Date of Award

Fall 2018

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Joachim Raeder

Second Advisor

Lynn Kistler

Third Advisor

Kai Germaschewski

Abstract

The connection between ionospheric conductivity and the dynamics of the magnetosphere was investigated, using several methods to change the ionospheric conductivity and then study the resultant changes to the magnetosphere. Computer simulations of the Earth's geospace environment were utilized using OpenGGCM coupled with an ionosphere model CTIM and a ring current model RCM.

Three methods were used to modify ionospheric conductivity. The incoming particle precipitation was modified by several orders of magnitude $\alpha=.01,.1,1,10$, the ionospheric conductivity was increased or decreased by factors $\beta=.25, .5, 1, 2,$ and $4$, and for the last method differing values of $F_{10.7}$, $70,110, 150, 200,$ and $250$ were used. Each of the methods is different because $F_{10.7}$ mostly affects the dayside, while precipitation mostly affects the nightside, then using the $\beta$ changes the conductivity over the whole ionosphere. This gives a good range for studying the effects of ionospheric conductivity on the magnetosphere.

The magnetospheric dynamics studied are: the dayside magnetopause location, the reconnection rate of the Earth's magnetosphere, X-line formation in the magnetotail, and substorm dynamics, both the frequency and magnitude of substorm occurrence.

To understand the effect of particle precipitation on conductivity two events were simulated, a calm period on 4 May 2005 and a strong storm period on 17 March 2013. Scaling the precipitation energy flux by several orders of magnitude, conductivities in the auroral oval were influenced which, in turn, influence the cross polar cap potentials. With the change in conductance, magnetospheric convection is enhanced or reduced, and the location of the subsolar distance of the magnetopause can change by up to one $R_E$. The investigation of the reconnection rate for the varying precipitation simulations using the Hesse-Forbes-Bern method shows that particle precipitation affects the magnetic reconnection rate in these two events. The most notable differences, up to 40\%, occur on short time scales, that is, hours. A relation for longer time scales (tens of hours) between precipitation and reconnection for these two events is more difficult to ascertain. Differences in cross polar cap potential (CPCP) and reconnection rate (R) can be explained by viscous interactions and polar cap saturation. When precipitation was decreased, polar conductance was decreased, viscous interactions are stronger, and CPCP is higher than R. For high precipitation, high conductance cases the polar cap is in the saturation regime and CPCP is lower than R. Hemispheric asymmetries were found in the cross polar cap potential and in the calculated reconnection rate derived from the Northern and Southern Hemispheres. The majority of this research has already been published in the Journal of Geophysical Research: Space physics, "Particle Precipitation Effects on Convection and the Magnetic Reconnection Rate in Earth's Magnetosphere" https://doi.org/10.1002/2017JA024030

For the whole ionospheric conductivity study, different values of $\beta=$, $.25, .5, 1, 2, 4$ were used to modify the ionospheric conductivity after it had been calculated by the ionosphere model. A moderate storm period, 16 May 2011 was simulated. Many of the same conclusions found in the precipitation study were found in this study as well, such as, CPCP decreasing as conductivity increases, the point at which the polar cap saturates decreases with increasing conductivity, and reconnection rates change on short time scales, but the overall average rate remains very similar. The incoming precipitation was used to identify auroral brightening that is linked with substorms. The criteria for auroral brightenings used in this study is where the maximum precipitation increased by at least $1 \ mW/m^2$ within 20 minutes. The criteria for substorms is that the maximum precipitation increases by 80\% within 20 minutes. Identifying all the auroral brightenings and substorms showed that as conductivity increased the maximum amount of precipitation decreased, and also the number and frequency of both the substorms and auroral brightenings decreased. The occurrence of extended X-lines in the magnetotail was analyzed, where if an earthward flow of greater than 50 km/s extended for greater than 10 $R_e$ in $Y_{GSE}$ was classified as an extended X-line. This is not to be confused with a bursty bulk flow or dipolarization front, which happen from reconnection but usually do not have a large extent in $Y_{GSE}$. Identifying extended X-lines in this manner showed a similar trend that as conductivity increased the number of extended X-lines decreased, and while there was not much of an indication if the size or location is affected much, the amount of time the simulation had extended X-lines present decreased.

For the $F_{10.7}$ study, using values of $70, 110, 150, 200,$ and $250$, the ionospheric conductivity was influenced mostly on the dayside. A day during the spring equinox was simulated with ideal solar wind conditions as well as the 16 May 2011 storm period. The main results found is that $F_{10.7}$ does not affect the system as much as the precipitation study, or the whole ionosphere conductivity study, but there are still some indications that show the same conclusions obtained previously.

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