An explanation for type 1 radar echoes from the midlatitude E‐region ionosphere

Abstract

Type 1 radar echoes from the midlatitude E‐region are comparatively rare, but we now have many nighttime examples. These observations imply that large electric fields, of the order of 15 mV/m or more, must exist at times in the plasma, even during quiet magnetic conditions. We suggest here that such fields can be generated by the same polarization process as at the magnetic equator, but with the geometry “turned on its side.” That is, we assume that there are sharp horizontal conductivity gradients associated with patchy nighttime metallic ion layers that play the same role that vertical gradients play at the equator. Sharp gradients in the zonal direction, particularly, could lead to polarization fields more than an order of magnitude greater than the ambient dynamo fields and sufficient to excite the two‐stream instability implied by the radar data. Only a small percentage of the metallic ion patches are like to have the right geometry and be at the right altitude, however, which perhaps explains why the type 1 echoes are seen only occasionally.