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Blue Jets, Red Sprites and Elves - Page 2

© Keith C. Heidorn

branching carrots, or tentacled octopi. The luminous body of the sprite can extend as high as 95 km (60 miles) with peak brightness between 50 and 75 km (30 and 47 miles). Downward draping tendrils often drop below 30 km (19 miles) altitude but do not reach the thundercloud tops. Rather than forming a narrow channel like the cloud-to-ground lightning with which they are associated, sprites are estimated to be around 10 metres (30 ft) across and often appear as clusters that illuminate a large volume, perhaps thousands of cubic kilometres spreading out over 150 km (93 miles) from their origin.

Sprites emerge high above very large thunderstorm systems, appearing at intervals of up to several minutes and lasting several milliseconds. They seem to associate with cloud-to-ground lightning flashes of large positive polarity (most, but not all, lighting strokes are of negative polarity). A diffuse disk-shaped glow lasting about a millisecond precedes some sprites. These sprite halos are less than 100 km (62 miles) wide, and propagate downward in altitude from about 85 to 70 km (53 to 44 miles). Columnar sprites sometimes emerge from the lower portion of the sprite halo's concave disk.

Researchers have discovered unique radio signals are emitted by the lightning stroke producing each sprite event. But using this property for detection rather than visual observations, researchers now believe sprites, once considered rare, appear to form during roughly one in every two hundred lightning strikes.

Blue Jets

In 1993, while flying above severe thunderstorms in Arkansas, Davis Sentman and Eugene Wescott of the University of Alaska's Geophysical Institute noted a new TLE. They were surprised to see blue light beams shooting upward directly out of cloud tops. Rising at speeds of over 100 km/sec (60 miles/sec), the beams reached heights of 40 to 50 km (25 to 30 miles) - two or three times the cloud heights - before fading away. Wescott and Sentman named these flashes blue jets.

Blue jets propagate from the cloud tops toward the ionosphere 20 to 50 km (12 to 30 miles) high and last from tenths to a full second. They are always blue and funnel-shaped: 1.6 to 3.2 km (1 to 2 miles) at their base and 8 to 10 km (5 or 6 miles) at the top. Simultaneous blue jets propagate slowly upward from the cloud tops, but extinguish simultaneously. The blue starter, a related phenomenon, may actually be a blue jet that fails to completely form. Blue jets appear to be very rare, but that may arise from the fact their faint blue light is quickly scattering by the

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