  The solar corona routinely exhibits explosive activity, in particular coronal
mass ejections and their accompanying eruptive flares, that have global-scale
consequences. These events and their smaller counterparts, coronal jets,
originate in narrow, sinuous filament channels. The key processes that form and
evolve the channels operate on still smaller spatial scales and much longer
time scales, culminating in a vast separation of characteristic lengths and
times that govern these explosive phenomena. In this article, we describe
implementation and tests of an efficient subgrid-scale model for generating
eruptive structures in magnetohydrodynamics (MHD) coronal simulations. STITCH
-- STatistical InjecTion of Condensed Helicity -- is a physics-based, reduced
representation of helicity condensation: a process wherein small-scale vortical
surface convection forms ubiquitous current sheets, and pervasive reconnection
across the sheets mediates an inverse cascade of magnetic helicity and free
energy, thereby forming the filament channels. STITCH abstracts these complex
processes into a single new term, in the MHD Ohm's law and induction equation,
which directly injects tangential magnetic flux into the low corona. We show
that this approach is in very good agreement with a full helicity-condensation
calculation that treats all of the dynamics explicitly, while enabling
substantial reductions in temporal duration especially, but also in spatial
resolution. In addition, we illustrate the flexibility of STITCH at forming
localized filament channels and at energizing complex surface flux
distributions that have sinuous boundaries. STITCH is simple to implement and
computationally efficient, making it a powerful new technique for event-based,
data-driven modeling of solar eruptions.
