Science Seminars

New understandings of energy-dependent dynamics of radiation belt electrons: Van Allen Probes observations

Speaker: Hong Zhao (LASP)
Date: Thursday, Sep 13, 2018
Time: 4:00 PM
Location: SPSC N100

Seminar Abstract:

The Earth’s radiation belt electrons consist of different populations, including source (10s of keV), seed (100s of keV), relativistic (~MeV), and ultrarelativistic (>~3 MeV) electrons. These electrons exhibit energy-dependent acceleration, transport and loss processes under the influence of various physical mechanisms. Understanding the energy-dependent behavior of radiation belt electrons and the effectiveness and relative importance of physical mechanisms on different populations is critical for understanding radiation belt dynamics. The dual-spacecraft Van Allen Probes mission provides radiation belt particle observations with wide energy coverage and high energy resolution, which gives us an unprecedented opportunity to study the energy-dependent dynamics of radiation belt electrons. Enabled by measurements from the Van Allen Probes, we report an unexpected type of “reversed” energy spectra with abundant high-energy and fewer low-energy electrons in the energy range of ~hundreds of keV – ~2 MeV (“bump-on-tail” spectra). This unanticipated feature is the most prevalent electron energy spectrum in the outer radiation belt inside the plasmasphere. Using two-dimensional Fokker-Planck diffusion simulations with a data-driven, time-dependent plasmaspheric hiss wave model, we demonstrate that hiss wave scattering explains well the formation of the bump-on-tail electron energy spectrum. On the other hand, though most radiation belt electron populations have been intensively studied, the ultrarelativistic electrons (E>~3 MeV) received limited attention prior to the Van Allen Probe era mainly due to sparse measurements. Using data from the Van Allen Probes, significant flux enhancements of ultrarelativistic electrons with energies up to 7.7 MeV during a small to moderate geomagnetic storm is reported and the underlying physical mechanisms are investigated. The results suggest that, during this storm, the acceleration mechanism for ultrarelativistic electrons in the outer belt is energy-dependent: local acceleration plays the most important role in the flux enhancements of ~3 – 5 MeV electrons, while inward radial diffusion is the main acceleration mechanism for ~7 MeV electrons at the center of the outer radiation belt. A statistical analysis is also performed to examine the effectiveness of various solar wind/geomagnetic parameters to ultrarelativistic electron flux enhancements and the energy-dependent features are further revealed. These results shed light on the energy-dependent dynamics of radiation belt electrons.​