Solar cycle variations in atmospheric noise at HF

Data are presented showing a clear solar cycle dependence of HF background noise. The background noise at HF originates from two sources: electrical discharges in the atmosphere and galactic noise. The atmospheric component may enter the receiving system via the ground wave from local thunderstorms but the dominant component at mid latitudes is frequently the result of skywave propagation from distant thunderstorms. Given the marked changes which occur in the ionosphere (and hence radiowave propagation) over a solar cycle it is not surprising that there are corresponding changes in the background noise level at HF