Daily variations and polarization properties
The terrestrial Auroral Kilometric Radiation (AKR) is an electromagnetic emission generated between 20 and 1000 kHz by sources located mainly in the auroral region of the Earth’s magnetosphere, primarily in the nightside sector. The emission is observed near the local gyrofrequency of electrons. The theory of electron cyclotron maser instability (ECMI) is a widely accepted explanation of AKR.
Daily variations of the AKR have been detected during analysis of quasi-continuous observations of the AKR (January - May, 2007) recorded by STEREO-B/WAVES. In particular, the occurrence frequency of the AKR, as well as its frequency range, are strongly modulated with a period of ~24 hours and depend on the orientation of the terrestrial magnetic dipole relative to the Sun. Fig.1 shows a normalized occurrence frequency of the AKR emitted from the Northern hemisphere as a function of the local time of the axis of the magnetic dipole. We suggest that observed daily variations in the AKR occurrence frequency, intensity, and frequency range are caused by the variation of the ambient plasma density due to the periodic change in the auroral ionosphere exposure to the Sun. Such periodic plasma density modulations may result in changes in the position and altitudinal size of the AKR source cavity as well as in the periodic variations of the field aligned potential drop, which is important for the generation of the AKR via the ECM process.
Polarization is an important property of AKR. Generally AKR is dominated by the extra-ordinary right-handed (R-X) wave mode with small contribution of the left-hand ordinary (L-O) mode. Data provided by Interball-2/Polrad experiment (a tri-axial polarimeter which was able to determine the direction of arrival and the polarization state of incident radio waves) have been used to determine the Stokes polarization parameters in the AKR wave plane. Fig. 2 shows an example of the dynamic spectra of the determined Stokes parameters. On the basis of 198 AKR events measured between Aug. 1996 and Jan. 1999 it has been statistically demonstrated that the AKR is fully circularly polarized. The small portions of the linear polarization component in AKR can be explained by measurement errors and numerical instabilities in the applied mathematical routines.
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