  The hemispheric pattern of solar filaments is considered using
newly-developed simulations of the real photospheric and 3D coronal magnetic
fields over a 6-month period, on a global scale. The magnetic field direction
in the simulation is compared directly with the chirality of observed
filaments, at their observed locations. In our model the coronal field evolves
through a continuous sequence of nonlinear force-free equilibria, in response
to the changing photospheric boundary conditions and the emergence of new
magnetic flux. In total 119 magnetic bipoles with properties matching observed
active regions are inserted. These bipoles emerge twisted and inject magnetic
helicity into the solar atmosphere. When we choose the sign of this
active-region helicity to match that observed in each hemisphere, the model
produces the correct chirality for up to 96% of filaments, including exceptions
to the hemispheric pattern. If the emerging bipoles have zero helicity, or
helicity of the opposite sign, then this percentage is much reduced. In
addition, the simulation produces a higher proportion of filaments with the
correct chirality after longer times. This indicates that a key element in the
evolution of the coronal field is its long-term memory, and the build-up and
transport of helicity from low to high latitudes over many months. It
highlights the importance of continuous evolution of the coronal field, rather
than independent extrapolations at different times. This has significant
consequences for future modelling such as that related to the origin and
development of coronal mass ejections.
