On the polarization and origin of auroral kilometric radiation

Abstract

Observations of radio emissions by the Hawkeye 1 satellite at low altitudes over the southern hemisphere have now provided measurements at radial distances from about 1.5 to 2.5 RE along the auroral field lines, in the region where the intense nightside auroral kilometric radiation is believed to be generated. These measurements provide new evidence on the mode of propagation and origin of the auroral kilometric radiation. At low altitudes the auroral kilometric radiation is consistently observed to have a low‐frequency cutoff at the local electron gyrofrequency fg−. Since the electron plasma frequency fp− is usually much smaller than fg− in the region where these observations are obtained, this cutoff corresponds closely with the propagation cutoff for the right‐hand mode of propagation. These observations therefore provide a strong indication that the auroral kilometric radiation is right‐hand polarized in agreement with previous conclusions made on the basis of the angular distribution of this radiation. In the local evening region, where the intense auroral kilometric radiation is believed to be generated, a few events have been detected for which no low‐frequency cutoff is evident. In these cases the auroral kilometric radiation appears to merge essentially continuously into a band of intense auroral hiss which extends downward to frequencies as low as 1 kHz. These observations suggest that the generation of the whistler mode auroral hiss and the escaping auroral kilometric radiation are very closely related. Possible mechanisms which could produce strong coupling between the whistler mode and the escaping free space electromagnetic modes are discussed.