Coincident bursts of auroral kilometric radiation and VLF emissions associated with a type III solar radio noise event

Abstract

This paper examines an isolated magnetospheric VLF/radio noise event that is highly suggestive of the triggering of terrestrial auroral kilometric radiation (AKR) by solar type III radio emission and of a close relation between AKR and broadband hiss. The solar type III burst was measured on polar HF riometers and was coincident with local dayside VLF/LF noise emission bursts at South Pole station. It was also coincident with AKR bursts detected on the AMPTE/IRM satellite, at the same magnetic local time as South Pole. On the basis of the close association of AKR and VLF bursts, and from geometrical considerations relating to wave propagation, it is likely that the AKR source was on the dayside and on field lines near South Pole station. The general level of geomagnetic activity was very low. However, an isolated magnetic impulse event (MIE) accompanied by a riometer absorption pulse was in progress when all of the VLF/radio noise bursts occurred. The very close association of the type III burst at HF with the AKR is consistent with external stimulation of the AKR, if a different, more immediate, triggering process than that implied by Calvert (1981) is invoked. It is suggested here that some of the HF solar radiant energy may decay into waves with frequencies comparable to those of the AKR by parametric excitation or some other process, thus providing the few background photons required for the generation of AKR by the Wu and Lee (1979) cyclotron maser instability. The AKR, perhaps by modifying the magnetospheric electron velocity distribution, might have produced the observed VLF emissions. Alternatively, the VLF emissions may have arisen from the same anisotropic and unstable electron distribution function responsible for the AKR. While the relationship of these emission features to the occurrence of the magnetic/absorption impulse may have been coincidental, the MIE could be a preconditioning agent for the ionosphere/magnetosphere plasma, making it susceptible to the external stimulation of the AKR.