Cosmic radio-noise absorption bursts caused by solar wind shocks

Abstract. Bursts of cosmic noise absorption observed at times of sudden commencements (SC) of geomagnetic storms are examined. About 300SC events in absorption for the period 1967-1990 have been considered. It is found that the response of cosmic radio-noise absorption to the passage of an interplanetary shock depends on the level of the planetary magnetic activity preceding the SC event and on the magnitude of the magnetic field perturbation associated with the SC (as measured in the equatorial magnetosphere). It is shown that for SC events observed against a quiet background (K_p<2), the effects of the SC on absorption can be seen only if the magnitude of the geomagnetic field perturbation caused by the solar wind shock exceeds a threshold value ΔB_th. It is further demonstrated that the existence of this threshold value, ΔB_th, deduced from experimental data, can be related to the existence of a threshold for exciting and maintaining the whistler cyclotron instability, as predicted by quasi-linear theory. SC events observed against an active background (K_p<2) are accompanied by absorption bursts for all magnetic field perturbations, however small. A quantitative description of absorption bursts associated with SC events is provided by the whistler cyclotron instability theory.