  On September 10 2017, a fast coronal mass ejection (CME) erupted from the
active region (AR) 12673, leading to a ground level enhancement (GLE) event at
Earth. Using the 2D improved Particle Acceleration and Transport in the
Heliosphere (iPATH) model, we model the large solar energetic particle (SEP)
event of 10 September 2017 observed at Earth, Mars and STEREO-A. Based on
observational evidence,we assume that the CME-driven shock experienced a large
lateral expansion shortly after the eruption, which is modelled by a double
Gaussian velocity profile in this simulation. We use the in-situ shock arrival
times and the observed CME speeds at multiple spacecraft near Earth and Mars as
constraints to adjust the input model parameters. The modelled time intensity
profiles and fluence for energetic protons are then compared with observations.
Reasonable agreements with observations at Mars and STEREO-A are found. The
simulated results at Earth differ from observations of GOES-15. Instead, the
simulated results at a heliocentric longitude 20 degree west to Earth fit
reasonably well with the GOES observation. This can be explained if the
pre-event solar wind magnetic field at Earth is not described by a nominal
Parker field. Our results suggest that a large lateral expansion of the
CME-driven shock and a distorted interplanetary magnetic field due to previous
events can be important in understanding this GLE event.
