  The source of high-energy protons (>500 MeV) responsible for the so-called
ground level enhancements (GLEs) remains an open question in solar physics. One
of the candidates is a shock wave driven by a coronal mass ejection, which is
thought to accelerate particles via diffusive-shock acceleration. We perform
physics-based simulations of proton acceleration using information on the shock
and ambient plasma parameters derived from the observation of a real GLE event.
We analyse the simulation results with the aim to find out which of the
parameters are significant in controlling the acceleration efficiency and to
get a better understanding of the conditions under which the shock can produce
relativistic protons. We use results of the recently developed technique to
determine the shock and ambient plasma parameters, applied to the 17 May 2012
GLE event, and carry out proton acceleration simulations with the Coronal Shock
Acceleration model. We have performed proton acceleration simulations for nine
individual magnetic field lines characterised by various plasma conditions.
Analysis of the simulation results shows that the acceleration efficiency of
the shock, i.e., its ability to accelerate particles to high energies, tends to
be higher for those shock portions that are characterised by larger values of
the scattering-centre compression ratio and/or the fast-mode Mach number. At
the same time, the acceleration efficiency can be strengthened due to enhanced
plasma density in the flux tube. Analysis of the delays between the flare onset
and the production times of protons of 1 GV rigidity for different field lines
in our simulations, and a subsequent comparison of those with the observed
values indicate a possibility that quasi-perpendicular portions of the shock
play the main role in producing relativistic protons.
