The dual jet aircraft Sprites94 campaign yielded the first color imagery and unambiguously triangulated physical dimensions and heights of upper atmospheric optical emissions associated with thunderstorm systems. Low light level television images, in both color and in black and white (B/W), obtained during the campaign show that there are at least two distinctively different types of optical emissions spanning part or all of the distance between the anvil tops and the ionosphere. The first of these emissions, dubbed “sprites” after their elusive nature, are luminous structures of brief (< 16 ms) duration with a red main body that typically spans the altitude range 50–90 km, and possessing lateral dimensions of 5–30 km. Faint bluish tendrils often extend downward from the main body of sprites, occasionally appearing to reach cloud tops near 20 km. In this paper the principal characteristics of red sprites as observed during the Sprites94 campaign are described. The second distinctive type of emissions, “blue jets,” are described in a companion paper [Wescott et al., this issue].

Initial observations of a newly documented type of optical emission above thunderstorms are reported. “Blue jets,” or narrowly collimated beams of blue light that appear to propagate upwards from the tops of thunderstorms, were recorded on B/W and color video cameras for the first time during the Sprites94 aircraft campaign, June‐July, 1994. The jets appear to propagate upward at speeds of about 100 km/s and reach terminal altitudes of 40–50 km. Fifty six examples were recorded during a 22 minute interval during a storm over Arkansas. We examine some possible mechanisms, but have no satisfactory theory of this phenomenon.

[1] The Los Alamos Sferic Array (LASA) recorded VLF/LF electric-field-change signals from over ten million lightning discharges during the period from 1998 to 2001. Using the differential-times-of-arrival of lightning sferics recorded by three or more stations, the latitudes and longitudes of the source discharges were determined. Under conditions of favorable geometry and ionospheric propagation, sensors obtained ionospherically reflected skywave signals from the lightning discharges in addition to the standard groundwave sferics. In approximately 1% of all waveforms, automated processing identified two 1-hop skywave reflection paths with delays indicative of an intracloud (height greater than 5 km) lightning source origin. For these events it was possible to determine both the height of the source above ground and the virtual reflection height of the ionosphere. Ionosphere heights agreed well with published values of 60 to 95 km with an expected diurnal variation. Source height determinations for 100,000+ intracloud lightning events ranged from 7 to 20 km AGL with negative-polarity events occurring above $15 km and positive-polarity events occurring below $15 km. The negativepolarity events are at a suprisingly high altitude and may be associated with discharges between the upper charge layer of a storm and a screening layer of charge above the storm. Approximately 100 of the intracloud events with LASA height determinations were also recorded by VHF receivers on the FORTE satellite. Independent FORTE source height estimates based on delays between direct and ground-reflected radio emissions showed excellent correlation with the VLF/LF estimates, but with a +1 km bias for the VLF/LF height determinations.

Citizen scientists, along with satellite and ground-based sensors, have revealed a new arc boundary at subauroral latitudes.