Observational facts in revealing a close relation between acoustic-gravity waves and midlatitude spread F

The main purpose of this paper is to provide observational phenomena which show a close relation between acoustic-gravity waves (AGWs) and blurred traces of HF Doppler shifts. This relationship is regarded as observational evidence of a close connection of AGWs with midlatitude spread F (MSF), analyzed with six years of HF Doppler records. It is found that the frequency shift traces often become very blurred after local sunset and this kind of echo is reasonably assumed to be caused by ionospheric irregularities, namely, spread F. In most of the cases, these blurred echoes follow leading AGWs. Generally, the periods of AGWs were in a range of 15-30 mins and the Doppler frequency shifts were around or greater than 0.5 Hz (roughly equivalent to the amplitude of AGW, about 30-50 m/s). Usually, during local night, when there were AGWs with larger amplitudes, indicated by a frequency shift up to 0.5 Hz and above, the blurred echoes followed or overlapped them. These phenomena lasted for hours in general, then weakened gradually with the wavelike echoes still existing or having ceased. Of course, this is not always the case; in a few instances, clear wavelike disturbances solely are without accompanying blurred echoes, or blurred echoes occurred with only small amplitude waves ahead. However, it should be stressed that such exceptions were quite few. Statistics show that 23.5% of the total night time records were with blurred traces and 96% of blurred traces were accompanied with AGWs. In this paper, some typical examples are provided for different background (such as typhoon, volcano eruption, and other situations whenever AGWs appeared). These observations can be regarded as evidence of AGWs' having a seeding role in exciting MSF.