Dynamics of High‐Latitude Energy Conversion in the Upper Thermosphere Based on 9‐Year Measurements From FPI and Dynasonde in Tromsø, Norway

Abstract

This study highlights the need to re‐evaluate energy transfer in the upper thermosphere at high latitudes through analysis of 9‐year measurements of neutral wind ( u ) from a Fabry‐Perot interferometer and ion velocity ( v ) from a Dynasonde in Tromsø, Norway. This study introduced a ratio R (thermospheric energy conversion ratio) of the Joule heating rate, j · E ′ , to the electromagnetic energy transfer rate, j · E , that is, j · E ′ / j · E , where j and E are the current density and the electric field, respectively, and E ′  =  E  +  u  ×  B . Evaluating the energy transfer process using the ratio R does not require ionospheric conductivity information, which is an advantage of this method. This study found that the sign of u · v is associated with the energy transfer process at F‐region altitudes. In the dusk‐midnight sector, we found 0 <  R  < 1 under moderately active conditions, indicating that electromagnetic energy is converted to thermospheric thermal energy through Joule heating and kinetic energy via wind acceleration by the Lorentz force. R  < 0 occurred under quiet conditions throughout the night and at post‐midnight across geomagnetic activity levels, indicating the Lorentz force performs negative work on the thermosphere. R can exceed 1 under quiet dusk‐to‐dawn and moderately active post‐midnight, with all instances of u · v  < 0 being classified as R  > 1. During evening and morning with u · v  < 0, sunward flow dominates ionospheric plasma convection, causing ion drag to oppose the day‐night pressure gradient. However, ion drag cannot overcome the pressure gradient due to low geomagnetic activity, resulting in opposing thermospheric wind and ionospheric plasma convection directions.