Polarization of Extensive Air Shower Emission at 6 MHz

Abstract

MEASUREMENTS of radio pulses from cosmic ray air showers in the frequency range 20 to 70 MHz have established the dominance of the geomagnetic mechanism^1–4 and there is strong evidence from these experiments that the radio field strength increases with decreasing frequency. There have, however, been few observations at frequencies below 15 MHz where the background noise is considerably higher. At 2 MHz Allan et al.⁵ found a field strength of ∼750 µV/m/MHz for showers with a threshold primary energy of 10¹⁷ eV. At the same frequency Stubbs⁶ found a field strength of ∼0.8 µV/m/MHz for a threshold energy of 2 × 10¹⁴ eV. Hough et al.⁷ reported measurements at 3.6 MHz and compared their results with those of Allan et al. and Stubbs by normalizing all measurements to those expected on the axis of a shower of primary energy 10¹⁷ eV and incident normal to the Earth's magnetic field.