On the origin of ionospheric sublayers in the lower thermosphere

Abstract

Some properties are examined of ionospheric sublayers usually two in number originating at sunrise in the lower thermosphere. The formation of the sublayers is found to be nearly global in nature though they are quickly transported upward over the dip-equatorial region as a result of vertical drifts existing there. The ionization associated with the layers once formed appears to remain constant for over several hours while the ambient ionization exhibits a solar zenith angle dependence. The sublayers at equatorial and low latitudes are often found capable of yielding echoes of radio waves transmitted from ground of frequencies much larger than corresponding to the ambient plasma density. This is shown to be due to generation of gradient type instabilities in these sublayers. The generation of the sublayers themselves is shown to be consistent with the concept of ionization at sunrise of neutral constituents deposited overnight from micrometeorites in the lower thermosphere.