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Mars lacks an intrinsic magnetic field, allowing the solar wind and its embedded interplanetary magnetic field to directly interact with the planet’s dayside ionosphere. During this interaction, energy and momentum from the solar wind are transferred to ions escaping from the ionosphere. This transfer is governed by the structure of the electric field and current systems induced by the solar wind interaction. In a previous study, we mapped the spatial distribution of the energy transfer rate – i.e. the power exerted by the electric field to accelerate planetary ions – using amitis, a high-performance GPU-based hybrid plasma model. In the present study, we further decompose the contributions of different electric field components in Mars’ induced magnetosphere: the motional, Hall, and ambipolar terms, under typical solar wind and ionospheric conditions. Our results show that the motional electric field dominates the energy transfer process across most of the interaction region via mass loading. In particular, the direct energy transfer between the solar wind and planetary ions via the motional electric field is a fundamental characteristic of the Martian induced magnetosphere. The Hall term becomes significant at the base of planetary ion plumes, where magnetic field lines are highly bent and ion densities are elevated. We also find that the motional term contributes more to the energization of planetary ion species with a higher m/q ratio compared to the Hall term. The ambipolar term is generally weak, but it can transport newborn ions to higher altitudes, where they become subject to other electric field components. We illustrate the contributions of each electric field term by tracing their associated power along the trajectories of test particles.

When the cone angle of the interplanetary magnetic field (IMF) becomes small, induced magnetospheres of unmagnetized planets degenerate. Using hybrid simulations, we study ionospheric ion escape in a 4° cone angle case and compare it with the nominal 55° cone angle (Parker spiral) case. The total escape rate is 1.7×1⁢025 s−1, nearly an order of magnitude higher than the nominal rate of 2.2×1⁢024 s−1. The escape probability is four times higher. The unique feature of the degenerate induced magnetosphere is the upstream escape driven by the ambipolar electric field, contributing 42% to the total escape, a channel absent in the nominal case. Additionally, 52% of escape occurs through a cross-flow plume, formed by the drift of ionospheric ions in the weak convective field and IMF. This channel is dominant, exhibiting an intensity seven times greater than that of the plume driven by the convective electric field in the nominal case.

The plasma package Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) onboard Mars Express has observed ions and electrons accelerated by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar when it operates in its active ionospheric sounding mode. To better study the processes involved, new operational modes for MARSIS have been developed. In the first, a fixed frequency mode, the transmitter does not sweep over a range of frequencies, as normal, but instead transmits pulses at a fixed frequency. This frequency has been chosen to be close to the fundamental frequency of the local ionosphere around the spacecraft, which in all cases is < 350 kHz. Also, an alternating mode has been introduced, where observations in the fixed frequency mode are interleaved with observations that sweep over frequencies in order to investigate how ions are accelerated. Here we describe the new operational modes and present the results of several tests performed in the years 2020, 2021, and 2023.

Spacecraft discharge time constants are calculated from measurements of electron differential flux before and during operation of an ionospheric sounding radar. Determining these time constants provides insight into how the operation of a sounding radar affects the surrounding plasma's interaction with the spacecraft. The analysis is enabled by the fixed-frequency operation mode of a sounding radar which enhances resonant interaction with the ambient plasma. This mode's effect on measured energy spectra of ion and electron fluxes is described. Measurements of electron fluxes disturbed by radar operation serve as input to a model of spacecraft discharge for calculating capacitive discharge time constants. A case study using electron fluxes measured at Mars yields discharge time constants in the range 0.6–0.8 ms and reveals that a residual potential around (Formula presented.) V remains on the spacecraft long after radar operation ceases. The minimum spacecraft potential cannot be determined with these data and model due to the narrow energy range of electrons in the ambient plasma.

Unmagnetized bodies with sufficiently dense ionospheres, such as Mars, form induced magnetospheres when interacting with the solar wind carrying the frozen-in interplanetary magnetic field (IMF). Mars Express equipped with the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) operating for 20 years over two solar cycles made fundamental contributions to our understanding of how the induced magnetosphere of Mars works. ASPERA-3 established the ion escape rate from 2 × 1024 s−1 to 4 × 1024 s−1 depending on the phase of the solar cycle. The measured empirical dependences of the escape rate on the solar wind dynamical pressure and UV fluxes allowed to determine the total atmospheric pressure lost over the past 4 billion years to be on the order of 10 mbars, i.e. small, contrary to long-standing expectations of a strong ion escape process. Comparing the measured escape rates from Mars with Venus and the Earth resulted in formulating the paradigm-shifting statement that the intrinsic magnetic field increases the escape rates and does not protect planetary atmospheres. Due to the long longevity of the mission, ASPERA-3 captured a number of extreme solar weather events and the unique encounter of Mars with the comet Siding-Spring. ASPERA-3 conducted the first-ever energetic neutral atom imaging of an induced magnetosphere, revealing the global periodic variability of the system, the significant precipitation of ENAs originating in the solar wind and magnetosheath, and the enhancement of ENA emissions from the Martian magnetic anomalies. ASPERA-3 conducted studies of the particles responsible for the discrete Martian aurora and characterized the precipitation of solar wind protons and alpha particles onto the atmosphere. The latter turned out to be a significant contribution to the helium balance on Mars. ASPERA-3 made several important findings outside its main science objectives among those are detection of the tentative signatures of backscattered ions from the Phobos surface, investigations for the first time of radar accelerated ions and electrons in non-magnetized environments, and measurements of heliospheric ENAs. Despite the significant progress following outstanding Mars Express results in the field of the Mars – solar wind interaction there is a broad spectrum of unsolved problems and unanswered questions to be addressed by future mission. The most fundamental one is the ionosphere – magnetosphere interactions.

We apply a recently proposed method to estimate heavy ion escape from Mars. The method combines in situ observations with a hybrid plasma model, which treats ions as particles and electrons as a fluid. With this method, we investigate how solar upstream conditions, including solar extreme ultraviolet (EUV) radiation, solar wind dynamic pressure, and interplanetary magnetic field (IMF) strength and cone angle, affect the heavy ion loss. The results indicate that the heavy ion escape rate is greater in high EUV conditions. The escape rate increases with increasing solar wind dynamic pressure, and decreases as the IMF strength increases. The ion escape rate is highest when the solar wind is parallel to the IMF and lowest when they are perpendicular. The plume escape rate decreases when the solar wind convective electric field increases.

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.

The present‐day Venusian atmosphere is dry, yet, in its earlier history a significant amount of water evidently existed. One important water loss process comes from the energy and momentum transfer from the solar wind to the atmospheric particles. Here, we used measurements from the Ion Mass Analyzer onboard Venus Express to derive a relation between the power in the upstream solar wind and the power leaving the atmosphere through oxygen ion escape in the Venusian magnetotail. We find that on average 0.01% of the available power is transferred, and that the percentage decreases as the available energy increases. For Mars the trend is similar, but the efficiency is higher. At Earth, the ion escape does not behave similarly, as the ion escape only increases after a threshold in the available energy is reached. These results indicate that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind.

We investigate the heavy ion density and velocity in the Venusian upper ionosphere near the North Pole, using the Ion Mass Analyzer, a part of the Analyzer of Space Plasmas and Energetic Atoms 4, together with the magnetic field instruments on Venus Express. The measurements were made during June-July 2014, covering the aerobraking campaign with lowered altitude measurements (similar to 130 km). The plasma scale heights are similar to 15 km below 150-km altitude and similar to 200 km at 150-400-km altitude. A clear trend of dusk-to-dawn heavy ion flow across the polar ionosphere was found, with speeds of similar to 2-10 km/s. In addition, the flow has a significant downward radial velocity component. The flow pattern does not depend on the interplanetary magnetic field directions nor the ionospheric magnetization states. Instead, we suggest a thermal pressure gradient between the equatorial and polar terminator regions, induced by the decrease in density between the regions, as the dominant mechanism driving the ion flow. Plain Language Summary We have calculated the ion density and velocities in the Venusian polar ionosphere using measurements from the Ion Mass Analyzer on board the Venus Express spacecraft. During June-July 2014 the periapsis was lowered to similar to 130 km, which allowed for measurements down to low altitudes of the ionosphere near the North Pole. The plasma scale heights are similar to 15 km below 150-km altitude and similar to 200 km at 150-400 km, which is similar to what was found near the equatorial region by the Pioneer Venus mission. In addition, there is a clear trend of dusk-to-dawn flow, along the terminator, for the heavy ions. This is surprising, as a general flow from day-to-night is expected for the Venusian ionosphere due to the long nights and significant heating of the dayside upper atmosphere. The interplanetary magnetic field direction does not appear to affect the ion flow pattern. Instead, we propose a thermal pressure gradient as the dominant accelerating mechanism, induced by the decrease in density from the equator toward the pole.

The solar wind interaction with Ceres is studied for a high water vapor release from its surface using a hybrid model including photoionization. We use a water vapor production rate of 6 kg/s, thought to be due to subsurface sublimation, corresponding to a detection on 6 March 2013 by the Herschel Space Observatory. We present the general morphology of the plasma interactions, both close to Ceres and on a larger scale. Mass loading of water ions causes a magnetic pileup region in front of Ceres, where the solar wind deflects up to 15 ∘ and slows down by 15%. The global plasma interaction with Ceres is not greatly affected by the source location of water vapor nor on gravity, only on the production rate of water vapor. On a global scale, Ceres has a comet-like interaction with the solar wind with observable perturbations farther than 250 Ceres radii downstream of the body.