IONOSPHERIC DISTURBANCES OBSERVED DURING THE PERIOD OF MOUNT-PINATUBO ERUPTIONS IN JUNE 1991

A series of climactic eruptions of Mount Pinatubo occurred in June 1991. Many quasi-periodic ionospheric disturbances were observed in the virtual height variations of the ionosphere at Chungli, in the total electron content variations at Lunping, Kaohsiung, and Chihpen, and in the differential Doppler shift data recorded at Lunping. These ionospheric disturbances were characterized by quasi-periodic wavelike trains lasting for several cycles with periods around 16 to 30 min, wavelengths between 160 and 435 km, and propagation speeds ranging from 131 to 259 m/s from the direction of Mount Pinatubo to the observation sites. The oscillation periods, downward movement of distortion, and inclined phase fronts all show that these ionospheric disturbances were manifested by the gravity waves that arrived from the direction of Mount Pinatubo. By use of a simplified isothermal atmosphere model, the propagation mode is interpreted as the freely propagating gravity waves.