Physical Processes Driving the Response of the F ₂ Region Ionosphere to the 21 August 2017 Solar Eclipse at Millstone Hill

The high-resolution thermosphere-ionosphere-electrodynamics general circulation model has been used to investigate the response of F ₂ region electron density (Ne) at Millstone Hill (42.61°N, 71.48°W, maximum obscuration: 63%) to the Great American Solar Eclipse on 21 August 2017. Diagnostic analysis of model results shows that eclipse-induced disturbance winds cause F ₂ region Ne changes directly by transporting plasma along field lines, indirectly by producing enhanced O/N ₂ ratio that contribute to the recovery of the ionosphere at and below the F ₂ peak after the maximum obscuration. Ambipolar diffusion reacts to plasma pressure gradient changes and modifies Ne profiles. Wind transport and ambipolar diffusion take effect from the early phase of the eclipse and show strong temporal and altitude variations. The recovery of F ₂ region electron density above the F ₂ peak is dominated by the wind transport and ambipolar diffusion; both move the plasma to higher altitudes from below the F ₂ peak when more ions are produced in the lower F ₂ region after the eclipse. As the moon shadow enters, maximizes, and leaves a particular observation site, the disturbance winds at the site change direction and their effects on the F ₂ region electron densities also vary, from pushing plasma downward during the eclipse to transporting it upward into the topside ionosphere after the eclipse. Chemical processes involving dimming solar radiation and changing composition, wind transport, and ambipolar diffusion together cause the time delay and asymmetric characteristic (fast decrease of Ne and slow recovery of the eclipse effects) of the topside ionospheric response seen in Millstone Hill incoherent scatter radar observations.