SPA-GRL Issue Highlights

During thunderstorms, free electrons naturally present in the atmosphere can attain very high energies under the action of strong electric fields. Brunetti et al. report first observations of downward gamma ray bursts in the MeV energy range that were produced during thunderstorm activity. The authors observe two kinds of gamma radiation events during summer 1996 at Gran Sasso, Italy: (1) slow increases of the radiation with energy below 3 MeV arising on account of radioactivity from aerosols transported to the ground by rain washout, and (2) fast increments of gamma rays up to 10 MeV interpreted as bremsstrahlung radiation from high energy electrons accelerated by strong thunderstorm electric fields. The authors suggest that an accelerating mechanism acts during thunderstorms that energizes the electrons.

Winterhalter et al. scan the magnetic field data from the Ulysses spacecraft for the period September 1994 to September 1995 (during which the spacecraft executed its fast latitude scan) to investigate the occurrence in the solar wind of "magnetic holes" or local minima in the magnitude of the interplanetary magnetic field. The authors examine the -80 to +80 helio-latitude range and find that at approximately 30 north and south latitude, the "hole count" rate drops to about 10% of the value in the lower latitude region. At high helio-latitudes the hole count is small but not zero. The authors find that the hole count is anticorrelated with solar wind speed and that high hole-counts occur in regions of large velocity gradients. They suggest that the holes could be relics of "mirror waves" generated in the corona.