Properties of Upstream Waves at the Proton Cyclotron Frequency at Mars from MGS Observations

We report the first experimental evidence for nonlinear origin of the very high coherence of low frequency waves at the proton cyclotron frequency (f_c) in the plasma environment of Mars from observations by the MAGER experiment onboard MGS upstream from the bow shock. These waves are revealed as wave packets embedded inside a reguler lower frequency wave 'envelope' at abpout f_c/10. They can have large amplitude (up to 5 nT peak-to-peak) and are observed even at large distance from the planet (more than 6 R_M). The possible generation mechanisms of these two low frequency wave signatures are discussed. The 'proton cyclotron waves' are usually interpreted as generated by a resonant helical beam instability fed by the planetary pick-up protons which leads to constraints on the neutral density values in the Martian exosphere. However, this gives rise to a major discrepancy with pickup ion densities derived from up-to-date exosphere models. Moreover, a satisfactory mechanism is also needed for both the lower frequency 'envelope' signature and the high coherency of the ion gyrofrequency signature. We show that these waves can be interpreted as nonlinear stationary structures (solitons) in the bi-ion plasma but apparent as temporal structures in the spacecraft frame recently described as 'oscillitons'. They can be produced (in the flow regimes in which they are permitted) by the existence of two ion populations (solar wind + planetary) with different densities, velocities and/or temperatures. This implies that direct connection between waves at the cyclotron frequency and neutral densities in the planetary exosphere is not always ensured.