OBSERVATIONS of radio emission from extensive air showers below 20 MHz are difficult to interpret because terrestrial noise, reflected from the ionosphere, is large compared with the magnitude of the radio pulses1. At frequencies below 4 MHz, however, D layer absorption during daylight hours may sufficiently decrease the noise background to enable observation of the radio emission to be made. Results of a preliminary experiment at 3.6 MHz by Prescott et al.1 gave a field strength of 0.5 µV m−1 MHz−1 for showers of size ~ 5×105 particles. In this analysis the noise and signal were subtracted linearly; correct quadratic subtraction yields a field strength estimate of ~1.5 µV m−1 MHz−1. Two further observations of radio emission have been made at low frequencies. Allan et al.2 at 2 MHz, found a field strength of ~750 µV m−1 MHz−1 for showers of primary energy Ep≳1017 eV. Stubbs3, also at 2 MHz, found a field strength of ~1 µV m−1 MHz−1 for showers >2×1014 eV.
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Stubbs, T. J., Nature Physical Science, 230, 172 (1971).
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HOUGH, J., PRESCOTT, J. & CLAY, R. Observation of Radio Pulses from Extensive Air Showers at 3.6 MHz. Nature Physical Science 232, 14–15 (1971). https://doi.org/10.1038/physci232014a0
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