Atmospheric Ionizations by Solar X-Rays, Solar Protons, and Radiation Belt Electrons in September 2017 Space Weather Event

Energetic particles from space deposit their energies on the Earth's atmosphere and contribute to variations in the concentration of neutral components such as ozone which controls the atmospheric temperature balance. Comprehensive understandings of their global impact on the atmosphere require whole pictures of spatiotemporal ionization distributions due to them. We first attempt to evaluate and summarize the altitude profiles of ionization for the September 2017 space weather event with cutting-edge space-borne and ground-based observations of different types of particle inputs. In early September 2017, the Sun showed notable activity, including X-class flares and solar proton events. During this period, ground-based radar observations have confirmed atmospheric ionization events by energetic particle precipitations of solar flare X-rays, solar protons, and radiation belt electrons, the main sources of ionization into the Earth's atmosphere. We estimate the altitude profiles of the ionization rate by using the Particle and Heavy Ion Transport code System (PHITS) with the input of the particle fluxes obtained by satellites. The estimates are then compared with measurements of the ionization altitude, ionization intensity, and electron density by the radars in the polar region, such as the PANSY radar at Syowa Station and the EISCAT in Tromso, Norway. We conclude that the PHITS simulation results reasonably reproduce (within the error of a factor of two) those ionizations measured by ground-based instruments with inputs of observed ionization sources by satellites. Space weather can affect the Earth's environment and human activities via electromagnetic radiation and charged particles. It is now known that energetic particles precipitating into the Earth's atmosphere from space also affect the variability of atmospheric composition, such as ozone depletion, which is responsible for changes in the Earth's climate and weather. To properly understand these effects on the atmosphere, this study first attempted to quantitatively assess the ionization of the atmosphere that occurs when particle energy is carried to the atmosphere. In September 2017, multiple solar flares triggered space weather events. During that period, transient atmospheric ionization was observed due to the major sources of energetic particles in the Earth's atmosphere: X-rays from solar flares, solar protons emitted during the flares, and energetic electrons due to solar wind disturbances. We reproduced the observed ionization by ground-based instruments for all three sources, based on the particle measured by satellites, roughly within the error of a factor of 2. Such quantitative connections between those energetic particles and the concurrent ionization provide a better understanding of the overall picture of energetic particle effects on the atmosphere. Atmospheric ionization by energetic particles is examined for September 2017 space weather eventSolar X-rays, solar protons, and radiation belt electrons as observed by satellites are used for Particle and Heavy Ion Transport code System (PHITS)-simulation inputsThose ionizations in the mesosphere measured by ground-based instruments are reasonably reproduced by the PHITS simulation