1.4 Contributions

The major contributions of this dissertation are outlined below. The body of work described herein is primarily experimental, but Sections 2.4, 2.5, 3.2.1, 4.1.1, and parts of 5.1 also relate results from computer simulations and theoretical interpretations of observed phenomena.

1. A new technique for the remote sensing and identification of elves through optical and VLF signatures was developed. Motivated by the theoretical modeling of Inan et al. [1996c] and by a single observation of an elve from orbit [Boeck et al., 1992], a new high-speed photometric array was designed and built. This instrument, named the ``Fly's Eye,'' is described in Section 3.4 and records the output of 13 photometers with temporal resolution better than 30 $\mu$s, along with an image-intensified CCD video camera and a very low frequency (VLF) radio receiver. Deployed in the central United States during summer storm months each year from 1996 to 1999, the Fly's Eye was used to observe a predicted relationship between the VLF pulse radiated by lightning and the optical signatures of a briefly energized lower ionosphere. The discovery of this signature of elves, which also facilitates the location of the lower boundary of the optical flash, was reported by Inan et al. [1997] and is described in Section 4.1.2.

2. The empirical extent and prevalence of elves, and their relationship to causative lightning, was quantified for the first time. A number of theoretical studies have focused on horizontal intracloud currents [Rowland et al., 1996] or currents in sprites [Roussel-Dupre et al., 1998] as the cause for elves. In addition, until the execution of measurements reported in this dissertation, elves were reported to be associated primarily or only with positive cloud-to-ground lightning discharges. Chapter 4 reports experimental evidence, published in part by Barrington-Leigh and Inan [1999], showing that observed elves were caused only by the return strokes of cloud-to-ground lightning, and with equal effectiveness by positive and negative discharges. In addition, a study of the spatial extent and frequency of this form of heating of the nighttime ionosphere's $D$ region suggests the possibility for sustained or cumulative effects on the electron density at altitudes of $\sim$80 to 95 km. Theoretical modeling described in Section 2.5.1 quantifies this effect.

3. A distinction between two observed classes of lower ionospheric optical flashes is detailed in Section 5.1 and published by Barrington-Leigh et al. [2000]. Modeling and new high time resolution video are used to point out a common misidentification made by many workers over the last $\sim$5 years. Modeled emissions of the diffuse upper portion of sprites agree closely in form and timing with the diffuse flash seen frequently in intensified video and recently [Barrington-Leigh et al., 2000] in a high speed imager. These ``sprite halos,'' previously assumed to be signatures of elves, have implications for the frequency of sprite occurrence and may also be the cause of some ``early/fast'' VLF scattering events.

4. Using the Fly's Eye optical array to record the photometric signatures of sprites, a quantifiable feature of the optical relaxation of bright sprites is identified and related to the in situ electron density decay in Section 5.3. This result may form the basis for a possible new method of remote sensing mesospheric electron density changes and moreover may allow a determination of the in situ electric field.