No abstract

[1] We report intensified high speed video observations of two mesospheric transient luminous events acquired at 5000 and 7200 frames per second. Downward streamers appear to initiate either spontaneously or from brightening inhomogeneities at the bottom of a halo, and branch as they propagate downward. Simultaneously, a brighter column expands upward and downward from the initiation point. This expansion is usually followed by the development of bright upward propagating streamers that originate from the bottom of the expanding bright column and that terminate in diffuse emissions. The lower portions of these upward streamers are typically brighter and more persistent and form the bright core of the sprite. A new phenomenon is observed in which the tips of downwardmoving sprite streamers are attracted to and, in some cases, collide with adjacent streamer channels. The points of streamer collision appear to become long-persisting sprite beads, which have been suggested previously to affect mesospheric chemistry. Other persistent beads appear to form spontaneously on the downward streamer channels near the lower edge of the bright upper portion of the sprite.

[1] The transient ELF ($50 -5000 Hz) magnetic field radiated by lightning discharges across North America was continuously measured at Duke University during the summer of 2000. In total, 881 sprite-associated lightning discharges over 17 days were analyzed. We report in detail on 76 sprites for which we could reliably determine the lightning charge moment change from the ELF data at the time of sprite onset. The charge moment change for the initiation of a sprite is found to be as low as 120 C km. By folding together the charge moment distributions of spriteproducing lightning and all positive lightning, we find that the probability of sprite generation for lightning with >1000 C km charge moment change in <6 ms is >90%, while the sprite probability for lightning with <600 C km charge moment change in <6 ms is <10%.

[1] Simultaneous measurements of high-altitude optical emissions and magnetic fields produced by sprite-associated lightning discharges enable a close examination of the link between low-altitude lightning processes and high-altitude sprite processes. We report results of the coordinated analysis of high-speed sprite video and wideband magnetic field measurements recorded simultaneously at Yucca Ridge Field Station and Duke University. From June to August 2005, sprites were detected following 67 lightning strokes, all of which had positive polarity. Our data showed that 46% of the 83 discrete sprite events in these sequences initiated more than 10 ms after the lightning return stroke, and we focus on these delayed sprites in this work. All delayed sprites were preceded by continuing current moments that averaged at least 11 kA km between the return stroke and sprites. The total lightning charge moment change at sprite initiation varied from 600 to 18,600 C km, and the minimum value to initiate long-delayed sprites ranged from 600 for 15 ms delay to 2000 C km for more than 120 ms delay. We numerically simulated electric fields at altitudes above these lightning discharges and found that the maximum normalized electric fields are essentially the same as fields that produce short-delayed sprites. Both estimated and simulation-predicted sprite initiation altitudes indicate that long-delayed sprites generally initiate around 5 km lower than short-delayed sprites. The simulation results also reveal that slow (5-20 ms) intensifications in continuing current can play a major role in initiating delayed sprites.

During the summer of 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) program deployed a three-dimensional Lightning Mapping Array (LMA) near Goodland, Kansas. Video confirmation of sprites triggered by lightning within storms traversing the LMA domain were coordinated with extremely low frequency (ELF) transient measurements in Rhode Island and North Carolina. Two techniques of estimating changes in vertical charge moment (M q) yielded averages of ϳ800 and ϳ950 C km for 13 sprite-parent positive polarity cloud-to-ground strokes (ϩCGs). Analyses of the LMA's very high frequency (VHF) lightning emissions within the two mesoscale convective systems (MCSs) show that ϩCGs did not produce sprites until the centroid of the maximum density of lightning radiation emissions dropped from the upper part of the storm (7-11.5 km AGL) to much lower altitudes (2-5 km AGL). The average height of charge removal (Z q) from 15 sprite-parent ϩCGs during the late mature phase of one MCS was 4.1 km AGL. Thus, the total charges lowered by spriteparent ϩCGs were on the order of 200 C. The regional 0ЊC isotherm was located at about 4.0 km AGL. This suggests a possible linkage between sprite-parent CGs and melting-layer/brightband charge production mechanisms in MCS stratiform precipitation regions. These cases are supportive of the conceptual MCS spriteproduction models previously proposed by two of the authors (Lyons and Williams).