Characteristics of 30-MHz interference noises observed by riometers

Abstract

In 1989,when the sun was highly active, interference noises were often observed by 30-MHz riometers at Asuka and Syowa Stations in Antarctica. The noises showed a systematic diurnal and seasonal variation. The noises were global phenomena because they were also seen in other riometer data at high and middle latitudes in the northern hemisphere. These noises were investigated in connection with ionospheric foF_2 data, geomagnetic Kp indices and sunspot numbers. The results are summarized as follows; 1) Interference noises consist mainly of ground radio waves reflected by the ionosphere. 2) The interference noise occurrence shows a good correlation with foF_2. 3) The noise intensity shows a tendency of decreasing on weekends, and this feature suggests that the noises are mainly of artificial origins. It is concluded, therefore, that the occurrence feature of the interference noises reflects the condition of the ionosphere, and can be used as an indicator of the global electromagnetic wave environment in HF range.