Observational evidence of Z and L‐O mode waves as the origin of auroral kilometric radiation from the Jikiken (EXOS‐B) satellite

Abstract

The Jikiken satellite, which had an initial perigee and apogee of 250 km and 30,050 km, respectively, and an orbit inclination of −31°, passed through critical regions where auroral kilometric radiation (AKR) spectra were obtained on at least five occasions during a period from January 31, 1979, to June 21, 1980. On all of these occasions the AKR spectra revealed cutoff phenomena at the local Z cutoff frequency, indicating a continuation of the spectra from the high‐frequency side down to the Z mode wave frequency range and crossing over the local X cutoff frequency rather than being cut off at that frequency. Thus when observations are made near the low‐latitude portion of the source region, the AKR waves generate spectra that continuously cover the frequency range corresponding to Z mode and L‐O mode waves. We argue that the most plausible mechanism that gives a consistent interpretation of this spectral characteristic is the mode conversion theory. In other words, the plasma waves that are generated in the hybrid mode in the source regions are converted into Z mode waves that propagate toward dense plasma regions where the wave frequency coincides with the local plasma frequency and where a part of their energy is transferred to waves of the L‐O mode that can escape toward outer space. By using the propagation characteristics of Z mode waves, this conversion mechanism also gives a self‐consistent interpretation of previously presented evidence that was reported as the cutoff of AKR near the local electron cyclotron frequency. Although there is a possible ambiguity in the location of the source region, there is no conflict between the conversion mechanism for AKR generation and previous polarization observations carried out by Voyager spacecraft.