Observations of small-scale flows in sunspot penumbrae

Abstract

In this thesis we analyze the temporal evolution and the physical properties of small-scale flux in sunspot penumbrae. The penumbra is a magnetized medium where convection should be inhibited. However, it is observed as a plethora of bright filaments of different sizes that evolve in a few minutes, leading us to ask how matter moves in the penumbra. Since the discovery of the Evershed ow in 1909, theoretical models and observational studies have attempted to explain what it is happening in the penumbra, but none have totally explained observations. Recently, 3D MHD simulations have enable us to test models and to compare them with observations. Results from the latest published simulations support overturning convection (Scharmer et al., 2008) as the responsible of the penumbral ow motions, which is begin questioned due to the necessity of deconvolve data with uncertain parameters for stray light compensation in order to detect the presence of lateral down ows. In addition, supersonic down ows in the penumbra are still a matter of debate. Thanks to improvements in instrumentation, they have been characterized in detail and more aspects are known. Nevertheless, (van Noort et al., 2013) have described supersonic down ows with LOS velocities of order 20 km s-1 and magnetic field strengths of about 7 kG, being these values the greatest ever obtained in the penumbra and raising doubts in the community. A novel aspect of this thesis is the use of time sequences of high spatial resolution and high cadence spectropolarimetric data of the Fe I 6173A spectral line obtained with the CRISP spectropolarimeter at the Swedish 1- m Solar Telescope (SST). This enables us to study small-scale ow motions and, moreover, spatially resolve structures. In addition, time series allow us to filter for subsonic oscillations that introduce an undesired imprint in the velocity field, specifically when searching for weak signals. By means of line-of-sight (LOS) velocities using the bisector technique and filtering them for subsonic oscillations, we have detected lateral down-

ows throughout the penumbra. This is the first time that it is achieved without using stray light compensated data. Lateral down ows appear close or at the edges of the penumbral filaments and move according to the ow channel which is associated with. They are intermittent, occurring permanently mergings and fragmentations. The LOS velocity of the lateral down-

ows is very weak, of order 200 m s-1, their lifetime is of 6 minutes and they move outward. Through their four Stokes profiles inversion, we have characterized also their magnetic field. The magnetic field strength is of about 1.5 kG. Their inclination does not suggest the presence of inverse polarity, but after examining the Stokes V profiles we have observed the existence of an additional red lobe that suggests the presence of reverse-polarity magnetic fields. The overturning convection, the convective rolls (Danielson, 1961) and the twisted ux magnetic tubes (Borrero, 2007) are compatible with our results, although we prioritize the overturning convection because it is supported by simulations. However, the velocity obtained from simulations are greater than that from observations, which raises doubts. It is important to emphasize that simulations show a polarity inversion between = 1 and 0.1, and it might be interesting to study in order to relate lateral down ows with inverse-polarity fields. From two-component inversions of the pixels within patches harboring supersonic downdlows, we have characterized their physical properties and their temporal evolution. These patches appear in the middle and outer penumbra and move outward. They coincide with bright intensity structures located at the end of penumbral filaments. The supersonic down ows are characterized for having strong LOS velocities (~8 km s-1), magnetic field strengths of about 1.5 kG with opposite polarity to the sunspot (45º). Their lifetime varies between one and ten minutes, and can be recurrent. Patches harboring them undergo mergings and fragmentations, changing their LOS velocity, shape and intensity. According to these results, supersonic Evershed down ows might be abruptly stopped in lower and denser layers occurring a shock. As a consequence, there might produce a temperature enhancement going upward, appearing bright intensity structures.