Abstract We present a study of changes in Martian magnetic topology induced by upstream solar wind ram pressure variations. Using electron energy spectra and pitch angle distributions measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we classify the topology of magnetic field lines in the Martian space environment across a range of solar wind conditions. We find that during periods of high solar wind dynamic pressure, draped fields are pushed to lower altitudes on the dayside of the planet, compressing closed fields. At the same time, open topology becomes more prevalent on the nightside due to the broadening of crustal cusp regions. The result is a decrease in closed topology at all locations around Mars, suggesting that the Martian ionosphere becomes significantly more exposed to solar wind energy input during high solar wind pressure. This could likely contribute to elevated levels of ion escape during these periods.
Abstract The magnetic field draping pattern in the magnetosheath of Mars is of interest for what it tells us about both the solar wind interaction with the Mars obstacle and the use of the field measured there as a proxy for the upstream interplanetary magnetic field (IMF) clock angle. We apply a time‐dependent, global magnetohydrodynamic model toward quantifying the spatial and temporal variations of the magnetic field draping direction on the Martian dayside above 500‐km altitude. The magnetic field and plasma are self‐consistently solved over one Mars rotation period, with the dynamics of the field morphology considered as the result of the rotation of the crustal field orientation. Our results show how the magnetic field direction on the plane perpendicular to the solar wind flow direction gradually departs from the IMF as the solar wind penetrates toward the obstacle and into the tail region. This clock angle departure occurs mainly inside the magnetic pileup region and tailward of the terminator plane, exhibiting significant dawn‐dusk and north‐south asymmetries. Inside the dayside sheath region, the field direction has the greatest departure from the IMF‐perpendicular component direction downstream of the quasi‐parallel bow shock, which for the nominal Parker spiral is over the dawn quadrant. Thus, the best region to obtain an IMF clock angle proxy is within the dayside magnetosheath at sufficiently high altitudes, particularly over subsolar and dusk sectors. Our results illustrate that the crustal field has only a mild influence on the magnetic field draping direction within the magnetosheath region.
Abstract The Mars Atmosphere and Volatile Evolution mission has obtained comprehensive particle and magnetic field measurements. The Solar Wind Electron Analyzer provides electron energy‐pitch angle distributions along the spacecraft trajectory that can be used to infer magnetic topology. This study presents pitch angle‐resolved electron energy shape parameters that can distinguish photoelectrons from solar wind electrons, which we use to deduce the Martian magnetic topology and connectivity to the dayside ionosphere. Magnetic topology in the Mars environment is mapped in three dimensions for the first time. At low altitudes (<400 km) in sunlight, the northern hemisphere is found to be dominated by closed field lines (both ends intersecting the collisional atmosphere), with more day‐night connections through cross‐terminator closed field lines than in the south. Although draped field lines with ~100 km amplitude vertical fluctuations that intersect the electron exobase (~160–220 km) in two locations could appear to be closed at the spacecraft, a more likely explanation is provided by crustal magnetic fields, which naturally have the required geometry. Around 30% of the time, we observe open field lines from 200 to 400 km, which implies three distinct topological layers over the northern hemisphere: closed field lines below 200 km, open field lines with foot points at lower latitudes that pass over the northern hemisphere from 200 to 400 km, and draped interplanetary magnetic field above 400 km. This study also identifies open field lines with one end attached to the dayside ionosphere and the other end connected with the solar wind, providing a path for ion outflow.
Abstract We present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of Marsward and tailward fluxes of suprathermal (>25 eV) ions in the near‐Mars (∼1–1.5 Mars radii downstream) magnetotail. Statistical results show that the Marsward proton flux and magnetic field draping pattern are well organized by the upstream motional electric field direction. We observe both significant Marsward proton fluxes and tightly wrapped magnetic field lines in the hemisphere pointed in the opposite direction to the upstream electric field. These characteristics are very similar to those observed at Venus. On the other hand, the net flux of oxygen ions points tailward on average in the Martian tail, while net Venusward flows of oxygen ions were observed frequently in the same hemisphere at Venus. The mechanism by which the Marsward proton flux is produced in the presence of tailward oxygen ion flux remains unclear.
The atmosphere of Mars, lacking a global magnetic field, is exposed to the precipitation of solar energetic particles (SEPs), resulting in impact ionization and the production of secondary electrons, some of which may escape the atmosphere. In this study, we examine upward traveling fluxes of superthermal electrons between ∼100 and 650 eV, measured by the Mars Global Surveyor Magnetometer/Electron Reflectometer at 400 km altitude during nine of the largest and clearest SEP events of the last solar maximum from November 2000 until the “Halloween” storms of late 2003. We subtract the contribution from backscattered low‐energy precipitating electrons and find that, for the highest and most rarely observed SEP fluxes, we detect a statistically significant flux of SEP‐produced superthermal electrons escaping the Martian atmosphere. The measured fluxes are found to be in broad agreement with a calculation of expected upward electron fluxes resulting from ionization of neutrals by energetic proton impact. Peak SEP ionization rates on the nightside from the Halloween storms are found to be comparable to (although lower than) typical dayside photoionization rates and at least 3 orders of magnitude higher than average nightside electron impact ionization rates. Further advances in our knowledge of SEP effects on the Martian ionosphere await data from the Radiation Assessment Detector (RAD) instrument on the Mars Science Laboratory rover in 2012 and the MAVEN orbiter in 2014.
Abstract The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft observed a strong interplanetary coronal mass ejection (ICME) reaching Mars on 13 September 2017. In this work we analyze the interaction between such an extreme event and the Martian‐induced magnetosphere by means of Laboratoire Atmosphères, Milieux et Observations Spatiales Hybrid Simulation (LatHyS) stationary runs and magnetic field and plasma observations obtained by MAVEN in a time interval from ∼ 5 hr before the ICME shock arrival to about 5.5 hr after the impact. Detailed comparisons between simulation results and such MAVEN measurements are performed and show that several stages during this interaction can be described through a combination of steady states. LatHyS results show the simulated bow shock is closer to the planet for higher magnetosonic Mach number and solar wind dynamic pressure conditions, in agreement with previous observational studies. MAVEN observations and LatHyS results also suggest a compression on the flanks of the magnetic pileup boundary. Finally, simulated H + and O + planetary escape rates increase by a factor ∼10 and ∼2.4, respectively, due to the ICME passage through the Martian magnetosphere.
Abstract We present the first measurements of Mars discrete aurora in the extreme ultraviolet (<110 nm) and the first synoptic aurora images in the far ultraviolet (110–180 nm). Auroral emission is detected in >75% of nightside images, with patterns shifting visibly over 15–20 min. Aurora is observed most frequently in regions of open magnetic topology (where crustal magnetic fields are very weak and/or vertical), with the brightest aurora where crustal fields are strongest. We present the first disk‐averaged spectrum of discrete aurora, with several O, C, and CO features as expected for electron impact primarily on CO 2 . We categorize discrete auroral morphology into three types: crustal field aurora, non‐crustal field patchy aurora, and a new type we call “sinuous” aurora, an elongated serpentine structure that stretches thousands of kilometers into the nightside from near midnight in the northern hemisphere. These observations point to a highly dynamic environment in Mars' magnetotail.
We use Lunar Prospector data to identify 990 magnetic enhancements, previously termed “limb shocks” or “limb compressions”, external to the lunar wake. We find that they are clearly associated with lunar crustal sources, and sometimes occur far upstream from the limb at altitudes of ∼100 km. This is inconsistent with compressional disturbances convecting downstream with the solar wind, and implies that crustal fields are sometimes strong and coherent enough to produce a fluid‐like interaction where compressional waves steepen to form a shock extending upstream from their source. The likelihood of observing enhancements partly depends on upstream solar wind conditions, with low proton gyroradius and low β particularly favored. Low Mach numbers, implying a larger shock standoff distance, are also favored for observations which suggest shock‐like behavior.
Abstract We present a time‐dependent MHD study of the controlling effects of the Mars crustal field on atmospheric escape. We calculate globally integrated planetary ion loss rates under quiet solar conditions considering the continuous rotation of crustal anomalies with the planet. It is found that the rotating crustal field plays an important role in controlling atmospheric escape. Significant time variation of ∼20% and ∼50% is observed during the entire rotation period for O + and for and , respectively. The control is exerted mainly through two processes. First, the crustal magnetic pressure over the subsolar regime controls solar wind penetration and mass loading and therefore the escaping planetary ion source. There is a strong negative correlation between the magnetic pressure and ion loss, with a time lag of <1 h for O + and ∼2.5 h for and . Second, the crustal magnetic pressure near the terminator region controls the cross‐section area between the induced magnetospheric boundary and 100 km altitude at the terminator. The change in day‐night connection regulates the extent to which planetary ions created on the dayside can be ultimately carried away by the solar wind and escape Mars. There is a strong positive correlation between the cross‐section area and ion loss, with no significant time lag. As the planet rotates, the dayside process and the terminator process work together to control the total amount of escaping planetary ions. However, their relative importance changes with the local time of the strong crustal field region.
The magnetometer and electron reflectometer experiment (MAG/ER) on the Mars Global Surveyor (MGS) spacecraft has obtained magnetic field and electron data which indicates that the solar wind interaction with Mars is primarily an ionospheric‐atmospheric interaction similar to that at Venus. However, the global‐scale electric currents and resulting magnetic fields due to the interaction at Mars are locally interrupted or perturbed over distance scales of several hundred kilometers by the effects of paleomagnetic fields due to crustal remanence. In this paper we compare the Mars‐solar wind interaction with the Venus‐solar wind interaction by selecting MGS orbits which do not show significant magnetic perturbations due to crustal magnetic anomalies, and demonstrate that a number of phenomena characteristic of the Venus‐solar wind interaction are also observable at Mars.
