SEPs

SEPs, short for Solar Energetic Particles, are high-energy charged particles released into the heliosphere by processes associated with the Sun. The majority are protons, with electrons and heavier ions also present, spanning energies from tens of keV to several GeV. SEPs originate primarily in two types of solar events: impulsive events linked to solar flares and gradual events driven by coronal mass ejection–driven shocks. Impulsive events are brief and often show enrichment in rare isotopes such as 3He and heavy ions; gradual events can last from days to weeks and involve large particle fluxes.

Acceleration mechanisms include magnetic reconnection in flares for impulsive SEPs and diffusive shock acceleration at CME-driven shocks for gradual SEPs. Transport through the inner heliosphere is governed by interactions with magnetic turbulence, producing pitch-angle scattering, energy-dependent anisotropy, and sometimes wide longitudinal spreads. The energy spectrum and composition of SEPs provide clues about the dominant acceleration processes and the solar environment at the time of release.

Space missions and ground-based instruments monitor SEPs. Spacecraft such as ACE, SOHO, STEREO, Parker Solar Probe and Solar Orbiter measure particle spectra, timing, and composition; neutron monitors on Earth help track the high-energy SEP component. Observed differences between impulsive and gradual events inform models of solar activity and particle transport.

SEPs pose radiation risks to astronauts and spacecraft electronics, and they contribute to atmospheric ionization, with broader implications for space weather. They are a central focus of forecasting efforts, risk assessment, and mission planning. Research continues to refine understanding of acceleration and transport processes and to improve predictive capabilities for SEP events.

Other uses of the acronym SEPs exist in fields such as hardware security and software engineering; this article focuses on Solar Energetic Particles.