June 2, 2017, 1:00 pm - 2:00 pm

June 2, 1:00 pm - 2:00 pm

Middle atmosphere dynamical sources of the semiannual oscillation in the thermosphere and ionosphere

McArthur Jones Jr. (NRC Postdoc/NRL), with John T. Emmert, Douglas P.Drob,and David E.Siskind

The thermosphere exhibits intra-annual variations (IAVs) in globally averaged mass density that noticeably impact the drag environment of satellites in low Earth orbit. Particularly the strong global semiannual oscillation (SAO) in thermospheric density has been observed for five decades, but definitive knowledge of its source has been elusive. Several mechanisms have been proposed to explain the SAO, but it has yet to be reproduced by first-principles modeling simulations. The leading hypothesis is that IAVs in lower thermospheric turbulent mixing (associated with breaking gravity waves) modulate upper thermospheric composition. However, less attention has been paid to the effect of lower and middle atmospheric drivers on the lower boundary of the thermosphere. Hence, we use the National Center of Atmospheric Research thermosphere-ionosphere- mesosphere electrodynamics general circulation model (TIME-GCM) to study how middle atmospheric dynamics generate the SAO in the thermosphere-ionosphere (T-I). The 'standard' TIME-GCM simulates, from first principles, SAOs in thermospheric mass density and ionospheric total electron content that agree well with observed climatological variations. Diagnosis of the globally averaged continuity equation for atomic oxygen ([O]) shows that the T-I SAO originates in the upper mesosphere, where an SAO in [O] is forced by nonlinear, resolved -scale variations in the advective, net tidal, and diffusive transport of O. Contrary to earlier hypotheses, TIME-GCM simulations demonstrate that intra-annually varying eddy diffusion by breaking gravity waves may not be the primary driver of the T-I SAO: A pronounced SAO is produced without parameterized gravity waves.