The drag analysis of low-perigee satellites such as Injun 3 and Explorer 17 at a time of low solar activity has revealed appreciable fluctuations in upper-atmosphere temperature during geomagnetically undisturbed intervals. These temperature variations are very nearly proportional to the variations in the Kp index, in contrast with the temperature variations during magnetic storms, which show proportionality with the ap index. When Kp<3, the temperature increase corresponding to a unit increase in Kp is about 35°. This corresponds to a value of dT/dap close to 7°, six or seven times larger than that found in magnetic storms. Since the average value of Kp outside magnetic storms is about 2, the temperature on relatively undisturbed days must be about 70° higher than that corresponding to Kp = 0. This again raises the question whether no heat source other than solar EUV radiation is present when Kp = 0.
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Jacchia et al. (1964) studied this question.
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