Vertical current densities and magnetic gradients in sunspots

Abstract

Aims. The vertical component of electric current densities and the height dependence of the vertical component of the magnetic field in sunspots were determined. Methods. Full-Stokes magnetograms of eight sunspots obtained from infrared spectro-polarimetric measurements were investigated. The magnetic field strength and the magnetic inclination and azimuth were obtained from an inversion code. Vertical current densities and the vertical derivative of the vertical component of the magnetic field strength resulted from Maxwell equations. Results. It is found that electric current densities and magnetic gradients depend on the finestructure of the sunspots. Typical values for current densities vary in the range ±40 mA m -2 , but their errors are of the same order. Disturbances in the radial structure of the penumbra are related to enhanced current densities up to 149 mA m -2 , and a maximum value of 166 mA m -2 was found in a light bridge. There are indications that the radial structure of the penumbra is related to current densities, but no correlation to the local intensity fluctuations was found in the outer penumbra. The vertical component of the magnetic field decreases by 0.5-1.5 G km -1 in the umbra. Mean values in the inner penumbra are somewhat smaller than in the umbra, and locally dark structures exhibit a faster decrease with height than bright ones. In the outer penumbra the vertical magnetic component increases, independent of the local intensity distribution. Conclusions. Electric current densities could be a diagnostic tool for understanding the penumbral finestructure, although the presently available spatial resolution is probably not good enough to avoid artifacts. Magnetic extrapolations might depend crucially on this problem.