Do Extensive Air Showers of Cosmic-Ray Secondaries Initiate Lightning, and If So, How Would We Know?

摘要

Extensive Air Showers (EAS) of energetic particles and gamma radiation produced by cosmic rays are the primary sources of ionization in the atmosphere below about 50 km altitude [MacGorman and Rust, 1998]. Runaway electron theory suggests that the resulting population of energetic electrons could serve to initiate electrical discharges when strong electric fields associated with thunderstorms are present. Review of relevant literature suggests that a "y es" answer to our title question has been accepted implicitly in some quarters as a forgone conclusion. Actually, the cosmic-ray EAS (CREAS) hypothesis has not yet been tested critically. We address the question of how it might be possible to determine objectively and quantitatively whether some or all lightning discharges are indeed initiated by such a process and we discuss the results of two preliminary investigations designed to test the hypothesis. Using balloon-borne instruments deployed in a thunderstorm, we have observed X-ray bursts in direct association with field changes, consistent with the occurrence of runaway discharges. We have also searched for coincidences between lightning and surface observations of CREAS. It turns out that there is only a very small chance that any of the currently deployed ground-based detectors would record an EAS that might have initiated a nearby lightning flash. Though the CREAS hypothesis for lightning initiation has some attractive features, it still needs to be tested rigorously.