Role of Lower-hybrid Resonances to Relativistic Electrons

Abstract:

The interplay of processes within Earth’s radiation belts and the near-Earth plasma sheet play a major role in the energization, transport, and loss of energetic particles in the region. Both MHD and kinetic processes are found active even during geomagnetically quiet times in the flow-through region connecting the radiation belts and near-Earth plasma sheet. Many observations and modeling studies supported the importance of the interactions of waves and particles in vortices and reconnection regions. In particular, the lower-hybrid resonances, resonance branches between the ion and electron cyclotron frequencies in the plasma wave dispersion relations, could play an important role in converting flow energy in MHD scale to waves and heat ions and electrons in kinetic scale or vice versa. Recent observations from the Van Allen Probes and THEMIS spacecraft show enhancements of such wave activities occurred during the main phase of the geomagnetic storms when the relativistic electrons were going through the depletion and recovery cycles. The total wave powers in these events are comparable or even greater than those of whistler mode resonances along, indicating the physical processes at the lower-hybrid resonances are also important in energy conversion between waves and particles, particularly during the main phase of geomagnetic storms.

We use multi-point measurements from the Van Allen Probes and THEMIS spacecraft to study the kinetic processes of how the flow energy is converted to heat or cool ions and electrons in the region connecting the radiation belts and near-Earth plasma sheet. Using the in-situ plasma measurements as inputs, we will run the Waves in Homogeneous and Anisotropic Multi-component Plasmas (WHAMP) plasma wave dispersion solver to study the growth rates and instabilities of these waves near the lower hybrid resonances as well as the Alfvenic and whistler branches at lower and higher frequencies, respectively. We will present studies of two major geomagnetic storm events, including the well known, extensively studied November 14th, 2012, and another event with whistler solitary waves using measurements from the Van Allen Probes and THEMIS conjunctions and WHAMP analysis of the plasmas waves at the resonances.