Energy Dissipation during Geomagnetic Storms
Abstract
Some fraction of the solar wind energy is transferred to the Earth magnetosphere
and ionosphere. Sometimes this transfer of energy is exceptionally large,
producing a magnetic storm. Storms occur when the Interplanetary Magnetic
Field (IMF) turns southward and remains southward for an extended period of
time. During the main phase of many magnetic storms, the solar wind Mach
number is low, and IMF magnitude is large. Under these conditions, the ionospheric
potential saturates, and it becomes relatively insensitive to further increases
in the IMF magnitude. On the other hand, the dayside merging rate and
the potential become sensitive to the solar wind density. This should result in
a correlation between the intensity of the auroral electrojets and the solar wind
density. In this study, I find several storm events to examine the effect of the
solar wind density on the intensity of the auroral electrojets (as measured by the
SME index) under the condition of low Mach number and steady IMF. As expected,
there is a positive correlation between solar wind density and the SME
index. I show that this correlation coefficient gets larger for smaller Mach number
when one would expect the effect of density to be more significant.
Furthermore, I study the role of solar wind density during an event with the
small northward IMF. In the case of northward IMF, since the reconnection regions
are limited, the changes of the ionospheric potential caused by the viscous
interaction can be greater/comparable to the reconnection-driven potential. I
show that the solar wind density and the SME index correlate with small northward
IMF during the event. Thus, the solar wind density correlations with the auroral electrojets have the same behavior under two conditions: 1) in the saturation
regime and 2) in the event with northward IMF, although very different
physics drove them.
Moreover, I provide a sample of 314 moderate to strong storms and investigate
the correlation between the Dst index and the energy dissipated in the
ionosphere. I show that, on average, for the lower Mach number, this correlation
decreases. I also show that the ionospheric indices of the storms with the
lower Mach number are less correlated to the geoeffectiveness of the solar wind
during these storms.
As a next step to studying the energy dissipation during the magnetic storms,
I study the energy dissipated in the ionosphere through frictional heating, generally
referred to as Joule heating. There are several empirical models to estimate
Joule heating based on ionospheric currents using the AE index. In this study, I
select 12 magnetic storms from the CCMC database and compare the integrated
joule heating with the results of empirical models. I also use the SWMF global
magnetohydrodynamic simulations for 13 storms to reproduce the correlation
between the simulated AE index and simulated Joule heating to examine the
empirical models. I find that the scale factor in the empirical model is half the
predicted using the SWMF simulations.
Finally, at the end of the dissertation, I point out the possible future studies.