Survey of Polar and Auroral Region Effects on HF Propagation

Abstract

The effects of the auroral and polar zone ionosphere on forward propagation in the HF through low‐VHF (3 to ∼30 MHz) portion of the spectrum are reviewed. Phenomena that particularly affect the propagation of HF/VHF signals in this region include: the non‐great‐circle (NGC) mode, sporadic‐E ionization, F1‐layer effects, F‐region irregularities, D‐region absorption, and reflectivity properties of the polar ice caps. Specifically, we show that: (1) The NGC mode can cause deviations from the great‐circle path up to 90° and can carry the maximum frequency on a polar circuit. (2) Sporadic‐E ionization occurs primarily during the winter‐night period and is sometimes associated with the aurora. (3) F1‐layer effects are quite important and frequently determine the dominant propagation mode during summer mornings at sunspot minimum. (4) Auroral zone absorption is quite ‘patchy’ in spatial and temporal behavior and probably attenuates HF signals up to 60 db in severe cases. (5) Polar cap absorption, although a relatively rare event, can attenuate HF signals up to 100 db at times. (6) For multi‐hop modes, the relatively high attenuation for reflection on the ice caps should be considered in the transmission loss equation. Research performed during the decade ending in 1968 is emphasized, including some quite recent results of investigations conducted at the Geophysical Institute at the University of Alaska, not previously reported in the literature.

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

“…2a shows no obvious example of this phenomenon, an effect which may be partly attributed to propagation via an enhanced auroral E-layer, as documented by several investigators (e.g. Hunsucker and Bates, 1969;Hunsucker et al, 1996;Milan et al, 1997).…”

“…For example, Milan et al (1996) show enhanced absorption of the order of ∼30 dB on 6.8 MHz associated with individual magnetic substorms, and investigate a correlation between the magnitude of the absorption and the degree of dipolarization associated with each substorm. Direction finding studies show large deviations in arrival bearing associated with ionospheric propagating HF signals during auroral activity Warrington et al, 1997), a signature of non-great-circle propagation (review by Hunsucker and Bates, 1969). Furthermore, as noted above, enhanced ionospheric propagation can be associated with auroral substorms due to the formation of an auroral sporadic E-layer supporting additional propagation modes with low absorption (e.g.…”

“…see Milan et al, 1996). Hunsucker and Bates (1969) reviews effects of auroral activity on ionospheric propagation such as non-great-circle propagation, auroral sporadic E-layer propagation, F 1 -layer effects, D-region absorption, and scattering by ionospheric irregularities. The enhanced absorption of ionospherically propagating natural and man-made signals during times of high geomagnetic activity shown in Fig.…”

“…With regard to magnetic storm related effects, the principal phenomenon is "Polar Cap Absorption" (PCA), readily identified as enhanced absorption in riometer or ionosonde data but also as a black-out of ionospheric propagation on LF/MF/HF in polar regions, caused by absorption due to enhanced ionization in the lower ionosphere, caused by bombardment of the polar cap ionosphere by ∼10 MeV solar energetic protons following a solar flare. Hunsucker and Bates (1969) reviews the copious early literature on this LF/MF/HF propagation phenomenon. More recent examples include: observations on MF by Hunsucker et al (1989) showing ionospheric paths Kotzebue-Fairbanks and San Francisco-Fairbanks that were wiped out for 3 and 8-9 days, respectively, during a May 1986 magnetic storm; and storm-associated decreases in received HF signals quantitatively analyzed by Milan et al (1998).…”

An autonomous adaptive low-power instrument platform (AAL-PIP) for remote high-latitude geospace data collection

“…At distances beyond a few kilometers HF radio propagation is an ionospheric phenomenon (Hunsucker and Bates, 1969;Blagoveshchensky et al, 2008). In the lower HF band a critical frequency can exist where a wave launched vertically is reflected.…”