Generation Mechanism of Planetary‐Wave‐Modulated Variations in the Sporadic E Layer Examined Using the GAIA‐Es Model

Abstract

This study elucidates the generation mechanism of planetary wave (PW) period variations in the sporadic E (Es) layer at low and middle latitudes. One of the causes of its day‐to‐day variations is wind variations owing to thermospheric PWs. Some mechanisms have been proposed for how PWs generate PW‐period variations in the Es layer, but there is still no common theory. In this study, we employed a three‐dimensional numerical model to assess two reasonable mechanisms for the generation of PW‐period variations in the Es layer. One theory considers that vertical ion convergence (VIC) variations owing to PWs generate the PW‐period Es layer variations in the Es layer formation. Another theory asserts that horizontal ion convergence (HIC) variations owing to PWs change background metal‐ion distributions, thereby generating PW‐period variations in the Es layer. Our simulation demonstrates that PW‐period variations in the Es layer are caused by PW‐modulated winds below 120 km, and the VIC driven by PWs is the more plausible cause than HIC. Both spatial and temporal variations in VIC by PWs are consistent with PW‐period variations in the Es layer, but those in HIC are not. Our result clarifies the generation mechanism of PW‐period variations in the Es layer and suggests the critical role of VIC in the day‐to‐day variability of the Es layer, especially in the winter.