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.