Midlatitude F-region densities and temperatures at sunspot minimum

Abstract

Abstract Results obtained for F-region densities and temperatures using the Millstone Hill Ionospheric Radar for the year 1964 are presented. The measurements were made during 30-hr periods which followed at intervals of about 2 weeks throughout the year. The data obtained in each month have been averaged to yield a mean electron density profile and mean ion and electron temperature curves for each hour in the day. These curves were in turn used to construct plots showing contours of constant temperature and density as functions of altitude and time for each month. In addition, the annual variation of the midday and midnight densities and temperatures was obtained. The seasonal anomaly in the F-region peak electron density is evident though less pronounced than at years of high sunspot number. The F-region layer thickness undergoes a smooth transition from winter to summer, being greatest in summer. It is shown that the daytime temperatures exhibit no marked seasonal dependence, and hence temperature effects (e.g. on reaction rates) cannot be invoked as the cause. The most striking nighttime phenomenon is the high electron temperature encountered in winter (Te/Ti ∼ 2.0) compared with summer. Evidence is presented for the existence of a flux of fast photoelectrons arriving from the conjugate ionosphere. The nighttime heating of the F-region is thought to be caused by heat conducted down from the protonosphere together, in winter months, with the flux fast photoelectrons from the conjugate hemisphere.