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Titel |
High-Frequency Microturbulence within the Foot of a Supercritical Perpendicular Shock |
VerfasserIn |
Laurent Muschietti, Bertrand Lembège |
Konferenz |
EGU General Assembly 2010
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 12 (2010) |
Datensatznummer |
250038198
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Zusammenfassung |
In supercritical shocks a fraction of the incoming ions is reflected at the steep front and
stream across the magnetic field within the foot, making it a site of plasma instabilities
excited by the relative drifts between beams of incoming and reflected ions, and incoming
electrons. The drift Ure of the reflected ions versus the electrons across the B-field can easily
destabilize waves in the electron cyclotron frequency range. By means of linear
analysis, it is shown that several Bernstein harmonics are unstable, the number
of harmonics being directly proportional to Ure yet limited by the reflected ions’
temperature.
By means of elecromagnetic PIC simulations restricted to the foot, we investigate the
nonlinear characteristics of these waves. Depending on the drift Ure they saturate at weak
intensity levels or they reach significant levels and exhibit a nonlinear evolution marked by
two phases. First, cyclotron harmonics with high kÏe values develop, in good agreement with
linear dispersion properties. Second, as the high-k modes saturate by trapping of the reflected
ion beam, an inverse cascade takes place whereby the spectral power shifts toward lower
k-modes to eventually accumulate on the fundamental harmonic branch at the cyclotron
frequency Ωce. The phase is characterized by the development of a magnetic component to
the spectrum that had so far been mostly electrostatic. It exhibits also a significant energy
transfer from the ion beam to the electrons, which experience a marked increase
in their temperature. The dynamics of the ions and the electrons is analyzed and
discussed. |
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