Sprites and their
Parent Discharges






by
Mark Alan Stanley






Submitted in partial fulfillment
of the requirements for the degree of


Doctor of Philosophy in Physics






Department of Physics
New Mexico Institute of Mining and Technology
Socorro, NM

May 5, 2000

Abstract:

A detailed analysis of sprites and their parent discharges is presented in this study. Observations at VHF of the 3-dimensional development of discharges in a mesoscale convective system (MCS) over central Florida showed that sprite-producing discharges usually began as ordinary bilevel intraclouds (ICs) within convective cells. The negative leaders in the upper portion of each IC dropped in altitude from $\sim$10-12 km above mean sea level (MSL) to 7-8 km MSL as they propagated out of the convective region into the stratiform region. These altitudes coincide with the upper positive charge layer in MCSs (MacGorman and Rust, 1998, pg. 267). The average velocity of negative leader propagation was $\sim1\,$-$\,2\times10^5$ m/s in the stratiform region, consistent with previous measurements of negative leader velocities in cloud-to-grounds (CGs) (Uman, 1987, pg. 83) and ICs (Shao and Krehbiel, 1996). The negative leaders propagated a distance of about 40-60 km and filled a portion of the $>$20 dBZ reflectivity region at 7-8 km altitude prior to the occurrence of a +CG.

The sprite-producing +CGs removed positive charge from the 7-8 km MSL altitude of the upper positive charge layer in the stratiform region, consistent with previous multi-station electric field measurements of +CGs in a Florida MCS (Krehbiel, 1981). The sprite-producing $+$CGs had average (downward) currents of $\simeq$28-37 kA within the initial 1 ms post-return stroke interval and $\simeq$13-16 kA within the initial 4 ms. The range of charge transfers to ground for these time intervals was $\simeq$28-37 C and $\simeq$52-64 C, respectively. A comparison of high-speed video of sprite development with ELF-based sferic measurements revealed that the charge moment required for sprite initiation was $\simeq$300 C$\cdot$km for delays of up to a few milliseconds after the +CG stroke, consistent with previous estimates of Cummer and Inan (1997) and Huang et al. (1999) as well as the charge moments based on static electric field changes presented in this work.

The sprite plan locations were only associated with the most recent ($\sim$200-300 ms) portion of the discharges, suggesting that older channel segments had become resistive. The plan locations of most sprites were correlated with the apparent periphery of the parent discharges. Assuming that sprite initiation is correlated with local maxima in the electric field at high altitude, it is speculated that the +CG removes charge primarily from the periphery of the discharge in association with the advancement of negative leaders which supply the continuing current.

High-speed video revealed that the initiation altitude of nighttime sprites was 76$\pm$6 km MSL with a maximum density at $\simeq$76-79 km MSL. The initiation altitude is consistent with conventional breakdown predictions (Pasko et al., 1997b; Fernsler and Rowland, 1996; Wilson, 1925), though the measured charge moments were about 1.4-2.3 times less than required to initiate conventional breakdown in a homogeneously stratified atmosphere. Sprites developed bidirectionally upwards and downwards from the point of initiation, consistent with experimental and theoretical observations of positive and negative streamers which propagated in opposite directions away from an electron avalanche region in long air gaps (Dhali and Williams, 1985; Loeb and Meek, 1940). The downward developing positive streamers in sprites typically attained peak velocities of $\sim\,1\times10^7$ m/s, in agreement with the predictions of Raizer et al. (1998) for the observed charge moment changes in excess of 300 C$\cdot$km.

Parent discharges with unusually large and rapid charge moment changes produced sprite halos consistent with the quasi-electrostatic heating of electrons (Pasko et al., 1997b). Angel (jellyfish) sprites initiated from enhanced luminous regions at the base of the sprite halos and the positive streamers propagated downwards at velocities which could exceed $3\times10^7$ m/s (10% of light).

On some sprites, inferred negative streamers were spawned from regions through which the positive streamers propagated previously. The negative streamers propagated and branched upwards and outwards, transforming the sprite from a columniform shape (with tendrils) to an upward-V, or ``carrot'', shape.

The measurements of sprites and their parent discharges were extended to the daytime via a unique sprite ELF signature which was used to detect the presence of daytime sprites. The charge moment threshold for daytime sprite initation was much higher than at night, consistent with the lower altitude of the base of the ionosphere and conventional breakdown theory (Fernsler and Rowland, 1996; Wilson, 1925). The 6100 C$\cdot$km charge moment change of the first daytime sprite-producing parent discharge, prior to the start of the sprite ELF signature, may have been sufficient for conventional breakdown below the base of the ionosphere at an altitude of $\simeq$54 km, assuming an experimentally measured ion conductivity profile of Holzworth et al. (1985). The daytime sprites themselves contained unusually large charge moment changes of $\simeq$2800 C$\cdot$km, $\simeq$1200 C$\cdot$km, and $\simeq$910 C$\cdot$km. These charge moments are larger than the largest nighttime sprite charge moment change published to date of $\sim$840 C$\cdot$km (Cummer and Stanley, 1999).