IAFE   05512
INSTITUTO DE ASTRONOMIA Y FISICA DEL ESPACIO
Unidad Ejecutora - UE
artículos
Título:
Upstream Ion Cyclotron Waves at Venus and Mars
Autor/es:
M. DELVA, C. MAZELLE, C. BERTUCCI
Revista:
SPACE SCIENCE REVIEWS
Editorial:
SPRINGER
Referencias:
Año: 2011
ISSN:
0038-6308
Resumen:
The occurrence of waves generated by pick-up of planetary neutrals by the solar
wind around unmagnetized planets is an important indicator for the composition and
evolution of planetary atmospheres. For Venus and Mars, long-term observations of the upstream
magnetic field are now available and proton cyclotron waves have been reported by
several spacecraft. Observations of these left-hand polarized waves at the local proton cyclotron
frequency in the spacecraft frame are reviewed for their specific properties, generation
mechanisms and consequences for the planetary exosphere. Comparison of the reported
observations leads to a similar general wave occurrence at both planets, at comparable locations
with respect to the planet. However, the waves at Mars are observed more frequently
and for long durations of several hours; the cyclotron wave properties are more pronounced,
with larger amplitudes, stronger left-hand polarization and higher coherence than at Venus.
The geometrical configuration of the interplanetary magnetic field with respect to the solar
wind velocity and the relative density of upstream pick-up protons to the background
plasma are important parameters for wave generation. At Venus, where the relative exospheric
pick-up ion density is low, wave generation was found to mainly take place under
stable and quasi-parallel conditions of the magnetic field and the solar wind velocity. This
is in agreement with theory, which predicts fast wave growth from the ion/ion beam instability
under quasi-parallel conditions already for low relative pick-up ion density. At Mars,
where the relative exospheric pick-up ion density is higher, upstream wave generation may
also take place under stable conditions when the solar wind velocity and magnetic field are
quasi-perpendicular. At both planets, the altitudes where upstream proton cyclotron waves
were observed (8 Venus and 11 Mars radii) are comparable in terms of the bow shock nose distance of the planet, i.e. in terms of the size of the solar wind-planetary atmosphere interaction
region. In summary, the upstream proton cyclotron wave observations demonstrate
the strong similarity in the interaction of the outer exosphere of these unmagnetized planets
with the solar wind upstream of the planetary bow shock.