Nonlinear Origin Of Coherent Waves At The Cyclotron Frequency At Mars

MAZELLE, C.; BERTUCCI, C.; SAUER, K.; DUBININ, E.; BRAIN, D.; MITCHELL, D. L.; ACUNA, M. H.

Congreso; EGS XXVII General Assembly; 2002

We report the experimental evidence for nonlinear origin of the very high coherence of low frequency waves at the proton cyclotron frequency (fc) in the plasma environ- ment of Mars reported from observations by the MAGER experiment onboard MGS upstream from the bow shock. These waves are revealed as wave packets embedded inside a regular lower frequency wave 'envelope' at about fc/10. The proton gyrofre- quency waves are left-handed circularly polarized in the spacecraft frame. Their wave vectors lie at moderate angles from the ambient magnetic field and the observed peri- ods (spacecraft frame) always closely fit the local proton cyclotron periods. They can have large amplitude (up to 5 nT peak-to-peak) even at large distance from the planet (more than 6 RM ). 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 pro- tons which leads to constraints on the hydrogen density in the Martian exosphere. However, the large observed wave amplitude seems difficult to reach from the nonlin- ear saturation of the proton cyclotron instability using pickup ion densities consistent with those derived from the 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. These waves can be interpreted as nonlinear stationary structures 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.