Mars' Energetic Plume Ion Escape Channel

[摘要] Mars is losing its atmosphere. The planet’s small size results in relatively low energy requirements for atmospheric particles to escape into deep space, and its lack of a planetary magnetic field allows the solar wind to directly interact with the upper atmosphere, providing an additional source from which particles may obtain this requisite energy. The escape of particles from Mar’s atmosphere over the course of billions of years is not only a story of atmospheric evolution; it is a story of the evolution of a global climate. It is now thought that oceans worth of liquid water may have existed on a warmer ancient Mars, and atmospheric escape of hydrogen and oxygen is one explanation of how such an ocean may have vanished. The research presented here revolves around the examination of one particular ;;loss channel;; for oxygen (and other ;;heavy;; ions) from Mars. This loss channel, known as the ;;energetic plume,;; consists of pickup ions, electrically charged planetary particles that, finding themselves in the solar wind flow past Mars, are accelerated in the direction of the solar wind;;s convective electric field (ESW). In the spatially zoomed out view, the acceleration in this direction is just the initial part of the first gyration of an ESW-cross-B drift in the direction of solar wind flow. Zoomed in closer to Mars, where ion-observing satellites have orbited, a result of the huge gyroradius of these pickup ions is that, in addition to having high energies, energetic plume particles have flight directions distinct from other escaping particles and are observed at locations not reached by other escaping particles. This dissertation introduces the Mars space environment and the problem of atmospheric escape generally before presenting the search for this distinct phase space signature of the energetic plume in ion data from the Mars Express satellite. It was found that despite the presence of obstacles to observing the energetic plume using the Ion Mass Analyzer (IMA) onboard Mars Express, it is possible to both identify unambiguous instances of energetic plume observations in IMA data and to see signatures of the energetic plume in statistical maps of the Mars space environment made using IMA observations. Furthermore, it was found that accounting for ;;weathervaning” – the subsolarward bending of magnetic field lines draped around the ionosphere – can be used to improve estimates of the direction of ESW. The resulting more accurate estimate for the direction of ESW improves statistical representations of the energetic plume in IMA data, and significant quantities of energetic plume type ions are observed by IMA ~ 60% more frequently in the newly estimated direction of ESW than in the previously estimated direction of ESW. We conclude that the improved method of estimating the direction of ESW should be used in place of previously existing proxies in studies concerning the variation of energetic plume fluxes for different solar conditions during the time period between Jan. 2004 and Oct. 2006.

[发布机构] University of Michigan

[效力级别] Mars Express observations

[关键词] Mars ion escape;Mars Express observations;Mars atmospheric loss;Atmospheric;Oceanic and Space Sciences;Science;Atmospheric, Oceanic & Space Science