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Oral Defense Abstracts

Date: Thursday, October 5, 2006
Time: 3:00 pm
Location: Packard #101

Special University Ph.D. Oral Examination
Ground-Based Measurements of Lightning-Induced Electron Precipitation
William Peter
Department of Electrical Engineering, Stanford University

The radiation belts are trapping regions of high-energy charged particles surrounding the Earth. The precipitation of radiation belt particles is detectable via the use of ground-based VLF receivers. We develop a framework to quantify VLF signatures of lightning-induced electron precipitation (LEP) events in terms of the spatial and temporal characteristics of the precipitation and ionospheric disturbance. Comparison of VLF experimental observations of LEP events with a comprehensive model of lightning-induced electron precipitation allows the measurement of such events with unprecedented quantitative detail. The model consists of three major components: a test-particle model of the gyroresonant whistler-induced electron precipitation [Borntik et al., 2006]; a Monte Carlo simulation of the energy deposition into the ionosphere resulting from the calculated precipitation flux [Lehtinen, 2001]; and a model of VLF subionospheric signal propagation that takes into account the disturbed ionospheric density profiles [Chevalier and Inan, 2006]. Observations of VLF signal perturbations associated with two representative LEP events and recorded on the Holographic Array for Ionospheric/Lightning Research (HAIL) are interpreted in terms of precipitating flux and ionospheric density enhancement. For both cases, the model predicts VLF signal amplitude and phase perturbations within a factor of three of those observed, within the expected variability in trapped energetic flux levels. The modeled, precipitated energy flux (E > 45 keV) peaks at ~1 x 10-2 [ergs s-1 cm-2], resulting in a peak loss of ~0.001% from a single flux tube at L ~ 2.2, consistent with previous satellite measurements of LEP events [Voss et al., 1998]. The use of ground-based VLF receivers to quantitatively measure precipitation events is a critical step in quantifying the role of LEP in radiation belt loss.

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