On the equinoctial asymmetry in the threshold height for the occurrence of equatorial spread F

•The asymmetric behavior of the thermospheric neutral density is brought out.•Consistently higher h’Fc’for vernal equinox compared to autumnal equinox.•Higher O/N2 in vernal equinox compared to autumnal equinox.•Solar flux dependence of h′Fc asymmetry and O/N2 asymmetry.

In the present study, the threshold height (h′Fc) for ESF occurrence irrespective of the polarity of the meridional winds during vernal and autumnal equinoxes is investigated using a large database spread over 1993–2008. The characteristics of the thermosphere during the two equinoxes have also been examined using TIMED/GUVI O/N2 data. The major aspects that have emerged are (i) presence of significant equinoctial asymmetry in h′Fc for the years examined, (ii) increase in the equinoctial asymmetry of h′Fc with solar activity, (iii) presence of significant asymmetry in O/N2 and TIEGCM simulations of normalized atomic oxygen mass density, between the two equinoxes.