Sunrise behavior of theFlayer at midlatitudes

Measurements of the F-layer electron and ion temperatures during the predawn and sunrise periods for February and March 1967 are presented. These were obtained, using the Millstone Hill Thomson scatter radar (42.6°N 71.5°W). By limiting the observations to a fixed altitude (375 ± 40 km), it has been possible to make repeated measurements at short time intervals (6 minutes). From these data we have been able to recognize instances of conjugate point heating of the local ionosphere. These are observed in March (and October) but give rise to only small temperature increases in contrast to stations at lower latitudes. This difference is attributed to the lower rate at which the sun rises at the conjugate point and the high predawn values of electron temperature (and hence thermal conductivity), maintained in the local ionosphere by the cooling of the protonosphere. The rapid increase in electron temperature at local sunrise has been examined and compared with the behavior of the electron concentration at this time. It is found that initially the electron temperature rises rapidly and that the electron concentration decreases. The electron concentration begins to rise only after the electron temperature has been rising for about 30 minutes. This behavior lends support to the idea that F-region loss rates increase rapidly with increasing electron temperature. Based upon this dependence and the effects of conjugate point heating, it becomes possible to explain two additional sunrise anomalies observed at midlatitudes, namely (1) the seasonal variation of dNmax/dt, and (2) the seasonal variation in the delay that follows ionospheric sunrise before Nmax begins to increase.