Solar activity variations of nighttime ionospheric peak electron density

Abstract

Monthly median N m F 2 (maximum electron density of the F 2 ‐layer) data at Okinawa, Yamagawa, Kokubunji, and Wakkanai have been collected to investigate the solar activity dependence of the nighttime ionosphere. The result shows that there are seasonal and latitudinal differences of the solar activity variation of nighttime N m F 2 . The main seasonal effects are as follows: nighttime N m F 2 increases with F 107 linearly in equinoctial months (March and September), and it tends to saturate with F 107 increasing in summer solstice month (June). What is peculiar is that there is an amplification trend of nighttime N m F 2 with F 107 in winter solstice month (December). The latitudinal difference is mainly displayed by the evolvement course of the variation trend between N m F 2 and F 107 . Using h m F 2 (peak height of the F 2 ‐layer) data and the NRLMSISE00 model, we estimated the recombination loss around the F 2 ‐peak at different solar activity levels. We found that the solar activity variation of the recombination processes around the F 2 ‐peak also shows seasonal dependence, which can explain the variation trends of nighttime N m F 2 with F 107 qualitatively, and field‐aligned plasma influx plays an important role in the equatorial ionization anomaly (EIA) crest region. During the first several hours following sunset in December, there are faster recombination processes around the F 2 ‐peak at medium solar activity level in mid‐latitude regions. This feature is suggested to be responsible for inducing the amplification trend in winter. In virtue of the calculation of neutral parameters at 300‐km altitude and h m F 2 data, the variation trend of the recombination processes around the F 2 ‐peak with F 107 can be explained. It shows that both the solar activity variations of h m F 2 and neutral parameters (neutral temperature, density, and vibrational excited N 2 ) are important for the variation trend of nighttime N m F 2 with F 107 . Furthermore, the obvious uplift of h m F 2 at low solar activity level following sunset in December is important for the amplification trend.