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In this study, we present the analysis of steepened ultra-low frequency (ULF) waves in the foreshock region upstream of Mars’ bow shock observed by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft at Mars. A survey of MAVEN magnetic field and plasma measurements shows quasi-periodic gradual increases followed by a sharp decrease in the magnetic field magnitude (Btotal). Higher frequency waves were also commonly, but not always, observed at the trailing edge of the large-amplitude increase in Btotal. These observations are consistent with the signatures of shocklets observed in the solar wind region upstream of Earth’s and planetary bow shocks. Shocklets are believed to be formed as a result of the steepening of fast magnetosonic waves generated by reflected ions in the quasi-parallel foreshock region. We also performed the minimum variance analysis (MVA) and statistical analysis technique to determine the wave properties (e.g. polarization, wave propagation, amplitude and frequency) of the shocklets and higher frequency waves observed in its trailing edge. Our results showed that these shocklets are left-handed polarized in the spacecraft frame, with mean amplitude δB/B of ~3.5 and time separation between adjacent shocklet events of ~40s. We also analyzed measurements (ions and electrons) from MAVEN’s plasma instruments to investigate the energization process of the particles during the observations of shocklets. We will discuss the possible generation mechanisms for these steepened ultra-low frequency waves at Mars, and any implications for the martian plasma environment downstream of Mars’ bow shock.

Understanding the properties and variability of the ionosphere is vital for understanding the atmosphere of Mars. The presence and property of waves provide insight into the plasma and magnetic environment. We conducted a search for 5 - 16 Hz signals below 400 km in magnetic field data from Mars Global Surveyor (MGS), the Mars Atmosphere and Volatile Evolution (MAVEN), and Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) missions. We discuss an analysis of 54 identified MAVEN events using multiple instruments and present a case study event. Nearly half the wave events occur near the cusps of strong crustal magnetic fields (CMFs). The stronger regions have fewer events and may be a result of stronger CMFs preventing draped field lines from reaching lower altitudes. A majority of the observed magnetic waves occur on the nightside, are associated with greater fluxes of electrons traveling downward along the local magnetic field compared to those traveling upward, and correspond to increases in thermal plasma density. These aspects indicate electron precipitation was present during these wave events. We conclude that these waves are observed under magnetic field conditions favorable for the penetration of electrons and waves into the lower ionosphere, but that the electron precipitation cannot solely account for the waves or plasma changes.We then discuss our null results regarding Schumann resonances, which are electromagnetic signals generated by lightning that, if they exist on Mars, are expected to propagate at 7-14 Hz. Future studies specifically looking for Schumann resonances will require higher sensitivity instruments and would benefit from additional missions that reach into the ionosphere of Mars. Finally, we comment on the inconsistency between identifying MGS events purely via by-eye analysis versus using quantitative methods for guidance.

Venus lacks a significant intrinsic magnetic field, and thus, its atmosphere and ionosphere interact directly with the solar wind flow and magnetic field from the Sun. Interplanetary magnetic fields (IMF) can penetrate into the ionosphere when the upstream solar wind dynamic pressure is stronger than the ionospheric plasma pressure. Magnetic topology can be inferred at Venus if it is defined as the magnetic connectivity to the collisional atmosphere/ionosphere, rather than connectivity to the planet’s surface. Magnetic topology can be inferred from the pitch angle and energy distribution of superthermal (> ~1 eV) electrons. More specifically, the presence of loss cones in electron pitch angle distributions infers connectivity to the nightside collisional atmosphere and the presence of ionospheric photoelectrons (identified from electron energy distributions) indicates connectivity to the dayside collisional ionosphere. We design automated procedures to determine magnetic topology with electron and magnetic field measurements by the Venus Express spacecraft over its entire mission (2006-2014). This allows us to provide the first statistical mapping of magnetic topology at Venus. We also examine how the upstream drivers affect the low-altitude magnetic topology, revealing different magnetized states of the Venus ionosphere. We find that open and closed (a surprising topology not expected at Venus) fields cluster around the terminator and draped fields dominate other regions. Our results also reveal that there is more dayside magnetic connectivity in the -E (solar wind motional electric field) hemisphere than the +E hemisphere, and during solar maximum. During solar minimum, however, there is more nightside magnetic connectivity. Last but not the least, to understand the true nature of these magnetic topologies and broadly speaking the planet-solar wind interaction, we need to think about possible ways to measure the deeply penetrated magnetic fields at Venus and Mars.

Introduction: Electrons emitted from the lunar surface include photoelectrons, cold secondary electrons, and backscattered electrons, all of which contribute to the surface-charging environment. Among these electron populations, secondary electrons have been well characterized by previous studies but not so much for the other two populations that make up the high-energy tail. Recently, we reported of oxygen Auger electron observations at the Moon by the ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) spacecraft, which provides a unique feature to identify photoelectrons emitted from the lunar surface. This Auger feature allows for the creation of empirical templates for photoelectrons and backscattered electrons to disentangle these two populations and examine their characteristics separately. Methodology: By utilizing the Auger electron feature, we identify cases that are separately dominated by either photoelectrons or pure backscattered electrons and then create energy spectral templates for each population, when the Moon is immersed in the solar wind. With such templates, we then fit the total measured electron spectrum from the lunar surface as observed by ARTEMIS from mid-2012 to the end of 2020 and determine the relative contributions of the two populations. Results: For photoelectrons, we find that (1) Auger electron fluxes are correlated with solar X-ray photon fluxes as expected; and, (2) there is a dawn-dusk asymmetry in Auger electron fluxes, caused by a selection bias as downward electrons are mainly halo electron on the dusk side, lower in flux than strahl electrons on the dawn side. For backscattered electrons, we find the backscattering ratio (the flux ratio of backscattered electrons and downward electrons at the same energy) varies geographically due to different proportions of surface backscattered electrons and non-adiabatic magnetic scattering by crustal magnetic fields. In addition, for surface backscattering, the backscattering ratio decreases with increasing energies, ranging from

Here we report on synoptic (or “disk”) observations of Martian discrete aurora in the extreme and far ultraviolet (

Mars has a patchwork of intense crustal magnetization that gives rise to magnetic fields that can dominate the plasma environment up to ~1000 km altitude in some locations. The strongest crustal fields, in the southern hemisphere centered near 180 deg longitude, form an arcade of reversing polarities that extends > 1000 kilometers east-west. The topology of these fields can be complex and dynamic, with closed loops connecting either nearby and distant crustal sources, and magnetic reconnection with the IMF temporarily forming open field lines. The boundary between closed magnetic loops and open field lines is expected to have a small spatial scale that has so far been unresolved by plasma instruments at Mars. Electron energy-pitch angle distributions measured by the MAVEN Solar Wind Electron Analyzer (SWEA) can be used to infer magnetic topology and thus reveal the large-scale configuration of the magnetic field from measurements at one location. During normal operation, the instrument has a 2-second measurement cycle that provides ~8 km spatial resolution, which is too large to probe the transition region between closed and open field lines, for example. (Topology changes appear sharp and instantaneous at that scale.) However, it is possible to operate the instrument in a non-standard way to measure the electron angular distribution at a single energy with a time resolution of 0.03 seconds and a spatial resolution of 125 meters, which is comparable to the 50-eV electron gyroradius in a 100-nT field. High resolution observations from ~150 to 800 km altitude over strong crustal fields near the evening terminator were obtained on 12 orbits on January 27, 2019, and from May 27 to June 1, 2020. A total of eight topological transitions were captured. Six of these were transitions from open to closed, or the reverse. For example, observations on January 27, 2019 revealed a transition region between closed and open field lines with a spatial scale of ~900 meters, which is equivalent to 5 electron gyroradii under the prevailing conditions. The transition region was traversed in 290 millisec, which is equivalent to four mirror times (from the spacecraft to the reflection point and back). Before the transition region, trapped mirroring electrons are observed, which is indicative of a closed crustal magnetic loop. After the transition region, a loss cone is present, which is formed when precipitating electrons are absorbed before they can reflect back up to the spacecraft. This indicates an open field line. Within the transition region itself, the angular distribution is more isotropic, which indicates that a finite time is needed for the electron population to transition from one to the other. The other two events show sharp transitions between neighboring closed crustal magnetic flux tubes with different sets of ionospheric footpoints. This reveals a structured ionosphere controlled by the magnetic topology connecting crustal magnetic sources.

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 particle responsible for producing these detectable aurorae, that is accelerated electrons or hotter solar wind electrons (termed “auroral electrons"). By utilizing empirical criteria to select auroral electrons established by Xu et al. [2022], we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and electron fluxes of auroral electrons. We find the occurrence rate is well organized and increases with upstream dynamic pressure and weakly depends on the interplanetary magnetic field strength. Meanwhile, the integrated auroral electron flux is somewhat insensitive to the upstream drivers. In the meantime, the auroral emission is not the sole effect of electrons impacting the collisional atmosphere. Auroral electrons are expected to cause significant ionization and enhance the plasma density locally. In this study, we also quantify the ionospheric impact of auroral electron precipitation, specifically the thermal ion (O+, O2+, CO2+) density enhancement, with MAVEN observations and also modeling. Our results show the ion density to increase up to 1-2 orders of magnitude at low altitudes.

International audience; 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+, and CO2+) 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.

The Martian crustal magnetic anomalies create a varied, asymmetric obstacle for the draped interplanetary magnetic field (IMF) to interact with. One possible result of this interaction is magnetic reconnection, a process by which anti-parallel magnetic field lines connect and reconfigure, transferring energy into the surrounding environment and mixing previously separated plasma populations. Here, we present an analysis to determine the draped IMF conditions that favor reconnection with the underlying crustal anomalies at Mars. First, we plot a map of the crustal anomalies’ strength and orientation compiled from magnetic field data taken throughout the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Second, we create “shear maps” which calculate and plot the angle of shear between the transverse component of the anomalies and a chosen overlaid draping direction. Third, we define a “shear index” which quantifies the susceptibility of a particular region to undergo reconnection based on a given draped IMF orientation and the resulting shear map for that region. We then compare the shear index for a variety of draped field orientations within different regions of the Martian magnetosphere. Our results suggest eastward/westward (horizontal) draped fields present regions that are more likely for anti-parallel magnetic reconnection to occur with the crustal anomalies than northward/southward (vertical) draped fields, with one notable exception being the strongest crustal anomalies located in the southern hemisphere ~180° longitude. An east/west draped field roughly corresponds to a +/- By IMF direction on the dayside, implying the rate of magnetic reconnection on the dayside of Mars may be enhanced for IMF field lines pointing in the +/- YMSO direction compared to that of IMF field lines pointing in the +/- ZMSO direction, with MSO referring to the Mars Solar Orbital coordinate system. Understanding the interplay between Mars’s crustal magnetic fields and the IMF is crucial to answer outstanding science questions regarding nightside magnetospheric activity at Mars, namely how IMF orientation affects the twisting of the magnetotail, open magnetic topology observations on the nightside, and discrete aurora observations in the southern hemisphere.

International audience; Discrete aurorae have been observed at magnetized planets such as Earth and Jupiter, triggered by accelerated electrons. Similar aurorae have also been observed at Mars with only localized strong crustal magnetisms. However, our understanding of this phenomenon at Mars is still limited. In particular, direct and quantitative comparisons of the auroral and its source electron events are lacking as these two types of observations are usually made at different times and/or locations. In this study, we establish empirical criteria to select electron events ("auroral electrons") that could trigger detectable auroral emissions with Mars Atmosphere and Volatile EvolutioN measurements, thereby enabling a direct statistical comparison. We find auroral electrons share similar statistical characteristics to those previously reported for discrete auroral events. This study bridges the gap between electron observations and auroral detections and enables collaborations across different Mars missions, as well as comparative planetary studies of discrete aurora.

We analyzed MAVEN observations of fields and plasma signatures associated with an encounter of fully-developed Kelvin–Helmholtz (K–H) vortices at the northern polar terminator along Mars’ induced magnetosphere boundary. The signatures of the K–H vortices event are: (i) quasi-periodic, “bipolar-like” sawtooth magnetic field perturbations, (ii) corresponding density decrease, (iii) tailward enhancement of plasma velocity for both protons and heavy ions, (iv) co-existence of magnetosheath and planetary plasma in the region prior to the sawtooth magnetic field signature (i.e. mixing region of the vortex structure), and (v) pressure enhancement (minimum) at the edge (center) of the sawtooth magnetic field signature. Our results strongly support the scenario for the non-linear growth of K–H instability along Mars’ induced magnetosphere boundary, where a plasma flow difference between the magnetosheath and induced-magnetospheric plasma is expected. Our findings are also in good agreement with 3-dimensional local magnetohydrodynamics (MHD) simulation results. MAVEN observations of protons with energies greater than 10 keV and results from the Walén analyses suggests the possibility of particle energization within the mixing region of the K–H vortex structure via magnetic reconnection, secondary instabilities or other turbulent processes. We estimated the lower limit on the K–H instability linear growth rate to be ~5.84 x 10-3 s-1. For these vortices, we estimate the lower limit of the instantaneous atmospheric ion escape flux due to the detachment of plasma clouds during the late non-linear stage of K–H instability to be ~5.90 x 1026 particles/s, which is agrees with earlier studies for the Venusian plasma clouds but ~two orders of magnitude larger than that calculated for Mars.

The Martian magnetotail exhibits a highly twisted configuration, shifting in response to changes in polarity of the interplanetary magnetic field's (IMF) dawn-dusk (BY) component. Here, we analyze ∼6000 MAVEN orbits to quantify the degree of magnetotail twisting (θTwist) and assess variations as a function of (a) strong planetary crustal field location, (b) Mars season, and (c) downtail distance. The results demonstrate that θTwist is larger for a duskward (+BY) IMF orientation a majority of the time. This preference is likely due to the local orientation of crustal magnetic fields across the surface of Mars, where a +BY IMF orientation presents ideal conditions for magnetic reconnection to occur. Additionally, we observe an increase in θTwist with downtail distance, similar to Earth's magnetotail. These findings suggest that coupling between the IMF and moderate-to-weak crustal field regions may play a major role in determining the magnetospheric structure at Mars.