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Fast Photometric Imaging of High Altitude Optical Flashes Above Thunderstorms
Christopher Barrington-Leigh
Department
of Applied Physics, Stanford University
Abstract
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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