High-latitude propagation studies using a meridional chain of LF/MF/HF receivers

摘要

For over a decade,Dartmouth College has operated programmable radio receiversat multiple high-latitude sites covering the frequency range100-5000kHz with about a 1-s resolution. Besides detectingradio emissions of auroral origin, these receiversrecord characteristics of the ionospheric propagation of natural andman-made signals, documentingwell-known effects, such as the diurnal variation in thepropagation characteristics of short and long waves, and alsorevealing more subtle effects.For example, at auroral zone sites in equinoctialconditions, the amplitudes of distant transmissions on MF/HF frequenciesare often enhanced by a few dB just before they fade away at dawn.The polarization and/or direction of the arrival of ionosphericallypropagating signals in the lower HF range (3-5MHz) show a consistentvariation between pre-midnight, post-midnight, and pre-dawn conditions.As is well known, magnetic storms and substorms dramatically affectionospheric propagation;data from multiple stations spanning the invariant latitude range67-79° reveal spatial patterns of propagationcharacteristics associated with magnetic storms and substorms.For example, in the hours preceding many isolated substorms,favorable propagation conditions occur at progressively lowerlatitudes as a function of time preceding the substorm onset.For some of these effects, explanations follow readily fromelementary ionospheric physics, but understanding others requiresfurther investigation.Key words. Magnetospheric physics (annual phenomena) –Radio science (ionosphere propagation; radio-wave propagation)6