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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.

We apply a recently proposed method to estimate heavy ion (O(Formula presented) ) escape from Venus, combining in situ spacecraft observations from Venus Express (VEX) with a hybrid plasma model that treats ions as particles and electrons as a fluid. Using this approach, we examine how various upstream solar conditions – including extreme ultraviolet (EUV) radiation, solar wind dynamic pressure, and the strength and cone angle of the interplanetary magnetic field (IMF) – influence heavy ion loss. Our results show that the heavy ion escape rate exhibits no clear dependence on EUV radiation, possibly due to transport-limited processes. Under low EUV conditions, the escape rate increases with solar wind dynamic pressure, whereas no consistent trend is observed under high EUV. The escape rate decreases with increasing IMF strength and is highest when the solar wind flow is aligned with the IMF (i.e. small cone angle), and lowest when the flow and field are perpendicular. In general, some of these findings agree with what has previously been found for Mars, but Venus appears to be less sensitive to solar variability than Mars.

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.

The solar wind interacts with all solar system bodies, inducing different types of dynamics depending on their atmospheric and magnetic environments. We here outline some key open scientific questions related to this interaction, with a focus on the Moon and Mars, that may be addressed by future Mars and Moon missions by the European Space Agency's Human and Robotic Exploration programme. We describe possible studies of plasma interactions with bodies with and without an atmosphere, using multi-point and remote measurements, and energetic particle observations, as well as recommend some actions to take.

We refine a recently presented method to estimate ion escape from non-magnetized planets and apply it to Mars. The method combines in-situ observations and a hybrid plasma model (ions as particles, electrons as a fluid). We use measurements from the Mars Atmosphere and Volatile Evolution (MAVEN) mission and Mars Express (MEX) for one orbit on 2015-03-01. Observed upstream solar wind conditions are used as input to the model. We then vary the total ionospheric ion upflux until the solution fits the observed bow shock location. This solution is a self-consistent approximation of the global Mars-solar wind interaction at this moment, for the given upstream conditions. We can then study global properties, such as the heavy ion escape rate. Here we investigate the effects on escape estimates of assumed ionospheric ion composition, solar wind alpha particle concentration and temperature, solar wind velocity aberration, and solar wind electron temperature. We also study the amount of escape in the ion plume and in the tail of the planet. Here we find that estimates of total heavy ion escape are not very sensitive to the composition of the heavy ions, or the amount and temperature of the solar wind alpha particles. We also find that velocity aberration has a minor influence on escape, but that it is sensitive to the solar wind electron temperature. The plume escape is found to contribute 29 % of the total heavy ion escape, in agreement with observations. Heavier ions have a larger fraction of escape in the plume compared to the tail. We also find that the escape estimates scales inversely with the square root of the atomic mass of the escaping ion specie.

Using Mars Express data from 2007 until 2020 we show how ion outflow from Mars varied over more than a solar cycle, from one solar minimum to another. The data was divided into intervals with a length of one Martian year, starting from 30 April 2007 and ending 13 July 2020. The net escape rate was about 5×10²⁴s⁻¹ in the first covered minimum, and 2−3×10²⁴s⁻¹ in the most recent minimum. Ion escape peaked at 1×10²⁵s⁻¹ during the intervening solar maximum. The outflow is a clear function of the solar cycle, in agreement with previous studies which found a clear relationship between solar EUV flux and ion escape at Mars. The outflow during solar maximum is 2.5 to 3 times higher than in the surrounding solar minima. The average solar wind dynamic pressure over a Martian year was investigated, but does not vary much with the solar cycle. The escape rate at solar maximum is in good agreement with some recent MAVEN studies, and dominated by low energy ions at most sampled locations. A simple linear fit to the data gives a prediction of the escape rate for the much stronger solar maximum during the Phobos mission in 1989 that is consistent with observations. The fit also implies a non-linear response of ion escape for low solar EUV, with a lower initial escape response for lower solar EUV levels than those of the studied data set.

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.