Date of Degree
PhD (Doctor of Philosophy)
Jack D. Scudder
The electron physics in the collisionless magnetic reconnection is studied using data from the Polar spacecraft. Among the types of discontinuities in space plasmas, the Electron Diffusion Region (EDR) at the center of the reconnection has the theoretically unique properties that its thickness is of order of the electron gyroradius, and in such a region electrons are demagnetized with a non-gyrotropic pressure tensor. These unusual properties of EDRs reflect the expected violation of guiding center theory for electrons and are exploited in this thesis. We use four dimensionless, diagnostic, single spacecraft observables derived from theoretical properties of EDRs to locate them. They are electron agyrotropy, out-of-interconnection-plane electron Mach number, and dimensionless thresholds for electric field strengths parallel and perpendicular to the magnetic field. These observables are constructible using the electron density, bulk velocity, pressure tensor, and the electromagnetic field data. With a 3-year survey using particle data from a slower version of Hydra's moment producing system, M3, the vast preponderance of these dimensionless parameters are below unity, which is consistent with the theoretical expectations for most space plasmas being strongly magnetized. The unusual outliers with the demagnetization parameters over unity (<1%) in the distribution are geophysically distributed near the magnetopause within 8-9 Re shells and collected as potential reconnection sites, although a number of other possibilities are also considered in this thesis such as data processing anomalies, systematic effects of data acquisition and aliasing. It is shown that plasma particle data with the highest time resolution possible are needed to improve the time aliasing issues, and to sense the rapidly changing and short scale current structure like the EDR. We use a recently developed algorithm G3/T1, which reduces the aliasing time of the 3D analysis of products of the Polar Hydra Hot Plasma Analyzer by a factor of 12. With this new technique, we have found that among these outliers some demagnetization signatures are ameliorated by higher time cadences and the ones caused by time aliasing effects are ruled out. The moment recoveries of G3/T1 at a 2.3 cadence are in excellent agreement with input distribution models over a considerably wide range of density, Mach number, electron anisotropy, and agyrotropy, provided that a suitable accurate inventory can be made in advance for the bulk velocity of these distributions. The 2 candidate reconnection events analyzed in this thesis by G3/T1 processing techniques demonstrate: (1) strong out-of-interconnection-plane electron flows along the separatrices also observed in 2D PIC simulations; (2) significant electron agyrotropy enhancements framing high thermal Mach number flow, proving excellent consistency with the agyrotropy islands predicted by PIC simulations of asymmetric reconnection geometry; and (3) measured thermal electron gyroscale current channels in patterns that are supported by PIC simulation models as resolved examples of EDRs with direct measures of the electron demagnetization.
agyrotropy, demagnetization, electron diffusion region, hydra, magnetic reconnection, polar
xvi, 153 pages
Includes bibliographical references (pages 145-153).
Copyright 2011 Shanshan Li Rodriguez