Alignment of high‐latitude ionospheric and thermospheric Lagrangian coherent structures

The storm-time ionospheric-thermospheric (IT) state is of great interest, since the IT dynamics change dramatically as energy is input and dissipated in the upper atmosphere. Lagrangian coherent structures (LCSs), which are objective ridges in time-evolving flows that describe the tendency of neighboring fluid elements to separate, provides a unique opportunity to infer the dynamics in the IT system. In this work, we model IT flows with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model and identify the LCSs. We compare the LCSs in the neutral winds and plasma drifts during quiet times versus during active times. We find that LCSs are largely aligned in the modeled IT flows, with a dawn-dusk asymmetry in their latitudinal position. During a geomagnetic storm, the thermospheric LCSs (T-LCSs) and ionospheric LCSs (I-LCSs) shift equatorward, align more closely with each other, and maintain a dawn-dusk asymmetry. The collocation of T-LCSs and I-LCSs and their analogous response to the geomagnetic storm provide evidence of energy input into the thermosphere and ionosphere simultaneously, and the ion drag is the dominant effect causing LCS alignment during a geomagnetic storm.