The Martian atmospheric ion escape rate dependence on solar wind and solar EUV conditions: 1. Seven years of Mars Express observations

Abstract

More than 7 years of ion flux measurements in the energy range 10 eV-15 keV have allowed the ASPERA-3/IMA (Analyzer of Space Plasmas and Energetic Ions/Ion Mass Analyzer) instrument on Mars Express to collect a large database of ion measurements in the Mars environment, over a wide range of upstream solar wind (density and velocity) and radiation (solar EUV intensity) conditions. We investigate the influence of these parameters on the Martian atmospheric ion escape rate by integrating IMA heavy ion flux measurements taken in the Martian tail at similar (binned) solar wind density (n sw ), velocity (v sw ), and solar EUV intensity (I EUV ) conditions. For the same solar wind velocity and EUV intensity ranges (v sw and I s constrained), we find a statistically significant decrease of up to a factor of 3 in the atmospheric ion escape rate with increased average solar wind density (5.6 × 10 24 s −1 to 1.9 × 10 24 s −1 for 0.4 cm −3 and 1.4 cm −3 , respectively). For low solar wind density (0.1-0.5 cm −3 ) and low EUV intensity, the escape rate increases with increasing solar wind velocity from 2.4 × 10 24 s −1 to 5.6 × 10 24 s −1 . During high solar EUV intensities the escape fluxes are highly variable, leading to large uncertainties in the estimated escape rates; however, a statistically significant increase in the escape rate is found between low/high EUV for similar solar wind conditions. Empirical-analytical models for atmospheric escape are developed by fitting calculated escape rates to all sufficiently sampled upstream conditions.

Solar wind interaction with the Martian upper atmosphere: Crustal field orientation, solar cycle, and seasonal variations

“…One possible explanation is that we did not include the neutral wind in our simulations, which can greatly affect the ion loss [Brecht and Ledvina, 2014b]. On the other hand, the recent paper published by Ramstad et al [2015] showed that the solar wind density and velocity can greatly affect the ratio of escape rate between low and high solar EUV conditions. They adopted more than 7 years of ion flux measurements in the energy range 10 eV to 15 keV from ASPERA-3/IMA instrument on board MEX.…”

Why an intrinsic magnetic field does not protect a planet against atmospheric escape

“…Its contribution is significant in the 10 24 −10 25 s −1 range for Mars and Venus (Lammer 2013;Ramstad et al 2015;Masunaga et al 2013), but decreases rapidly when the magnetosphere grows with increasing magnetic moment (Appendix A.2 and Fig. 3).…”

Mars: a small terrestrial planet

“…The authors also derived an empirical expression for the Martian escape rate versus solar activity F10.7 index and the sunspot number, which is a useful tool to derive the accumulated ion escape rate from Mars based on historical records of solar activity, with big potentials to back to the young Sun epoch. More recently, Ramstad et al (2015) have further quantified the ion escape rate with respect to upstream solar wind density, velocity, and EUV conditions. The most interesting trend is the decrease Dubinin et al (2011) of the atmospheric erosion with increasing solar wind density, in particular during low solar EUV intensity conditions.…”