"The hydromagnetic structure of the polar cap and its interaction with " by Kevin D. Urban

Document Type

Dissertation

Date of Award

5-31-2016

Degree Name

Doctor of Philosophy in Applied Physics - (Ph.D.)

Department

Physics

First Advisor

Andrew Gerrard

Second Advisor

Louis J. Lanzerotti

Third Advisor

Denis L. Blackmore

Fourth Advisor

Jacob Bortnik

Fifth Advisor

Wenda Cao

Sixth Advisor

Dale E. Gary

Seventh Advisor

Jerry W. Manweiler

Eighth Advisor

Barbara J. Thompson

Ninth Advisor

Allan Thomas Weatherwax

Abstract

At this time it is uncertain as to how and to what extent ultra-low frequency (ULF) structures in the solar wind (periodicities ranging between several seconds to several hours) impact the coupled ionosphere-thermosphere-magnetosphere (ITM) system, especially in the high-latitude open flux region, where these structures can directly drive polar cap phenomena (e.g., via the direct injection of hydromagnetic energy along open field lines). First, a historical overview of open and closed models of the magnetosphere is provided, which develops the motivations and concepts associated with the expanding/contracting polar cap (ECPC) model that prevails today. In the ECPC model, the open flux region in the polar ionosphere grows and decays in size dependent on interplanetary plasma and electromagnetic conditions at the dayside magnetopause and in the magnetotail. In the first paper presented, following a body of theoretical and observational work, it is presumed that coherent waves on closed field lines near the polar cap have strong-enough amplitudes to often distinguish them from presumably non-resonant-or at least lower-powered-oscillations that may occur on open field lines. Guided by this principle, a residual spectral analysis of magnetic variation data acquired at each site in a high-latitude, Antarctic network of ground-based magnetometers is used to synoptically estimate the ULF wave power in the Pc5 (~2.5-10 minute) and Pc6 (~10-60 minute) bands as a function of time. By inspecting the statistical wave power at the poleward- and equatorward-most sites, an open/closed discriminant for each band is estimated, and synoptic-scale open/closed field line determinations are produced at a 10-minute resolution; it is shown that the technique has strong potential for continuously estimating the evolution of the open-closed boundary. Motivated by possible weaknesses in this scheme and to better understand the structure and dynamics of ULF wave and noise power in the polar cap region, the next paper looks at the day-to-day, long-term, and statistical features of Pc3-Pc7-band ULF power at ground sites throughout the nominal polar cap region. The analysis suggests that the corrected geomagnetic (CGM) coordinate description of the ground-level polar cap does not meaningfully organize the observed hydromagnetic spatial structure. It is argued that enigmatic asymmetries observed in CGM latitude and longitude are merely apparent, and not due to physical processes as suggested by previous authors; they arise only if assuming that CGM coordinates physically correspond to overhead magnetospheric and ionospheric regions. An alternative, observationally-motivated system of “polar cap latitudes” is provided that resolves nearly all observed asymmetries. A remaining mismatch between the synoptic ULF observations and the data-determined coordinates is cleared up by nudging the polar cap centroid geographically-westward-the result of which corresponds to the location of the eccentric dipole axis (EDA) pole. Thus, it is shown that coordinates assigned with respect to the EDA pole naturally organize the polar cap’s hydromagnetic structure. In the last paper presented, the direct influence of solar wind ULF structures in the vicinity of the open flux region is investigated over a 63-day interval of varied solar wind conditions (including several corotating interaction regions (CIRs) and coronal mass ejections (CMEs)). By analyzing a time series of lagged correlation sequences (“dynamic correlation functions”) between upstream solar wind and ground-based ULF power, one deep polar cap site uniquely stands out as having the capacity to remotely-sense hydromagnetic power in the solar wind, nearly independent of solar wind conditions. The analysis confirms the data-driven description of the polar cap is at odds with CGM coordinates. The observations presented herein give the first direct evidence that a magnetometer site deep in polar cap can unambiguously observe the IMF along open field lines for days, weeks, and even months at a time, allowing for a data product useful to nearly any study of the ITM system.

Included in

Physics Commons

Share

COinS
 
 

To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.