Storm-Time Strip-Like Plasma Density Bulges at Middle Latitudes Shaped by Meridional Wind Gradients

Prior studies identified a fine structure in the middle latitude ionosphere known as the strip-like plasma density bulge. These bulges emerge during geomagnetic storms, exhibiting a broad longitudinal span of over 150° and a narrow latitudinal extent of 1°~5°. The observations from the DMSP and ICON satellites reveal stronger equatorward ion drifts and neutral winds on the poleward side of bulges compared to the equatorward side. Using the Sami2 is Another Model of the Ionosphere (SAMI2), the bulge feature was reproduced for the storm of 4~6 November 2021 by amplifying the default meridional winds. Numerical simulations indicate that global wind disturbances establish a sharp meridional wind gradient within the lower mid-latitude region. This gradient, in turn, drives a divergence in ion transport parallel and perpendicular to the magnetic field lines, which ultimately results in the localized accumulation of plasma. The phenomenon is most pronounced in the vicinity of ±30° quasi-dipole latitude. This region is characterized by a magnetic inclination angle of approximately 45°, a configuration where the meridional wind component acts most efficiently to elevate ions vertically.