Storm time global thermosphere: A driven-dissipative thermodynamic system

Orbit-averaged mass densities $\overline{\rho and exospheric temperatures $\overline{T _∞ inferred from measurements by accelerometers on the Gravity Recovery and Climate Experiment (GRACE) satellites are used to investigate global energy E_th and power Π_th inputs to the thermosphere during two complex magnetic storms. Measurements show $\overline{\rho, $\overline{T _∞, and E_th rising from and returning to prevailing baselines as the magnetospheric electric field $\varepsilon$ _VS and the Dst index wax and wane. Observed responses of E_th and $\overline{T _∞ to $\varepsilon$ _VS driving suggest that the storm time thermosphere evolves as a driven-but-dissipative thermodynamic system, described by a first-order differential equation that is identical in form to that governing the behavior of Dst. Coupling and relaxation coefficients of the E_th, $\overline{T _∞, and Dst equations are established empirically. Numerical solutions of the equations for $\overline{T _∞ and E_th are shown to agree with GRACE data during large magnetic storms. Since $\overline{T _∞ and Dst have the same $\varepsilon$ _VS driver, it is possible to combine their governing equations to obtain estimates of storm time thermospheric parameters, even when lacking information about interplanetary conditions. This approach has the potential for significantly improving the performance of operational models used to calculate trajectories of satellites and space debris and is also useful for developing forensic reconstructions of past magnetic storms. The essential correctness of the approach is supported by agreement between thermospheric power inputs calculated from both GRACE-based estimates of E_th and the Weimer Poynting flux model originally derived from electric and magnetic field measurements acquired by the Dynamics Explorer 2 satellite.