Summer-winter hemispheric asymmetry of the sudden increase in ionospheric total electron content and of the O/N2 ratio: Solar activity dependence

摘要

The solar activity dependence of the summer-winter hemispheric asymmetry (SWHA) of the sudden increase in total electron content (SITEC) due to solar flares and of the O/N2 ratio is statistically analyzed using global GPS-total electron content data and TIMED Global Ultraviolet Imager column O/N2 ratio data. We focus on observations with nonnegligible residuals of the solar zenith angle (SZA) dependency of SITEC. We examined 109 SITEC events associated with flares larger than M5 X-ray class flare from 2000 through 2006 and compared the residual SITEC (δ) to the O/N2 ratio. We observed that the latitude gradient of δ has not only an annual variation but also a year-to-year variation that is similar to those of the O/N2 ratio. The SWHA magnitude (defined as the annual maximum of latitude gradient) of both δ and O/N2 decreases as the solar activity declines toward its minimum. The correlation coefficient between the annual SWHA magnitudes of δ and those of O/N2 is 0.92, indicating strongly that the SWHA of O/N2 is responsible for that of SITEC in both the annual and year-to-year variations. The X-ray classes of the solar flares have no clear correlation with the solar activity, F10.7 index. We observe that the SWHA magnitude of δ does not depend on the magnitude of solar flare but rather on the background solar activity through the SWHA magnitude of the O/N2.