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

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\textendash2008. 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 inh 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.