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Oral Defense Abstracts

Date: Tuesday, July 29, 1997
Time: 1:00 pm (Refreshments at 12:45 pm)
Location: Durand 450

Special University Ph.D. Oral Examination
Lightning and Ionospheric Remote Sensing Using VLF/ELF Radio Atmospherics
Steven A. Cummer
Department of Electrical Engineering

Lightning discharges radiate the bulk of their electromagnetic energy in the Very Low Frequency (VLF, 3-30 kHz) and Extremely Low Frequency (ELF, 3-3000 Hz) bands. This energy, contained in impulse-like signals called radio atmospherics or sferics, is guided for long distances by multiple reflections from the ground and lower ionosphere. These two facts suggest that observed sferic waveforms radiated from lightning and received at long distances (>1000 km) from the source stroke contain a great deal of information about both the state of the ionosphere along the propagation path and the dynamics of the current in the lightning return stroke.

To measure the characteristics of the ionosphere along the sferic propagation paths, broadband VLF (3-22 kHz) magnetic field observations are compared to results from a general ELF/VLF propagation model. I will demonstrate that this technique can provide accurate, path-averaged measurements of the nighttime D region of the ionosphere (altitudes of 60-90 km), a region which is extremely difficult to probe by other means.

To measure the source current waveforms of individual lightning discharges, measurements of broadband ELF (10-1500 Hz) radio atmospherics are interpreted with the same propagation model. Of particular interest are those lightning discharges which cause sprites, the recently discovered transient flashes which occur at altitudes from approximately 40-80 km above thunderstorm regions. It has been hypothesized that an unusually large cloud-to-ground transfer of charge in the lightning discharge is responsible for the creation of sprites, but the results of this work show that the actual magnitude of this charge movement to the ground is significantly smaller than predicted by existing theories.

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