Direction‐of‐arrival measurements of auroral kilometric radiation and associated ELF data from Isis 1

TLDR: The spin-modulated noise from the dipole connected to the sounder receiver on the Isis 1 spacecraft has been used along with the computation of ray paths to deduce the direction of propagation of auroral kilometric radiation (AKR) as mentioned in this paper.

Abstract: The spin-modulated noise from the dipole connected to the sounder receiver on the Isis 1 spacecraft has been used along with the computation of ray paths to deduce the direction of propagation of auroral kilometric radiation (AKR). On the basis of three passes with fixed-frequency measurements at 0.48 MHz it is concluded that in the generation region at about 3500-km altitude the wave vectors make an angle between about 60° and 90° with respect to the local magnetic field. That is, they are predominantly downcoming. Also, in using the sounder records the plasma frequency fN is found to be very much less than the gyrofrequency fH in the generation region (fN ≲50 kHz, fH ≃450 kHz). When these Isis observations are combined with previous evidence on AKR, from which it was concluded that it propagates predominantly in the right-hand polarized X mode, serious disagreement with a number of theories, particularly those that predict the left-hand-polarized O mode, results. However, two theories of a quite different nature, direct cyclotron radiation and a three-wave interaction involving an ELF/VLF wave, give some prospect of accommodating all the observations.