Global-scale observations of changes in ionospheric echo occurrence and convection during periods of increased solar wind activity and increased geomagnetic activity

Abstract

This thesis presents an investigation of ionospheric plasma convection and HF radar echoes during periods of enhanced geomagnetic activity. The work was split into two studies: the ionospheric response to periods of prolonged geomagnetic activity in the form of geomagnetic storms and to abrupt changes in geomagnetic activity during sudden commencement (SC) events. A new statistical analysis technique was developed and applied to ionospheric plasma velocity measurements to categorize sunward and antisunward ionospheric plasma drifts. The global effects on the ionosphere during periods of abrupt changes in geomagnetic activity were investigated using the Super Dual Auroral Radar Network (SuperDARN).

The first study investigated 136 geomagnetic storms. The influences of the main and recovery phase of geomagnetic storms on HF radar echoes and ionospheric plasma velocities were examined. It is important to study the HF radar echoes because they reveal important information regarding the state of the ionosphere. The geomagnetic storms were divided into three storm classes, and a superposed epoch analysis was performed. It was found that the number of SuperDARN echoes varied during the storm. For weak-moderate and strong storms, the number of echoes decreased during the main phase until well into the recovery phase. In contrast, the intense storms exhibited a marked increase in the number of echoes seen during the main phase in the 09-15 MLT sector and a reduction in the 21-03 MLT sector. SuperDARN recorded faster antisunward velocities in the 09-15 MLT sector before and during the main phase. A very good correlation between the minimum Sym-H at the end of the main phase and the minimum IMF B$_z$ that occurred roughly 1 hour earlier was found. This indicated a continuum of storm intensities that suggested the division of geomagnetic storms into classes based on intensity is arbitrary and unnecessary. The ring current decay time calculated using the high time resolution Sym-H index for the 136 geomagnetic storms was T=7.2 hours. This agreed with the \citet{burton1975} value of T=7.7 hours.

The second study focused on 205 SC events. The SC events were identified using ground-based magnetometer data from the years 2000 through 2007. Irrespective of whether or not the SC was followed by a geomagnetic storm, there was excellent correlation between the strength of SC events and the magnitude of the jump in the solar wind dynamic pressure. HF radar velocities and echo occurrence rates in the noon sector increased in response to the jump in solar wind dynamic pressure. In contrast, the number of SuperDARN echoes in the midnight sector decreased as the solar wind dynamic pressure increased, although the average drift speed in the midnight sector increased. The ionosphere and ring current evolved differently following the arrival of the solar wind pressure pulses. The Sym-H index, which represents changes in both the magnetopause and ring currents responded immediately and either rapidly returned to pre-SC values or progressed into the main phase of a geomagnetic storm. The SuperDARN velocities were affected for a much longer time period. This research revealed that the ring current reacts to a sudden compression of the magnetosphere on a time scale of 10 minutes, while the ionospheric convection velocities and echo occurrence was affected for as long as the increase in solar wind dynamic pressure was sustained, or until a geomagnetic storm was triggered.