Skywave modes of radio communication operate by bending ( refracting ) radio waves ( electromagnetic radiation ) through the Ionosphere . During the "peaks" of the solar cycle, the ionosphere becomes increasingly ionized by solar photons and cosmic rays . This affects the propagation of the radio wave in complex ways that can either facilitate or hinder communications. Forecasting of skywave modes is of considerable interest to commercial marine and aircraft communications , amateur radio operators and shortwave broadcasters . These users occupy frequencies within the High Frequency or 'HF' radio spectrum that are most affected by these solar and ionospheric variances. Changes in solar output affect the maximum usable frequency , a limit on the highest frequency usable for communications.