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

Abstract

AbstractA 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.