Equatorial B0 anomaly in September under extremely low solar activity

This study examines the variations in F2 layer thickness parameter (B0) in September from 2003 to 2008. The B0 data are derived from the observed ionograms at Jicamarca (12°S, 76.9°W), located near the dip equator. Results show the maximum B0 (B0max) at 1200 LT decreases from 2003 to 2007 with a deceasing solar activity, except 2008. It is noted that the solar activities are extremely low in September 2008. This anomaly, appearing in B0max, does not occur in two other F2 layer parameters, the density and height of F2 peak. The plasma densities at fixed altitudes are studied to explore what causes the B0max anomaly in September 2008. There is an obvious discrepancy appearing in the rate of decrease (R) of plasma density between at the F2 peak and 200 km. Through considering the relationship of terms in the analytical function of bottomside F2 layer profile, this R discrepancy would be mainly responsible for the B0max anomaly in September 2008. Key Points The B0 of September 2008 does not decrease with a decreasing solar activity A B0 anomaly in the equatorial ionosphere is found for the first time The B0 anomaly is related to the changes in the bottomside electron profile