Temporal Evolution of Three-Dimensional Structures of Metal Ion Layer Around Japan Simulated by a Midlatitude Ionospheric Model

A regional numerical ionospheric model with neutral winds, corresponding to the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy model, was used to investigate the temporal evolution of 3-D structures of metal ion layers (MILs) around Japan. The MILs that appear specifically in the ionospheric E region, called "sporadic E," display complicated multi-layer structures and intense density variations. Although the wind shear theory elucidates the basic formation mechanism of MILs, it does not fully explain the physical mechanism of 3-D MIL structures, especially their sporadic behavior. Herein, we present two simulation cases in which the effect of the monthly mean and day-to-day variations of wind on the MILs around Japan was investigated. The temporal evolutions of the complicated MIL structures were classified into the following four phases: (1) Above 110 km, MILs are generally constrained vertically and horizontally at the zonal-wind shear null. (2) Below 110 km, MILs lag behind the zonal-wind shear null, and the 3-D MIL structures are affected not only by the wind shears but also by the magnitude and direction of the horizontal winds. (3) When stagnating metal ions exist below a descending MIL, some metal ions ascend and merge with the descending MIL, thereby increasing its density. (4) MILs stagnate at around 100 km or descend to <100 km depending on the strength of the vertical winds. Our results reveal that Phases (2-4) are crucial for the formation of complicated MIL structures.