Subjects: Geosciences >> Space Physics submitted time 2017-01-22
Abstract： In this paper, the observational investigation shows that the blurred echoes traces, as the expression of occurrence of spread-F, are occurred on the day time at mid-latitude regions based on the analyses of the HF Doppler frequency shift data; Simultaneously, the other ionospheric disturbances accompanied with or immediately before or after or following spread-F are also recorded. The results show that there are several aspects of the characteristics of the daytime spread-F-like phenomena observed by the HF Doppler sounding as following: 1) occurrence time cover almost the whole daytime, such as: the time period of morning, from morning to noon and the time period of afternoon; 2) the morphological features are the blurred echoes traces (similar as the nighttime spread-F), the shifts of which tend to be positive, and sometimes are accompanied by traveling ionospheric disturbances (TIDs) that are not the characteristics of the one generated by typhoon; 3) duration time is much longer, the longest one lasts about five hours; 4) it appears accompanying with the other ionospheric disturbances such as TIDs, solar flare etc.. The characteristics show that the HF Doppler sounding is useful tool for detecting the daytime spread-F-like. Although the daytime spread echoes phenomenon occasionally occur and the occurrence rate is very low, the extensiveness of temporal and spatial distribution of the daytime spread-F-like implies that its effect on the space weather can not be ignored and its properties can not be replaced by the one of the usual spread-F. Therefore, the study of the daytime spread-F-like has very important significance.
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