Author ORCID Identifier

https://orcid.org/0000-0002-2633-627X

Semester

Spring

Date of Graduation

2024

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Weichao Tu

Committee Member

Katherine Goodrich

Committee Member

Piyush Mehta

Committee Member

Paul Cassak

Abstract

Earth’s inner magnetosphere is a highly dynamic region with various charged particle populations and current systems. The radiation belts, composed of relativistic electrons and protons, is an environment that can pose significant risks to both spacecraft and humans in space; while the fluctuations of ring current, an electric current flowing around the earth consisting of energetic electrons and ions, can lead to severe disruptions in ground-based electrical systems. In this dissertation, we first modeled the long-term evolution of ring current protons based on the measurements of Van Allen Probes. By implementing a 1D radial diffusion model with charge exchange loss, we find that radial diffusion is the dominant source mechanism for >75 keV protons at L*=3.5-5.5. The observed fast decay at lower energies (μ=30 MeV/G) is well explained by charge exchange, while the fast drops at higher energies (>100 keV) and lower L* region, possibly induced by electromagnetic ion cyclotron wave (EMIC) scattering or field line curvature scattering effect, are not captured by the model. Interestingly, the observed 100s keV proton fast drops sometimes occur concurrently with fast dropout of MeV radiation belt electrons. Theoretically, two main loss mechanisms are proved to be responsible for the fast losses of both populations, including the precipitation loss due to EMIC wave scattering and magnetopause shadowing (MPS) combined with outward radial diffusion. To quantify the role of each loss mechanism on the simultaneous dropout of both populations, two storm events are studied, respectively. The first storm event is on 27 February 2014, when concurrent dropouts between the two populations are accompanied by in-situ EMIC wave measurements within ~ 40 min. The wave and particle measurements during the period of the most intense EMIC waves at L ∼ 5.2 are used to calculate the quasilinear diffusion coefficients and simulate the evolution of both energetic electrons and protons. Our 2D pitch angle and energy diffusion model well captures the dropout of electrons with energies >1 MeV and pitch angles < 75°, and the concurrent dropout of protons with energies >200 keV and pitch angles > 40°. However, even though this study reveals the important role of EMIC wave scattering to the concurrent localized dropout of both populations, it only focused on one L shell and was based on a drift-averaged model without a local time dependence. To quantify the relative contribution of EMIC wave scattering to the global variations of both particle populations, we further simulated this storm event by employing the global kinetic ring-current-atmosphere interactions model (RAM) coupled with 3-D Euler potential-based plasma equilibrium code (SCB). The results indicate that by including EMIC wave scattering loss, especially by the He-band EMIC waves, the model aligns closely with data for both populations. Additionally, we investigate the simulated pitch angle distributions (PADs) for both populations. Including EMIC wave scattering in our model predicts a more 90° peaked PAD for electrons with stronger losses at lower pitch angles, while protons exhibit an isotropic PAD with enhanced losses at pitch angles above 40°. Furthermore, our model predicts considerable precipitations of both particle populations, predominantly confined to the afternoon to midnight sector (12 hr < MLT < 24 hr) during the storm's main phase, corresponding closely with the presence of EMIC waves.

The other storm event we simulated is during 27-28 May 2017, which focuses on investigating the magnetopause shadowing (MPS) effect to the simultaneous dropout of both radiation belt electrons and ring current protons. A radial diffusion model with an event-specific last closed drift shell is used to simulate the MPS loss of both populations. The model well captures the fast shadowing loss of both populations at L* > 4.6, while the loss at L* < 4.6, possibly due to the electromagnetic ion cyclotron wave scattering, is not captured. The observed butterfly pitch angle distributions of electron fluxes in the initial loss phase are well reproduced by the model. The initial proton losses at low pitch angles are underestimated, potentially also contributed by other mechanisms such as field line curvature scattering.

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