Low Frequency Radio Emission from Extensive Air Showers

Abstract

RADIO pulses associated with extensive air showers have been observed over a frequency range of 2 MHz (refs 1 and 2) to 550 MHz (ref 3). Up to now measurements over the range of 20 MHz to 100 MHz have proved to be those most easily made⁴. At the highest frequencies a decrease in the lateral extent of the radiation, a decrease in its amplitude and increasing receiver noise have limited observations. At the lowest frequencies the overriding experimental problem is background noise. Only the use of highly directional antennas² or the reduction in daytime noise due to ionospheric D-layer absorption¹ have enabled measurements to be made down to 2 MHz. But the lowest frequencies hold particular interest since the large amplitudes reported seem to be in gross disagreement with theory⁵. Also, long wavelength radiation might prove to have a large lateral spread which could be exploited in the form of a large air shower detecting array.