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

J Labelle

doi:10.5194/angeo-22-1705-2004

Abstract

Abstract. For over a decade, Dartmouth College has operated programmable radio receivers at multiple high-latitude sites covering the frequency range 100-5000kHz with about a 1-s resolution. Besides detecting radio emissions of auroral origin, these receivers record characteristics of the ionospheric propagation of natural and man-made signals, documenting well-known effects, such as the diurnal variation in the propagation characteristics of short and long waves, and also revealing more subtle effects. For example, at auroral zone sites in equinoctial conditions, the amplitudes of distant transmissions on MF/HF frequencies are often enhanced by a few dB just before they fade away at dawn. The polarization and/or direction of the arrival of ionospherically propagating signals in the lower HF range (3-5MHz) show a consistent variation between pre-midnight, post-midnight, and pre-dawn conditions. As is well known, magnetic storms and substorms dramatically affect ionospheric propagation; data from multiple stations spanning the invariant latitude range 67-79° reveal spatial patterns of propagation characteristics associated with magnetic storms and substorms. For example, in the hours preceding many isolated substorms, favorable propagation conditions occur at progressively lower latitudes as a function of time preceding the substorm onset. For some of these effects, explanations follow readily from elementary ionospheric physics, but understanding others requires further investigation.Key words. Magnetospheric physics (annual phenomena) – Radio science (ionosphere propagation; radio-wave propagation)6

Highlights

For over a decade, Dartmouth College has operated programmable radio receivers at high-latitude sites, in an effort to systematically study radio emissions of auroral origin
In addition to tracking the latitude dependence of auroral absorption over several days during magnetic storms, the meridional chain of radio receivers effectively measures spatial and temporal variations in radio-wave propagation conditions associated with substorms over shorter intervals, albeit with relatively crude latitude resolution implied by five observatories covering 13 degrees of latitude
Many shorter time scale ionospheric effects are detectable in LF/MF/HF radio propagation data at multiple high-latitude sites

Summary

Instrumentation

The receivers used in this study involve one of two antenna types: electric or magnetic dipoles In both cases, the antenna is nonresonant at all frequencies detected. The magnetic dipole consists of a 10-m2 magnetic loop antenna oriented vertically at an angle which nulls out the strongest local interference signal. In both cases, active preamplifiers at the antenna couple the signals to coaxial cables for transmission to the receiver in a building several hundred feet away. At Churchill during 1997–1998, two magnetic loop antennas in the vertical plane oriented perpendicular to each other were deployed, and the phase between them recorded at each frequency, to determine wave polarization as described. Detection of the polarization of whistler mode auroral hiss, which is known to be right-hand polarized, confirms that the technique works (Shepherd et al, 1997)

Diurnal variations of propagation characteristics

Terminator effects on radio-wave propagation

Magnetic storm and substorm effects

Conclusions