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As the solar wind reaches Mars, planetary ions mass-load the flow, slowing it down and creating a bow shock upstream of the planet. The convective electric field, coming from the solar wind flow and solar wind magnetic field, results in a potential difference across the conducting ionosphere that in turn results in induction currents flowing through the conductor (unipolar induction). The magnetic fields associated with the induction currents cancel (or reduce due to finite conductivity) the magnetic field inside the ionosphere (Lenz’s law). Above the ionosphere, the induced fields act on the solar wind plasma by deviating it, thus a void called an induced magnetosphere is created. Without a global magnetic field, Mars’ atmosphere is eroded by the solar wind, an ongoing atmospheric escape that has significantly influenced its climatic evolution. For present Mars, the dominant escape of atmospheric neutrals is through four channels: Jeans escape, photochemical reactions, sputtering and electron impact ionization, while ions above the exobase are accelerated by the solar wind convective electric field to escape.

In this study, we introduce a new method for estimating heavy ion (O+, O+2, and CO+2) escape rates from Mars, combining a hybrid plasma model with observational data. We use observed upstream solar wind parameters as input for a hybrid plasma model, where the total ion upflux at the exobase is a free parameter. We then vary this ion upflux to find the best fit to the observed bow shock location. This method gives us a self-consistent description of the Mars-solar wind interaction, which enables broader analyses of the interaction’s properties, beyond just escape.

We investigate the influence of external factors, solar EUV radiation, solar wind dynamic pressure, interplanetary magnetic field (IMF) strength, and IMF cone angle on Martian heavy ion escape. Our results reveal that ion escape increases with stronger EUV radiation and solar wind dynamic pressure, but decreases with a higher IMF strength and cone angle. In an extreme case study when the solar wind flowis nearly aligned with the solar IMF, the induced magnetosphere of Mars degenerates, and the bow shock on the dayside disappears, ions flowing towards the sun are accelerated by the ambipolar field, and a large-scale E×B cross-flow structure forms, dramatically increasing ionescape. We therefore call this type of interaction a degenerate induced magnetosphere. Finally, we compare the interactions of Mars and Venus in response to similar solar wind conditions, finding significant similarities in their responses as unmagnetized planets, further informing our understanding of atmospheric escape and solar wind interactions with unmagnetized bodies. 

Using over eight years of Mars Atmosphere and Volatile EvolutioN (MAVEN) data, from November 2014 to May 2023, we have investigated the Martian nightside ionospheric magnetic field distribution under the influence of upstream solar wind drivers, including the interplanetary magnetic field intensity (|BIMF|), solar wind dynamic pressure (PSW), solar extreme ultraviolet flux (EUV), and Martian seasons (Ls). Our analysis reveals pronounced correlations between magnetic field residuals and both |BIMF| and PSW. Correlations observed with EUV flux and Ls were weaker — notably, magnetic field residuals increased during periods of high EUV flux and at Mars perihelion. We find that the IMF penetrates to an altitude of 200 km under a wide range of upstream conditions, penetrating notably deeper under high |BIMF| and PSW conditions. Our analysis also indicates that EUV flux and IMF cone angle have minimal impact on IMF penetration depth. Those findings provide useful constraints on the dynamic nature of Martian atmospheric escape processes and their evolution, suggesting that historical solar wind conditions may have facilitated deeper IMF penetration and higher rates of ionospheric escape than are observed now. Moreover, by establishing criteria for magnetic ‘quiet’ conditions, this study offers new insights into the planet’s magnetic environment under varying solar wind influences, knowledge that should help refine models of the Martian crustal magnetic field.

The interaction between planets and stellar winds can lead to atmospheric loss and is, thus, important for the evolution of planetary atmospheres¹. The planets in our Solar System typically interact with the solar wind, whose velocity is at a large angle to the embedded stellar magnetic field. For planets without an intrinsic magnetic field, this interaction creates an induced magnetosphere and a bow shock in front of the planet². However, when the angle between the solar wind velocity and the solar wind magnetic field (cone angle) is small, the interaction is very different³. Here we show that when the cone angle is small at Mars, the induced magnetosphere degenerates. There is no shock on the dayside, only weak flank shocks. A cross-flow plume appears and the ambipolar field drives planetary ions upstream. Hybrid simulations with a 4° cone angle show agreement with observations by the Mars Atmosphere and Volatile Evolution mission⁴ and Mars Express⁵. Degenerate, induced magnetospheres are complex and not yet explored objects. It remains to be studied what the secondary effects are on processes like atmospheric loss through ion escape.

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.

When the solar wind reaches the Mars obstacle, mass loading by planetary ions slows down the solar wind and raises the bow shock. The Martian atmosphere is undergoing the a scavenging by the solar wind without the protection of a global magnetic field. Atmospheric escape is an important process for the evolution of the Martian climate. For present Mars, the dominant escape of atmospheric neutrals is through four channels: Jeans escape, photochemical reactions, sputtering and electron impact ionization. Ions above the exobase get accelerated by the solar wind electric field and can escape.

We here apply a new method for estimating heavy ion (O⁺, O⁺₂, and CO⁺₂) escape rates at Mars, which combines a hybrid model and observations. We use observed upstream solar wind parameters as input for a hybrid plasma model, where the total ion upflux at the exobase is a free parameter. We then vary this ion upflux to find the best fit to the observed bow shock location. This method gives us a self-consistent description of the Mars-solar wind interaction, which can be used to study other properties of the solar wind interaction besides escape.

När solvinden stöter på Mars så tyngs den ner av joner från planeten, vilket bromsar solvinden och expanderar bogshocken. Mars atmosfär eroderas av solvinden eftersom planeten saknar ett globalt magnetfält. Atmosfärsförlust är en viktig process i hur Mars klimat förändras. För nuvarande Mars är det fyra dominerande processer för förlust av neutrala atomer: Jeans förlust, fotokemiska reaktioner, sputtering och elektronkollisionsjonisering. Joner ovan exobasen accelereras av solvinden och kan förloras. Här använder vi en ny metod för att uppskatta förlusten av tunga joner (O⁺, O⁺₂ , and CO⁺₂) vid Mars, som kombinerar en hybridmodell och observationer. Vi använder observerade solvindsparametrar som indata till en hybrid plasmamodell, där totalt jonuppflöde vid exobasen är en fri parameter. Vi varierar sedan detta jonuppflöde för att hitta bästa passningen till den observerade positionen för bogshocken. Metoden ger en självkonsistent beskrivning av Mars växelverkan med solvinden, som kan användas till att studera andra egenskaper av växelverkan, förutom jonförlust.

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.