Case study of apparent longitudinal differences of spread F occurrence for two midlatitude stations

Spread F is a widely studied subject, and the occurrence of spread F is affected by many factors. One of these factors is acoustic gravity waves (AGWs) which are very important in seeding spread F. Since most of the AGWs in the ionosphere originate from the lower atmosphere, there should be some regional features of spread F due to the different meteorological or ground conditions immediately beneath the local ionosphere. In this paper, a data set with a time coverage of one solar cycle from two Chinese stations located at exactly the same latitude and a 38 degrees separation in longitude, and having sharp contrasts of ground meteorological conditions, are used to make comparisons of spread F occurrence rates. The results showed that the total number of occurrence or occurrence percentage at Changchun station (very near the coast) is always much higher than that at Urumqi station (in the very center of the Europe-Asia continent). The annual maxima of spread F occurrence are in summer and winter. Other features of spread F occurrence at these two stations are in agreement with known properties of spread F. However, the great difference of occurrence frequency between the two stations is striking and worth further studying.