Dissertation
Suprathermal electron characteristics observed by the Parker Solar Probe
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2025
DOI: 10.25820/etd.008021
Abstract
The solar wind electron typically comprises core, halo, and strahl electrons. In this study, we focused on the suprathermal (halo and strahl) electron characteristics observed by the Parker Solar Probe (PSP). In the first study, we investigate localized deviations from the nominal Parker spiral magnetic field in the solar wind called switchbacks. We explore electron parameters in relation to the angle of rotation of the magnetic field from radial to determine whether electron distributions observed within switchbacks have any differences from those outside of switchbacks. Our observations reveal several trends in the suprathermal electron populations inside switchbacks. The sunward deficit in the electron velocity distribution function (eVDF) typically observed near the Sun is filled in at larger rotation angles. This results in the suprathermal electron density and heat flux in the anti-strahl direction changing from a negative to a positive value. On many days, we also observe a positive correlation between the halo density and rotation angle, which may suggest that the growth of the halo may fill in the sunward deficit. We also find that strahl distributions have an increased average angular spread at large magnetic field rotation angles. The increase in suprathermal electron flux in the anti-strahl direction, and the increase in strahl width, together could suggest that enhanced scattering occurs inside switchbacks. Electron core beta values tend to increase with the magnetic field rotation angle, mainly due to a decrease in magnetic pressure. An increase in electron beta may favor the growth of instabilities inside switchbacks. The PSP observations therefore support an enhanced role for wave-particle interactions in switchbacks. Two possible wave-particle interactions that could alter the eVDF inside switchbacks are whistler waves and kinetic Alfve ́n waves (KAW).
In the second study, we utilize Parker Solar Probe (PSP) measurements from the first nine perihelia to investigate suprathermal electron scattering near the Sun. We employ a normalized isotropy parameter to identify pitch angle scattering (PAS) regions in the inner heliosphere and compare the plasma conditions during these periods to the background (BG) solar wind. Suprathermal electron scattering was also observed in partial/complete heliospheric current sheet crossings (PCS/HCS). PCS/HCS crossing times were obtained from other literature. We find slightly higher electron collisional ages (Ae) in the PAS and PCS/HCS regions than in the BG regions. However, we conclude that Coulomb collisions alone likely cannot explain the observed suprathermal scattering. We investigate plasma wave modes that could play a role in suprathermal electron scattering and identify trends in wave occurrence in BG, PAS, and PCS/HCS regions. We find higher occurrence rates of narrowband whistler-mode waves with frequencies of 0.04-0.19 f/fce and a higher occurrence of larger magnetic field wave power in this frequency band in the PAS and PCS/HCS regions. These observations support the hypothesis that whistler-mode waves play a role in suprathermal electron scattering at moderate distances. However, closer to the Sun, narrowband whistlers are more rarely observed. Instead, we find higher occurrence rates of broadband electrostatic waves with frequencies of 0.1-4.4 f/flh and a higher occurrence of larger electric wave power in this band in the near-Sun PAS and PCS/HCS regions. These observations suggest a role for broadband electrostatic waves in suprathermal electron scattering closer to the Sun.
Details
- Title: Subtitle
- Suprathermal electron characteristics observed by the Parker Solar Probe
- Creators
- Raaman Nair
- Contributors
- Jasper S Halekas (Advisor)Casey DeRoo (Committee Member)Phyllis L Whittlesey (Committee Member)Gregory G Howes (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Physics (Astronomy)
- Date degree season
- Spring 2025
- DOI
- 10.25820/etd.008021
- Publisher
- University of Iowa
- Number of pages
- xv, 140 pages
- Copyright
- Copyright 2025 Raaman Nair
- Language
- English
- Date submitted
- 04/06/2025
- Description illustrations
- illustrations, graphs, tables
- Description bibliographic
- Includes bibliographical references (pages 120-140).
- Public Abstract (ETD)
- The solar wind (plasma emanating from the Sun) electrons comprise a less energetic population called the core electrons and a highly energetic population of electrons called suprathermals (halo and strahl electrons). The first study investigated how the three electron populations vary inside switchbacks compared to the ambient solar wind. Switchbacks are localized rotations in the solar winds. We explore the various electron parameters in relation to the angle of rotation of the magnetic field from radial to determine whether electron distributions observed within switchbacks differ from those outside switchbacks. Our observations reveal several trends in certain populations of electrons inside switchbacks, specifically in two types of electrons, the halo and strahl electrons. In the second study, we investigated strahl electron scatterings in the first nine PSP spacecraft encounters with the Sun. We categorized our observations based on three regions of the solar wind: ambient/background solar wind (BG), pitch angle scattered regions (PAS), and partial/complete heliospheric current sheet crossings (PCS/HCS). This study aims to understand what mechanisms may be scattering strahl electrons closer to the Sun and how these three regions differ. We found particle-particle collisions (electron Coulomb collisions) to play a minor role in scattering the strahl electrons. Instead, our observations suggest whistler waves scatter the strahl electrons via wave-particle interactions. Closer to the Sun, whistler waves are rare, and we observed the possibility of broadband electrostatic waves scattering strahl electrons. Our observations support wave-particle interactions as the primary process that scatters strahl electrons in the inner heliosphere.
- Academic Unit
- Physics and Astronomy
- Record Identifier
- 9984831023502771
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