Generation of the Auroral Kilometric Radiation in plasma cavities—I. Experimental study

Abstract

According to the Viking observations, the sources of the Auroral Kilometric Radiation (AKR) are plasma cavities, filled by a hot and tenuous plasma, separated from the denser and colder external plasma by sharp density gradients. These plasma inhomogeneities constitute interfaces that affect the wave propagation and then could play an important role in the generation of the radiation. These effects on the propagation are analyzed using Viking measurements obtained during crossings of AKR sources. It is shown that the electromagnetic energy, generated on the X mode, in the close vicinity of the local electron gyrofrequency, is first confined inside the cavity structure. Indeed, in a small frequency domain that extends a few kHz above the local electron gyrofrequency the “internal” X mode waves can only be connected to “external” Z or O mode waves. The corresponding transmission coefficients are measured as being an attenuation larger than 25 dB. At higher frequencies, the internal waves are connected to the external X mode with a transmission coefficient close to 1. However, this connection is only possible after an upward propagation of the electromagnetic energy inside the source. The progressive rotation of the wave vector associated to the wave refraction is estimated. An analysis of the polarization also shows that the radiating diagram likely does not have a cylindrical symmetry with respect to the geomagnetic field: the radiation is mainly emitted tangentially to the laminar structure constituted by the source. The existence of a preferential direction in the radiating diagram is a direct observable consequence of the finite extension of the sources. This could have important consequences for remote sensing studies.