On Seeding Equatorial Spread F During Solstices

Abstract

Interpretation of the morphology of equatorial spread F (ESF) is usually given in terms of factors that modulate the growth rate of the Rayleigh-Taylor instability. That interpretation is, however, incomplete because only one means for growth-rate enhancement has been included; that is, the strength of the post-sunset rise of the F layer could become enhanced, when the solar terminator aligns with geomagnetic field lines. Maxima in ESF occurrence observed near equinoxes seem accountable by this means, but maxima observed near solstices require another source of enhancement. Seasonal migration and latitudinal alignment of the inter-tropical convergence zone (ITCZ) with the magnetic dip equator is suggested as the missing source, and shown able to fill that void.