Ionospheric TEC disturbances based on GPS observations

To indicate the ionospheric TEC variability, it is introduced in this paper an ionospheric disturbance index DI, which is defined as the relative deviation of the vertical total electron content. Based on the GPS observations at Qingdao and 5 stations along 116 degree longitude, specifications of the TEC variations are discussed, particularly for the upper and lower 5% values of DI. The criteria for identifying TEC storm event are proposed by combining the intensity and the duration of the DI variations. Moreover, the characters of TEC storm events in China region are presented.