Relative contributions of momentum forcing and heating to high‐latitude lower thermospheric winds

We discuss the significance of potential vorticity in the thermosphere and quantify the relative contributions of momentum forcing and heating to its total time derivative in the high‐latitude lower thermosphere during the southern hemisphere summertime for negative interplanetary magnetic field (IMF) B z conditions on the basis of numerical simulations. A term analysis of the potential vorticity equation for weak or strong southward IMF ( B z  = −2.0 nT or −10.0 nT) gives the following results: the ratios of the momentum forcing term to the heating term at 142, 123, and 111 km altitudes for IMF B z  = −2.0 nT are roughly 6:1, 4:1, and 2:1, respectively, indicating that the momentum forcing term makes the larger contribution to the total time derivative of the potential vorticity, although the relative contribution of the momentum forcing weakens with descending altitude. The ratios of the momentum forcing term to the heating term at 142, 123, and 111 km altitudes for IMF B z  = −10.0 nT are roughly 3:1, 2:1, and 1:1, indicating that, at higher altitudes, the momentum forcing term makes the larger contribution to the total time derivative of the potential vorticity, but the relative contributions of momentum forcing and heating are comparable at lower altitudes. A comparison of the heating term and the momentum forcing term for IMF B z  = −2.0 nT and IMF B z  = −10.0 nT conditions indicates that the heating term increases more significantly than the momentum forcing term as IMF B z becomes more negative.