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Study of ionospheric disturbances during solar flare events using the superdarn hokkaido radar

Advances in Polar Science 2013年第1期24卷 12-18页

作者： Daiki Watanabe Nozomu Nishitani Solar-Terrestrial Environment Laboratory Nagoya University

It is well known that many types of ionospheric disturbances occur during solar flare events. The sudden increase in total electron content （SITEC） has been studied for several decades, but total electron content （TEC） data do not provide information on the altitudinal distribution of electron density changes. Previous studies used HF Doppler system data to investigate the contributions of the D-region and F-region ionospheric electron density changes by examining the HF radio wave frequency dependence on the Doppler shift values. In this study we examined the dependence of the elevation angle of the Doppler shift of ground scatter echoes using the superdarn hokkaido radar. We analyzed solar flare events from Dec 2006 to Mar 2012. A sudden fade-out of echoes was observed in almost all the events we analyzed, which was the result of the radio absorption associated with a significant increase in electron density within the D-region ionosphere. In addition, we discovered positive Doppler shifts just before the sudden fade-out of echoes. The Doppler shift is negatively correlated with the elevation angle of received radar waves. It indicates that variation of electron density in the D-region ionosphere is dominant during solar flare events. This result is consistent with a previous study. We also compared the irradiation by X-ray and extreme ultra violet rays observed by the GOES-14 and GOES-15 satellites, which generated Doppler shifts. A positive Doppler shift is consistent with a change of X-ray

关键词： solar flare superdarn hokkaido radar Doppler shift D-region F-region GOES satellite

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Northward-propagating nighttime medium-scale traveling ionospheric disturbances observed with superdarn hokkaido HF radar and GEONET

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Advances in Polar Science 2013年第1期24卷 42-49页

作者： Akimitsu Ichihara Nozomu Nishitani Tadahiko Ogawa Takuya Tsugawa Solar-Terrestrial Environment Laboratory Nagoya University National Institute of Information and Communications Technology

We report on the characteristics of nighttime medium-scale traveling ionospheric disturbances （MSTIDs） propagating northward observed with the superdarn hokkaido HF radar, which has a field of view to the north of Japan, and occasionally with the GNSS Earth Observation NETwork （GEONET）, which provides total electron content （TEC） data over Japan. From statis- tical analysis of MSTIDs observed with the hokkaido radar during nighttime （1700-0700 LT） from January 2007 to July 2009, we find that these MSTIDs traveling northward, although rare in comparison with those traveling southwestward, have a relatively high occurrence rate after sunset and around midnight in May and August, which is partly consistent with the occurrence rate of MSTIDs over Japan observed with GEONET in 2002, when the MSTID event database is available. We also use the data from simultaneous observation of nightside MSTIDs by the hokkaido radar and GEONET to find that when the HF radar observed northward-propagating MSTIDs, GEONET did not always observe such MSTIDs with the same propagation direction. Judging from this result and considering the HF radar field of view located to the north of the GEONET coverage area, we speculate that some physical parameters of the ionosphere/thermosphere over Japan differ from those to the north of Japan, which may result in the inconsistency of MSTID propagation direction. The present results provide new knowledge of MSTIDs propagating northward using the hokkaido radar, whose field of view was not covered by GEONET.

关键词： superdarn hokkaido radar GEONET MSTID

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