Local and global properties of plasma waves at the proton cyclotron frequency upstream from Mars

ROMANELLI, N.; BERTUCCI, C.; GOMEZ, D. O.; MAZELLE, C. X.; DELVA, M.; COWEE, M.

Congreso; American Geophysical Union, Fall Meeting 2011; 2011

A comprehensive study on the properties of electromagnetic ULF plasma waves upstream from Mars is presented. This study is based on reliable upstream measurements provided by the magnetometer and electron reflectometer (MAG/ER) onboard the Mars Global Surveyor (MGS) spacecraft during the Science Phasing Orbit period. We find that a large majority of these waves is characterized - in the spacecraft frame - by a left-hand circular polarization and a frequency which coincides with the local proton cyclotron frequency. Minimum variance analysis of the magnetic field indicates that these waves propagate almost parallel to the background magnetic field and comparison with suprathermal electron fluxes measured by ER suggests a small degree of compressibility. A survey based on more than 300 MGS elliptical orbits indicates that these waves are almost permanent and their spatial distribution does not seem to depend on the orientation of the solar wind convective electric field assuming a flow along the Sun-Mars line. Also, we find that their amplitude decreases with radial distance from the planet, supporting the idea that their source is Mars. Theoretical (analytical and numerical) approaches suggest that these properties are compatible with plasma waves originating in the pick-up of ionized exospheric hydrogen. We discuss these theoretical results and their implications for Mars and other atmospheric unmagnetized objects such as Venus.