HEAT-BUDGET OF THE THERMOSPHERE AND TEMPERATURE-VARIATIONS DURING THE RECOVERY PHASE OF A GEOMAGNETIC STORM

Poststorm temperature variations have been simulated by using a two-dimensional, time-dependent model of coupled dynamics and composition. The numerical simulation results have been analyzed to obtain the characteristic time scale of the temperature relaxation as a function of pressure level and latitude. The relaxation time was defined by the e-folding time of temperature variations in response to the cessation of geomagnetic activity, specified by a decrease in the TIROS/NOAA power index level. The time variation of the power index level was described by a step function. The causal mechanisms responsible for the recovery of the perturbed temperature have been investigated from a term analysis of the energy conservation equation. The relative importance of the different terms of the energy conservation equation has been evaluated from the solutions of model equations as a function of time. By using the calculated individual heating/cooling rates, a time constant is derived from the heat balance between the major heating/cooling rates at a particular simulation time as a function of time. The principal results include the following: (1) the relaxation time of the temperature is in the range from 12 to 15 hours in the upper thermosphere around 350 km and in the range from 15 to 21 hours in the lower region around 150 km; (2) the relaxation time of the temperature is longer within the source region at high latitudes than at middle and low latitudes; (3) the time constants derived from the heat balance at each simulation time increase with time, particularly at high latitudes; (4) the major cooling mechanism in the upper thermosphere is the downward molecular heat conduction; (5) at lower altitudes the radiative cooling from the 5.3-mum nitric oxide (NO) band is dominant, but the vertical heat advection is also important because of the vertical winds associated with the meridional circulation; and (6) the vertical winds in the lower region also contribute to the adiabatic heating at high latitudes and to the adiabatic cooling at middle latitudes. The relaxation of the temperature during the recovery phase of a geomagnetic storm is not determined solely by the local heat balance but is also influenced by the global dynamical and thermal coupling between various regions of the thermosphere.