Search

The impact of solar flares on the upper atmosphere of Mars presents an opportune environment for investigating the ionosphere's response to the rapid and high‐energy dynamics of our Sun, with implications for characterizing atmospheric escape and evolution. The ionosphere of Mars is strongly influenced both by the levels of ionizing solar irradiance it receives and the associated behavior of the neutral thermosphere within which it is embedded. Here, we characterize density and composition changes in the topside ionosphere of Mars due to the X8.2‐class solar flare on 10 September 2017 from MAVEN spacecraft observations roughly 90 min after the onset of flare irradiance at Mars, by which time the neutral thermosphere had experienced significant thermal expansion. By contrasting altitude and pressure‐based representations of the ionospheric changes during the flare with the neutral thermospheric changes in both representations and basic photochemical expectation, we interpret and quantify the ionospheric behavior through a lesser explored lens of the neutral thermosphere's response to a flare. We find that the high‐altitude ionospheric density enhancements during this flare are larger than many identified from other flares, and that the neutral thermosphere's expansion accounts for a majority of the cumulative ion density changes observed. The ion density and compositional changes exhibit corresponding behavior with the neutral thermosphere's changes from both thermal expansion and photochemical effects. The identified characteristics of the ionosphere's response to a solar flare provide observational benchmarks for assessing and aiding numerical modeling efforts in accurately reproducing the behavior of the environment of Mars. Key Points: At high altitudes, ion density as much as doubles due to this solar flare in contrast with the little change identified for other flaresIon species exhibit flare‐driven changes unique both from one another and between altitude and pressure‐based representationsThermal expansion of the thermosphere accounts for the majority of ion density and composition changes during these flare event observations [ABSTRACT FROM AUTHOR]

Unusually large peak electron density values of ∼5 × 1010 m−3 are sporadically observed in the nightside ionosphere of Mars in regions of strong and open crustal magnetic field. However, the associated vertical structure of the ionosphere has not previously been observed. Here, we present three vertical electron density profiles from the nightside ionosphere of Mars that have comparably large values of peak electron density. They were acquired by the Radio Occultation Science Experiment on the MAVEN spacecraft on 23, 24, and 29 March 2021. In these profiles, the peak density is large (2–4 × 1010 m−3), the peak altitude is low (60–90 km), and the plasma layer is broad (full width at half maximum greater than 100 km, Chapman layer lengthscale of ∼30 km). Even if the peak altitude is at the upper end of this range, 90 km, the plasma density at 60 km would still be large, approximately 70% of the peak density. These findings hold even if the ionosphere was patchy, not spherically symmetric, during these observations. These three profiles were acquired at times when fluxes of energetic protons, but not electrons, were enhanced above background levels. The apparent association of large nightside electron density values with protons, rather than electrons, is contrary to previous findings. Key Points: Nightside ionospheric layers with peak density of 2–4 × 1010 m−3, peak altitude of 60–90 km, and thicknesses >100 km are seen on MarsThese layers are seen in association with enhanced fluxes of energetic protons, but not electronsThese findings hold if the ionosphere is patchy, not spherically symmetric, over the observed region [ABSTRACT FROM AUTHOR]

Discrete aurorae have been observed at Mars by multiple spacecraft, including Mars Express, Mars Atmosphere and Volatile EvolutioN (MAVEN), and most recently the United Arab Emirates Hope spacecraft. Meanwhile, there have been studies on the source particles responsible for producing these detectable aurorae (termed "auroral electrons"). By utilizing empirical criteria to select auroral electrons established by Xu et al. (2022, https://doi.org/10.1029/2022GL097757), we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and fluxes of auroral electrons. We find the occurrence rate increases with upstream dynamic pressure and weakly depends on the interplanetary magnetic field strength. Meanwhile, the integrated auroral electron flux somewhat decreases with increasing upstream drivers. Auroral electron precipitation also occurs more frequently and is more intense over regions of strong crustal fields compared to weak crustal fields. Aside from emissions, auroral electrons are expected to cause significant impact ionization and enhance the plasma density locally. In this study, we also quantify the nightside ionospheric impact of auroral electron precipitation, specifically the thermal ion (O+, O2+ ${O}_{2}^{+}$, and CO2+ $C{O}_{2}^{+}$) density enhancement, with MAVEN observations. Our results show that the ion density is increased by up to an order of magnitude at low altitudes. The crustal effects on ion density profiles for nominal electron and auroral electron precipitation are also discussed. Plain Language Summary: Localized auroral emissions have been observed at Mars by multiple spacecraft, including Mars Express, Mars Atmosphere and Volatile EvolutioN (MAVEN), and most recently the United Arab Emirates Hope spacecraft. Meanwhile, there have been studies on the source particles responsible for producing these detectable aurorae, termed "auroral electrons." By utilizing previously established criteria to select auroral electrons, we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and also the intensity of auroral electrons. In the meantime, auroral emission is not the sole effect of electrons impacting the collisional atmosphere, but also an enhanced local ionization. Using both MAVEN observations, we show that the ion density is increased by up to an order of magnitude at low altitudes comparing auroral electron and nonauroral electron events. Key Points: The occurrence rates of auroral electrons increase but their integrated energy fluxes somewhat decrease with increasing upstream driversWe quantify the planetary ion density enhancement resulting from auroral electron precipitation to be up to an order of magnitudeAuroral electron precipitation occurs more frequently and is more intense over strong crustal fields than weak crustal fields [ABSTRACT FROM AUTHOR]

Previous missions underlined the complex influence of the crustal magnetic fields on the Martian environment, including the plasma boundaries. Their influence on the bow shock is however poorly constrained, with most studies showing North/South differences attributed to the crustal fields, with various conclusions from little to strong variabilities. We analyze for the first time in detail the influence of crustal fields on the Martian shock location based on a multi‐mission analysis (MAVEN and MEX). We introduce the angular distance to the strongest crustal field region in the southern hemisphere that induces the largest influence (but not unique, with a minimum pressure threshold analyzed). Its impact is at large scale (>40–60° around), is modulated by the local time of the strongest source region (with no influence beyond terminator), and maximizes when the Interplanetary Magnetic field (IMF) is stable during the preceding hours. We introduce a technique, that is, partial correlations, to provide a coherent picture for both MAVEN/MEX due to existing cross correlations with Extreme UltraViolet (EUV). A composite parameter is proposed, that represents the combined influence of EUV, magnetosonic mach number (two major drivers) and crustal fields, the latter having an impact of hundreds of km. The influence of crustal fields on the shock appears seasonal and correlated with the Total Electronic Content, revealing a large scale coupling between the crustal fields, the ionosphere and the shock. The crustal field influence on the shock is thus significant and complex, with a coupling to both the ionosphere below and the IMF above. Plain Language Summary: The Mars Express (MEX) and Mars Atmosphere and Volatile Evolution (MAVEN) observations underlined the complex interaction between the solar wind plasma flowing out from the Sun and the martian environment. The supersonic flow creates a bow shock upstream of the planet, before it is deflected around the conductive ionized atmosphere of the planet that acts as an obstacle to the flow. Contrary to Earth, Mars lost its intrinsic global dynamo magnetic field that could protect efficiently the atmosphere from the incident solar wind sputtering, thus eroding the atmosphere with time. However, the planet possesses remnant crustal magnetic fields locked up in its crust, in particular in the southern hemisphere, that are known to play a major role in the martian interaction with the incident solar wind. We investigate here the influence of these crustal fields on the martian bow shock location, and show they are a significant driver according to MAVEN and MEX observations, despite being less important than the solar Extreme UltraViolet fluxes or magnetosonic mach number of the solar wind. We also show that that this influence varies with season, revealing a strong coupling between the crustal fields, the ionosphere and the bow shock location. Key Points: The influence of crustal magnetic fields on the Martian shock is significant based on the first multi‐mission studyThe strongest crustal field region has a major influence in a large angular range, when close to noon and when the interplanetary magnetic field is stableThe crustal field influence varies with season, showing a coupling between crustal fields, the ionosphere and the shock [ABSTRACT FROM AUTHOR]

This paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3‐D time‐dependent magnetohydrodynamic model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform eight simulation cases under solar minimum quiet‐solar‐wind conditions (four equinox/solstice seasons, each with two interplanetary magnetic field polarities) and in an actual interplanetary coronal mass ejection (ICME) case to assess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that (a) the ionosphere is magnetically connected with high altitudes through open field lines and (b) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small‐scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is ∼0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. Modeling shows the occurrence probability dramatically increases during the ICME event, particularly by more than an order of magnitude in weak crustal field regions. Our model results reasonably agree with NASA Mars Atmosphere and Volatile EvolutioN and Mars Express observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp‐like process but due to the recycling of nightside magnetospheric electrons. Plain Language Summary: This paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora at Mars on a planetary scale. Discrete aurora is a patchy and sporadic phenomenon, posing a daunting challenge for understanding how and where auroral electron precipitation takes place in the ionosphere. To address this challenge, we apply a state‐of‐the‐art global model to characterize the spatial and temporal dynamics of near‐Mars plasma and magnetic field environments. We simulate the penetration of energetic electrons from high altitudes to the nightside ionosphere through open magnetic field lines, a process responsible for auroral emissions. Our results show that during quiet solar activity, the overall chance of observing a discrete auroral event on the nightside is only ∼0.77% across the globe, but the odds are significantly higher over strong crustal field regions. The occurrence probability is enhanced globally during an extreme space weather event, especially in weak crustal field regions. This study reveals a different source mechanism from the prevailing hypothesis that discrete aurora may be excited by the direct entry of magnetosheath plasma in a cusp‐like process. Instead, we propose that it is caused by the recycling of nightside magnetospheric electrons that return to the ionosphere. Key Points: Modeling shows that discrete aurora occurs more likely on small‐scale patches embedded inside strong crustal field regionsAuroral occurrence probability greatly increases during space weather events, particularly in weak crustal field regionsDiscrete aurora is not caused by the direct entry of magnetosheath plasma but due to the recycling of nightside magnetospheric electrons [ABSTRACT FROM AUTHOR]

We present the first long‐term characterization of ionization layers in the lower ionosphere of Mars (below ∼90 km), a region inaccessible to orbital in‐situ observations, based on an analysis of radar echo blackouts observed on Mars Express and the Mars Reconnaissance Orbiter from 2006 to 2017. A blackout occurs when the expected surface reflection is partly or totally attenuated for portions of an observation. Enhanced ionization at altitudes of 60–90 km, below the main ionospheric electron density peak, leads to increased absorption of the radar signal, resulting in the blackouts. We find that (a) MARSIS, operating at frequencies between 1.8 and 5 MHz, suffered more blackouts than SHARAD, which has a higher carrier frequency (20 MHz), (b) there is a clear correlation of blackout occurrence with solar cycle, (c) there is no apparent relationship between blackout occurrence and crustal magnetic fields, and (d) blackouts occur during both nightside and dayside observations, although the peak occurrence is deep on the nightside. Analysis of Mars Atmosphere and Volatile EvolutioN Solar Energetic Particle electron counts between 20 and 200 keV demonstrates that these electrons are likely responsible for attenuating the radar signals. We investigate the minimum SEP electron fluxes required to ionize the lower atmosphere and produce measurable attenuation. When both radars experience a blackout, the SEP electron fluxes are at their highest. Based on several case studies, we find that the average SEP spectrum responsible for a blackout is particularly enhanced at its higher energy end, that is, above 70 keV. Plain Language Summary: The orbital radars that study the surface and subsurface of Mars can suffer a near‐total loss of received signal during periods when high‐energy electrons from the Sun enter the Martian atmosphere, causing enhanced ionization at altitudes between 60 and 90 km. We have studied these radar blackouts using radars on the Mars Express and Mars Reconnaissance Orbiter spacecraft. The radars on the two spacecraft work at different frequencies, allowing us to test the theory that a larger effect is predicted at lower frequency. Indeed, we do see more blackouts of the radar operating at lower frequency (1.8–5 MHz) than at the higher frequency (20 MHz). We also find that more blackouts occur for both radars during periods of higher solar activity that is, more solar flares and Coronal Mass Ejections. The radar blackouts occur on the nightside and dayside and over all locations on the planet's surface, indicating a global effect of the solar particle events. We have also investigated the electron fluxes measured by the Mars Atmosphere and Volatile EvolutioN spacecraft and find they are higher during blackouts that include the 20 MHz frequency. These observations provide a new window on the levels of ionization at low altitudes. Key Points: First long‐term characterization of the lower ionosphere of Mars with Mars Express, Mars Reconnaissance Orbiter, and Mars Atmosphere and Volatile EvolutioNRadar sounder blackouts caused by energetic electrons occur globally at MarsRadar sounder blackouts correlate with the solar cycle [ABSTRACT FROM AUTHOR]

Mars Atmosphere and Volatile Evolution (MAVEN)'s Imaging Ultraviolet Spectrograph has identified 278 occurrences of discrete aurora events on Mars, which are patchy, sporadic ultraviolet emissions emanating from the upper atmosphere. We confirm prior results finding that emissions are highly correlated with crustal magnetic fields results, with the brightest and most frequent occurrences located around strong crustal fields in the southern hemisphere. The six‐year data set shows that events can also occur globally, in regions of weak or absent crustal fields. We find that events occur primarily in evening hours, especially during favorable orientations of the interplanetary magnetic field. Under these conditions, auroral events probably occur nightly and last for hours. Optical counterparts to these UV emissions would probably be detectable with present‐day instrumentation, and would be visible to future astronauts. Plain Language Summary: Mars has an unusual magnetic field: it lacks a global field but retains the vestiges of an ancient field locked in regions of the crust. These fields are sufficient to focus charged particle precipitation into the atmosphere and cause aurora. The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft carries an ultraviolet instrument capable of mapping aurora emissions and correlating them with conditions that favor aurora. We have detected hundreds of auroral events allowing the first statistical study of their behavior. We find that events occur primarily in evening hours, especially during favorable orientations of the interplanetary magnetic field. Under these conditions, auroral events probably occur nightly and last for hours. Astronauts, whenever they reach Mars, would have a good chance to see this natural wonder from above or below. Key Points: Mars Atmosphere and Volatile Evolution (MAVEN)/Imaging UltraViolet Spectrograph (IUVS) has detected hundreds of transient, patchy auroral emission events at Mars, mostly near strong crustal magnetic fieldsEvents are triggered in evening hours during favorable orientations of the interplanetary magnetic field, and may last for hoursVisible counterparts to the ultraviolet emissions could potentially be imaged by spacecraft and eventually seen by astronauts [ABSTRACT FROM AUTHOR]

We present an analysis of hundreds of middle ultraviolet auroral spectra collected at the limb with the Imaging UltraViolet Spectrograph (IUVS) instrument on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. While the companion paper by Schneider et al. (2021), https://doi.org/10.1029/2021JA029428 focuses on the detection, location, and occurrence frequency of discrete auroral events, this study addresses the spectral properties and vertical profiles of the auroral emissions. Our independent selection of events is based on a combination of automatic and manual detection methods with adequate signal‐to‐noise ratio of both the CO Cameron bands and the CO2+ ultraviolet doublet (UVD) at 190–270 and 288–289 nm, respectively. We find that the ratio of these two features remains quasi‐constant for UVD intensities exceeding ∼200 rayleighs (R), but the CO Cameron/CO2+ UVD ratio may become increasingly large for low UVD intensities. Three weak N2 Vegard‐Kaplan bands are identified in the Martian aurora for the first time. Limb profiles of the [OI] line at 297.2 nm indicate that the visible oxygen green line brightness may reach a few kilorayleighs. The distribution of the altitude of the emission peaks in the aurora is identical in and out of the region of crustal magnetic field located in the southern hemisphere. Comparisons of in situ measurements of electron energy spectra and ultraviolet auroral detections have been made for five optical detections. They generally show temporal coincidence but not necessarily quantitative agreement with the altitude and brightness expected from the characteristics of the measured electron energy spectra. Plain Language Summary: The MAVEN spacecraft has been orbiting Mars since September 2014 to study its upper atmosphere, ionosphere, and interaction with the solar wind. We focus on the discrete aurorae, previously discovered from the Mars Express orbiter. Discrete aurorae are observed on the nightside and are localized both in space and time. We analyze the ultraviolet spectra observed at the limb with the Imaging Ultraviolet Spectrograph to determine the main characteristics of these aurorae. We show that the emission peaks occur at ∼130 km. They are composed of several emissions, such as CO, CO2+, N2, and atomic oxygen. We find that they occur more often in the presence of strong crustal magnetic field. We also deduce that the altitude of the aurora is independent of many parameters except for seasons. We analyze the intensity ratio of these emissions and show that anomalies occur in the ratio of the CO Cameron bands to the CO2+ UV doublet at low intensity. Finally, we compare the detection of auroral events with the presence of strong electron precipitation measured on board MAVEN. A temporal correlation is generally observed but association of the electron flux and specific UV auroral detection is not always clear because measurements are not colocated. Key Points: N2 Vegard‐Kaplan bands have been detected in the ultraviolet Martian auroraThe presence of crustal magnetic field does not influence the altitude of the auroral emission peakGreen line intensities up to a few kilorayleighs are expected in regions of crustal magnetic field and may be visible with the naked eye [ABSTRACT FROM AUTHOR]

Superthermal electron precipitation is one of the main sources supporting the Mars nightside ionosphere. It is expected that solar wind electron fluxes are to increase significantly during interplanetary coronal mass ejections (ICMEs) and therefore an enhanced nightside ionospheric density. This study is to quantify the variation of the precipitating and deposited electron fluxes during five extreme ICMEs encountered by Mars Global Surveyor (MGS). We find that energy fluxes correlate better with the upstream dynamic pressure proxy than number fluxes and electron fluxes increase more at high energies, which means that electrons tend to have a lower peak production altitude during storm times. The precipitating and net/deposited fluxes are increased up to an order of magnitude from low to high dynamic pressures. The estimated total electron content (TEC) is a few times of 1014 m−2 for quiet times and on the order of 1015 m−2 for storm times, with an enhancement up to an order of magnitude locally near strong crustal fields. Crustal magnetic fields have an effect on the deposited fluxes with more prominent magnetic reflection over strong magnetic fields during quiet periods, which is significantly reduced during storm times. Lastly, we estimate a global energy input from downward fluxes of 1.1 × 108 and 5.5 × 108 W and the globally deposited energy from net fluxes of 2.0 × 107 and 1.6 × 108 W for quiet and storm time periods, a factor of 5 and 8 enhancement globally, respectively, but up to an order of magnitude locally near strong crustal fields. Key Points: We quantify the variation of nightside precipitating and deposited electron fluxes during five extreme ICME events encountered by MGSThe estimated TEC is on the order of 1014 m−2 for quiet periods and 1015 m−2 for storm periods because of enhanced electron precipitationCrustal magnetic fields reduce deposited fluxes during quiet time because of magnetic reflection, which is largely reduced during storm time [ABSTRACT FROM AUTHOR]

The effects of solar flares on the upper atmosphere of Mars are large but poorly constrained by observations. These effects are an important aspect of the response of Mars to space weather events and may also influence the escape of volatiles from Mars, particularly in the solar system's early history. Here we report the effects of the X8.2 flare on 10 September 2017 on the density and composition of the thermosphere of Mars. This analysis uses neutral number densities of He, O, N2, CO, Ar, and CO2 from the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS). From these observations, we investigate how the enhanced solar irradiance during the flare produced changes in the neutral upper atmosphere of Mars due to atmospheric heating and photochemistry. Flare‐produced photochemical changes in the neutral thermosphere of Mars have been previously implied but not quantified. We find that at fixed altitudes, the number densities of all species barring He increase and the O/CO2 ratio decreases by ∼15–45%, indicating thermal expansion. However, when viewed at fixed total number densities, the densities of CO2 and Ar decrease, and the O/CO2 ratio increases by up to a factor of ∼3. The photodissociation of CO2 is one photochemical process that produces changes resembling those identified. The quantified changes will help to constrain the shifting chemical makeup of the upper atmosphere due to these impactful flare events and may aid modeling of flare behavior and the impact of flares on the evolution of the Martian climate. Key Points: Mars thermospheric densities and composition are affected by this solar flareThermospheric responses at fixed altitude (heating‐based changes) and fixed number density (chemistry‐based changes) differThe O/CO2 ratio nearly triples at low number densities during this flare [ABSTRACT FROM AUTHOR]

We report results from a study of two consecutive Martian years of imaging observations of nitric oxide ultraviolet nightglow by the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission spacecraft. The emission arises from recombination of N and O atoms in Mars' nightside mesosphere. The brightness traces the reaction rate as opposed to the abundance of constituents, revealing where circulation patterns concentrate N and O and enhance recombination. Emissions are brightest around the winter poles, with equatorial regions brightening around the equinoxes. These changes offer clear evidence of circulation patterns transitioning from a single cross‐equatorial cell operating during solstice periods to more symmetric equator‐to‐poles circulation around the equinoxes. Prominent atmospheric tides intensify the emissions at different longitudes, latitude ranges, and seasons. We find a strong eastward‐propagating diurnal tide (DE2) near the equator during the equinoxes, with a remarkably bright spot narrowly confined near (0°, 0°). Wave features at the opposite winter poles are dissimilar, reflecting different circulation patterns at perihelion versus aphelion. LMD‐MGCM simulations agree with the patterns of most observed phenomena, confirming that the model captures the dominant physical processes. At the south winter pole, however, the model fails to match a strong wave‐1 spiral feature. Observed brightnesses exceed model predictions by a factor of 1.9 globally, probably due to an underestimation of the dayside production of N and O atoms. Further study of discrepancies between the model and observations offers opportunities to improve our understanding of chemical and transport processes controlling the emission. Plain Language Summary: The MAVEN spacecraft orbiting Mars has obtained a new type of imaging data which reveals the effects of global‐scale winds and waves in the upper atmosphere. Ultraviolet emissions confirm the expected change of wind patterns with season and pinpoint an unusually bright spot on Mars' nightside. Computer model runs successfully match many of the observed features and allow an examination of the underlying physical causes. Differences between the model and observations offer insights on how to improve the model. Key Points: The MAVEN spacecraft orbiting Mars has obtained a new type of nightside imaging data which reveals the effects of global‐scale winds and waves in the upper atmosphereUltraviolet nightglow emissions confirm the expected change of circulation patterns with season and pinpoint an unusually bright spot on Mars' nightsideA Mars general circulation model successfully matches many of the observed features and allows an examination of the underlying physical causes [ABSTRACT FROM AUTHOR]

The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical, and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our solar system: It is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H‐dominated thermosphere for astrophysicists or the Sun‐like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so‐called Xenon paradox. Plain Language Summary: In addition to having the right surface temperature, a planet needs an atmosphere to keep surface liquid water stable. Although many planets have been found that may lie in the right temperature range, the existence of an atmosphere is not guaranteed. In particular, for planets that are kept warm by being close to dim stars, there are a number of ways that the star may remove a planetary atmosphere. These atmospheric escape processes depend on the behavior of the star as well as the nature of the planet, including the presence of a planetary magnetic field. Under certain conditions, a magnetic field can protect a planet's atmosphere from the loss due to the direct impact of the stellar wind, but it may actually enhance total atmospheric loss by connecting to the highly variable magnetic field of the stellar wind. These enhancements happen especially for planets close to dim stars. We review the complete range of atmospheric loss processes driven by interaction between a planet and a star to aid in the identification of planets that are both the correct temperature for liquid water and that have a chance of maintaining an atmosphere over long periods of time. Key Points: The different escape processes at planets and exoplanets are reviewed along with their mathematical formulationThe major parameters for each escape processes are described; some escape processes negligible in the solar system may be major source at exoplanets, or for the early solar systemA magnetic field should not be a priori considered as a protection for the atmosphere [ABSTRACT FROM AUTHOR]

The nighttime limb intensity of diffuse auroral emission of CO2+ (B2Σu+ ‐ X2πg) Ultraviolet Doublet (UVD) is observed in the northern hemisphere of Mars during 17–21 December 2014 from Imaging Ultraviolet Spectrograph instrument onboard Mars Atmosphere and Volatile Evolution. We have used hybrid model and four‐dimensional yield spectrum approach based on Monte Carlo simulation to calculate the ionization rate, limb intensity, and ion and electron densities of diffuse aurora due to precipitation of solar energetic particle and proton‐hydrogen (H+‐H) fluxes in the nighttime ionosphere of Mars. It is found that the production rates of atmospheric ions (CO2+, N2+, and O+) are dominant in the upper ionosphere at about 100–150 km due to impact of H+‐H. The solar energetic particle formed auroral ionosphere (CO2+, NO+, and O+) in the middle ionosphere between 50 and 100 km due to precipitation of monoenergetic electrons of energies 25 to 100 keV. The simulated limb intensities of CO2+UVD due to impact of H+‐H and auroral electrons are compared with Imaging Ultraviolet Spectrograph observations. Our model results are overestimating the observations, but 100 keV electrons deposited maximum energy around 75 km, closer to the observed altitude of the maximum emission. The densities of upper ionosphere (O2+, NO+, and CO2+) due to impact of H+‐H are smaller by one to two orders of magnitude than that produced by auroral electrons in the middle ionosphere. Key Points: Causative mechanisms of diffuse auroraSEP electron and H+‐H precipitation on Mars [ABSTRACT FROM AUTHOR]

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express, which operates between 0.1 and 5.5 MHz, suffered from a complete blackout for 10 days in September 2017 when observing on the nightside (a rare occurrence). Moreover, the Shallow Radar (SHARAD) onboard the Mars Reconnaissance Orbiter, which operates at 20 MHz, also suffered a blackout for three days when operating on both dayside and nightside. We propose that these blackouts are caused by solar energetic particles of few tens of keV and above associated with an extreme space weather event between 10 and 22 September 2017, as recorded by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Numerical simulations of energetic electron precipitation predict that a lower O2+ nighttime ionospheric layer of magnitude ~1010 m−3 peaking at ~90‐km altitude is produced. Consequently, such a layer would absorb radar signals at high frequencies and explain the blackouts. The peak absorption level is found to be at 70‐km altitude. Plain Language Summary: Several instrument operations, as well as communication systems with rovers at the surface, depend on radio signals that propagate throughout the atmosphere of Mars. This is the case also for two radars that are currently working in Mars' orbit, sounding the ionosphere, surface, and subsurface of the planet. In mid‐September 2017, a powerful solar storm hit Mars, producing a large amount of energetic particle precipitation over a 10‐day period. We have found that high‐energy electrons ionized the atmosphere of Mars, creating a dense layer of ions and electrons at ~90 km on the Martian nightside. This layer attenuated radar signals continuously for 10 days, stopping the radars to receive any signal from the planetary surface. In this work, we assess the properties of this layer in order to understand the implications of this kind of phenomenon for radar performance and communications. Key Points: A large space weather event caused negatively impacted radar performance for 10 daysSolar electron precipitation created a low ionospheric layer at ~90 km on the nightsideThe nightside ionization is comparable to dayside values [ABSTRACT FROM AUTHOR]

The persistent two‐peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies, respectively, determine their ionization altitude. A third low‐altitude transient layer (previously referred to as M3 or Mm) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring nonglobally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bow shock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers' wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN Radio Occultation Science Experiment in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed. Plain Language Summary: The ionosphere of a planet couples the neutral atmosphere to the exosphere and solar wind. Created by solar ionizing radiation, the ionosphere on the dayside of planet is expected to be hemispherically symmetric. When Mars Express discovered transient low‐altitude ionospheric layers in 2005, they were initially predicted to be due to meteor ablation. However, recent observations indicate that they are due to a geometric observational effect related to local, not hemispheric, ionization. Observations from three instruments on Mars Atmospheric and Volatile EvolutioN are used concurrently to understand this phenomenon. These transient layers have been observed at Venus and Titan as well, and this hypothesis indicates novel physics for those ionospheric systems. Key Points: Low‐altitude transient ionospheric layers may be explained by a geometric observational effect from localized, not hemispheric, ionizationA radio occultation of a transient ionospheric layer was obtained with observations of variable proton aurora and penetrating solar windTransient layers observed at Mars, Venus, and Titan are consistent with the presence of solar wind impact ionization at those planets [ABSTRACT FROM AUTHOR]

Magnetic topology is important for understanding the Martian plasma environment, including particle precipitation, energy transport, cold ion escape, and wave‐particle interaction. In this study, we combine two independent but complementary methods in order to determine magnetic topology based on superthermal electron energy and pitch angle distributions. This approach removes ambiguities that result from using either energy or pitch angle alone, providing a more accurate and comprehensive determination of magnetic topology than previous studies. By applying this combined technique, we are able to identify seven magnetic topologies, including four types of closed field lines, two types of open field lines, and draped. All seven topologies are present in the Mars environment and are mapped in longitude, latitude, solar zenith angle, and altitude with the combined technique near the terminator. We find that closed field lines with double‐sided loss cones are frequently present over stronger crustal field regions at higher altitudes. We also show that the cross‐terminator closed field lines are more spatially confined over strong crustal regions, likely connecting nearby magnetic crustal patches. In contrast, cross‐terminator closed loops over weak crustal regions have more distantly separated foot points, most likely connecting distant crustal patches. Key Points: A technique combining e‐ pitch angle and energy distribution is developed to most accurately infer up to seven magnetic topologies at MarsClosed magnetic loops with trapped electrons occur frequently over stronger crustal field regions at higher altitudesCross‐terminator closed loops are more spatially confined over strong crustal regions and distantly separated over weak crustal regions [ABSTRACT FROM AUTHOR]

Understanding how the Mars plasma environment responds to space weather events provides insights into the early Sun‐Mars interaction and helps constrain the extrapolation of atmospheric loss back in time. The 2003 Halloween interplanetary coronal mass ejection (ICME) event is one of the most extreme space weather events encountered by Mars during the last two decades. Mars Global Surveyor's circular orbit at ∼400 km allows us to observationally study the magnetic topology response to this extreme event globally for the first time. We analyze the suprathermal electron data and magnetic field measurements from Mars Global Surveyor to infer magnetic topology before, during, and after this ICME event and quantify the variation of the topology over both weak and strong crustal regions and correlate the variation with the upstream dynamic pressure. Over weak crustal regions, more draped field lines and fewer closed field lines are observed under high dynamic pressures, implying a deeper interplanetary magnetic field penetration during the ICME event. Over dayside strong crustal regions, the dominant magnetic topology switches from closed field lines during quiet periods to open field lines during disturbed periods, suggesting more reconnection occurring between the crustal magnetic fields and interplanetary magnetic field. This mixed magnetic topological response is consistent with predictions from magnetohydrodynamic simulations for ICME events in previous studies. Key Points: We analyze Martian magnetic topology response to the 2003 Halloween ICME event and correlate the variation with upstream driversA deeper IMF penetration is found over weak crustal regions but more open field lines over strong crustal regions during the eventThis mixed magnetic topological response confirms previous predictions from MHD simulations for ICME events [ABSTRACT FROM AUTHOR]