>LP2-LP3</td><td>LP3-LP4</td><td></td></tr><tr><td>Mode 1 [2D]</td><td>LP1-LP4</td><td>LP1-LP3</td><td>LP3-LP4</td><td></td></tr><tr><td>Mode 2 [2D]</td><td>LP1-LP2</td><td>LP2-LP4</td><td>LP1-LP4</td><td></td></tr><tr><td>Mode 3 [1D]</td><td>LP1-LP4</td><td>LP1-LP4</td><td>LP1-LP4</td><td></td></tr></tbody></table>

<table><caption>**Table 6** Electric field vector (E) transformation matrix for Mode 0</caption><thead><tr><th>Voltages U [V]</th><th colspan="3">Transformation matrix M [m]</th><th>E-field [V/m]</th></tr></thead><tbody><tr><td>ΔU<sub>12</sub></td><td>1.029</td><td>6.133</td><td>-0.926</td><td>E<sub>x</sub></td></tr><tr><td>ΔU<sub>23</sub></td><td>0.0</td><td>0.0</td><td>7.153</td><td>E<sub>y</sub></td></tr><tr><td>ΔU<sub>34</sub></td><td>4.223</td><td>-5.924</td><td>-6.281</td><td>E<sub>z</sub></td></tr><tr><th>E-field [V/m]</th><th colspan="3">Inverse transformation matrix M<sup>-1</sup> [m<sup>-1</sup>]</th><th>Voltages U [V]</th></tr><tr><td>E<sub>x</sub></td><td>0.1852</td><td>0.1923</td><td>0.1917</td><td>ΔU<sub>12</sub></td></tr><tr><td>E<sub>y</sub></td><td>0.1320</td><td>-0.0112</td><td>-0.0322</td><td>ΔU<sub>23</sub></td></tr><tr><td>E<sub>z</sub></td><td>0.0</td><td>0.1398</td><td>0.0</td><td>ΔU<sub>34</sub></td></tr></tbody></table>

For the default LP Mode 0 [3D], to obtain the electric field vector, *E*, in S/C coordinates, from the measured voltage differences, $\Delta U$, the transformation matrix M and its inverse $M^{-1}$ ($\Delta U = ME$ or equivalently $M^{-1}\Delta U = E$), are given in Table 6.

The SCM and sensor axes are shown in Fig. 20, where with W-axis of the SCM unit points towards the spacecraft. The SCM and RWI (SPICE) sensor positions in spacecraft coordinates are given in Table 7 and 8.

## 2.7 Surface Charging of JUICE

The interaction between a spacecraft and its space environment will charge the spacecraft and give it a surface potential (e.g., DeForest 1972; Wahlund et al. 1999a,b; Eriksson et al. 1999; Eriksson and Wahlund 2006). Differential surface charging will occur since there are different dielectric surfaces on the spacecraft, such as the HGA and the radiators. Since spacecraft surface charging modifies the near spacecraft plasma environment it can im-
**Fig. 20** SCM sensor directions. The SCM W sensor is parallel with the MAG boom and the U and V sensors are rotated by 45 degrees around this axis. The spacecraft frame is not given here

<figure><img src="image_22.png" /><figcaption>**Table 7** SCM (SPICE) mechanical positions in spacecraft coordinates and rotation matrix from sensor frame to S/C frame. The position of the SCM is in the centre of the unit</figcaption></figure>

<table><caption>**Table 8** RWI (SPICE) mechanical positions in spacecraft coordinates and rotation matrix from sensor frame to S/C frame. The position of the RWI is the reference hole of the unit reference frame (URF)</caption><thead><tr><th>SCM</th><th colspan="3">Spacecraft coordinates [mm]</th></tr></thead><tbody><tr><th scope="row">SCM position</th><td>6541</td><td>-857</td><td>-6205</td></tr><tr><th scope="row">B-field [T] S/C</th><th colspan="3">Rotation matrix (SPICE)</th></tr><tr><th scope="row">B<sub>X</sub></th><td>-0.0677</td><td>0.7742</td><td>-0.6293</td></tr><tr><th scope="row">B<sub>Y</sub></th><td>-0.9962</td><td>-0.0872</td><td>0</td></tr><tr><th scope="row">B<sub>Z</sub></th><td>-0.0548</td><td>0.6269</td><td>0.7771</td></tr></tbody><thead><tr><th></th><th colspan="3">B-field [T] SCM</th></tr></thead><tbody><tr><th scope="row">B<sub>U</sub></th><td></td><td></td><td></td></tr><tr><th scope="row">B<sub>V</sub></th><td></td><td></td><td></td></tr><tr><th scope="row">B<sub>W</sub></th><td></td><td></td><td></td></tr></tbody></table>

<table><thead><tr><th>RWI</th><th colspan="3">Spacecraft coordinates [mm]</th></tr></thead><tbody><tr><th scope="row">RWI position</th><td>6750</td><td>-836</td><td>-6719</td></tr><tr><th scope="row">E-field [V/m] S/C</th><th colspan="3">Rotation matrix (SPICE)</th></tr><tr><th scope="row">E<sub>X</sub></th><td>-0.8749</td><td>0.2683</td><td>-0.4031</td></tr><tr><th scope="row">E<sub>Y</sub></th><td>0.3303</td><td>-0.2779</td><td>-0.9020</td></tr><tr><th scope="row">E<sub>Z</sub></th><td>-0.3541</td><td>-0.9224</td><td>0.1546</td></tr></tbody><thead><tr><th></th><th colspan="3">E-field [V/m] RWI</th></tr></thead><tbody><tr><th scope="row">E<sub>U</sub></th><td></td><td></td><td></td></tr><tr><th scope="row">E<sub>V</sub></th><td></td><td></td><td></td></tr><tr><th scope="row">E<sub>W</sub></th><td></td><td></td><td></td></tr></tbody></table>

pact the measurements of both particles and fields. In preparation for the RPWI operations we have performed several studies of the spacecraft environment and surface charging of JUICE. These studies include conditions in the Jovian magnetosphere, the ionospheres of Jupiter's moons Ganymede and Europa, and the solar wind at 1 AU. For these simulations (Holmberg 2020, 2023, Bochet et al. 2023) we have used the Spacecraft Plasma Interaction Software (SPIS, Sarrailh et al. 2015); a 3D model of the JUICE spacecraft; a catalogue
<figure><img src="image_23.png" /><figcaption>**Fig. 21** The surface potential of the JUICE spacecraft in sunlight (blue) and eclipse (black) at 9.5, 15, and 26.3 $R_J$ from the centre of Jupiter, in the densest region of the plasma disk. The bars give the potential range of the different spacecraft surfaces</figcaption></figure>

of each spacecraft surface materials and its properties; and descriptions of each simulated environment. Examples of JUICE spacecraft surface charging typically encountered in the Jovian magnetosphere is presented in Fig. 21. The charging within Ganymede's and Europa's ionospheres at 400 km can reach down to -20 V. The modification of the spacecraft plasma environment from such surface potentials, even at the distant locations of the Langmuir probes, is important to understand. For RPWI the effects are mitigated by the Langmuir probe sweep range of up to ±80 V, when estimating the thermal plasma properties. The surface charging of JUICE in the solar wind at 1 AU is expected to vary between -42 V to +12 V for typical solar wind conditions (Holmberg et al. 2024). One of the first RPWI Langmuir probe sweep measurements in the solar wind during near Earth commissioning indicated an average spacecraft surface potential of about +5 V (see Fig. 9). This is in excellent agreement with the SPIS simulations, which shows an average spacecraft surface potential of around +6 V for a typical solar wind environment (Holmberg et al. 2024).

## 2.8 Summary of RPWI Capabilities

The RPWI science products and related performances are given in Table 9. RPWI ensures that the *electron density*, $n_e$, can be determined in all plasma environments encountered by JUICE. This is achieved by measuring $n_e$ through several complementing techniques, ensuring good measurements in all plasma regions as well as accuracy evaluation by method comparisons:

* LP-PWI bias voltage sweeps (see Fig. 10, for $n_e > 10 \text{ cm}^{-3}$).

* Derive the upper hybrid resonance frequency ($f_{uh}$) from natural upper hybrid emission bands (for $n_e > 0.01 \text{ cm}^{-3}$).

* Monitoring the spacecraft potential ($U_{SC}$) and calibrating toward $f_{uh}$ (or possibly the PEP electron spectrometer). (for $n_e < 1 \text{ cm}^{-3}$)

* Continuous high time resolution sampling of the probe current (allows ms time resolution, $\delta n/n$, for $n_e > 10 \text{ cm}^{-3}$).

* Active mutual impedance measurements (MIME, for $n_e > 0.01 \text{ cm}^{-3}$).

* Refractive index measurement near radio frequency wave cut-offs (e.g., during ionosphere remote sounding, for $n_e > 10 \text{ cm}^{-3}$).

The *ion density*, $n_i$, can be determined by

* Langmuir probe sweeps in denser plasmas ($n_i > 1 \text{ cm}^{-3}$).

The *thermal electron temperature* ($T_e$) is determined through two techniques:
<table><caption>**Table 9** RPWI science products and performance verified mostly from ground measurements and pending in-flight calibrations</caption><thead><tr><th>RPWI Measured Quantity</th><th>Range/Product</th><th>Error/Sensitivity</th></tr></thead><tbody><tr><td>Electric field vector, δE(f)</td><td>DC – 1.5 MHz<br>Dust detection.<br>Polarization with δB(f).<br>Poynting flux with δB(f).</td><td>DC: ≈ 0.9 μV/m (instrument)<br>(Calibration for S/C influence needed in flight)<br>Spectral (differential):<br>&lt;1 μV/m/√Hz (&gt;500 Hz)</td></tr><tr><td>Electric field vector, δE(f)</td><td>80 kHz – 45 MHz<br>Wave vector (δk)<br>Polarization</td><td>30 – 50 nV/m/√Hz (at 10 MHz)<sup>1)</sup><br>~1° (at 5 MHz)<br>~10% (at 5 MHz)<br>(Direction finding &amp; sensitivity calibration needed in-flight)</td></tr><tr><td>Magnetic field vector, δB(f)</td><td>0.1 Hz – 20 kHz<br>Polarization with δE(f).<br>Poynting flux with δE(f).</td><td>8 pT/√Hz (at 1 Hz)<br>0.6 pT/√Hz (at 10 Hz)<br>0.06 pT/√Hz (at 100 Hz)<br>20 fT/√Hz (500 Hz-10 kHz)</td></tr><tr><td>Electron density (n<sub>e</sub>)</td><td>10<sup>-4</sup> – 10<sup>5</sup> cm<sup>-3</sup></td><td>&lt;10% (&gt;10 cm<sup>-3</sup>)<br>Leak current ≈0.5pA &lt;100 Hz</td></tr><tr><td>Density fluctuations (δn/n)<br>δE or δn interferometry</td><td>DC – 10 kHz<br>&lt;1000 km/s</td><td>&lt;5% below 1 kHz<br>Phase accuracy &lt;1° for &lt;20 kHz.<br>Phase response correctable.</td></tr><tr><td>Ion density (n<sub>i</sub>)</td><td>1 – 10<sup>5</sup> cm<sup>-3</sup></td><td>&lt;20%</td></tr><tr><td>Electron temperature (T<sub>e</sub>)</td><td>0.01 – 100 eV</td><td>&lt;20%</td></tr><tr><td>Ion drift speed (V<sub>di</sub>)</td><td>0.1 – 200 km/s</td><td>&lt;20%</td></tr><tr><td>Ion temperature (T<sub>i</sub>) upper constraints</td><td>0.02 – 20 eV</td><td>Constrained by &lt;mV<sub>di</sub><sup>2</sup>/2e</td></tr><tr><td>Spacecraft potential (Usc)</td><td>±80 V</td><td>&lt;10%</td></tr><tr><td>Integrated solar EUV flux</td><td>&lt;1 Hz</td><td>Res. 0.003 Gphotons/cm<sup>2</sup>/s</td></tr><tr><td>PSSR: Max Ice depth / Resolution</td><td>&lt;20 km (depend on ice conductivity/salinity)</td><td>&lt;1 km</td></tr></tbody></table>

* LP-PWI bias voltage sweeps (see Fig. 10, $0.01 < T_e < 20$ eV).

* Active mutual impedance measurements (MIME, $0.01 < T_e < 100$ eV).

The absolute value of ion drift speed ($V_{di}$), and limits on the ion temperature ($T_i$), is determined from

* LP-PWI bias voltage sweeps (see Fig. 10).

* $E \times B$ drift (together with J-MAG $B$ data).

The combined measurements by RPWI allow determination of:

* Wave vector, energy flux (Poynting vector), and wave polarization estimates of different plasma waves and radio waves.

* Remote radio source location.

<footnote n="1">¹) Need in-flight verification.</footnote>
* Interferometry of plasma waves and density inhomogeneities ($\delta n/n$), giving phase speeds and plasma drift speeds.

* Poynting flux associated with field-aligned currents.

* Dust distribution (> 1 µm size).

The *spacecraft potential*, $U_{SC}$, can be determined in most plasma environments by LP- RPWI, i.e., where the spacecraft charges within +/−80 V. The active measurement tech- niques (MIME), applied to the RPWI sensors, will furthermore allow to determine the effec- tive antenna electric lengths, deployment lengths and electric sensor calibration to increase the accuracy of the passive RPWI measurements.

## 3 The RPWI Science Objectives

The main RPWI science objectives are listed below as science topics for easy reference. It is by no means to be a complete list or a complete account for the science to be made by RPWI. The intention is instead to give a resumé of the main science questions that need an answer, and how RPWI intends to answer these questions with what measured parameters.

The main science focus of RPWI is to monitor the energy and momentum transfer pro- cesses dealing with electromagnetism in the whole Jovian system, especially near the icy moons. This includes many aspects of the information gained by measuring the vector elec- tric fields (near DC to 45 MHz), vector magnetic fields (0.1 Hz–20 kHz) and bulk plasma properties. In short, the RPWI will focus on characterizing electric currents and its dynamos, electric acceleration structures, e.g., in auroral energisation regions, plasma boundary pro- cesses, and all types of waves, e.g., dispersive Alfvén waves, Whistler waves, plasma waves and their role in transporting energy and momentum in the magnetospheres and ionospheres. The dynamics and particle energisation processes can further be monitored by radio waves, which also will be used to monitor the icy moons ionospheres and sub-surface properties. Most processes related to coupling between the magnetospheres, ionospheres, charged dust, thin atmospheres, surfaces, and the conductive interior of the icy moons will be addressed by RPWI. One major objective is to contribute to the study of sub-surface ocean structure(s) and possibly mapping of ocean flows. The focus will also be to determine the properties of bulk plasma and dust in the space environment in the magnetosphere of Jupiter and in the regions near the icy moons. Here the ionospheric structure (and chemistry), conductiv- ity and transport near the icy moons is of special interest, and their role in closing electric current systems. The dust particle size distribution in the Europa and Io tori as well as po- tential exhaust plumes will be studied. This will allow for studies of possible dust-plasma interaction and acceleration of charged dust by electric fields. Table 10 summarize the main

<table><caption>**Table 10** Main RPWI science goals</caption><thead><tr><td>Addressed RPWI science process</td><td>Experiment contribution</td></tr></thead><tbody><tr><td>Sub-surface induction at icy moons</td><td>GANDALF, flybys<br/>Sect. 3.1</td></tr><tr><td>Sub-surface ocean tides at Ganymede</td><td>GANDALF, in Ganymede orbit<br/>Sect. 3.1</td></tr><tr><td>Auroral oval “wiggling” at Ganymede</td><td>GANDALF, in Ganymede orbit<br/>Sect. 3.2</td></tr></tbody></table>
<table><caption>**Table 10** (Continued)</caption><thead><tr><td>Addressed RPWI science process</td><td>Experiment contribution</td></tr></thead><tbody><tr><td>Structure of icy crust of icy moons</td><td>JENRAGE, flybys, in Ganymede orbit</td></tr><tr><td>Jovian and Ganymede magnetospheric plasma waves, their boundary processes</td><td>Sect. 3.3</td></tr><tr><td>Ionosphere–magnetosphere couplings at Ganymede</td><td>GANDALF, SAMWISE, near Ganymede, in Jupiter orbit and approach, high latitude</td></tr><tr><td>Structure and variability of the ionosphere and magnetosphere of Ganymede, electrodynamics, Alfvén wings, thermal plasma outflow</td><td>Sect. 3.5</td></tr><tr><td>Ganymede auroral processes</td><td>GANDALF, MIME, SAMWISE, near Ganymede</td></tr><tr><td>Europa's ionosphere, related electrodynamics, ion outflows, interaction with Jovian magnetosphere</td><td>Sect. 3.6</td></tr><tr><td>Europa plume dust, structure, and electrodynamics</td><td>GANDALF, MIME, SAMWISE, JENRAGE, near Ganymede</td></tr><tr><td>Europa torus</td><td>Sect. 3.7</td></tr><tr><td>Dust near icy moons</td><td>GANDALF, MIME, SAMWISE, near Ganymede</td></tr><tr><td>Callisto's ionosphere, related electrodynamics, ion outflows, interaction with Jovian magnetosphere</td><td>Sect. 3.8</td></tr><tr><td>Co-rotation breakdown in the Jovian magnetosphere</td><td>All RPWI experiments, Europa flybys</td></tr><tr><td>Jovian radio emissions & magnetosphere dynamics</td><td>Sect. 3.9</td></tr><tr><td>Io torus dynamics</td><td>GANDALF, MIME, SAMWISE, Europa flybys</td></tr><tr><td>Wave emissions from Jupiter atmosphere lightning</td><td>Sect. 3.10</td></tr><tr><td>Cruise science</td><td>GANDALF, SAMWISE, Sect. 3.11</td></tr><tr><td>Upstream solar wind</td><td>SAMWISE, near icy moons</td></tr><tr><td>Global Jovian magnetosphere structure and convection</td><td>Sect. 3.11</td></tr><tr><td>Jovian current/plasma sheet</td><td>All RPWI experiments, Callisto flybys</td></tr><tr><td></td><td>Sect. 3.12</td></tr><tr><td></td><td>GANDALF, SAMWISE, Jupiter orbit, high latitude phase</td></tr><tr><td></td><td>Sect. 3.13</td></tr><tr><td></td><td>JENRAGE, Jupiter orbit, high latitude phase</td></tr><tr><td></td><td>Sect. 3.14</td></tr><tr><td></td><td>JENRAGE, Jupiter orbit</td></tr><tr><td></td><td>Sect. 3.15</td></tr><tr><td></td><td>SAMWISE, JENRAGE, close to Jupiter</td></tr><tr><td></td><td>Sect. 3.16</td></tr><tr><td></td><td>All RPWI experiments, Sect. 3.17</td></tr><tr><td></td><td>GANDALF, SAMWISE</td></tr><tr><td></td><td>GANDALF</td></tr><tr><td></td><td>GANDALF, SAMWISE</td></tr></tbody></table>
<figure><img src="image_24.png" /></figure>

**Fig. 22** The induction process coupling the local space plasma where the spacecraft is situated to the electric currents in a conductive sub-surface ocean. The red parameters are measured in orbit (or during flybys) of the icy moons by RPWI and J-MAG onboard JUICE. The quasi-periodic magnetospheric induction disturbance dominantly from Jupiter's magnetodisc propagates down to the sub-surface ocean in an Alfvénic manner, and there generates an induction current. The ocean (e.g., tidal) motion also contributes to an electric current. The total induction response in the ocean propagates outward through the ionosphere to the magnetosphere, where the JUICE spacecraft measures the disturbed electric and magnetic fields at many frequencies (days, hours, minutes). Various other electric current sources, such as ionospheric electric currents, also contribute to the measured electric and magnetic fields and must be separated out from the contribution of the sub-surface ocean. For instance, the induction response itself generates electric currents in the ionosphere, and for Callisto, this ionospheric induction response can dominate the measurements

processes that RPWI focus on. This can be compared with more general science goals for JUICE (Fletcher et al. 2023; Masters et al. 2024; Tosi et al. 2024; Van Hoolst et al. 2024; Witasse et al. 2025, all in this collection).

## 3.1 Mapping of Sub-Surface Induction Electromagnetic Fields at the Icy Moons, and Ocean Tides at Ganymede

The near dipole magnetic field of Jupiter is tilted by about 9.8 degrees to the planet's spin axis and the magnetosphere rotates roughly with the planet's sidereal period (9.93 h), at least out to the co-rotation breakdown in the middle magnetosphere (e.g., Kennel and Coroniti 1975; Hill 1979, 2001; Cowley and Bunce 2001). A magnetodisc is formed mainly due to mass loading stress of volcanic material ejected from Io (e.g., Bagenal and Sullivan 1981; Khurana and Kivelson 1999; Khurana et al. 2004). The magnetodisc will therefore undergo a quasi-periodic motion past the large icy moons of Jupiter, i.e., Europa, Ganymede and Callisto. There are also other magnetospheric processes that will contribute to a highly time-variable magnetospheric environment (e.g., Kivelson et al. 2002; Duling et al. 2014) in the icy moons Keplerian and equatorial orbits around Jupiter (cf. Masters et al. 2024; Van Hoolst et al. 2024, both in this collection).

According to Faraday's law of induction (see Fig. 22) a time-varying magnetic field causes a rotation of the electric field, which in the presence of a conducting (salty) ocean gives rise to induced electrical currents in this ocean that in turn generate a secondary (induced) and opposing magnetic field (e.g., Khurana et al. 1998; Neubauer 1998; Kivelson et al. 2000; Saur et al. 2010, 2015). The primary magnetic field change in the magnetosphere and the induced secondary response in the conducting material will propagate via an Alfvénic disturbance (Alfvén 1942) at the Alfvén speed.

Using induction models and data from the Galileo spacecraft, the existence of an ocean at Ganymede was confirmed (Kivelson et al. 1999). It was however not yet constrained in
terms of its depth and conductivity. To achieve that, suitable coverage (over a wide range of longitudes and latitudes) of the surface of Ganymede is needed for the largest oscillation signals. This was somewhat achieved by Kivelson et al. (2002) by calculating variations in the internal dipole field over a full sidereal rotation period of Jupiter and the five available Galileo flybys. Lack of data did however not yet allow for a clear distinction between a time-varying dipole and a collection of fixed multipoles. This is an issue that JUICE will easily resolve. At Europa, similar methods have already confirmed the presence of a large ocean-induced dipole via the study of the dominant 11 h rotation period oscillation (Kivelson et al. 2000).

RPWI will measure the near DC electric (E, and δE) field and the density (δn/n) components of the disturbance, map the ionosphere and magnetosphere currents and their contribution to the induction, and monitor the plasma density and (E × B) convection of the surrounding variable Jovian (and Ganymede) magnetosphere on a continuous basis. J-MAG will measure the near DC magnetic (B, and δB) field. RPWI (and J-MAG) will, for this purpose, use both flybys and the orbital phases of JUICE at Ganymede.

The amplitude and phase delay of the induction response, in the case of a spherical conducting shell and a uniform oscillating magnetic field across it, can be used to determine the radial conductivity structure of the shell. The largest variation in Jupiter's inducing magnetic field is caused by the dipole tilt and occurs for the synodic rotation period (around 10 h for the icy moons), but smaller effects are expected for the orbital periods of the icy moons due to their orbital eccentricities. However, a broad spectrum of electromagnetic variability will affect the icy moons, and this can also be used for sounding the interior impedance at various depths (e.g., Grimm et al. 2020, 2021). In this case where the magnetotelluric method (e.g., Vozoff 1991; Simpson and Bahr 2005; Chave and Jones 2012) is applied, the horizontal electric (E_x) and the orthogonal magnetic (B_y) fields in a broad frequency range is measured above the surface of the icy moons. The source field does not need to be known with this method. The apparent conductivity is calculated from

$$\sigma_a(\omega) = (\omega/\mu) |B_y/E_x|$$

where the conductivity with depth σ(z) is calculated from the σ_a(ω) by, e.g., Occam's inversion (Constable et al. 1987). The frequency range from micro-Hz to 100 Hz is reachable with RPWI E-field and J-MAG B-field measurements. The JUICE spacecraft will be in near polar orbit around Ganymede toward the end of the mission, slowly pre cessing above the surface so as it covers all longitudes by time. It is therefore possible to construct a map of the inferred conductivity profile with depth.

To infer internal ocean properties, the measurements from JUICE RPWI and J-MAG will drive and constrain theoretical induction models which calculate the ocean induction and secondary electric and magnetic fields as a function of the inducing (Jovian) field and ocean properties. Accurate knowledge of the inducing magnetic field is of essence, obtained partly from local measurements and from theoretical models. Note that because the region of interest here is located well within the magnetosphere, random variations in the current sheet due to the solar wind or other external factors are limited. Over the long term, irregular ejections of plasma from Io's volcanic surface may produce additional variations in the plasma-sheet that may not be predicted in any reliable way. Another source of influence on the induced signal may lie within the moons' conductive ionospheres and may account for all the induction signal detected near the moon Callisto (Hartkorn and Saur 2017). Theoretical calculations of ocean induction in the presence of an ionosphere have revealed that the electric field does not seem to decay due to the presence of an ionosphere: large increases in the Alfvén speed counteract the (1/R³) decay in the electromagnetic perturbation.
Mapping of tidal flows in Earth's salty conductive ocean have been made by the Swarm and CHAMP spacecraft by using a spherical harmonic analysis (Egbert and Erofeeva 2002; Grayver et al. 2016, 2017; Tyler et al. 2017; Grayver and Olsen 2019). We plan to combine the RPWI and J-MAG measurement using a similar method to infer the sub-surface ocean tides of Ganymede (if existing) when JUICE is in near polar circular orbit of this moon at 200 and 500 km altitude respectively for numerous Ganymede orbits around Jupiter (a total of 1000+ such orbits are currently planned). This would open the possibility to study also other suggested ocean movements, like a suggested electromagnetically driven equatorial jet (Gissinger and Petitdemange 2019).

## 3.2 Sub-Surface Ocean Conductivity Determination by Mapping of Auroral Processes

The magnetosphere and the auroral ovals of Ganymede are moving near the surface in latitude as a response to the periodical Jovian magnetodisk flapping motion past this icy moon. An electrically conductive ocean would cause the externally time-varying magnetosphere to reduce the response in Ganymede's magnetosphere due to the induction response within the sub-surface ocean. This has been demonstrated by observation with HST of the oval oscillatory position (Saur et al. 2015), suggesting that a sub-surface ocean indeed does exist at Ganymede and giving constraints on the conductivity of such an ocean.

The JUICE spacecraft will be in near polar orbit around Ganymede toward the end of the mission, slowly pre-cessing above the surface so as it covers all longitudes by time. This means the JUICE spacecraft will pass the south and north auroral ovals 4 times each orbit (see Fig. 23). The position of the auroral oval can be determined by, e.g., the near DC electric field convection identification of the so called low-latitude boundary layer (LLBL, e.g., Morooka et al. 2004; Vines et al. 2017) separatrix to an accuracy of a few km. A sharp poleward boundary was detected by the JUNO Ultraviolet Spectrograph (UVS) (Greathouse et al. 2022). Other auroral indicators can be used as well. Determining the oval position requires ±10 minutes of electric field measurements over the predicted auroral zone boundary crossings. RPWI need at least 100 crossings (out of at least 4000+ planned such JUICE crossings) to gain enough statistics to determine the auroral oval wiggling variation with longitude and Jovian plasma disk variations, and thereby determine the average internal conductivity variation with longitude, thus further constraining the internal structure of Ganymede's sub-surface and ocean.

## 3.3 Structure of Icy Crusts

RPWI is designed to be operated as a passive radar sounder to explore the icy moons interior (Fig. 24; Romero-Wolf et al. 2015; Hartogh and Ilyushin 2016; Kumamoto et al. 2018; Schroeder et al. 2016; Carrer et al. 2020; Gerekos et al. 2019; Di Paolo et al. 2020; Roberts et al. 2021; Gassot et al. 2021). The science objectives for RPWI are to characterise the icy moons ice shells and to look for past and present activity. The wide frequency range of RPWI enables us to carry out these radar observations by using intense Jovian radio emissions, such as DAM/HOM (0.4–40 MHz) and bKOM (20–400 kHz), which were observed during the Jupiter flybys by Voyager and Cassini (e.g., Zarka et al. 2004; Cecconi et al. 2012, 2021). The Jovian emission locations can be predicted in advance (Louis et al. 2019a; Cecconi et al. 2021). A correlation is made between the recorded incoming signal from Jupiter with its reflections from the surface and subsurface of a given target icy moon. From the correlation output, it is possible to locate the target features and their respective depths. The
<figure><img src="image_25.png" /><figcaption>Fig. 23 Simplified sketch of selected magnetic field lines and locations of auroral ovals when Ganymede is above (dashed lines) and below the current sheet (solid lines), respectively. The ovals are located where the open-closed field line boundary (OCFB) intersects Ganymede’s surface. The JUICE spacecraft in polar orbit and slowly pre-cessing around Ganymede will encounter a periodically motion of the auroral ovals of Ganymede due to the Jovian magnetodisk motion past this moon. The oscillatory varying location of the oval can be determined with in-situ measurements by RPWI with very high accuracy, thereby giving constraints on the internal conductivity of the moon (picture and partly Figure text from Saur et al. 2015)</figcaption></figure>

frequency range of RPWI observations make it also possible to characterize the roughness of the surface. The possibility to detect subsurface feature by a passive radio acquisition depends on several conditions: (i) the Jovian radio flux density value, (ii) the radar bandwidth, (iii) the integration time, (iv) the surface roughness (v) the attenuation in the ice, (vi) the total electron content (TEC) of the icy moons’ ionospheres, and (vii) the orbit altitude.

The RPWI freedom of frequency choice allows us to select frequencies where the noise from the S/C is minimized, but also allows for a correction for the ionosphere delay. One mode of operation centres around 1 MHz and use a bandwidth of about 300 kHz. The TEC of Ganymede’s ionosphere was reported to be up to $0.6 \times 10^{16} \text{ el./m}^2$ (Buccino et al. 2022), but can also be provided by the RIME measurements (Bruzzone et al. 2025, this collection). The equivalent distance to the ionospheric delay can be estimated by $c\tau = 40 \text{ TEC} / f^2$ where f is frequency of the radio wave. It is < 240 km at 1 MHz.

The amplitude of the echoes from the subsurface boundaries will be affected by attenuation in the ice and roughness of the boundaries. Since the conductivity of the ice doesn’t depend significantly on the frequency (e.g., Evans 1965; Fujita et al. 2000), low attenuation cannot be expected by changing the observation frequency. On the other hand, the roughness of surface and subsurface boundaries determines the frequency dependence of the echo power, which can be estimated by the radar equation for targets with rough surfaces (Bruzzone et al. 2011):

$$P_{RX} \propto \int_{0}^{\infty} \exp(-2s^2 k^2 t^{2H}) t dt,$$
<figure><img src="image_26.png" /><figcaption>**Fig. 24** (a) Illustration of passive radar using Jovian radio waves in wide frequency range by RPWI. (b) The view of Jupiter, the moons, and visible Jovian radio wave sources from RPWI around Ganymede</figcaption></figure>

where *k* is wave number, *s* is RMS height for 1 m distance, and *H* is Hurst exponent (an incident angle of 0 is assumed for simplicity). Typical roughness parameters *H* and *s* in several areas of Europa were reported as (*H*, *s*) = (0.75, 0.2), and (0.5, 1.5) (Nimmo and Schenk 2008; Steinbrugge et al. 2018). The ratio of the echo power *P*<sub>RX</sub> at 1 MHz with respect to 10 MHz is estimated to be 20 dB in the former area while it is estimated to be 40 dB in the latter area. The RPWI observations covering a wide frequency range will thereby be useful to determine roughness of the surface, but also characterise subsurface boundaries.

RPWI plans to operate in three different passive subsurface radar modes, PSSR-1, 2 and 3. When the radio wave propagates from Jupiter to the spacecraft orbiting around the icy moon, and if the radio wave is continuous and its duration is longer than the round-trip delay between the spacecraft and icy moon surface (1.3 ms or 3 ms for a S/C height of 200 km or 500 km respectively, at the sub-Jovian position), the direct radio wave is superimposed by the radio waves reflected at the surface and subsurface boundaries of the icy moon. Due to the superposition of the direct and reflected waves, the amplitude of the wave becomes larger or smaller depending on the phase difference, which also depends on the wavelength. The radio wave spectrum with the interference patterns can be measured in PSSR-1 mode (Kumamoto et al. 2018). Similar interference was observed by the Kaguya spacecraft in orbit around Earth's moon (Ono et al. 2010). In the spectrogram of auroral kilometric ra-
diation (AKR), interference patterns between AKR directly from the Earth and reflected at the surface of the Moon was observed. For Jovian radio bursts with a shorter duration than the round-trip delay in PSSR-2 or continuous noise bursts in PSSR-3, the waveforms of the bursts are sampled directly from Jupiter and reflected at the surface and subsurface boundaries of the icy moons (for monitor and burst observations, respectively). By applying correlation and autocorrelation analysis, time delays, Doppler and polarization (utilizing the three RWI antennas) of the surface and subsurface echoes can then be determined.

## 3.4 Mapping Jovian Magnetosphere Processes

RPWI will nominally monitor the Jovian magnetosphere continuously in a low telemetry mode, where the electric field (E) is sampled at 24 sps and any waves sampling (16k snapshots) and active Langmuir probe or MIME sweeps are made with a long duty cycle (once per 10 minutes or more). Triggered burst measurements (for example using an enhanced DC electric field power trigger, and/or selective downlink of burst data) will be carried out during bow shock and magnetopause crossings, and occasional radio burst measurements will occur during observations of Jovian auroral acceleration events when JUICE is in the high latitude phase. The RPWI measurements in the Jovian magnetosphere aims to study the following conditions/processes:

* Upstream solar wind.

* Global magnetosphere mapping of plasma density ($n_e$), convection ($E \times B$, together with J-MAG), and Jovian radio emissions and their dynamics.

* Current/plasma sheet.

* Co-rotation breakdown (see Sect. 3.13).

* Io torus (remote radio, Sect. 3.15) and Europa torus (in-situ, Sect. 3.11) plasma transport.

* Jupiter aurora (radio, see Sect. 3.14).

* Electromagnetic and plasma wave energy transport (see Sect. 3.5).

* Bow shock and magnetopause (see Sect. 3.5).

* Reconnection in Jovian magnetotail and magnetopause (see Sect. 3.5).

* Whistler and radio waves from Jovian lightning (see Sect. 3.16).

We will not describe these magnetospheric processes in detail here but refer to the given sub-sections and to the associated JUICE review (Masters et al. 2024, this collection).

## 3.5 Characteristics of Alfvén Waves, Whistler Waves, Plasma Waves in the Jovian and Ganymede Magnetospheres Including Their Boundary Processes

There have been sporadic reports from plasma wave measurements within the Jovian magnetosphere from many spacecraft (Pioneers, Voyagers, Ulysses, and Galileo) in the past (e.g., Gurnett et al. 1983a; Stone et al. 1992; Shprits et al. 2018), but few in-depths interpretations. Somewhat more efforts have been put into Ultra Low Frequency (ULF) waves and turbulence, such as electromagnetic ion cyclotron waves (EMIC), and their role in Jupiter's magnetosphere (e.g., Petkaki and Dougherty 2001; Tao et al. 2015; Manners and Masters 2020). Also there exist several reports on plasma waves during close encounters with the Galilean moons (see further Sects. 3.7 through 3.12).

However, recent Juno observations in the auroral acceleration regions closer to Jupiter (Sulaiman et al. 2022; and references within) have shown that auroral bidirectional energetic electron acceleration, proton acceleration, and field-aligned currents are associated with large amplitude broadband Alfvén wave activity and plasma density cavities, and it has
been suggested that an ionospheric Alfvén resonator (Polyakov 1976; Sato 1978; Belyaev et al. 1990) can account for the broad energy of the field-aligned electrons (e.g., Saur et al. 2018; Lysak et al. 2021) and electric currents are set up in the ionosphere (auroral electrojet, Boström 1964). On Earth plasma auroral density cavities are related to auroral acceleration processes (e.g., Persoon et al. 1988; Paschmann et al. 2003; Alm et al. 2014, 2015), and it is further related to two main types of acceleration processes; inverted-V electric potential structures (Alfvén 1958; see further Sect. 3.8) and ionospheric dispersive Alfvén wave cascade (Hasegawa 1976; see further Sect. 3.8). The latter process is believed to be more important for the generation of Jupiter’s aurora (Clark et al. 2018). Dispersive Alfvén waves on Earth generate broadband emissions (e.g., Gurnett and Frank 1977; Chmyrev et al. 1989; Boehm et al. 1990; Louarn et al. 1994; Wahlund et al. 1994, 1998, 2003; Stasiewicz et al. 2000a,b; Chaston et al. 2001, 2008; Streltsov and Karlsson 2008) and are observed together with upgoing transversely heated ionospheric ions (e.g., Temerin and Roth 1986; André et al. 1988) and down going field-aligned electrons (e.g., Boehm et al. 1990). This has been interpreted as a positive feedback mechanism, whereby small-scale Alfvén waves cascade to higher ion mode frequencies and erode the auroral ionosphere by facilitating transverse ion acceleration and resulting ion outflow, which in turn leads to deeper density cavities that maintain the production of small-scale Alfvén waves via refraction and phase mixing of incoming large-scale Alfvén waves (e.g., review by Haerendel 2011, and references therein; see further Sect. 3.8). The inertial Alfvén waves are associated with a parallel electric field component, which accelerates electrons as beams along the magnetic field.

JUICE will during its high-inclination phase reach between 33–38° jovi-graphic latitude during 15–18 flybys (depending on finally selected JUICE trajectory), using gravity assist by Callisto each occasion. This high latitude phase allows for the first prolonged studies of the mid-latitudes of the Jovian magnetosphere with high-resolution in situ measurements at distances near to ~25R<sub>j</sub> at ~45° magnetic latitude. This would allow, for example, for a long temporal baseline study of the Jovian auroras including remote sensing in UV and IR wavelengths in combination with monitoring of magnetospheric dynamics through RPWI Alfvén and plasma waves measurements. It also allows for a detailed analysis of the plasma flows at mid-high magnetic latitudes which will magnetically map to the outer magnetosphere and tail, tackling the major question of solar wind versus co-rotation effects. This, in turn, will make it possible to study the correlation of reconnection or injection events in the magnetosphere with resulting auroral emissions and detailed studies of the moon footprints. Furthermore, it facilitates detailed studies of the region of the magnetosphere where the rigid corotation of the plasma flow breaks down (region of field-aligned current maps into the main auroral oval – inner pair of dashed green lines in Fig. 25, see further Sect. 3.13), and beyond to higher latitudes. The high-latitude orbits give coverage of the field lines and field-aligned current systems well poleward of the main oval into the polar region - to the edge of the open field lines according to this simple model.

RPWI will be able to determine the full electrodynamic and electromagnetic environment along the JUICE trajectory. For instance, all waves will be sampled with the full vector electric and magnetic fields giving polarization and Poynting flux as well as waveform, and in some operation modes also δn/n, near-continuously below 180 Hz, and with a duty cycle below 20 kHz. The electric waveform can be sampled up to 1.5 MHz, giving information on electron wave modes. RPWI therefore provides excellent capabilities to measure plasma waves and plasma density which will allow characterization of the boundary processes in the magnetosphere of Jupiter and Ganymede. Measurements of the full vector electric (δE) and magnetic (δB) components of waves in a broad frequency range together with high resolution plasma density measurements (by several methods) allow for identification of
<figure><img src="image_27.png" /><figcaption>**Fig. 25** Jupiter's magnetic field (black and green) in magnetic coordinates, where the green field lines (dashed and solid) mark the regions planned to be covered by JUICE in its high-latitude and equatorial phases at a 25*R<sub>J</sub>* conjugate radial position (using Callisto flybys to reach the higher latitudes). The regions where field-aligned currents map into the main auroral oval is marked with pairs of dashed green lines. Trajectory coverage show the arc (purple) of a circular orbit at 25*R<sub>J</sub>* that will be covered by a 30°inclined phase in magnetic coordinates. The heavy dashed purple line near the equator shows the coverage for an equatorial orbit, i.e., ±9.5° about the equator as the magnetic dipole rotates. The lighter dashed purple parts correspond to a 30° orbit, which extends the range to ±39.5° about the equator. An example Juno orbit is shown in red for comparison (adapted from Cowley et al. 2008 by Emma Bunce, credit Stan Cowley, University of Leicester)</figcaption></figure>

low-frequency waves as kinetic Alfvén waves (KAW), medium frequency waves such as lower hybrid drift waves (LHDI), whistlers, electrostatic solitary waves, and high frequency Langmuir and upper-hybrid waves, and beam wave modes. All these waves provide important information about the kinetic plasma processes related to plasma heating and particle acceleration, as well as energy and plasma transport at magnetospheric boundaries.

Waves can be used for identification and characterization of plasma boundaries. RPWI plasma density measurements will allow to identify density gradients. High-resolution measurements of magnetic field measurements by SCM and J-MAG will allow to identify thin current sheets. It is difficult to determine the boundary speed and thus the spatial scale, and presence of strong lower-hybrid drift waves can be used as an indicator of thin ion-scale boundaries with strong density gradients (Graham et al. 2019), such as the magnetopause boundary close to a reconnection site. Waves around reconnection regions provide a unique footprint, and this can be used to identify reconnection regions (Khotyaintsev et al. 2019). Detection of Whistler waves, knowledge of their wave-normal angle and direction of propagation from the Poynting flux can be used to identify open-closed field boundaries. Reconnection leads to creation of a loss-cone electron distribution close to the open-closed boundary; such distributions are difficult to resolve in the particle data but can be identified
through whistlers propagating towards the reconnection site. Also, KAWs can indicate that a region is connected along the magnetic field lines to an active reconnection X-line. Sepa-ratrices - the regions where field-aligned electrons beams are often present can be identified from Langmuir waves, and electrostatic waves (Viberg et al. 2013). Regions of active mag-netic flux pile-up like at the fronts of reconnection jets can be identified using field-aligned Whistler waves (Khotyaintsev et al. 2011).

Plasma waves which will be measured by RPWI will allow us to characterize several different processes in different boundary regions. At the magnetopause the waves processes participate in plasma entry and mixing across the boundary, cross-field diffusion, reconnec-tion, finite gyro-radius effects, unsteady reconnection. The magnetopause of Ganymede will be crossed multiple times, so we will collect a large database to quantify these processes (see further Sects. 3.6 and 3.7). In the magnetotail the waves contribute to plasma heating and acceleration, particle dynamics at reconnection jet fronts, pitch-angle scattering, energy transport by wave Poynting flux (whistlers and KAWs). In the Alfvén wings we expect the waves drive plasma heating and ion outflow.

## 3.6 Determine the Magnetospheres-Ionosphere Couplings at Ganymede

Ganymede has, unlike most other moons in the solar system, an intrinsic magnetic field strong enough to produce a mini magnetosphere of its own residing in the vastly much larger magnetosphere of Jupiter (Kivelson et al. 1996). As the ambient plasma flow in Jupiter's magnetosphere is both sub-Alfvénic and sub-magnetosonic at the orbit of Ganymede there is no bow shock forming but instead Alfvén wings are formed (Neubauer 1998). The ionosphere of Ganymede (see Sect. 3.7) is created by ionisation of its atmosphere. Magnetosphere-ionosphere coupling involves the process of transporting energy and mo-mentum from one region within a plasma environment to another. This typically involves electric currents, often mediated by Alfvén disturbances, that travel along magnetic field lines on a global scale. The global current systems that are set up in this system are strongly affected by the coupling between Jupiter's magnetosphere and Ganymede's magnetosphere. An interaction region upstream, between the southward directed magnetic field of Jupiter and the northward directed magnetic field of Ganymede (not unlike the situation arising be-tween a southward magnetic field in the solar wind and Earth's magnetosphere for instance) sets up a system of complex electromagnetic interactions. At the magnetopause upstream of Ganymede, magnetic reconnection is likely to occur, as are the formation of current sheets (e.g., Tóth et al. 2016; Zhou et al. 2020). Accelerated particles triggered by the reconnec-tion processes, including acceleration by dispersive Alfvén waves and electrostatic potential structures on auroral field-lines (see Sect. 3.8), can travel to either the atmosphere of Jupiter or Ganymede itself and cause auroral emissions there. Tailward of the moon, a magnetic wake forms with somewhat stretched out magnetic field lines, allowing for field-aligned currents to flow and possibly also cause ion outflows via the plasma sheet (e.g., Jia et al. 2009). As the Jovian magnetospheric flow is sub-magnetosonic the magnetosphere will be-come cylinder-shaped with the axis aligned with the external magnetic field direction, in contrast to the parabola-shaped magnetosphere of a body embedded in the solar wind.

JUICE instruments including RPWI will sample this interaction region via several close flybys (12 planned) and during the Ganymede orbiting phases toward the end of the mission. These begins with 122 days (≈240 orbits) in highly elliptic (300 km–10,000 km) or high altitude (5000 km) orbits, followed by circular orbits at 500 km and 200 km altitude for 156 days (≈1150 orbits). This may change somewhat depending on the final trajectory decision still to be made. Thus, there are many passes through relevant regions of the magnetosphere
of Ganymede where the dynamic electrodynamic coupling can be studied, and wherefrom a coherent observational picture can be formed.

RPWI is planned to do continuous measurements of the electric field and thermal plasma near Ganymede and will investigate the physical processes occurring in the interaction region between Ganymede and the ambient Jovian magnetosphere by utilizing a wide range of measurements and associated data products. Cold plasma (ion and electron) density, temperature and flow velocity and associated (thermal) pressure terms will be measured using the GANDALF and MIME experiments. Three-dimensional electric (E, δE) and magnetic fields (δB) using both GANDALF and SAMWISE will be measured which will allow for detection of certain wave modes present that are involved in the energy and momentum transfer in this coupling region (see further below and Sect. 3).

## 3.7 Mapping Structure and Variability of the Ionosphere and Magnetosphere of Ganymede and Related Electrodynamics and Thermal Plasma Outflow

The ionosphere and magnetosphere of Ganymede was detected by the Galileo plasma wave and magnetic field instruments (Gurnett et al. 1996; Kivelson et al. 1996), while the on-board radio sounding experiment gave upper constraints for the electron density near the surface of a few $1000 \text{ cm}^{-3}$ and the uncertainty was attributed to the possible non-spherical geometry of the limb observation toward the wake (Kliore 1998). Further analysis of the plasma wave and plasma analyser (Frank et al. 1997) data and associated preliminary modelling gave an $O_2^+ - O^+$ dominated ionosphere with near surface densities not more than $2500 \text{ cm}^{-3}$ for the two closest Galileo G1 and G2 flybys (Eviatar et al. 2001). The plasma wave observations furthermore showed a variety of emissions common in planetary magnetospheres, like whistlers, cyclotron waves, upper hybrid waves, broadband noise at the lower frequencies below the ion plasma frequency, but also possible radio waves created in the magnetosphere of Ganymede (Gurnett et al. 1996; Kurth et al. 1997).

Recent measurements by the Juno spacecraft during a distant flyby (c/a 1046 km) provided both new plasma (Allegrini et al. 2022; Valek et al. 2022), waves (Kurth et al. 2022) and new ionospheric occultation results (Buccino et al. 2022). The wave data revealed broadband emissions (dispersive Alfvén wave cascade?) in the magnetospheric wake possibly related to reconnection and heavy ions ($3-24 \text{ cm}^{-3}$), Whistler waves and electron cyclotron harmonics emissions, and an obliquely propagating chorus associated with few 100 eV precipitating electrons contributing to Ganymede’s aurora. The two-limb radio occultations gave no ionospheric peak signatures but rather an exponential increase toward the surface with maximum electron densities of about $1000-2000 \text{ cm}^{-3}$ close to the surface. The error in the egress signal (on closed magnetic field lines) was large though and could be a non-detection. The ingress signal on the other hand was a detection and occurred for open magnetic field lines, indicating that electron impact ionization was important. The ion composition data revealed ionospheric ions ($O_2^+, O^+, H_2^+, H^+, \text{ and } H_3^+$), but the inferred positively charged ion density was ~2.5 times larger than the electron density from the plasma wave data (Allegrini et al. 2022; Valek et al. 2022). These ions were flowing outward along the magnetic field and belonged to a cold ionospheric plasma (thermal energies up to tens of eV). The peak outward flux of $O_2^+$ was about $5 \times 10^{11} \text{ ions/m}^3/\text{s}$. Detailed theory investigations have studied the atmosphere-ionosphere-magnetosphere coupling processes and their variability under different conditions (e.g., Leclercq et al. 2016; Fatemi et al. 2016; Leblanc et al. 2017; Poppe et al. 2018; Carnielli et al. 2019, 2020a,b; Vorburger et al. 2022; Leblanc et al. 2023). Photo- and particle-impact ionization of the atmosphere gives rise to an ionospheric plasma, which fills most of the magnetosphere (Galand et al. 2025). For an
atmosphere dominated by O₂ close to the surface, the most abundant ionospheric species inferred are O₂⁺ followed by O⁺, H₂⁺, and H₂O⁺ (e.g., see Table 3 in Carnielli et al. 2019). Though a large part of the oxygen ions impacts the surface and constitute a source for the atmosphere (Carnielli et al. 2020b), the rest of the oxygen ions as well as most of the other ionospheric ions flows outward and escapes the magnetosphere through the Alfvén wings or through the plasma tail (in co-rotation direction).

Cold plasma with temperatures only slightly higher than the corresponding atmospheric source of ions can dominate the number density of the space environment of the icy moons and within their tails along their orbits within the Jovian magnetosphere. Such cold plasma outflow can drastically change the Alfvén speed and the conditions for magnetic reconnection (Toledo-Redondo et al. 2021) and it is in this respect interesting that broadband dispersive Alfvénic activity seems to exist in the wake of Ganymede (Kurth et al. 2022). The (cold) plasma also significantly influences the spacecraft charging (e.g., Morooka et al. 2009; Bergman et al. 2020), which will be continuously monitored by RPWI. Hence mapping the locations, transport and energisation of cold plasma may be complicated but is important for understanding the Jovian system.

RPWI will study the ionosphere and magnetosphere of Ganymede with several in-situ techniques (cf. Sect. 2.8), but also apply ionosphere occultation cut-off of Jovian radio emissions (with a radio burst mode, cf. Sect. 2.5.1 and 5.8.2). The RPWI instrument has several important advantages for measuring the densities of the bulk ion and electron components, including the possibility of detecting the negatively charged ion and nm-sized dust populations (if existing). This will be sampled nominally every 30–600 s with Langmuir probe sweeps (cf. Sect. 2.2) and mutual impedance sweeps (cf. Sect. 2.3), but density monitoring can, if needed, be sampled continuously at a rate of 763 sps to detect sharp density gradients. Different Langmuir probes can simultaneously sample the electron and positive ion densities separately. These can usually be cross calibrated against upper hybrid, Langmuir wave and other plasma wave emissions. The plasma convection (E × B) can be inferred from combined RPWI and J-MAG measurements. This will give needed information of plasma transport within the Jovian plasma disc and Ganymede magnetosphere, the icy moons outflow flux and other plasma flow patterns. The RPWI full vector capabilities of measuring electrostatic and electromagnetic waves (δE and δB) up to 20 kHz will be used to study the energy transport (Poynting flux, polarization) between local particle populations, or over long distances with (dispersive) Alfvén waves.

## 3.8 Characterize Ganymede Auroral Processes

The first indications of auroral emissions from Ganymede was made by UV oxygen emission lines by the HST (Hall et al. 1998). It was inferred from the analysis of these lines (130.4 and 135.6 nm) that they were produced by electron impact ionizations of an O₂ dominated atmosphere. Further HST observations revealed the auroral ovals, and these were located at a higher latitude on Ganymede's hemisphere facing the co-rotation plasma stream of the Jovian magnetosphere and that the emissions varied strongly with time and longitude (Feldman et al. 2000; McGrath et al. 2013; Molyneux et al. 2018; Marzok et al. 2022). MHD models of the Jovian magnetosphere interaction with the magnetosphere of Ganymede supported the idea that the Ganymede aurora occurred near the boundary between open and closed magnetic field lines (e.g., Jia et al. 2009; Zhou et al. 2019). The higher resolution Juno UVS measurements showed an intense narrow auroral curtain (<5°) with a sharp poleward boundary (<2°) near the open-closed magnetic field boundary, and there are less intense emissions equatorward of the curtain on closed magnetic field lines (Greathouse et al.
2022). The emissions were inferred to originate from within 25–50 km of the surface of Ganymede. The JUNO in-situ plasma wave measurements (Kurth et al. 2022) showed both broadband (Alfvénic?) emissions in the wake magnetosphere of Ganymede possibly related to reconnection, and an obliquely propagating chorus associated with few 100 eV precipitating electrons contributing to Ganymede’s aurora. JUNO encountered only open magnetic field-lines.

It is tempting to compare with the auroral processes on Earth. However, the interaction with Jupiter’s nearly co-rotating magnetosphere is sub-magnetosonic (and sub-Alvénic), causing the Ganymede magnetosphere to develop Alfvén wings from the polar regions and a bow shock should not form upstream the magnetopause (Neubauer 1998; Kivelson et al. 1998, 2004). The Alfvén wings are connected via magnetic flux tubes to Jupiter itself, and there cause an auroral footprint. These emissions produced in the Jovian upper atmosphere are caused by dispersive Alfvén waves with Poynting fluxes of ~100 mW/m², creating electron acceleration up to several keV toward Jupiter’s atmosphere (Szalay et al. 2020b). This Alfvén wave activity should also propagate in the other direction along the magnetic flux tube toward Ganymede and there these waves would become inertial when hitting its polar ionosphere (increased Alfvén speed) and produce field-aligned precipitating electrons. The Juno UVS did not detect corresponding emissions in the Ganymede polar cap, but could be due to the electron energies involved, and this need be studied more in detail by JUICE.

On Earth there are two main auroral forms: diffuse aurora and discrete arcs. It could be reasonable to relate these to the two forms detected by JUNO UVS (Greathouse et al. 2022). Diffuse aurora (which can pulsate) is mostly associated with precipitating electrons or ions injected from mostly the plasma sheet and scattered into a loss cone by plasma waves (e.g., Sandahl et al. 1980; Newell et al. 2009; Nishimura et al. 2020). Scattering of electrons by chorus waves accounts for intense inner magnetosphere electron precipitation (e.g., Cully et al. 2011), while electrostatic electron cyclotron harmonic waves dominate the nightside at high L-shells (e.g., Ni et al. 2011; Kurita et al. 2014). Scattering by electromagnetic ion cyclotron (EMIC) waves is also a lead candidate for ion precipitation (e.g., Yahnin et al. 2021).

Discrete aurora is also associated with several different acceleration processes of charged particles. The suggestion that parallel electric fields, aligned with Earth’s magnetic field, accelerate particles producing aurora was first made by Alfvén (1958). Experimental evidence of parallel electric fields has since been presented from both sounding rockets and satellite missions (e.g., Mozer and Bruston 1967; Block 1972; Mozer et al. 1977; Lindqvist and Marklund 1990; Chust et al. 1998; Mozer and Kletzing 1998; Ergun et al. 1998, 2002, 2004; Marklund et al. 2001, 2011), and the resulting acceleration of electrons creates what is often called an “inverted-V” structure. The theory behind auroral cavities and electrostatic potential drops are in good agreement with theoretical modelling (Block and Fälthammar 1968; Knight 1973; Janhunen et al. 2002), and reviews on the subject exist (e.g., Karlsson et al. 2020; Borovsky et al. 2020; Lysak et al. 2020).

Another acceleration process is caused by inertial Alfvén waves and broadband Extremely Low Frequency (ELF) waves. Hasegawa (1976) showed that Alfvén waves with small perpendicular scale lengths could support a finite fluctuating parallel electric field in the magnetosphere or topside ionosphere above the aurora. Broadband ULF/ELF emissions have been observed on many spacecraft in the boundary between the auroral oval and the polar cap, being the transition between open and closed magnetic field lines (e.g., Gurnett and Frank 1977; Chmyrev et al. 1989; Boehm et al. 1990; Louarn et al. 1994; Wahlund et al. 1994, 1998, 2003; Seyler et al. 1998; Stasiewicz et al. 2000a,b; Chaston et al. 2001, 2008; Streltsov and Karlsson 2008). These emissions have been interpreted as turbulent Alfvénic
activity (or Alfvénic aurora) propagating down the magnetic field lines and breaking up to higher frequency components below the ion plasma frequency and is characterized by large downward Poynting flux (e.g., Volwerk et al. 1996; Wahlund et al. 2003; Keiling et al. 2002, 2019a,b) and a transfer of its energy into precipitating electrons up to keV energies, transverse ion acceleration and resulting ion outflow in a divergent magnetic field. The theory behind dispersive Alfvén waves (DAW) and its turbulent cascade is reasonably understood (e.g., Goertz 1984; Lysak and Lotko 1996; Schukla and Stenflo 2000; Stasiewicz et al. 2000b; Chaston et al. 2008; Lysak 2023), but the observed processes also include several non-linear wave-wave and wave-particle interactions which needs further studies.

The JUICE mission can characterize the processes responsible for the aurora on Ganymede, where RPWI, J-MAG, PEP and UVS will all contribute to understand this phenomenon to a knowledge level we already have of aurora at Earth. The near polar orbital phase of JUICE with a slow precession around Ganymede, and at different altitudes from 5000 km down to 200 km will globally cover the study of Ganymede auroral acceleration regions, both the in-situ processes and remote effects. There are four crossings of the auroral oval each orbit, and RPWI will do high-resolution measurements ±10 minutes across each. The RPWI sensors measuring plasma and electromagnetic waves (δE, δB) can identify the wave characteristics like polarization and Poynting flux, and the near DC electric field (E) can identify both large and small scale electrostatic potential drops on auroral magnetic field lines, and RPWI (together with J-MAG and PEP) can infer the electric currents and related instabilities causing the particle precipitation and plasma outflow processes in detail.

## 3.9 Determine the Structure of the Europa Ionosphere & Related Electrodynamics and Ion Outflows During the Flybys

The Jovian magnetospheric interactions with the icy moons are characterized by the formation of Alfvén wing structures (e.g., Drell et al. 1965; Neubauer 1998, 1999; Kivelson et al. 1999, 2004; Volwerk et al. 2007). The interaction is a result of a sub-Alfvénic magnetized plasma flow with a conducting body, e.g., with its ionosphere or a sub-surface conducting material. In the case of Europa, ion pickup mass-loads the magnetic field lines and divert and slow them down, creating electric currents which exert a j × B force doing the deceleration. The flow is basically un-affected at the poles of Europa, and this forms a velocity shear along a bended structure from the poles. The electromagnetic disturbance is propagated along the polar flux tubes by the Alfvén speed, and Alfvén wings are created. The bending angle is roughly given by $\theta_A = \tan^{-1}(v_{flow}/v_A)$. More detailed MHD and hybrid interaction scenarios of involved processes have been presented (e.g., Blöcker et al. 2016; Harris et al. 2021; Addison et al. 2021). At the Jupiter endpoint of the Alfvén wing flux tubes of the icy moons (the magnetic footprints) there exist auroral like emissions (e.g., Bonfond et al. 2017, and references therein). The electron precipitation caused by, and associated field-aligned currents carried by, dispersive Alfvén waves near these magnetic footprints of the icy moons have been observed by the JUNO spacecraft instruments (e.g., Szalay et al. 2020a; Allegrini et al. 2020b; Rabia et al. 2023).

Much published knowledge of the thermal plasma environment of Europa comes from Galileo flybys and numerical simulations, but also a recent Juno flyby. The review by Bageal and Dols (2020) summarises many relevant studies and their Figs. 1 and 10 give an overview of the Io-Europa space environment and the thermal ion sources at Europa, respectively. A major source of ions at Europa is the impact of electrons giving approximately 20 kg/s $O_2^+$, with photo ionization being a less important source (Saur et al. 1998; Dols et al. 2016). Simulations suggest that a few times $10^{24}$ thermal (about 100 eV) $O^+$ ions per second
**Fig. 26** The first JUICE flyby of the moon Europa. The second one occurs in the south, but with a similar path through the tail. Detailed operation requests and related planning have so far only been carried out for the first flyby. RPWI measures in electric field burst mode for the in-situ measurements with the Langmuir probes and SCM. Radio burst (pink) is used for Jovian radio occultation measurements of the ionosphere range profile. The Europa wake region is marked with circles in the Z-Y-plane projection. JUICE will spend a long time in the Europa wake

<figure><img src="image_28.png" /></figure>

from the plasma disc co-rotating with Jupiter cause sputtering of particles from the moon's surface, while another significant part of the plasma disc flow is diverted due to interaction with the ionosphere (Harris et al. 2022).

The ionosphere of Europa was detected during five (of six) limb radio occultation measurements onboard Galileo (Kliore et al. 1997), which showed a rather dense ionospheric plasma up to $10^4 \text{cm}^{-3}$ close to the surface. The altitude profiles assume a spherically symmetric configuration, which is probably not the case for Europa's ionosphere, so the exact altitude profiles can be inaccurate. This ionosphere is also a conductive region affecting the electrodynamic interaction with the magnetosphere of Jupiter (e.g., Saur et al. 1998) and where there are electric currents flowing in response to the corotation convection electric field across Europa and where the field-aligned currents in the Alfvén wings close. The interaction causes the ionosphere to be strongly asymmetric by the magnetospheric convection and dragged through the tenuous atmosphere (and frictionally heated) and out into a tail “behind” the moon (in corotation direction). The Galileo plasma wave measurements during nine flybys detected electron cyclotron harmonics on the upstream ram side, widespread Whistler (or chorus) emissions and upper hybrid resonance enhancements within $2R_E$ of the moon, as well as highly variable electrostatic noise in the plasma wake (Gurnett et al. 1998; Kurth et al. 2001). Recent Juno plasma wave observations during a high inclination flyby at 355 km distance and through the wake region (Kurth et al. 2023) also detected chorus emissions, upper hybrid band and broadband electrostatic waves (ESW) below a few kHz with a magnetic component at the lowest frequencies. The ESW coincide with periods of bi-directional magnetic field-aligned electron beams at energies below 300 eV. The ESW could therefore be the cascade of Alfvénic turbulence to higher frequencies, like what is observed in the auroral region of Earth (cf. Sect. 3.8).

Two flybys of the moon Europa at 400 km altitude are planned for JUICE. These occur over geologically active regions near closest approach and pass inside and along the
plasma wake (Fig. 26). For the first flyby RPWI focus on the near DC electric field (E) and waves (δE, δB) and electrodynamics. Continuous electric field measurements up to 763 sps, and 16k snapshots up to 694ksps are planned. For the second flyby, see Sect. 3.10. Associated with the JUICE spacecraft moving along the trajectory, Jovian radio emission sources (<1 MHz) will gradually become occulted by the ionospheric plasma near the surfaces of the icy moons. From the radio cut-off frequencies of these emissions, we hope to gain range profiles of the topside ionosphere, with the cooperation of the occurrence of Jovian radio emissions. This remote sensing capability can provide the vertical distributions of the ionospheric densities, as well as episodic plume ejections triggered by crustal activities (cf. Sect. 5.8.2).

## 3.10 Characterize the Plasma, Dust, and Electrodynamics of Possible Plumes

There are putative Europa plume candidates observed in far-ultraviolet spectroscopy and imaging observation by the Hubble Space Telescope (HST, e.g., Saur et al. 2011; Roth et al. 2014a; Sparks et al. 2016) as well as Galileo spacecraft in-situ measurements (Jia et al. 2018; Arnold et al. 2019). However, the HST spectroscopic image detection could not be repeated (Roth et al. 2014b), and the pure imaging observations are associated with uncertainties and was explained by purely statistical fluctuations (Giono et al. 2020). Therefore, there is no definitive evidence for plumes at Europa from the UV observations. The in-situ measurements by Galileo stems from magnetic and electron density measurements in reasonable agreement with MHD models applied to one event (Kurth et al. 2001; Kivelson et al. 2009; Jia et al. 2018). The electron density is estimated from plasma wave data but consist of only two enhanced samples, while the magnetic field data indicate the presence of local electric currents nearby the spacecraft. It is clear though that a close encounter provides the best opportunities to resolve the issue and give detailed information of the physics and chemistry state of potential plume(s) (e.g., Vorburger and Wurz 2021). JUICE aims to target its two flybys above active surface regions of Europa, and its current trajectory seems to encounter possible plume material (Winterhalder and Huybrighs 2022). The JUICE Jupiter tour, however, need freeze by mid-2028, so any tweaks of the JUICE trajectory due to a detection of more exact plume location will be restricted.

A great deal of information on how to probe active plumes can be gained from the detailed studies by the Cassini spacecraft of the geysers emanating from the geologically active south polar region on the small icy moon Enceladus. From there gas and ice grains are continuously being expelled and form a water rich plume (Dougherty et al. 2006; Porco et al. 2006; Spahn et al. 2006; Waite et al. 2006; Morooka et al. 2011; Postberg et al. 2018). Many in depth studies have been conducted with various instruments regarding the plume physics based on direct fly-through observations by the Cassini spacecraft (e.g., Spitale and Porco 2007; Cravens et al. 2009; Krupp et al. 2012; Engelhardt et al. 2015). The plume material is for instance salty and contains phosphates (e.g., Postberg et al. 2009, 2011, 2023). The plume has also been found to contain partially ionized material including negatively charged nanograins (e.g., Tokar et al. 2006, 2009; Jones et al. 2009; Morooka et al. 2011; Shafiq et al. 2011; Hill et al. 2012; Farrell et al. 2010; Dong et al. 2015), where the densities of electrons, ions and negatively charged dust increase to several orders of magnitude compared to the surrounding magnetosphere of Saturn. Newly ejected material from the plume (both ions and charged dust) becomes picked up and accelerated by the Kronian magnetospheric flow (e.g., Tokar et al. 2008; Pontius and Hill 2006; Fleshman et al. 2010; Farrell et al. 2012, 2014). Enceladus is the primary source of sub-micron sized dust in the E-ring (e.g., Kurth et al. 2006; Spahn et al. 2006), which consists predominantly of negatively charged water
ice (Kempf et al. 2006, 2008, 2010; Hillier et al. 2007), and plays an important role in the dust plasma interaction there (Wahlund et al. 2005, 2009; Farrell et al. 2017). Enceladus and its electrically conductive plume act as an obstacle to Saturn's magnetospheric plasma flow and causes large scale perturbations around the moon (e.g., Dougherty et al. 2006; Saur et al. 2007; Engelhardt et al. 2015). The magnetospheric flow slows down due to mass loading of material from the plume, and results in a magnetic field pileup region upstream of Enceladus' plume (Dougherty et al. 2006; Morooka et al. 2011). The neutral-ion collisions generate perpendicular (Pedersen and Hall) currents inside the plume, which are then thought to close along the magnetic field lines (Kriegel et al. 2011; Simon et al. 2014; Engelhardt et al. 2015) and cause an auroral footprint in Saturn's ionosphere (Pryor et al. 2011). Clear signatures of accelerated electrons are observed at the edges of the plume, which are also associated with auroral hiss emissions (Gurnett et al. 2011; Leisner et al. 2013) also closer to Saturn (Sulaiman et al. 2018). These are along wedge shaped regions (the so-called Alfvén wings) like those observed at Io (Neubauer 1980). These Alfvén wings are the magnetic wakes caused by a moving plasma flow around a stationary conductive obstacle, in this case the plume. Many of the findings from Enceladus can with advantage be considered also for the Europa plume since both are the results from expulsion of dominantly water from a subsurface and therefore should have some general physical similarities. There are also distinct differences: Due to Europa's larger mass and gravity, most of the ejected material will return to the surface. Only the fastest particles, with speeds exceeding the 2 km/s escape speed will be fed directly into the environment. Moreover, Europa possesses a global atmosphere, maintained by charge particle sputtering and sublimation of the surface and consisting primarily of O₂ (Hall et al. 1995, 1998; Johnson et al. 2009; McGrath et al. 2009). Near the sub-solar point, H₂O might be equally or more abundant than O₂ (Roth 2021; de Kleer et al. 2023), also affecting the ionosphere (Li et al. 2020). Therefore, any signals affected by the gas and plasma environment must be interpreted carefully to distinguish between different sources of the gas, including possible active plumes.

The RPWI, with its ambitious *in-situ* sensor suite, can further the science of the electro-dynamics and structure of a Europa plume to an unprecedented level not possible by any other spacecraft instrumentation. Potential cracks in the ice on any of the large icy moons and potential associated plume formation can be detected much more easily by *in-situ* studies close to these moons. For Europa, RPWI considers two different sets of measurements for the two JUICE flybys, one focusing on the near DC electric field (E) and waves (δE, δB) and electrodynamics, the other focusing on high time resolution simultaneous electron (n_e) and ion (n_i) density and inhomogeneity measurements (δn_e/n_e and δn_i/n_i, at 763 sps and 49ksps) and the structure of a potential dust-plasma interactions in a water exhaust plume. Both sets include µm-sized dust measurements from RPWI.

## 3.11 Characterize the Plasma and Dust of Europa Torus and Near the Icy Moons

The plasma environment near the moon Europa (and Io) has recently been reviewed (Bagenal et al. 2015; Bagenal and Dols 2020; Masters et al. 2024, this collection). A dominant source of plasma in the orbit of Europa originates from the volcanic moon Io. An extensive cloud of neutrals and dust is expelled from Io (estimates are 1–2 ton/s), which is partly ionized and expand outward toward the orbit of Europa in a few tens of days and fills most of the magnetosphere of Jupiter with a heavy mass plasma (e.g., O+, S+, SO+, SO₂, Na+, H+, and multiple charge states, Bagenal et al. 2017a; Huscher et al. 2021). Typical densities in the disk range from about 30 to 0.1 cm⁻³ at 10 to 30 R_j, and thermal energies increase from about 100 eV in the Io torus to several keV at tens of R_j. This Io torus is also generating powerful Alfvén waves mediating strong electric currents along the magnetic field
toward Jupiter, radio, and plasma waves. Charge exchanges between co-rotating plasma and this neutral cloud further produce a sizable amount of energetic neutral atoms detectable by, e.g., the JUICE PEP-Hi instrument (see Barabash et al. 2025, this collection). The characteristics of the plasma and dust in the orbit of Europa is also affected by expelled (and ionized) material from the moon Europa itself which forms a possible additional torus in its orbit (e.g., Mauk et al. 2003; Smith et al. 2019; Szalay et al. 2022; Roth et al. 2023) consisting mostly of molecular hydrogen and oxygen and being ionized mostly to H<sub>2</sub><sup>+</sup>, H<sup>+</sup>. This constitutes the Europa torus, which is embedded within the Io torus variations.

A dust component of µm-sized grains may also exist within both the Io and Europa tori, but also close to the icy moons (e.g., Krüger et al. 1999, 2003; Thiessenhusen et al. 2000; Krivov et al. 2002; Liu and Schmidt 2019; Kurth et al. 2023), which are different populations as compared to the electric antenna detection of similar sized dust around the ring plane near Jupiter (Ye et al. 2020) by the JUNO spacecraft. The belief here is that hypervelocity impacts of interplanetary micrometeorids ejects dust particles from the moons. RPWI measurements electric pulses with four Langmuir probes can reveal from where and when the ejected dust particles leave the surface of the moons. There exists clear decrease of 0.2–2 µm-sized grains outward with radial distance from Jupiter (Soja et al. 2013) with a few 100 grains/km³ close to Europa. This number was predicted to increase by 2–3 orders of magnitude during flybys of the icy moons at closest approach.

The RPWI sensors will monitor the electric field (E), wave activity, plasma density, and dust hits within the orbit of Europa and close to Europa. Several JUICE flybys through the predicted Europa torus exist including two close flybys of Europa itself, and frequent close encounter visits exist of Ganymede and Callisto, including the orbital phase of Ganymede. While the plasma density variation of the Europa plasma torus could be hard to separate out from the plasma disc variations, the wave activity and electric fields affecting charged particles (and charged dust) will be continuously monitored. The dust counting will be made with the four Langmuir probes in electric field modes as well as when sampling currents (density mode). This is very similar to the method of electric field spikes detected with antenna on several other missions (e.g., Aubier et al. 1983; Gurnett et al. 1987, 1983b, 1991; Kurth et al. 2006; Meyer-Vernet et al. 1996, 2009; Zaslavsky et al. 2012; Ye et al. 2014, 2016, 2019, 2020). The dust counting algorithm that RPWI uses is based on the one used on Solar Orbiter (Zaslavsky et al. 2021; Soucek et al. 2021). An important difference here is that RPWI will make use of the four Langmuir probes instead of the RWI antenna elements, and the Langmuir probes make use of both differential and single ended measurements simultaneously and almost continuously at 49ksp/s (up to 25% duty cycle). With this method we expect 0.01–10 dust hits/s to be detected in the relevant regions.

## 3.12 Determine the Structure of the Callisto Ionosphere & Related Electrodynamics and Ion Outflows During the Flybys

Callisto has a dense (~10⁴ cm⁻³), asymmetric, and highly variable ionosphere (Kliore et al. 2002) that interacts with the nearly corotating and highly time variable magnetosphere of Jupiter (e.g., Khurana et al. 1998) forming Alfvén wings and a plasma wake around Callisto (e.g., Neubauer 1999). The plasma environment at the Callisto orbit has been observed by Galileo and Juno (Bagenal et al. 2016; Kim et al. 2020; Huscher et al. 2021). The Jovian plasma disc at this orbital distance includes O⁺, S²⁺ (about 60% of the total charge density) together with other charge states of O and S, with typical densities and energies of about 0.3 cm⁻³ and 1–10 keV. Much of the predictions of the local plasma around Callisto comes from simulations (e.g., Lindkvist et al. 2015; Galli et al. 2022). Atmospheric models indicate
densities from $10^6$ to above $10^8$ cm$^{-3}$ (depending on conditions and assumptions) for H$_2$, H$_2$O and O$_2$ at altitudes of a few hundred km (e.g., Carberry Mogan et al. 2022).

The ionosphere was best detectable on the magnetospheric nearly co-rotating ram side at the same time this hemisphere was sunlit (Kliore et al. 2002), indicating that both plasma impact ionization and photoionization in an impact sputtered atmosphere are important for its formation. This ram-dayside ionosphere had a clear peak at about 25–50 km altitude and similar scale heights. The Galileo magnetic field data also for this icy moon revealed a strong induction signature indicative of induced electric currents in a salty conductive sub-surface ocean (Khourana et al. 1998). However, results from an atmosphere-ionosphere model of Callisto (Hartkorn et al. 2017; Hartkorn and Saur 2017) pointed to that induced electric fields and electric currents in a conductive ionosphere could instead account for a large part of the observed induction response. The salty sub-surface ocean would in this case not exist or be situated deeper in Callisto's interior. On the other hand, the asymmetry of Callisto's ionosphere and atmosphere, affecting the Pedersen and Hall conductivities, together with an applied magnetospheric motional electric field (E ~ 10 mV/m) and a highly time variable electric field, $\delta E(t)$, induced by the streaming and variable Jovian magnetosphere give rise to complex electrodynamic system not easily inferred by theory alone. A detailed mapping of the ionosphere-atmosphere-surface and its dynamic interaction with the Jovian magnetosphere and inferring the electric current system around Callisto must be carried out to resolve the issues.

Some Galileo spacecraft flybys, using the plasma wave instrument, show plasma densities up to $400 \text{ cm}^{-3}$ at altitudes just above 500 km (Gurnett et al. 2000). This is about three orders of magnitude higher than the ambient thermal plasma density in the Jovian plasma disc, showing that significant plasma outflow may be expected. The JUICE trajectory during more than 20 flybys of Callisto will often have a closest approach around 200 km, which is well above the ionospheric peak, but RPWI in-situ measurements can nevertheless give information on cold plasma properties like the topside ionospheric plasma density, electron temperature and scale heights. Galileo plasma wave observations also reveal upper hybrid band emissions and broadband electrostatic wave activity (Gurnett et al. 2000).

RPWI will employ the full in-situ electric field and thermal plasma measurements during most Callisto flybys (see Sect. 2.7). Here the 22 JUICE flybys where several are as close as 200 km altitude will be extra important. An example is shown in Fig. 27. An example RPWI will also carry out radio occultation measurements using Jovian radio emission cut-offs. A coverage of these with respect to solar zenith angle (SZA) and plasma ram angle is displayed in Fig. 28.

### 3.13 Characterize Processes Behind the Co-Rotation Breakdown in the Jovian Magnetosphere

As a consequence of Jupiter's intense internally generated magnetic field and rapid rotation, much of Jupiter's magnetosphere is in a so-called "rotationally-dominated" regime, in which the dynamics of the magnetospheric plasma are largely controlled by coupling via the ionosphere to the rotating neutral atmosphere. However, the (radial) extent of this control is ultimately limited, such that enforcement of a state of rigid corotation is maintained only out to distances of ~15–20 R$_J$, beyond which the plasma sub-corotates, having azimuthal velocities that are only a fraction of the local corotation velocity (McNutt et al. 1979). Prior to the arrival of Juno at Jupiter, the generally accepted framework held that mechanical stresses, transferred along magnetic field lines between the polar ionosphere(s) and equatorial magnetosphere by field-aligned currents were primarily responsible for the main auroral
<figure><img src="image_29.png" /><figcaption>**Fig. 27** RPWI planned operations near a distant flyby of Callisto. We plan to do in-situ burst operations for Langmuir probes in electric field modes and SCM at closest approach (red). Ionospheric occultation of Jovian radio emissions is planned at special times when these radio waves pass through the ionosphere of Callisto (two cases, black)</figcaption></figure>

emissions observed at Jupiter (see e.g., summaries provided by Achilleos et al. 2015 and Bagenal et al. 2017b). This paradigm has been extensively explored in models ranging from one-dimensional purely analytical approximations to spatially resolved numerical simulations (e.g., Hill 1975, 1979; Cowley and Bunce 2001; Nichols 2011; Chané et al. 2013), and was generally found to reproduce well both the available in-situ particle and field data from missions such as Voyager and Galileo, as well as remote observations of auroral emissions made, e.g., by HST (e.g., Clarke et al. 2009). However, more recent observations made by Juno instrumentation of auroral structures, plasma waves, precipitating particle distributions and the Jovian field at high latitudes have in some respects challenged the accepted model of the Jovian main emissions (e.g., Bonfond et al. 2020). Specifically, evidence in both particles and fields for significant energy fluxes into the polar ionosphere carried by broadband-accelerated electrons and Alfvenic fluctuations appears to suggest a component of the Jovian magnetosphere-ionosphere-atmosphere coupling due to aspects not originally considered (see e.g., Gershman et al. 2019; Kotsiaros et al. 2019; Allegrini et al. 2020a; Mauk et al. 2020).

The measurements to be made by RPWI in the Jovian magnetosphere are expected to contribute to studies of the process of corotation breakdown in the equatorial plasma in several ways, all of which are intrinsically related to studies of the general spatial and temporal structure of the equatorial magnetosphere and magnetodisc. For example, quasi-continuous monitoring of the local plasma density is obtained by RPWI using various methods, which together can be used to construct maps of the magnetospheric thermal plasma structure, as well as its thermal and kinetic energy content (dominated by its azimuthal velocity). Spatial
<figure><img src="image_30.png" /><figcaption>**Fig. 28** Coverage of ionosphere measurements from RPWI radio occultations of Jovian decametric radio emissions. Panel A: The magnetospheric plasma ram arrives from –X (+X is Callisto wake). The different coloured lines are the radio occultation traces with altitude up to 1500 km. The ionosphere plasma is expected to be encountered below about 300 km with a peak in the 50–100 km range. Both the ram-side and the wake-side can be covered. Panels B & C show the coverage of radio occultations with respect to solar zenith angle (SZA) and where B is North hemisphere and C the South hemisphere</figcaption></figure>
gradients and inhomogeneities in these parameters yield information as to the forces acting on and within the plasma in the magnetodisc. Such observations can be used to provide further constraints on, e.g., the action of the interchange instability and radial plasma transport, the spatial (local time and radial) extent over which corotation breakdown occurs, as well as externally and internally forced temporal variability in these processes. Measurements of the quasi-DC electric field combined with J-MAG data can give the **E** × **B** drift velocity of the ambient plasma. From this, departures from rigid corotation can be quantified both in terms of their magnitude and spatial variation throughout the regions sampled by JUICE, as well as monitoring any temporal variations associated, e.g., with magnetospheric compressions and expansions. Similarly, in certain circumstances, interferometry techniques can be employed to study the motion of plasma structures (δn/n) across the spacecraft, yielding measurements of ambient plasma flow over a limited range of relative velocities (e.g., LaBelle et al. 1986; Kintner et al. 1987; Holmgren and Kintner 1990; Wahlund et al. 1998, 2009; and references therein; see also Sect. 2.2.2).

Simultaneous measurements of Jovian auroral radio emissions provide a mechanism to infer both the internal state of the magnetosphere, as well as detecting any transient changes associated with external forcing by solar wind variations - critical drivers of the system that may otherwise generally only be inferred by extrapolated solar wind conditions from other solar system missions and heliospheric modelling. Local plasma wave measurements over a broad frequency range, including wave propagation characteristics including polarization and Poynting flux provide information as to the transfer of energy between different regions, which can be analysed together with, e.g., estimates of quasi-DC field aligned currents provided by J-MAG.

## 3.14 Determination of Dynamics of the Jovian Magnetosphere Through Radio Emissions

The first measurements of radio waves from the neighbourhood of Jupiter were reported by Burke and Franklin (1955) and since then many different types of observations have been made in the radio spectrum both from ground and from spacecraft. The JENRAGE experiment will have the sensitivity to detect all components of the Jovian magnetospheric radio emissions (high latitude auroral and satellite-induced Cyclotron-Maser (CM) emissions, low-latitude plasma emissions, e.g., from the Io torus, atmospheric lightning radio waves) from the kilometer to the decimeter wavelength range (Zarka 1998; Zarka et al. 2004; Louis et al. 2021). Figure 29 summarizes the source location and beaming for most of these components.

JENRAGE only has a marginal capability of detecting the fast-drifting S-bursts (Zarka et al. 1996; Mauduit et al. 2023), but all other emissions varying slower than a few seconds will be observed in flux density and polarization together with the direction of arrival of the waves, thanks to JENRAGE's goniopolarimetric capability. Dynamic spectra of CM emissions will be compared to simulations done using the code ExPRES (Louis et al. 2019a) and relying on the most recent Jovian magnetic field model (Connerney et al. 2022). Refracted propagation paths from the sources, especially at low frequencies, will be studied using the ARTEMIS-P code (Gautier 2013). Finally, RPWI radio measurements will be complemented or correlated with measurements from other remote sensing instruments onboard JUICE (particles, UV aurorae, magnetic field, etc.), as well as with other ground-based and space radio observations.

As its trajectory will not bring JUICE much closer to Jupiter than the orbit of Europa, radio measurements will rather be of synoptic nature. They will allow us to address the
<figure><img src="image_31.png" /><figcaption>**Fig. 29** Jovian natural radio sources observable by JUICE/RPWI. The radio emissions are mostly produced in the jovian auroral regions, by energetic particles traveling along magnetic field lines and resulting in unstable distribution functions. The radio emission pattern is not isotropic: the radio waves are emitted along a thin hollow cone represented in the Figure. The approximate location of the io-controlled decametric (Io-DAM), auroral decametric (non-Io-DAM), hectometric (HOM), narrow-band kilometric (nKOM), brand-band kilometric (bKOM) and quasi-periodic bursts (QP) components. For instance, Io-DAM emissions are only visible when the emission cone is illuminating the observer. The longitudinal lag between the magnetic longitude of Io and that of the Io-controlled radio sources (the so-called lead-angle) is represented by the δ angle. Vector M and Ω represent the magnetic and rotation axes of Jupiter, respectively. In the bottom-right corner, sub-figure shows an HST UV image of the Jovian auroral components (Galilean moon footprints, main oval and polar emissions), which are produced by the same populations of particles as the radio components. The lightning symbol in Jupiter represents the electromagnetic counterpart of atmospheric lightnings. This plot is modified from Zarka 2000 & Cecconi et al. 2012</figcaption></figure>

following objectives concerning the dynamics of the Jovian magnetosphere, to understand better:

(1) the internally controlled dynamics by observing the so-called energetic events (Louarn et al. 2014, 2015), that correspond to outward centrifugal plasma transport and particle injections associated to radio intensifications; RPWI measurements will allow us to compare the variations of all Jovian radio components together with plasma variations, and to document the recurrence time of these events and their relationship with Io's volcanic activity (see Sect. 3.15). It will explore the origins of the so-called system IV period, related to corotation lag within Io's torus (Reiner et al. 1993) but also affecting more surprisingly some auroral decameter emissions (Panchenko et al. 2010, 2013).

(2) the externally controlled dynamics by measuring the response of radio emissions to interplanetary shocks (forward and reverse) (Hess et al. 2014; Louis et al. 2023), the delay between excitation and response, the different behaviour of the different parts of the magnetosphere (dawn- or dusk-side).

Following Zarka et al. (2021) who proposed a method for comparing quantitatively the level of internal versus external control for each radio component, Boudouma et al. (2023) established that the hectometer (HOM) emission mainly originates from the dusk side of Jupiter's magnetosphere, where the plasma disc is thicker and more turbulent (Kivelson and
Southwood 2005) and the aurora brighter (Bonfond et al. 2015). The question remains open for other components. Combined with ground-based and Juno radio observations, RPWI observations may allow us to address longer-term variations depending on Jovian season or the solar cycle.

JENRAGE measurements will also address questions related to the generation of radio waves:

* The strongly needed long-term (3.5 years) goniopolarimetric measurements will allow us to precisely identify the source location of all radio components, from decametric to kilometric range (in particular the surprising but well-defined decameter components A' and A", Marques et al. 2017). Lower frequency radio components, such as QP-bursts (MacDowall et al. 1993; Morioka et al. 2006; Kimura et al. 2012), or the narrowband low-frequency – nLF – component (Louis et al. 2021), will be observed with GANDALF since JENRAGE only observes down to 80 kHz. CM radio sources will be related to the auroral morphology (diffuse aurora, zones I & II, Louis et al. 2019b; Sulaiman et al. 2022; Collet et al. 2023). The radio beaming, measured along with source location, is important both for, e.g., improving ExPRES simulations and for being applied to exoplanetary radio emissions to derive emitted power from measured flux densities (Callingham et al. 2021).

* Measurements up to ~30° latitude may allow us to identify the transition from observed circular to elliptical polarization (Reiner et al. 1995) as a function of the frequency (with measurable Faraday rotation when elliptical polarization is present (Lecacheux et al. 1991), that can be modelled with ARTEMIS-P and has deep implications on the wave generation and near-source plasma content (Lecacheux 1988).

* CM radio emissions seem to be mostly produced at an oblique angle relative to the magnetic field in the sources (Hess et al. 2008; Mottez et al. 2010), via the loss-cone driven CM instability (Louarn et al. 2017) whereas growth rates are generally higher for shell-driven CM instability (Hess et al. 2007, 2008).

* What is the origin of peculiar emissions in Jovian radio dynamic spectra such as Zebra (Panchenko et al. 2018) or the high frequency “splitting” at the upper frequency of decameter radio emission (Leblanc and Rubio 1982)?

## 3.15 The Io Torus Dynamics and Related Radio Emissions

Radio emissions induced by the interaction of Galilean satellites with Jupiter's magnetic field are not limited to the case of Io. Ganymede- and Europa-induced emissions have been unambiguously identified (Louis et al. 2017; Zarka et al. 2018; Jacome et al. 2022) and their detailed characteristics will be studied by JENRAGE and compared to the plasma environment of these satellites. For such radio sources restricted to a single magnetic flux tube, JENRAGE observations will be used together with an accurate Jovian internal field model (Connerney et al. 2022) and be compared to ExPRES simulations to give strong constraints on the radio beaming angle (possibly azimuth-dependent (Galopeau and Boudjada 2016; Louis et al. 2017)), the Alfvén propagation time between the satellite and Jupiter and the energy of emitting electrons (Lamy et al. 2022, 2023; Zheng et al. 2023). Despite the tentative detection of a UV footprint associated to Callisto (Bhattacharyya et al. 2018), the existence of CM radio emissions induced by Callisto remains an open question which will be tackled by JENRAGE.

The torus also generates emissions at or near the local plasma frequency, such as the narrowband kilometer - nKOM (Reiner et al. 1993) and perhaps the nLF (Louis et al. 2021)
components. The variations of nKOM are related to the system IV period and to energetic events (see previous section). The dynamics of the torus, and that of the Jovian magnetosphere at large fed by the torus plasma, can be affected by Io's volcanism (Yoneda et al. 2010, 2013). This variability can be studied via that of the Io- and Io torus related radio components (nKOM, Io-decimeter).

## 3.16 Characterize Wave Emissions from Lightning at Jupiter

The first detections of lightning in Jupiter's turbulent atmosphere were made with the Voyagers spacecraft when bright spots on Jupiter's nightside were imaged with the camera (Cook et al. 1979) and whistlers were detected with the radio instrument onboard Voyager 1 (Gurnett et al. 1979). Those were long dispersion whistlers lasting for several seconds, recorded at frequencies of a few kHz in the Io plasma torus between 5 to 6 Jovian radii. Surprisingly, none of the radio instruments on-board the Voyagers, Ulysses, Galileo, Cassini or Juno ever detected sferics (radio signals from the atmosphere) at Jupiter in a frequency range of 1 to 40 MHz. This is in marked contrast to Saturn, where Saturn Electrostatic Discharges are ~$10^4$ times more intense than terrestrial lightning radio emissions in this high frequency (HF) range (Fischer et al. 2008). The reason for this is unclear, but it could be due to strong absorption of these radio waves in Jupiter's lower ionospheric layers (Zarka 1985). Farrell et al. (1999) attributed the non-detection of the HF component to the decrease of the spectral power with increasing frequency. However, this is in contradiction to the surprising detection of prevalent lightning sferics at 600 MHz with the Juno Microwave Radiometer (Brown et al. 2018). New Jovian lightning detections with Juno were also made below 150 kHz: The so-called Jupiter dispersed pulses (JDPs) were recorded by the Juno Waves instrument at frequencies between 10 and 150 kHz with dispersions in the millisecond range at distances below 5.5 Jovian radii. They might propagate in the free-space ordinary mode through low density regions of Jupiter's ionosphere (Imai et al. 2019). Furthermore, close to Jupiter, rapid whistlers at frequencies from 50 Hz to 20 kHz with dispersions from units of milliseconds to tens of milliseconds were also detected (Kolmašová et al. 2018). Recently, groups of radio pulses (JDPs and rapid whistlers) with typical time separations of one millisecond were identified in the Juno Waves data. This finding suggests step-like extensions of Jovian lightning channels and indicates that lightning initiation processes at Jupiter are similar to the initiation of intracloud lightning at Earth (Kolmašová et al. 2023). All these detections with Juno have led to the largest data set on Jovian lightning collected to date. Due to the large periapsis distances of JUICE (~9 to 20 Jovian radii) it will be much more difficult for JUICE to detect lightning radio pulses compared to Juno. However, it is not excluded that RPWI will detect whistlers from very high latitudes, JDPs passing through ionospheric holes, or maybe the missing HF component. Further details about optical thunderstorm and lightning observations with JUICE can be found in the paper by Fletcher et al. (2023, in this collection).

## 3.17 Cruise Science by RPWI

Any cruise science by JUICE is for the moment on best effort basis by the project. The launch of JUICE (April 14, 2023) allows though for a close flyby of the moon, a Venus flyby, three flybys of Earth and measurements through the asteroid belt. Conjunctions are also looked for with another spacecraft such as Solar Orbiter, Parker Solar Probe, BepiColombo and the soon to be launched Europa Clipper. We here provide a short list of the RPWI cruise science opportunities.
* *Earth flyby*:

o Compare with radio emissions & whistlers from lightning ±20 R<sub>Earth</sub>

o Compare with measurements from Wind/WAVES and Cluster allowing for two-point
measurements of the beaming of AKR

o Part of calibration runs near Earth

* *Moon flyby*:

o Test Passive Sub-Surface Radar (PSSR) with AKR as source

o Lunar wake measurements

* *Solar system dust & solar wind interaction characterization:*

o LP-PWI & SCM to do continuous monitoring of plasma and dust counts

o ... in the orbits of Venus and Earth

o ... make a dust count profile through the asteroid belt.

* *Conjunctions with another spacecraft for solar wind studies:*

o BepiColombo, Solar Orbiter, Parker Solar Probe, Europa Clipper conjunctions within 5° latitude/longitude and within 0.1 AU to be able to probe the same solar wind plasma. Here RPWI will monitor solar wind ion flux, **E** × **B** (with J-MAG), and plasma turbulence.

# 4 Instrument Design

## 4.1 General Design

Figure 30 shows the RPWI overall block diagram, where a redundant set of Data Processing Units (DPU) and Low Voltage Power Supplies (LVPS) controls the four main experiments of the instrument package. Physically, the SCM and RWI preamplifiers are installed inside the SCM and RWI sensors boxes (chassis), while the LP-PWI preamplifiers are situated at the roots of the LP-PWI booms near the S/C surface (i.e., not within the spherical Langmuir probes).

The RPWI consists of products, as shown in Fig. 30.

* EBOX, which is the main electronic unit where all data from the sensors is collected and processed as well as power distribution and TM/TC communication to the S/C.

<figure><img src="image_32.png" /><figcaption>**Fig. 30** RPWI physical architecture</figcaption></figure>
<figure><img src="image_33.png" /><figcaption>**Fig. 31** Main RPWI units. Top left: main electronics box (EBOX), Top right: LP-PWI boom unit and preamplifier. Bottom left: Search-coil magnetometer (SCM). Bottom right: RWI antenna system (stowed). The SCM and RWI boxes are covered by their multi-layer insulations (MLI)</figcaption></figure>

o LVPS (A&B), Low Voltage Power Supply. Primary and redundant side.

o DPU (A&B), Digital Processing Unit. Primary and redundant side.

o LF (Low Frequency) Receiver.

o LP-MIME (Langmuir Probe & Mutual Impedance Measurement) Transceiver.

o HF (High Frequency) Receiver.

• LP-PWI, which consists of four Langmuir probes and (deployable) booms.

• LP-PWI Preamplifiers, which consists of four LP preamplifiers, and external harnesses.

• SCM, which consists of the tri-axes search coil antenna and preamplifier with an external harness along the MAG boom.

• RWI, which consists of the three collocated (deployable) dipole antennas and preamplifiers, with its external harness along the MAG boom.

The RPWI subsystems are shown in Fig. 31.
<figure><img src="image_34.png" /><figcaption>**Fig. 32** LVPS and DPU power switching philosophy</figcaption></figure>

The RPWI system is built up around a main Electronics Box (EBOX), which houses dual redundant Low Voltage Power Supplies (LVPS A&B) and Digital Processing Units (DPU A&B). These serve the Low Frequency (LF) receiver, the High Frequency (HF) receiver, and the Langmuir Probe & Mutual IMpedance Experiment (LP-MIME) transceiver. The duplicated (redundant) LVPS and DPU adds to robustness and immunity to single point failure. LVPS A is hardwired to DPU A and LVPS B is hardwired to DPU B. The power switching philosophy is shown in Fig. 32.

A backplane inside the EBOX distributes the analogue and digital signals, the different voltages, and a clock signal, which is generated by dual redundant crystal oscillators on the LVPS circuit board. The LP-MIME transceiver is connected internally to the LF receiver, for which it provides single-ended and differential signals from the Langmuir probes. It also contains driver circuits for the MIME transmitter. The MIME drivers can furthermore be synchronized to the mixer frequency of the HF receiver, for mutual impedance measurements at high frequencies. External harnesses connect the LP-PWI, SCM, and RWI sensors to the circuit boards inside the EBOX.

The EBOX is housed inside the payload vault of the spacecraft, where it is relatively well protected from the space environment. The RPWI sensors are mounted on the outside and face a more severe temperature and radiation environment. For optimal performance, all preamplifiers are mounted as close as possible to the sensors. While the sensors themselves are immune to the harsh radiation and thermal environments around Jupiter, the preamplifier electronics is less resilient. Special considerations have therefore been taken as to their radiation hardness, shielding, and thermal management. The units mounted on the outside will be subject to both high and low temperatures and are therefore protected by MLI.

Each LP-PWI sensor has a deployable boom with the Langmuir probe at its end and with the preamplifier mounted close to its root. The LP-PWI preamplifier housings are attached directly to the spacecraft body on standoffs and under MLI. Survival heaters are mounted
just under the preamplifier housings. This protects them from radiation from one side and simplifies their thermal management.

The SCM is mounted on the MAG boom. Its preamplifier is a custom-made ASICs, which is designed to withstand the radiation levels and to work in a wide temperature range. They are mounted inside the mechanical structure that supports the three sensors and acts as the boom interface. The SCM preamplifier ASIC has been tested down to -196 °C.

The RWI antenna subsystem is also mounted on the MAG boom close to the SCM. Its preamplifiers are housed inside a preamp box that is designed to provide radiation shielding and thermal balance. The early engineering models (mechanically representative chassis and electrically representative preamplifiers but without any antennas) was successfully turned on and worked in thermal vacuum between -180 °C and +90 °C. The preamplifier box supports the antenna elements and acts as a mechanical interface to the boom. Survival heaters are mounted on the RWI box.

Deployment power for the deployable units (including heaters) is the direct bus power provided by the spacecraft. However, RPWI is designed to measure very low-level electric and magnetic signals, which makes it special in this respect. Any cable that is galvanically connected from the spacecraft to the RPWI sensors is a potential source of noise (even after deployment). For this reason, the deployment lines are referenced to RPWI instrument ground when deployment is not active.

## 4.2 Electronics Box

The EBOX, which is made of 8 mm thick CNC machined Aluminium, houses seven A5 sized (210 × 148 mm) printed circuit boards (PCB), which are interconnected by a backplane with floating connectors. The backplane block diagram is shown in Fig. 33. The FM PCBs are displayed in Fig. 34. As can be seen in the diagram, each receiver (LP-MIME, LF, and HF) has its own PCB, the DPU A & B have separate PCBs, and the LVPS A & B have separate PCBs. Each unit is mounted in a separate frame. They are stacked vertically, with thin Aluminium plates between them for electromagnetic shielding. The frames are held together by long bolts. This construction makes the EBOX very rigid and practically insensitive to shocks and vibrations. Using frames is also very convenient during the development of the different subsystems, as the frames provide mechanical protection for the PCBs and support for electric connectors, which makes it much safer and easier to handle them than compared to handling bare PCBs.

The backside of each frame has connectors for the backplane and the front has connectors for the sensors, which makes it possible to connect individual PCBs safely to laboratory test equipment during the development phase, for instance to the electric ground support equipment (EGSE) during tests. External harnesses connect the sensors to the different receivers inside the EBOX.

### 4.2.1 Backplane

The purpose of the backplane inside the EBOX is to route all the signals and power lines between the instruments, the DPUs and the LVPS PCBs. A block diagram of the backplane is shown in Fig. 35. The backplane ground, with 3 dedicated planes, creates a low resistance star grounding point between all the stacked PCBs, and the EBOX chassis, which has single connection to the spacecraft. A star ground topology minimizes potential EMC problems between the instruments and the use three full layers makes it easier to design and route high speed digital signals needed for the communication between the DPU and instrument
<figure><img src="image_35.png" /><figcaption>Fig. 33 RPWI functional block diagram. Red marked texts correspond to instrument single point failure chains</figcaption></figure>

PCBs. In addition, care has been taken to avoid crosstalk on the backplane. Analogue signals are all contained in strip-lines with coplanar ground borders with via stitching. Noisy nets, such as digital nets, were kept well separated on the other half of the backplane from any analogue nets.
<figure><img src="image_36.png" /><figcaption>**Fig. 34** EBOX PFM PCBs, Backplane (top left), HF (top right), LP-MIME (middle left), LF (middle right), DPU (Bottom left) and LVPS (bottom right)</figcaption></figure>

### 4.2.2 Harness

A block diagram of the harness is shown in Fig. 36. The whole LP-PWI harness system is matched in length using analogue signals.

The different harnesses are parts of the sensor subsystems (LP-PWI, SCM, and RWI). The harnesses have been constructed in such a way as to minimize EMC impact to adjacent signals in the same harness as well as to nearby harnesses. To achieve this, differential signals make use of twisted pairs with an overall shield and single ended low-level signals use coaxial cables. Special care has also been taken when designing harness for any heaters and power supply lines to external sensors, twisting the source with return to minimize current loops. In addition, the routing of harnesses has been made to try minimizing pickup of noise, especially cables routed to the external sensors, where potential noise may contaminate the signals RPWI intend to measure.
<figure><img src="image_37.png" /><figcaption>Fig. 35 Internal EBOX backplane block diagram</figcaption></figure>

### 4.2.3 LVPS

The RPWI power generation is secured by the dedicated Low Voltage Power Supply, comprising of two independent DC/DC converters isolating the S/C PCDU out of the RPWI subsystems power inputs, Power Distribution Unit and Housekeeping Monitors (voltage, current sensing on both primary and secondary sides of the converter). Insight to the concept of the LVPS is given in block diagram in Fig. 37.

The converter is based on the step-down converter (Buck regulator) serving as a constant voltage feed pre-stage for the push-pull driver of the main power transformer. Thus, the converter topology is called Voltage-Fed Isolated Push-Pull Converter. Due to a lack of proper rad-hard semiconductors intended for isolated DC feedback on the market, the converter works with fixed duty cycle without any secondary to primary feedback loop, taking the output and internal parasitic resistance as acceptably low to provide the output load variation between required boundaries. This directly leads to positive impact on PCB footprint, mass savings and increased reliability (no optocouplers used, etc.).

The over-voltage and over-current protection are applied on both input circuitry as well as all outputs, except for the DPU, which needs to stay biased after start-up with all other loads implicitly set to OFF. Short circuit protection of dedicated loads or the ON/OFF switching are based on solid state MOSFET switches with low resistance between Drain and Source
<figure><img src="image_38.png" /><figcaption>Fig. 36 RPWI Harness and interconnections</figcaption></figure>

when turned ON ($R_{DSON}$). The analogue current sensing circuitry is used to limit the maximum output current per each output, whilst is continuously measured via FPGA. In case the limit is reached, for a limited amount of time, the current mode is activated. Dedicated digital current sensing then decides with at least three consecutive samples over the threshold to switch off the line permanently or keep ON.
<figure><img src="image_39.png" /><figcaption>Fig. 37 LVPS block diagram</figcaption></figure>

The whole RPWI package is synchronized to a 50 MHz low jitter crystal oscillator clock source distributed to all subsystems via LVDS bus, further divided locally to a switching frequency of 250 kHz by the FPGA. The switching clock is then back fed to the primary side via pulse transformer and replaces the startup R/C oscillator, once sync clock is available. An artificial delay of ~300 ms is introduced to let the RC network stabilize and bias the LVPS core circuitries, prior the clock synchronization takeover is initiated. Housekeeping measurements and TM/TC engine is realized as FPGA softcore. Isolation of the clock signal and DPU-LVPS data link is based on dedicated LVDS driver to prevent the FPGA against the failure propagation from any other instrument subsystem, even if the LVDS peripherals are available in target FPGA at appropriate voltage/timing levels.

Two cold redundant LVPS power converters contain a switch matrix to fulfil the ON/FF function for dedicated lines for each RPWI subsystem separately. Output of both redundant parts are interconnected together as a summing circuitry making it a single point failure free design on ideal diode principle. Moreover, fast solid-state switches are used as a current chopper, controlled by the pulse trains (quasi PWM) delivered by the FPGA with duty cycle and length variable on the fly via telecommand. A series of pulses controlling the MOSFET line switch decrease the initial start-up current into bulk capacitances (order of millifarads) on the load side resulting in damped inrush currents observed on the main 28 V S/C power bus.

Difficulties with high current distribution of low voltage lines (1.5 V and 1.8 V) are covered locally at each subsystem by dedicated Point-of-Load (PoL) or Low Drop Out (LDO) regulators to produce the required voltages (CPU/FPGA core voltages). These PoLs and LDOs are powered from the 3.7 V rail to save power needed for their quiescent currents when not loaded by full computational load. Switch Mode PoL regulators are synchronized to 1 MHz clock derived from the distributed 50 MHz LVDS signal to minimize the frequency random walk of PoL's internal oscillators. The isolation between primary and secondary power lines in the RPWI are ≥ 1Mohm, 150nF. Power required for DPU, and instruments is routed via LVPS frame to the backplane to each of the subsystems. The deployment power
<figure><img src="image_40.png" /><figcaption>Fig. 38 DPU block diagram</figcaption></figure>

is not controlled by the LVPS, however the 28 V is routed straight through the LVPS to the backplane and made available to the LP and HF PCBs.

### 4.2.4 DPU

The Digital Processing Unit (DPU) provide all required RPWI instrument control, health checkout, monitoring and high-level data processing functions. A block diagram of the DPU is shown in Fig. 38. To avoid "Single Point Failure" the RPWI instrument include two separate and fully autonomous DPU modules (primary A, redundant B). DPU A is supplied from LVPS A and DPU B is supplied from LVPS B, i.e., without commutation or crossing. Each DPU module communicate independently with the satellite on board data handling (OBDH) system TM interfaces. However, only one of the DPU modules is switched on at any given time, following the Hot/Cold rule. The DPU module can be silted into several
functional blocks as depicted in the high-level block diagram shown in Fig. 38. The DPU architecture is centred around a fault tolerant ASIC (the dual core GR712 RC from Aeroflex Geisler) which implements a LEON3 FT SPARC V8 processor core and several required peripheries, working as Board Supervisor Unit (BSU) The BSU features cover all functions required for minimum instrument functionality. TC & TM are supplied by SpW from the S/C where the LVDS buffers are powered with overvoltage protected 3.3 V to avoid failure propagation to the S/C in case of buffer failure.

The tasks to be handled by the on-board software are:

* Internal mode control.

* Switching and commanding of measurements measurement modules and LVPS.

* Tele command (TC) verification, validation, and execution.

* Software patch download, verification, and execution.

* Data collection and buffering (reduced functionality).

* Telemetry (TM) packet formatting and sending.

* Housekeeping data collection.

Some parts of this unit have been implemented in the FPGA together with supervisor part. The base 50 MHz clock is used to generate the reference time pulses (PPS) which are PLLed to the external S/C SpW time pulses. These time pulses are used by the other instrument boards to synchronize the sampling and time keeping.

There are three fully independent cores for communication with LP, LF or HF modules. Each one includes command and data FIFOs implemented inside the FPGA. The FIFO was based on RAM blocks and behavioral VHDL written FIFO controller. FIFOs will be 32-bit wide and 2048 words depth to optimize data transfer time (to or from CPU) and simplify CPU interfacing. The I/O registers access is always 32-bit wide. Data FIFO will be able to hold whole transmitted packet. So, no CPU involvement is required during one (requested or pushed) data packet transmission.

The Housekeeping acquisition unit (HKU) performs all functionality required to acquire data from analogue and thermistor instrument check points. The HKU is controlled by the BSU and includes ramp-compare ADC and analogue electrical interfaces to resistive or voltage sensors. This solution reduces the radiation sensitive of analogue hardware parts significantly since the ADC counter and control logic is implemented in an RTAX2000 FPGA (BSU processor peripheral). The sensor interface input circuitry can be unified to only a few types as presented in Table 11.

The analogue to digital conversion is based on comparison of measured values to generated voltage ramp signal by a Delta-Sigma DAC core implemented in FPGA. When the ramp starts, a timer starts counting to the ramp voltage until it matches the input. A comparator then fires, and the current ramp value is recorded.

The implemented method of data acquisition is useful only for signals that are changing at a slow rate. If the analogue input voltage changes during the sampling process, it effectively causes the sample point to move randomly. This adds a noise component, which becomes larger as the input frequency increases. However, most of the HK "checkpoints" values change relatively slow (like temperature).

The memory unit is one of the units in every processor-based system that is sensitive to single event upset errors. In the DPU, external (from the viewpoint of the FPGA embedded processor) memory is used for storing:

* Application software code

* Application data structures

* Configuration and calibration tables
<table><caption>**Table 11** HK sensor types (Voltage and Temperature) and interfaces</caption><thead><tr><th>Sensor</th><th>Range</th><th>Interface</th></tr></thead><tbody><tr><td>Voltage</td><td>0 - 5.0 V</td><td></td></tr><tr><td>Temperature</td><td>-65 °C to 100 °C<br>(NTC sensors)<br>and<br>-200 °C to 200 °C<br>(PT1000 sensor)</td><td></td></tr></tbody></table>

<figure><img src="image_41.png" /></figure>

<figure><img src="image_42.png" /></figure>

* Software patches

* Configuration bit streams of DPU FPGA

* Telemetry packet buffer

Data stored in memory are sensitive to errors induced by radiation. For instance, application software code, data structures, parameters, lookup tables, etc, are very sensitive and any content alteration may end up with catastrophic errors. On the other hand, information such as data samples, buffered TM packets, etc, are not as sensitive and may not require the same amount of error protection. To avoid complete software reloading, the flight software code is resident in ROM and FLASH and copied to RAM for execution. This implementation offers the advantage of allowing executable code modifications during the mission and autonomous code error corrections without restarting of program execution. For the code and data RAM memory segment, a “Correct Always” structure is used (EDAC-protected using a (39,7) BCH code). A block diagram of the memory module is depicted in Fig. 39.

The Boot SW can run directly from PROM (slow access time) or can be copied into RAM (fast access time) and executed from there. The BSW, loaded to SRAM or executed directly from PROM, cover all functions required for minimum instrument functionality.

Application specific software (ASW) will be stored in a rewritable MRAM memory bank that hold at least two images of complete code. The internal MRAM EDAC will be transparent for DPU design instead software image correctness will be controlled by segmented CRC or EDAC support for 8-bit memory provided by the LEON memory controller. This implementation offers the advantage of allowing executable code modifications during the mission and autonomous code error corrections without restarting of program execution.

The PROM and MRAM memory could be switched OFF (stays unpowered) when copied and run from SRAM (external or On-chip) to power saving at normal operation.

The Signal Processing is carried out by the dual core CPU (GR712RC). It includes data stream processing IP cores dedicated to each measurement module. The implemented
<figure><img src="image_43.png" /><figcaption>Fig. 39 DPU memory module block diagram</figcaption></figure>

processing algorithms cover data accumulation, signal filtering and spectral analysis (Fast Fourier Transform).

The DPUs support the following Space Wire link characteristics.

* Supported link speed is 100 Mbps.

* Maximum packet size supported is 4kBytes.

* Minimum TM buffer size 100 kByte supporting 100 ms latency.

* Peak data traffic 2MByte/s.

* Traffic profile average rate is <40kByte/s during typical measurement cycles.

### 4.2.5 LP-MIME Transceiver

The LP-MIME transceiver consists of the LP receiver and the MIME driver circuits for filtering, switching and transmission. MIME and LP are two sandwiched boards that together make up one in the frames of the EBOX. A block diagram of the LP analogue section is shown in Fig. 40. The LP-MIME digital block diagram is shown in Fig. 41.

Up to four selected analogue signals are captured by the ADCs and routed to the FPGA for further filtering, mixing, and processing. The data is then sent to the DPU for further processing and storing. Each ADC has the possibility to choose from a subset of eight analogue signals (differential, single ended, E-field or Density).

The basic principles underlying plasma density (Langmuir probe – LP) and electric field (E-field) measurements with the LP receiver are identical to that of a digital multi-meter measuring in current and voltage mode, respectively (see Fig. 41). The plasma density is a scalar quantity, and one probe measurement is sufficient to obtain the local density at the probe. The E-field, on the other hand, is a vector quantity, which requires a minimum of four probes since one E-field component is obtained as the potential difference between two probes divided by the separation distance. The LP receiver employs four spherical probes, 10 cm in diameter, mounted at the tips of 3-meter-long deployable booms. The booms are allocated on the spacecraft such that three-dimensional E-field measurements are enabled. Each probe can be individually configured in Density or E-field mode; to support all expected sciences modes.

The spacecraft will become charged due to its interaction with the environment. The final surface potential of the spacecraft depends on the characteristics of the environment, but for many environments the potential difference between two insulated conductive spacecraft