Date of Award

Spring 2019

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Lynn M Kistler

Second Advisor

Harlan Spence

Third Advisor

Kai Germaschewski

Abstract

The transport and acceleration of energetic particles into the magnetosphere during geomag- netic storms energizes and enhances the ring current, the current system in the inner magnetosphere driven by the pressure gradients of charged particles. The enhancement of this current creates a disturbance to Earth’s magnetic field. While there have been extensive measurements of the en- hanced current, how the particles are transported and accelerated into the ring current is still an active question. While methods such as direct injection, particle-wave interactions, and radial diffusion have been proposed to account for the transport and acceleration, enhancement of the convection electric field during storms has been consistently shown to contribute to the formation of the storm-time ring current. While during geomagnetically quiet times the ring current consists of primarily H+, during storms O+ becomes a major contributor to the ring current, with its contri- bution increasing with increasing storm strength. Because of the large amount of O+ in the inner magnetosphere during storms, it has been proposed that there is an inner source of O+ ions or O+ acceleration inside the inner magnetosphere. This dissertation uses Van Allen Probes data com- bined with modeling to address whether the enhancement of the convection electric field alone, transporting and accelerating plasma from the magnetotail, is sufficient to explain the observed enhancements of the ring current during geomagnetic storms and to what extent electric field mod- els are able to reproduce the spectral features observed in the storm-time ring current during large geomagnetic storms.

Using measurements of both the near-earth plasma sheet source and the ring current itself, we are able to show the inward adiabatic convection of the plasma sheet into the inner magnetosphere is sufficient to explain the measurements of the storm-time ring current. We show that there are multiple effects that can lead to the preferential enhancement of O+ over H+ in the ring current. First, the O+ spectrum in the plasma sheet can be steeper than the H+ spectrum leading to a higher O+/H+ ratio at low energies. As the population gets accelerated as it moves inward, these lower energies dominate the energy density. Second, the O+ in the plasma sheet source region is much more variable than the H+. Large O+ enhancements, convected inward, can also lead to the O+ dominance. Further, we find the electric field models are unable to reproduce the ion spectral features during large storms, even after increasing the drivers of the field. We find that additional changes to the potential patterns is necessary for highly disturbed time periods.

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