Peculiar features of ionospheric F3 layer during prolonged solar minimum (2007–2009)

We present the seasonal and local time occurrence of ionospheric F-3 layer over Tirunelveli (geographic longitude 77.8 degrees E, geographic latitude 8.7 degrees N, dip 0.7 degrees) during extremely low and prolonged solar activity period (2007-2009). Canadian Advanced Digital Ionosonde observations from this station are used in the present study. We find that the occurrence of F-3 layer is nearly 3 times higher during 2009 (approximate to 48%) as compared to that during 2007 (approximate to 16%). The increase of this order just within the low solar activity period is unusual. In earlier studies similar increase in F-3 occurrence has been reported when solar activity changes from high (F-10.7=182) to low (F-10.7=72). The other important feature is the presence of postnoon F-3 layers which are observed dominantly during summer solstice of 2009. Such occurrence of postnoon F-3 layers was nearly absent during summer solstice of the previous solar minimum (1996) over nearby dip equatorial station Trivandrum. We take equatorial electrojet (EEJ) as a proxy for eastward electric field. It is noticed that the EEJ strength and the maximum rate of change of EEJ are higher for F-3 days as compared to those on non-F-3 days. We find that the peak occurrence of prenoon F-3 layer closely coincides with the time of maximum rate of change of EEJ. It is in general accordance with the theory proposed by Balan et al. (1998) that suggests the formation of F-3 through vertically upward E x B drift in presence of equatorward neutral wind. The present study reveals that the rate of change of eastward electric field (dE/dt) as well plays an important role in the formation of F-3 layer.