Ionospheric Topside Diffusive Flux and the Formation of Summer Nighttime Ionospheric Electron Density Enhancement Over Millstone Hill

Ionospheric F-region electron density is anomalously higher in the evening than during daytime on many occasions summer geomagnetic mid-latitude regions. This unexpected ionospheric diurnal variation has been studied for several decades. The underlying processes have suggested to be related meridional winds, topside influx arising from sunset collapse, and other factors. However, substantial controversies remain unresolved. Using a numerical model driven by statistical O+ diffusive flux Millstone Hill incoherent scatter radar data, we provide new insight into competing roles of flux, neutral electric fields forming peak. Simulations indicate that while which turn equatorward before sunset, are essential sustain ionization near dusk, critically important formation timing