PhD Thesis

The Impact of Magnetosonic Waves on Earth’s Radiation Belt Electrons: Observations and Simulations

Jinxing Li (Space Physics)

Directed by Prof. Zuyin PU and Suiyan FU

ABSTRACT

Earth’s radiation belts are filled with energetic particles trapped by Earth’s magnetic field. The electron radiation belt is comprised of the inner belt, the outer belt and a ‘slot’ in between. Responding uniquely to various plasma waves including the ULF, chorus, hiss, EMIC, ECH and magnetosonic waves, radiation belt electrons are highly dynamic due to the imbalance between the acceleration, loss and transport processes. However, how the magnetosonic waves impact the energetic electrons is yet to be uncovered. There had been no direct observations or dynamic simulations about the impact of the magnetosonic waves on the radiation belts.

During the 29 June 2013 geomagnetic storm, the Van Allen Probes measured the formation process of the energetic electron butterfly distributions in the inner radiation belt and the slot region. A strong magnetosonic wave enhancement was also detected in the same region. Based on multi-point high-quality wave measurements from both Van Allen Probes, this dissertation, for the first time, constructed an accurate magnetosonic wave model with a harmonic structure, a distribution of wave normal angle and a spatial distribution over magnetic local times. By combining test particle calculations and the Fokker-Planck simulations, this dissertation reveals that the magnetosonic wave can scatter the near 90o pitch angle electrons to the medium pitch angles via a parallel acceleration related to the Landau resonance, and thereby create the electron butterfly distributions.

A very intense magnetosonic wave can even create the ~100 keV electron butterfly distributions in the heart of the outer radiation belt in just 1 minutes, as observed by Van Allen Probe A on 21 August 2013, and we successfully reproduced this process based on the wave and electron measurements. During the recent 17 March 2015 large geomagnetic storm, the Van Allen Probes observed that when the magnetosonic waves were enhanced, the ultrarelativistic electrons with energies up to 5.2 MeV exhibited butterfly distributions. As the magnetosonic waves intensified to 4.1 Earth Radii, the electron butterfly distributions extended to the same altitude. Based on the Van Allen Probe observations and the wave-particle interaction simulations, this dissertation reveals that scattering by equatorial magnetosonic waves is a significant cause for the formation of energetic electron butterfly distributions in the inner radiation belt, the slot region and the outer radiation belt. Because of the existence of magnetosonic waves, the high energy electrons are not concentrated near the magnetic equator but also populated in the medium and high latitudes, making the high latitudes also radioactive.