A Statistical Study of High Amplitude Whistler-mode Waves Using 5 Years of Van Allen Probes Data and 4 Case Studies of Magnetotail Reconnection Events Using Cluster
2018-11
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A Statistical Study of High Amplitude Whistler-mode Waves Using 5 Years of Van Allen Probes Data and 4 Case Studies of Magnetotail Reconnection Events Using Cluster
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2018-11
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I present in this thesis the first statistical study of large-amplitude whistler-mode waves using 100% duty-cycle and non-averaged wave amplitudes, a new method for analyzing filter bank data from the Van Allen Probes, and a unique study of energy partitioning in the magnetotail using integrated fluxes and including oxygen dynamics. Although the presence of whistler-mode waves in the radiation belts has been known for decades, most spacecraft measurements of these waves has been limited to time-averaged amplitudes due to data telemetry limitations. Averaging wave amplitudes over time results in a averaging-out of short-lived but very large-amplitude waves. As such, the presence of large-amplitude whistler-mode waves within the magnetosphere is a relatively recent discovery. Since 2003, waves which are orders of magnitude larger than the average amplitude have been observed in short interval, high-cadence waveform data. While high-cadence waveform data is capable of studying individual events, it has a very low duty-cycle. The Electric Field and Waves (EFW) filterbank data set from the Van Allen Probes offers the first 100% coverage, opportunity to determine the prevalence, location, and distribution of these high-amplitude whistler-mode waves in the radiation belt region. Filterbank data records the maximum and average wave amplitude observed by the EFW instrument within a set of 13 or 7 frequency bins every 1/8 second. No time averaging is performed on the peak amplitudes, thus preserving the true maximum of the wave without the constraint of storing and telemetering high-cadence burst data. Because the frequency resolution of the data is very low, we present here an interpolation method by which we can improve the frequency resolution and amplitude accuracy of the filterbank data. This method is demonstrated to improve the sensitivity of the data such that we can determine whether or not observed high-amplitude peaks fall within the expected frequency range of magnetospheric chorus waves or other whistler-mode waves. Using this data set, we find that high amplitude whistler-mode waves with electric field peaks > 5 mV/m are observed primarily above L of 3.5 and MLT from 0 to 7 with occurrence rates of around 1-4%, while whistler-mode waves with magnetic field peaks > 50 pT are spread more widely in MLT sectors and at higher L (> 4.5) at rates of around 1-6%. Although most of the high-amplitude waves occur within 5 degrees of the magnetic equator, the day-side population of large magnetic field peaks are primarily seen at higher magnetic latitudes. The largest of the electric field peaks (> 50 mV/m) were seen almost exclusively at L less than 4 in the pre-dawn to dawn sector, while the largest magnetic field peaks (> 500 pT) were spread between midnight and MLT of 7, and at L greater than 4.5. This indicates that the largest whistler-mode waves observed at low L are primarily quasi-electrostatic and oblique. The location of these waves corresponds with an area of deep erosion of the plasmasphere during geomagnetically active conditions. I also present an analysis of 4 reconnection events in the magnetotail observed by Cluster, focusing on the energy partitioning between integrated Poynting flux, enthalpy flux, and kinetic energy flux for electrons, H+ ions, and O+ ions. We find that Poynting flux, electron enthalpy flux, H+ enthalpy flux, H+ kinetic energy flux, and O+ enthalpy flux can all be important or dominant portions of the energy outflow depending on distance from the current sheet, direction of flow, and local conditions. Our results indicate that oxygen energization may not be neglected when considering the energy partitioning from tail reconnection. We also examine the waves and structures which carry Poynting flux from the reconnection region and find that fluctuations with periods on the order of current sheet surface waves and periods on the order of current sheet flapping dominate the energy outflow.
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University of Minnesota Ph.D. dissertation. November 2018. Major: Astrophysics. Advisor: Cynthia Cattell. 1 computer file (PDF); xxxi, 185 pages.
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Tyler, Evan. (2018). A Statistical Study of High Amplitude Whistler-mode Waves Using 5 Years of Van Allen Probes Data and 4 Case Studies of Magnetotail Reconnection Events Using Cluster. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/201715.
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