Energy Flow in the Polar F-region Ionosphere-Thermosphere System through an Indirect Momentum-Energy Coupling Mechanism

Abstract:

Thermospheric neutral properties, such as wind, temperature, and neutral mass density, are driven by momentum and energy sources. Much work has focused on the response of the thermosphere when driven by energy sources from the magnetosphere, resulting in direct heating of the neutral gas. This leads to a dynamical response of the thermosphere described as a direct wind circulation. However, sources that change the momentum transfer to the neutral gas can also lead to thermal changes through a dynamical response that would be characterized as an indirect wind circulation. The ionosphere-thermosphere (I/T) system is tightly coupled by momentum and energy. The path for a direct energy source is to change the energy and then the momentum, while the route for an indirect energy source is to change the momentum and then the energy. For the indirect energy source, changes in the ion drag force cause a divergence in the neutral wind field, which results in thermal changes due to adiabatic heating or cooling via vertical winds. All of the important forces and processes exist for describing both the direct and indirect energy sources, but it is the time evolution of the processes that will define how the thermosphere responds. In this work, we present an indirect energy mechanism that can alter the thermospheric neutral mass density and temperature in the polar F-region. Similar to energy partitioning within the high-latitude I/T from an external energy source, energy can be transferred from kinetic to internal energy from a momentum source. We use the NCAR TIEGCM to analyze quantitatively how energy flows through the polar I/T system due to an internal momentum modification. These results illustrate how changes in the ion drag force can affect the thermal energy of the polar F-region I/T system, and create neutral mass density and temperature anomalies.