Summer‐winter hemispheric asymmetry of sudden increase in ionospheric total electron content induced by solar flares: A role of O/N2 ratio

Abstract

Latitudinal and local time dependences of the sudden increase in total electron content (SITEC) of the ionosphere induced by solar flares were statistically studied. We analyzed 91 SITEC events associated with solar flares larger than M5 X‐ray class from January 2000 to May 2005. Global total electron content (TEC) data from the Scripps Orbit and Permanent Array Center (SOPAC) GPS networks and X‐ray flux data obtained by the Geosynchronous Operational Environmental Satellites (GOES) were used in this study. The global TEC maps revealed that the SITEC value basically depends on the cosine of solar zenith angle (SZA). However, we found that there were significant residuals δ from the linear fitting to the SZA for almost all the flare events. The value of δ is statistically larger in the winter hemisphere than in the summer hemisphere. The latitudinal difference of δ is ∼0.5 TECU between 50°N and 50°S in the solstice. On the other hand, there was not a clear statistical difference in δ between the morning and evening sectors. The similar summer‐winter hemispheric asymmetry was also seen in the daytime distribution of the F region O/N2 density ratio in the MSISE‐90 model. This result indicates that the SITEC phenomena induced by solar flares depend not only on the SZA but also on the O/N2 density ratio. The daytime F region electron density and the background TEC is well known to be determined basically by the O/N2 ratio. It is reasonable that the O/N2 ratio affects the sudden increase in TEC in the same sense as it determines the background TEC.