Thermal coupling of conjugate ionospheres and the tilt of the Earth's magnetic field

Abstract

It is known, from predawn observations conducted over 20 years ago, that there is strong thermal coupling between conjugate ionospheres at mid‐latitudes. Nevertheless, most previous theoretical studies of interhemispheric coupling have concentrated on mass flux coupling, which has been universally found to be weak. Little attention has been paid to interhemispheric thermal coupling and the effects of the displacement of the magnetic and geographic axes. While these previous studies provided useful insights into the process of plasma exchange between conjugate ionospheres, several interesting phenomena have been overlooked. In this paper we examine the thermal coupling between conjugate ionospheres and confirm that it is very strong. Solar illumination in one hemisphere is accompanied immediately by heating in the conjugate hemisphere, and because the electron density is usually small in the dark hemisphere, the heating from the conjugate hemisphere can produce electron temperatures which are 1000°K higher in the dark than in the sunlit hemisphere. The high temperatures have little effect on NmF2 but cause a sharp rise in the O+ density in the topside ionosphere and the consequent rapid expulsion of H+ into the plasmasphere. The morning, upward H+ fluxes in the dark ionosphere that are produced as a result of conjugate heating are as large as the daytime fluxes. The tilt of the earth's magnetic field means that the conjugate illumination effects can actually be observed in the dark hemisphere 2 hours in local time before they appear in the sunlit hemisphere, and 5 hours before local illumination. In addition to the above effects that are caused by the longitudinal displacement of the conjugate points, there is a strong seasonal asymmetry in the overall behavior of the H+ fluxes at the longitude studied due in part to the differing day lengths at the conjugate points, which differ by 15° in latitude.