Physics – Plasma Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995jgr...10017189d&link_type=abstract
Journal of Geophysical Research, Volume 100, Issue A9, p. 17189-17204
Physics
Plasma Physics
26
Magnetospheric Physics: Plasma Waves And Instabilities, Magnetospheric Physics: Energetic Particles, Precipitating, Space Plasma Physics: Numerical Simulation Studies
Scientific paper
We present results from self-consistent, one- and two-dimensional, eletromagnetic simulations of the electron whistler mode instability relevant to the near-Earth nightside plasma sheet region during geomagnetically disturbed times. Specifically, we study the evolution of energetic, anisotropic (T⊥>T∥) electron distributions that are injected into the nightside ring current region at geomagnetically disturbed times, the resulting growth of electron whistler mode waves, and subsequent electron pitch angle diffusion via electron whistler wave-particle interactions. Growth of whistler mode waves from an initial pitch angle anisotropy (T⊥~4T∥) is studied in the strong pitch angle diffusion regime (defined as having scattering times much shorter than a typical electron bounce time in the near Earth's dipolar field). The quasi-linear and subsequent nonlinear evolution of waves and the corresponding migration of electrons in velocity space is followed over timescales such that ion motion may be neglected. Our simulations contain wave frequencies and growth rates that are a significant fraction of the electron gyrofrequency (ω~0.5ωce, γ~0.1ωce being typical) and the simultaneous evolution of waves propagating both parallel and nonparallel to the ambient magnetic field direction.
Effects due to these are not usually accounted for in applications of quasi-linear theory to the problem of electron whistler wave-particle interactions, so that our self-consistent simulations of the electron whistler instability provide an important insight into the applicability of quasi-linear theory to the velocity space diffusion of electrons due to electron whistler wave-particle interactions. We examine the dependence of whistler mode wave growth rates, nonlinear wave mode saturation, and pitch angle diffusion rates on the β value of the hot electron species which contains the resonant population, and we compare the differences between results of one- and two-dimensional simulations. In general, we have found significantly larger average growth rates in a one-dimensional than in a two-dimensional geometry, by up to a factor of ~2-3, with the difference between such growth rates becoming larger as β increases. As a result, we find pitch angle diffusion rates are significantly larger (by up to a factor of ~10) in a one-dimensional geometry, and pitch angle diffusion rates increase as β increases. During the early wave growth period of the instability, pitch angle diffusion rates Dα have been found to scale to magnetic wave energy B2w, approximately Dα~B2w.
We also show that in the self-consistently evolved system (containing several wave modes), the pitch angle diffusion is still consistent with a usual quasi-linear approach in which resonant particles are taken to diffuse along surfaces of constant wave frame energy.
Chapman Sandra C.
Devine P. E.
Eastwood James W.
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