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
Abstract
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.
|
< back to top