Investigation of the Role of Cosmic-Ray Extensive Air Showers in the Initiation of Lightning

Abstract:

There has been few efforts at experimental validation of the Relativistic Runaway Electron Avalanche (RREA) model as an explanation of lightning initiation. Similarly, there have been few efforts to determine the energetic secondary electron environment from cosmic ray extensive air showers (CREAS) in order to verify if sufficient numbers electrons exist at thunderstorm altitudes to initiate lightning by this model. We are attempting to experimentally validate the RREA model by correlating time- and location-resolved lightning initiation data from the Oklahoma Lightning Mapping Array (OKLMA) with time- and location-resolved cosmic ray muon count rate data. Secondary cosmic ray muons are used as a proxy for relativistic seed electrons produced in CREAS. An array of four ground-based cosmic ray muon detectors have been designed, fabricated, and deployed in the shape of a 200 m square within the central area of coverage of the OKLMA. Each detector measures the muon count rate with sub-millisecond resolution and data from the four detectors are combined to identify large CREAS. Data from the cosmic ray muon detectors obtained while thunderstorms pass over the array are then compared to OKLMA data.

Using the CORSIKA Monte Carlo cosmic ray transport code, we have determined the energy spectrum and ambient flux of secondary electrons in both fair weather and thunderstorm atmospheric conditions <12 km above sea level. The secondary electron energy spectrum contains a population of >100 MeV electrons, which are able to generate many additional relativistic secondary electrons. The ambient flux of secondary electrons increases exponentially with altitude with a value of 260 electrons m^-2 sr^-1 s^-1 at 8 km, but the electron flux can be >10⁵ m^-2 inside the core of a 10¹⁶ eV air shower.