text: what are jets and sprites above earth's highest clouds?   characteristics of red sprites  sprites are massive but weak luminous flashes that appear directly above an active thunderstorm system and are coincident with cloud-to-ground or intracloud lightning strokes. their spatial structures range from small single or multiple vertically elongated spots, to spots with faint extrusions above and below, to bright groupings which extend from the cloud tops to altitudes up to about 95 km. sprites are predominantly red. the brightest region lies in the altitude range 65-75 km, above which there is often a faint red glow or wispy structure that extends to about 90 km. below the bright red region, blue tendril-like filamentary structures often extend downward to as low as 40 km. sprites rarely appear singly, usually occurring in clusters of two, three or more. some of the very large events, such as shown in figure 1, seem to be tightly packed clusters of many individual sprites. other events are more loosely packed and may extend across horizontal distances of 50 km or more and occupy atmospheric volumes in excess of 10,000 cubic km.   high speed photometer measurements show that the duration of sprites is only a few ms. current evidence strongly suggests that sprites preferentially occur in decaying portions of thunderstorms and are correlated with large positive cloud-to-ground lightning strokes. the optical intensity of sprite clusters, estimated by comparison with tabulated stellar intensities, is comparable to a moderately bright auroral arc. the optical energy is roughly 10-50 kj per event, with a corresponding optical power of 5-25 mw. assuming that optical energy constitutes 1/1000 of the total for the event, the energy and power are on the order of 10-100 mj and 5-50 gw, respectively.  if sprites are only barely detectable by the unaided human eye, in intensified television images obtained from the ground and from aircraft they appear as dazzlingly complex structures that assume a variety of forms  characteristics of blue jets  blue jets are a second high altitude optical phenomenon, distinct from sprites, observed above thunderstorms using low light television systems. as their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. following their emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15 degrees full width at vertical speeds of roughly 100 km/s (mach 300), fanning out and disappearing at heights of about 40-50 km. their intensities are on the order of 800 kr near the base, decreasing to about 10 kr near the upper terminus. these correspond to an estimated optical energy of about 4 kj, a total energy of about 30 mj, and an energy density on the order of a few mj/m^3. blue jets are not aligned with the local magnetic field.