# Cold and Hot Electron Populations Observed by Juno
## -JADE in the Jovian Magnetodisc

Y. Sarkango, J. Szalay, A. Poppe, Quentin Nénon, Z. Liu, F. Allegrini, D.

Mccomas, R. Ebert, P. Kollmann, G. Livadiotis, et al.

► To cite this version:

Y. Sarkango, J. Szalay, A. Poppe, Quentin Nénon, Z. Liu, et al. Cold and Hot Electron Populations Observed by Juno -JADE in the Jovian Magnetodisc. Geophysical Research Letters, 2025, 52 (17), 10.1029/2025GL118209. [hal-05378558](https://HAL Id: hal-05378558)

Submitted on 24 Nov 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2025GL118209

**Key Points:**

* Electron distributions in the Jovian magnetodisc are well fit with two anisotropic kappa-distributions

* Estimates of density, temperature, anisotropy, and kappa index for cold and hot electrons are provided

* Electron temperature increases non-adiabatically with M-shell until M ~ 20

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

Y. Sarkango,
sarkango@umn.edu

**Citation:**

Sarkango, Y., Szalay, J. R., Poppe, A. R., Nénon, Q., Liu, Z. Y., Allegrini, F., et al. (2025). Cold and hot electron populations observed by *Juno*-JADE in the Jovian magnetodisc. *Geophysical Research Letters*, *52*, e2025GL118209. https://doi.org/10.1029/2025GL118209

Received 17 JUL 2025

Accepted 12 AUG 2025

© 2025. The Author(s).
This is an open access article under the
terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.

<figure><img src="image_2.png" /></figure>

## Cold and Hot Electron Populations Observed by Juno-JADE in the Jovian Magnetodisc

Y. Sarkango¹,² ID, J. R. Szalay¹ ID, A. R. Poppe³ ID, Q. Nénon⁴ ID, Z. Y. Liu⁴ ID, F. Allegrini⁵,⁶ ID, D. J. McComas¹ ID, R. W. Ebert⁵,⁶ ID, P. Kollmann⁷ ID, G. Livadiotis¹ ID, L. Y. Khoo¹ ID, N. André⁴,⁸ ID, J.-Z. Wang⁹ ID, F. Bagenal⁹ ID, S. Schlegel¹⁰ ID, J. Saur¹⁰ ID, W. S. Kurth¹¹ ID, and A. H. Sulaiman² ID

¹Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA, ²School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA, ³Space Sciences Laboratory, University of California, Berkeley, CA, USA, ⁴IRAP: Institut de Recherche en Astrophysique et Planétologie, CNRS-UPS-CNES, Toulouse, France, ⁵Southwest Research Institute, San Antonio, TX, USA, ⁶Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, NJ, USA, ⁷The Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA, ⁸Institut Supérieur de l'Aéronautique et de l'Espace (ISAE-SUPAERO), Université de Toulouse, Toulouse, France, ⁹Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA, ¹⁰Institut für Geophysik und Meteorologie, Universität zu Köln, Cologne, Germany, ¹¹Department of Physics and Astronomy, University of Iowa, Iowa, IA, USA

**Abstract** Despite their importance to a variety of plasma processes in Jupiter's magnetosphere, properties of the thermal and suprathermal electrons have been sparsely reported in past literature. To address this gap, we used data from *Juno's* Jovian Auroral Distributions Experiment Electron sensors to survey electron properties in Jupiter's magnetodisc between M-shells of M = 9–40 and magnetic latitudes less than 20° within years 2016–2023. We fit the electron measurements with a sum of two anisotropic kappa distributions and provide estimates of the electron density, temperature, anisotropy, and thermodynamic kappa for the cold and hot populations, separately. We find that the electron temperature increases non-adiabatically with M-shell and the cold electron temperature anisotropy is most extreme near M = 15. We also provide estimates of the ambient electron properties near the Galilean moons, which should be treated with caution considering the many caveats of the fitting process, which is often associated with large uncertainties.

**Plain Language Summary** Electrons play a critical role in Jupiter's magnetosphere, where they produce and respond to different physical processes. We surveyed properties of the electrons measured in Jupiter's inner and middle magnetosphere using the unprecedented amount of data gathered by the *Juno* spacecraft in this region. We fit a model distribution to the *Juno* observations to better understand the electron environment. We found that the electron temperature increases non-adiabatically with M-shell and electron temperature anisotropy is most extreme near the transition from the inner to the middle magnetosphere. We also provide estimates of the electron densities and temperature near the icy moons, and caution that the fitting is often imperfect and associated with large uncertainties.

## 1. Introduction

Electrons in Jupiter's heavy-ion dominated magnetosphere play an important role in various processes, such as wave excitation, wave-particle interactions (e.g., Menietti et al., 2012), and pressure balance in the magnetodisc and current sheet (Nichols et al., 2015). They are also the primary drivers of radiolysis (Carmack & Loeffler, 2024) and electron-impact ionization (Bagenal & Dols, 2020; Carberry Mogan, Johnson, et al., 2023) at the Jovian moons, which ultimately determines the rate at which neutrals escape from the moons and how they are ionized. However, electron flux measurements made in the energy range of few eV to tens of keV in Jupiter's magnetodisc have been only sparsely reported in past literature, as described below.

It was not clear if the plasma electron measurements made by *Pioneer 10* were indicative of photoelectrons or an ambient electron population, hence they were inconclusive (Grard et al., 1977; Intriligator & Wolfe, 1974). Scudder et al. (1981) studied plasma electron distributions (0.01–5.95 keV) in Jupiter's magnetodisc using the Plasma Science Experiment (PLS) instrument during the *Voyager 1* flyby. They determined that electron distributions were not Maxwellian and could be better described as the sum of a thermal (or cold) component with temperatures $T_c \sim 20$ eV and a suprathermal (or hot) component with temperatures $T_h \sim 1–2$ keV. These
hot electrons were only a small fraction (e.g., <1%) of the total electron population at Io's orbit (M = 5.9) (Sittler & Strobel, 1987), but they represented a larger fraction (~50%) of the total electrons with increasing radial distance, and with increasing latitude or distance from the central plasma sheet. Similar observations were made during the Ulysses flyby of Jupiter (Phillips et al., 1993). These cold and hot electron populations were co-located and spread over a large M-shell range. It is important to note that these populations are distinct from the hot and cold Io tori, present at M < 5.5 for the “cold” torus with thermal temperatures of <1 eV and M = 5.5–7.6 for the “hot” torus with temperatures of 5–30 eV for the core thermal population (Bagenal et al., 1980; Nerney, 2025; Nerney et al., 2024; Sittler & Strobel, 1987). The hot and cold Io tori located inward of M = 8 are not the focus of the present work. In contrast, we use the terms cold and hot to refer to electrons with temperatures on the order of 10–100 eV and >1 keV, respectively.

The observations made by *Cassini's* CAPS/ELS (0.56–26 keV) and MIMI/LEMMS (15–884 keV) instruments in Saturn's magnetosphere also showed a similar abundance of non-Maxwellian electrons that were best described as the sum of two kappa-distributions with different densities, temperatures and thermodynamic kappa index (Livi et al., 2014; Schippers et al., 2008). Notably, the bi-kappa distribution was found to better fit to the data over a bi-Maxwellian or Maxwellian-kappa combination. The non-Maxwellian nature of the measured distributions can also be identified visually by the presence of a power-law decline in the flux at the higher energies. Livi et al. (2014) also study the variation of hot and cold electron properties with Saturnian L-shell.

In addition to being non-Maxwellian, electron distributions are anisotropic, and their pitch-angle anisotropy also varies with different M-shells (note: a temperature anisotropy would manifest as a pitch-angle anisotropy in measured flux). Observations from both *Galileo* at Jupiter and *Cassini* at Saturn show a similar trend of pancake-like pitch-angle distributions (with peak flux at 90° pitch-angle) in the inner magnetosphere transitioning to mostly field-aligned distributions (with peak flux at 0° and 180° pitch angle) in the middle and outer magnetosphere (Mauk & Saur, 2007; Schippers et al., 2008; Tomás, Woch, et al., 2004a, 2004b). A similar trend was observed for the electrons (0.19–297 keV) measured by *Juno's* JADE and JEDI instruments (Ma et al., 2021). Analysis of *Juno* observations suggests that this trend could be due to adiabatic transport of energetic electrons due to radial transport (Liu et al., 2025). Similar trends in the pitch-angle distribution have also been observed for the energetic proton populations in Jupiter's magnetosphere and perhaps result from a similar adiabatic process (Sarkango et al., 2023; Shen et al., 2022; Tomás, Woch, et al., 2004a, 2004b). The potential influence of adiabatic radial transport on creating pitch angle anisotropy was also noted by Kivelson (2016) and Vogt et al. (2014), though in their model particles remain on the same flux tube, which is distended due to centrifugal effects, and there was no transport across field lines.

In this work, we report on observations made by *Juno's* Jovian Auroral Distributions Experiment Electron (JADE-E) sensors (McComas et al., 2017) between M-shells of M = 9–40. The main objective of this study was to characterize electron distributions in Jupiter's magnetodisc, which vary with energy, pitch angle, location (e.g., M-shell and latitude), and time. We describe our methodology in Section 2 and in the Supporting Information S1 and discuss and summarize our results and their implications on Jupiter's magnetosphere in Sections 3 and 4.

## 2. Methodology

We analyzed JADE-E data 4 days prior to and 1 day after each *Juno* perijove until PJ55 (2016–2023). Due to *Juno's* trajectory, these times near perijove usually cover a range of M-shells within M < 50. We limit the analysis to M = 9–40 and magnetic latitudes ±20° from the magnetic equator. Data from M < 9 was excluded due to excessive contamination of the JADE-E sensors due to penetrating radiation. We used three different distributions to fit the observations—(a) a single anisotropic κ-distribution, (b) a sum of anisotropic Maxwellian and κ-distributions, and (c) a sum of two anisotropic κ-distributions. We discuss our fitting methodology in detail in Supporting Information S1 (Text S1). Out of the three models, we only show results in this text from Case (3) (the sum of two κ-distributions). Results from the other models, along with a comparison of reduced chi-square between each, can be found in Supporting Information S1. The anisotropic bi-κ-distribution contains eight free parameters — density (n) [cm⁻³], thermal speed (θ) [m s⁻¹], temperature anisotropy (T⊥/T‖), and thermodynamic κ, for both hot and cold electrons.

An example of this two-dimensional fit is shown in Figure 1, where panel (a) shows the measured phase space density at the JADE-E180 detector during a 10-min interval centered on 2019, DOY 201 07:33 UTC for different
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Fitting the sum of two anisotropic kappa distributions to the Jovian Auroral Distributions Experiment Electron data during a 10-min interval centered on 2019, DOY 201, 07:33. (a) The measured phase-space density $f$ [$s^3 cm^{-6}$] from the JADE-E180 sensor. (b) The “cold” component of the best-fit bi-kappa distribution ($f_1$). (c) The “hot” component of the best-fit bi-kappa distribution ($f_2$). (d) The best-fit bi-kappa distribution, summed for both hot and cold components ($f_1 + f_2$). (e) Comparison of the measured (dots) and the fitted (dashed lines for $f_1$ and $f_2$, solid black line for $f_1 + f_2$) kappa distribution along the perpendicular direction (pitch angle = 90°). (f) Comparison of the measured and fitted bi-kappa distribution along the field-aligned (pitch angle = 0°, in red) and anti-field-aligned direction (pitch angle = 180°, in blue). The parameter values for the cold (1, dashed blue curve) and hot (2, dashed yellow curve) populations are noted.</figcaption></figure>

parallel and perpendicular velocities. Panels (b) and (c) show the best-fit distributions for the cold ($f_1$) and hot ($f_2$) populations to the data shown in panel (a), and panel (d) shows the sum of these two populations ($f_1 + f_2$). Slices of the two-dimensional fitted phase space density along the parallel (pitch-angles = 0°, 180°) and perpendicular (90°) directions were extracted and are shown in panels (e) and (f) along with the measurements as a function of energy. The interval analyzed in Figure 1 was typical of a good fit; it illustrates the presence of two populations and the different phase space density profiles in the parallel versus perpendicular directions. For this interval, the reduced chi-square of the fitted distribution with respect to the observations is $\chi^2 = 0.4$.

## 3. Results

The distributions of best-fit parameters (density, temperature, temperature anisotropy, and thermodynamic kappa) for the anisotropic bi-kappa distribution are shown in Figure 2 as a function of M-shell and colored by magnetic latitude. Fit parameters vary with M-shell and magnetic latitude, but there is also a significant temporal, or orbit-by-orbit variability (i.e., significantly different distributions at the same M-shell and latitude were observed at different times), which makes it difficult to identify any latitudinal trends. Each point in Figure 2 represents the parameter value that best represents a 10-min interval for one JADE-E sensor, and is scaled by the inverse of its variance, that is the smaller points are ones with large variance. Also shown in yellow/orange are the inverse-variance weighted averages for each parameter in different M-shell bins, along with the 1 $\sigma$ standard deviation (Note: all averaging and error calculations are done in log-space). Details for this statistical analysis are provided in Supporting Information S1 (Text S2). The data shown in Figure 2 is provided as Supporting Information in the form of CSV files.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Distributions of the fit parameters for the cold (left) and hot (right) electrons between M-shells $M = 8–40$ and magnetic latitudes $|λ_m|≤20°$ (a–b) Density $n$ in [cm$^{-3}$] (c–d) Temperature $T$ in [eV] (e–f) Temperature anisotropy $T_⊥/T_‖$, and (g–h) thermodynamic kappa ($κ$). Each data point represents the parameter values during a 10 min interval, and the size of each point reflects the inverse sigma, i.e., the reliability of the fit. Yellow points and error ranges represent the inverse-variance weighted average and standard deviation within each bin (see Supporting Information S1). Large errors in temperature usually correspond to $κ → 3/2$. For context, the range of M-shells sampled by Europa (E), Ganymede (G), and Callisto (C) are also annotated.</figcaption></figure>

The densities of cold and hot electrons decrease with increasing M-shell (panel a–b). Hot electron density is a factor of ~100 smaller than the cold electron density near $M = 10$ but is comparable to the cold electron density beyond $M \sim 20$. The temperature of the “cold” population increases from ~10 eV near $M = 10$ to ~1 keV near $M = 30$ (b). The cold and hot populations are mostly anisotropic. The cold population is isotropic ($T_⊥/T_‖ \sim 1$) inward of $M \sim 12$, achieves the highest anisotropy with $T_⊥/T_‖ \sim 0.4$ near $M \sim 20$, and is only weakly anisotropic at the outer M-shells near $M \sim 30–40$ with $T_⊥/T_‖ \sim 0.8–1.0$ (panel e). Meanwhile, the hot population is mildly anisotropic inward of $M \sim 20$ with $T_⊥/T_‖ \sim 1–1.3$, and mildly anisotropic outward of $M \sim 20$ with $T_⊥/T_‖ \sim 0.8–1.0$ (panel f). The thermodynamic kappa for the hot and cold population is predominantly close to the lower bound of 3/2.

In Figure 3 we show the distribution of the ratio of the hot-to-cold electron densities (panel a) and compare it to the ratio of the hot to cold electron pressure (assuming $p = nk_B T$) (panel b). We also show the distributions of the parallel and perpendicular pressure in panels (c–h) for the cold and hot populations separately. The fraction of the hot electrons is smaller by three orders of magnitude near the $M = 9$ but increases and approaches $n_H/n_C \sim 1$ near
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Distributions of the derived parameters - (a) ratio of hot to cold electron densities, (b) hot to cold electron pressure (p = nk_BT) (c), (d) cold and hot electron pressures in [nPa] (e), (f) cold and hot parallel electron pressure (g), (h) cold and hot perpendicular electron pressure, as a function of M-shell in a similar format as in Figure 2.</figcaption></figure>

$M = 40$. However, the hot electron pressure is often larger than the cold electron pressure at nearly all M-shells between $M = 9–40$ by an order of magnitude.

**4. Discussion**

There are some commonalities between our JADE-E electron survey and previous surveys of the JADE-I ions (Kim et al., 2020a, 2020b; Wang et al., 2024). Much like the cold electron densities shown in Figures 2a and 2b, ion densities decrease with M-shell, which is expected and has been observed since the Voyager-era (Bagenal & Delamere, 2011). The shared ion temperature near $M = 20$ was found to be ~2–5 keV, which is also comparable to hot electron temperature reported in this work. Likewise, the densities from the bi-kappa fits also agree with the densities derived by Huscher et al. (2021) using numerical moments of the JADE-E data.
<table><caption>**Table 1**

Ambient Electron Parameters in the Vicinity of the Icy Jovian Moons Calculated by Fitting the JADE-E Observations</caption><thead><tr><th rowspan="2"></th><th colspan="2">Density [cm<sup>-3</sup>]</th><th colspan="2">Temperature [eV]</th><th colspan="2">Anisotropy<br>T<sub>⊥</sub>/T<sub>‖</sub></th><th colspan="2">Thermodynamic<br>κ</th></tr><tr><th>Cold</th><th>Hot</th><th>Cold</th><th>Hot</th><th>Cold</th><th>Hot</th><th>Cold</th><th>Hot</th></tr></thead><tbody><tr><td>Europa</td><td>270<sup>+1600</sup><sub>-230</sub></td><td>0.84<sup>+3.50</sup><sub>-0.68</sub></td><td>17<sup>+12.0</sup><sub>-7.1</sub></td><td>7080<sup>+?</sup><sub>-7000</sub></td><td>1.0<sup>+0.0</sup><sub>-0.0</sub></td><td>1.1<sup>+0.3</sup><sub>-0.2</sub></td><td>2.5<sup>+2.1</sup><sub>-1.5</sub></td><td>1.5<sup>+0.0</sup><sub>-0.0</sub></td></tr><tr><td>Ganymede</td><td>2.9<sup>+25</sup><sub>-2.6</sub></td><td>0.24<sup>+1.80</sup><sub>-0.21</sub></td><td>137<sup>+12000</sup><sub>-135</sub></td><td>8800<sup>+?</sup><sub>-8700</sub></td><td>0.6<sup>+0.0</sup><sub>-0.2</sub></td><td>0.9<sup>+0.8</sup><sub>-0.4</sub></td><td>1.5<sup>+0.9</sup><sub>-0.0</sub></td><td>1.5<sup>+2.0</sup><sub>-0.0</sub></td></tr><tr><td>Callisto<sup>a</sup> (single-κ fit)</td><td></td><td>0.05<sup>+0.32</sup><sub>-0.05</sub></td><td></td><td>2000<sup>+1.0E4</sup><sub>-1.7E3</sub></td><td></td><td>0.8<sup>+0.7</sup><sub>-0.4</sub></td><td></td><td>1.5<sup>+3.6</sup><sub>-0.0</sub></td></tr></tbody></table>

Note. The values shown are averages weighed by the inverse variance of each successful fit and the errors denote ±1σ in log-scale. M-shell intervals considered for the various moons are—*M* = 9.2–10.0 for Europa, *M* = 14.8–17.7 for Ganymede, and *M* = 25.1–40.0 for Callisto. The M-shell ranges considered here may differ by Δ*M* ~ ±0.1 for each moon. <sup>a</sup>Note that Callisto traverses larger M-shells than what was considered for this study.

In the present electron survey, we have found that the transition from $T_{\perp} > T_{\parallel}$ to $T_{\perp} < T_{\parallel}$ occurs near *M* = 12 for the cold electrons and broadly between *M* = 15–20 for the hot electrons. Electron temperature anisotropy ($T_{\perp} > T_{\parallel}$) can provide free energy for the growth of whistler-mode waves, including the two-band chorus observed at Earth, Jupiter, and Saturn (J. Li et al., 2019; Menietti et al., 2012). Various studies have reported high whistler wave activity at lower magnetic latitudes <25° and between M-shells *M* ~ 5–15, with maximum intensity near *M* = 10 (W. Li et al., 2020; Ma, Li, Zhang, Bortnik, et al., 2024; Ma, Li, Zhang, Kang, et al., 2024; Menietti et al., 2016; Shprits et al., 2018). Hence, the observed anisotropic electron distributions inward of *M* ~ 20 may provide the free energy for the growth of whistler-mode waves identified by previous studies in the same regions. The whistler-mode waves may then scatter energetic electrons in the atmosphere of Jupiter and accelerate electrons to relativistic energies (Ma et al., 2020).

Electron pressure also contributes to the force balance of the magnetodisc and warrants discussion. For cold electrons, JADE-E observations show the temperature anisotropy to be most extreme near *M* = 18–20 with $T_{\perp}/T_{\parallel}$ ~ 0.5. In Figure 3 we also show that, despite smaller densities, hot electron pressure usually exceeds that of cold electrons because of their higher temperatures. These findings, along with the trends in anisotropy observed for the cold and hot populations in Figures 1e and 1f are broadly consistent with previous observations using Voyager PLS and LECP or Galileo PLS or EPD data (Bagenal, 1994, p. 199; Mauk & Krimigis, 1987; Paranicas et al., 1991), although note that most past work focused on the ion anisotropy or data at sparse intervals. Artemyev et al. (2023) showed that electrons that have anisotropic distributions can additionally contribute to the pressure balance of the Jovian current sheet by providing a parallel current. Liu et al. (2024) also showed that the pressure contribution of the electrons is smaller compared to the ions, although it is not negligible, and the pressure anisotropy of the different plasma species would be the dominant force in the magnetodisc. Kivelson (2016) proposed that ion pressure anisotropy of $p_{\perp} > p_{\parallel}$ in the inner magnetosphere results from pickup ions, whereas the opposite anisotropy of $p_{\perp} < p_{\parallel}$ in the middle and outer magnetosphere results from the influence of the centrifugal force that creates and maintains the magnetodisc configuration. Since the centrifugal force increases with radial distance, Kivelson (2016) hypothesized that the ratio of $p_{\perp}/p_{\parallel}$ would continue to decrease and the disc would become increasingly flatter with increasing M-shell. However, our work shows that the most extreme anisotropy for the cold electrons (which contribute <~10% of the electron pressure) is reached at *M* ~ 20 and diminishes with radial distance, which may result from sub-corotation (and hence diminishing centrifugal force with radial distance). The hot electrons contribute most of the electron pressure, however we find that $T_{\perp}/T_{\parallel}$ for the hot electrons hovers around 0.8–1.0 and does not decrease further with radial distance (see Figure 3b). The above discussion is qualitative and assumes that the electron and ion pressure anisotropies follow similar trends, which may not necessarily be the case. Lastly, cold electron temperature anisotropy is the only parameter in which we can discern latitudinal trends, with the most extreme anisotropy ($T_{\parallel} > T_{\perp}$) observed near the equator than at higher latitudes.

Using the derived fit parameters, we estimated the ambient electron properties in the vicinity of the icy moons, shown in Table 1. At the icy moons, electron precipitation induces radiolysis of the water ice that contributes to the moons' $O_2$ exospheres and possible neutral tori, and electron-impact ionization of these neutrals creates pickup ions that are incorporated into the magnetosphere, where they are a minor, but unique, plasma source (Szalay et al., 2024). Hence, characterizing the electron environment of the moons is crucial. For example, Carberry
Mogan, Liuzzo et al. (2023), Carberry Mogan, Johnson et al. (2023) used the electron flux measurements from JADE-E near Callisto to model electron precipitation onto Callisto's exobase and estimate O₂ production rates. Pelcener et al. (2024) combined JADE-E and JEDI electron data to model the variability of the electron plasma environment at Ganymede's orbit and compare it with past observations. Reddy et al. (2024) propose a mechanism for charging Europa's surface, assuming different values for the magnetospheric cold and hot electron (and ion) populations.

The icy Galilean moons experience different electron environments. Hot electrons are <1% of the total electron density at Europa but increase to nearly ~40% at *M* ~ 25, the inner M-shell boundary sampled by Callisto and are comparable or larger than the “cold” electrons at *M* ~ 40. Electron anisotropy also differs between the moons. Cold electrons have *T*⊥/*T*‖ of about ~1 near Europa and Callisto but it decreases to 0.5 at Ganymede. Meanwhile, the hot electron anisotropy is about *T*⊥/*T*‖ ~1.2 near Europa but decreases to 1.1 and ~0.9 near Ganymede and Callisto. We found that the electron environment at the outer M-shells can be well described by a single anisotropic κ-distribution (see Supporting Information S1 for comparison with a single κ-distribution), and so we have refrained from separating into the cold and hot populations in Table 1 for Callisto.

The thermodynamic kappa index is close to 3/2 for the cold electrons, except near *M* = 10, and for the hot electrons inward of *M* = 20. This means that the energy spectra for both cold and hot populations are, usually, as shallow in energy as possible in a κ-distribution such that it has the most possible contribution from higher energies that such a distribution could have. For a κ-distribution, κ → ∞ represents a Maxwell-Boltzmann distribution with no correlations between the particles (e.g., in classical thermal equilibrium), a state with maximum entropy. The lower limit at κ → 3/2 represents a state with maximum correlations, or a state with minimum entropy (Livadiotis & McComas, 2023b). The limit of κ → 3/2 also corresponds to a *E*⁻³/² decline in differential intensity [cm⁻² sr⁻¹ keV⁻¹ s⁻¹] as a function of energy (spectral index of 1.5) and to a ν⁻⁵ decline in phase space density [s³ m⁻⁶] as a function of speed. Such distributions have been observed previously in many different environments in space plasma physics, for example, in the solar wind (Drake et al., 2013; Fisk & Gloeckler, 2006, 2014). Electron distributions near the icy moons are not Maxwellian (κ → ∞) and approach the κ → 3/2 limit.

At the κ = 3/2 limit, temperature values are undefined. This manifests during the fitting process via large standard errors for the fit parameters, and high covariance (and correlation) between κ and T. We have also found occasions of high correlation between density and temperature, which may be a result of fitting a κ-distribution onto a power-law distributed data. These higher-uncertainty fits are accounted for in our statistics via the inverse-variance averaging (see Supporting Information S1 for more details).

We also fitted power-law trends to the density and temperature variation with M-shell (shown in Figure S4 in Supporting Information S1). We find that the cold electron densities decrease as ∝*r*⁻⁸.⁵ and ∝*r*⁻³.⁸ inward and outward of *M* ~ 19, respectively. Cold electron temperatures increase as ∝*r*⁻³.⁸ for *M* < 19 but remain fairly constant for *M* > 19 (proportionate trend). The hot electron densities and temperatures fall as ∝*r*⁻².² and ∝*r*⁻⁰.⁴ for *M* > 19 (large errors in hot electron temperature prevent us from doing this calculation for *M* < 19). Using these, we calculated the associated polytropic index ν ∝ log *n*/log *T*, which is related to the polytropic index (γ) as γ = 1 + 1/ν (Livadiotis & McComas, 2023a). Using this expression and the variation of density and temperature we find that the cold electrons have a polytropic index of γ ~ 0.5 inward of *M* ~ 19 and γ ~ 1.0 outward of *M* ~ 19, that is if moving outward, they are being heated non-adiabatically (Livadiotis & McComas, 2023a). The hot electrons have a polytropic index of γ ~ 1.1, which is comparable to the cold electrons outward of *M* ~ 19, perhaps indicating some ambiguity between the hot and cold populations.

Although some findings reported here have been discussed previously in the literature using sparse *Voyager* PLS or *Galileo* PLS data, the vast amount of data gathered by *Juno* allows for a statistical survey of the <10 keV electron plasma distributions and for a separate characterization of the cold and hot populations. The observations here point to the pervasive influence of electrons on wave-particle interactions and pressure balance of the magnetodisc. Results from this survey may be useful for modeling studies that require knowledge of the electron environment in Jupiter's magnetodisc, such as near the Galilean moons. In this letter, we mainly discuss variation of electron parameters close to the magnetic equator as a function of M-shell and do not consider the variation with longitude, local time, or with distance from the centrifugal equator, which should be considered for future work.
## 5. Summary

We used data from the JADE electron sensors to survey electron properties in the Jovian magnetodisc between the years 2016–2023 M-shells $M = 9–40$, and magnetic latitudes $\pm 20^\circ$ from the magnetic equator. We fitted the sum of two anisotropic kappa-distributions and examined the trends in fit parameters—density, temperature, temperature anisotropy, and thermodynamic kappa, with M-shell and latitude. Note that there is significant variability in each parameter and no latitudinal trend is discernible, even if it may be expected. In future work, we will study the latitudinal variation for each current sheet crossing separately and explore longitudinal variations using fits made at a higher cadence. Key findings are summarized below.

1. The fraction of hot electrons is <1% inward of $M = 10$ but is comparable to the cold electrons near and outward of $M = 25$.

2. The cold electron temperature increases from ~10 eV near $M = 10$ to ~1 keV near $M = 30$. The hot electron temperatures are less well captured for $M < 20$ but hover around ~1 keV within $M = 20–40$.

3. Both cold and hot electron temperatures are anisotropic. Cold electron anisotropy is maximum near $M \sim 12$ with $T_\perp/T_\parallel \sim 0.4$. Hot electrons are mildly anisotropic with $T_\perp/T_\parallel \sim 1–1.5$ inward of $M \sim 20$ and $T_\perp/T_\parallel \sim 0.8–1.0$ outward of $M \sim 20$. These findings are consistent with the occurrence of whistler-mode emissions reported in earlier studies.

4. Hot electrons, despite being a fraction of the cold electrons, contribute significantly more to the electron pressure, often by an order of magnitude or more.

5. The thermodynamic kappa for hot and cold electrons is small, usually less than 3 with the majority of distributions having $\kappa$ approaching the lower bound of 3/2.

6. The Galilean moons experience different electron environments that may influence radiolysis and electron-impact ionization. Electrons near Europa are more isotropic. In contrast, Ganymede experiences electron fluxes which are on average more field-aligned, and Callisto experiences an environment that is more dominated by higher energy electrons than the other moons, possibly leading to less efficient electron impact ionization.

7. Outward of $M \sim 30$, the JADE observations cannot distinguish between a cold and hot population and the electrons can be described as a single-$\kappa$ distribution equally well. This does not indicate an absence of hot electrons and could be related to JADE's measurement capabilities.

## Data Availability Statement

All JADE data analyzed in this work is publicly available from the NASA Planetary Data System Plasma Interactions Node and can be found at Allegrini, F. et al. (2024).

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## Acknowledgments

This work was supported by NASA grant NFDAP 80NSSC21K0823, and through the Juno mission's JADE instrument science team. French co-authors acknowledge the support of CNES for the Juno mission. RWE was also funded by grant 80NSSC23K0658.
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