  Forecasting solar energetic particles (SEPs), and identifying flare/CMEs from
active regions (ARs) that will produce SEP events in advance is extremely
challenging. We investigate the magnetic field environment of AR 12673,
including the AR's magnetic configuration, the surrounding field configuration
in the vicinity of the AR, the decay index profile, and the footpoints of
Earth-connected magnetic field, around the time of four eruptive events. Two of
the eruptive events are SEP-productive (2017 September 4 at 20:00~UT and
September 6 at 11:56~UT), while two are not (September 4 at 18:05~UT and
September 7 at 14:33~UT). We analysed a range of EUV and white-light
coronagraph observations along with potential field extrapolations and find
that the CMEs associated with the SEP-productive events either trigger null
point reconnection that redirects flare-accelerated particles from the flare
site to the Earth-connected field and/or have a significant expansion (and
shock formation) into the open Earth-connected field. The rate of change of the
decay index with height indicates that the region could produce a fast CME ($v
>$ 1500~km~s$^{-1}$), which it did during events two and three. The AR's
magnetic field environment, including sites of open magnetic field and null
points along with the magnetic field connectivity and propagation direction of
the CMEs play an important role in the escape and arrival of SEPs at Earth.
Other SEP-productive ARs should be investigated to determine whether their
magnetic field environment and CME propagation direction are significant in the
escape and arrival of SEPs at Earth.
