Vertical Motion Field in the Middle Thermosphere from Satellite Drag Densities

The vertical motion field in the thermosphere is calculated from the continuity equation. This calculation ik based on a field of horizontal winds and an intermediate model of the thermospheric temperature field consistent with the density structure inferred from satellite drag data. The vertical motion consists of a component due to rise and fall of constant pressure surfaces and a component due to horizontal mass divergences, both components being of the order of 1 m. set.-' Only the latter component is of importance for thermodynamic considerations. The adiabatic warming associated with the diurnally variable part of the vertical motion due to mass divergence gives a second heat source which is of magnitude comparable to the heating by solar radiation. The time-averaged meridional circulation also implies large adiabatic warming and cooling. This computed mean meridional circulation cannot be reconciled with the heat balance of the thermosphere. The thermospheric temperature field at low levels in high latitudes can be changed so as to reverse the direction of the mean meridional pressure gradient and thus to give a mean meridional circulation consistent with heat balance considerations. Existing global thermospheric models could be improved by adjustment of the temperature field at low levels in such a way that vertical motions computed from horizontal winds give a plausible adiabatic heating field.