Triggering of Whistler Fallers by a Nonlinear Absolute Instability (Invited)

Details Triggering of Whistler Fallers by a Nonlinear Absolute Instability (Invited) Triggering of Whistler Fallers by a Nonlinear Absolute Instability (Invited)

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm51c..04l&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SM51C-04

Physics

[2753] Magnetospheric Physics / Numerical Modeling, [2772] Magnetospheric Physics / Plasma Waves And Instabilities, [2774] Magnetospheric Physics / Radiation Belts, [2799] Magnetospheric Physics / General Or Miscellaneous

Scientific paper

We consider the instability of a coherent whistler wave packet propagating from south to north along a geomagnetic field line, in the presence of a cold plasma background as well as a beamlike distribution of high energy electrons moving from north to south. We present simulations and analysis showing that linear instability (growing exponentially in time) is initiated at two locations, symmetrically located in the northern and southern hemispheres, where the electrons are cyclotron resonant with the wave. The instability in the southern hemisphere saturates at the onset of trapping. In the northern hemisphere, however, nonlinear growth continues to much larger amplitude. The growth, which is linear as a function of time, is fueled by energy extracted from untrapped resonant electrons which are propagating along the field line toward the smaller geomagnetic field at the equator. The resonant electrons which have streamed through, and become modulated at the resonant point, drive new waves at a lower frequency which is locally resonant with the electrons, permitting growth of the triggered waves all the way from the linearly resonant point to the equator. Due to these wake electrons, the instability grows in the direction opposite to linear wave propagation, leading to an absolute instability. The instability eventually saturates in amplitude, but the large-amplitude triggered waves continue to broaden spatially, as wave propagation carries energy away from the resonant region as fast as it is extracted from the resonant electrons. The instability causes a sharp drop in the pitch angle of the resonant electrons. This work was supported by Office of Naval Research.

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