Robert J. Lillis

Variability of the altitude of the Martian sheath

Received 31 March 2005; revised 18 August 2005; accepted 25 August 2005; published 24 September 2005. [1] Using electron energy spectra, we identify time periods when the Mars Global Surveyor (MGS) spacecraft is in or above the Martian magnetic pileup boundary (MPB). We use more than five years of data to develop a statistical picture of the location of the MPB relative to the MGS mapping altitude near 400 km. We show for the first time that the MPB location is sensitive to interplanetary magnetic field (IMF) orientation and to Martian season, and confirm a dependence upon solar wind pressure. We confirm that crustal magnetic sources raise the altitude of the MPB, and demonstrate that sheath electrons populate magnetic cusp regions in the southern hemisphere. During southern summer strong crustal fields near the subsolar point raise the altitude of the MPB over the entire dayside, implying that Martian crustal fields modify the solar wind interaction globally. Citation: Brain, D. A., J. S. Halekas, R. Lillis, D. L. Mitchell, R. P. Lin, and D. H. Crider (2005), Variability of the altitude of the Martian sheath, Geophys. Res. Lett., 32, L18203,

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).

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.

MAVEN observations of solar wind hydrogen deposition in the atmosphere of Mars

Mars Atmosphere and Volatile EvolutioN mission (MAVEN) observes a tenuous but ubiquitous flux of protons with the same energy as the solar wind in the Martian atmosphere. During high flux intervals, we observe a corresponding negative hydrogen population. The correlation between penetrating and solar wind fluxes, the constant energy, and the lack of a corresponding charged population at intermediate altitudes implicate products of hydrogen energetic neutral atoms from charge exchange between the upstream solar wind and the exosphere. These atoms, previously observed in neutral form, penetrate the magnetosphere unaffected by electromagnetic fields (retaining the solar wind velocity), and some fraction reconvert to charged form through collisions with the atmosphere. MAVEN characterizes the energy and angular distributions of both penetrating and backscattered particles, potentially providing information about the solar wind, the hydrogen corona, and collisional interactions in the atmosphere. The accretion of solar wind hydrogen may provide an important source term to the Martian atmosphere over the planets history.

MAVEN insights into oxygen pickup ions at Mars

Since Mars Atmosphere and Volatile EvolutioN (MAVEN)s arrival at Mars on 21 September 2014, the SEP (Solar Energetic Particle) instrument on board the MAVEN spacecraft has been detecting oxygen pickup ions with energies of a few tens of keV up to ~200 keV. These ions are created in the distant upstream part of the hot atomic oxygen exosphere of Mars, via photoionization, charge exchange with solar wind protons, and electron impact. Once ionized, atomic oxygen ions are picked up by the solar wind and accelerated downstream, reaching energies high enough for SEP to detect them. We model the flux of oxygen pickup ions observed by MAVEN SEP in the undisturbed upstream solar wind and compare our results with SEPs measurements. Model-data comparisons of SEP fluxes confirm that pickup oxygen associated with the Martian exospheric hot oxygen is indeed responsible for the MAVEN SEP observations.

Discovery of diffuse aurora on Mars

Planetary auroras reveal the complex interplay between an atmosphere and the surrounding plasma environment. We report the discovery of low-altitude, diffuse auroras spanning much of Mars’ northern hemisphere, coincident with a solar energetic particle outburst. The Imaging Ultraviolet Spectrograph, a remote sensing instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, detected auroral emission in virtually all nightside observations for ~5 days, spanning nearly all geographic longitudes. Emission extended down to ~60 kilometer (km) altitude (1 microbar), deeper than confirmed at any other planet. Solar energetic particles were observed up to 200 kilo–electron volts; these particles are capable of penetrating down to the 60 km altitude. Given minimal magnetic fields over most of the planet, Mars is likely to exhibit auroras more globally than Earth.

MAVEN measured oxygen and hydrogen pickup ions: Probing the Martian exosphere and neutral escape

Soon after the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft started orbiting Mars, the SEP (Solar Energetic Particle), SWIA (Solar Wind Ion Analyzer), and STATIC (Supra-Thermal and Thermal Ion Composition) instruments onboard the spacecraft detected planetary pickup ions. SEP can measure energetic (>60 keV) oxygen pickup ions, the source of which is the extended hot oxygen exosphere of Mars. Model results show that these pickup ions originate from tens of Martian radii upstream of Mars and are energized by the solar wind motional electric field as they gyrate back towards Mars. SWIA and STATIC can detect both pickup oxygen and pickup hydrogen with energies below ~30 keV and created closer to Mars. In this study, data from the SEP, SWIA, and STATIC instruments containing pickup ion signatures are provided and model-data comparisons are shown. During the times when MAVEN is outside the Martian bow shock and in the upstream undisturbed solar wind, the solar wind velocity measured by SWIA and the solar wind (or interplanetary) magnetic field measured by the MAG (magnetometer) instrument can be used to model pickup oxygen and hydrogen fluxes. By comparing measured pickup ion fluxes with model results, the Martian thermal hydrogen and hot oxygen neutral densities can be probed outside the bow shock, providing a helpful tool in constraining estimates of neutral oxygen and hydrogen escape rates. Our analysis reveals an order of magnitude density change with Mars season in the hydrogen exosphere, whereas the hot oxygen exosphere was found to remain steadier.

Hot oxygen corona at Mars and the photochemical escape of oxygen: Improved description of the thermosphere, ionosphere, and exosphere

The Mars Adaptive Mesh Particle Simulator model is coupled with the Mars Global Ionosphere Thermosphere Model for the first time to provide an improved description of the Martian hot O corona based on our modeling studies of O2+ dissociative recombination. A total of 12 cases comprising three solar activity levels and four orbital positions is considered to study the solar cycle and seasonal variability. The newly coupled framework includes two additional thermospheric species and adopts a realistic forward scattering scheme using the angular differential cross sections. We present the effects of these changes on the resulting hot O corona and escape rate. A comparison between the simulated hot O corona and the recent observations from the ALICE/Rosetta instrument showed a reasonable agreement, considering the large uncertainties in the data. We assume that some discrepancies near the transition altitude may be originated from the averaging over the altitude range, where the cold and hot O densities become comparable. The revised O escape rates by our new coupled framework range from ~1.21 × 1025 s−1 to ~5.43 × 1025 s−1.

Computing uncertainties in ionosphere‐airglow models: II. The Martian airglow

[1] One of the objectives of spectrometers onboard space missions is to retrieve atmospheric parameters (notably density, composition and temperature). To fulfill this objective, comparisons between observations and model results are necessary. Knowledge of these model uncertainties is therefore necessary, although usually not considered, to estimate the accuracy in planetary upper atmosphere remote sensing of these parameters. In Part I of this study, “Computing uncertainties in ionosphere-airglow models: I. Electron flux and species production uncertainties for Mars” (Gronoff et al., 2012), we presented the uncertainties in the production of excited states and ionized species from photon and electron impacts, computed with a Monte-Carlo approach, and we applied this technique to the Martian upper atmosphere. In the present paper, we present the results of propagation of these production errors to the main UV emissions and the study of other sources of uncertainties. As an example, we studied several aspects of the model uncertainties in the thermosphere of Mars, and especially the O( 1 S) green line (557.7 nm, with its equivalent, the trans-auroral line at 297.2 nm), the Cameron bands CO(a 3 P), and CO2 (B 2 Su ) doublet emissions. We first show that the excited species at the origin of these emissions are mainly produced by electron and photon impact. We demonstrate that it is possible to reduce the computation time by decoupling the different sources of uncertainties; moreover, we show that emission uncertainties can be large (>30%) because of the strong sensitivity to the production uncertainties. Our study demonstrates that uncertainty calculations are a crucial step prior to performing remote sensing in the atmosphere of Mars and the other planets and can be used as a guide to subsequent adjustments of cross sections based on aeronomical observations. Finally, we compare the simulations with observations from the SPICAM spectrometer on the Mars Express spacecraft. The production of excited species at the origin of the green line, the CO Cameron bands and the CO2 (B) doublet is found to be on the dayside, consistent with photon and electron impact on CO2 as the main source of excitation of the three emissions, in contrast to the findings of Huestis et al. (2010) for the O( 1 S) case. Moreover, we re-examine the cross section for the production of the Cameron bands by electron impact on CO2.