COINCIDENT BURSTS OF AURORAL KILOMETRIC RADIATION AND VLF EMISSIONS ASSOCIATED WITH A TYPE-III SOLAR RADIO NOISE EVENT

This paper examines an isolated magnetospheric VLF/radio noise event t hat is highly suggestive of the triggering of terrestrial auroral kilo metric 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 coinci dent with AKR bursts detected on the AMPTE/IRM satellite, at the same magnetic local time as South Pole. On the basis of the close associati on of AKR and VLF bursts, and from geometrical considerations relating to wave propagation, it is likely that the AKR source was on the days ide and on field lines near South Pole station. The general level of g eomagnetic activity was very low. However, an isolated magnetic impuls e event (MIE) accompanied by a riometer absorption pulse was in progre ss when all of the VLF/radio noise bursts occurred. The very close ass ociation of the type III burst at HF with the AKR is consistent with e xternal stimulation of the AKR, if a different, more immediate, trigge ring process than that implied by Calvert (1981) is invoked. It is sug gested here that some of the HF solar radiant energy may decay into wa ves with frequencies comparable to those of the AKR by parametric exci tation or some other process, thus providing the few background photon s required for the generation of AKR by the Wu and Lee (1979) cyclotro n maser instability. The AKR, perhaps by modifying the magnetospheric electron velocity distribution, might have produced the observed VLF e missions. Alternatively, the VLF emissions may have arisen from the sa me 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 coinciden tal, the MIE could be a preconditioning agent for the ionosphere/magne tosphere plasma, making it susceptible to the external stimulation of the AKR.