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.

Polarimetry of auroral kilometric radiation with a triaxial nonorthogonal antenna system

“…The first inferences about AKR polarization were obtained from Hawkeye [Gurnett and Green, 1978] and ISIS-1 [Benson and Calvert, 1979;James, 1980]. The antenna systems of these spacecraft were not designed to provide direct measurements of the polarization parameters.…”

Ordinary mode auroral kilometric radiation, with harmonics, observed by ISIS 1

“…One theoretical approach, based on direct amplified cyclotron emission [Melrose, 1976;Wu and Lee, 19791, predicts radiation in both extraordinary and ordinary modes, both -•ith harmonics, with the fundamental x-mode being by far the dominant signal in the low density source regions of intense AKR (Melrose et el., 1982]. Most observations have inferred that the main radiation is in the x-mode [Gurnatt and Green, 1978;Kaiser et al, 1978;Benson and %,alvert, 1979;Shawhan and Gurnett, 19821. The plasma wave receiver on the Jikiken satellite (EPOS-B) also detects AKR [Morioka et al, 19811.…”

Dynamic spectra of the Stokes parameters for the dayside and nightside auroral kilometric radiation

“…A theory of a cyclotron maser instability (CMI) [Wu and Lee, 1979] is widely accepted since it predicts the observed domination of the righthanded extra-ordinary (R-X) mode of AKR emission. This has been deduced first from the observed cutoff frequencies for AKR propagating in a magnetized plasma [Gurnett and Green, 1978], [Benson and Catvert, 1979], [James, 1980], and later from intensity measurements of the right-handed and left-handed components of AKR made on Voyagers and DE-1 s/c [Kaiser et at., 1978], [Shawhan and Gurnett, 1982]. …”