High-resolution spectro-polarimetry of a flaring sunspot penumbra

Abstract

We present simultaneous photospheric and chromospheric observations of the trailing sunspot in NOAA 10904 during a weak flare eruption (GOES magnitude B7.8), obtained with the Swedish Solar Telescope (SST) in La Palma, Canary Islands. High-resolutionCa ii H images show a typical two-ribbon structure that has been hitherto only known for larger flares, and the flare appears in a confined region that is discernible by a bright border. The underlying photosphere shows a disturbed penumbral structure with intersecting branches of penumbral filaments. High-resolution Doppler-and vector-magnetograms exhibit oppositely directed Evershed flows and magnetic field vectors in the individual penumbral branches, resulting in several regions of magnetic azimuth discontinuity and several islands where the vertical magnetic field is reversed. The discontinuity regions are co-spatial with the locations of the onset of the flare ribbons. From the results, we conclude that the confined flare region is detached from the global magnetic field structure by a separatrix marked by the bright border visible in Ca ii H. We further conclude that the islands of reversed vertical field appear because of flux emergence and that the strong magnetic shear appearing in the regions of magnetic azimuth discontinuity triggers the flare.

Lambda-shaped jets from a penumbral intrusion into a sunspot umbra: a possibility for magnetic reconnection

“…065 /pixel. More details on the data acquisition and reconstruction are described in Hirzberger et al (2009). Measurement of instrumental polarization effects of the SST optical setup was done by inserting calibration optics, consisting of a rotating polarizer and a quarter wave plate into the beam.…”

“…Measurement of instrumental polarization effects of the SST optical setup was done by inserting calibration optics, consisting of a rotating polarizer and a quarter wave plate into the beam. A code developed by Selbing (2005) was used to determine the demodulation matrix (see Hirzberger et al 2009, for more details. )…”

“…Speckle interferometric techniques (see, Hirzberger et al 2009) were used to reconstructed the G-continuum, the Ca II H core and Ca II H wing time series and SOUP polarization data. In addition, the HeLIx inversion code (Lagg et al 2004) was used to invert the Stokes vector data assuming a simple one-componentplus-straylight Milne-Eddington atmosphere.…”

Velocity fields in and around sunspots at the highest resolution

“…Shear flows are not just limited to horizontal flow fields. In a spectropolarimetric study of a B7.8 flare in active region NOAA 10904, Hirzberger et al (2009) detected (supersonic) downflows of up to 7 km s −1 in the penumbra. Twisting and interlaced penumbral filaments are no longer radially aligned with respect to the major sunspot and become almost tangential.…”

Near-infrared spectropolarimetry of aδ-spot

“…At lower spectral and spatial resolution, similar downflows of 1.5-1.7 km s −1 have been detected by Takizawa et al (2012) in regions of penumbral decay. Evershed flows of opposite polarity umbrae converge near the PIL and have to pass each other, which is indicative of an interleaved system of magnetic field lines (Lites et al 2002;Hirzberger et al 2009). Moreover, significant up-and downflows in excess of 10 km s −1 are encountered during flares (Fischer et al 2012).…”