Solar wind measurements by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission provide samples of the heliosphere at 1.38–1.67 AU, and of the upstream conditions that drive numerous processes in the near-Mars plasma environment. We reduce ion measurements from MAVEN’s Solar Wind Ion Analyzer (SWIA), using contextual magnetic field measurements, to 13 independent macroscopic plasma parameters by fitting a convolution of SWIA’s 3-dimensional response function and a superposition of phase-space bi-kappa distribution functions to each measured distribution using an iterative Poisson optimization scheme. This ensemble of parameters represents the solar wind H+ core, H+ beam, and He2+ (alpha) populations, effectively separating each population’s contribution to any measured distribution. Sporadic plasma frequency measurements from MAVEN’s Langmuir Probe and Waves (LPW) instrument are used to calibrate the SWIA measurements such that ion charge densities match LPW-derived electron charge densities. The resulting dataset is effectively ground-truthed, largely corrected for instrumental particularities, and provides a rich timeline of solar wind properties at Mars, including composition, velocities, temperature anisotropies, differential drifts, and degree of thermalization.

Abstract The Martian ionosphere hosts complex current systems driven both by solar wind interactions and neutral wind dynamo processes. While the global patterns of these current systems have been statistically examined, their detailed structures and orientations remain poorly constrained. Using magnetic field observations from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft during the Deep Dip 2 campaign, we resolve the horizontal components of dayside ionospheric currents over regions without strong Crustal Magnetic Fields (CMF). The currents are most prominent between 130 and 150 km altitude, with peak magnitudes of ∼1 μA/m 2 , and are oriented counterclockwise from, and roughly perpendicular to, the draped Interplanetary Magnetic Field in the horizontal plane when viewed from above. Theoretically calculated neutral wind‐driven dynamo currents show significant discrepancies from the derived currents, likely reflecting uncertainties in neutral winds and conductivities. In contrast, simulated magnetohydrodynamic (MHD) ionospheric currents, which are purely solar wind‐induced in the model, share similar features with the derived currents. We conclude that, for the four cases examined here, which were sampled in near‐subsolar regions with weak CMF, the derived ionospheric currents are more consistent with a solar wind‐induced origin, whereas neutral wind‐driven dynamo currents may play only a minor role under these conditions.

Atmospheric escape is an important process that influences the evolution of planetary atmospheres. A variety of physical mechanisms can contribute to escape from an atmosphere, including thermal escape, ion escape, photochemical escape, and sputtering. Here we estimate escape rates via each of these processes for a hypothetical Mars-like exoplanet orbiting Barnard's star (an old, inactive M dwarf star). We place the planet at an orbital distance that receives the same total stellar flux as it does in our solar system. We use the measured stellar extreme ultraviolet (EUV) spectrum and assumptions on the star's magnetic field to determine both the high-energy radiation and the stellar wind environment around the planet. This information is used to model the response of the planet's thermosphere, exosphere and magnetosphere using a variety of models that have been validated against solar system observations. We find overall escape rates that are dominated by thermal processes and elevated by 2-5 orders of magnitude relative to present-day Mars, suggesting that a Mars-like planet orbiting Barnard's star would not retain a significant atmosphere for more than 10's of millions of years. Recently reported planets around Barnard's star should also not have retained significant atmospheres. By extension, Mars-like planets orbiting any M dwarf near the 'Habitable Zone' should not retain atmospheres for extended periods of time.

We investigate the global electric current systems of Mars, a non-magnetized planet with an induced magnetosphere, using a hybrid plasma model.We find that the bow shock currents are primarily confined to planes perpendicular to the interplanetary magnetic field. These currents flow along the bow shock in a direction opposite to the solar wind convective electric field.Regarding current closure, the simulations reveal a system where the bow shock currents close partially through the magnetosheath into the ionosphere. However, a significant portion of the bow shock and magnetotail currents do not close near the planet; instead, they level off and close at far distances, a pattern previously observed in lunar wake current systems. This topology aligns with multi-spacecraft observations at Earth and recent magnetohydrodynamic (MHD) studies of Mars, though it differs from some earlier interpretations of MAVEN data.Additional features identified include: Ionospheric currents that flow along meridians in the direction of the solar wind convective electric field. An ion plume current directed along the convective electric field. Tail currents consisting of two loops enclosing a central current sheet.

Mars was impacted by a series of Interplanetary Coronal Mass Ejections (ICMEs) during a period of intense solar activity in May 2024. A particularly strong ICME arrived on May 17, and its impact was observed by plasma and fields instruments on the Mars Atmosphere and Volatile EvolutioN (MAVEN) and Mars Express (MEX) orbiters. At this time, MEX was well-positioned to observe the upstream solar wind, while MAVEN's orbit had fortuitously precessed to cover the deep induced magnetotail, providing an unusually clear-cut view of escaping atmospheric plasma and the effects of the ICME impact. We compare MAVEN and MEX measurements of solar wind and escaping ions before, during, and after the May 17 ICME impact to investigate how the evolution of the upstream solar wind properties affected the acceleration and resulting fluxes of escaping atmospheric ions. Our analysis of this event reveals non-linear time-dependent effects of the solar wind properties on the ion escape process.

The plasma environment of our neighboring planets, Venus and Mars, differs significantly from Earth’s. Although neither of them possesses a dominant intrinsic dipolar magnetic field, there are still induced magnetospheres forming around the two planets, due to the interaction of the solar wind and the interplanetary magnetic field (IMF) with their conductive ionospheres, exospheres, and the localized crustal magnetic fields in the case of Mars. Induced magnetospheres, their associated plasma environments, and the physical processes within them are particularly susceptible to the changing upstream conditions. The increasing number of successful and long-lived missions during the last few decades has been key for describing the fundamental structures and processes comprising the induced magnetospheres of the two planets. Nevertheless, their induced magnetotails have been more challenging to probe, due to the restrictions of the orbital geometry of planetary missions. Here, we present the latest discoveries and a comprehensive comparison between the Venusian and Martian induced magnetotails, and we highlight the need for further exploration of these regions. Atmospheric escape and energy transfer processes and paths are inextricably linked with the climate history and the disappearance of water at Mars, though there are many unknowns still in the case of Venus. Past and current missions utilizing particle and fields instruments have explored a great part of the plasma environments of Venus and Mars. Several plasma boundaries, shaped by both internal and external factors, divide the planetary environments and magnetospheres into different plasma regimes and have been described by observations and models. Simulations and observations have also been utilized to investigate the magnetotail structure of the two planets, which appears to be governed mainly by the IMF, the solar wind dynamic pressure, and the crustal magnetic fields in the case of Mars. At Mars, the presence of the crustal magnetic fields, which are regions of crustal magnetization on the surface of the planet clustered mostly in the southern hemisphere, further complicates the interaction of the solar wind and the IMF with the planet’s plasma environment, thus justifying the term ‘hybrid’ – instead of induced – that is often used to describe the Martian magnetosphere. The existence of a magnetotail twist, as well as a first approach on mapping the structure of the current systems, has been reported at Mars, whereas different types of magnetotail current sheet flapping motion have been observed at both Mars and Venus. The magnetic topology, which describes the morphology of closed, open, and draped magnetic field lines over a planet, has also been inferred and explained for both planets. Escape processes, escape rates and their response to space weather have been reported, and we now have a better idea of the differences between the two planets. Escaping structures, contributing with a bulk removal of plasma, have also been observed in their magnetotails. Nevertheless, much still remains unknown, for example the specifics of how individual processes respond to solar drivers. Mars and Venus are not the only solar system bodies with no global intrinsic magnetic field. Induced magnetotails are formed around Saturn’s moon Titan and comets too. A comparison between those bodies and Venus and Mars will provide a broader and general picture of induced and hybrid magnetotails, which could help future investigations of the plasma environments and tails of exoplanets. Lastly, in this review paper, we also summarize the questions that remain unanswered, emphasizing the need for future missions.

At planets that possess strong dipole magnetic fields, the Zwan-Wolf effect acts to squeeze plasma along magnetic flux tubes, aiding in the deflection of the solar wind flow about the planet. While the effect has been most studied at Earth, candidate observations have also been made at the outer planets. Here we present observations of the Zwan-Wolf effect occurring at Mars, an unmagnetized planet that lacks a dipole magnetic field. Our analysis of observations made by NASA's Mars Atmosphere and Volatile EvolutioN spacecraft suggest that while the Zwan-Wolf effect is likely continuously active within the Martian ionosphere, it operates below detection thresholds of typical plasma analyzers most of the time. However, an interplanetary coronal mass ejection impact at Mars in December 2023 greatly enhanced the Zwan-Wolf effect within the ionosphere, allowing it to be observed, and highlighting the importance of space weather events for these unmagnetized planetary systems.

On 13 March 2023, when the Parker Solar Probe was situated on the far side of the Sun as seen from Earth, a large solar eruption took place creating a strong solar energetic particle (SEP) event observed by multiple spacecraft (S/C). The energetic event was observed at six well-separated locations: Parker Solar Probe, Solar Orbiter, BepiColombo, STEREO A, near-Earth S/C, and MAVEN. An in-situ shock crossing and a related energetic storm particle (ESP) event were observed at all inner-heliospheric S/C, suggesting that the interplanetary coronal mass ejection (CME)-driven shock extended all around the Sun. However, the solar event was accompanied by a series of pre-event CMEs. We aim to characterize this extreme widespread SEP event and to provide an explanation for the unusual observation of a circumsolar interplanetary shock and corresponding circumsolar ESP event. We analyse data from seven space missions to characterize the solar eruption at the Sun, the energetic particle event, and the interplanetary context at each observer location as well as the magnetic connectivity of each observer to the Sun. We employ magnetohydrodynamic simulations of the solar wind in which we inject various CMEs that were launched before as well as contemporaneously with the solar eruption under study. In particular, we test two different scenarios that could have produced the observed global ESP event: 1) a single circumsolar blast-wave-like shock launched by the associated solar eruption, and 2) the combination of multiple CMEs driving shocks into different directions. By comparing the simulations of the two scenarios with observations we find that both settings are able to explain the observations. However, the blast-wave scenario performs slightly better in terms of the predicted shock arrival times at the various observers.

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 , nearly an order of magnitude higher than the nominal rate of . 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.

Mars, being a small planet with a tenuous atmosphere, does not have a sharp boundary between regions dominated by solar wind plasma and planetary plasma. Instead, this transition is typically extended, allowing the interplanetary magnetic field (IMF) to penetrate into the Martian ionosphere. However, the depth of this penetration is not well understood. Using 6 years of MAVEN data, we statistically assess locations where a transition exists between the dominance of magnetic versus cold (< 1 eV), thermal plasma pressure to better understand the reach of the IMF. We identify the presence or absence of pressure transitions from 200 to 800 km altitude for each MAVEN orbit and find a clear transition in similar to 55% of cases. The pressure transition locations are mapped in different coordinate systems that provide insight into the solar and planetary driving conditions that cause a detected transition region. Transitions are more likely to occur under weak-to-nominal solar wind conditions, away from strong crustal magnetic fields, near the terminator, on the dusk side of the planet compared to the dawn side, and in the negative solar wind motional electric field hemisphere. We speculate on possible causes for asymmetries that arise in the mapped locations of these pressure transitions and the effect that penetrated IMF may have on driving plasma dynamics in the Martian ionosphere.

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.

This study combines the observations of the Venus Express Venus Radio science experiment (VeRa), Mars Express Mars Radio Science experiment (MaRS), and MAVEN Radio Occultation Science Experiment (ROSE) at Venus and Mars with proxies for environmental parameters, insolation and solar wind characteristics to compare the effects of individual drivers on the variability of the topside ionospheres of both planets.The average extent and variability of the ionospheric topside of both planets decrease with increasing solar wind dynamic pressure. On Venus, the average altitude and variability of the ionospheric topside increase with rising solar EUV irradiation. On Mars, the extent of the ionospheric topside on the northern hemisphere under very low solar wind conditions is similarly affected by solar EUV, while the southern hemisphere is influenced by the presence of strong crustal magnetic fields and heliocentric distance. The relationship between the ionosphere on the southern hemisphere and heliocentric distance indicates that increased solar irradiation and atmospheric dust affect the extent of the ionospheric topside during southern summer.A wide variety of ionospheric topside features, including traditional ionopause gradients, are observed in low noise level and near-vertical electron density profiles on the ionospheric dayside of both planets. All topside structures observed in the Venus ionosphere during the VEX mission are also seen at Mars. This suggests that despite the influence of unique drivers on each planet's ionosphere, the overall interaction of Mars with the solar wind shows many similarities to that of Venus during a weak solar cycle.

Although Venus appears to present a predominantly ionospheric obstacle to the solar wind, the magnetic connectivity between the solar wind and the Venus ionosphere, or magnetic topology, is important for characterizing the Venus space environment. In particular, magnetic connectivity is relevant to the magnetization state of the ionosphere, particle precipitation into the atmosphere causing ionization and auroral emissions, and planetary ion escape at Venus. The spatial distributions of different magnetic topologies were statistically analyzed, with some unexpected results. Here, we build on those results by investigating how the external factors of solar cycle phase and upstream conditions affect the occurrence rates of the three magnetic topologies and consider their implications regarding the state of Venus's induced magnetosphere. We find that both the solar cycle phase and upstream dynamic pressure variations control its expansion or contraction. Under solar minimum conditions, the interplanetary magnetic field (IMF) more deeply penetrates into the collisional atmosphere, increasing the occurrence rates of open and closed topologies at low altitudes and in Venus's wake. We also find hemispheric differences in the occurrences of dayside‐connected and nightside‐connected open fields, likely related to mass loading of the near‐Venus plasma environment by planetary pickup ions.

Atmospheric ion escape plays a crucial role in the evolution of planetary climate and habitability. While Mars has been the focus of extensive in-situ spacecraft observations, our understanding of ion escape at Mars has been constrained by single-point spacecraft measurements, which fail to distinguish spatial and temporal variability. Observations from NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission and China's Tianwen-1 mission provide complementary observations the Martian space environment and a unique opportunity to study the variability of ion escape. Here, we report that ion escape at Mars exhibits unexpected spatial-temporal variability under steady and weak external solar wind conditions. In the hemisphere where the solar wind electric field is directed toward the planet, a condition that usually hinders ion escape into space, we instead observe the transient appearance of escaping planetary ions with high energies and strong escape fluxes. This finding underscores that planetary ion escape can be unsteady and dynamic, even under stable external conditions.

We present Mars Express (MEX) observations of heavy pickup ions (HPUI) at Mars and a new method to derive interplanetary magnetic field (IMF) properties. The MEX HPUI measurements were organized using the upstream IMF directions measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Since these HPUIs are mostly accelerated by the electric field in the upstream and magnetosheath regions, the IMF component perpendicular to the solar wind, both directions and magnitudes, can be derived from the HPUI flux directions, energies, and positions measured by MEX. The derived IMF properties were compared with the simultaneous IMF measurements by MAVEN, which showed good agreements with each other with the accuracy comparable to most previous IMF proxies based on MAVEN data. This new IMF proxy does not require any magnetic field data and hence can be applied to the long time period prior to the MAVEN mission.

The interaction between the solar wind and Mars plays a crucial role in driving atmospheric escape and shaping the Martian space environment. Despite its importance, the electromagnetic energy transport and conversion associated with this interaction remain poorly characterized. In this study, we construct global maps of electromagnetic energy transport and conversion at Mars using 9 years of magnetic field and plasma data from NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Our results reveal that the bow shock serves as an electromagnetic generator, converting the kinetic energy of the solar wind into electromagnetic energy. In contrast, the induced magnetotail acts as a load region, where electromagnetic energy is converted back into particle energy. The magnetosheath exhibits a spatially variable role: it functions as a generator in regions where the interplanetary magnetic field (IMF) is draped around the planet but transitions into a load region as the draped fields are diverted toward the magnetic poles. Planetary oxygen ions are persistently energized throughout the system, with particularly strong energization observed in the ion plume region. We also identify a pronounced hemispheric asymmetry in energy transport and conversion. While our qualitative results are robust, the quantitative analysis reveals an imbalance between the calculated energy transport and conversion terms. This discrepancy suggests that unresolved small-scale electric fields or currents and nonlinear processes may be missing from the present analysis, underscoring the need for future high-resolution, multi-point observations to better constrain the electromagnetic energy budget and enhance our understanding of the Martian space environment.

Fig. 1: Topside ionospheres of Venus and Mars as seen by (a) VEX VeRa, (b) MEX MaRS and (c) MAVEN ROSE.radio science.While the orbital and environmental parameters of Venus and Mars show significant differences, their planetary ionospheres show notable similarities (Figure 1). The photochemically dominated regions of the undisturbed dayside ionospheres of Venus and Mars are both characterized by two major features. The ionospheric main peak region (V2 at Venus, M2 at Mars) is a result from photoionization by solar EUV irradiation. The weaker secondary V1/M1 region originates from the primary and secondary ionization of the neutral atmosphere caused by solar X-ray radiation. The upper region of the Venus and Mars dayside ionospheres is governed by transport processes.The extent and shape of the ionospheric topsides is observed with the radio science experiments Venus Express Radio Science (VeRa) onboard Venus Express (VEX) [1] at Venus and Mars Express Radio Science (MaRS) [2] onboard Mars Express (MEX) and the Radio Occultation Science Experiment (ROSE) onboard the MAVEN spacecraft [3] at Mars. The observed electron density exhibits substantial variability on temporal scales and ranges from an undisturbed exponential decay (Fig. 1b) to strongly compressed shapes (Fig. 1a, c). This work combines 9 years of VEX-VeRa (2006-2014), 18 years of MEX-MaRS (2004-2021) and 8 years of MAVEN-ROSE (2014-2021) radio occultation observations to investigate the variability of the topside ionospheres of Venus and Mars on the planetary dayside. The derived ionospheric characteristics will be compared to accompanying observations of the solar wind dynamic pressure (from VEX-ASPERA4 [4], MEX-ASPERA3 [5, 6] and MAVEN instruments [7]), solar irradiation flux (FISM-V2 model [8], MAVEN EUV monitor [9]) and a model of the crustal magnetic field for Mars [10] to improve our understanding of the solar wind interaction of planets without a global magnetic field.References[1] Häusler et al. (2006) PSS 54 (13-14)[2] Pätzold et al. (2004) in Fletcher (2004) Mars Express. The scientific payload. ESA[3] Withers et al. (2018) JGR Space Phys. 123 (5)[4] Barabash et al. (2007) PSS 55 (12)[5] Barabash et al. (2006) SSR 126[6] Ramstad et al. (2015) JGR Planets 120 (7)[7] Halekas et al. (2015) SSR 195[8] Chamberlin et al. (2020) Space Weather 18 (12)[9] Thiemann et al. (2017) JGR Space Physics 122 (3)[10] Morschhauser et al. (2014) JGR Planets 119 (6)

We examine the newly discovered phenomena of sinuous aurora on the nightside of Mars, using images of 130.4 and 135.6 nm oxygen emission measured by the Emirates Mars Mission EMUS ultraviolet spectrograph, and upstream measurements from the MAVEN and Mars Express spacecraft. They are detected in similar to 3% of observations, totaling 73 clear detections. These emissions are narrow, elongated (1,000-6,000 km), cross Mars' UV terminator, and are oriented generally toward the anti-solar point, clustering into north, south, east, and west-oriented groups. Diverse morphologies are observed, though some spatial features, such as broad curves, may in some cases be due to temporal aliasing of aurora motion as each image is built up over 15-20 min. Sinuous aurora form away from Mars' strongest crustal magnetic fields and can be interrupted by moderate crustal fields. Sinuous aurora occurrence increases strongly with solar wind pressure, though brightness shows only a weak positive dependence on pressure. Interplanetary magnetic field (IMF) clock angle affects their occurrence and orientation: sinuous aurora show a broad range of orientations centered on the solar wind convection electric field (Econv) direction and forming in the +Econv hemisphere, although with moderate clockwise and counterclockwise average "twists" for westward and eastward IMF, respectively. From these features we infer a link between sinuous aurora and electron energization in Mars' magnetotail current sheet, where field geometry on the +Econv side of the sheet is more organized and symmetric. Determination of specific triggering conditions for sinuous aurora requires further investigation. Sinuous aurora are narrow, extended patterns of UV emission caused by long, thin channels of energized electrons striking Mars' nightside upper atmosphere. We study images of these aurora taken by the Emirates Mars Mission EMUS instrument. 73 cases of sinuous auroras were found (similar to 3% occurrence rate) with lengths ranging from 1,000 to 6,000 km. These auroras usually cross Mars' day-night boundary and extend in the direction opposite to the Sun. They tend to cluster into groups oriented toward the north, south, east, and west directions with a diverse array of shapes. Sinuous aurora generally form away from Mars' strongest crustal magnetic fields. They occur more frequently for higher solar wind pressure. Their orientations are affected by the interplanetary magnetic field (IMF), displaying a broad range of orientations centered on the direction of the electric field in the solar wind, and forming in the hemisphere to which this electric field points, although with moderate counterclockwise and clockwise average "twists" for eastward and westward IMF, respectively. From these features we infer a link between sinuous aurora and a sheet of current in Mars magnetic tail, wherein the aurora-causing electrons may be energized before falling into the upper atmosphere to produce aurora. These narrow emission features form away from strong crustal fields, oriented anti-sunward, cross the terminator, are detected in 3% observations Occurrence increases with solar wind pressure, certain interplanetary magnetic field orientations, and in the positive motional electric field hemisphere Sinuous aurora may be related to magnetotail asymmetry and electron energization processes occurring in the tail current sheet

We explore the ability of an unmagnetized planet to retain an atmosphere as a function of its radius. We use a particle-in-cell hybrid code to simulate the global plasma interaction of unmagnetized terrestrial planets at 1 au under average solar wind conditions. We vary the radius of the planet (R-p) from Mars-sized (3390 km) to super-Earth-sized (9390 km). We inject hydrogen and oxygen ion outflows from the ionosphere and quantify how the ion escape, recirculation, solar wind deposition, and net atmospheric mass flux vary as a function of planetary radius. We find that as the radius and the corresponding ionospheric outflow rate are varied, the fraction of outflowing H+ that escapes remains at 15.5 +/- 1.0 percent, while the rest recirculates back towards the planet. The fraction of produced O+ that escapes from a Mars-sized planet is 27 +/- 1 percent, and decreases to 7 +/- 1 percent for super-Earth, suggesting that smaller planets are less able to retain heavy ions. We find, however, that larger planets have lower solar wind deposition fractions because their bow shocks are at greater distances from the surface of the planet. The ionospheric outflow rate at which mass deposition is equal to mass escape is found to be proportional to R-p(2). Lastly, we propose that the bulk gyration of the solar wind at the induced magnetosphere can lead to differential escape trajectories of light and heavy ions.

When the cone angle of the solar interplanetary magnetic field (IMF) becomes small, induced magnetospheres of unmagnetized planets degenerate, resulting in a markedly different mode of the interaction. In this case, solar wind protons penetrate all the way to the top of the atmosphere on the dayside of Mars. Ions from the ionosphere propagate upstream in the solar wind, toward the Sun, with a substantial flow perpendicular to the solar wind flow. We investigate the ionospheric ion escape from such an object. This study specifically concentrates on hybrid simulations of the ionospheric ion escape from Mars in the case of the 4° cone angle with the other solar wind conditions typical. The total escape rate is found to be almost one order of magnitude higher than for the typical Parker spiral case. The unique feature of the degenerate induced magnetosphere is the upstream escape driven by the ambipolar field, contributing 42% to the total escape rate, fully absent in the Parker spiral case. Additionally, 52% of the total escape occurs through the cross-flow plume, arising from the drift of ionospheric ions in the weak convective field and IMF. This channel dominates and is seven times more intense than the plume driven by the convective field in the nominal case. Understanding how degenerate magnetospheres operate is important not only for the planets in the solar system, but also for exoplanets.