Properties Of Waves At The Proton Cyclotron Frequency Upstream From Mars

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

Congreso; AGU Fall Meeting 2012; 2012

We present a study on the properties of electromagnetic plasma waves in the region upstream of the Martian bow shock, detected by the magnetometer and electron reflectometer (MAG / ER) onboard the Mars Global Surveyor (MGS) spacecraft during the period known as Science Phasing Orbits (SPO). The waves found display frequencies in the reference system of MGS (SC) which are close to the local proton cyclotron frequency. Minimum variance analysis (MVA) shows that these 'proton cyclotron frequency' waves (PCWs) are characterized - in the SC frame - by a left-hand, elliptical polarization and propagate almost parallel to the background magnetic field. They also have a small degree of compressibility and an amplitude that decreases with radial distance from the planet. The latter result supports the idea that the source of these waves is Mars. In order to determine the instability which could produce the observed waves, we studied the relation between their polarization and the direction of the mean magnetic field relative to the solar wind velocity direction. In addition, we find that these waves are not associated with the foreshock and that their spatial distribution does not depend on the orientation of the solar wind convective electric field. We also find a clear difference in the waves occurrence rate between SPO1 and SPO2 subphases and we discuss these results in the context of possible changes in the pick up conditions associated with seasonal variations.