  Multi-spacecraft observations of solar energetic particle (SEP) events not
only enable a deeper understanding and development of particle acceleration and
transport theories, but also provide important constraints for model validation
efforts. However, because of computational limitations, a given physics-based
SEP model is usually best-suited to capture a particular phase of an SEP event,
rather than its whole development from onset through decay. For example,
magnetohydrodynamic (MHD) models of the heliosphere often incorporate solar
transients only at the outer boundary of their so-called coronal domain --
usually set at a heliocentric distance of 20-30 $R_{\odot}$. This means that
particle acceleration at CME-driven shocks is also computed from this boundary
onwards, leading to simulated SEP event onsets that can be many hours later
than observed, since shock waves can form much lower in the solar corona. In
this work, we aim to improve the modelled onset of SEP events by inserting a
"fixed source" of particle injection at the outer boundary of the coronal
domain of the coupled WSA-Enlil 3D MHD model of the heliosphere. The SEP model
that we employ for this effort is SEPMOD, a physics-based test-particle code
based on a field line tracer and adiabatic invariant conservation. We apply our
initial tests and results of SEPMOD's fixed-source option to the 2021 October 9
SEP event, which was detected at five well-separated locations in the inner
heliosphere -- Parker Solar Probe, STEREO-A, Solar Orbiter, BepiColombo, and
near-Earth spacecraft.
