Properties of Supersonic Evershed Downflows

Abstract

We study supersonic Evershed downflows in a sunspot penumbra by means of high spatial resolution spectropolarimetric data acquired in the Fe I 617.3 nm line with the CRISP instrument at the Swedish 1-m Solar Telescope. Physical observables, such as Dopplergrams calculated from line bisectors and Stokes V zero-crossing wavelengths, and Stokes V maps in the far red wing, are used to find regions where supersonic Evershed downflows may exist. We retrieve the LOS velocity and the magnetic field vector in these regions using two-component inversions of the observed Stokes profiles with the help of the SIR code. We follow these regions during their lifetime to study their temporal behavior. Finally, we carry out a statistical analysis of the detected supersonic downflows to characterize their physical properties. Supersonic downflows are contained in compact patches moving outward, which are located in the mid and outer penumbra. They are observed as bright, roundish structures at the outer end of penumbral filaments that resemble penumbral grains. The patches may undergo fragmentations and mergings during their lifetime, even some of them are recurrents. Supersonic downflows are associated with strong and rather vertical magnetic fields with a reversed polarity compared to that of the sunspot. Our results suggest that downflows returning back to the solar surface with supersonic velocities are abruptly stopped in dense deep layers and produce a shock. Consequently, this shock enhances the temperature and is detected as a bright grain in the continuum filtergrams, which could explain the existence of outward moving grains in the mid and outer penumbra.

15 pages, 16 figures. Accepted for publication in ApJ. Movies are available at http://dubshen.astro.su.se/~sara

References in corpus (14)

• CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
• CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter
• Numerical sunspot models: Robustness of photospheric velocity and magnetic field structure
• Spatially coupled inversion of spectro-polarimetric image data I: Method and first results
• Spectral Line Selection for HMI: A Comparison of Fe I 6173 and Ni I 6768
• Peripheral downflows in sunspot penumbrae
• Multi-frame blind deconvolution with linear equality constraints
• Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope
• Evershed clouds as precursors of moving magnetic features around sunspots
• On the velocity field of sunspot penumbrae - I. A global view
• Temporal evolution of the Evershed flow in sunspots. I. Observational characterization of Evershed clouds
• The velocity field of sunspot penumbrae II. Return flow and magnetic fields of opposite polarity
• Lateral downflows in sunspot penumbral filaments and their temporal evolution
• The Evershed Flow and the Brightness of the Penumbra

Cited by in corpus (8)

• STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar observations
• Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)
• Detection of the strongest magnetic field in a sunspot light bridge
• Statistical Investigation of Supersonic Downflows in the Transition Region above Sunspots
• Magnetic Properties and Flow Angle of the Inverse Evershed Flow at Its Downflow Points
• Three-lobed near-infrared Stokes V profiles in the quiet Sun
• Temporal evolution of short-lived penumbral microjets
• The MODEST catalog of depth-dependent spatially coupled inversions of sunspots observed by Hinode/SOT-SP