Classification of circular polarization Stokes profiles in a sunspot using k-means clustering

Abstract

The magnetic and velocity fields in sunspots are highly structured on small spatial scales which are encoded in the Stokes profiles. The Stokes profiles are in turn, derived from a sequence of polarization modulations on the incoming light that are imaged using an analyser-detector combination. Our aim is to identify Stokes profiles in a sunspot which exhibit spectral characteristics that deviate from those associated with the Evershed flow and their corresponding spatial distribution. To that end, we employ the k-means clustering routine to classify Stokes V spectra in the penumbra of a regular, unipolar sunspot, that also comprises a granular and a filamentary light bridge. We find that 75% of the penumbral region, corresponding to about 93500 pixels, is dominated by profiles comprising two, nearly anti-symmetric lobes, while 21% of the area is occupied by three-lobed profiles that are associated with the Evershed flow returning to the solar photosphere. The remaining 4% of the penumbral area is dominated by four groups/families of profiles - Group 1: three-lobed profiles in which both the rest and strong downflowing (sometimes supersonic) component have the same polarity as the host sunspot and seen exclusively in the filamentary light bridge. Group 2: single, red-lobed profiles occupying an area of about 2% located at the outer penumbra in discrete patches that possibly signify the downflowing leg of an Ω-loop. Group 3: three-lobed or highly asymmetric profiles, in which the rest component and the strong downflowing component have an opposite polarity as the sunspot. These occupy about 1.4% of the penumbra’s area and are seen in conspicuous, elongated structures or isolated patches in the outer penumbra and at the penumbra-quiet Sun boundary. Group 4: three lobed-profiles, in which the rest component has the same polarity as the sunspot and a weaker, upflowing component with a polarity opposite that of the sunspot. These profiles are located close to the entrance of the filamentary light bridge and are found in only 0.12% of the penumbral area. These minority groups of profiles could be related to dynamic phenomena that could also affect the overlying chromosphere. The simplicity and speed of k-means can be utilized to identify such anomalous profiles in larger data sets to ascertain their temporal evolution and the physical processes responsible for these inhomogeneities.