An Analytical Model for Fluted Sunspots and a New Interpretation of Evershed Flow and X-Ray Anemones

Abstract

view Abstract Citations (19) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Analytical Model for Fluted Sunspots and a New Interpretation of Evershed Flow and X-Ray Anemones Martens, Petrus C. H. ; Hurlburt, Neal E. ; Title, Alan M. ; Acton, Loren W. Abstract We present a force-free constant-α model for the magnetic field in and above so-called "fluted" sunspots. This model is motivated by recent high-resolution observations of Title et al. at the Swedish Solar Observatory in La Palma. They observed that the inclination angle of the magnetic field in the penumbra of sunspots oscillates rapidly with azimuth, with a period of about 60 and an amplitude of about 18°. They further find that there is little variation in the radial direction and in absolute field strength. The resulting phenomenon of interlocking high- and low-inclination field lines was called "flutedness. In our model, the parameters are chosen to reproduce the La Palma magnetograms, and an analytical expression is obtained for the three-dimensional magnetic field emanating from the sunspot's umbra and penumbra. The model correctly reproduces the azimuthal variation in inclination angle, as well as the mean constancy of the magnetic field strength, and the appearance of a highly corrugated neutral line on the limb side of off-center sunspots. We find that the "flutedness" results in a highly complex topology in a boundary layer extending from the photo sphere into the chromosphere, while the coronal field is uniform. Title et al. demonstrated that the Evershed flow occurs in regions of nearly horizontal magnetic field, and tacitly assumed, as is done in most of the literature, that the dark filaments in which the flow is observed form individual magnetic flux tubes. Our magnetic field solution suggests that the regions of nearly horizontal field at the photo spheric boundary may not form individual magnetic flux tubes, but rather a series of short horizontal loops bridging a neutral line that is stretched in the radial direction along the penumbra, up to the outer penumbral boundary. Hence, the Evershed flow could not be a simple siphon flow in the radial direction, but would consist of phase-coordinated flows along the many short loops bridging the neutral line. However, the assumption of a force-free field breaks down in this region of the atmosphere, and the topology suggested by it may not materialize in reality. We further demonstrate that there are large variations in the photospheric cross sections of coronal loops, due to the complexity of the field near their photospheric footpoints. Under the assumption of constant energy input per unit surface area into these loops, the variation in cross section is qualitatively consistent with the variation in X-ray brightness of loops in penumbral "anemones" observed by Yohkoh. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177251 Bibcode: 1996ApJ...463..372M Keywords: MAGNETOHYDRODYNAMICS: MHD; SUN: MAGNETIC FIELDS; SUN: X-RAYS; GAMMA RAYS; SUN: SUNSPOTS full text sources ADS |