Exchange of energy between the ionosphere and the protonosphere

Abstract

A theory of non-local heating of the topside ionosphere and the protonosphere by photoelectrons which escape the F2-layer is developed. Use is made of the expression for the rate of heat input to a protonospheric field tube to obtain an estimate of the rate at which thermal energy is conducted downward across the 1000 km surface at middle geomagnetic latitudes during the day. The expression for the rate of heat input is combined with the steady-state heat conduction equation to calculate the temperature distribution along the axis of the field tube. It is shown that, near sunspot maximum, heating by photoelectrons which escape the F2-layer produces a steep temperature gradient along a middle-latitude field tube; near sunspot minimum, however, the protonosphere is found to be very nearly isothermal in the region enclosed by the magnetic shell intersecting the 1000 km surface at 40° geomagnetic latitude. A time-dependent model describing the night-time cooling of a middle-latitude protonospheric field tube is presented. The model supports the conclusion that a downward flux of some 10 8 eV cm −2 sec −1 into the ionosphere can be maintained throughout most of the night. The electron temperature profile calculated for this heat flux is in good agreement with measurements of night-time electron temperature in the topside ionosphere, leading to the conclusion that a downward flux from the protonosphere is the only significant source of heat input to the quiet night-time ionosphere at middle latitudes.