ONE CASE STUDY ABOUT THE IONOSPHERIC RESPONSE TO ONE GREAT SOLAR FLARE IN THE SUN-LIT HEMISPHERE

The characteristics of the ionospheric response to one great solar flare was studied using the total electron content derived from GPS observational stations in the whole sun-lit hemisphere under International GPS Service for Geodynamics （IGS）. It was found that the largest enhancement of the Sudden Increase of Total Electron Content （SITEC） during this flare is about 2.6 TECU （1 TECU= 10¹⁶/m²）. The effects of solar flare radiation on the ionosphere can be recognized even in the region at 0600 LT or 1800 LT. Due to the ionospheric scintillation, the TEC enhancement couldn’t be derived from temporal TEC variation curves in the high latitudes. On the other hand, the synoptic picture of the sun-lit ionospheric response to the flare was obtained and the results showed the relationship of TEC enhancements with solar zenith angles. The larger the solar zenith angle, the smaller the TEC enhancement. It was worth mentioning that even the minor fast changes in TEC during this flare can be revealed from the changing rate of the temporal TEC variation curves in mid- and low latitudes. These minor and global synchronous disturbances of TEC were compared with the solar X-ray flux of this flare observed by satellites, and a close correlation between those minor ionospheric disturbances and the hard X-ray flux fluctuations is found.