First‐principles physics of cusp/polar cap thermospheric disturbances

This first‐principles examination of physics driving the cusp/polar upper thermosphere response to significant input energy impulses discloses previously unappreciated factors essential to thermospheric input‐response relationships. The physics essential to coupling of cusp input‐response processes is detailed, to make previously unexplained up‐to‐doubling of air density and drag near 400 km not only understandable but expected, if not inevitable. Presented as a common natural consequence of magnetic reconnection near the magnetopause, this energy‐coupling from sun to upper atmosphere is through familiar processes, but by inadequately appreciated linkages. The underlying physics applies more broadly than this. We trace a logic path that should clarify the input‐response, and lay out a path which if followed should enable most existing time‐dependent 3‐D global thermospheric models to significantly improve the realism of their representation and prediction of cusp/polar thermosphere disturbances to transient energy sources. We illustrate the concept with a sample model‐run incorporating representative data.