Detection of sea-serpent field lines in sunspot penumbrae

Abstract

We investigate the spatial distribution of magnetic polarities in the penumbra of a spot observed very close to disk center. High-spatial resolution, high-cadence magnetograms taken with the Narrowband Filter Imager aboard Hinode are used in this study. They provide continuous and stable measurements in the photospheric Fe I 630.25 line for long periods of time. We discover small-scale, elongated, bipolar magnetic structures that appear in the mid penumbra and move radially outward across the penumbra. They occur in between the more vertical fields of the penumbra, and can be associated with the horizontal fields that harbor the Evershed flow. Many of them cross the outer penumbral boundary, becoming moving magnetic features in the sunspot moat. We determine the properties of these structures, including their sizes, proper motions, footpoint separation, and lifetimes. The bipolar patches can be interpreted as being produced by sea-serpent field lines that originate in the mid-penumbra and eventually leave the spot in the form moving magnetic features. The existence of such field lines has been inferred from Stokes inversions of spectropolarimetric measurements at lower angular resolution, but this is the first time they are imaged directly. Our observations add another piece of evidence in favor of the uncombed structure of penumbral magnetic fields.

4 pages, 3 figures, 1 movie. Letter accepted by Astronomy & Astrophysics

References in corpus (6)

• MHD simulations of penumbra fine structure
• Magnetostatic penumbra models with field-free gaps
• Vector spectropolarimetry of dark-cored penumbral filaments with Hinode
• Temporal evolution of the Evershed flow in sunspots. I. Observational characterization of Evershed clouds
• The velocity structure of moving magnetic feature pairs around sunspots: support for the U-loop model
• Temporal evolution of the Evershed flow in sunspots. II. Physical properties and nature of Evershed clouds

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• Spectropolarimetery of umbral fine structures from Hinode: Evidence for magnetoconvection
• Penumbral decay observed in active region NOAA 12585
• Magnetic topology of a naked sunspot: Is it really naked?
• Interaction of solar jets with filaments: Triggering of large-amplitude filament oscillations
• The dynamics of 3-minute wavefronts and their relation to sunspot magnetic fields
• Expulsion of counter Evershed flows from sunspot penumbrae
• The formation and decay of sunspot penumbra in Active Region NOAA 12673