Physics – Plasma Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm12b..05m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SM12B-05
Physics
Plasma Physics
[7829] Space Plasma Physics / Kinetic Waves And Instabilities, [7851] Space Plasma Physics / Shock Waves, [7863] Space Plasma Physics / Turbulence, [7867] Space Plasma Physics / Wave/Particle Interactions
Scientific paper
Supercritical shocks are characterized by a fraction of the incoming ions which is reflected at the steep front and stream across the magnetic field in the foot. These ions accumulate and are responsible for the shock front self-reformation. The drift of the reflected ion beam versus the electrons can easily destabilize waves in the electron cyclotron frequency range. By means of linear analysis, we show that several Bernstein harmonics can be unstable, their number being proportional to the drift, yet limited by the ion beam's temperature. Separate electromagnetic PIC simulations restricted to the ion and electron populations of the foot region are performed for various drifts in order to investigate with high spatial resolution and high particle statistics the nonlinear characteristics of these waves. For drifts less than (T_e/m)^{1/2}, the waves saturate at weak intensity levels. For larger drifts, yet still below the ion acoustic threshold, they reach significant levels and exhibit an interesting nonlinear evolution. First, high cyclotron harmonics develop in good agreement with linear dispersion properties and over timescales much shorter than the period of the shock front self-reformation. Second, as the high k-modes saturate by trapping ions of the reflected beam, the spectral power shifts toward lower k-modes to eventually accumulate on the first harmonic in a process that appears like an inverse cascade. Third, one surprising result in the late phase is the development of a magnetic component to a spectrum that had so far been mostly electrostatic. The late phase also exhibits a significant energy transfer from the ion beam to the electrons which experience a marked increase in temperature.
Lembege Bertand
Muschietti L.
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