Solar energetic particle variations Original Research Article

In the largest solar energetic-particle (SEP) events, acceleration occurs at shock waves driven out from the Sun by coronal mass ejections (CMEs). In fact, the highest proton intensities directly measured near Earth at energies up to ∼1 GeV occur at the time of passage of shocks, which arrive about a day after the CMEs leave the Sun. CME-driven shocks expanding across magnetic fields can fill over half of the heliosphere with SEPs. Proton-generated Alfvén waves trap particles near the shock for efficient acceleration but also throttle the intensities at Earth to the “streaming limit“ early in the events. At high energies, particles begin to leak from the shock and the spectrum rolls downward to form an energy-spectral “knee” that can vary in energy from ∼1 MeV to ∼1 GeV in different events. All of these factors affect the radiation dose as a function of depth and latitude in the Earthʹs atmosphere and the risk to astronauts and equipment in space. SEP ionization of the polar atmosphere produces nitrates that precipitate to become trapped in the polar ice. Observations of nitrate deposits in ice cores reveal individual large SEP events and extend back ∼400 years. Unlike sunspots, SEP events follow the ∼80–100 year Gleissberg cycle rather faithfully and are now at a minimum in that cycle. The largest SEP event in the last 400 years appears to be related to the flare observed by Carrington in 1859, but the probability of SEP events with such large fluences falls off sharply because of the streaming limit.