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Transient luminous events (sprites, blue jets, elves) above large mesoscale convective systems (MCSs) over the U.S. High Plains have been routinely monitored from the Yucca Ridge Field Station near Fort Collins, Colorado using ground-based low-light video systems. We analyzed 36 sprites above the Nebraska MCS of August 6, 1994. The results lend further support to the hypothesis that sprites are almost uniquely associated with positive cloud-to-ground (+CG) lightning flashes. Sprite-associated +CGs also averaged substantially larger peak currents than the remaining +CG population (81 kA versus 30 kA in this storm system). There is some evidence that sprite-associated +CGs also have higher stroke multiplicity. This study yields no evidence of sprites associated with negative CG events. In the central United States an additional requirement appears to be that the parent MCS has a contiguous radar reflectivity area exceeding 20-25,000 km 2. The majority of the sprites occur above the large stratiform precipitation region and not the high-reflectivity convective core of the MCS. Triangulation of a limited number of paired images (from September 7, 1994) suggests that the sprite is generally centered within 50 km of the parent +CG. Assuming the +CG provides the range, single-image photogrammetric analyses provide estimates of the maximum vertical extent of the sprites. For this storm the sprite tops averaged 77 km with a maximum of 88 km. The bases averaged 50 km but with a few sprite tendrils extending as low as 31 km. Introduction While anecdotal reports of unusual forms of "lightning" discharging into the "stratosphere" have been reported world wide for over a century [Toynbee and Mackenzie, 1886; Lyons and Williams, 1993] and theoretically postulated [Wilson, 1956], it remained for a chance observation in 1989 from a low-light television (LLTV) camera to actually document that such events existed [Franz et al., 1990]. Subsequently, LLTVs on board the Space Shuttle [Vaughan et al., 1992; Boeck et al., 1995], on aircraft [Sentman and Wescott, 1993; Sentman et al., 1995; Wescott et al., 1995] and at fixed ground stations [Lyons, 1994 a,b; 1996] have revealed that stratospheric and mesospheric luminous events above mesoscale convective systems (MCSs) are rather common. In 1993, observations from the Yucca Ridge Field Station near Fort Collins, Colorado documented over 600 such transient luminous events, now called sprites, on 11 nights [Lyons, 1994 a]. On July 7, 1993, a particularly active storm system in Kansas and Nebraska produced over 25,000 cloud-to-ground (CG) discharges and more than 240 sprites. Yet ongoing LLTV monitoring of numerous MCSs has demonstrated that sprites do not occur for all storms exhibiting extremely high CG rates. Why do certain storms generate this phenomenon while others with apparently similar CG activity fail to manifest sprites? Are sprites related to convective storm type or specific CG characteristics, such as polarity or peak current? These and related issues were more completely addr...

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In two summertime mesoscale convective systems (MCSs), mesospheric optical sprite phenomena were often coincident with both large-amplitude positive cloud-to-ground lightning and transient Schumann resonance excitations of the entire Earth-ionosphere cavity. These observations, together with earlier studies of MCS electrification, suggest that sprites are triggered when the rapid removal of large quantities of positive charge from an areally extensive charge layer stresses the mesosphere to dielectric breakdown.

[1] Gravity waves in the mesopause region (80-105 km) may induce perturbations in OH Meinal Band emissions at $87 km. These perturbations can be observed by ground-based OH airglow imagers. In this paper, we present observations of concentric gravity waves (CGW) by the all-sky OH imager at Yucca Ridge Field Station (40.7°N, 104.9°W) near Fort Collins, Colorado. We find that expanding rings of concentric gravity waves were observed on 9 out of 723 clear nights from 2003 to 2008. In particular, on 11 May 2004, concentric rings were observed for $1.5 h, with nearly perfect circular rings entirely in the field of view during the first 30 min. The centers of the concentric rings occurred at the geographic locations of two strong convective plumes which were active in the troposphere $1 h earlier. We measured the horizontal wavelengths and periods of these gravity waves as functions of both radius and time. These results agreed reasonably well with the internal Boussinesq gravity wave dispersion relation with an assumed zero background wind. Similarly, for the other 8 cases, strong convective plumes occurred prior to the CGW observations near the apparent center of each of the arcs or rings. For the 7 out of the 9 cases, radiosonde data were available up to z = 30-35 km. These data showed that the wind speeds from the tropopause to $30-35 km were smaller than $20-30 m/s. Because 8 of the 9 cases occurred when the total horizontal mean winds were weak and because the horizontal winds below $87 km were less than $20 m/s on 11 May 2004 (according to radiosonde and TIME-GCM model data), we postulate that weak background horizontal winds are likely a necessary condition for gravity waves excited from convective overshooting to be observed as concentric arcs or rings in the OH layer.