Discovery Metadata System

Summary

Abstract:
Whistler-mode chorus waves play a key role in driving radiation belt dynamics by enabling both acceleration of electrons to relativistic energies as well as their loss into the atmosphere via pitch-angle scattering. The ratio between the electron plasma frequency (fpe) and the electron gyrofrequency (fce) significantly influences the efficiency of these processes, with electron acceleration being most effective during periods of low fpe/fce. In this study, a combined total of approximately 24.5 years of THEMIS wave data are analyzed to show how chorus wave intensity and spatial location vary with relative frequency, geomagnetic activity and fpe/fce. Results demonstrate that the strongest chorus emissions are observed during active conditions. At these times, equatorial chorus at low relative frequencies (flhr<f<0.1fce) is strongest when fpe/fce is high (fpe/fce>10) primarily in the region 5<L*<8, from 22:00-12:00 MLT. In sharp contrast at high relative frequencies (0.5fce<f<0.7fce), the equatorial chorus is strongest when fpe/fce is low (fpefce<6) mainly in the region 4<L*<6 from 21:00-09:00 MLT. At intermediate relative frequencies (0.3fce<f<0.4fce), equatorial chorus is strongest in the region 3.5<L*<8 from 21:00-12:00 MLT. In the off-equatorial region the strongest waves are seen in the frequency range (0.1fce<f<0.3fce) between 5<L*<8 and 06:00-15:00 MLT and again are mostly independent of fpe/fce. We show that the location of the strongest waves can be largely explained in terms of the source electrons being in the required energy range for resonance and the absence of Landau damping and highlight the regions where electron acceleration to relativistic energies is likely to be mostly significant.

Keywords:
Radiation belts, Wave-particle interaction, Whistler-mode chorus