Unusual Stokes V profiles during flaring activity of a delta sunspot

TL;DR

This study investigates the magnetic and velocity field changes in a delta sunspot during a flare, revealing localized magnetic restructuring, asymmetric Stokes V profiles, and possible sub-resolution fine structures associated with flare activity.

Contribution

It provides detailed analysis of Stokes profile asymmetries and magnetic restructuring during a solar flare, highlighting localized fine structures in the photosphere.

Findings

Signs of magnetic restructuring near the PIL during the flare

Detection of asymmetric Stokes V profiles co-spatial with flare footpoints

Identification of localized sub-resolution fine structures in the photosphere

Abstract

We analyze a set of full Stokes profile observations of the flaring active region NOAA 10808 recorded with the Vector-Spectromagnetograph (VSM) of the SOLIS facility. We aim to quantify transient and permanent changes in the magnetic field and velocity field. The results are put in context with MDI magnetograms and reconstructed RHESSI X-ray images. We find signs of restructuring of the photospheric magnetic field during the flare close to the polarity inversion line (PIL) at the flaring site. At two locations in the umbra we encounter strong fields (approx. 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 approx. 12km/s to 14 km/s 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 approx. 5 arcsec in the photospheric umbra, co-spatial with a flare footpoint, exhibits a sub-resolution fine structure.