Solar energetic particles : a modern primer on understanding sources, acceleration and propagation

In a field overflowing with beautiful images of the Sun, solar energetic particle (SEP) events are a hidden asset, perhaps a secret weapon, that can sample the solar corona and carry away unique imprints of its most bizarre and violent physics. Only recently have we found that the abundances of the elements in SEPs carry a wealth of data, not only on their own acceleration and history, but on plasma temperatures at their source, and on aspects of the genesis of the corona itself. SEPs are the tangible product of differing energetic outbursts at the Sun. They come in extremes. Little "impulsive" SEP events from magnetic reconnection in solar jets (also in flares), have most unusual 1000-fold resonant enhancements of 3He and of heavy elements like Au or Pb, while large "gradual" SEP events accelerated at shock waves driven by coronal mass ejections (CMEs), sample the composition of the corona itself, but also accelerate GeV protons that threaten Mars-bound astronauts with hazardous radiation. Direct SEP measurements plus solar images provide complimentary, "multi-messenger" data on high-energy physics at the Sun.