Spread-F is a manifestation of ionospheric irregularities and generally takes place at nighttime. However, it can also be observed seldom at daytime. It is recognized that acoustic gravity waves (AGWs) play an important role in triggering plasma instability which results in spread-F in the ionosphere. The typhoon is a main source of the AGWs. In this paper, two cases of ionospheric daytime spread-F in the period of typhoon were analyzed. One case was on July 29, 1988 and the other was on August 01, 1989. The results showed the following: 1) There were some wave-like disturbances appearing in the HF Doppler records firstly, consequently the Doppler echo traces became scattered, which indicated that the ionospheric spread-F was triggered; 2) the blurred echo traces in the both two cases appeared in the morning (08:30-11:30 Beijing time) and lasted for more than two hours; 3) with the blurred echoes gradually weakening, the traveling ionospheric disturbances (TIDs) still existed and became clearer; 4) the frequency shifts in the two cases were both positive, implying the effective reflecting surface of the radio wave in the ionosphere moved downwards. These results provide good observational evidence for daytime spread-F during the typhoon period in Asian region.
