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Register a new userConvective Nature of Sunspot Penumbral Filaments: Discovery of Downflows in the Deep Photosphere
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We study the velocity structure of penumbral filaments in the deep photosphere to obtain direct evidence for the convective nature of sunspot penumbrae. A sunspot was observed at high spatial resolution with the 1-m Swedish Solar Telescope in the deep photospheric C I 5380 {AA} absorption line. The Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) method is used for image restoration and straylight is filtered out. We report here the discovery of clear redshifts in the C I 5380 {AA} line at multiple locations in sunspot penumbral filaments. For example, bright head of filaments show larger concentrated blueshift and are surrounded by darker, redshifted regions, suggestive of overturning convection. Elongated downflow lanes are also located beside bright penumbral fibrils. Our results provide the strongest evidence yet for the presence of overturning convection in penumbral filaments and highlight the need to observe the deepest layers of the penumbra in order to uncover the energy transport processes taking place there.
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The sunspot penumbra comprises numerous thin, radially elongated filaments that are central for heat transport within the penumbra, but whose structure is still not clear. To investigate the fine-scale structure of these filaments, we perform a depth-dependent inversion of spectropolarimetric data of a sunspot very close to solar disk center obtained by Hinode (SOT/SP). We have used a recently developed spatially coupled 2D inversion scheme which allows us to analyze the fine structure of individual penumbral filaments up to the diffraction limit of the telescope. Filaments of different sizes in all parts of penumbra display very similar magnetic field strengths, inclinations and velocity patterns. The similarities allowed us to average all these filaments and to extract the physical properties common to all of them. This average filament shows upflows associated with an upward pointing field at its inner, umbral end and along its axis, downflows along the lateral edge and strong downflows in the outer end associated with a nearly vertical, strong and downward pointing field. The upflowing plasma is significantly hotter than the downflowing plasma. The hot, tear-shaped head of the averaged filament can be associated with a penumbral grain. The central part of the filament shows nearly horizontal fields with strengths of ~1kG. The field above the filament converges, whereas a diverging trend is seen in the deepest layers near the head of the filament. We put forward a unified observational picture of a sunspot penumbral filament. It is consistent with such a filament being a magneto-convective cell, in line with recent MHD simulations. The uniformity of its properties over the penumbra sets constraints on penumbral models and simulations. The complex and inhomogeneous structure of the filament provides a natural explanation for a number of long-running controversies in the literature.
Recent numerical simulations and observations of sunspots show a significant amount of opposite polarity magnetic field within the sunspot penumbra. Most of the opposite polarity field is associated with convective downflows. We present an analysis of 3D MHD simulations through forward modeling of synthetic Stokes profiles of the Fesci 6301.5 AA~ and Fesci 6302.5 AA~ lines). The synthetic Stokes profiles are spatially and spectrally degraded considering typical instrument properties. Line bisector shifts of the Fesci 6301.5 AA~ line are used to determine line-of-sight velocities. Far wing magnetograms are constructed from the Stokes V profiles of the Fesci 6302.5 AA~ line. While we find an overall good agreement between observations and simulations, the fraction of opposite polarity magnetic field, the downflow filling factor and the opposite polarity-downflow association are strongly affected by spatial smearing and presence of strong gradients in the line-of-sight magnetic field and velocity. A significant fraction of opposite polarity magnetic field and downflows are hidden in the observations due to typical instrumental noise. Comparing simulations that differ by more than a factor of two in grid spacing we find that these quantities are robust within the simulations.
We study the velocity field of umbral dots at a resolution of 0.14. Our analysis is based on full Stokes spectropolarimetric measurements of a pore taken with the CRISP instrument at the Swedish 1-m Solar Telescope. We determine the flow velocity at different heights in the photosphere from a bisector analysis of the Fe I 630 nm lines. In addtion, we use the observed Stokes Q, U, and V profiles to characterize the magnetic properties of these structures. We find that most umbral dots are associated with strong upflows in deep photospheric layers. Some of them also show concentrated patches of downflows at their edges, with sizes of about 0.25, velocities of up to 1000 m/s, and enhanced net circular polarization signals. The downflows evolve rapidly and have lifetimes of only a few minutes. These results appear to validate numerical models of magnetoconvection in the presence of strong magnetic fields.
Sunspot penumbrae show high-velocity patches along the periphery. The high-velocity downflow patches are believed to be the return channels of the Evershed flow. We aim to investigate their structure in detail using Hinode SOT/SP observations. We employ Fourier interpolation in combination with spatially coupled height dependent LTE
Penumbral filaments and light bridges are prominent structures inside sunspots and are important for understanding the nature of sunspot magnetic fields and magneto-convection underneath. We investigate an interesting event where several penumbral filaments intruded into a sunspot light bridge for more insights into magnetic fields of the sunspot penumbral filament and light bridge, as well as their interaction. The emission, kinematic, and magnetic topology characteristics of the penumbral filaments intruding into the light bridge and the resultant jets are studied. At the west part of the light bridge, the intruding penumbral filaments penetrated into the umbrae on both sides of the light bridge, and two groups of jets were also detected. The jets shared the same projected morphology with the intruding filaments and were accompanied by intermittent footpoint brightenings. Simultaneous spectral imaging observations provide convincing evidences for the presences of magnetic reconnection related heating and bidirectional flows near the jet bases and contribute to measuring vector velocities of the jets. Additionally, nonlinear force-free field extrapolation results reveal strong and highly inclined magnetic fields along the intruding penumbral filaments, consistent well with the results deduced from the vector velocities of the jets. Therefore, we propose that the jets could be caused by magnetic reconnections between emerging fields within the light bridge and the nearly horizontal fields of intruding filaments. They were then ejected outward along the stronger filaments fields. Our study indicates that magnetic reconnection could occur between the penumbral filament fields and emerging fields within light bridge and produce jets along the stronger filament fields. These results further complement the study of magnetic reconnection and dynamic activities within the sunspot.
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