The formation and characteristics of E-region thin ionization layers at high latitudes

Abstract

Thin layers of ionization are often observed in the E region ionosphere. The layers are 1 to 3km in thickness and are of high density, often as high as $5\times10\sp5$ cm$\sp{-3}$. The density of the layers is a large enhancement over the background density. The layers are primarily composed of metallic ions, which have long lifetimes and can accumulate to high densities. Thin ionization layers at mid-latitudes are formed by the action of neutral wind shears redistributing the background ionization. At high-latitudes wind-shears are not as effective at the redistribution of ionization, and it is found that the high-latitude electric fields may cause layer formation. Here the mechanism for high-latitude layer formation is studied through numerical simulation and incoherent-scatter radar observations. A one-dimensional simulation examined high-latitude layer formation in detail, in particular it examined the effects of the direction of the electric field. It was found that thin ionization layers formed for electric fields directed in the north-west and south-west quadrants. The layers formed by electric fields in the south-west quadrant form at altitudes which are consistent with observations, while fields in the north-west quadrant resulted in layer altitudes which were higher than usually observed. It was also found that the neutral wind acting in concert with the electric field may affect the altitude and thinness of layers. The one-dimensional simulation was extended to a three-dimensional model of the polar cap ionosphere. The three dimensional simulation showed that large areas of thin ionization layers may form for widely varying geophysical conditions. Incoherent-scatter radar observations showed the presence of thin ionization layers on 12 out of 16 nights of observation. High-resolution spectral data showed that the average ion mass within the layers was higher than that in the background ionization, demonstrating the presence of heavy metallic ions. Concurrent observation of thin layers and electric fields showed layers present for field directions between 40$\sp\circ$ and 140$\sp\circ$ west of magnetic north, which agrees with the simulation however the range of angles is more limited than was predicted. Antenna scanning observations examined the latitudinal extent of the layers, and found the layers were of a limited extent; the largest extent observed was about 200km.