S. M. Curry

MAVEN observations of the response of Mars to an interplanetary coronal mass ejection

Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

The spatial distribution of planetary ion fluxes near Mars observed by MAVEN

We present the results of an initial effort to statistically map the fluxes of planetary ions on a closed surface around Mars. Choosing a spherical shell ~1000 km above the planet, we map both outgoing and incoming ion fluxes (with energies >25 eV) over a 4 month period. The results show net escape of planetary ions behind Mars and strong fluxes of escaping ions from the northern hemisphere with respect to the solar wind convection electric field. Planetary ions also travel toward the planet, and return fluxes are particularly strong in the southern electric field hemisphere. We obtain a lower bound estimate for planetary ion escape of ~3 × 1024 s−1, accounting for the ~10% of ions that return toward the planet and assuming that the ~70% of the surface covered so far is representative of the regions not yet visited by Mars Atmosphere and Volatile EvolutioN (MAVEN).

Strong plume fluxes at Mars observed by MAVEN: An important planetary ion escape channel†

The accepted version of the above article was posted prematurely on September 11, 2015. The final version of record will be made fully available at later date along with a special collection of related papers.

Early MAVEN Deep Dip campaign reveals thermosphere and ionosphere variability

The Mars Atmosphere and Volatile Evolution (MAVEN) mission, during the second of its Deep Dip campaigns, made comprehensive measurements of martian thermosphere and ionosphere composition, structure, and variability at altitudes down to ~130 kilometers in the subsolar region. This altitude range contains the diffusively separated upper atmosphere just above the well-mixed atmosphere, the layer of peak extreme ultraviolet heating and primary reservoir for atmospheric escape. In situ measurements of the upper atmosphere reveal previously unmeasured populations of neutral and charged particles, the homopause altitude at approximately 130 kilometers, and an unexpected level of variability both on an orbit-to-orbit basis and within individual orbits. These observations help constrain volatile escape processes controlled by thermosphere and ionosphere structure and variability.

Response of Mars O+ pickup ions to the 8 March 2015 ICME: Inferences from MAVEN data‐based models

We simulate and compare three phases of the Mars-solar wind interaction with the 8 March interplanetary coronal mass ejection (ICME) event using Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observations in order to derive heavy ion precipitation and escape rates. The MAVEN observations provide the initial conditions for three steady state MHD model cases, which reproduce the observed features in the solar wind density, velocity, and magnetic field seen along the MAVEN orbit. Applying the MHD results to a kinetic test particle model, we simulate global precipitation and escape maps of O+ during the (1) pre-ICME phase, (2) sheath phase, and (3) ejecta phase. We find that the Case 1 had the lowest precipitation and escape rates of 9.5 × 1025 and 4.1 × 1025 s−1, Case 2 had the highest rates of 9.5 × 1025 and 4.1 × 1025 s−1, and Case 3 had rates of 3.2 × 1025 and 1.3 × 1025 s−1, respectively. Additionally, Case 2 produced a high-energy escaping plume >10 keV, which mirrored corresponding STATIC observations.

Marsward and tailward ions in the near‐Mars magnetotail: MAVEN observations

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.

Implications of MAVEN Mars near‐wake measurements and models

Mars is typically viewed as a member of the category of weakly magnetized planets, with a largely induced magnetosphere and magnetotail produced by the draped fields of the solar wind interaction. However, selected MAVEN suprathermal electron and magnetic field observations in the near wake, sampled along its elliptical orbit during the early prime mission at altitudes ranging from its ~150 km periapsis to the tail magnetosheath, reinforce a picture seen in an MHD model where magnetic fields are rooted in the planet throughout much of the Martian magnetotail.  The Mars-solar wind interaction has often been viewed as a largely induced (Venus-like) magnetosphere type.  In contrast, MHD models of the interaction suggest much of its wake magnetic flux may be rooted in Mars.  MAVEN suprathermal electron anisotropy measurements, together with magnetic field measurements show some support for this alternate picture, at least at the present epoch.

Test particle comparison of heavy atomic and molecular ion distributions at Mars

This study uses the Mars Test Particle simulation to create virtual detections of O+, O2+, and CO2+ in an orbital configuration in the Mars space environment. These atomic and molecular planetary pickup ions are formed when the solar wind directly interacts with the neutral atmosphere, causing the ions to be accelerated by the background convective electric field. The subsequent ion escape is the subject of great interest, specifically with respect to which species dominates ion loss from Mars. O+ is found to be the dominant escaping ion because of the large sources of transported ions in the low-energy ( 1 keV) range. O2+ and CO2+ are observed at these energy ranges but with much lower fluxes and are generally only found in the tail between 10 eV and 1 keV. Using individual particle traces, we reveal the origin and trajectories of the low-energy downtail O+ populations and high-energy polar O+ populations that contribute to the total escape. Comparing them against O2+ and CO2+ reveals that the extended hot oxygen corona contributes to source regions of high- and low-energy escaping ions. Additionally, we present results for solar minimum and maximum conditions with respect to ion fluxes and energies in order to robustly describe the physical processes controlling planetary ion distributions and atmospheric escape.

Martian magnetic storms

The response of Mars to the major space weather events called interplanetary coronal mass ejections (ICMEs) is of interest for both general planetary solar wind interaction studies and related speculations on their evolutionary consequences—especially with respect to atmosphere escape. Various particle and field signatures of ICMEs have been observed on Phobos-2, Mars Global Surveyor (MGS), Mars Express (MEX), and now Mars Atmosphere and Volatile EvolutioN (MAVEN). Of these, MAVENs combined instrumentation and orbit geometry is particularly well suited to characterize both the event drivers and their consequences. However, MAVEN has detected only moderate disturbances at Mars due in large part to the general weakness of the present solar cycle. Nevertheless, the strongest event observed by MAVEN in March 2015 provides an example illustrating how further insights can be gained from available models. Here we first look more closely at what previously run BATS-R-US MHD simulations of the combined MAVEN observations tell us about the March 2015 event consequences. We then use analogous models to infer those same responses, including magnetic field topology changes and ionospheric consequences, to a hypothetical extreme ICME at Mars based on STEREO A measurements in July 2012. The results suggest how greatly enhanced, yet realistic, solar wind pressure, magnetic field, and convection electric field combine to produce strong magnetospheric coupling with important consequences for upper atmosphere and ionosphere energization.

Martian planetary heavy ion sputtering of Phobos

The Martian moons, Phobos and Deimos, have long been suspected to be the sources of tenuous neutral gas tori encircling Mars. While direct outgassing has been ruled out as a strong source, micrometeoroid impact vaporization and charged particle sputtering must operate based on observations at other airless bodies. Previous models have addressed solar wind sputtering of Phobos; however, Phobos and Deimos are also subject to a significant, yet temporally variable, flux of heavy planetary ions escaping from Mars. In this report, we use a combination MHD/test-particle model to calculate the planetary heavy ion flux to Phobos and the ensuing neutral sputtered flux. Depending on ambient solar wind conditions and the location of Phobos, heavy ion sputtering of Phobos generates neutral fluxes up to and exceeding that from solar wind sputtering. We model pickup ions from the Phobos torus itself with applications for observations by the upcoming Mars Atmospheric and Volatile Evolution mission.