Solar Energetic Particles

Abstract

Solar energetic particles (SEPs) have been studied for over seven decades, initially from ground-based instrumentation and now routinely from spacecraft throughout the heliosphere. Our knowledge of the acceleration and transport of these particles from close to the Sun througzh the interplanetary medium has advanced dramatically in the last 40 years as the instrumentation and modeling capabilities have become more sophisticated. However, a full understanding of the conditions and processes that cause variability in SEP event characteristics remains elusive due to the scarcity of in situ measurements well inside 1 AU. This chapter reviews our current understanding of the properties of SEP events, how they are accelerated and transported, and the impact of those processes on event variability. It is not an exhaustive review, but provides a snapshot of the state of our understanding before the near-Sun observations from the Parker Solar Probe and Solar Orbiter missions reveal surprises that challenge this understanding.