The Vector Magnetic Field, Evershed Flow, and Intensity in a Sunspot

Abstract

We present results of simultaneous observations of the vector magnetic field, Evershed flow, and intensity pattern in a nearly axisymmetric sunspot, made with the Advanced Stokes Polarimeter at the Vacuum Tower Telescope at NSO (Sacramento Peak). The vector magnetic field is determined from the Stokes profiles of the magnetically sensitive lines Fe I 630.15 and 630.25 nm, and Doppler velocities and intensities are measured in several lines including the weak C I 538.03 nm line, formed in the deepest layers of the atmosphere. The strength of the magnetic field decreases with increasing zenith angle (angle of inclination to the local vertical), and this decrease is nearly linear over most of the range of values in the sunspot. Magnetic field strength and continuum intensity are inversely related in the sunspot in a manner similar to the characteristic nonlinear relationship found by Kopp & Rabin in the infrared line Fe I 1564.9 nm. A different relationship is found between magnetic field strength and core intensity (in Fe I 630.25 nm), however, with the curve doubling back to give two distinct values of field strength at the same core intensity in the penumbra—the higher and lower field strengths corresponding to the inner and outer penumbra, respectively. In the penumbra the magnetic field pattern consists of spokelike extensions of stronger, more vertical magnetic field separated by regions of weaker, nearly horizontal magnetic field, as found by Degenhardt & Wiehr and Lites et al. The penumbral magnetic field extends outward beyond the outer continuum boundary of the sunspot, forming a canopy at the height of formation of Fe I 630.25 nm. Our results for the Evershed flow confirm the discovery by Rimmele that this flow is generally confined to narrow, elevated channels in the penumbra. In the Fe I 630.25 nm line and other strong photospheric lines we see isolated, radially elongated channels of Evershed flow crossing the outer penumbra. These flow channels lie in regions of the penumbra where the magnetic field is very nearly horizontal. In the weak C I 538.03 nm line (formed at a height h = 40 km) the flow pattern shows small, isolated patches of upflow, lying at the inner end of the Fe I flow channels where the magnetic field is more inclined to the horizontal. These patches presumably correspond to the upstream footpoints of the arched magnetic flux tubes carrying the Evershed flow. For some of the flow channels we find isolated patches of strong downflow in the C I line just outside the penumbra that might correspond to the downstream footpoints of these flux tubes. There is a weak association between the Evershed flow channels and the dark filaments seen in continuum intensity in the penumbra, but a much stronger association between the flow and the dark filaments seen in core intensity measured in the same spectral line.