Date of Award

Fall 2021

Document Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Engineering Physics

Department

Physical Sciences

Committee Chair

Anatoly V. Streltsov

First Committee Member

Evgeny Mishin

Second Committee Member

Katariina Nykyri

Third Committee Member

Matthew Zettergren

Abstract

This doctoral dissertation presents the results of investigation of the Ultra-Low Frequency (ULF) waves at middle latitudes during substorms. The dissertation consists of two major parts, observations and simulations. The research in this dissertation proposes that the main role in the generation of ULF waves at middle latitudes during substorm belongs to the plasmapause.

The first part of the dissertation presents results of the data analysis of 84 intense substorm events as well as an overview of space observation programs such as CRRES, Van Allen Probes and DMSP. Data used in this study are from the ACE satellite taken measurements in the solar wind and ground magnetometers at high (L = 5.76), middle (L = 2.46), and low ( L = 1.87) latitudes. We estimated correlations between fluctuations of the magnetic field measured at all sources and correlations between fluctuations of the solar wind ion density and the magnetic field at high, middle, and low latitudes.

The main conclusion is that the dominant frequencies of oscillations of the magnetic field are in the range of 0.45-0.80 mHz across all the sources, and 0.45-0.55 mHz are the most common ones across all sources. Among 84 events, 33 events have a good match of dominant frequencies in power spectral density; 43 events have a cross-correlation of r > 0.2 of the detected waves across all sources; and 22 events feature both. Also, seven out of the nine GEM events have a strong correlation between the variation of the solar wind speed and the Dst index. Therefore, the results suggest that the variations of the magnetic field in the solar wind are one of the main drivers of the ULF magnetic field pulsations with frequencies less than 1 mHz detected in three different latitudes during substorms. The novelty of this research is that it is among the first studies that investigate the ULF waves in the solar wind and at high, middle and low latitudes on the ground.

The research also shows that in the amplitude of the ULF waves observed at low latitudes is higher compare with the amplitude of ULF waves detected at middle latitudes. This feature suggests that these waves are generated by the disturbances in the ring current, which are driven by the disturbances of the magnetic field in the solar wind. The coupling between the solar wind and the inner magnetosphere of the Earth can occur at the night side and on the day side of the Earth. The exact mechanism of coupling between the oscillations in the solar wind and the magnetic pulsations detected in these parts of the magnetosphere is discussed.

In the second part of the dissertation, we also provide the theoretical background of the Field-Line Resonance and Ionospheric Feedback Instability as well as the methodology for the 2D RMHD two-fluid simulations at mid latitudes. The results from the numerical study presented in the dissertation demonstrate that the plasmapause plays a very important role in the generation of ultra-low frequency waves detected with ground magnetometers at middle latitudes. The strong gradient in the plasma density associated with the plasmapause converts the electromagnetic energy of the fast magnetosonic waves propagating across the magnetic field to the energy of the field-aligned surface Alfvén waves.

The efficiency of this mode conversion depends on the parameters of the plasmapause. In particular, the time-dependent simulations of the two-fluid MHD model show that the amplitude of the surface waves is linearly proportional to the gradient of the plasma density and the magnitude of the variation of the density across the plasmapause.

The research uses the observations from the ground magnetometers at Palmer station in Antarctica to identify the frequency of the driver and frequencies of the waves generated by the ionospheric feedback instability. It also uses the observations of the plasma density conducted by the the NASA Van Allen Probes satellite in the equatorial magnetosphere on the same date. The satellite observations confirm that during substorms the plasmasphere can be strongly eroded away and the plasmapause moves to middle latitudes.

The model uses the observed frequency of 1.1 mHz to drive the surface waves on the plasmasphere. The simulations show that the large scale electric field modulated with this frequency drives the ionospheric feedback instability generating waves with frequencies < 25 mHz. The waves with the same frequencies are also observed in the measurements of the Palmer magnetometer. The numerical results and the ground observations also reveal waves with the frequencies in the range 0.5-1.0 Hz, which correspond to the frequencies of the ionospheric Alfvén resonator. This is a very expectable outcome because the main driver of the waves inside the IAR is the same ionospheric feedback instability as the one used in this study to drive the waves in the global magnetospheric resonator. The simulations also demonstrate that the instability develops quite efficiently from the small-scale ionospheric irregularities associated with the numerical noise in the simulations which can be considered as a proxy for the random thermal fluctuation of the plasma density in the real ionosphere.

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