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Oral Defense Abstracts

Date: Friday, July 21, 2000
Time: 2:00 pm
Location: Packard #101

Special University Ph.D. Oral Examination
Fast Photometric Imaging of High Altitude Optical Flashes Above Thunderstorms
Christopher Barrington-Leigh
Department of Applied Physics, Stanford University

Lightning in the Earth's troposphere is among the largest impulsive energy sources within the bounds of the magnetosphere, and with 50 to 100 cloud-to-ground discharges per second globally, provides a steady source of excitation. Lightning effects on the magnetosphere in the form of whistler-mode waves have been recognized for decades, and they are known also to cause lightning electron precipitation in the ionosphere. Recently, however, a range of spectacular and more immediate lightning effects on the lower ionosphere and the mesosphere have been discovered. These were first detected by very low frequency (VLF) radio remote sensing, which inspired studies of possible optical effects at about the same time as two fortuitous discoveries in 1989 and 1992 revealed remarkable visual evidence of direct electrodynamic coupling between lightning and the upper atmosphere [1, 2]. These new phenomena were soon to be called "elves" and "sprites".

A novel photometric array with a high-speed triggered data acquisition system, bore-sighted image-intensified CCD video camera, and VLF radio receiver was built to detect a predicted signature of elves, the lower ionosphere (80 to 95 km altitude) flash due to heating by an impinging electromagnetic pulse launched by intense lightning currents. The narrow individual photometer fields-of-view of (2.2 degree by 1.1 degree) provide a spatial resolution of ~20 km at a range of 500 km, enabling the documentation of expansion occurring over a horizontal range of 200 km with a time resolution of 15 microseconds [3].

In 1997 data acquired by the array (named the "Fly's Eye") settled several questions regarding the relationship between elves and lightning and, by measuring the spatial extent of ionospheric heating and the frequency of occurrence of elves, demonstrated their significance in causing sustained and cumulative modification of the nighttime lower ionosphere electron density profile over large thunderstorm systems [4].

The Fly's Eye, along with a telescopic imaging system developed in 1998, was also used to investigate sprites. Sprites are highly structured discharges lasting 5 to 100 ms and extending from 40 to 85 km altitude which result from intense electric fields following a major redistribution of electric charge in the troposphere -- usually a positive cloud-to-ground return stroke. Photometric, video, and radio (30 Hz to 20 kHz) measurements were used to detect the first sprites directly associated with negative cloud-to-ground lightning, implying a breakdown process that can propagate in upward and downward electric fields; this is consistent with only a subset of the theoretical descriptions for sprites [5]. In addition, telescopic imagery shows clear evidence of both positive and negative corona streamer propagation in a sprite.

Detailed electromagnetic (finite difference time domain) modeling of both elves and sprites is used to interpret observations. Three events recorded by a high-speed (3000 frames per second) imaging system in 1997, combined with modeling results, led to the recognition of a widespread confusion in interpreting video signatures of elves and sprites and identified for the first time the diffuse upper portion of sprites, a hard-to-measure but likely ubiquitous form of heating and ionization in the upper mesosphere. [6]

[1] Boeck, W.L., O.H. Vaughan, Jr., R. Blaskeslee, B. Vonnegut and M. Brook, 1992: Lightning induced brightening in the airglow layer. Geophys. Res. Letters, 19, 99-102.

[2] Franz, R.C., R.J. Nemzek and J.R. Winckler, 1990: Television image of a large upward electrical discharge above a thunderstorm system. Science, 249, 48-51.

[3] Inan, U.S., C. P. Barrington-Leigh, S. Hansen, V.S. Glukhov, T.F. Bell, and R. Rairden, ``Rapid Lateral Expansion of Optical Luminosity in Lightning-Induced Ionospheric Flashes Referred to as `Elves,' '', Geophys. Res. Lett., 24, 5, 1997.

[4] Barrington-Leigh, C. P., and U. S. Inan, ``Elves triggered by positive and negative lightning discharges'', Geophys. Res. Lett. 26, No. 6, March 1999.

[5] Barrington-Leigh, C. P., U. S. Inan, M. Stanley, and S. A. Cummer, ``Sprites triggered by negative lightning discharges'', Geophys. Res. Lett. 26, No. 24, December 1999.

[6] Barrington-Leigh, C., U. Inan, and M. Stanley, ``Identification of Sprites and Elves with Intensified Video and Broadband Array Photometry,'' in press, Journal of Geophysical Research.

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