  Shock waves associated with fast coronal mass ejections (CMEs) accelerate
solar energetic particles (SEPs) in the long duration, gradual events that pose
hazards to crewed spaceflight and near-Earth technological assets, but the
source of the CME shock-accelerated plasma is still debated. Here, we use
multi-messenger observations from the Heliophysics System Observatory to
identify plasma confined at the footpoints of the hot, core loops of active
region 11944 as the source of major gradual SEP events in January 2014. We show
that the elemental composition signature detected spectroscopically at the
footpoints explains the measurements made by particle counting techniques near
Earth. Our results localize the elemental fractionation process to the top of
the chromosphere. The plasma confined closest to that region, where the coronal
magnetic field strength is high (a few hundred Gauss), develops the SEP
composition signature. This source material is continually released from
magnetic confinement and accelerated as SEPs following M-, C-, and X-class
flares.
