Search

Discrete aurora at Mars, characterized by their small spatial scale and tendency to form near strong crustal magnetic fields, are emissions produced by particle precipitation into the Martian upper atmosphere. Since 2014, Mars Atmosphere and Volatile EvolutioN's (MAVEN's) Imaging Ultraviolet Spectrograph (IUVS) has obtained a large collection of UV discrete aurora observations during its routine periapsis nightside limb scans. Initial analysis of these observations has shown that, near the strongest crustal magnetic fields in the southern hemisphere, the IUVS discrete aurora detection frequency is highly sensitive to the interplanetary magnetic field (IMF) clock angle. However, the role of other solar wind properties in controlling the discrete aurora detection frequency has not yet been determined. In this work, we use the IUVS discrete aurora observations, along with MAVEN observations of the upstream solar wind, to determine how the discrete aurora detection frequency varies with solar wind dynamic pressure, IMF strength, and IMF cone angle. We find that, outside of the strong crustal field region (SCFR) in the southern hemisphere, the aurora detection frequency is relatively insensitive to the IMF orientation, but significantly increases with solar wind dynamic pressure, and moderately increases with IMF strength. Interestingly however, although high solar wind dynamic pressures cause more aurora to form, they have little impact on the brightness of the auroral emissions. Alternatively, inside the SCFR, the detection frequency is only moderately dependent on the solar wind dynamic pressure, and is much more sensitive to the IMF clock and cone angles. In the SCFR, aurora are unlikely to occur when the IMF points near the radial or anti-radial directions when the cone angle (arccos(B x /|B|)) is less than 30° or between 120° and 150°. Together, these results provide the first comprehensive characterization of how upstream solar wind conditions affect the formation of discrete aurora at Mars.

Using Mars Atmosphere and Volatile Evolution observations, we characterize the variability of water vapor in the Martian thermosphere during Mars Years 32–36. Near a fixed atmospheric pressure level of ∼5 × 10−7 Pa, the typical water density is 1.7 (±1.0) × 103 cm−3 and the typical water mixing ratio is 14.6 (±9.0) ppm. Thermospheric water levels are higher during the southern spring and summer seasons when Mars is near perihelion and there is significant dust loading in the lower atmosphere. However, the seasonal variation is not the same from year‐to‐year, likely due to annual differences in dust loading. Water vapor abundance is highly correlated with the amount of dust in the lower atmosphere, and increases during both regional and global dust storms. Additionally, there is a local time asymmetry with higher mixing ratios observed during morning local times, potentially caused by atmospheric circulation. Our results support previous work that found increased dust levels allow more water to be supplied directly to the thermosphere. Plain Language Summary: Water vapor in the atmosphere of Mars can be transported to high altitudes and ultimately lost to space. Hence, understanding the distribution and variability of water throughout the atmosphere is critical for determining the processes that control water loss at Mars. We present new results on the variability of water vapor in the thermosphere of Mars, a region where few water observations exist. We find there is more water during the southern spring and summer seasons when Mars is closest to the Sun and there is a significant amount of dust in the lower atmosphere. We also find that the amount of water is highly correlated with the amount of dust in the lower atmosphere. These results validate previous studies that have shown atmospheric heating from increased dust levels allows more water to be transported to high altitudes without condensing. Key Points: We characterize the variations of thermospheric water with season, local time, and atmospheric dust content in Mars Years 32–36There is an asymmetry with respect to local time, with higher mixing ratios observed during morning local times (6–9 hr)The water abundance is highly correlated with the amount of dust in the lower atmosphere [ABSTRACT FROM AUTHOR]

Discrete aurora are sporadic emissions of light originating in Mars upper atmosphere. We report nadir imaging observations from MAVEN's Imaging UltraViolet Spectrograph which identify the conditions which trigger electron precipitation causing these events. Prior studies have shown that discrete aurora events in the strong crustal magnetic field region in the southern hemisphere are the brightest and most repeatable compared to events occurring outside the region. Our new data set offers a more complete and accurate characterization of aurora in this area. The region of strongest crustal fields is composed of two distinct magnetic regions, with magnetic fields in opposite directions; discrete aurora events trigger in one region after dusk and in the other before dawn. Magnetic reconnection in these two adjacent regions with the draped interplanetary field may open the crustal fields in these regions during opposing local times. Particle precipitation can then cause discrete aurora at the observed times and locations. Plain Language Summary: Mars has a surprising variety of types of aurora, all different from Earth's "northern lights." Mars lacks the familiar high‐latitude aurora because it no longer has the same kind of global magnetic field Earth does. This study examines "discrete aurora" events that occur in region in Mars southern hemisphere that has retained some of the ancient magnetic field. It takes the form of long arcades of magnetic loops that resemble a set of arches. As Mars rotates, these arcades are carried around the planet as the solar wind and its imbedded magnetic field are carried past the planet. We show that when conditions are favorable, the magnetic field locked in the solar wind can interact and "reconnect" with Mars magnetic loops, allowing energetic particles to spiral down the field lines into the atmosphere to cause discrete aurora. Key Points: New data confirm that Mars discrete aurora events occur most frequently near strong crustal fields and vary with local timeAuroral events in adjacent regions with opposite magnetic polarity occur before or after midnight depending on local magnetic field directionMagnetic reconnection between crustal fields and the draped interplanetary field appears to control regional and local time behavior [ABSTRACT FROM AUTHOR]

At Mars, charge exchange between solar wind protons and neutral exospheric hydrogen produces energetic neutral atoms (ENAs) that can penetrate into the collisional atmosphere, where they can be converted through collisions into H+ and H−. The Mars Atmosphere and Volatile EvolutioN mission observed a population of negatively charged particles at low altitudes, whose energies, angular distribution, and dependence on the upstream solar wind were consistent with H− produced by solar wind hydrogen ENAs. The highest fluxes of H− were observed near perihelion and the southern summer solstice. We calculated an average ratio of ∼4% between H− density and H+ density, implying a slightly smaller relative abundance than reported previously (∼10%). We found that the fraction of H ENAs converted to H− increases with the solar wind energy, in agreement with laboratory measurements of the H–CO2 electron capture cross section. Plain Language Summary: At Mars, interactions between solar wind protons and neutral hydrogen in the outer atmosphere produces energetic neutral atoms that travel into the inner atmosphere, where collisions with atmospheric gas can produce H+ ions and H− ions. The Mars Atmosphere and Volatile EvolutioN mission observed H− ions in the inner atmosphere, with energies and velocities that matched the solar wind. The highest fluxes of H− ions were seen when Mars was closest to the Sun, during the southern summer. We found that the relative amount of H− ions to H+ ions was smaller than previous studies had found. We also found that the amount of H− ions produced depended on the solar wind energy, a result that agrees with laboratory experiments. Key Points: We observed H− ions at low altitudes in the Martian atmosphere whose energies and angular distribution matched those of the upstream solar windFluxes of H− varied seasonally, with a maximum near perihelion and the southern summer solsticeWe found a ratio of about four percent between H− density and H+ density, lower than previously reported ratios [ABSTRACT FROM AUTHOR]

Using observations from Mars Atmosphere and Volatile EvolutioN's Neutral Gas and Ion Mass Spectrometer, we characterize the seasonal, solar zenith angle (SZA), and solar flux dependent variations of the O+ peak and the O+/O 2+ ratio in the topside ionosphere of Mars. We find that the O+ peak is between 220 and 300 km and forms at a roughly constant neutral atmospheric pressure level of 10( − 8.7 ± 0.4) Pa. The O+ peak altitude also decreases with increasing SZA near the terminator and varies sinusoidally with an amplitude of 26 km over a period of one Mars year in response to the changing solar insolation. The O+ peak altitude reaches a maximum near Northern Winter solstice and Mars perihelion. The O+ peak density on the dayside has an average value of (1.1 ± 0.5) × 103 cm−3, has no dependence on SZA for SZAs up to ∼90°, and is mainly controlled by the thermospheric O/CO2 ratio as predicted by photochemical theory. Above the O+ peak, the O+/O 2+ ratio in the dayside ionosphere approaches a constant value of 1.1 ± 0.6, decreases with increasing SZA, and is highly variable on timescales of days or less. We discuss why the O+ peak is different than the main (M2) peak at Mars and why it is similar to the F2 peak at Earth. Key Points: The O+ peak forms at a roughly constant neutral pressure level on the daysideThe O+ peak density is controlled by the neutral O/CO2 ratio as predicted by photochemical theoryThe topside O+/O 2+ ratio decreases with increasing SZA and is highly variable on timescales of days or less [ABSTRACT FROM AUTHOR]

Solar wind protons can interact directly with the hydrogen corona of Mars through charge exchange, resulting in energetic neutral atoms (ENAs) able to penetrate deep into the upper atmosphere of Mars. ENAs can undergo multiple charge changing interactions, leading to an observable beam of penetrating protons in the upper atmosphere. We seek to characterize the behavior of these protons in the presence of magnetic fields using data collected by the Mars Atmosphere and Volatile EvolutioN spacecraft. We find that backscattered penetrating proton flux is enhanced in regions where the magnetic field strength is greater than 200 nT. We also find a strong correlation at CO 2 column densities less than 5.5 × 10 14  cm -2 between magnetic field strength and the observed backscattered and downward flux. We do not see significant changes in penetrating proton flux with magnetic field strengths on the order of 10 nT., (© 2022. The Authors.)

The objective of this White Paper, submitted to ESA's Voyage 2050 call, is to get a more holistic knowledge of the dynamics of the Martian plasma system, from its surface up to the undisturbed solar wind outside of the induced magnetosphere. This can only be achieved with coordinated multi-point observations with high temporal resolution as they have the scientific potential to track the whole dynamics of the system (from small to large scales), and they constitute the next generation of the exploration of Mars analogous to what happened at Earth a few decades ago. This White Paper discusses the key science questions that are still open at Mars and how they could be addressed with coordinated multipoint missions. The main science questions are: (i) How does solar wind driving impact the dynamics of the magnetosphere and ionosphere? (ii) What is the structure and nature of the tail of Mars' magnetosphere at all scales? (iii) How does the lower atmosphere couple to the upper atmosphere? (iv) Why should we have a permanent in-situ Space Weather monitor at Mars? Each science question is devoted to a specific plasma region, and includes several specific scientific objectives to study in the coming decades. In addition, two mission concepts are also proposed based on coordinated multi-point science from a constellation of orbiting and ground-based platforms, which focus on understanding and solving the current science gaps., Competing Interests: Conflicts of interest/Competing interestsNot applicable., (© The Author(s) 2021.)