The Variation Characteristics of the Spread-F Occurrences at Chongqing in China

In the post-sunset ionosphere, plasma depleted bubbles with associated irregularity structures of scale sizes varying from centimeters to kilometers are generated due to plasma instability processes which causes the ionospheric irregularities. Ionospheric spread-F was discovered some decades ago. The variation in spread-F occurrence percentages continues to be an active area for scientific investigations at the present time. The spread-F presents a large dependence on the solar flux, the local time, the season, the latitude and longitude and the magnetic disturbances. In this paper, we present simultaneous ionosonde observations carried out at Chongqing (29.51°N, 106.42°E), which is located at the middle latitude in China, to study the characteristics of spread-F occurrence percentages and variations with local time, season and solar activity during 1997 to 2016. The results show that the onset time of spread-F seems to start at around 20 LT and continue till 05 LT at middle latitude. The spread-F occurs during summer most frequently at Chongqing. The spread-F occurrences decrease with the increase of solar and geomagnetic activity at middle latitude. These observations may improve our understanding of the factors responsible for the generation, growth and dynamics of the middle latitude spread-F events.