On the 20 August 2024 the JUICE spacecraft made its first gravity assist maneuver at Earth with a closest approach altitude of 6850 km corresponding to a L-shell of 2.31 RE. At this distance from Earth the spacecraft encountered the Earth plasmasphere and several sensors – originally designed for measurements at Jupiter – were able to make new observations of the Earth plasmasphere. Here we report on the measurements of the Jovian Electron and Ion spectrometer (JEI) which is one of the thermal plasma sensors of the Particle Environment Package (PEP, ) onboard JUICE. The observations are supported by measurements made by the Radio & Plasma Wave Investigation (RPWI, ).

The Earth plasmasphere is a region of dense cold plasma extending between +1.5 and about 6 Earth radii (RE) around the center of Earth . The plasmaspheric ion density is usually > 10³/cm3 within 3 RE with a plasma temperature < 1 eV . Because the plasmaspheric ion population is rotating close to corotation with Earth, it has very low energy (< 10 eV) in the frame of typical spacecraft observations. This means the ion properties can only be measured in-situ with retarding potential analyzer (RPA) techniques or when the spacecraft charges negatively. The first detailed analysis of the effect of the spacecraft potential on the plasmaspheric spectrum was provided by using data obtained by the RPA instrument on the OGO-3 satellite. The most relevant cold ion composition measurements available until today were made by the RPA instrument RIMS onboard Dynamics Explorer 1 (DE 1) between 1981 and 1984 . The RIMS instrument could in principle measure ions up to mass 32 amuq-1 ; the continuously downlinked dataset contains only observations of H+, He+, He++, O+ and O++ . This dataset was reviewed by where it is stressed that specifically the oxygen density of the outer plasmasphere (> 2 RE) is highly variable and can reach beyond 1000/cm3 such that a specific oxygen torus might form. But on a single specific orbit during a magnetic storm the RIMS instrument was operated in a mode which allowed observation of heavier ions up to mass 46 . On this orbit a strong flux of ions with 28, 20 and 32 amuq-1 with similar relative abundance was observed stretching from L = 1.1 to 3 RE. These were interpreted as N2+, NO+ and O2+ ions. Interestingly the flux at larger distances was observed to be accelerated along the magnetic field. These observations agreed with earlier observations by the ion mass spectrometer on the ISIS-2 spacecraft during a magnetic storm at 1400 km altitude . The accelerated thermal plasma was even sometimes observed at the Earth magnetopause . Recently also reported on observations of plasmaspheric cold ions by the the Van-Allen-Probe HOPE instrument. All observations of molecular ion outflow from the ionosphere up to 2022 were reviewed by and . From these reviews it becomes clear that all more recent observations are for energized outflow (with energies > 100 eV) while the cold plasmaspheric composition has not been observed again after the DE-1 measurements in 1984. A problem with the DE-1 measurements was that the spacecraft potential was not observed by an independent instrument during the observations . Ion temperature and potential were obtained by an iterative fit to the data resulting in a plasmaspheric ion temperature of about 3000–5000 K (0.27–0.45 eV) at L = 2 RE . But the authors emphasize that without knowledge of the potential the ion temperature may be overestimated. The importance of the cold ion population for magnetospheric dynamics was discussed by : the cold plasma dominates the solar wind contribution in several magnetospheric regions, it affects the reconnection and instability rates, controls the generation of waves and pulsations. Nevertheless the cold plasma is difficult to observe and observations are rare. A comprehensive review of the instrumental limitations of cold ion observations can be found in .

The rather high plasmaspheric temperatures (> 0.5 eV) compared to ionospheric temperatures (< 0.1 eV) observed by these early RPA instruments did pose a problem for plasma theory because no sufficient heating mechanism could be identified . Proton temperatures were again determined from 1996–2001 by the RPA instrument on the MAGION-5 sub-satellite of the INTERBALL 2 mission. Analysis of this data set by showed that the temperature of the inner plasmasphere is closely correlated to the temperature of the upper ionosphere. Local temperature is anti-correlated to local density and drops to values below 2000 K (0.18 eV) for densities > 2000/cm3 confirming that the earlier RPA analyses may have overestimated the temperatures. The plasmaspheric temperature observations were discussed again in context of the Van-Allen-Probe mission where proton temperatures were determined by indirect methods for two orbits confirming the results of . We note that the Van-Allen-Probe spacecraft charged positive in the plasmasphere which inhibited the direct observation of cold ions .

The ESA spacecraft JUICE was launched on 14 April 2023 on its long mission to planet Jupiter. Between the 19 and 21 August 2024 the spacecraft conducted the first ever Lunar–Earth gravity assists (LEGA) maneuver. The trajectory of the spacecraft during the Lunar–Earth flyby is shown in Fig.  projected onto the geocentric solar ecliptic (GSE) XY-plane. Overplotted are the orbit of the Moon (red crosses), the expected position of the Earth plasmapause (red dots), magnetopause (green) and bowshock (cyan). The JEI sensor of the PEP instrument suite was switched on only during 4 time intervals of length 40 min. These 4 intervals are indicated by the coloring along the JUICE trajectory which shows the count rate observed by channel 15 of the JEI sensor, which shows the highest count rates. The intervals were chosen to obtain electron measurements around the Moon and magnetopause and ion measurements in the plasmasphere and around the bow shock. In this paper we only discuss the observations made during the plasmasphere crossing. An enlarged plot of this crossing is shown in Fig.  in the same coordinate plane as Fig. . The JEI sensor obtained science data on 20 August 2024 from 21:19 to 21:56 UTC just before closest approach to the Earth at 21:56:14 UTC. At this point the JUICE spacecraft had a distance of 13 211 km or altitude of about 6850 km at geographic latitude 24.0° and L-shell 2.31 RE.

Figure  shows the densities of H+, He+ and O+ predicted for the position of JUICE by the Earth global core plasmasphere model for a Kp value of 1.7 which was prevalent during the JUICE closest approach (https://kp.gfz.de/en/, last access: 16 October 2026). The model predicts a sharp inbound plasmapause. The authors of the model note that specifically the O+ density is highly variable and the model assumes constant initial ratios of 0.1 for He+ and 0.01 for O+ relative to H+. Molecular ions are not contained in the model. We may just consider the total peak ion density value of about 4000/cm3 predicted by the model and note that the SPICED model for the equatorial plasmasphere predicts for L = 2 RE and MLT = 6 h an average density of 2200/cm3 (, their Fig. 12).

To illustrate the general effect of a potential with small Debye length on a plasma flow with different ion masses we show in Fig.  a Simion^® (https://simion.com/, last access: 16 October 2026) simulation of ion trajectories in a linear and planar potential of Vp = -5 V at spacecraft surface and instrument and 0 V at 50 cm distance. Ions are launched parallel to the spacecraft surface at 50 cm distance with mass per charge of 1 amuq-1 (left) and 16 amuq-1 (right) and a velocity of vx = 8 kms-1. One can see that the lighter ions reach the JEI sensor at a higher polar angle than the heavy ions. We note that Fig.  is for illustration of the principle only. The Simion software can not simulate a potential with exponential fall off caused by Debye shielding. The important assumptions are that the electric field is mainly vertical and ions are launched from a field-free point. In this case the ion acceleration is mainly in vertical direction such that the distance from which ions are launched has no influence on the arrival angle. A more realistic simulation could be done using the SPIS model, cited above.

To treat the effect analytically we assume that the potential is planar with respect to the spacecraft nadir deck. In this case the acceleration of ions by the potential is only in -Z direction. The energy gain is given by Ep=qVp, where Vp is the spacecraft potential and q the ion charge. Since the acceleration is only in -Z direction, we can calculate the observed vertical velocity by vz=2Ep/m-sign(vz0)vz02, where vz0 is the initial undisturbed vertical velocity. The sign of the vertical velocity is introduced to cover both ions moving initially towards the spacecraft deck and away from the spacecraft deck. Under the assumption of a planar potential we can then calculate the polar angle θp relative to the vertical axis by 1tan⁡θp=vx/vz=vx2qVp/m-sign(vz0)vz02, where (vx,vz0) are the initial velocity components parallel and perpendicular to the spacecraft surface, and m/q is the ion mass per charge. The expected total observed energy per charge is E/q=m|v|22q+Vp. In Table  we calculate the expected mean inclination angles for different ion species for the conditions valid at around 21:50 UTC where the spectra shown in Figs.  and  were observed. From Fig.  we can read the approximate initial velocities either assuming that the plasma is at rest relative to Earth in the GSE frame or rotating with corotation speed. We here compare expected angles for a spacecraft potential of -5.0 and of -6.5 V as reported by RPWI (Fig. ). The polar angle of the inflow, when undisturbed by the potential, is about 45°. By Eq. () and illustrated in Fig.  the expected polar angle becomes smaller with increasing potential. The maximum ion mass we consider is 32 amuq-1 (O2+). Earlier observations of heavy molecules in the plasmasphere suggest that molecules with mass 28 and 30 amuq-1 (N2+ and NO+) may be more abundant than O2+. The energy resolution of the JEI sensor does not allow to separate these masses such that O2+ is considered only as representative for the heavy molecular component. Similar spatial effects of the spacecraft potential on the ion distribution were discussed in context of the Rosetta mission .

We can see from Fig.  that ions are observed only for polar angles smaller than 45° in agreement with the expectation that the spacecraft potential decreases the inclination angle. We can also see that spectra observed at inclination angles < 25° show only a single or double Maxwellian distributions as the example shown in Fig.  (right panels). Table  shows that spectra observed at lower angles should be dominated by light ions – mainly H+, He+ and He++. The high energy tail seen at θ = 14.4° may be contributed by the helium ions. Only at inclination angles > 25° we can see spectra with higher mean energy per charge and separate peaks in the spectrum as shown in Fig. . We interprete these as being dominated by heavier ions. Since the peak with maximum energy per charge is observed at 15.0 eV we may conclude from the values shown in the table that: 1.

the energy of the peak count rate observed by the JEI sensor suggests a spacecraft potential of around -5.0 about 1.5 V less negative than the value reported by RPWI, though this can be an effect of difference in the ground reference voltage,

To better quantify the contribution to the spectra shown in Figs.  and  we made a sensor response calculation for the specific ion species and geometries listed in Table . We assume for all ions a common initial velocity of 7.5 kms-1, an acceleration by a -6.5 V potential, a temperature of 0.1 eV and a density of n0 = 1000/cm3 for each species. We take the same density for each species to later allow a scaling to the observed count rate. We derive the expected phase space distribution for each species as a function of angular distance from the beam center as explained in Appendix . From the phase space distribution we derive the particle flux using the base geometric factor given in Table . But here we also take the polar angle (θ) dependence of the efficiency into account which reduces the geometric factor at smaller polar angles. We also include the dependence of the CEM efficiency on ion mass and pre-acceleration voltage (CEM_head) using the general formula for MCPs from .

Figure  shows the expected counts in each energy bin for the inclination angles listed in the first line of Table . The integration time per energy bin is 3 ms. The plots divide the polar field-of-view of JEI (10–75°) into 16 sectors of 4.4° width. The JEI sensor was operated during this observation pass with a division of the polar field-of-view into only 8 sectors, but the polar full width half maximum (FWHM) of the sensor for a cold beam is only about 5° such that only every second sector shown in Fig.  was observed by JEI. The mean angles of the JEI polar sectors are given in Figs.  and .

In Fig.  we overlayed the predicted contributions of the different ion species to the spectrum observed under the top 8 polar simulation sectors – only 4 of these sectors have been observed by the JEI sensor. These distributions can now be compared with the 4 observed spectra shown in Figs.  and . Note, that the simulation uses the same density of 1000.0/cm3 for each species. We can see that the spectrum observed at 14.4° can have contributions of H+ and He+ where the density of 177 ± 45/cm3 obtained by fitting shown in Fig.  can not be directly compared because is does not contain the dependence of the efficiency on elevation angle and pre-acceleration voltage of the sensor. To obtain a more realistic density value for H+ (assuming that He+ is minor) we may multiply the assumed density n0 by the ratio of the peak counts in Figs.  and  for the respective θ angles. We then get: 2θ=14°:nH+=n0×(100.0±10)/50.0=2000±200/cm3,θ=19–24°:nHe+=n0×(80.0±9)/60.0=1330±150/cm3,θ=28–33°:nO+=n0×(65.0±8)/30.0=2170±270/cm3,θ=28–33°:nO2+=n0×(60.0±8)/20.0=3000±400/cm3.

The errors shown contain just the error of the counting statistics, but there are several other possible sources of error in these estimations such as: value of spacecraft potential, corotation speed of plasmasphere, polar angle response of the geometric factor, CEM efficiency dependence on mass and energy. Especially the latter may lead to an overestimate of the heavy ion densities. For this reason we repeated the calculation by assuming the efficiency of H+ for all ions as shown in Fig. . One can see that this makes a significant difference. If we now calculate densities based on this response assumption we get: 3θ=14°:nH+=n0×(100.0±10)/90.0=1110±110/cm3,θ=19–24°:nHe+=n0×(80.0±9)/110.0=730±80/cm3,θ=28–33°:nO+=n0×(65.0±8)/200.0=325±40/cm3,θ=28–33°:nO2+=n0×(60.0±8)/350.0=170±23/cm3.

This comparison shows the significant influence of the CEM efficiency on the calibration for heavy ions. Unfortunately the JEI sensor operated during the observation with a rather low post-acceleration voltage CEM_head of -250 V which does not provide enough acceleration for heavy ions. The nominal post-acceleration for observations at Jupiter in ion mode is -1000 V but for safety reasons this was not applied during the JEI operations during the LEGA. To solidify the quantitative density estimates for these observations laboratory tests of the CEM efficiency at -250 V post-acceleration are needed. In 2017 ion beam tests of the JEI CEMs were done at -370 V total ion energy with H2+ and N2+ ions. These showed that the efficiency of the heavier ions was smaller than for the light ions by a factor of only ∼ 0.8 (Istvan Szemerey, personal communication, 2026). The respective ratio for this energy returned by the MCP formula of is ∼ 0.1. This indicates that the assumption of a mass independent efficiency can be more realistic for the JEI CEMs. In addition to these considerations a specific correction of the observed densities taking into account the shape of the spacecraft potential may be applied as explained in Appendix . But this correction is expected to be small compared to the influence of the CEM efficiency.

Nevertheless the simulation results (Eq. ) show that the heavy ion contribution (O+ and O2+) to the total density during the JUICE plasmasphere passage is at least 25 %. Additionally, the spectral peak at 15 eVq-1 is difficult to explain without considering molecular heavy ions.

The negative charging of the JUICE spacecraft allowed a rare direct measurement of the plasmaspheric cold ion distribution by the PEP JEI electrostatic analyzer. This is a novel measurement as compared to the multiple observations by RPA instruments flown in the earlier phases of magnetospheric science. The advantage of the electrostatic analyzer is a rather high energy and spatial resolution as compared to RPA instruments. More recent missions (Cluster, MMS, Van-Allen-Probe) used electrostatic analyzers but were inhibited by tuning of the instruments to higher energies or by positive charging of the spacecraft. The PEP JEI sensor is a pure energy analyzer and has no built-in mass discrimination. As we have demonstrated above the high spatial resolution of the sensor allows to use the negative spacecraft potential as a mass filter for cold ions when they have a common bulk speed determined by the spacecraft velocity. The location of the flux peak for H+ and He+ are with 5.2 and 5.6 eV lower than expected for a spacecraft potential of -6.5 V as measured by RPWI. The reason may either lie in a difference between spacecraft ground and JEI electronics ground or a potential deviation at the specific location of the JEI sensor . In our calculations we assume a value of -5.0 V for the potential.

Coming back to the statistical occurrence rate of the spectral peaks with energies above 10 eVq-1 we mentioned above that this type of spectrum was only observed on 28 out of 108 polar scans obtained during the plasmasphere passage. If we assume that the plasmaspheric composition did not change significantly during this 9 min interval the reason for not observing the peaks on every scan must be the narrow polar field of view of only about 5° for a mono-directional beam. Since the 80° polar field-of-view is stepped through in only 8 steps the instrument frequently misses the heavy ion part of the plasma distribution. This effect can be estimated from Fig.  when considering that only every second polar sector shown in the Figure is actually observed.

One topic we have not yet discussed is the influence of the magnetic field on the observations. Observations in the outer plasmasphere (L > 3 RE) indicate in addition to the cold ion distribution a significant warm ion distribution with temperature > 1 eV and mean energy > 10 eV . Unfortunately at the time of writing no offset-corrected magnetic field data are available from the JUICE magnetometers. For this reason we have included in Figs.  and  only the predicted magnetic field vector from the Tsyganenko96 model. As shown in Fig.  the predicted vector is perpendicular to the spacecraft velocity vector. A field parallel acceleration could in principle be explained by a strong electron density gradient (polar wind effect). But the observed distribution is not field parallel and any such acceleration mechanism would create a power law spectral shape and not the observed cut-off at 15 eV. It can also not explain the peaks in the spectrum which coincide with the modeled location for the different ion masses. Also the azimuthal angle ϕ of the predicted magnetic field is at 90° (Fig. ), while the observed azimuth is in the range -20 to +20°.

The flux peak observed in the energy spectrum around 15 eV (Fig. ) indicates the presence of heavy molecular ions in the mass range 30–32 amuq-1. The steep fall off above 15 eV indicates that no heavier molecules than mass 32 amuq-1 are present. Since the efficiency of the JEI CEMs at the very low post-acceleration voltage of -250 V is not well known further calibrations are needed to quantify the relative contributions of the different ion species to the observed spectra.

The observed ion temperatures (Fig. ) should in principle not depend on the spacecraft potential when the negative potential is lower than the undisturbed minimum energy for a specific ion species. But this is not the case for the spectra observed at θ = 14.4° and θ = 23.8° where the observed mean energy is very close to the spacecraft potential reported by the RPWI instrument. In this case the part of the spectrum which is lower than the spacecraft potential will be accumulated in a single energy bin and the Maxwellian fit in energy space results in too low temperatures. We discuss this effect in Appendix  with the conclusion that the temperature should only be derived from the upper part of the energy distribution. But this can not explain why the temperature values reported in Fig.  are lower than those obtained by earlier measurements using RPA instruments .

If we now consider the implications of our analysis for observations by the JUICE spacecraft in the plasma environment of the Jovian moons we first note that a negative spacecraft charging was also observed by the PLS instrument on the Galileo spacecraft . calculated spacecraft potentials for the Ganymede and Jupiter magnetospheric phases of the JUICE mission using the SPIS potential simulation software. For the Ganymede phase they predict a negative potential of -15.5 V at a Debye length of 1.2 m such for this phase the methods described in our analysis may be applied with some modification. For the Jupiter magnetospheric phase predict a positive potential of +4 V at a Debye length of 120 m. In this case the common assumption of a spherical potential geometry can be applied.

During the first gravity assist of JUICE at Earth on 20 August 2024 the spacecraft passed through the Earth plasmasphere for about 2 h. Before closest approach at a distance of 2.1 RE the Jovian Electron and Ion spectrometer (JEI) of the PEP instrument suite was switched on for 40 min in an ion mode test configuration. The high plasma density of about 3000/cm3, as observed by the RPWI plasma wave instrument, led to a negative charging of the spacecraft which allowed a rare observation of the cold and dense plasmaspheric ion population. Since the ions are only corotating with the Earth with a velocity of about 1 kms-1 at this distance the observed ion speed is dominated by the spacecraft velocity of about 8 kms-1. For this reason ions with different masses are separating in the energy spectrum observed by JEI. In addition the spacecraft potential leads to specific filtering of ion masses in the observed angular distribution. By calculating the sensor response function for this specific geometry it is possible to quantify densities of the different ion species. This response calculation depends critically on the response of the JEI CEMs to the ion velocity and mass. Since the sensor was operated with a low post-acceleration, further laboratory calibrations may be needed for this specific setup. Still we can already conclude from the observations that ions with mass > 4 amuq-1 contribute at least 25 % to the observed total density. A flux peak observed in the energy spectrum at 15 eVq-1 can only be explained by the presence of heavy molecular ions with mass ∼ 30 amuq-1. Molecular ions have only been rarely detected in the outer Earth plasmasphere. With respect to the performance and operations of PEP JEI, these observations show that PEP JEI can achieve sensitive ion composition measurements in the exospheres of the moons of Jupiter.