New insights into the complex interplay between drag forces and its thermospheric consequences

Drag forces, ion and viscous, are evaluated as modifiers of global wind and temperature structure in the upper thermosphere, shedding new light on their relative roles in neutral dynamics and energetics. Exploiting the coupling of an ionosphere-thermosphere model, it is discovered that ion and viscous drag forces lead to sustained divergent winds, adjustments in mass, modification of pressure gradients, and a redistribution of the radiatively forced thermal energy. The interplay between the relative magnitudes of the ion and viscous drag forces and its effect on the ionosphere-thermosphere system has not yet been addressed and results in diverse behavior in the neutral wind and temperature structures of the upper atmosphere, dependent upon the type of drag acting on the gas. It is found that viscous drag is more efficient in cooling the dayside thermosphere and heating the nightside than the ion drag force in solar maximum and under quiet geomagnetic activity, resulting in a 150 K day-night temperature difference. The ion drag force inhibits this effective day-to-night energy circulation, culminating in a dynamically induced difference of about 400 K in the day-night temperature difference. It is demonstrated that the resultant wind and thermal structure greatly depends on the type of drag force environment, and a mechanism is introduced whereby ion and viscous drag forces can alter the energy budget of the upper thermosphere system. For example, in solar minimum, viscous drag is significant relative to other forces and more effectively cools the dayside and warms the nightside, thereby reducing the day-night temperature gradient.