Long-term variations of the nighttime electron density enhancement during the ionospheric midlatitude summer

This study, for the first time, presented the long-term variations of Midlatitude Summer Nighttime Anomaly (MSNA) in the two hemispheres by using 66 ground-based ionosonde observations from 1957 to 2010. MSNA is characterized by the feature of higher nighttime electron density than daytime density in the midlatitude region during local summer months. Observations from 66 ionosonde stations were used to calculate the MSNA index which is defined by the difference between nighttime and noontime NmF2 values. The MSNA occurrence is determined by positive value of the MSNA index. The global distribution map of the MSNA index shows that there are three regions of intense MSNA. Three ionosonde stations in each of active MSNA regions were chosen to study the long-term variation of MSNA covering longer than one solar cycle. One station in the southern hemisphere is AIJ6N (Argentine IS; 65.2 degrees S, 64.3 degrees W geographic) and two stations in the northern hemisphere are LN047 (Lannion; 48.8 degrees N, -3.4 degrees E geographic) and MG560 (Magadan; 60.0 degrees N, 151.0 degrees E geographic). Results show that there is a clear solar activity negative dependence of the MSNA index, high MSNA in the low solar activity condition and low MSNA in the high solar activity condition. The seasonal and solar activity variations of the MSNA index are explained by the combined effects of the vertical plasma drift induced by the neutral wind and photoionization during the nighttime.