The GPS measured SITEC caused by the very intense solar flare on July 14, 2000

This work studies the sudden increases in total electron content of the ionosphere caused by the very intense solar flare on July 14, 2000. Total electron content (TEC) data observed from a Global Positioning System (GPS) network are used to calculate the flare-induced TEC increment, dTECf, and variation rate, dTECf/dt. It is found that both dTECf/dt and dTECf are closely related with the solar zenith angles. To explain the observation results, we derived a simple relationship between the partial derivative of the flare-induced TEC, oTECf/ot, which is a good approximation for dTECf/dt, and the solar zenith angle v, as well as the effective flare radiation flux If, according to the well-known Chapman theory of ionization. The derived formula predicted that oTECf/ot is proportional to If and inverse proportional to Chapman function ch(v). This theoretical prediction not only explains the correlation of dTECf/dt and dTECf with v as shown in our TEC observation, but also gives a way to deduce If from TEC observation of GPS network. Thus, the present work shows that GPS observation is a powerful tool in the observation and investigation of solar flare effects on the ionosphere, i.e., the sudden ionospheric disturbances, which is a significant phenomenon of space weather. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.