Numerical experiments on possible impact of substorms on energetic electrons in the inner magnetosphere

Abstract:

The abrupt reconstruction of the outer radiation belt is often observed after substorms, and is believed to result from internal acceleration or external transport. The internal acceleration is thought to take place outside the plasmapause through Doppler shifted cyclotron resonance, or relativistic turning acceleration with whistler mode chorus waves. Electrons are thought to be accelerated by the waves when characteristic pitch angle distribution, hard energy spectrum and earthward gradient of phase space density are identified, but it seems that direct observational evidence for the energy transfer from waves to electrons has not been explicitly provided. The external transport is thought to take place when electrons are accelerated by strong electric fields. We have solved bounce-averaged drift transport equations under the electric and magnetic fields given by the recently developed global MHD simulation. We reproduced the sequence of a substorm, and determined onset as a sudden decrease in the AL index and a sudden increase in the ionospheric conductivity (a proxy of aurora). Near the onset, a strong electric field is formed in the inner magnetosphere in a longitudinally narrow region with a thickness of the order of earth radius (Re), which rapidly transported relativistic electrons inward. Simultaneously, keV electrons were also injected inward, which may become a seed of relativistic electrons. Temperature anisotropy becomes large near the leading edge of the injected hot electrons. As the plasmapause shrinks, the ratio of the plasma frequency to the cyclotron frequency becomes small outside the plasmapause, which may favor the growth of chorus waves. We estimated the evolution of the phase space density of electrons due to the interaction with chorus waves under the assumption that the wave amplitude is small. We will demonstrate the results of numerical experiments on the energy spectrum, pitch angle distribution and radial gradient of the phase space density of energetic electrons for the purpose of identifying physical processes responsible for the observable signatures and providing a guide to understanding the abrupt reconstruction of the outer radiation belt.