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Examining auroral downward current region processes using ground based data.

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Call No.	QC971.7.A4 M53 2007
Author	Michell, Robert Gregory.
Title	Examining auroral downward current region processes using ground based data.
Date	2007
Abstract	This thesis presents data from ground camera observations of the aurora conducted from Alaska during the winters of 2005 through 2007. In conjunction with these camera observations, conjugate radar observations of the auroral ionosphere were conducted during the winters of 2006 and 2007, using the Poker Flat Advanced Modular Incoherent Scatter Radar (AMISR). Two main aspects of auroral downward-current (DCR) and time-varying/polar cap boundary regions are investigated, namely, dark auroral signatures in the camera data, and naturally enhanced ion acoustic lines (NEIALs) in the radar data. An auroral event containing a distinct dark stripe occurred on 06 March 2005 over Kaktovik, Alaska. This event displayed many characteristics of an auroral DCR, as compared to in situ studies of DCRs using data from the FAST and Cluster satellites. It is shown that this dark stripe is the auroral signature of a downward-current region. This stripe was found to widen at a rate of 750 m/s, which is consistent with the predictions from the theory and model of DCRs ( Streltsov and Marklund, 2006). The second aspect of these auroral regions investigated here is the occurrence of NEIALs in the radar data. Observations of NEIALs are presented from two separate nights, displaying different auroral context. On 08 February 2007, one short-lived NEIAL event was observed with AMISR using high-time resolution (∼19 ms). These raw data reveal that the large returns associated with the NEIALs come from the dark region immediately adjacent to an active dynamic auroral arc. It was also found that propagation in altitude of the NEIALs occurs at or near the Alfvén velocity. Furthermore, it was found that the enhanced up- and down-shifted shoulders of the NEIALs most often occurred independently of one another on a 19 ms timescale. On 31 March 2006, a moderately intense auroral arc, (∼10 kR at 557.7 nm), was located in the local magnetic zenith at Poker Flat, Alaska. During this event the radar observed 7 distinct periods of NEIAL activity. These times correspond to (a) when the polar cap boundary of the auroral arcs passed through the magnetic zenith and (b) when small-scale filamentary dark structure was present in the magnetic zenith. These observations are consistent with NEIALs occurring within the same auroral morphology which is known to contain broad-band extremely low frequency (BBELF) wave activity. By comparing the densities at which NEIALs have been observed by previous studies, it is found that NEIALs occur at densities roughly between 5 and 30 x10 10 m -3. The observations presented in this dissertation support the hypothesis that NEIALs and BBELF are differently observed aspects of the same auroral phenomenon. It is a goal of this thesis to connect these ground-based observations with previous in situ measurements and identify DCRs from the ground. A relation between NEIALs and the in situ signature of BBELF wave activity, provides a link between the in situ measurements and the ground-based observations. There are no actual ground-based/in situ conjugate events in this study. The focus here is to compare the optical observations to the morphology and auroral context of DCRs as measured in situ by previous studies. The use of ground-based observational techniques for observing DCR processes has many implications. These include identifying and following the temporal and spatial evolution of DCRs as well as being able to identify regions of potential ion outflow on a large spatial and temporal scale using ground based optical observations.
Description	xxii, 185 leaves : ill. 28 cm.
Format	application/pdf
Advisor	Kristina A. Lynch
Subject	Auroras -- Alaska -- Observations. Magnetospheric currents.
College	Dartmouth College -- Department of Physics and Astronomy.
Issuing Body	Dartmouth Dissertations are made available through the Dartmouth Digital Library.
Degree	Thesis (Ph.D.)--Dartmouth College.
Copyright	All rights reserved. No part of this book may be reproduced in any form by any electronic or mechanical means without permission in writing from the author.
DOI	http://dx.doi.org/10.1349/ddlp.139

Title	Page 1
Format	application/pdf
Issuing Body	Dartmouth Dissertations are made available through the Dartmouth Digital Library.
Degree	Thesis (Ph.D.)--Dartmouth College.
Full Text	EXAMINING AURORAL DOWNWARD CURRENT REGION PROCESSES USING GROUND BASED DATA A Thesis Submitted to the Faculty in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics and Astronomy by Robert Gregory Michell DARTMOUTH COLLEGE Hanover, New Hampshire 25 July 2007 Examining Committee: Kristina A. Lynch William Lotko Robyn M. Millan Joshua L. Semeter Charles K. Barlowe, Ph.D. Dean of Graduate Studies

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