Further analysis of the Viking RPA data has now provided measurements of the thermal electron temperature in the upper Martian ionosphere. It is found that Te is several thousand degrees K, i.e., only of the order of twice the ion temperature. The sum of all the measured partial plasma pressures, including ions and suprathermal electrons, has a minimum value of ∼5×10−10 dyn cm−2 near 350 km and is found to be insufficient to balance the measured electron pressure in the shocked solar wind near 1000 km altitude, by a factor of the order of 4. Thus there is no doubt that a magnetic field of at least 30 to 40 nT permeates the ionosphere. This conclusion is not inconsistent with previous assessments, but it now has a firm observational basis. These data do not uniquely establish whether the magnetic field is intrinsic or induced, but our assessment is that a significant intrinsic moment is not required.

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The equilibrium photoelectron distribution in the upper atmosphere of Mars is calculated and compared with a typical theoretical model of the photoelectron distribution in the upper atmosphere of the earth. The neutral upper atmosphere and ionosphere models used in t,•e calculations for Mars are based on the Viking 1 neutral mass spectrometer and retarding potential•analyzer data. The corresponding models in the case of the earth are based on Arecibo incoherent scatter data. while the primary photoelectron spectra in Mars' and earth's ionospheres are remarkably similar in spectral shape and finer structure features, the Martian equilibrium photoelectron spectrum is more structured and much softer than the earth's. The equilibrium photoelectron spectrum is used to investigate the loss of the photoelectron kinetic energy to the various constituents of the upper atmosphere of Mars, including the heating of the ambient electron gas. The ionization rates by photoelectron impact are calculated and are found to contribute about 30% to the total ionization rate on Mars. Photoelectron impact excitation rates of the A2IIu and B2•u + states of CO2 + and the aSH and Axil states of CO are calculated and compared with the corresponding excitation rates by other mechanisms. Photoelectron impact excitation is found to contribute 20-30% to the CO2 + and CO airglow of Mars. The solar dectromagnetic radiation in the extreme ultraviolet (EUV) constitutes a major energy source of the upper atmospheres of the planets. Knowledge of the absorption and the partition of this energy among the various atmospheric constituents is required for the understanding of the composition of the thermal structure, the dynamics, the airglow, and several other important aspects of planetary atmospheres. The neutral mass spectrometers on the Viking landers have provided detailed information about the composition and thermal structure of the upper atmosphere of Mars. The neutral Martian atmosphere, in the altitude range 120-200 km, consists mainly of CO•., with trace-detectable quantities of Ne, Ar, CO, O•., O, and NO [Nier and McElroy, 1976]. The observed neutral gas altitude profiles have been analyzed by McElroy et al. [1976] to deduce the neutral particle temperature over the same altitude range. The temperature profiles obtained from that analysis are highly structured and significantly lower than the thermospheric temperatures deduced from the analysis of the airglow data of the Mariner spacecraft [Barth et al., 1972; Stewart, 1972; Strickland et al., 1972, 1973]. In addition, the Viking landers provided the first in situ measurements of the ionic composition and temperature of the ionosphere of that planet. The results of the ionosphere composition and thermal structure experiments have been presented by Hanson et al. [1977]. Briefly, the ionosphere of Mars is an Fx layer with peak ion concentration of about 105 cm -a, near 130-km altitude, consisting of about 90% O•. + and 10% CO?. The ion temperature profile measured by Viking 1 increases from a ...

The thermal response of the nighttime F region ionosphere to local heating by HF radio waves has been observed with the incoherent scatter radar at Arecibo, Puerto Rico. The observations consist of high‐resolution space and time variation of the electron temperature as a high‐power HF transmitter is switched on and off with a period 240 s. As soon as the HF transmitter is turned on, the electron temperature begins to rise rapidly in a narrow altitude region near 300 km, below the F2 layer peak. The electron temperature perturbation subsequently spreads over a broader altitude region. The observations are compared with the anticipated thermal response of the ionosphere based on numerical solutions of the coupled time‐dependent heat conduction equations for the electron and composite ion gases and are found to be in good agreement over the entire altitude region covered by the observations. The calculations show that heat conduction is responsible for the spreading of the electron temperature enhancement outside the region where the radio wave energy is deposited. They also show that a smaller, but experimentally observable, ion temperature enhancement takes place while electron temperatures are enhanced by 40% or more of their equilibrium values. For the data presented they also show ionospheric absorption of roughly half the radiated HF energy, with anomalous absorption roughly equal to deviation absorption heating.

Airglow enhancement observations have been considered as supporting evidence of electron acceleration in ionosphere heating experiments by high‐power HF waves. Here we analyze some of the 6300‐Å airglow data from the Platteville, Colorado, heating experiments of 1970, employing new electron impact excitation rates for the O(1D) state and empirical, but in accord with experimental and theoretical constraints, plasma heating rates and show that these airglow enhancements should be attributed to excitation by thermal electrons. An important aspect of the present analysis is the excellent agreement of the observed and the calculated airglow enhancements over several complete transmitter on/off cycles of several minutes duration and an increasing airglow trend of 1 hour duration. The fact that the OI red line may be thermally excited and the scarcity of observations of simultaneous OI red and green line enhancements imply that electron acceleration, even to a few eV, may require very special experimental and ionospheric conditions that are not very often realized.