Abstract
Spread-F is known as the electron density inhomogeneous structures in F layer of ionosphere and can usually be classified as frequency spread-F (FSF) and range spread-F (RSF). Few studies have reported on the statistical characteristics of spread-F occurrences at midlatitudes in Eastern Asia, particularly the comparison of spread-F occurrences between China and Japan. In this paper, we used spread-F data recorded by ten ionosondes located between 25°N and 45°N from 1997 to 2016, to investigate the longitudinal differences in the statistical characteristics of spread-F occurrence and the probable mechanism for its occurrence at midlatitudes in Eastern Asia. Variations in the spread-F occurrences with the solar and geomagnetic activities, season and local time are presented. The main conclusions are as follows: (a) the occurrence percentage of FSF is higher than that of RSF, of which the former is anti-correlated with the solar or geomagnetic activities; (b) higher FSF occurrence percentages mostly appeared during summer, while RSF occurred more frequently in winter near 45°N latitude such as Urumqi, Changchun and Wakkanai; (c) the maximum of the FSF occurrence percentages mostly appeared between 01:00 and 02:00 LT approximately, whereas that of RSF appeared near 00:00 LT; (d) the spread-F occurrence percentages in the coastal or marine areas are higher than those in the inland region between 35°N and 45°N latitudes; however, this phenomenon is not obvious at lower latitudes; and (e) both the mean occurrences of FSF and RSF reach the minimum around 31°N latitude. These above results are helpful for understanding variations in spread-F occurrence at midlatitudes in Eastern Asia.
Highlights
Spread-F has been widely studied since it was first defined on the ionogram in the early 1930s (Booker and Wells 1938)
Summary and conclusions In this study, we investigated the variations in the frequency spread-F (FSF) and range spread-F (RSF) occurrences, and the possible mechanisms for the longitudinal differences in the spread-F occurrence percentages at midlatitudes in Eastern Asia include the years of the data used in 23rd and 24th solar cycles
The major conclusions are summarized as follows: (1) The occurrence percentages of FSF in these ten stations are higher than RSF
Summary
Spread-F has been widely studied since it was first defined on the ionogram in the early 1930s (Booker and Wells 1938). Bhaneja et al (2018) investigated the seasonal and solar cycle variations in midlatitude spread-F at five different North American sites spanning between Puerto Rico (18.5°N, 67.1°W) and California (34.8°N, 120.5°W). They found that spread-F events occurred more frequently during solar minimum years for all five stations. The minimum spread-F occurrence percentages happened near spring equinox for all the sites except Vandenberg. They found that the influence of geomagnetic on the spread-F occurrence was weak
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