Condign volumes 1 to 3

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# UNCLASSIFIED

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SPRITES, ELVES AND BLUE JETS

INTRODUCTION

1. Three types of transient optical luminous vertically-orientated phenomena have been observed at high altitudes above thunderstorms: 'Elves' at roughly 90km which is an enhanced airglow, 'Sprites', a reddish glow from 50-90km and an upward-moving bluish beam emission from cloud tops, known as 'Jets', below 40km. Large area multi-cell thunderstorms lead to the formation of vertically-orientated cylindrical structures of gravity waves closely resembling those observed in optical emissions associated with transient luminous glows. These are believed to occur as a result of transfer of large amounts of charge (from clouds at altitudes of 5-10km) to the ground. Apart from the visual effects, these charge transfers are known to generate upper ELF Band signals (>300kHz). The Sprite path can also be located at high altitudes between the ionosphere and cloud tops. The luminous emission at low altitude is red. The likely explanation is gas breakdown by the additional electromagnetic fields of lighting discharges. Some Sprite 'bluish tendrils' were observed to extend down to 20-30km altitude. Horizontal magnetic field variations have also been observed in the frequency range of the earth-ionosphere cavity resonance. These phenomena have been studied recently in the USA, Germany and in the Antarctic.

## SPRITES

2. Sprites are the older known phenomenon, first being reported 100 years ago. They last tens of milliseconds and can thus be sensed by the human eye. They are sometimes described as 'fan shaped' or

'carrot-like' in shape. They can extend laterally for several kilometres. Other observers have reported 'upward forks', due to divergence of the striations.

3. Electric field discontinuities can occur over stratiform clouds at 10-16km altitude and this is believed to be due to the positive cloud-to-ground discharges. Satellite-based instruments have also detected gamma-ray flashes (see para. 7 below). Current theories suggest that Sprites, Gamma-ray Flashes and Blue Jets are all types of discharge produced by a run-away air breakdown.

4. Measurements A TV slit spectrograph recently recorded 25 events and measured optical wavelengths in a band from 540 to 840 $ \mu m $ , although other measures have spanned from 450-800 nanometer. The fleeting columns of luminosity have also been imaged by CCD cameras. Sprites are found to have a bright core less than 2km in horizontal dimension, they followed the associated lighting (below the (cumulonimbus) cloud) within a few milliseconds, and rose to peak intensity in 0 to 3 ms, fading a few tens of milliseconds later. Sprites can easily be observed from aircraft and from the ground, and have been seen from spacecraft.

5. More than 150 discharges measured (in the USA) in August 1993 extended exceptionally, down to cloud tops below 20km altitude. In 1994, 56 Blue Jets were recorded at one location in just 22 minutes. They can extend from anvil cloud tops either partially or fully up to the ionosphere.