Solar forcing and planetary ion escape from Mars

Abstract

The variability of planetary ion escape from Mars is studied using data from the Ion Mass Analyzer, IMA, on Mars Express (MEX). 42 orbits were selected during 17 months for different solar wind conditions, focusing on the low energy (≈30 – 800 eV) heavy ion (e.g. O + , O 2 + and CO 2 + ) outflow. A strong correlation is found between solar wind forcing of the obstacle, the cross‐sectional area enclosing the ion outflow from Mars and the total heavy ion escape flux. The at least one order of magnitude changes of the ion outflow on the short term (hours, days), is directly connected with the variability of solar wind, solar soft x‐ray and solar EUV (XEUV). The latter was first inferred from an analysis of how the obstacle size changes with changing solar wind and solar XEUV forcing. The 17‐month trend of decreasing ion outflow with EUV during a declining phase of solar cycle 23, the EUV determined from the Neutral Particle Imager (NPI) on MEX, illustrates the influence of solar EUV forcing. On the basis of this we conclude that changes in solar wind‐ and solar XEUV forcing governs the variable ion escape from Mars. Both forcing terms appear to be equally important for the escape rate. Considering the difference in travel time for XEUV and the solar wind to Mars, the XEUV effect will precede the solar wind effect by several (3–9) days.