Ionospheric Spread-F and couplings between thermosphere and lower atmosphere (Invited)

Ionospheric Spread-F is a widely studied subject due to its significance in understanding physical essentials of the ionospheric irregularities and applications in scintillations prediction of radio wave propagation. The occurrence of Spread-F is affected by many factors such as density gradient below the F2 peak, neutral wind, electric field and so on. All these factors are necessary as the background for Spread-F to occur while not sufficient. There seems some triggering mechanisms are needed for spread-F to onset. Among the triggering factors, acoustic gravity wave is very important in seeding Spread-F occurring. In this paper, observational evidences based on ionospheric HF Doppler measurements are provided to show that in many cases spread-F are accompanied with wave-like disturbances, especially, when the amplitudes of such disturbances are large. These kinds of wave-like disturbances are known as acoustic gravity waves. We recognized these waves are originated from severe disturbances in the lower atmosphere such as typhoon, volcano eruption and some post-earthquake effects. Although many statistics indicate that Spread-F is correlated with solar and geomagnetic activities the occurrence of it is not controlled directly by any solar events such as flares or CME. Thus, in a sense, Spread-F is a manifestation of couplings between thermosphere and lower atmosphere and it might be a hint for us to explain some regional features of thermosphere by the different conditions of lower atmosphere under thermosphere.