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

Abstract

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.

The Effects of Vertically Propagating Tides on the Mean Dynamical Structure of the Lower Thermosphere

“…This suggests that there could be a net imbalance between how much eastward and westward momentum is deposited by the dissipating tides into the zonal-mean circulation (i.e., momentum flux divergence), and/or additional mechanisms (i.e., one other than direct tidal deposition of momentum) associated with tidal dissipation contributing to the zonal-mean zonal wind differences in Figure 1b. Above ∼200 km, vertical gradients in the zonal-mean zonal winds and zonal wind differences become much smaller due to the presence of exponentially increasing kinematic viscosity (Hsu et al, 2016); thus, our diagnosis of the zonal-mean momentum balance in the presence of dissipating tides is limited to below 200 km. Although the zonal-mean zonal winds in the lower thermosphere change seasonally (e.g., Figures 1 and S2) largely due to the spectrum of waves reaching the thermosphere, we will focus on diagnosing the mechanisms through which atmospheric tides influence the zonal-mean circulation in March when the tidally induced differences are largest (for reference the supporting information includes latitude-height cross sections of zonal-mean zonal wind results for other solstice and equinox seasons, Figure S2).…”

WITHDRAWN: Longitudinal variations of noontime thermospheric winds in response to IMF Bz temporal oscillations: broken mean circulation and standing feature

“…For instance, the pressure gradient, ion drag, viscosity, Coriolis, and momentum advection. Similar to Hsu et al (2016), we perform a term analysis of these forcing in meridional momentum equation. In this work, only the ion drag and pressure gradient are shown, because they are dominant over the other drivers.…”

A Comparison of Quiet Time Thermospheric Winds Between FPI Observations and Model Calculations

“…There are a number of processes that drive the global circulation in the thermosphere, including pressure gradients and ion drag. Hsu et al () showed that ion drag could have a significant effect on global circulation. Without ion drag, the wind circulation is much stronger and the day‐to‐night temperature difference is smaller.…”