Bombardment of the polar‐cap ionosphere by solar cosmic rays

Solar flares often produce bursts of energetic particles (mainly protons but also heavier nuclei and electrons). Protons from these flares with energies greater than about 0.8 Mev are of interest because they enter the earth's atmosphere at polar latitudes and produce ionization below 90‐km altitude at a rate that far exceeds the ionization rate due to normal solar and galactic sources. This increased ionization produces many interesting ionospheric effects. Comparison of ion and electron concentration profiles in the lower ionosphere (specifically in the D region) with inferences drawn from both ground‐based radio data and satellite particle data lead to the conclusion that the ion chemistry at such times is much simpler than during undisturbed times. The diverse methods seem to be compatible in defining major characteristics and reactions of the D region during periods when it is bombarded by solar‐flare protons. Thus these events provide the framework for a detailed investigation of D region chemical processes.