Equinoctial asymmetry during solar minima at low to mid latitude

Abstract

Equinoctial asymmetry (EA) of F2-layer critical frequency (foF2) during minimum years of solar cycle 22–23 (1996) and 23–24 (2008) has been studied for low and mid latitude stations ranges from 20°N - 55° N. Year 2008 provided an opportunity to study this phenomenon in extreme low EUV conditions. Pronounced/reasonable increase in EA strength at low/mid latitudes from 1996 to 2008 indicates role of EUV in EA. In addition to an increase in strength of EA from 1996 to 2008, variation in its strength and sign (positive if Mar Equinox foF2means > Sep Equinox foF2means and vice versa) with latitude and longitude are also observed during both years. Similar to previous studies, EA is observed in daytime during both years. However at low latitude stations, two peaks of foF2 are also observed at night: the first peak in summer and the second peak in the beginning of the September equinox which shows superposition of semi-annual and annual components. Hourly analysis showed asymmetry (Sep Equinox foF2means > Mar Equinox foF2means) from sunset to mid-night at low mid to mid latitude. It appears that the Mid latitude Summer Nighttime Anomaly (MSNA) is extending to September equinox with decreasing magnitudes from Summer to Sep equinox. IRI-16 showed EA during daytime only with over- and under- estimation as compared to observed data. Serious discrepancy between observed and IRI-2016 model values especially during deep minimum suggests modification of model for extreme low EUV condition as well as incorporation of parameters related to global wind pattern in the model.