Temporal Evolution of Velocity and Magnetic Field in and around Umbral Dots

Abstract

We study the temporal evolution of umbral dots (UDs) using measurements from the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Scans of the magnetically sensitive 630 nm iron lines were performed under stable atmospheric conditions for 71 min with a cadence of 63 s. These observations allow us to investigate the magnetic field and velocity in and around UDs at a resolution approaching 0.13". From the analysis of 339 UDs, we draw the following conclusions: (1)UDs show clear hints of upflows, as predicted by magnetohydrodynamic (MHD) simulations. By contrast, we could not find systematic downflow signals. Only in very deep layers we detect localized downflows around UDs, but they do not persist in time. (2) We confirm that UDs exhibit weaker and more inclined fields than their surroundings, as reported previously. However, UDs that have strong fields above 2000 G or are in the decay phase show enhanced and more vertical fields. (3)There are enhanced fields at the migration front of UDs detached from penumbral grains, as if their motion were impeded by the ambient field. (4) Long-lived UDs travel longer distances with slower proper motions. Our results appear to confirm some aspects of recent numerical simulations of magnetoconvection in the umbra (e.g., the existence of upflows in UDs), but not others (e.g., the systematic weakening of the magnetic field at the position of UDs.)

27 pages, 26 figures, accepted for publication in the Astrophysical Journal

References in corpus (5)

• Magneto-convection in a sunspot umbra
• MHD simulations of penumbra fine structure
• Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope
• Brightness, distribution, and evolution of sunspot umbral dots
• Properties of Umbral Dots from Stray Light Corrected Hinode Filtergrams

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• High Resolution Observations of Chromospheric Jets in Sunspot Umbra
• Dynamics in Sunspot Umbra as Seen in New Solar Telescope and Interface Region Imaging Spectrograph Data
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• The Observational Evidence for the Internal Excitation of Sunspot Oscillations Inferred from the Fe I 5435 A Line
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• Proper Motions of Sunspots' Umbral Dots at High Temporal and Spatial Resolution
• Ti I lines at 2.2 $\mu$m as probes of the cool parts of sunspots
• Hinode 7: Conference Summary and Future Suggestions
• Similarities of magnetoconvection in the umbra and in the penumbra of sunspots