  Evidence for two different physical mechanisms for acceleration of solar
energetic particles (SEPs) arose 50 years ago with radio observations of type
III bursts, produced by outward streaming electrons, and type II bursts from
coronal and interplanetary shock waves. Since that time we have found that the
former are related to "impulsive" SEP events from impulsive flares or jets.
Here, resonant stochastic acceleration, related to magnetic reconnection
involving open field lines, produces not only electrons but 1000-fold
enhancements of 3He/4He and of (Z>50)/O. Alternatively, in "gradual" SEP
events, shock waves, driven out from the Sun by coronal mass ejections (CMEs),
more democratically sample ion abundances that are even used to measure the
coronal abundances of the elements. Sometimes residual impulsive suprathermal
ions contribute to the seed population for shock acceleration, complicating the
abundance picture, but this process has now been modeled theoretically.
Initially, impulsive events define a point source on the Sun, selectively
filling few magnetic flux tubes, while gradual events show broad acceleration
that can fill half of the inner heliosphere, beginning when the shock reaches
~2 solar radii. Shock acceleration occurs as ions are scattered back and forth
across the shock by resonant Alfven waves amplified by the accelerated protons
themselves. These waves also can produce a streaming-limited maximum SEP
intensity and plateau region upstream of the shock. Behind the shock lies the
large expanse of the "reservoir", a spatially extensive trapped volume of
uniform SEP intensities with invariant energy-spectral shapes where overall
intensities decrease with time as the enclosing "magnetic bottle" expands
adiabatically. These reservoirs now explain the slow intensity decrease that
defines gradual events, once erroneously attributed solely to slow outward
diffusion.
