Phenomenology

Since initial documentation in 1989, three types of phenomena occurring above thunderstorms have been documented: sprites, blue jets, and elves, as illustrated in Figure 1.

Figure 1. The three types of transient optical events above thunderstorms recorded in the past decade are summarized. Sprites are brief emissions occurring between approximately 40-95 km, associated with large positive cloud-to-ground lightning discharges. Blue jets are cones of light which propagate upward from the top of the electrically active convection core of thunderstorms at an average velocity of 112 km/s to a terminal altitude of ~37 km. Elves occur over both positive and negative CG's at the base of the ionosphere and expand radially to horizontal diameters as large as ~500 km and vertical widths of several km.

Also shown in this figure are typical vertical temperature and electron density profiles which provide context for the three types of phenomena. Two characteristics separating the phenomena into these three groups are duration and altitude. Sprites span the altitude between ~40-95 km and last ~>10-100 ms. Elves, while not currently triangulated for accurate altitude determination, are estimated to occur between 75-95 km altitudes, lasting less than 1 ms. Blue jets appear from cloud tops (~20 km) and propagate upwards in the shape of an expanding cone to altitudes of ~40 km, over a period of ~200-300 ms. Blue starters appear to propagate upward from storm tops, and are similar to jets. However, at altitudes of ~25 km or less, the starters extinguish rather than propagating to 40 km as with a blue jet.

Lightning discharges are classified into four types, based on direction of propagation (cloud-to-ground or ground-to-cloud) and the net charge removed (positive or negative) (Berger(1978)). Negative cloud-to-ground discharges remove negative charge from the thunderstorm to ground and constitute approximately 90% of lightning activity. Positive cloud-to-ground discharges constitute almost all the remainder of lightning activity (positive and negative ground-to-cloud discharges are rare, occurring from tall human structures or mountains). Positive cloud-to-ground discharges often occur away from the most electrically active convective cores of thunderstorms, and generally occur during the later stages of large thunderstorms. Sprites generally occur in association with large positive cloud-to-ground (CG) lightning rather than over the more electrically active convective core (Boccippio et al.(1995)).

Great progress has been made in the theoretical understanding of sprites, elves, blue jets and blue starters since the first video recordings. These phenomena can be classified by the source of the electric fields that generate them. Experimentally sprites and elves are clearly associated with lightning discharges, which can be detected by instruments on the ground (e.g., the National Lightning Detection Network - NLDN). Jets and starters are the least understood of the phenomena, don't appear to be associated with detectable lightning, and may be associated directly with breakdown near charge centers in the storm (similar to normal lightning). In a thunderstorm, electrical charges are accumulated in a number of charge centers by a slow process, typically minutes or 10's of minutes (MacGorman and Rust(1998)). A lightning discharge is a rapid rearrangement of these charges and consequently intense electric fields are generated. These fields have two primary sources, one electromagnetic and one electrostatic.

The current change (di/dt) in a lightning stroke radiates a large electromagnetic pulse (EMP). The EMP fields propagate upward, with amplitude decreasing inversely with distance 1/r (like the far field of an antenna) until reaching the bottom of the ionosphere. These fields are not large enough to cause breakdown in the atmosphere at low altitudes, but the atmospheric density and, thus, the field required for breakdown decreases exponentially with altitude (the scale height in the middle atmosphere is ~7 km). The theoretical studies discussed below (originating with work by (Inan et al.(1991))) show that breakdown from the EMP field will occur in the region between the bottom of the ionosphere at about 95 km and about 75 km, consistent with some of the larger current changes associated with lightning. The EMP fields are associated with the generation of elves. The 100 us duration of elves is close to the 10-100 us duration of the high current portions of typical lightning strokes. Elves also have a large horizontal extent (100's of km) matching expectations from the 1/r drop in field strength.

Sprites are associated with the Quasi-Electrostatic (QE) fields of lightning, as first suggested by (Wilson(1925)). Their sources are the electrostatic fields from the charges in thundercloud. A lightning stroke rapidly removes charge from the thunderstorm, resulting in a rapid change in the charge configuration of the storm giving rise to an electrostatic field. These `near-fields' fall-off as r<sup>-3</sup> from the source (the dipole made up of cloud charge and ground image) and have an amplitude proportional to the charge moment (Q x L). If the upper atmosphere were not conducting, only a change in the electrostatic field level that is delayed by the speed of light propagation time would be observed. However, in the stratified conducting atmosphere the behavior is more complex. Free charge carriers in the atmosphere respond to the new electrostatic field configuration and `shield-out' the field. This process can be accomplished on approximately the local relaxation time scale ( $\sigma/\epsilon_0$) which is 10's of ms at 80 km and a few seconds at 40 km altitude. Because of the very different time scales for cloud charging (minutes), electrical relaxation (milliseconds to seconds) and lightning discharge (microseconds to milliseconds) the QE fields in the upper atmosphere have a rapid onset followed by a decay to a lower level. The field from the original cloud charge configuration does not significantly penetrate to high altitudes because a shielding charge configuration has developed during the slow thunderstorm charge buildup. A lightning stroke changes the electrostatic field of the thunderstorm. The initial amplitude of the QE field is essentially the same as the electrostatic field change expected in a non-conducting atmosphere since the atmosphere does not have time to relax. The typical lifetime of sprites (10's of milliseconds) corresponds well with the relaxation time at higher altitudes 60-90 km. Positive cloud-to-ground lightning flashes tend to neutralize greater charge from higher altitudes than other types of lighting, partially explaining the strong correlation between sprites and positive flashes.