Statistical analysis of ionospheric responses to solar flares in the solar cycle 23

In this study, we studied the ionospheric responses to solar flares during 1999–2006 by using GOES 0.1–0.8 nm X‐ray, 26–34 nm EUV, and GPS/total electron content (TEC). The solar zenith angle (SZA) dependence was quantitatively investigated by analyzing global TEC enhancements during about 100 X‐class flares. The mean ratio of ΔTEC at SZA = 90° to ΔTEC at SZA = 0° is about 0.39. The statistical results show that a limb flare has less effect on the ionosphere than a central flare does because the main ionization source of the ionosphere, solar EUV radiation, can be absorbed by thick solar gas due to large central meridian distance (CMD), which is called the CMD effect. Furthermore, the CMD effect decreases with decreasing flare X‐ray class. The results show that TEC responses are not highly related to solar X‐ray flux enhancement with correlation coefficient of 0.6, but more closely related to solar EUV flux enhancement with correlation coefficient of 0.91 for 26–34 nm EUV. The combination of X‐ray flux and flare location is also a good indicator for TEC response: The correlation coefficient of ∆X‐ray*cos(CMD) and ∆TEC is as high as 0.95. The seasonal dependence in TEC response is also investigated. There are larger responses in equinoxes than in solstices. The seasonal variation of neutral density is considered to be a main cause of the season dependence of TEC response. Key Points The solar zenith angle dependence was quantitatively investigated Flare location effect decreases with decreasing solar flare class Statistically analyzed the seasonal dependence of TEC responses