Voyager 1 Observations of Galactic Cosmic Ray Anisotropies in the Local Interstellar Medium

Author: Rankin, Jamie Sue

Year: 2019

Degree: Dissertation (Ph.D.)

Advisor: Stone, Edward C.

Committee Members: Stone, Edward C.; Hallinan, Gregg W.; Hopkins, Philip F.; Patterson, Ryan B.

Option: Physics

DOI: 10.7907/WX3S-1D86

Abstract

Since crossing the heliopause on August 25, 2012, Voyager 1 has observed reductions in galactic cosmic ray counting rates caused by a time-varying depletion of particles with pitch angles near 90-deg, while intensities of particles with other pitch angles remain unchanged. Between late 2012 and mid-2017, three large-scale, durable events occurred, lasting roughly 100 to 630 days. Omnidirectional and directional data from the Cosmic Ray Subsystem's high energy telescopes are used to report observations of the cosmic ray intensity variations. Omnidirectional (>20 MeV) proton-dominated measurements show up to a ~3.8% intensity reduction. Bi-directional (>70 MeV) and unidirectional (~18 to ~70 MeV) proton-dominated measurements are analyzed using data taken from various spacecraft orientations, including during magnetometer roll calibrations and 70-deg-offset maneuvers. The anisotropy is characterized as a "notch" in an otherwise uniform pitch-angle distribution of varying depth and width centered about 90-deg in pitch angle space. The notch averages ~22-deg wide and ~15% deep -- signifying a depletion region that is broad and shallow. However, electron observations reveal that there is only a weak, at most, evidence of pitch angle anisotropy in cosmic-ray electrons with energies of ~3 to ~105 MeV, indicating that the generation of the notch or its evolution differs between electrons and protons, or varies with rigidity. There are indications that the anisotropy is formed by a combination of magnetic trapping and adiabatic cooling in associated shocks or compression regions.

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