ON FIBRILS AND FIELD LINES: THE NATURE OF H<i>α</i>FIBRILS IN THE SOLAR CHROMOSPHERE
Abstract
Observations of the solar chromosphere in the linecore of the H line show dark elongated structures called fibrils that show swaying motion. We performed a three-dimensional radiation-MHD simulation of a network regionand computed synthetic H images from this simulation to investigate the relation between fibrils and the magnetic field lines in the chromosphere. The periods, amplitudes, and phasespeeds of the simulated fibrils are consistent with observations. We find that some fibrils trace out the same field line along the fibril's length, while other fibrils sample different field lines at different locations along their length. Fibrils sample the same field lines on a timescale of 200 s. This is shorter than their own lifetime. Fibril-threading field lines carry slow-mode waves, as well as transverse waves propagating with the Alfvn speed. Transverse waves propagating in opposite directions cause an interference pattern with complex apparent phase speeds. The relationship between fibrils and field lines is governed by constant migration and swaying of the field lines, their mass loading and draining, and their visibility in H. Field lines are visible where they lie close to the optical depth unity surface. The location of the latter is at a height at which the column mass reaches a certain fixed value. The visibility of the field line is thus determined by its own mass density and by the mass density of the material above it. Using the swaying motion of fibrils as a tracer of chromospheric transverse oscillations must be done with caution.