Image_fluxel

Image fluxel

Photospheric boundary conditions in FLUX are enforced by the method of image fluxels. Each fluxel interacts not just with other fluxels in the simulation but also with images of itself reflected through the boundary plane. Spherical and cylindrical boundaries are treated as polyplanar surfaces - each fluxel's image is reflected through the plane tangent to the boundary at the boundary's closest approach to the fluxel itself.

For planar boundaries, image fluxels limit the extent of each real fluxel's Voronoi cell to the appropriate half-space; this is a convenient way of preventing the fluxons from passing through the boundary plane.

The first (and last) fluxel of each fluxon presents a problem -- fluxels that are anchored on the plane are forced by this technique to be perpendicular to the plane. To avoid that problem, image fluxels are not generated for line-tied endpoint fluxels. That allows the endpoints to settle to the correct entry angle at the boundary.