Date of Award

Spring 2018

Project Type

Thesis

Program or Major

Physics

Degree Name

Master of Science

First Advisor

Eberhard Moebius

Second Advisor

Martin Lee

Third Advisor

John Dawson

Abstract

We present the first systematic analysis to determine pickup ion (PUI) cutoff speed variations, during general compression regions identified by their structure, shock fronts, and times of highly variable solar wind (SW) speed or magnetic field strength. This study is motivated by the attempt to remove or correct for these effects on the determination of the longitude of the interstellar neutral gas flow from the flow pattern related variation of the PUI cutoff with ecliptic longitude. At the same time, this study sheds light on the physical mechanisms that lead to energy transfer between the SW and the embedded PUI population. Using 2007-2014 STEREO A PLASTIC observations we identify compression regions and shocks in the solar wind and analyze the PUI velocity distribution function (VDF). We developed a routine to identify stream interaction regions and CIRs, by locating the stream interface and the successive velocity increase in the solar wind speed and density. Characterizing these individual compression events and combining them in a superposed epoch analysis allows us to analyze the PUI population under similar conditions and find the local cutoff shift with adequate statistics. The result of this method yields substantial cutoff shifts in compression regions with large solar wind speed gradients. Additionally, through sorting the entire set of PUI VDFs at high time resolution, we obtain a noticeable correlation of the cutoff shift with gradients in the SW speed and interplanetary magnetic field strength. We discuss implications for the understanding of the PUI VDF evolution and the PUI cutoff analysis of the interstellar neutral gas flow.

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