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| Titel |
Microturbulence in Front of a Supercritical Shock: Stimulation and Inverse Cascade of Waves in the Electron Cyclotron Frequency Range |
| VerfasserIn |
Laurent Muschietti, Bertrand Lembège |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047893
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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 can be unstable, the number of harmonics
being directly proportional to Ure yet limited by the temperature of the reflected
ions.
By means of electromagnetic PIC simulations restricted to the foot, we investigate the
nonlinear characteristics of these waves. While the waves saturate at weak intensity levels for
Ure < (Te-m)1-2, for Ure ~ (Te-m)1-2 they reach significant levels and exhibit a
nonlinear evolution marked by two phases. First, high cyclotron harmonics develop in good
agreement with linear dispersion properties. Second, as the high-k modes saturate by trapping
ions of the reflected beam, an inverse cascade takes place whereby the spectral power shifts
toward lower k-modes to eventually accumulate on the first harmonic branch with 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 that experience a marked increase
in their temperature. The dynamics of the ions and the electrons is analyzed and
discussed. |
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