A critical survey of ionospheric temperatures

Abstract

The relative order of importance of the various methods available for the evaluation of temperature from an ionospheric parameter is (a) temperature and collisional frequency, (b) temperature and auroral scale heights, (c) temperature and electron concentration, (d) temperature and the influx function, and (e) temperature and the recombination coefficient. Method (e), still in its infancy, does not yet permit accurate temperature evaluations in the ionosphere. Some indication of a diurnal temperature variation may be obtained from method (d). The probable temperature-altitude relationship in the atmospheric regions 100-400 km., as determined by several techniques, is tabulated below: Upper Atmospheric Temperatures (°K.) Altitude (km.) Method of Determination Collisional frequencies Auroral scale heights Electron concentration 100 300 219 - 150 825 531 - 200 1350 1580 - 250 2175 2073 - 300 2400 2455 - 330 - - 1530-2680 350 3225 2704 - 400 3450 - - 500 - - 2200-3910 These values are considered to represent mean annual temperatures at middle latitudes. The electron and gas temperatures are essentially identical. If the free electrons have an excess of energy (as may result during an ionization process) this energy is quickly distributed among the gas and other particles. Thermal equilibrium and therefore a Maxwellian velocity distribution of the atmospheric particles exists among the gaseous constituents in the range 100-400 km. The kinetic temperature is therefore equivalent to the gas temperature. Fluctuation phenomena and deviations of the particles from a Maxwellian velocity distribution probably do not occur in the terrestrial atmosphere until much higher altitudes. A totally ionized atmosphere below 400 km. does not agree with the experimental evidence inasmuch as the temperature deduced on such a basis will not support the observed number density. Tentatively it has been shown that no large diurnal temperature variations are found in the altitude range 100-400 km. Preliminary estimates of a probable rate of heating based upon the dissipation of excess energy absorbed during an ionization process appear to confirm the preceding statement. However, the rate of heating attributable to non-detectable absorption processes, to large electrical currents in the ionosphere or to the bombardment by large numbers of high energy electrons is unknown. The existence of temperatures much higher than those given in the above Table would be difficult to explain in the terrestrial atmosphere. Indeed, it may be qualitatively concluded that the maximum atmospheric temperatures occur in the ionospheric regions at an altitude near or somewhat above 400 km. Above this temperature maximum, a general decrease probably occurs to meet the atmospheric boundary temperature with interstellar space. A temperature maximum near the ionospheric regions probably arises from the various absorption or heating processes together with a sufficient number density of particles.

The Structure of the F Region of the Ionosphere

“…Indeed, from several independent sources (see, e.g. Gerson 1951) there is evidence of a temperature gradient between the E and the F 2 regions.…”

On the Use of Ionization Gage Devices at Very High Altitude

“…Thermal equilibrium among gas particles can be assumed externally (16) as well as internally with small error. Internally, however, it is assumed that the particle temperatures will be set by the wall temperature, adopting a unity accommodation coefficient.…”