Ultra-low frequency (ULF; 1 mHz - 1 Hz) waves are known to play an important role in the transfer of energy from the solar wind to Earth's magnetosphere and ionosphere. The Super Dual Auroral Radar Network (SuperDARN) is an international network consisting of 35 low-power high frequency (HF: 3-30 MHz) coherent scatter radars at middle to polar latitudes that look into Earth's upper atmosphere and ionosphere. In this study, we use Doppler velocity measurements obtained by the SuperDARN radars and coordinated spacecraft observations to investigate the occurrence statistics and driving mechanisms of ionospheric ULF waves. We begin in Chapter 2 with a case study of Pi2 pulsations which are short-duration (5-15 min) damped geomagnetic field oscillations with periods of 40-150 s. Simultaneous observations of Pi2 pulsations from THEMIS spacecraft, midlatitude SuperDARN radars, and ground magnetometers, together with analysis of their longitudinal polarization pattern and azimuthal phase propagation, confirmed that they are consistent with a plasmaspheric virtual resonance excited by a longitudinally localized source near midnight. In Chapter 3, to further investigate the overall occurrence of ionospheric ULF signatures, a comprehensive statistical study was conducted using an automated detection algorithm to identify ionospheric signatures of Pc3-4 and Pc5 waves over 7 years of high time resolution SuperDARN radar data. Specifically, we have investigated their spatial occurrence, frequency characteristics, seasonal factors, and dependence on solar wind and geomagnetic conditions. We note two particular findings: (i) an internal wave-particle interaction source is most likely responsible for Pc4 waves at high latitudes in the duskside ionosphere; and, (ii) a source associated with magnetotail dynamics during active geomagnetic times is suggested for Pc3-4/Pi2 waves at midlatitudes in the nightside ionosphere. These findings are further expanded in Chapter 4 which investigates the hypothesis that internal wave-particle interactions are an important source for generation of these waves. A case study of long-lasting poloidal waves was conducted using coordinated observations with the GOES and THEMIS satellites to examine the generation and propagation of waves observed in the dayside ionosphere by multiple SuperDARN radars. The source of wave excitation is suggested to be bump-on-tail ion distributions at 1-3 keV. Collectively, these research findings provide better constraints on where and when ionospheric ULF waves occur, their source mechanisms, and how they might affect magnetospheric and ionospheric dynamics. / Doctor of Philosophy / Earth’s magnetic field, approximates that of a bar magnet. It is an effective barrier to charged particles originating directly from the Sun and protects us against harmful space weather influences. The geomagnetic field lines can oscillate in ultra-low frequencies (ULF: 1 mHz - 1 Hz). These natural oscillations of closed magnetic field lines, analogous to vibrations on a stretched string, are also called geomagnetic pulsations or ULF waves. The interaction between matter and electromagnetic fields emitted from the Sun and the Earth’s outer atmosphere and magnetic field form a magnetic shield named the Earth’s magnetosphere. ULF waves play a key role in the transfer of energy from outside this shield to regions inside it, including Earth’s upper atmosphere and ionosphere (a region extending from about 60 km to 1000 km above the Earth’s surface). In this study, we use Doppler velocity measurements obtained by the Super Dual Auroral Radar Network (SuperDARN) radars and coordinated spacecraft observations to investigate the occurrence statistics and driving mechanisms of ionospheric ULF waves. We begin in Chapter 2 with an event study of a type of irregular pulsations (Pi2) which are short-duration (5-15 min) damped geomagnetic field oscillations with periods of 40-150 s. Simultaneous observations of Pi2 pulsations from NASA THEMIS spacecraft, midlatitude SuperDARN radars, and ground magnetometers, together with further analysis of wave spectra and propagation, confirmed their driving mechanism as a type of magnetic resonance, analogous to striking a bell. In Chapter 3, to further investigate the overall occurrence of ionospheric ULF signatures, a statistical study was conducted using an automated detection algorithm to identify ionospheric signatures of ULF waves over 7 years of high time resolution SuperDARN radar data. Specifically, we have investigated their spatial occurrence, frequency characteristics, seasonal factors, and dependence on solar and geomagnetic activity. We obtained findings regarding the different driving sources of waves observed in different regions. The findings are further expanded in Chapter 4 which investigates the generation of waves through energy exchange with charged particles. A case study of long-lasting (2-3 days) waves was conducted using coordinated observations with the GOES and THEMIS satellites to examine the generation and propagation of waves observed in the dayside ionosphere by multiple SuperDARN radars. The source of wave excitation is suggested to be unstable particle distributions in the magnetosphere. Collectively, these research findings provide better constraints on where and when ULF waves occur, their source mechanisms, and how they affect dynamics in the geospace environment.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/100904 |
| Date | 30 May 2019 |
| Creators | Shi, Xueling |
| Contributors | Electrical Engineering, Ruohoniemi, J. Michael, Baker, Joseph B. H., Srinivasan, Bhuvana, Buehrer, R. Michael, Scales, Wayne A., Clauer, C. Robert |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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