Re-evaluation of thermosphere heating by solar EUV and UV radiation 1This article is part of a Special issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career.

Abstract

This paper presents a new calculation of neutral gas heating by solar extreme ultraviolet and ultraviolet radiation with updated solar irradiances and photochemistry. It is found that the heating rate of the neutral gas is significantly different from some previous determinations. Neutral gas heating arises from the many exothermic chemical reactions that take place from the ions and excited species created by energetic electrons. The calculations show that less than half the energy initially deposited ends up heating the neutral gases. The rest is radiated or lost in the dissociation of O2 because the O atoms do not recombine in the thermosphere above 100 km. At high altitudes, the heating rates are sensitive to the thermal electron density and long-lived species. The calculations show that, while most of the energy is deposited at low altitudes, the heating per neutral particle is greatest at high altitudes. This paper also examines the consequences of different thermal electron cooling rates to O2 vibration and O fine structure. Although there are large differences between the individual rates, the overall cooling rates have similar magnitudes and there is little effect on the calculated electron temperature. The concept of the neutral gas heating efficiency is a useful way of capturing the flow of energy in the thermosphere and was used in early global models to reduce computation. It is defined at a particular altitude as the rate of energy transferred to the neutral gas divided by the energy deposited rate at that altitude. The heating efficiency peaks at about 0.5 just below 200 km altitude. The heating efficiency is relatively insensitive to the solar input and thermosphere conditions when plotted against pressure rather than altitude coordinates and when thermal electron heating is calculated separately.