Spin-up circulation of high-latitude ion drag-driven gyres

We have used a model of thermospheric gyres with simplified geometry (azimuthally symmetric cylindrical coordinate) to study dynamical adjustment for high‐latitude gyres spun‐up into rapid motion by ion drag. In our simulations, winds are spun‐up for an hour subject only to circumgyre ion drag forcing from strong radial electric fields with peak values of ±75 mV m−1 centered at a radial distance of 1500 km. The winds in the core of the jets approach 500 m s−1. The major finding is that the imbalance between the inertial forces (centrifugal and Coriolis) and the pressure gradient force during the spin‐up of high‐latitude gyres drives a strong radial circulation, and this circulation is a significant contributor to the radial circulation forced by all sources. This agradient circulation attempts to establish a gradient‐wind balance between the inertial and pressure gradient forces. A feature of general circulation model simulations of the high‐latitude lower thermosphere is that, subtracting the warming of the whole polar cap due to Joule heating, the counterrotating gyres driven into motion by ion convection have opposite thermal polarities, with the cyclonic gyre developing a cold core (low density) and the anticyclonic gyre a warm core (high density). We suggest that this is accomplished by the radial circulation forced by the aforementioned imbalance between the inertial and pressure gradient forces. While diabatic heating over the polar cap acts to elevate the temperature (raise the density) over the polar cap as a whole, the changes induced by the dynamically induced circulation account for the fact that cyclonic gyres in the lower thermosphere are relatively colder and denser and the anticyclonic gyres are relatively warmer and less dense. We also examine the radial circulation forced by the initial stages of spin‐down and show that the spin‐down circulation is significant.