CHANGE OF THE THERMOSPHERECOMPOSITION AND DENSITY DURING SEVEREGEOMAGNETIC ACTIVITIES

In this papers the Ap values for three geomagnetic activities of Jul. 6, 1974,Mar. 2, 1982 and Sept. 6, 1982, and thermospheric composition data from AECand DE-Bare used to search the change of the thermospheric composition anddensity during severe geomagnetic activities. The results show that the fluctuationof thermospheric density is very clear during severe geomagnetic activities. Andthe magnitude of this change increases as the altitude increases. At near 600km,the change can be four times of the original values, the change of N2 density is thelajrgest in all of the thermospheric compositions; while the abundance of O (No/N_all)decreases notably during severe geomagnetic activities.