Three‐dimensional simulation of the coupled Perkins and <i>E</i>_s‐layer instabilities in the nighttime midlatitude ionosphere

<jats:p>Plasma density structures and associated irregularities in the nighttime midlatitude ionosphere are frequently observed as frontal structures elongated from northwest to southeast (NW‐SE) in the Northern Hemisphere. The frontal structures and the coupling process between the <jats:italic>E</jats:italic> and <jats:italic>F</jats:italic> regions are studied with a three‐dimensional numerical model, which can simulate two instability mechanisms: Perkins instability in the <jats:italic>F</jats:italic>‐region and sporadic‐<jats:italic>E</jats:italic> (<jats:italic>E</jats:italic><jats:sub>s</jats:sub>)‐layer instability in the <jats:italic>E</jats:italic> region. The fastest growth of the coupled instability occurs when the unstable conditions on NW‐SE perturbation are satisfied in both regions. The perturbation of <jats:italic>F</jats:italic>‐region integrated conductivity grows much faster than the isolated Perkins instability. The meridional component of a rotational wind shear blows an existing <jats:italic>E</jats:italic><jats:sub>s</jats:sub> layer southward, and the <jats:italic>F</jats:italic>‐region structure follows the <jats:italic>E</jats:italic>‐region drift velocity. The NW‐SE structure in the <jats:italic>E</jats:italic> region can be formed from random perturbation regardless of the <jats:italic>F</jats:italic>‐region condition. When the <jats:italic>F</jats:italic> region is unstable on the NW‐SE perturbation, however, the NW‐SE structure is formed in both regions with a common scale length. We conclude that (1) the <jats:italic>E</jats:italic><jats:sub>s</jats:sub>‐layer instability plays a major role in seeding NW‐SE structure in the <jats:italic>F</jats:italic> region, and the Perkins instability is required to amplify its perturbation; (2) the rotational wind shear in the <jats:italic>E</jats:italic> region produces southwestward phase propagation of the NW‐SE structure in both the <jats:italic>E</jats:italic> and <jats:italic>F</jats:italic> regions; and (3) the coupling process has a significant effect on the scale of the <jats:italic>E</jats:italic><jats:sub>s</jats:sub>‐layer perturbation rather than the growth rate of the <jats:italic>E</jats:italic><jats:sub>s</jats:sub>‐layer instability.</jats:p>