ScienceShot: Broadcast From On High

NASA (Astronaut Photo ISS031-E-10712)

Red sprites, the enigmatic electrical discharges that sometimes occur high over thunderstorms in the wake of strong lightning strikes in the lower atmosphere, can emit low-frequency radio waves, a new analysis suggests. Most work on sprites—fan-shaped, fireworklike flashes that last no more than a few millisecond—hasn't noted such emissions, possibly because researchers weren't looking for them. In the few studies that have detected them, the research hasn't definitively identified the short-lived sprites (denoted by an arrow in this image taken from the International Space Station over southeastern Asia) as the source. Now, analyses of how highly charged molecules generated by sprites would behave suggest that individual, kilometers-long streamers within sprites—somewhat analogous to single bolts of lightning—could indeed act as antennas, generating radio waves at many different frequencies. At an altitude of 75 km, streamers would emit radio waves at frequencies below 3 kilohertz, or 3000 cycles per second, the researchers reported last month in Geophysical Research Letters. A sprite streamer at an altitude of 40 km, where the atmosphere is much thicker, might generate radio waves with frequencies up to 300 kHz. Also, the team estimates, the stronger the electric field in a sprite, the higher the frequency of radio emissions will be.

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