High frequency (HF) radio communications use reflections from the ionosphere to send signals long distances around the curvature of the Earth. This is still used in polar regions where line-of-sight VHF links or satellite communications cannot be used. An increase in HF radio use has been prompted by the need for air traffic control centres to talk to the increasing number of commercial airlines flying on trans-polar routes. Unfortunately HF radio communications can be affected by space weather disturbances: solar X-ray flares produce shortwave fadeouts on the sunward side of the Earth, energetic protons pentrate into the ionosphere at high latitudes producing polar cap absorption, and electrons entering the ionosphere in a ring around each pole cause auroral absorption. The increased absorption of radio signals can cause a loss of radio communication for trans-poalr flights. This paper reviews the characteristics of these disturbances and presents the space weather services that are being put in place to mitigate the impact on airline operations.
High frequency (HF) radio communications (also referred to as shortwave radio) operates on frequencies from 3 to 30 Mhz. In this frequency band signals are reflected from the ionosphere and are thus able to travel round the curvature of the Earth (Figure 1). Radio waves are reflected by the upper E and F layers of the ionosphere, but are absorbed by the lower D region. With a normal ionosphere there is a window of frequencies, between those absorbed in the D region and the upper limit for reflection, that can be used for long-distance communication. However, during space weather disturbances extra absorption by the D region reduces that window and can cause a complete loss of radio communication.
fig 1. Reflections from the ionosphere enable radio communication around the curve of the Earth