Unusual Stokes V profiles during flaring activity of a delta sunspot

¹ ESA/ESTEC RSSD, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
e-mail: cfisher@rssd.esa.int
² Sterrewacht Leiden, Universiteit Leiden, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
³ School of Physics and Astronomy, SUPA, University of Glasgow, Glasgow G12 8QQ, UK
⁴ Instituto de Astrofísica de Canarias, Avda Vía Láctea S/N, La Laguna 38200, Tenerife, Spain

Received: 23 March 2012
Accepted: 4 September 2012

Abstract

Aims. We analyze a set of full Stokes profile observations of the flaring active region NOAA 10808. The region was recorded with the Vector-Spectromagnetograph of the Synoptic Optical Long-term Investigations of the Sun facility. The active region produced several successive X-class flares between 19:00 UT and 24:00 UT on September 13, 2005 and we aim to quantify transient and permanent changes in the magnetic field and velocity field during one of the flares, which has been fully captured.

Methods. The Stokes profiles were inverted using the height-dependent inversion code LILIA to analyze magnetic field vector changes at the flaring site. We report multilobed asymmetric Stokes V profiles found in the δ-sunspot umbra. We fit the asymmetric Stokes V profiles assuming an atmosphere consisting of two components (SIR inversions) to interpret the profile shape. The results are put in context with Michelson Doppler Imager (MDI) magnetograms and reconstructed X-ray images from the Reuven Ramaty High Energy Solar Spectroscopic Imager.

Results. We obtain the magnetic field vector and find signs of restructuring of the photospheric magnetic field during the flare close to the polarity inversion line at the flaring site. At two locations in the umbra we encounter strong fields (~3 kG), as inferred from the Stokes I profiles, which, however, exhibit a low polarization signal. During the flare we observe in addition asymmetric Stokes V profiles at one of these sites. The asymmetric Stokes V profiles appear co-spatial and co-temporal with a strong apparent polarity reversal observed in MDI-magnetograms and a chromospheric hard X-ray source. The two-component atmosphere fits of the asymmetric Stokes profiles result in line-of-sight velocity differences in the range of  ~12 km s^-1 to 14 km s^-1 between the two components in the photosphere. Another possibility is that local atmospheric heating is causing the observed asymmetric Stokes V profile shape. In either case our analysis shows that a very localized patch of  ~5″ in the photospheric umbra, co-spatial with a flare footpoint, exhibits a subresolution fine structure.