Analysis of the steady state available energy budget in the high-latitude lower thermosphere

We evaluate the budgets of available energy (AE) production, transport, and loss under steady state forcing of the high-latitude lower thermosphere during the southern summer time for weak or strong southward interplanetary magnetic field (IMF, B-z = -2.0 or -10.0 nT), using the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model. The AE below 150-km altitude is partitioned into commensurable reservoirs of kinetic energy (KE) and available potential energy (APE). KE density is larger than APE density above 123 km, but they are comparable below 123 km. Whereas Pedersen ion drag generates strong winds and KE, vertical winds and adiabatic cooling above 123 km tend to offset temperature perturbations caused by Joule heating, thereby reducing APE. Below 123 km Hall ion drag and associated vertical motions are important for creating temperature perturbations and APE. With increasingly negative IMF B-z values APE density intensifies more significantly than KE density above about 123 km, while KE density intensifies more significantly than APE density below. KE is generated primarily where the ion-drag force associated with plasma convection accelerates the neutral gas and is destroyed primarily where the ion-drag force opposes the wind. APE is generated primarily where Joule heat is deposited in regions of elevated temperatures and destroyed where the heat is deposited in regions of reduced temperatures. Ion drag is generally more important than Joule heating for generating AE for steady-state conditions, but the relative contribution of the ion-drag forcing compared with heating decreases with descending altitude. AE generation by Joule heating intensifies more significantly than generation by ion drag with increasingly negative IMF B-z values. Transport of AE by horizontal and vertical winds is a significant component of the AE budget. Conversion of APE to KE, and of KE to APE, constitutes an important part of their budgets.