Date of Award

Spring 2007

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Joachim Raeder

Abstract

The state of the plasma sheet in the magnetosphere is usually observed to be hot (1-10 keV) and tenuous (∼0.1 cm-3). However, sometimes part of it is observed to be colder (< 1 keV) and denser (∼1 cm-3), and the plasma flow is almost stagnant. Much higher density (∼10 cm-3) plasma material (superdense plasma sheet) is also sometimes observed near the geosynchronous orbit. The cold dense plasma sheet (CDPS) is usually observed after a period of northward interplanetary magnetic field (IMF), which is also a necessary condition for the formation of a superdense plasma sheet (SDPS). Since the CDPS is generally absent of a cold O+ component, and the ionospheric outflow is strong only under southward IMF condition, the source of the CDPS is thought to be the solar wind.

Usually, solar wind plasma and energy entry into the magnetosphere is considered to occur mainly during the southward IMF condition through reconnection processes that first occur at the dayside magnetopause and then in the magnetotail. However, the formation of CDPS suggests that there are also certain processes that let solar wind enter the magnetosphere when the IMF is northward. The purpose of this dissertation study is to find out the mechanism that transports solar wind plasma into the magnetosphere under northward IMF conditions, and thus to find out the mechanism of the formation of CDPS and SDPS.

To study the solar wind entry mechanism, I use global simulations of the magnetosphere in conjunction with the analysis of observation data. The model used here is the Open Global Geospace Circulation Model (OpenGGCM), which is a magnetosphere MHD model with a stretched grid that has higher grid resolution near the Earth. The simulation is driven by the upstream solar wind input. I run simulations for several CDPS events to validate the model by comparing the simulation results with observations. I then establish that the double high-latitude reconnection process is the dominant process that leads to the entry of solar wind plasma under northward IMF conditions, and that it is sufficient to form the CDPS. With the successful simulation of CDPS events, I continue to study a SDPS event in detail using simulation and observations from a series of spacecraft. I find that the southward IMF following a long period of northward IMF condition compresses the preexisting CDPS, and sets off the near-tail reconnection that causes the compressed CDPS to be pushed and accelerated toward the Earth and form the SDPS near the geosynchronous orbit.

I further systematically study how the solar wind plasma enters the magnetosphere due to double high-latitude reconnection for various solar wind, northward IMF and geomagnetic dipole conditions. I trace flow paths from the solar wind and study the variation of the magnetic field line topology along the flow paths. I find that there is an entry window through which the solar wind plasma can enter the magnetosphere as a result of double high-latitude reconnection under northward IMF conditions. I show how the entry window depends on solar wind, IMF and geomagnetic dipole parameters. With the entry window, I estimate the solar wind plasma entry rate for various conditions. I find that the entry rate under northward IMF conditions is of the order of 1026 to 1027 particles per second. I also estimate the conditions at which solar wind plasma entry is most efficient.

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