Heliospheric energetic particle variations

As observed from the ecliptic plane and at a distance of 1 AU from the Sun, the energetic particle population of the heliosphere drastically changes from solar maximum to solar minimum. The energetic particle populations in the inner heliosphere include: (1) Galactic cosmic rays (GCRs) originated in the interstellar medium and able to penetrate into the heliosphere. (2) Anomalous cosmic rays (ACRs) that originate as interstellar neutral atoms traveling into the heliosphere, ionized by solar UV and carried out as pickup ions in the solar wind to be finally accelerated to energies as high as ∼100 MeV/nucleon presumably close to the solar wind termination shock or in the heliosheath. (3) Solar energetic particles (SEPs) that originate near the Sun in association with solar flares and/or large coronal mass ejections (CMEs). As CMEs expand outward from the Sun, they may be able to drive interplanetary shock waves that can reaccelerate SEPs to form large gradual SEP events. Occasionally, SEP events are observed at very high energies reaching ∼GeV for protons and ∼100 MeV for electrons. (4) Energetic particles accelerated by other shocks and disturbances in the solar wind such as shocks formed in the solar wind stream interaction regions (SIs) or corotating interaction regions (CIRs). (5) Energetic particles accelerated in planetary magnetospheres, such as Jovian electrons observed in the inner heliosphere at energies from a few hundred keV to less than about 30 MeV.