Solar wind erosion of the polar regions of the Mars ionosphere

Abstract

[1] Measurements conducted with the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) instrument of the Mars Express spacecraft provide data of plasma fluxes that stream away from the polar regions of the Mars ionosphere with energy spectra whose peak value increases with distance from the planetary surface. The observed energy distribution reveals a velocity boundary layer with ionospheric plasma that is eroded from the polar regions of the Mars ionosphere and that extends in the downstream direction within a geometry similar to that present along the polar flanks of the Venus ionosheath. The direction of motion of the ionospheric particles in those fluxes is close to that of the solar wind velocity and is not also oriented in a transverse direction as would have been expected if they were solely accelerated by the convective electric field of the solar wind. The ionospheric plasma eroded and deviated by the solar wind within the boundary layer forms a region whose shape is compatible with that of the asymmetric Mars plasma halo that was inferred from the X-ray emission lines measured with the reflecting grating spectrometer of the XMM-Newton telescope. The latter emission is interpreted as resulting from thermal dissipative processes associated with the transport of solar wind momentum to the polar upper ionosphere where both plasma populations interact with each other. Different conditions are applicable throughout most of the dayside hemisphere where the enhanced interplanetary magnetic field intensities that are observed within the ionosphere should make the interaction of the oncoming solar wind plasma with the ionospheric material less efficient.

Rosetta and Mars Express observations of the influence of high solar wind pressure on the Martian plasma environment

“…As reported by Dubinin et al (2009) and Nilsson et al (2009) the ion outflow from Mars increases during high pressure solar wind. Planetary ion outflow was, in fact, also directly observed to occur during the time of the Rosetta swingby (Pérez-de-Tejada et al, 2009). In this paper, we have combined these results and further strengthened the evidence that high pressure solar wind causes an increase of planetary ion outflow from Mars.…”

“…The shape asymmetry lasts until about two orbits after the solar wind feature has passed. As reported by Pérez-de-Tejada et al (2009), the exosphere of Mars, as seen in X-ray images from the XMM/Newton satellite, is also very asymmetric during this interval. The X-ray emissions from the exosphere are more intense over the poles and also indicate that the exosphere over the poles is tilted in an anti-sunward direction.…”

“…The escaping ions, as observed by MEX, gained in energy in the direction, as indicated by Rosetta MAG measurements, of the convective electric field. The exosphere was extended over both poles and the ion outflow was observed over the Northern Hemisphere (Pérez-de-Tejada et al, 2009). In summary, we conclude the high pressure solar wind enables the increased erosion of the ionosphere of Mars at this time.…”

“…MEX can also provide sporadic monitoring of the solar wind during the intervals spent outside the BS. Not reported in the study by Pérez-de-Tejada et al (2009) is that their observations take place during a period of very active solar wind of high pressure. In this paper we observe asymmetric shapes of the BS and MPB and link this to the outflowing ions and asymmetric exosphere reported by Pérez-de-Tejada et al (2009) by arguing that the same effect took place as reported by Dubinin et al (2009) during a high pressure CIR.…”

“…Nilsson et al (2009) have also performed a statistical study showing that the outflow of ions increases with increased solar wind dynamic pressure. Pérez-de-Tejada et al (2009) performed a study where they observe escaping planetary ions on the 25 February 2007 in MEX data. Simultaneous XMM/Newton X-ray images show evidence of an extended and asymmetric exosphere of Mars.…”