Observations of Plasma Waves at the Earth's Dayside Magnetopause

Abstract

Plasma waves near the magnetopause are of considerable interest due to the possible role which wave-particle interactions may play in the diffusion and transport of plasma across the magnetopause and the possible effects of plasma turbulence on energy dissipation and magnetic reconnection. Large amplitude plasma waves in a variety of frequency bands are often observed during crossings of the magnetopause current sheet when diagnostics indicate that reconnection is occurring. The studies herein were performed using plasma wave electric and magnetic fields and particle data primarily from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite to investigate the possible generation mechanisms of different wave modes and the roles that the wave modes play in the process of magnetic reconnection and magnetopause boundary layer formation. The main advantages of the THEMIS data set are the long intervals of high time resolution three-dimensional electric and magnetic field burst waveforms. The work began with observations of large amplitude waves in a well-defined electron diffusion region at the subsolar magnetopause. These waves identified as whistler-mode waves, electrostatic solitary waves, lower-hybrid waves and electrostatic electron cyclotron waves, are observed in the same 12 s waveform capture and in association with signatures of active magnetic reconnection. The large amplitude waves in the electron diffusion region are coincident with abrupt increases in electron parallel temperature suggesting strong wave heating. The whistler-mode waves are at the electron scale and enable us to probe electron dynamics in the diffusion region. The energetic electrons ($\sim$30 keV) within the electron diffusion region have anisotropic distributions with $T_{e\perp}/T_{e\parallel}>1$ that may provide the free energy for the whistler-mode waves. The energetic anisotropic electrons may be produced during the reconnection process. The whistler-mode waves propagate away from the center of the ``X-line'' along magnetic field lines, suggesting that the electron diffusion region is a possible source region of the whistler-mode waves. Another study was the identification of large amplitude electrostatic ion cyclotron waves near the Earth's dayside magnetopause at MLT of $\sim$ 14. The electrostatic ion cyclotron waves were identified in a boundary layer in the magnetosphere adjacent to the magnetopause where reconnection was occurring. The electrostatic ion cyclotron wave power was primarily at 2$f_{cH}$ (where $f_{cH}$ is the hydrogen cyclotron frequency) and simultaneously observed with perpendicular ion heating. The electrostatic ion cyclotron waves had electric field amplitudes as large as 30 mV/m peak-to-peak with significant power both perpendicular and parallel to the magnetic field. These amplitudes were greater than those of previously observed ion cyclotron harmonics at the nightside magnetopause. The electrostatic ion cyclotron waves occurred during an interval of enhancements in the quasi-static electric field and fluctuations in the background magnetic field, plasma density and temperatures. The observations indicate that a plasma density gradient is a possible source of free energy for the electrostatic ion cyclotron waves. The observed flow shears are not large enough to drive the waves. Whistler-mode waves were identified near the electrostatic ion cyclotron wave region but closer to the magnetopause in a region with slightly higher ion and electron temperatures. Further investigation was on simultaneous observations of these waves at the low-latitude boundary layer of the Earth's magnetopause. The waves were identified through auditory analysis in the high resolution (16384 samples/s) electric field burst data and occurred at the same time as large fluctuations of plasma density and temperature (at time scales of $\sim$3 to 4 minutes) at a location of 9.3 Re, 14.4 magnetic local time, and 5.8 degrees magnetic latitude. Large fluctuations in the interplanetary magnetic field and solar wind flow speed were observed associated with this wave event and could be responsible for the variations seen in the low-latitude boundary layer. The particle distribution functions show that lower-energy ions ($<$1.3 keV) are anisotropic with $T_{i\perp} > T_{i\parallel}$ while lower-energy ($<$300 eV) electrons are anisotropic with $T_{e\perp} < T_{e\parallel}$. In addition, electrons show a double-peaked distribution, i.e., bi-streaming beams. These distributions are consistent with instability mechanisms proposed for the observed waves. The results provide insights into wave coupling near the magnetopause and suggest that coupling processes may be more important than usually thought. The work presented in this thesis has helped increase understanding of the microphysics of reconnection and boundary layer formation through investigation of the role of waves.