Results of the HF forward and backscatter program at College since 1963

Abstract

This report describes the results of the HF backscatter and forward sounding project conducted at College, Alaska, from 1963 to 1966. The primary finding was that with the exception of slant-F, meteor and groundscatter echoes, the oblique backscatter echoes observed from the auroral zone and polar cap ionospheres were produced by the aurora. When combined with satellite data, this result led to the conclusion that the auroral precipitation markedly increases the electron density at all ionospheric heights, and an auroral model incorporating this notion explains most of the observed details of auroral backscatter, and of auroral sidescatter on forward-oblique paths. The absence of groundscatter from beyond the auroral belt is explained as being due, in part, to the enhanced ionization of the E region by the auroral precipitation. Aspect-sensitive auroral backscatter can be obtained from the south as well as from the north in the northern hemisphere. A single sounder installed at each geomagnetic pole could provide real-time information on the position of the auroral precipitation completely about the pole. Auroral morphology studies made from radio data are independent of the weather and the season. An examination of a year’s data shows that no marked difference exists between the behavior of the summer and winter positions of the auroral precipitation; the northern limit of the south boundary of the aurora during the day was found to be approximately 1.5° geomagnetic latitude north of that during the winter. The radio data for 1965 agrees closely with published optical data for the period of 1957 to 1959 when the level of magnetic activity is considered. This finding shows that the position of the southern boundary of the auroral precipitation for a given level of magnetic activity does not depend appreciably upon the level of solar activity. The excellent correspondence between the optical and radio determinations of the auroral position and occurrence rates is added evidence that the radio method provides reliable auroral tracking information.