To determine the approximate temperatures of free electrons in the sunlit ionospheric D region an experiment was conducted at a site near College, Alaska, during the summer and fall of 1960. A dual polarized antenna whose beam was directed at the zenith was used to observe alternately the incident radio noise fluxes in the ordinary and extraordinary polarizations at the frequency 2.89 MHz. The 2.89-MHz radio noise flux measured on a vertical dipole antenna was used to check for propagated interference from low elevation angles. The results show that, during the five-month observing period, the extraordinary-mode antenna registered a median daytime temperature of 260°K, with a standard deviation of about ±23°K, which indicates that the electrons in the D region are approximately in thermal equilibrium with the neutral gas, even at the auroral zones. The theoretically anticipated antenna temperatures, based on assumed ionospheric parameters, agree fairly well with the observed experimental values. In particular, the observed variations of antenna temperature with solar zenith angle can be matched by use of reasonable theoretical models.
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