High-resolution spectro-polarimetry of a flaring sunspot penumbra (0908.3803v1)

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-resolution \ion{Ca}{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 \ion{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.

• The paper details high-resolution observations that uncover intricate magnetic field structures and reconnection zones using full Stokes polarimetry.
• The study employs simultaneous photospheric and chromospheric imaging to capture dynamic flow interactions and asymmetric Evershed patterns in the sunspot penumbra.
• The findings underscore that small-scale flares can exhibit complex two-ribbon structures, challenging traditional views on flare size and energy scales.

High-Resolution Spectro-Polarimetry of a Flaring Sunspot Penumbra

This paper explores the detailed spectro-polarimetric observations of a flaring sunspot in the trailing region of NOAA 10904, conducted using high-resolution instruments at the Swedish Solar Telescope (SST). The paper provides crucial insights into solar flare dynamics at a more granular level than traditionally achieved. By capturing the photospheric and chromospheric emissions concurrently, the researchers offer a comprehensive view of the small flare activity (GOES magnitude B7.8), characteristically detailed by its two-ribbon structure. This contrasts with previous perceptions that only larger flares typically exhibit such structures.

Observational Setup

The observational setup involved multiple wavelength imaging and spectro-polarimetric techniques. The SST was employed with instrumentation allowing for observations in the \ion{Ca}{ii}\,$H$ band, capturing both the photospheric and chromospheric phenomena around the flare. A narrow-band SOUP instrument was used in conjunction with full Stokes polarimetry to measure the magnetic vector fields. Despite some technical hitches, the setup provided temporal and spatial resolutions sufficient to resolve fine-scale features in the solar atmosphere.

Magnetic Field and Flow Dynamics

The results underscore a notably complex magnetic topology in the sunspot's penumbra, where intersecting penumbral fibrils indicated oppositely directed Evershed flows. Utilizing Milne-Eddington inversions of the Stokes profiles, the researchers identified regions with magnetic azimuth discontinuities and islands of reversed vertical magnetic fields. These are hypothesized to be regions of magnetic reconnection and intense magnetic shear, likely triggering the flare onset.

The photospheric flow analyses reveal divergent and convergent flows via local correlation tracking, showing velocities which suggest dynamic interactions between penumbral branches. The penumbral branches exhibited asymmetric redshift and blueshift patterns, often exceeding typical Evershed flow velocities. Importantly, the analysis points to strong magnetic shearing and flux emergence in the sunspot's penumbra as potential flare initiators.

Chromospheric Behavior

In the chromosphere, the \ion{Ca}{ii}\,$H$ images provided clear evidence of a flare confined by a separatrix delineated by a bright boundary. Notably, the temporal evolution showcased filamentary structures and chromospheric fibrils connecting disparate magnetic regimes. The observed flare initiation at the intersection of two branches hints at locale-specific energy accumulation and impulsive release mechanisms typical of magnetic reconnection processes.

Implications and Future Directions

This paper offers a significant contribution to understanding solar flare mechanisms, particularly through the lens of small-scale flare dynamics. The intricate interplay between magnetic field configurations and flow dynamics documented herein provides insights that could refine theoretical models of flare initiation and evolution. Moreover, the observation that smaller flares can exhibit similar structures to larger flares gives rise to new questions on energy scale and flare productivity in solar physics.

Future research could expand upon these findings by exploring the temporal evolution of these magnetic structures further, employing even higher spectral resolution and leveraging contemporary techniques like machine learning for pattern recognition within these complex datasets. Such advancements can potentially decipher the tangled webs of penumbral fibrils and their interaction with flaring events, enhancing predictive capabilities for solar activity and its terrestrial impacts.