Delay of the thermospheric wind response to a sudden westward turning of the ionospheric velocity at a pseudo breakup and irresistible force of the thermosphere in the energy dissipation process

Thermospheric wind response at F-region altitude to a sudden westward turning of the ion velocity at high latitude was studied by analyzing data obtained during a conjunction event with Fabry-Perot interferometer (FPI; 630 nm), Dynasonde and Swarm A/C satellites in northern Scandinavia. The event was found during a period of geomagnetically quiet conditions with Kp of 0 to 1 through the night, but some auroral activity in the north. From the Swarm measurement, the ionospheric trough was identified at 70-72N. The collocated FPI and Dynasonde measured thermospheric winds (U) and ionospheric plasma velocities (V), respectively, at the equatorward edge of the trough. A notable scientific message that was revealed by this study is a possible role of the thermospheric wind in the energy dissipation process. Negative UV indicates that kinetic energy of the thermosphere is dissipated into the ionosphere by particle collisions, and this can occur when the thermospheric wind is not able to follow instantly a sudden V change due to inertia. At a pseudo breakup during the conjunction event, the Dynasonde-measured V suddenly changed the direction from eastward to westward within 10 min. The FPI-measured U was also accelerated westward after the pseudo breakup, but its development was more gradual than that of V, so that U remained eastward for a while after the pseudo breakup. During this transition interval of about 10 min, UV was negative. An irresistible force of the thermosphere such as seen in this study should be found frequently at high latitudes because sudden direction change in V is a typical ionospheric feature at the substorm onset. Sign of UV may be used as the indicator to find time and location where the thermospheric inertia plays a role in the energy dissipation process.