# He + ions in the vicinity of mercury observed by the MESSENGER and BepiColombo spacecraft

Markus Fränz, Harald Krüger, Jim Raines, Austin Glass, Daniel Gershman, Fabio Prencipe, Norbert Krupp, Lina Hadid, Dominique Delcourt, Sae Aizawa, et al.

► To cite this version:

Markus Fränz, Harald Krüger, Jim Raines, Austin Glass, Daniel Gershman, et al. He + ions in the vicinity of mercury observed by the MESSENGER and BepiColombo spacecraft. Planetary and Space Science, 2025, 265, pp.106152. 10.1016/j.pss.2025.106152. hal-05158242

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# He⁺ ions in the vicinity of mercury observed by the MESSENGER and BepiColombo spacecraft

Markus Fränz a,*, Harald Krüger a,b, Jim M. Raines c, Austin N. Glass c, Daniel J. Gershman d,
Fabio Prencipe a, Norbert Krupp a, Lina Z. Hadid e, Dominique Delcourt e, Sae Aizawa e,
Shoichiro Yokota f, Yuki Harada g, Yoshifumi Saito h

## ARTICLE INFO

Dataset link: https://doi.org/10.5281/zenodo.14591537

**Keywords:**

MESSENGER

BepiColombo

Interstellar helium

Planetary helium

Pick-up ions

Fast Imaging Plasma Spectrometer (FIPS)

Mercury Mass Spectrum Analyzer (MSA)

Mercury

## ABSTRACT

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft was launched in 2004, and between March 2011 and April 2015 it was the first spacecraft in orbit around Mercury. The Fast Imaging Plasma Spectrometer (FIPS) instrument on board MESSENGER measured the ion composition in the vicinity of Mercury and in the inner solar system.

We aim to determine the origin of He⁺ ions in the inner solar system and in the environment of Mercury, continuing earlier work by Gershman et al., (2013).

We have analyzed measurements of He⁺ and He²⁺ ions made by the FIPS instrument during the interplanetary cruise phase of MESSENGER and its entire orbital mission at Mercury. We determined the spatial distributions of He⁺ ions in the regions sampled by MESSENGER during that period and compare the spectra to a similar observation by the Mass Spectrum Analyzer instrument which is part of the Mercury Plasma Particle Experiment (MPPE-MSA) onboard BepiColombo. We consider two possible sources of He⁺: (1) interstellar neutral helium ionized close to Mercury and (2) solar He²⁺ ions converted close to or at the surface of Mercury. We also compare the observed densities with a simple model of the ionization of the interstellar helium flow.

The FIPS data show a continuous evolution of the He⁺ energy spectra from solar wind towards Mercury - changing from a shape typical for pick-up ions to a thermalized spectrum. This could mean that interstellar He⁺ is concentrated at Mercury by increased electron impact ionization close to the planet. We find a remarkably similar high mean ratio of He⁺/He²⁺ ions in the upstream solar wind and in the inner magnetosphere, while the ratio is reduced in the magnetosheath.

Outside Mercury's magnetosphere the source of He⁺ ions is interstellar helium, while inside the planet's magnetosheath and the magnetosphere both interstellar helium and solar wind helium may be of similar magnitude. The observed median upstream He⁺ densities are in good quantitative agreement with a simple model — though the existence of an upstream density crescent cannot yet be confirmed.

## 1. Introduction

Helium is one of the trace elements in the atmospheres of the terrestrial planets. The only planetary source is radioactive decay of uranium and thorium in the interior of the planets. Neutral helium

atoms then slowly seep through the rock and escape to the atmospheres. Additional sources may include helium captured from the solar wind (Krasnopolsky and Gladstone, 2005; Chanteur et al., 2009), or interstellar helium (Gershman et al., 2013). To better understand the

<footnote>a Max-Planck-Institut für Sonnensystemforschung, 37077, Göttingen, Germany</footnote>

<footnote>b Planetary Exploration Research Center, Chiba Institute of Technology, Narashino, Chiba, 275-0016, Japan</footnote>

<footnote>c Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA</footnote>

<footnote>d Geospace Physics Laboratory at NASA Goddard Space Flight Center, Greenbelt, MD, USA</footnote>

<footnote>e Laboratoire de Physique des Plasmas (LPP), Ecole polytechnique, 91120, Palaiseau, France</footnote>

<footnote>f Osaka University Institute of Space and Astronautical Science, Osaka, Japan</footnote>

<footnote>g Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, Japan</footnote>

<footnote>h ISAS-JAXA, Sagihamara, Japan</footnote>

<footnote>* Corresponding author.
E-mail address: fraenz@mps.mpg.de (M. Fränz).</footnote>
physics of helium at Mercury we first give an overview of helium observations in the inner solar system.

## 1.1. Helium in the inner solar system

Helium atoms in the atmospheres of Earth, Mars and Venus originate from outgassing following radioactive decay of uranium and thorium in the planetary crust as described above. Models suggest, however, that the neutral helium formed this way is not sufficient to explain the amounts of helium actually observed in the atmospheres of Mars and Venus considering observed escape rates (Krasnopolsky and Gladstone, 2005; Chanteur et al., 2009). The thermal escape rates of helium from these atmospheres are negligibly small, and the main loss process from the planetary atmosphere is ionization of helium atoms above the ionopause followed by interaction with the solar wind sweeping out the freshly produced $He^+$ ions (Gérard et al., 2011). The ionization process can be by photons, charge exchange with protons or electron impact. Photo-ionization is the dominant process — except in regions of high electron density and temperature as for example closer to the Sun than 0.5 AU. The efficiency of this loss process depends on the helium density in the upper atmosphere and the solar wind conditions. Thus, precipitating solar wind ions may transfer matter, energy and momentum from the solar wind to the upper atmosphere. The precipitating solar wind $He^{2+}$ ions ($\alpha$ particles) could be an important source of atmospheric helium for both Mars and Venus (Stenberg-Wieser et al., 2015).

### 1.1.1. Mercury

Mercury has a tenuous exosphere where the particle density is so low that the exospheric molecules essentially do not collide with each other. Given that Mercury does not have a denser atmosphere underneath, the exosphere is in direct contact with the planetary surface (it is thus called a "surface-bounded exosphere"). Hence, the surface acts as both a source and a sink for exospheric molecules, and the exospheric composition is strongly linked to the composition and structure of Mercury's surface (e.g., McClintock et al., 2018). Helium, hydrogen and oxygen were discovered in the exosphere by Mariner 10 airglow spectrometer observations (Broadfoot et al., 1974, 1976) with a derived helium abundance of $6 \times 10^3 \text{ cm}^{-3}$ (Hunten et al., 1988). Since then, several other species were discovered by ground-based observations and by observations with the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, so that the total inventory of confirmed exospheric neutral species now includes H, He, Na, K, Ca, Mg, Al, Fe, and Mn (see McClintock et al., 2018, for a recent review).

Several processes are believed to be likely source mechanisms for surface-bounded exospheres, including Mercury's, leading to space weathering of these surfaces. These include thermal desorption, photon-stimulated and electron-stimulated desorption, and ion and chemical sputtering. The significance of each of these processes is still under debate (see, e.g., the reviews by Milillo et al. (2005), Domingue et al. (2014), Killen et al. (2018), Grava et al. (2021b), Leblanc et al. (2022). A fraction of the volatiles released into the exosphere is also produced by regolith vaporization following hypervelocity impacts of (micro)meteoroids onto the planetary surface (e.g., Wurz et al., 2022). The continuous bombardment of the surface by micrometeoroids is also believed to be an important cause of the regolith gardening, i.e., the pulverization and overturn of the upper crust providing fresh material in the upper layers to be further released to the exosphere (Killen et al., 2018). Micrometeoroid impacts release ejecta, melt, and vapor. Impacts of larger projectiles (meteoroids) are much rarer but can excavate deeper layers, thereby exposing fresh material to the external environment and to eroding agents like solar radiation and ions. The significance of (micro)meteoroid impacts for exosphere formation at Mercury was recently reviewed by Krüger et al. (2024). Ionized species detected at Mercury include $H^+$, $He^+$ and $He^{2+}$, among heavier ions

which were detected with the Fast Imaging Plasma Spectrometer (FIPS; Raines et al., 2013) and the UV/Visible Spectrometer which is part of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS-UVVS; Vervack et al., 2016) on board MESSENGER. Finally, the Probing the Hermean Exosphere by UV Spectroscopy instrument (PHEBUS) on board the BepiColombo spacecraft measured helium in Mercury's exosphere during the spacecraft's first flyby at the planet in 2021 (Quémerais et al., 2023). Recent observations of higher abundances of $He^+$ ions in Mercury's plasma tail by BepiColombo initiated this analysis and are discussed below.

### 1.1.2. Venus

The first detection of helium in the Venus atmosphere was made with the ultraviolet spectrometer on board Mariner 10 and later confirmed by the Venera 10/11, the Extreme Ultraviolet Explorer (EUVE) spacecraft from Earth orbit, and by the Cassini spacecraft during its flyby at Venus. These measurements provided the helium abundance in the planetary atmosphere at a reference altitude of 145 km from the remote observation of the He I resonance line at $584 \AA$ in the range of $2-8 \times 10^6 \text{ cm}^{-3}$ (Gérard et al., 2011, their Tab. 1 and references therein). In situ measurements of the helium abundance were also performed with the Pioneer Venus mission, providing a value of $7 \times 10^6 \text{ cm}^{-3}$ in agreement with the remote measurements (von Zahn et al., 1980). Finally, helium ions in the vicinity of Venus were measured inside the planet's induced magnetosphere with the Ion Mass Analyzer (IMA) which is part of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA) 4 instrument onboard Venus Express (Barabash et al., 2007). These measurements showed that the precipitation of $He^{2+}$ ions onto the upper atmosphere of Venus is not enough to replace the helium escaping from the planet (Stenberg-Wieser et al., 2015). A recent review of the Venus upper atmosphere was provided by Gérard et al. (2017) showing that helium is an important constituent. The origin is likely mainly volcanic with a minor contribution by solar wind, and a possible interstellar component.

In this context it is very interesting that helium plays a major role in the photo-chemistry of the Venus ionosphere. Observations with the Solar and Heliospheric Observatory (SOHO) satellite showed for the first time that carbon ions can escape in greater quantity from Venus into space (Grünwaldt et al., 1997). This was recently confirmed by observations of the BepiColombo Mass Spectrum Analyzer instrument which is part of the Mercury Plasma Particle Experiment (MPPE-MSA; Delcourt et al., 2016; Murakami et al., 2020; Saito et al., 2021; Hadid et al., 2024b). The reason for the high abundance of atomic carbon at high altitudes at Venus was not discussed in these observational papers. However, it is caused by the chemical reaction ($He^+ + CO \rightarrow He + C^+ + O$), as described in the ionospheric model by Fox (1992). Thus, helium is an important chemical catalyst at Venus.

### 1.1.3. Mars

At Mars, helium was for the first time detected remotely by EUVE from Earth orbit (Krasnopolsky et al., 1994) and later confirmed by Phobos 2 using the $584 \AA$ line. The first in-situ measurements of ionized planetary helium picked up by the solar wind were carried out onboard the Phobos 2 spacecraft (Barabash et al., 1995). Similar to the case of Venus, these measurements implied that outgassing of radiogenic helium may be a minor source of helium on Mars because volcanic activity there is low and the radiogenic surface activity died out. 70% of the required source flux is supplied by the capture of solar wind $He^{2+}$ ions (Krasnopolsky and Gladstone, 1996). This was later confirmed by measurements of $He^{2+}$ ions in the Martian ionosphere with the ASPERA 3 instrument on board the Mars Express spacecraft which showed that solar wind $He^{2+}$ particles are very likely a source of helium in the Martian atmosphere (Stenberg et al., 2011). At Mars no significant carbon escape is observed since the helium abundance in the ionosphere is much smaller than at Venus.
### 1.1.4. Earth Moon

As a large solar system body without an atmosphere and without volcanic activity the helium exosphere of the Earth Moon may be best comparable to the helium exosphere of Mercury. The helium budget of the lunar surface was studied since the Apollo 17 mission, and observations have been reviewed by Grava et al. (2021b). The most recent observational study (Grava et al., 2021a) reports that about 19% of the observed neutral helium is of endogenic origin while the rest is generated by implantation of solar wind $He^{2+}$ ions.

But there are four major differences in the case of Mercury:

1. The transport of endogenic helium from the planetary mantle through the crust may be stronger due to the tidal activity of the Mercury crust.

2. The surface of Mercury is better protected against solar wind implantation by the Mercury magnetic field.

3. On the other hand, the solar wind flux at Mercury is higher by about a factor of 8.

4. The interstellar helium flux at Mercury is higher than at the Moon as discussed below.

### 1.2. Interstellar helium

Neutral helium atoms from the local interstellar medium (LISM) penetrate the heliosphere as the solar system moves with approximately $26 \text{ km s}^{-1}$ through the local interstellar environment surrounding our solar system (Witte et al., 2004; Swaczyna et al., 2023). Unlike neutral interstellar hydrogen, which is ionized before reaching the inner heliosphere, neutral helium atoms largely survive their heliospheric transit. These atoms are not subject to strong solar radiation pressure forces, hence their trajectories are altered only by the Sun's gravitational force. In the downwind region of the interstellar flow, converging trajectories of atoms result in a region of enhanced neutral helium density, the so-called “helium gravitational focusing cone” (Fahr, 1971; Thomas, 1978; Gershman et al., 2013; Galli et al., 2022, and references therein). During their transit, neutral helium atoms can become singly charged $He^{+}$ ions primarily through photo-ionization and, at heliocentric distances less than approximately 0.3 AU, by electron-impact ionization processes (McMullin et al., 2004; Bzowski et al., 2013).

Once an interstellar neutral atom becomes ionized, it is picked up by the magnetic and motional electric fields of the solar wind plasma, which moves much faster than the interstellar neutrals in the solar inertial frame. This results in the production and incorporation of interstellar pick-up ions (PUIs) at highly non-thermal speeds in the reference frame of the solar wind (Holzer and Axford, 1971; Vasyliunas and Siscoe, 1976; Zirnstein et al., 2022, and references therein). In addition to interstellar PUIs, an inner PUI source close to the Sun is predicted to be formed through the interaction with interplanetary dust (Geiss et al., 1994, 1995; Rivera et al., 2020). Interstellar PUIs can be distinguished from solar wind plasma by their velocity distribution functions (VDFs) in phase space. The interstellar PUIs do not exhibit a Maxwellian profile typical of the solar wind, but instead are governed by wave-particle interaction which leads to isotropization over time (Möbius et al., 1985; Gloeckler et al., 1993; Drews et al., 2016). The number density of PUIs in the solar wind depends on the local neutral density, the rate of neutral ionization, charge exchange reactions, and the accumulation of PUIs closer to the Sun that are transported outwards with the radially propagating solar wind (Zirnstein et al., 2022). $He^{+}$ PUIs dominate the PUI source near Earth due to their high ionization potential and ability for interstellar helium atoms to propagate closer to the Sun before experiencing ionization.

After their discovery (Möbius et al., 1985), a suite of spacecraft in the heliosphere have provided a wealth of interstellar PUI measurements spanning nearly four decades. These measurements have revealed both the ubiquitous presence of PUIs in the solar wind and their diagnostic capabilities at inferring properties of the local interstellar medium (e.g., Gloeckler et al., 2004). The in situ observations

of interstellar PUIs by spacecraft in the heliosphere were recently reviewed by Zirnstein et al. (2022) and Zank et al. (2022), and the present theory and modeling approaches for PUIs were summarized by Sokól et al. (2022).

Dedicated in situ space measurements of energetic neutral atoms at energies between approximately 10 eV and 10 keV in the heliosphere and from interstellar space have so far been performed by the Ulysses interplanetary spacecraft (Wenzel et al., 1992) and, most importantly, by the Interstellar Boundary Explorer Mission (IBEX) in Earth orbit (McComas et al., 2009). In addition to helium, IBEX also detected interstellar neutral hydrogen, oxygen and neon atoms. One important result of the IBEX measurements was the determination of the inflow vector of the interstellar neutral helium flow at ($\lambda$ = 255.73° ± 0.19°, $\beta$ = 5.04° ± 0.15°) in ecliptic coordinates, speed $v$ = 26.63 ± 0.17 $\text{kms}^{-1}$, and temperature $T$ = 7.350 ± 110 K (Swaczyna et al., 2023), confirming earlier Ulysses findings (Witte et al., 2004). Finally, a limited number of measurements were also performed by the Mars Express and Nozomi missions en route to Mars (Chicarro et al., 2004; Noda et al., 2001) and by Venus Express en route to Venus, respectively (Svedhem et al., 2009), and observations of the UV resonance line of neutral helium at 584 Å were recently performed in lunar orbit by the Lunar Reconnaissance Orbiter (Grava et al., 2018). A comprehensive review of the observations by all these spacecraft was recently given by Galli et al. (2022). In the recent analysis by Quémérais et al. (2023) the contribution of the interstellar source to the neutral helium observed at Mercury was also discussed.

There have been many attempts to model the distribution of interstellar helium and $He^{+}$ in the inner heliosphere starting from simple analytical models (Holzer and Axford, 1971) to models taking into account the full phase space distribution (Rucinski et al., 2003). The latter models have been refined into sophisticated computer simulations in the context of recent observations by the IBEX mission at 1AU solar distance (Bzowski et al., 2023). But the distribution of helium and $He^{+}$ at distances below 0.5 AU was rarely considered in these models. Only Drews et al. (2012) predict that when considering the ionization time and velocity of helium atoms on their path to the Sun, a higher density of helium and $He^{+}$ should be produced upstream of the Sun. They named this upstream density increase a *crescent* because it forms over a wider range of ecliptic longitudes than the downstream gravitational focusing cone. We investigated this idea in a simple cylindrical symmetric simulation (see Appendix). The resulting densities at the orbit of Mercury are shown in Fig. 1, confirming the increase at the downstream focusing cone and the upstream density crescent. The simulation also shows the strong dependence on solar activity caused by a change in the UV photon flux.

In this paper we analyze measurements of $He^{+}$ ions made by the FIPS instrument during the interplanetary cruise phase of MESSENGER and its entire orbital mission at Mercury. We determine the spatial distributions of $He^{+}$ and $He^{2+}$ ions in the regions sampled by MESSENGER during that period. We consider two possible sources of $He^{+}$: (1) interstellar neutral helium ionized close to Mercury and (2) solar $He^{2+}$ ions converted close to or at the surface of Mercury. We assume that any internal source must be very weak.

## 2. MESSENGER and BepiColombo missions to Mercury

### 2.1. MESSENGER and FIPS instrument overview

MESSENGER was launched on 3 August 2004, and after one flyby at Earth, two at Venus and three at Mercury, it became the first spacecraft to orbit Mercury from March 2011 until April 2015. MESSENGER's trajectory about Mercury was highly eccentric, with an initial periapsis about 200 km above the surface and an apoapsis of approximately 15,000 km altitude, with an orbital period of 12 h. The orbital inclination was about 80°, and the periapsis was at approximately 60° northern latitude. More details about the spacecraft and its orbital
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Predicted density of interstellar helium (black) and He⁺ (red) as a function of ecliptic longitude along the orbit of Mercury from a simple cylindrical symmetric model. Upper helium density values are for solar maximum, lower values for solar minimum. The model is described in the Appendix.</figcaption></figure>

mission at Mercury were given by Solomon et al. (2001), Solomon and Anderson (2018).

The Fast Imaging Plasma Spectrometer (FIPS) on board MESSENGER measured energy per charge (E/q), time-of-flight (TOF), and the incident angle of ions in Mercury's space environment (Andrews et al., 2007). The FIPS sensor measured ions with E/q from 0.05 to 13 keV e⁻¹. When inside the magnetosphere the time resolution was approximately 10 s; outside of the magnetosphere the time resolution was about 1 min. The instrument can resolve mass per charge (m/q) from 1 to 40 amu e⁻¹. FIPS has a very large instantaneous field-of-view (FOV) of approximately 1.4π sr with an approximate angular resolution of 15° throughout the entire FOV.

FIPS was placed on the spacecraft to maximize the collection of heavy ions from Mercury's surface and space environment, taking into account important thermal constraints. Therefore, a portion of the FOV was blocked due to its mounting position behind the spacecraft sunshade (Leary et al., 2007). To protect the spacecraft from overheating, the Sun had to be kept within a certain range in directions, and the spacecraft sunshade was oriented towards the Sun at all times, even in the shadow of the planet. This means that the FOV within 12° of the Sun-spacecraft vector was obstructed at all times. The FIPS FOV extended from directly behind the sunshade to up to 45° to the rear. An additional fraction of the FOV was obstructed by one of the two solar panels, which was subject to changes in orientation. Neither the anti-sunward direction nor the sunward direction were ever directly within the FIPS FOV. The instrument's placement on the spacecraft, its FOV obstructions, and their impact on measured ion distribution functions were described by Raines et al. (2011).

## 2.2. BepiColombo and MPPE-MSA instrument overview

After Mariner 10 and MESSENGER, the third mission to investigate Mercury will be BepiColombo. The mission consists of two spacecraft,

the Mercury Planetary Orbiter (MPO; Benkhoff et al., 2010, 2021), and the Magnetospheric Orbiter (Mio; Murakami et al., 2020). The two spacecraft were launched on 19 October 2018. During interplanetary cruise both spacecraft are stacked together on top of the Mercury Transfer Module (MTM), and are 3-axis stabilized. In addition, the Mio spacecraft needs to be shielded by a sunshield (MOSIF) during cruise. After one flyby at Earth, two at Venus and a total of six flybys at Mercury (Mangano et al., 2021), the two spacecraft will be separated and injected into individual highly eccentric polar orbits around Mercury in November 2026. Once in Mercury orbit, Mio will be spin-stabilized and MPO will be 3-axis stabilized.

One of the five science instruments or instrument packages on board Mio is the Mercury Plasma Particle Experiment (MPPE; Saito et al., 2021). MPPE studies plasma and neutral particles from the planet, the magnetosphere, and the solar wind using seven sensors, one of them being MSA. MSA measures the mass identified 3-dimensional phase space density of low energy ions over a wide energy range between 1 eV e⁻¹ and 38 keV e⁻¹ with an instantaneous FOV of 5° × 260° (Saito et al., 2021). During cruise, the FOV of MSA is partially shielded by the MOSIF. The instrument houses a 2-dimensional mass spectrometer with a time-of-flight (TOF) chamber which measures the plasma composition with a mass resolution *m*/*δ*m* > 40 over a large range of masses up to 60 amu.

## 3. Data processing

In this paper we analyze ion measurements from the FIPS instrument (Andrews et al., 2007) onboard MESSENGER obtained during the interplanetary mission phase and the entire orbital mission of the spacecraft at Mercury. During the orbital mission the data were taken on an orbit-by-orbit basis and thus measured at altitudes ranging from 200 km to approximately 15,000 km over 16 Mercury years (about 4,000 MESSENGER orbits). Given that the MESSENGER orbital
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** FIPS raw event counts accumulated in the first month after orbit insertion at Mercury as a function of time-of-flight [ns] and energy [keV/e]. We show only the TOF range from 30 ns to 130 ns containing the traces of protons (H⁺), He²⁺, O⁶⁺ and He⁺ ions as described in Fig. A2 of Raines et al. (2013). Note that events with TOF < 60 ns have been only partially transmitted to avoid saturation of the telemetry. Overplotted are the expected average energy vs. TOF lines for protons (H⁺), He²⁺ and He⁺, respectively.</figcaption></figure>

plane was approximately fixed in inertial space, the periapsis of the orbit moved around the planet during each Mercury year. Thus, a particular local time of periapsis was always sampled at the same part of Mercury's orbit around the Sun, so that local time variations may involve contributions from seasonal variations of the exosphere.

The instrument measured the energy spectra of five ion types: H⁺, He⁺, He²⁺ as well as two groups of heavy ions grouped together into *m/q* ranges to improve signal-to-noise. These latter are the O⁺ group (m/q ≈ 16–20 amu e⁻¹, including O⁺ and water-group ions) and the Na⁺ group (m/q ≈ 21–30 amu e⁻¹, including Na⁺, Mg⁺, and Si⁺). Approximately 97% of the plasma ions measured by FIPS are protons (Raines et al., 2013), but in this paper we consider the ion counts for He⁺ and He²⁺ only.

The FIPS instrument uses a double-coincidence technique to signifi-
cantly reduce background noise. However, spurious double-coincidence
counts still occur. These counts originate from two main sources: the
extension of very high count proton measurements into other times of
flight, and the release of small numbers of ions from surface processes
within the instrument (Gershman, 2012). Here we analyze the He⁺
ion species from the instrument raw pulse-height-analysis (PHA) data
(Messenger-Dataset, 2021). While for each recorded event its energy
and time-of-flight bin are transmitted, a respective mass bin was allo-
cated by ground processing in the published raw data set for a subset
of all recorded events. This post processing excluded events which
were regarded as caused by electronic noise or other effects by an
automatic algorithm (Ho et al., 2017). As we will discuss further down
this processing may have been too restrictive.

An example of the energy vs. TOF accumulated raw measurement
count rate per month of FIPS is shown in Fig. 2. It shows the E/q
of each event plotted as a function of TOF, with curved tracks corre-
sponding to different ion species: The highest intensity track (≈ 10⁴
counts/bin) at TOF ≈ 50 ns represents the proton (H⁺) count rate, the
next one (≈ 10⁴ counts/bin) at 60–70 ns is the track of He²⁺ ions, and
the rightmost track (≈ 50–100 counts/bin) at approximately 80–100 ns
is the count rate of He⁺ ions (see also Raines et al., 2013). The expected
average energy vs. TOF lines for protons (H⁺), He²⁺ and He⁺ are
overplotted as black, red and blue lines, respectively. At solar wind
energies (0.4–1.5 keV e⁻¹) the figure shows a broad horizontal yellow
and green stripe which is due to saturation of the TOF system by high
proton fluxes at the respective energy levels. This means that the peaks
for species of higher mass per charge (m/q) need to be corrected for
this effect (Gershman, 2012).

**Fig. 3** shows the accumulated raw data in different spatial bins
around Mercury from the entire MESSENGER orbital mission in the
Mercury-centered Solar Orbital (MSO) coordinate system in which the
x-axis points from Mercury to the Sun, the z-axis is perpendicular to
the orbital plane pointing northward, while the y-axis completes the
right-handed system. Far from the planet there is a clear He⁺ trace in
the FIPS data which is to be expected because interstellar He⁺ should
always be there. The He⁺ trace is well visible up to high energies both
inside of the magnetosphere and at large distances from the planet. At
solar wind energies (0.4–1.5 keV e⁻¹) the trace may be contaminated by
TOF saturation of FIPS. For this reason we restrict most of the analysis
in this paper to the energy range above 2 keV e⁻¹. Interestingly, there is
also a clear trace at mass per charge ratio of approximately 2.7 which
corresponds to O⁶⁺ ions. The O⁶⁺/He²⁺ abundance ratio in the solar
wind is about 1% (Ogilvie and Coplan, 1995; Reisenfeld et al., 2013)
which agrees with the ratio observed by FIPS. Other multiply charged
ions may also contribute to this trace. Analysis of these data may be
worth a separate paper.

Fig. 4 shows a new analysis of the MESSENGER FIPS raw data. The He⁺ events were filtered into new energy spectra using the original mass allocation in the published data set. Previous studies have often used derived energy spectra (called ‘ESPEC’, part of the published FIPS data set (Messenger-Dataset, 2021) also derived from the PHA event data but only covering observations inside of the bow shock and excluding part of the energy range. Here we go back to the original PHA data set and include observations outside of the bow shock and over the full energy range. This results in much better event statistics as compared to previous works. Energy spectra derived in this way from all PHA event data with the original mass allocation we call ‘PHA-SPEC’. We could even go one step further and define new TOF windows for each species neglecting the original mass allocation stored in the published FIPS PHA data set. For this we apply the calculation of the energy loss in carbon foils by Allegrini et al. (2006) to the FIPS instrument. This results in a spectral product (which we call light ion spectra ‘LSPEC’) which has better statistics than the ‘ESPEC’ or ‘PHA-SPEC’ products. The reason for this difference is that the original mass allocation logic omitted many events based on their spatial sector allocation, while statistics show that these events are on valid mass tracks. But in the range below 2 keV it suffers from contamination by events caused by high proton fluxes. For this reason we restrict our analyses using this product to energies above 2 keV.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Raw FIPS event data accumulated close to Mercury during the entire Mercury orbital mission (April 2011 to April 2015) in different spatial bins in MSO cylindrical coordinates. Each inset shows the energy vs. time-of-flight spectrogram accumulated in the respective spatial region of size $2.5R_M \times 2.5R_M$ around Mercury, i.e. in the intervals $[0, 2.5R_M]$, $[2.5R_M, 5.0R_M]$, and $[5.0R_M, 7.5R_M]$, respectively. Small gray numbers on the right axis indicate the energy [keV], small red numbers on the bottom axis the TOF bin [ns]. The parameter shown on the Y axis is $R_{\text{cyl}} = \sqrt{Y^2 + Z^2}$ in units of $R_M$. We show only the TOF range from 30 ns to 110 ns containing the traces of protons, $He^{2+}$ and $He^+$ as described in Fig. A2 of Raines et al. (2013). Mercury's surface, the nominal magnetopause and the bow shock (Winslow et al., 2013) are overplotted in black, red and green, respectively. Background removal has not been done for the data shown in this figure.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** $He^{2+}$ and $He^+$ events extracted from FIPS raw data using the PHA product mass allocation. The curved tracks show the expected average energy vs. TOF lines for protons ($H^+$), $He^{2+}$ and $He^+$ ions.</figcaption></figure>

## 4. Interstellar dependence

In Fig. 5 we show the $He^+$ density overplotted onto the MESSENGER trajectory. From 2007–2011 this covers the interplanetary mission

phase of MESSENGER. From 2011–2015 when the spacecraft was in orbit around Mercury, the trajectory of the planet and the spacecraft are identical in this plot. The figure shows that the highest $He^+$ density is observed downstream of the Sun along the interstellar flow direction,
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** MESSENGER heliocentric trajectory in ecliptic coordinates with median of MESSENGER/FIPS He⁺ density above 2keV in different spatial bins superimposed, for the interplanetary mission phase (2007–2011; top right orbital arc and the four outer orbits mostly appearing in blue and pink) and the orbital mission phase at Mercury (2011–2015; inner orbit appearing mostly in green). The red arrow indicates the interstellar neutral helium flow direction. Vernal equinox is to the right.</figcaption></figure>

where a gravitational focusing cone of neutral helium was observed before (e.g. Fahr, 1971; Thomas, 1978; Gershman et al., 2013). By co-incidence, this cone is oriented in the direction of Mercury’s perihelion which may explain the larger angular spread of densities exceeding $4 \times 10^{-4} \text{ cm}^{-3}$. This confirms earlier results by Gershman et al. (2013) which showed the downstream focusing cone. In the opposite (up-stream) direction a slightly higher He⁺ density can be discerned by the lighter green color of the inner orbits which accumulate the Mercury orbital phase data. We will discuss later whether or not these reveal the upstream crescent predicted by Drews et al. (2012). The uppermost orbit in the figure contains observations from early 2007 when different instrument modes were tested, leading to the irregular density pattern. In the earlier analysis by Gershman et al. (2013) the dependence of the He⁺ density on the distance from the Sun was rescaled to achieve a flat longitudinal profile on the upstream side (see Section 4.2 of Gershman et al. (2013)). This also may have removed a possible density increase on the upstream side from their Fig 9. But the density increase by a factor of 9 in the focusing cone reported in this paper agrees well with our simple model calculation shown in Fig. 1.

Fig. 6 shows the median of the He⁺ density as a function of ecliptic longitude and distance from Mercury measured during MESSENGER's orbital mission at the planet from 2011 until 2015. The highest intensity is observed at the helium gravitational focusing cone at an ecliptic longitude $\lambda \approx 70^\circ$ which coincides with Mercury's perihelion. In the opposite direction ($\lambda \approx 260^\circ$) a higher intensity is observed as well at distances exceeding $2R_M$. However, in this type of plot several dependencies are not considered: (1) Mercury's perihelion approximately

coincides with the helium focusing cone such that the distance from the Sun plays a role, (2) because the MESSENGER orbit was approximately fixed in inertial space, it precessed around the north pole of the planet w.r.t. local time when Mercury moved around the Sun, implying that the coverage of the Mercury plasma tail varies with longitude, (3) the field-of-view of FIPS is centered at 45° w.r.t. the solar direction which can cause a bias depending on the ion distribution function (Raines et al., 2011). (4) the He⁺ velocity distribution changes with distance from the Sun since the distribution consists of a mixture of freshly picked-up interstellar ions and ions picked-up closer to the Sun and convected to the location of observation by the solar wind.

To exclude observations inside the Mercury magnetosphere we sampled in Fig. 7 data as a function of ecliptic longitude and distance from the nominal Mercury bow shock. This distance is calculated for each point of observation by calculating the surface perpendicular to the nominal mean bow shock (Winslow et al., 2013). Positive distances are outside of the bow shock, negative ones inside. In Fig. 7 (top) we show the median He⁺ density. We here use the median because mean values are often dominated by outliers. The map shows a broad density increase around the longitude of the focusing cone but no such increase on the upstream side where the crescent would be expected. The median upstream density of about $0.002 \text{ cm}^{-3}$ is in good quantitative agreement with our simple model shown in Fig. 1. However, the density increase in the focusing cone is only about a factor of 4 as compared to the factor of 8 of our model and reported in the cruise phase analysis by Gershman et al. (2013). The reason for the discrepancy could either
<figure><img src="image_8.png" /><figcaption>Fig. 6. MESSENGER FIPS He⁺ density above 2 keV as a function of ecliptic longitude and distance from Mercury from the in-orbit phase from 2011 until 2015 (median of LSPEC data product).</figcaption></figure>

be in a weakness of our cylindrical model (see Appendix) or in the
restricted field-of-view of the FIPS instrument which can only catch part
of the denser distribution in the cone. But since the cruise phase data
are in better agreement with the model and suffer from the same field-
of-view restriction it is more probable that He²⁺ conversion at Mercury
causes a higher He⁺ background even at some distance from the planet.

Since both the solar He⁺ and the He²⁺ density fall off approximately quadratically with distance from the Sun, we can remove the solar distance bias by analyzing the mean ratio between the He⁺ and the He²⁺ density. Fig. 7 (bottom) shows this ratio above 2 keV e⁻¹. In this ratio we can see a local maximum at the focusing cone (λ ≈ 70°) and a broad increase in the opposite direction (λ ≈ 260°). One could interpret this broad increase of the He⁺/He²⁺ density ratio as an indication of the interstellar helium upstream crescent described in Fig. 12 and by Drews et al. (2012). But it may as well be caused by the FIPS field-of-view bias since the phase space distributions of He⁺ and He²⁺ are very different.

5. Distribution of He⁺ in Mercury's magnetosphere

In Fig. 8 we compare the median fluxes of He²⁺ and He⁺ measured by FIPS in different spatial bins around Mercury for the entire orbital mission of MESSENGER at the planet from 2011 to 2015. These and the following maps were created by collecting 10 minute averages of the fluxes, and sampling all values in a spatial data matrix. From this data matrix we can then derive median, mean, standard deviation and total counts in each spatial bin. Fluxes are integrated over the energy range 2–14 keV.

For both species, He²⁺ and He⁺, we observe a homogeneous flux
distribution outside of the bow shock, and a flux increase by one order
of magnitude between the bow shock and the magnetopause. This can
be explained on the one hand by an actual increase in flux caused by the
deflection around the obstacle, and on the other hand by the restricted
field-of-view of FIPS which can better observe a heated non-radial flow.
We also observe a flux cavity below the magnetopause on the dayside
and a flux increase on the nightside within 2 R<sub>M</sub> distance (Mercury
radius R<sub>M</sub> = 2,440 km), or specifically in the near-planet shadow.
We here need to keep in mind that the He⁺ velocity distribution is
mainly convected from the inner solar system but certainly not fully

thermalized (see below), which biases the comparison of total fluxes
and densities between He⁺ and He²⁺.

Fig. 9 (top) shows a map of the FIPS He⁺/He²⁺ flux ratio above 2keV in 10 min averages for the complete MESSENGER in-orbit phase from 2011 to 2015. Fig. 9 (bottom) shows the sampling statistics for each spatial bin. At the minimum observable flux, FIPS sampled only 1 count per accumulation interval (1 min). We regard a sampling of more than 30 samples per bin as statistically sufficient since the count statistics follow a Poisson distribution.

One can see that the ratio of He⁺/He²⁺ outside of the bow shock in the solar wind and in the inner magnetosphere inside the magnetopause are rather similar with a range of 0.1–0.3, but the ratio drops to 0.05 in the magnetosheath. This indicates that electron capture of He²⁺ from the hot electron environment in the magnetosheath does not play a major role, but the probability to observe in the FIPS field-of-view the deflected solar wind flow in the magnetosheath increases. Also the He⁺/He²⁺ flux ratio increases close to the planetary surface, indicating either a planetary source or an increased conversion of He²⁺ to He⁺ close to or at the surface - via neutralization and subsequent ionization. The similarity of the ratio inside and outside of the magnetosphere may indicate that the surface source is not stronger than the interstellar source. But this measurement is certainly biased by the fact that inside of the magnetopause the ratio between thermal and bulk speed of both the He⁺ and He²⁺ distributions gets higher and thus the distribution is easier to observe by FIPS than in the solar wind. The figure also shows an increase of the He⁺/He²⁺ ratio in the solar wind downstream of the Mercury terminator plane outside of the bow shock (upper right corner of the figure). This can be interpreted as a higher ionization probability by heated electrons or a higher He⁺ plasma wave heating rate in the foreshock region as compared to the undisturbed solar wind.

In Fig. 10 we show the mean FIPS PHA-SPEC He²⁺ and He⁺ dif-
ferential flux spectral data in different spatial bins around Mercury
for MESSENGER's entire orbital mission at the planet. The energy
range shown is from 0.1 to 14 keV e⁻¹. The He⁺ spectra in the solar
wind and the magnetosheath peak at around 500 keV e⁻¹, and show
an exponential fall-off at higher energies, while He²⁺ shows a typical
thermal spectrum peaking at 800 keV e⁻¹. A similar He⁺ spectral shape
is also observed far away from Mercury, indicating that the origin may
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Median density of He⁺ (top) and mean ratio of MESSENGER FIPS He⁺/He²⁺ density (10 min sampling) (energy 2–14 keV/e) as a function of ecliptic longitude and distance from Mercury's nominal bow shock measured during the spacecraft's orbital mission around Mercury in 2011–2015.</figcaption></figure>

be interstellar helium (Holzer and Axford, 1971; Vasyliunas and Siscoe, 1976). Inside of the magnetopause the He²⁺ and He⁺ spectra are getting more proportional to each other, specifically the He²⁺ distribution gets hotter. In this region a direct conversion of He²⁺ to He⁺ by electron capture from neutrals could also play a role.

But these spectra also show the general problem of observations with restricted field-of-view: while for a thermalized spectrum with known bulk velocity (He²⁺) the total flux may be reconstructed from the observations, the asymmetric velocity distribution of He⁺ needs a more sophisticated model to be included in future studies. With the methods of the current study a further quantification of the ratio between interstellar and solar contribution does not seem possible.

For comparison we show in Fig. 11 the observation of an intense He⁺ spectrum observed by the MPPE-MSA instrument during the second flyby of BepiColombo at Mercury which initiated our new analysis of the MESSENGER data. Similar spectra were observed also on the third flyby of BepiColombo (Hadid et al., 2024a). The underlying E/q-TOF matrix product in low mode (Time-of-flight Stop Low telemetry product, TSTL; Saito et al., 2021) is only transmitted to Earth every 18 min with some records missing such that spectra could only

be collected within the time windows 09:14–09:30, 09:31–09:49 and 10:07–10:24 UTC, indicated in the left figure. The figure shows differential flux spectra in the same energy range and units as for MESSENGER FIPS in Fig. 10. Contamination of the matrix by secondary electrons created by protons hitting the top part of the chamber has been removed from the data set. The most intense He⁺ spectrum was observed in the second time interval corresponding to the close tail passage of the spacecraft. The flux intensity and the slope of this spectrum are comparable to the averaged FIPS spectra obtained in the same spatial region with the exception that BepiColombo observed a higher flux at low energies. The field-of-view of MSA is directed perpendicular to the solar direction and restricted to about 10° × 30° which may explain why the He²⁺ flux is even less visible than for FIPS, though in the foreshock region at 10:07–10:24 UTC, solar wind spectra are partially observed by MSA.

## 6. Discussion

Raines et al. (2013) analyzed the initial three Mercury years of MESSENGER measurements with FIPS in orbit around Mercury. They
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Median of MESSENGER FIPS LSPEC He²⁺ (top) and He⁺ (bottom) total flux [cm⁻²s⁻¹sr⁻¹] above 2 keV in different spatial bins around Mercury 2011–2015 in cylindrical MSO coordinates. Mercury’s surface, nominal magnetopause and bow shock (Winslow et al., 2013) are overplotted in black, red and blue, respectively.</figcaption></figure>

observed that the distribution of He⁺ is very different from that of O⁺ and Na⁺. They speculate that He⁺ at Mercury may be created by charge exchange between He²⁺ and any neutral atom or by ionization of exospheric neutral helium released from the planetary surface. He⁺ created by charge exchange should show enhancements in regions where the He²⁺ density is the highest. Fig. 8 shows that this is the case in the magnetosheath, qualitatively consistent with charge exchange being a major driver for He⁺ production.

Processes for helium release from the surface also include radioac-
tive decay of uranium and thorium from rocks and subsequent release
of He²⁺ (α particles) from the surface (Krasnopolsky and Gladstone,
2005; Chanteur et al., 2009). Since there is no known volcanic activ-
ity on Mercury and the surface is rather old, this source should be
small, similar to, e.g. Mars and Venus. Several additional processes
are believed to be likely source mechanisms for surface bounded exo-
spheres, including thermal desorption, photon-stimulated and electron-
stimulated desorption, and ion and chemical sputtering, and the sig-
nificance of each of these processes is still under debate (see, e.g. the

review by Wurz et al., 2022). None of these processes, however, is likely
relevant for the release of helium.

We note that the He⁺ spectra outside of the bow shock have a peak flux intensity of about 10³cm⁻²s⁻¹sr⁻¹ (Fig. 10), while the spectra inside of the magnetopause peak intensity is about a factor 5 higher. If we now consider that, (1) the density ratios between He⁺ and He²⁺ shown in Fig. 9 are comparable inside and outside of the magnetosphere, and (2) that the He⁺ observed outside is mainly of interstellar origin as shown by Fig. 7, we may conclude that the interstellar contribution to the He⁺ density observed inside of the Hermean magnetosphere is of similar order as the solar contribution.

Gershman et al. (2013) discussed the appearance of the focusing
cone in the downstream direction of the interstellar helium behind
the Sun. The possible He⁺ crescent on the upstream data set was
not visible in Gershman et al.’s data set caused by the solar distance
bias. But also in the data set presented here an upstream crescent
cannot be confirmed in the He⁺ density statistics. The increase of
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Mean of MESSENGER FIPS LSPEC He⁺/He²⁺ average total flux (cm⁻²s⁻¹sr⁻¹, 2.0–14 keV) ratio (top) and number of 10 min samples (bottom) in different spatial bins around Mercury 2011–2015 in cylindrical MSO coordinates. Mercury's surface, nominal magnetopause and bow shock (Winslow et al., 2013) are overplotted in black, red and blue, respectively.</figcaption></figure>

the He⁺/He²⁺ ratio observed upstream possibly because of a field-of-view bias. Further analysis requires modeling of the phase space distributions.

Recent measurements (Krupp et al., 2024) by the ion energy MSA-MPPE instrument on board the BepiColombo/Mio spacecraft showed a He⁺ signature on the night side of Mercury during BepiColombo's second flyby at the planet on 23 June 2022, and during measurements in interplanetary space far away from Mercury in March 2022. The latter must be due to interstellar helium. More data are expected to come during the spacecraft's orbital mission at the planet beginning in November 2026 (Milillo et al., 2020; Mangano et al., 2021).

## 7. Conclusions

The distribution of He⁺ in the vicinity of Mercury has so far not been studied in greater detail. Raines et al. (2013) gave a first overview of

observations of the distribution of He⁺ inside of the Mercury magnetosphere by the MESSENGER FIPS instrument. In this earlier analysis a possible interstellar contribution was neglected. On the other hand, the earlier studies by Gershman et al. (2013, 2014) only analyzed observations of He⁺ in the inner solar system during the cruise phase of MESSENGER. In the present study for the first time we use the complete MESSENGER FIPS data set to address the questions:

1. How strong is the interstellar contribution to the observed He⁺ outside and inside of the Hermean magnetosphere?

2. Are energy spectra of He⁺ ions from the interstellar source different from the spectra produced by ionized helium emanating from the planetary surface?

3. Can the existence of an interstellar He⁺ crescent upstream of the Sun predicted first by Drews et al. (2012) be confirmed?
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Mean MESSENGER FIPS PHA-SPEC $He^{2+}$ (black) and $He^+$ (green) differential flux [$cm^{-2} s^{-1} sr^{-1} keV^{-1}$] spectra as a function of energy/charge [$keV e^{-1}$] in different spatial bins around Mercury for in-orbit mission data in cylindrical MSO coordinates. Mercury's surface, nominal magnetopause and bow shock (Winslow et al., 2013) are overplotted in black, red and blue, respectively.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Left: Orbit of the BepiColombo spacecraft during its second Mercury flyby on 23 June 2022 in MSO coordinates with nominal plasma boundaries. Overplotted along the track is the total $He^{2+}$ flux observed by MPPE-MSA at 64 s resolution (M2L product). The boxes indicate the sampling intervals of the mass resolved TSTL product. Right: Energy-flux spectra obtained by the MPPE-MSA instrument for $H^+$, $He^{2+}$ and $He^+$ ions in low mode (Time-of-flight Stop Low telemetry product, TSTL; Saito et al., 2021) at 3 time intervals during the flyby.</figcaption></figure>

The majority of the $He^+$ observed at Mercury may originate from solar $He^{2+}$ (Raines et al., 2013) but our new analysis shows that the increase in the observed $He^+$ close to Mercury may be due to a concentration of interstellar neutral helium close to the planet, leading to an increased $He^+$ density. This agrees with the qualitative analysis by Quémerais et al. (2023). On the other hand, the $He^+$ energy spectra show a continuous evolution from the solar wind towards Mercury. This could mean that interstellar $He^+$ is concentrated at Mercury by increased ionization (and gravitation).

Our analysis of the $He^+$ density indicates a density increase at Mercury's perihelion and possibly at aphelion as well – though this aphelion increase appears only inside of the Mercury magnetosphere and the observations may be biased by the orbit of MESSENGER. It confirms earlier results by Gershman et al. (2013, 2014), namely the

existence of the interstellar helium focusing cone at approximately 0.3–0.5 AU downstream of the Sun. However, we also find a higher $He^+/He^{2+}$ ratio in the upstream hemisphere at Mercury orbit — if we only take observations outside of the Mercury bow shock. Whether this confirms the prediction by Drews et al. (2012) and our own simple model of the interstellar distribution in the inner solar system needs to be clarified by simulations of the phase space distributions in relation to the FIPS field-of-view.

Inside of the Mercury bow shock we observe a depletion of the $He^+/He^{2+}$ ratio, indicating a higher observed $He^{2+}$ flux. It may also mean that the production of $He^+$ by electron capture of $He^{2+}$ from either neutral atoms or the warm electron background may not play a major role in this region. Inside of the Mercury magnetopause we observe a $He^+/He^{2+}$ ratio that is comparable to the observed ratio
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Cylindrical symmetric model of the interstellar neutral Helium and He⁺ distribution in the inner solar system. The model is aligned with the interstellar flow direction. The orbit of Mercury is indicated by the red circle. The color bar (density range) is valid for both figures.</figcaption></figure>

in the solar wind close to Mercury with a further increase towards Mercury surface and in the shadow. In this region the He⁺ production may be dominated by neutralization of He²⁺ at or close to the surface and subsequent ionization. But even close to the planet the interstellar component may be important. Coming back to the three question posed above, we can state:

1. Our analysis shows that the interstellar contribution to the He⁺ observed around Mercury is probably on the same order as the solar contribution. The interesting point for planetary science is of course whether there is any indication of an internal planetary source of helium at Mercury. Our preliminary answer is the observed He⁺ intensities can probably be explained by the solar and interstellar sources only.

2. The energy spectra of He⁺ outside of Mercury's bow shock are typical for pick-up ions, while those observed inside are thermalized. This can be explained by the stronger *vxB* field outside of the bow shock.

3. The existence of an interstellar He⁺ crescent upstream of the Sun cannot be confirmed.

Modeling the conversion of He²⁺ and the concentration of interstellar He⁺ close to Mercury will be an interesting future investigation in the context of the coming observations of the BepiColombo mission.

### CRediT authorship contribution statement

**Markus Fränz:** Writing – original draft, Visualization, Software, Methodology, Formal analysis, Data curation, Conceptualization. **Hарald Krüger:** Writing – original draft, Methodology, Investigation, Conceptualization. **Jim M. Raines:** Validation, Investigation, Data curation. **Austin N. Glass:** Validation, Data curation. **Daniel J. Gershman:** Writing – review & editing, Validation, Data curation. **Fabio Prencipe:** Writing – review & editing, Validation, Investigation, Conceptualization. **Norbert Krupp:** Writing – review & editing, Supervision, Project administration, Investigation, Conceptualization. **Lina Z. Hadid:** Writing – review & editing, Validation, Project administration, Data curation. **Dominique Delcourt:** Project administration, Funding acquisition. **Sae Aizawa:** Investigation, Data curation. **Shoichiro Yokota:** Validation, Data curation. **Yuki Harada:** Validation, Software, Data curation. **Yoshifumi Saito:** Supervision, Project administration, Funding acquisition, Data curation.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Markus Fraenz reports financial support was provided by German Space Agency DLR. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

We would like to thank all the people who contributed to the success of the MESSENGER FIPS and BepiColombo MPPE instruments. Work at MPS was supported by the Max Planck Society and by the German Space Agency DLR (Grant number 50 QW 2101). Raw data of the MESSENGER FIPS instrument used in this study are available through the NASA PDS PPI archive (https://pds-ppi.igpp.ucla.edu/index.jsp). BepiColombo MPPE and FIPS processed data are available through the Zenodo archive at (https://doi.org/10.5281/zenodo.14591537).

### Appendix. The simple interstellar Helium model

Fig. 12 shows a simple cylindrical model of the interstellar He⁺ density distribution in the inner solar system. The model follows Rucinski et al. (2003) for the interstellar neutral helium flux and includes photo-ionization after Sokół et al. (2022) and electron impact ionization after Chen et al. (2013). The model works by launching parcels (phase space volumes) of neutral helium with the interstellar density of 0.015 cm⁻³ and velocity of 25.3 km s⁻¹ along the perpendicular axis at a distance of 5 AU. The temperature is initially not considered in this simple model. At each timestep the parcels are accelerated by solar gravity and partly ionized. The phase space volume of each parcel is maintained at each time step. The singularity at the focusing cone is avoided by limiting the increase of velocity space volume by the ratio between thermal and bulk velocity. Retrograde orbits of the parcels are also considered.

The He⁺ density observed at a given solar distance is composed of the density produced by local pick-up and the density convected from the smaller distances. On the downstream side the helium focusing cone is formed but on the upstream side also a broader density increase is formed by two factors: first by retrograde neutral helium orbits and second by a longer exposure time of helium to photo-ionization on close impact orbits. These effects form the so-called interstellar He⁺ crescent first described by Drews et al. (2012). The figure also shows the Mercury orbit crossing both cone and crescent.

### Data availability

Data not publicly available have been shared via Zenodo: https://doi.org/10.5281/zenodo.14591537.
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