Author ORCID Identifier

https://orcid.org/0000-0002-7105-4634

Semester

Summer

Date of Graduation

2023

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Weichao Tu

Committee Member

Paul Cassak

Committee Member

Piyush Mehta

Committee Member

Earl Scime

Abstract

Energetic charged particles trapped in the Earth’s radiation belt form a hazardous space environment for artificial electronic systems and astronauts. The study of Earth's radiation belt is becoming increasingly important with the development of communication technology, which plays a significant role in modern society. Earth’s radiation belt is highly dynamic, and the electron flux can drop by several orders of magnitude within a few hours which is called radiation belt dropout. The fast dropout of energetic electrons in the radiation belt, despite its significance, has not been thoroughly studied. One of the most compelling outstanding questions in Earth's radiation belt studies is: "What physical mechanisms cause these rapid and substantial drops of radiation belt electron flux?" Apart from well-studied processes like wave-particle interaction, which contribute to the loss of radiation belt electrons through the processes including magnetopause shadowing and atmospheric precipitation, the effects from an anomalous process called drift orbit bifurcation (DOB) have not yet been fully understood. DOB has been suggested to play a major role in the loss and transport of radiation belt electrons since it violates the particles’ second adiabatic invariant and makes the third invariant undefined. In our first study of this dissertation, using guiding-center test particle simulations based on the Tsyganenko-1989c magnetic field model we show that DOB could affect a broad region of the outer radiation belt. It can penetrate inside the geosynchronous orbit at Kp ≥ 3, where Kp is a geomagnetic index that quantifies the general disturbance level of Earth’s magnetosphere. Moreover, DOB effects are more significant further away from Earth, at higher Kp, and for higher electron energies. Specifically, the short-term simulation results after one electron drift show both traditional and nontraditional DOB transport of electrons, with the nontraditional DOB, caused by a third minimum of the magnetic field strength near the equator, reported by us for the first time. Moreover, our results show large ballistic jumps in the second invariant and radial distance for electrons at high equatorial pitch angles after one drift. In addition, long-term DOB transport coefficients of electrons over many drifts are calculated based on our simulation results. We find that the pitch angle and radial diffusion coefficients of electrons due to DOB could be comparable to or even larger than those caused by electron interactions with chorus and Ultra-Low-Frequency waves, respectively. Meanwhile, the last closed drift shell (LCDS) has been identified as a crucial parameter for investigating the magnetopause shadowing loss of radiation belt electrons. However, the DOB effects have not been physically incorporated into the LCDS calculation. In the second study of this dissertation, we calculate the event-specific LCDS using different approaches to dealing with the DOB effects, i.e., tracing field lines ignoring DOB, tracing test particles rejecting DOB, and tracing test particles including DOB, and then incorporate them into a radial diffusion model to simulate the fast electron dropout observed by Van Allen Probes in May 2017. The model effectively captures the fast dropout at high L* (the third adiabatic invariant) and exhibits the best agreement with data when LCDS is calculated by tracing test particles and including DOB effects more realistically. This study represents the first quantitative modeling of the DOB effects on the radiation belt magnetopause shadowing loss via a more physical specification of LCDS. In summary, our results demonstrate that DOB could cause effective loss and transport of radiation belt electrons even in the absence of waves.

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