IAFE   05512
INSTITUTO DE ASTRONOMIA Y FISICA DEL ESPACIO
Unidad Ejecutora - UE
artículos
Título:
Proton cyclotron wave generation mechanisms upstream of Venus
Autor/es:
M. DELVA; C. MAZELLE; C. BERTUCCI; M. VOLWERK; Z. VÖRÖS; T.L. ZHANG
Revista:
JOURNAL OF GEOPHYSICAL RESEARCH
Editorial:
AMER GEOPHYSICAL UNION
Referencias:
Año: 2011 p. 1 - 10
ISSN:
0148-0227
Resumen:
Long-term observations of proton cyclotron waves in the upstream region
of Venus raise the question of under which general solar wind conditions
these waves are generated and maintained. The waves are characterized by
their occurrence at the local proton cyclotron frequency and left-hand
polarization, both in the spacecraft frame. Magnetometer data of the
Venus Express spacecraft for two Venus years of observations are
analyzed before, during, and after the occurrence of these waves. The
configuration of the upstream magnetic field and the solar wind velocity
is investigated, to study if the waves are generated from a ring
distribution of pickup ions in velocity space or from a parallel pickup
ion beam, i.e., for quasi-parallel conditions of solar wind velocity and
magnetic field when the solar wind motional electric field is weak. It
is found that stable and mainly quasi-parallel magnetic field conditions
for up to 20 min prior to wave observation are present, enabling
sufficient ion pickup and wave growth to obtain observable waves in the
magnetometer data. Persistent waves occur mainly under quasi-parallel
conditions. This is in agreement with linear theory, which predicts
efficient wave growth for instabilities driven by field-aligned
planetary ion beams, already for low pickup ion density. The occurrence
of highly coherent waves at 4 RV upstream toward the Sun
implies that planetary neutral hydrogen is initially picked up at least
5 RV toward the Sun from a sufficiently dense Venus hydrogen
exosphere.