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


Rankin, Jamie Sue (2019) Voyager 1 Observations of Galactic Cosmic Ray Anisotropies in the Local Interstellar Medium. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/WX3S-1D86.


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.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Voyager 1; Voyager Mission; interstellar medium; galactic cosmic rays; solar physics; Cosmic Ray Subsystem; heliophysics; astro-particle physics; sun; interstellar space
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Stone, Edward C.
Group:Space Radiation Laboratory
Thesis Committee:
  • Stone, Edward C. (chair)
  • Hallinan, Gregg W.
  • Hopkins, Philip F.
  • Patterson, Ryan B.
Defense Date:23 August 2018
Funding AgencyGrant Number
Record Number:CaltechTHESIS:09022018-124448728
Persistent URL:
Rankin, Jamie Sue0000-0002-8111-1444
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11168
Deposited By: Jamie Rankin
Deposited On:06 Sep 2018 22:27
Last Modified:24 Jun 2024 20:19

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