F. Leblanc

Mercury's sodium exosphere

Abstract Mercurys neutral sodium exosphere is simulated using a comprehensive 3D Monte Carlo model following sodium atoms ejected from Mercurys surface by thermal desorption, photon stimulated desorption, micro-meteoroid vaporization and solar wind sputtering. The evolution of the sodium surface density with respect to Mercurys rotation and its motion around the Sun is taken into account by considering enrichment processes due to surface trapping of neutrals and ions and depletion of the sodium available for ejection from the surfaces of grains. The change in the sodium exosphere is calculated during one Mercury year taking into account the variations in the solar radiation pressure, the photo-ionization frequency, the solar wind density, the photon and meteoroid flux intensities, and the surface temperature. Line-of-sight column densities at different phase angles, the supply rate of new sodium, average neutral and ion losses over a Mercury year, surface density distribution and the importance of the different processes of ejection are discussed in this paper. The sodium surface density distribution is found to become significantly nonuniform from day to night sides, from low to high latitudes and from morning to afternoon because of rapid depletion of sodium atoms in the surfaces of grains mainly driven by thermal depletion. The shape of the exosphere, as it would be seen from the Earth, changes drastically with respect to Mercurys heliocentric position. High latitude column density maxima are related to maxima in the sodium surface concentration at high latitudes in Mercurys surface and are not necessarily due to solar wind sputtering. The ratio between the sodium column density on the morning side of Mercurys exosphere and the sodium column density on the afternoon side is consistent with the conclusions of Sprague et al. (1997, Icarus 129, 506–527). The model, which has no fitting parameters, shows surprisingly good agreement with recent observations of Potter et al. (2002, Meteor. Planet. Sci. 8, 3357–3374) successfully explaining their velocity and column density profiles vs. heliocentric distance. Comparison with this data allows us to constrain the supply rate of new sodium atoms to the surface. We also discuss the possible origins of the strong high latitude emissions (Potter and Morgan, 1990, Science 248, 835–838; 1997a, Adv. Space Res. 19, 1571–1576; 1997b, Planet. Space Sci. 45, 95–100; Sprague et al., 1998, Icarus 135, 60–68) and the strong variations of the total content of the sodium exosphere on short (Potter et al., 1999, Planet. Space Sci. 47, 1441–1449) and long time scales (Sprague et al., 1997, Icarus 129, 506–527).

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.

Observations of aurorae by SPICAM ultraviolet spectrograph on board Mars Express: Simultaneous ASPERA‐3 and MARSIS measurements

We present a new set of observations of Martian aurorae obtained by Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) on board Mars Express (MEX). Using nadir viewing, several auroral events have been identified on the Martian nightside, all near regions of crustal magnetic fields. For most of these events, two to three consecutive events with variable intensities and separated by a few seconds to several tens of seconds have been observed, whereas simultaneous observations with Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) have been possible. In this paper, we present the data set for these events and discuss the possible correlation between the measured UV emission by SPICAM, the measured downward and/or upward flux of electrons by ASPERA-3 and the total electron content recorded by MARSIS. Despite the limited coverage of SPICAM ultraviolet spectrograph (UVS) on the Martian nightside (essentially in regions of high crustal magnetic fields), there is however a very good correlation between the regions with the locally smallest probability to be on closed crustal magnetic field lines, as derived from Mars Global Surveyor/Electron Reflectometer (MGS/MAG-ER), and the position of an aurora event. This suggests that the crustal magnetic fields, when organized into cusp-like structure, can trigger the few aurorae identified by SPICAM UVS. It confirms also the good probability, in the cases where SPICAM UVS measured UV emissions, that the increase in the measured total electron content by MARSIS and the simultaneous measured precipitating electron flux by the ASPERA-3/Electron Spectrometer may be related to each other.

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.

Martian corona: Nonthermal sources of hot heavy species

[1] We have studied the production of hot O and C atoms, and hot CO 2 and CO molecules in the Martian upper atmosphere and exosphere by dissociative recombination (DR) of O 2 + and CO + ions, and sputtering of the atmosphere by incident O + pick-up ions. Production and collisional thermalization of the hot particles in the upper atmosphere are described by using a unique Monte Carlo test particle approach to simulate both nonthermal processes. Velocity distributions, atmospheric loss rates, and density profiles are derived for suprathermal 0, C, CO, and CO 2 at low and high solar activity. At high solar activity the hot oxygen escape rate estimated from DR of O 2 + is found to be less than two times the sputtering rate. Sputtering is found to efficiently populate the corona with molecular species such as CO and CO 2 at high solar activity and also to produce a carbon escape rate that is comparable to that derived from the major photochemical sources. Dissociation of CO 2 molecules by the impacting pick-up ions flux are found to produce about 50% of the sputtered exospheric oxygen density at high solar activity. Collisions of the background atmospheric gas with hot O atoms produced by DR of O 2 + produce densities of hot CO 2 and CO molecules larger than 10 2 cm -3 for altitudes lower than 1000 km, at both high and low solar activity. Interestingly, the hot CO 2 density scale height is observed to be process dependent. The hot oxygen energy distributions associated with sputtering and DR near the exobase are also found to follow distinct decreasing energy laws. We suggest that the effects of the solar zenithal angle (SZA), crustal magnetic fields, and atmospheric tides on the ionospheric structure may produce exospheric signatures.

Monte Carlo model of electron transport for the calculation of Mars dayglow emissions

[1] A model of the photoelectron collision-induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on a stochastic solution of the Boltzmann equation using the direct simulation Monte Carlo method. The neutral atmosphere is taken from outputs of a global circulation model, and recent inelastic collision cross sections are adopted. The calculated vertical profiles of the CO Cameron bands and CO2 doublet emissions integrated along the line of sight compare well with the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) limb profiles observed with the SPICAM spectrograph on board Mars Express made at Ls = 166 during the summer season at northern midlatitudes. The comparison shows agreement to within the uncertainties of the excitation cross sections. Seasonal changes in the brightness and the altitude of the emission peaks are predicted with intensity variations in the range 15–20%.

Solar energetic particle event at Mercury

Abstract Mercurys magnetosphere only partially protects its surface from solar particles. When a solar energetic particle (SEP) event encounters Mercury, a significant flux of energetic particles will reach Mercurys surface which can change Mercurys exosphere. In this paper, we describe some of the consequences of the encounter of a SEP event with Mercury before and few hours after the shock associated with such an event reaches Mercury. Energetic ions and electrons with energy between 10 keV and 10 MeV are followed across a model of Mercurys magnetosphere (J. Geophys. Res. 103 (1998) 9113). The effects of such an encounter on the ion and neutral exospheres are estimated for one particular gradual proton event reported by Reames et al. (Astrophys. J. 483 (1997a) 515) and scaled to Mercurys orbit. After the arrival of these SEP at Mercury, a population of quasi-trapped energetic ions and electrons is expected close to Mercury which is stable for hours after their arrival at Mercury. A significant dawn/dusk charge separation is observed. A fraction of the initial energetic particles (∼10%) impact the surface with a spatial distribution that exhibits north/south and dawn/dusk asymmetries. The flux of particles impacting the surface and the ability of a quasi-trapped population to be maintained near Mercury are highly dependent on the Bz sign of the interplanetary magnetic field. The impacting SEP can eject a non-uniform distribution of sodium atoms into Mercurys exosphere and can be the origin of several exospheric features observed during last decade.

Space Weather Storm Responses at Mars: Lessons from A Weakly Magnetized Terrestrial Planet

Much can be learned from terrestrial planets that appear to have had the potential to be habitable, but failed to realize that potential. Mars shows evidence of a once hospitable surface environment. The reasons for its current state, and in particular its thin atmosphere and dry surface, are of great interest for what they can tell us about habitable zone planet outcomes. A main goal of the MAVEN mission is to observe Mars’ atmosphere responses to solar and space weather influences, and in particular atmosphere escape related to space weather ‘storms’ caused by interplanetary coronal mass ejections (ICMEs). Numerical experiments with a data-validated MHD model suggest how the effects of an observed moderately strong ICME compare to what happens during a more extreme event. The results suggest the kinds of solar and space weather conditions that can have evolutionary importance at a planet like Mars.

The 2π charged particles analyzer: All-sky camera concept and development for space missions

Increasing the temporal resolution and instant coverage of velocity space of space plasma measurements is one of the key issues for experimentalists. Today the top-hat plasma analyzer appears to be the favorite solution due to its relative simplicity and the possibility to extend its application by adding a mass-analysis section and an electrostatic angular scanner. Similarly, great success has been achieved in MMS mission using such multiple top-hat analyzers to achieve unprecedented temporal resolution. An instantaneous angular coverage of charged particles measurements is an alternative approach to pursuing the goal of high time resolution. This was done with FONEMA 4-D and, to a lesser extent, by DYMIO instruments for Mars-96 and with the FIPS instrument for MESSENGER mission. In this paper we describe, along with precursors, a plasma analyzer with a 2π electrostatic mirror that was developed originally for the Phobos-Soil mission with a follow-up in the frame of the BepiColombo mission, and is under development for future Russian missions. Different versions of instrument are discussed along with their advantages and drawbacks.