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 electrodynamic excitation. Lightning effects on the magnetosphere in the form of whistler-mode waves have been recognized for decades, and whistlers 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 1990 revealed remarkable visual evidence of direct electrodynamic coupling between lightning and the upper atmosphere [Boeck et al., 1992; Franz et al., 1990]. These new phenomena were soon to be called ``sprites'' and ``elves.''

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 ionospheric (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 $^\circ\times$1.1$^\circ$) provide a spatial resolution of $\sim$20 km at a range of 500 km, enabling the documentation of rapid expansion occurring over a horizontal range of 200 km with a time resolution of $\sim$15 $\mu$s [Inan et al., 1997].

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 ionospheric electron density profile over large thunderstorm systems [Barrington-Leigh and Inan, 1999].

The Fly's Eye, along with a telescopic imaging system developed in 1998 [Gerken et al., 2000], 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 [Barrington-Leigh et al., 1999a]. 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 which is now called the sprite halo [Barrington-Leigh et al., 2000].

a new array of horizontally spaced photometers boresighted with a low-light-level camera provide the first measurement of the rapid lateral expansion of optical luminosity in lightning-induced ionospheric flashes referred to as `elves', occurring over time scales substantially less than 1 ms. The narrow individual fields-of-view of (2.2 $^\circ\times$1.1$^\circ$) provide a spatial resolution of $\sim$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 $\sim$30$\mu$s. The observed dynamic features of elves are consistent with a model in which the optical output is produced as a result of heating by the electromagnetic pulse (EMP) from a lightning discharge.

Large electrical currents flowing between thunderclouds and the ground have recently been found to have several immediate effects on the overlying atmosphere and lower ionosphere. These include "elves," the  1 ms heating of the lower ionosphere (75 to 95 km) by the electromagnetic pulse launched by the lightning current, and "red sprites," complex discharges lasting 5 ms to 100 ms at altitudes of 40 to 85 km.

Both of these effects can cause changes in ionization levels on time scales of about one minute, which are of interest in particular to those studying and using sub-LF (below 30 kHz) radio propagation over large distances around the globe.

In addition, both sprites and elves cause optical emissions. By designing an array of high speed (15 microsecond resolution) photometers that are recorded along with a VLF radio receiver, we have identified a diagnostic signature of elves, distinguishing the flash from that of normal lightning and from that of sprites. This photometric array, named the "Fly's Eye," has been deployed in Colorado and New Mexico during the peak summer storm months since 1996. Observations have revealed a ubiquity and spatial extent of elves that were not apparent in other observations. When combined with results from numerical modeling of the electromagnetic heating of the ionosphere, our studies show the likelihood of sustained and cumulative modification of the electron density profile of the lower ionosphere over large thunderstorm sytems.

In 1998 the Fly's Eye was deployed along with a telescopic imaging system for observing the optical structure in sprites. Results from these two instruments show that the plasma streamers constituting sprites can propagate both upwards and downwards and in both upward and downward pointing electric fields. This is consistent with only a subset of theories describing the breakdown in sprites, and also constrasts with the observational fact that sprites are almost exclusively seen above positive cloud to ground lightning.

Confusion in the interpretation of standard-speed video observations of optical flashes above strong cloud-to-ground lightning discharges has persisted for a number of years. New high speed (3000 frames per second) image-intensified video recordings are used along with theoretical modeling to elucidate the optical signatures of elves and sprites. In particular, a brief diffuse flash sometimes observed to accompany or precede more structured sprites in standard-rate video is shown to be a normal component of sprite electrical breakdown and to be due entirely to the quasi-electrostatic thundercloud field (sprites), rather than the lightning electromagnetic pulse (elves). These ``sprite haloes'' are expected to be produced by large charge moment changes occurring over relatively short time scales ($\sim$1 ms), in accordance with their altitude extent of $\sim$70 to 85 km. The relatively short duration of this upper, diffuse component of sprites makes it difficult to detect and to discriminate from elves and Rayleigh-scattered light using normal-speed video systems. Modeled photometric array signatures of elves and sprites are contrasted and shown to be consistent with observations. The diffuse portion of sprites may be a cause VLF scattering phenomena known as early/fast VLF events.