INVESTIGADORES
BERTUCCI Cesar
congresos y reuniones científicas
Título:
Properties of Upstream Waves at the Proton Cyclotron Frequency at Mars from MGS Observations
Autor/es:
MAZELLE, C.; BERTUCCI, C. L.; SAUER, K.; BRAIN, D. A.; MITCHELL, D. L.; REME, H.; ACUNA, M. H.
Reunión:
Congreso; AGU Fall Meeting 2001; 2001
Resumen:
We report the first experimental evidence for nonlinear origin of the
very high coherence of low frequency waves at the proton cyclotron
frequency (fc) 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 fc/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 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 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.