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Abstract: We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All hybrid results, however, showed two main channels for oxygen ion escape (a pickup ion ‘plume’ in the hemisphere toward which the solar wind convection electric field is directed, and a channel in the opposite hemisphere of the central magnetotail), while the MHD models showed one (a roughly symmetric channel in the central magnetotail). Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars’ past using models. [Copyright &y& Elsevier]

Abstract: The analyser of space plasma and energetic atoms (ASPERA-3) neutral particle imager (NPI) on board Mars Express (MEX) is devoted to energetic neutral atom (ENA) detection within the Martian environment. These ENAs originate from the interaction between the energetic ions flowing inside the Martian environment and the exospheric neutral gas, thus providing crucial information about the dynamics of this interaction. NPI records the instantaneous angular distribution of the energy-integrated ENA signal. In order to identify recurrent ENA signals in the Martian environment, we have performed a statistical analysis of the NPI data. Count rates have been averaged using different methods in order to be able to discriminate signals coming from the planet, from a selected direction, or from specific planetographic regions at the planetary surface. Possible recurrent ENA signals (about 5×106 (cm2 srs)−1) are found coming from the terminator direction and above the atmosphere toward nightside when the spacecraft was inside the planetary shadow, mainly close to the shadow edge. Some significant signal was found from the anti-Mars directions in 2005. No statistically significant signal related to pick-up ions from the atmosphere or related to magnetic anomalies above the sensor intrinsic error (estimated as 3×106 (cm2 srs)−1) was observed. Our analysis shows that particular attention should be given to the use of NPI data when performing statistical studies; in fact, the sensor has some intrinsic limitations due to inadequate UV suppression, difficulties in sector inter-calibrations, and variations in the sector response versus time. [Copyright &y& Elsevier]

Although the Mars Express (MEX) does not carry a magnetometer, it is in principle possible to derive the interplanetary magnetic field (IMF) orientation from the three dimensional velocity distribution of pick-up ions measured by the Ion Mass Analyser (IMA) on board MEX because pick-up ions' orbits, in velocity phase space, are expected to gyrate around the IMF when the IMF is relatively uniform on a scale larger than the proton gyroradius. During bow shock outbound crossings, MEX often observed cycloid distributions (two dimensional partial ring distributions in velocity phase space) of protons in a narrow channel of the IMA detector (only one azimuth for many polar angles). We show two such examples. Three different methods are used to derive the IMF orientation from the observed cycloid distributions. One method is intuitive (intuitive method), while the others derive the minimum variance direction of the velocity vectors for the observed ring ions. These velocity vectors are selected either manually (manual method) or automatically using simple filters (automatic method). While the intuitive method and the manual method provide similar IMF orientations by which the observed cycloid distribution is well arranged into a partial circle (representing gyration) and constant parallel velocity, the automatic method failed to arrange the data to the degree of the manual method, yielding about a 30° offset in the estimated IMF direction. The uncertainty of the derived IMF orientation is strongly affected by the instrument resolution. The source population for these ring distributions is most likely newly ionized hydrogen atoms, which are picked up by the solar wind. [ABSTRACT FROM AUTHOR]