A three-dimensional, time-dependent simulation of the global dynamical response of the thermosphere to a geomagnetic substorm

Abstract

Some of the major features of the atmospheric circulation predicted to occur following a geomagnetic substorm in the earth's thermosphere are described on the basis of a three-dimensional, time-dependent simulation. The circulation changes and the gravity wave oscillatory phenomena are traced at two levels of the thermosphere, 120 and 240 km, over a period of 4 1 2 h following the substorm onset. For both aspects of the response, and for each height, the characteristics, magnitudes and propagation are quite distinct, and the three-dimensional model shows that very large differences occur between one local time and another. The night-time period, and particularly that between 0000 and 0600 LT, shows, at middle latitudes, the largest wind and wave amplitudes, and the most complex response. In general, the response at any location to an individual substorm can be considered as a series of wave-like disturbances. This is due to the nature of the impulsive disturbance which is concentrated in an annular ring corresponding to the auroral oval, of which each part generates both poleward-travelling as well as equatorward-travelling components, with approximate circular symmetry. Observing the response at 240 km, it appears that focusing occurs of the initial poleward wavefront, and in subsequent poleward wavefronts, the latter arising from the apparent ‘reflection’ of waves initially propagating sunward as they interact with the mean anti-sunward flow on the dayside. Three major waves can therefore be identified in the night-time mid-latitude thermosphere—the initial wave from the nearby auroral oval, a second wave following about 90 min later originating in the dayside auroral oval. This wave is stronger than the first and, being focused and guided by the enhanced mean anti-sunward flow, is capable of generating larger waves. About another 90 min later a third wave arrives due to the dayward propagating wave being reflected, refocused and guided by the mean flow, which can also cause comparable disturbances throughout the thermosphere. There is a strong enhancement of the contra-rotating vortices of the high latitude region—providing sunward flow in the morning and evening auroral ovals with a strong return anti-sunward flow over the polar cap, which extends well into the mid-latitude regions in the midnight and early morning period. In general, amplitudes of the wind and wave disturbances increase continuously with altitude above about 100 km, even allowing for the omission of propagating tidal winds from our model (generated by the lower atmosphere). For the modest disturbance simulated here—a substorm we consider typical of a period when K p ~ 6-winds of 200–250 m s −1 occur at 120 km, increasing to 500 m s −1 at 240 km, and near 400 km the winds exceed 600 m s −1. While at 240 km the response is dominated by oscillations superimposed on an enhancement of the general thermospheric circulation, at 120 km there is strong evidence that two long-lasting vortices may be created by the dynamical and energetic effects of the substorm, while there is yet no direct observational counterpart of such vortices—an anticyclone centred between 2200 and midnight, and a cyclone centred between 1000 and midday both in the auroral oval—they may be quite characteristic features of the slow recovery of the thermosphere from the impulsive effects of the additional energy input during geomagnetic substorms.