A simulation study on the time delay of daytime thermospheric temperature response to the 27�day solar EUV flux variation

In this study, the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamic General Circulation Model is used to explore the physical mechanisms of the time delay of daytime thermospheric temperature response to the 27-day solar extreme ultraviolet (EUV) flux variation. The contributions from different physical processes to the time delay of thermospheric temperature response, the peak perturbation of the thermospheric temperature with respect to the peak of 27-day solar EUV flux variation, were examined by diagnostically analyzing the thermospheric energy equation in a fixed local time frame. It was found that the time delay of the thermospheric temperature response to the 27-day solar EUV flux variation corresponds to the time needed for the total heating and cooling rates to be balanced. This time is slightly later than the times of the peaks of both the heating and cooling rates. Furthermore, the global circulation in the upper atmosphere plays a significant role in the delayed response of thermospheric temperature to solar EUV flux variations. Our simulation results show that the time delay of thermospheric temperature response to the 27-day solar EUV flux variation is about 0.5-0.8 day, depending on the amplitude of solar EUV flux variations.