Evershed effect

Abstract: Results of a 3D MHD simulation of a sunspot with a photospheric size of about 20 Mm are presented. The simulation has been carried out with the MURaM code, which includes a realistic equation of state with partial ionization and radiative transfer along many ray directions. The largely relaxed state of the sunspot shows a division in a central dark umbral region with bright dots and a penumbra showing bright filaments of about 2 to 3 Mm length with central dark lanes. By a process similar to the formation of umbral dots, the penumbral filaments result from magneto-convection in the form of upflow plumes, which become elongated by the presence of an inclined magnetic field: the upflow is deflected in the outward direction while the magnetic field is weakened and becomes almost horizontal in the upper part of the plume near the level of optical depth unity. A dark lane forms owing to the piling up of matter near the cusp-shaped top of the rising plume that leads to an upward bulging of the surfaces of constant optical depth. The simulated penumbral structure corresponds well to the observationally inferred interlocking-comb structure of the magnetic field with Evershed outflows along dark-laned filaments with nearly horizontal magnetic field and overturning perpendicular (`twisting') motion, which are embedded in a background of stronger and less inclined field. Photospheric spectral lines are formed at the very top and somewhat above the upflow plumes, so that they do not fully sense the strong flow as well as the large field inclination and significant field strength reduction in the upper part of the plume structures.

Abstract: We present the results of numerical 3D magnetohydrodynamic (MHD) simulations with radiative energy transfer of fine structure in a small sunspot of about 4 Mm width. The simulations show the development of filamentary structures and flow patterns that are, except for the lengths of the filaments, very similar to those observed. The filamentary structures consist of gaps with reduced field strength relative to their surroundings. Calculated synthetic images show dark cores like those seen in the observations; the dark cores are the result of a locally elevated τ = 1 surface. The magnetic field in these cores is weaker and more horizontal than for adjacent brighter structures, and the cores support a systematic outflow. Accompanying animations show the migration of the dark-cored structures toward the umbra, and fragments of magnetic flux that are carried away from the spot by a large-scale "moat flow." We conclude that the simulations are in qualitative agreement with observed penumbra filamentary structures, Evershed flows, and moving magnetic features.

Abstract: An observational study of the inclination of magnetic fields and flows in sunspot penumbrae at a spatial resolution of 0. �� 2 is presented. The analysis is based on longitudinal magnetograms and Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma using the Lockheed Solar Optical Universal Polarimeter birefringent filter. Data from two sunspots observed at several heliocentric angles between 12 ◦ and 39 ◦ were analyzed. We find that the magnetic field at the level of the formation of the Fe -line wing (630.25 nm) is in the form of coherent structures that extend radially over nearly the entire penumbra giving the impression of vertical sheet-like structures. The inclination of the field varies up to 45 ◦ over azimuthal distances close to the resolution limit of the magnetograms. Dark penumbral cores, and their extensions into the outer penumbra, are prominent features associated with the more horizontal component of the magnetic field. The inclination of this dark penumbral component - designated B - increases outwards from approximately 40 ◦ in the inner penumbra such that the field lines are nearly horizontal or even return to the solar surface already in the middle penumbra. The bright component of filaments - designated A - is associated with the more vertical component of the magnetic field and has an inclination with respect to the normal of about 35 ◦ in the inner penumbra, increasing to about 60 ◦ towards the outer boundary. The magnetogram signal is lower in the dark component B regions than in the bright component A regions of the penumbral filaments. The measured rapid azimuthal variation of the magnetogram signal is interpreted as being caused by combined fluctuations of inclination and magnetic field strength. The Dopplergrams show that the velocity field associated with penumbral component B is roughly aligned with the magnetic field while component A flows are more horizontal than the magnetic field. The observations give general support to fluted and uncombed models of the penumbra. The long-lived nature of the dark-cored filaments makes it difficult to interpret these as evidence for convective exchange of flux tubes. Our observations are in broad agreement with the two component model of Bellot Rubio et al. (2003), but do not rule out the embedded flux tube model of Solanki & Montavon (1993).

Abstract: A diffraction-limited 120 minute time sequence of Evershed flows along penumbral filaments was obtained using high-order adaptive optics in conjunction with postprocessing. We observe individual Evershed flow channels and study their evolution in time. The vast majority of flow channels originate in bright, inner footpoints of size 02-04 with an upflow. The upflow turns into a horizontal outflow along a dark penumbral filament within fractions of 1'' (300-500 km). The time sequence clearly shows that both (bright) upflow and (dark) horizontal flow move around and evolve as a unit, indicating that they are part of the same feature. The inner footpoints are brighter than the average quiet photosphere and move inward at 0.5-1 km s-1. Our observations provide strong evidence that penumbral grains are the inner footpoints of Evershed flows where a hot upflow occurs. We observe an Evershed flow channel as it appears to emerge near the outer penumbra and track the flow over a period of about 100 minutes as it moves toward the penumbra-umbra edge, where it disappeared. We observe a steep decline (≤02) of the velocity at outer end of individual flow channels, even for flow channels that end well within the penumbra. This sharp outer edge of the flow channels is also observed to move inward toward the penumbra-umbra boundary. Flows in dark-cored penumbral filaments appear to be produced by the Evershed effect. We discuss our observational results in the context of models of the Evershed effect. Some aspects of our observations provide strong support for the moving tube model of the Evershed flow.