Date of Award

Spring 2015

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Joachim Raeder

Second Advisor

Charlie Farrugia

Third Advisor

Martin Lee

Abstract

In this dissertation, we study the influence of interplanetary (IP) shock impact angles in the IP shock geoeffectiveness focusing on simulations and observations. In our simulations, we use OpenGGCM global MHD code to study the nightside magnetospheric, magnetotail, and ionospheric responses to IP fast forward shocks. Three cases are presented in this study: two inclined oblique shocks, hereafter IOS-1 and IOS-2, where the latter has a Mach number twice stronger than the former. Both shocks have impact angles of 30$^o$ in relation to the Sun-Earth line. Lastly, we choose a frontal perpendicular shock, FPS, whose shock normal is along the Sun-Earth line, with the same Mach number as IOS-1. We find that, in the IOS-1 case, due to the north-south asymmetry, the magnetotail is deflected southward, leading to a mild compression. The geomagnetic activity observed in the nightside ionosphere is then weak. On the other hand, in the head-on case, the FPS compresses the magnetotail from both sides symmetrically. This compression triggers a substorm allowing a larger amount of stored energy in the magnetotail to be released to the nightside ionosphere, resulting in stronger geomagnetic activity. By comparing IOS-2 and FPS, we find that, despite the IOS-2 having a larger Mach number, the FPS leads to a larger geomagnetic response in the nightside ionosphere. As a result, we conclude that IP shocks with similar upstream conditions, such as magnetic field, speed, density, and Mach number, can have different geoeffectiveness, depending on their shock normal orientation. In the second part of this dissertation, we present a survey of fast forward IP shocks using WIND and ACE satellite data from January 1995 to December 2013 to study how IP shock geoeffectiveness is controlled by IP shock impact angles. A shock list covering one and a half solar cycle is compiled. The yearly number of IP shocks is found to correlate well with the monthly sunspot number. We use data from SuperMAG, a large chain with more than 300 geomagnetic stations, to study geoeffectiveness triggered by IP shocks. The SuperMAG SML and SME indices, enhanced versions of the familiar AL and AE indices, are used in our statistical analyses to quantify substorm strength and auroral power (AP) intensity, respectively. The jumps of the SML index and the calculated AP intensity triggered by IP shock impacts on the Earth's magnetosphere are investigated in terms of IP shock orientation and speed. We find that, in general, strong (high speed) and almost frontal (shock normal almost parallel to the Sun-Earth line) shocks are more geoeffective than inclined shocks with low speed. The highest correlations (correlation coefficient R = 0.78 for SML, and R = 0.79 for AP) occur for fixed IP shock speed and varying the IP shock impact angle. We attribute this result, predicted previously by simulations, to the fact that frontal shocks compress the magnetosphere symmetrically from all sides, which is a favorable condition for the release of magnetic energy stored in the magnetotail, which in turn can produce moderate to strong auroral substorms, which are then observed by ground based magnetometers. These results confirm our previous numerical simulations.

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