The variability, morphology and composition of Martian ionospheric plasma escape
Rickard Lundin
Swedish Institute of Space Physics
Abstract
ASPERA-3 measurements from Mars Express demonstrate that Martian ionospheric plasma escapes in a comet-like fashion. Low-energy (cold) ionospheric plasma is swept from the dayside, expanding into the nightside/tail of Mars. The primary energization processes brings ionospheric plasma to just above escape velocity (5 - 20 km/s). In analogy with the polar wind of the Earth, ionospheric plasma is expected to become energized by waves and electric fields generated by solar wind energy and momentum transfer processes. The escape of low-energy ionospheric plasma, streaming along the external sheath flow, suggest a ?viscous-like? coupling between the sheath plasma and the expanding ionospheric plasma. The ionospheric ion outflow is very structured, fan-like and modulated in the ULF frequency range. The ionospheric densities measured vary correspondingly with time, altitude, latitude and local time. A similar variability of solar wind ions is found in the Martian magnetosheath. This implies that magnetosheath wave activity is involved, transferring energy and momentum to ionospheric ions. We demonstrate that the wave activity modulating ions and electrons, reaches down to the MEX pericenter (≈300 km), suggesting that heating/energization of ionospheric plasma extends deep into the ionosphere.
An interesting aspect of the plasma escape from Mars is the large abundance of molecular ions, implying that the outflow source region extends down to the lower ionosphere where molecular ions dominate. We also find a fair amount of ionized molecular hydrogen, H2+, in the ion outflow, the H2+ outflow corresponding to some 1/10 of the H+ outflow. Because the cold ionospheric ion outflow is dominated by H+, H2+, O+ and O2+, the average CO2+ outflow corresponding to about 10% of the heavy ion outflow, we have made a stoichiometric analysis of the ion escape. Adding the total outflow of hydrogen and oxygen respectively, we get H/O = 0.6 - 1.3. Altogether this suggests that the ultimate origin of the bulk ion escape from Mars is a minor constituent in the Martian atmosphere - water.
Created 2009-07-30 17:07:16 by Mats Holmström Last changed 2009-09-10 09:26:42 by Rick McGregor
