  The underlying origin of solar eruptive events (SEEs), ranging from giant
coronal mass ejections to small coronal-hole jets, is that the lowest-lying
magnetic flux in the Sun's corona undergoes the continual buildup of stress and
free energy. This magnetic stress has long been observed as the phenomenon of
"filament channels:" strongly sheared magnetic field localized around
photospheric polarity inversion lines. However, the mechanism for the stress
buildup - the formation of filament channels - is still debated. We present
magnetohydrodynamic simulations of a coronal volume that is driven by
transient, cellular boundary flows designed to model the processes by which the
photosphere drives the corona. The key feature of our simulations is that they
accurately preserve magnetic helicity, the topological quantity that is
conserved even in the presence of ubiquitous magnetic reconnection. Although
small-scale random stress is injected everywhere at the photosphere, driving
stochastic reconnection throughout the corona, the net result of the magnetic
evolution is a coherent shearing of the lowest-lying field lines. This highly
counter-intuitive result - magnetic stress builds up locally rather than
spreading out to attain a minimum energy state - explains the formation of
filament channels and is the fundamental mechanism underlying SEEs.
Furthermore, this mechanism may be relevant to other astrophysical or
laboratory plasmas.
