Emission of Radio Pulses from Cosmic Ray Extensive Air Showers

THERE have been several attempts to ascertain the mechanism by which the ionized disk of a cosmic ray extensive air shower may emit a radio pulse, as observed at frequencies between 22 MHz and 560 MHz (ref. 1), but no conclusive result has yet been found. The high intensity requires that there must be coherent radiation from a spatial distribution of charge; it has been suggested that this occurs as a result of the separation of charges of opposite sign in the Earth's magnetic field 2 or by the selective absorption of positrons leaving an excess negative charge 3 . Other possible mechanisms 4 seem to give much lower intensities. Experiments at 22 MHz (ref. 5) and 30 MHz (ref. 6) support the magnetic separation hypothesis, but experiments at 44 MHz (refs. 6 and 7) indicate that the magnetic field may be unimportant. This communication describes a further experiment at 42 MHz which suggests that the relative importance of the two mechanisms may differ between large and small radio pulses.