Viscosity and the vertical profile of horizontal thermospheric winds

It is generally assumed that viscosity smoothes out all vertical gradients of the horizontal thermospheric winds above about 150 km, and thus observations of neutral winds at one height can be used at other altitudes in the thermosphere. In this paper we present neutral wind simulations of the May 1997 geomagnetic storm using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. We found that: 1) there are significant variations in the vertical profiles of horizontal neutral winds above 150 km, indicating that previous understandings of vertical gradients of horizontal winds need to be revisited; 2) viscosity is less important when compared with other forcing processes during the storm main phase; and 3) there are strong temporal and spatial changes in both neutral winds and forcing terms, and the relative importance of viscosity in the neutral momentum balance increases during the storm recovery phase.