A comet‐like escape of ionospheric plasma from Mars

Abstract

New measurements of the ion escape from Mars display a mantle of low‐energy ionospheric ions swept from the dayside over the terminator, expanding into the tail in a comet‐like fashion. The finding is based on data obtained with new energy settings for the ASPERA‐3 ion mass analyzer (IMA), enabling us to also measure cold ionospheric ions. By including the comet‐like contribution of low‐energy ions (<200 eV), we obtain a heavy ion escape rate of 3.3 · 1024 (±0.6 · 1024) s−1. While a modest energization characterizes the draped comet‐like outflow of low‐energy ions, ion pickup and acceleration above magnetic anomalies leads to the energetic and structured ion fluxes observed in the Martian tail. Compared to the previous measurements, where the flow of accelerated ionospheric ions is asymmetric, controlled by the solar wind electric field, the low‐energy ion escape is symmetric, emerging from the dayside and expanding towards the tail along the tail flank. An analysis of the escape rate versus tail distance from the planet displays a gradual energization of ions, yet maintaining an almost constant escape rate. We finally note that the planetary heavy ion escape rate is measured during solar minimum. The solar maximum value is yet to be determined, but it may very well exceed 1025 ions/s.