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Spacecraft charging is a well‐known effect that occurs when a spacecraft is located in a charged environment such as plasma. In this process, the surface of the spacecraft acquires an electrostatic potential through the accumulation and emission of positive and negative charges. In addition to causing severe electrostatic discharges, it also significantly affects low‐energy particle measurements performed by instruments onboard the spacecraft as it causes a change in energy and a distortion of the Field Of View (FOV) of the instrument by modifying the trajectory of measured particles. Spacecraft charging is therefore an important aspect to consider for Jovian plasma Dynamics and Composition analyzer (JDC), an instrument which aims to perform cold plasma measurements around the Galilean moons onboard JUpiter ICy moons Explorer (JUICE). In this study we use SPIS to perform simulations to study the FOV distortion of JDC for positive ions caused by spacecraft charging in two environments of the JUICE mission: the ionosphere of Ganymede and the Jovian magnetosphere. We show that the resulting distortion of the instrument FOV is highly space dependent and varies in shape and intensity from a pixel to another. However, we show that in both environments the complexity of the interactions between measured positive ions and the spacecraft can be decomposed and described as a superposition of a finite number of elementary interactions (i.e., modes). We show that each mode is caused by a specific element of the spacecraft and leads to a characteristic distortion of the instrument FOV. This study constitutes a first step toward necessary spacecraft potential corrections of the measurements performed by JDC. Key Points: The spacecraft potential affects charged particle measurements by distorting the instrument field of viewInteractions between JUpiter ICy moons Explorer spacecraft and Jovian plasma Dynamics and Composition analyzer positive ion measurements are simulatedThe spacecraft potential causes complex and highly geometry dependent distortions, especially for particles with low energy [ABSTRACT FROM AUTHOR]

Jovian magnetospheric plasma irradiates the surface of Ganymede and is postulated to be the primary agent that changes the surface brightness of Ganymede, leading to asymmetries between polar and equatorial regions as well as between the trailing and leading hemispheres. As impinging ions sputter surface constituents as neutrals, ion precipitation patterns can be remotely imaged using the Energetic Neutral Atoms (ENA) measurement technique. Here we calculate the expected sputtered ENA flux from the surface of Ganymede to help interpret future observations by ENA instruments, particularly the Jovian Neutrals Analyzer (JNA) onboard the JUpiter ICy moon Explorer (JUICE) spacecraft. We use sputtering models developed based on laboratory experiments to calculate sputtered fluxes of H2O, O2, and H2. The input ion population used in this study is the result of test particle simulations using electric and magnetic fields from a hybrid simulation of Ganymede's environment. This population includes a thermal component (H+ and O+ from 10 eV to 10 keV) and an energetic component (H+, O++, and S+++ from 10 keV to 10 MeV). We find a global ENA sputtering rate from Ganymede of 1.42 × 1027 s-1, with contributions from H2, O2, and H2O of 34%, 17%, and 49% respectively. We also calculate the energy distribution of sputtered Energetic Neutral Atoms (ENAs), give an estimate of a typical JNA count rate at Ganymede, and investigate latitudinal variations of sputtered fluxes along a simulated orbit track of the JUICE spacecraft. Our results demonstrate the capability of the JNA sensor to remotely map ion precipitation at Ganymede. Plain Language Summary Particles trapped by Jupiter's magnetic field interact with Jupiter's moons. Ganymede, the largest of those moons, lacks a dense atmosphere to protect its surface from these energetic Jovian particles, but Ganymede's magnetic field is strong enough to influence their trajectory: charged particles are deflected away from equatorial regions to polar regions, resulting in uneven particle precipitation patterns at the surface of Ganymede. When ions hit the surface of Ganymede, they eject particles from the surface, in a process referred to as sputtering. Those particles are mostly neutral and therefore unaffected by Ganymede's magnetic fields, so we can image where ions hit the surface of Ganymede by measuring ejected neutral particles. The Jovian Neutrals Analyzer (JNA) will fly onboard the JUpiter ICy moon Explorer (JUICE) spacecraft and will measure sputtered neutrals in the vicinity of Ganymede. To help interpret the data to be collected by JNA, we used models derived from laboratory experiments to simulate what JNA will observe at Ganymede. Our results show that JNA will be able to show us where ions hit the surface of Ganymede, which is important as uneven ion precipitation is thought to explain why Ganymede's poles are brighter than its equatorial regions. [ABSTRACT FROM AUTHOR]

Velocity distribution functions (VDFs) are a key to understanding the interplay between particles and waves in a plasma. Any deviation from an isotropic Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer (MAG) onboard Venus Express, we study proton distributions in the plasma environment of Venus. We focus on the temperature anisotropy, that is, the ratio between the proton temperature perpendicular (T⊥) and parallel (T‖) to the background magnetic field. We calculate average values of T⊥ and T‖ for different spatial areas around Venus. In addition we present spatial maps of the average of the two temperatures and of their average ratio. Our results show that the proton distributions in the solar wind are quite isotropic, while at the bow shock stronger perpendicular than parallel heating makes the downstream VDFs slightly anisotropic (T⊥/T‖ > 1) and possibly unstable to generation of proton cyclotron waves or mirror mode waves. Both wave modes have previously been observed in Venus's magnetosheath. The perpendicular heating is strongest in the near‐subsolar magnetosheath (T⊥/T‖≈3/2), which is also where mirror mode waves are most frequently observed. We believe that the mirror mode waves observed here are indeed generated by the anisotropy. In the magnetotail we observe planetary protons with largely isotropic VDFs, originating from Venus's ionosphere. Key Points: We present maps of the perpendicular and parallel proton temperatures and their ratio in the plasma environment around VenusThe largest perpendicular temperature anisotropy with a median temperature ratio of about 3/2 is found in the near‐subsolar magnetosheathThe region with the largest observed temperature anisotropy coincides with observations of proton cyclotron and mirror mode waves [ABSTRACT FROM AUTHOR]