Improved Millstone Hill exospheric temperature measurements: Evidence for a seasonal variation of the magnetic activity effect

An analysis has been performed of a new collection of exospheric temperature data, gathered by using the ‘two‐pulse’ incoherent scatter radar experiment at Millstone Hill. The results are compared with previously reported ‘one‐pulse’ data from Millstone Hill and related to differences previously noted between these one‐pulse and Saint Santin results. Implications of these differences for current thermospheric models also are discussed. The two‐pulse data appear to be of improved accuracy in relation to the one‐pulse data. Both the two‐pulse data and a recent revision of the older one‐pulse results imply that the previously reported one‐pulse exospheric temperatures were too low by about 50 K, on average. Otherwise, the two‐pulse data provide generally improved agreement with the seasonal and diurnal variations observed at Saint Santin. The primary exception to this rule is that the two‐pulse data confirm a larger seasonal variation magnitude at Millstone than at Saint Santin. It is shown that this difference might be accounted for by a strong seasonal dependence in the magnetic activity effect at the Millstone location. The two‐pulse data indicate that the temperature response to magnetic activity is four times stronger in summer than in winter at the Millstone location. This type of variation is not included in current thermospheric models.