Solar protons are accelerated to high energies in processes related to solar flares and coronal mass ejections (CMEs). These high energy protons can access the Earth’s magnetosphere and precipitate into the polar ionospheres. Increased fluxes of high energy protons can damage satellites or disturb satellite operations including navigation satellites. Furthermore, they cause absorption of radio waves in the polar regions up to a complete radio blackout for HF communication systems. Energetic protons also enhance the radiation dose received by astronauts in orbit and by passengers and aircrew onboard high-altitude polar flights.
The unshielded proton flux is typically measured with geostationary satellites (for example NOAA GOES) at different energies. Times when the >= 10 MeV integrated proton flux exceeds 10 particle flux units (pfu) are called Solar Energetic Particle Events (SEPEs) or Solar Proton Events (SPEs). This corresponds to the lowest S-scale threshold (S1) as defined by NOAA Space Weather Prediction Center. Note that lower than S1-level >= 10 MeV integrated proton flux can cause moderate HF signal absorption in the solar cap, especially in sunlit conditions.