Solar activity swallows HF radio signals
Of course it's not only scientific instruments that are affected by the enhanced D-region absorption. The D-region absorption may also severely impact on users of HF radio communications. HF radio communications are used extensively in the aviation and maritime industries, and also in military operations and by emergency services. During solar proton events, disruptions to HF communications in the polar regions may last for several days. It is therefore critical for HF communicators to have other methods for maintaining communications when the ionospheric conditions become unfavourable. For example, the Global Maritime Distress and Safety System (GMDSS) requires that sea vessels travelling in the polar regions have MF, HF and VHF radios onboard.