Possible effects of ionospheric beating for the formation of Pc1 pearl structures based on 6-year ground observations in Canada, Russia and Japan

Abstract:

We investigate pearl structures (amplitude modulation) of Pc1 pulsations simultaneously observed at Athabasca (ATH, 54.7N, 246.7E, L=4.3) in Canada, Magadan (MGD, 60.1N, 150.7E, L=2.6) in Russia, and Moshiri (MOS, 44.4N, 142.3E, L=1.5) in Japan. From a 6-year period of ground observations, from 2008 to 2013, we selected 150 events at longitudinally separated stations (ATH and MGD, group 1), 782 at latitudinally separated stations (MGD and MOS, group 2), all with high coherence of Pc1 waveforms (r > 0.5). As a result, we found that the peak occurrence rates of simultaneous Pc1 events were at 12-18 UT in group1, when ATH was in the morning sector and MGD in the midnight sector. In group 2, the peak was at 18-21 UT, with MGD and MOS in the morning sector. Using cross-correlation analysis, we confirmed the similarity of Pc1 pearl structures at different stations during Pc1 event timing. 82 % of Pc1 events in group 1 were less than 40 % of similarity of Pc1 pearl structures. In addition, 18 % of Pc1 events in group 1, which are over than 40 % of similarity of Pc1 pearl structures, were concentrated at 10-18 UT. According to polarization angle distribution in group 1, 57 % of Pc1 events occurred between ATH and MGD. Most of those events had high similarity of Pc1 amplitude envelopes. We investigated the relationship between the similarity of Pc1 pearl structures and geomagnetic activities (AE and SYM-H indices). The AE index seemed suddenly to start decreasing before the Pc1 onset and increasing after 80 min. The variation of SYM-H index was increasing 2 hours before Pc1 onset. From our statistical analysis, we suggest that the beating process in the ionosphere could be the dominant generation mechanism of Pc1 pearl structures in the ionosphere. It seems that the ionospheric duct in dawn sector of the ionosphere is well-defined during Pc1 pulsation propagating from ionospheric sources to stations. In order to understand the beating process in the ionosphere more clearly, we are going to take into account the possible effects such as distributed ionospheric source, attenuation effect in the ionosphere, dispersive propagation in the ionosphere and Pc1 source amplitude. Using the same procedure, we will also investigate Pc1 pearl structures simultaneously observed at latitudinally separated ground stations (MGD and MOS).