Theoretical model simulations for an intense magnetic storm show the creation of a low-latitude electron density arch aligned along the geomagnetic field created by strong uplift of the F-2 layer that is driven by the penetration electric field. When the arch forms during the day, a new F-2 layer is created at the original altitude by photoionization, and a density hole can be created between this new F-2 layer and the arch. When the arch forms during the night, the F-2 layer is not recreated and no hole forms. In a vertical profile of electron density, the daytime elevated ionospheric layer can appear distinctly from the recreated F-2 layer, in which case the elevated layer is called the F-3 layer. A latitude cut through the night-side arch shows the characteristics of an equatorial electron density trough, bounded to the north and south by enhanced densities associated with plasma that has diffused down along the geomagnetic field from the elevated layer. It is pointed out that the signature of the night-side arch has been seen in low Earth orbit spacecraft data, but has not been previously associated with an arch structure.
