Space Weather Influences on HF, UHF, and VHF Radio Propagation

This Chapter considers space weather effects of the ionosphere on the propagation of radio frequencies classed by the International Telecommunications Union as HF (3-30 MHz), VHF (30-300 MHz) and UHF (300-3000 MHz). The ionosphere and its variation is important for these radio frequencies. The ionosphere is the part of the upper atmosphere above 70-80 km altitude, where the density of the neutral atmosphere is low and the ionisation is significant enough to affect radio propagation. Even though the amount of ionisation is still below 1% of the neutrals, it is sufficient to exert an important effect on radio signals. In this Chapter the ionosphere is introduced, then a description of the theory of radio propagation at HF, VHF and UHF is given. The effects of solar storms and space weather on the ionosphere and hence on radio propagation are outlined. Finally, anomalous propagation conditions and outstanding problems and questions are also discussed.