INVESTIGADORES
BERTUCCI Cesar
congresos y reuniones científicas
Título:
Nonlinear Origin Of Coherent Waves At The Cyclotron Frequency At Mars
Autor/es:
MAZELLE, C.; BERTUCCI, C.; SAUER, K.; DUBININ, E.; BRAIN, D.; MITCHELL, D. L.; ACUNA, M. H.
Reunión:
Congreso; EGS XXVII General Assembly; 2002
Resumen:
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.