Different peak response time of daytime thermospheric neutral species to the 27�day solar EUV flux variations

Previous work suggested that the peak response time of the mass densities of atomic oxygen (O) and molecular nitrogen (N-2) in the thermosphere had more than a 1-day difference relative to the peak of the 27-day periodic variation of solar extreme ultraviolet (EUV) flux. In this study, we used the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) to explore the physical mechanisms responsible for the different peak response times of the daytime thermospheric neutral species. It was found that the peak response time of O or N(2)mass density corresponds to the time of equilibrium between the contributions from the barometric effect and the change in its abundance. The peak response time of O is shorter than that of thermospheric temperature Tn, due to a dynamic change in the circulation that acts to cancel out the contribution from the barometric process prior to the peak of Tn. On the contrary, the change of N(2)abundance contributes further to a decrease of N(2)mass density on a constant pressure surface when the thermosphere is expanding. The change of chemical loss leads to a longer peak response time of N(2)abundance than that due to barometric motion. Therefore, an equilibrium is reached after the barometric effect turns from expansion (contraction) to contraction (expansion), so that the peak response time of N(2)is longer than that of Tn. Moreover, the meridional circulation in the thermosphere modulates the latitudinal dependence of the peak response time of thermospheric neutral species.