The detailed intensity distribution of the AKR emission cone

Abstract

Auroral kilometric radiation (AKR) observations in the northern hemisphere from 3.5 years of IMP 6 data and 4 years of Hawkeye data, at large radial distances, are combined to produce detailed intensity distributions of the AKR emission pattern below 200 kHz. All Hawkeye and IMP 6 AKR observations are normalized to a radial distance of 7 earth radii (RE) from the average source region reported by Gallagher and Gurnett (1979). The normalization is done to compensate for the expected radial variations in the intensity. Unlike previous emission pattern studies which showed the AKR frequency of occurrence above a preset power flux threshold, intensity contours of spectral power flux are presented. The AKR observations are well ordered in the source‐centered coordinates which provides further support for auroral zone generation of this emission at those altitudes. AKR is found to be emitted into a large emission cone pattern whose size decreases with frequency from the AKR spectral peak to low frequencies, consistent with previous studies. The emission cones have solid angles of 4.6, 3.3, and 2.7 steradians at 178, 100, and 56.2 kHz, respectively. Unexpected in these results is that there is less than a 10 dB variation in the illumination intensity across nearly the entire time‐averaged AKR emission patterns, at 178 and 100 kHz. In addition, simultaneous observations by Hawkeye and IMP 6 reveal that individual bursts of AKR can easily illuminate the northern emission cone uniformly. No systematic AKR intensity difference in longitude or latitude, within the cone at 178 kHz, is found during the simultaneous two spacecraft observations. This is not consistent with the hollow emission cone pattern reported by AKR at low frequencies (see Calvert, 1981c) near the source region.