On Fibrils and Field Lines: the Nature of Hα Fibrils in the Solar Chromosphere
2015 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 802, no 2, 136Article in journal (Refereed) Published
Observations of the solar chromosphere in the line. core of the H alpha line show dark elongated structures called fibrils that show swaying motion. We performed a three-dimensional radiation-MHD simulation of a network region. and computed synthetic Ha images from this simulation to investigate the relation between fibrils and the magnetic field lines in the chromosphere. The periods, amplitudes, and phase. speeds 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 similar to 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 Alfven 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 Ha. 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.
Place, publisher, year, edition, pages
2015. Vol. 802, no 2, 136
magnetohydrodynamics (MHD), Sun: atmosphere, Sun: chromosphere, Sun: oscillations
Astronomy, Astrophysics and Cosmology
IdentifiersURN: urn:nbn:se:su:diva-117486DOI: 10.1088/0004-637X/802/2/136ISI: 000353014500064OAI: oai:DiVA.org:su-117486DiVA: diva2:815485