Computer simulation of polarization electric fields as a source of midlatitude field‐aligned irregularities

Abstract

The generation of quasi‐periodic (QP) echoes and the associated polarization electric fields in the midlatitude E‐region are considered by using a computer simulation. It is shown that the horizontal structure of a sporadic‐E (Es) layer plays an important role in the generation of polarization electric fields. Where rod‐like enhancements of plasma density exist in an Es layer, a large polarization electric field can be generated by an ambient zonal electric field or meridional neutral wind. The generated polarization electric fields map along the geomagnetic field up to the higher E‐region and form a field‐aligned plasma density structure. The growth rate of the gradient‐drift instability becomes positive up to an altitude of 120 km. Meridional neutral winds can also produce striated QP echoes as a trace of the field‐aligned structure. Even if the Es layer is uniform, a polarization electric field is generated by atmospheric gravity waves, especially those propagating northward, which support the resemblance of a period and a wavelength between the gravity waves and the QP echoes. The large polarization electric fields associated with the Es layer are thought to be a common phenomenon in the midlatitude and play a key role in the generation of the QP echoes.