Periodicity of Twisting Motions in Sunspot Penumbral Filaments

Solar Physics (Impact Factor: 3.26). 07/2009; 257(2):251-260. DOI: 10.1007/s11207-009-9373-2

ABSTRACT We study the periodicity of twisting motions in sunspot penumbral filaments, which were recently discovered from space (Hinode) and ground-based (SST) observations. A sunspot was well observed for 97 minutes by Hinode/SOT in the G-band (4305 Å) on 12November 2006. By the use of the time – space gradient applied to intensity space – time
plots, twisting structures can be identified in the penumbral filaments. Consistent with previous findings, we find that the
twisting is oriented from the solar limb to disk center. Some of them show a periodicity. The typical period is about ≈ four
minutes, and the twisting velocity is roughly 6 km s−1. However, the penumbral filaments do not always show periodic twisting motions during the time interval of the observations.
Such behavior seems to start and stop randomly with various penumbral filaments displaying periodic twisting during different
intervals. The maximum number of periodic twists is 20 in our observations. Studying this periodicity can help us to understand
the physical nature of the twisting motions. The present results enable us to determine observational constraints on the twisting

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    ABSTRACT: Aims. We study the recently discovered twisting motion of bright penumbral filaments with the aim of constraining their geometry and the associated magnetic field.Methods. A large sunspot located 40°  from disk center was observed at high resolution with the 1-m Swedish Solar Telescope. Inversions of multi-wavelength polarimetric data and speckle reconstructed time series of continuum images were used to determine proper motions, as well as the velocity and magnetic structure in penumbral filaments.Results. The continuum movie reveals apparent lateral motions of bright and dark structures inside bright filaments oriented parallel to the limb, confirming recent Hinode results. In these filaments we measure upflows of ${\approx}$1.1 km s$^{-1}$ on their limbward side and weak downflows on their centerward side. The magnetic field in them is significantly weaker and more horizontal than in the adjacent dark filaments.Conclusions. The data indicate the presence of vigorous convective rolls in filaments with a nearly horizontal magnetic field. These are separated by filaments harbouring stronger, more vertical fields. Because of reduced gas pressure, we see deeper into the latter. When observed near the limb, the disk-centerward side of the horizontal-field filaments appear bright due to the hot wall effect known from faculae. We estimate that the convective rolls transport most of the energy needed to explain the penumbral radiative flux.
    Astronomy and Astrophysics 09/2008; · 5.08 Impact Factor
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    ABSTRACT: We interpret penumbral filaments as due to convection in field-free, radially aligned gaps just below the visible surface of the penumbra, intruding into a nearly potential field above. This solves the classical discrepancy between the large heat flux and the low vertical velocities observed in the penumbra. The presence of the gaps causes strong small-scale fluctuations in inclination, azimuth angle and field strength, but without strong forces acting on the gas. The field is nearly horizontal in a region around the cusp-shaped top of the gap, thereby providing an environment for Evershed flows. We identify this region with the recently discovered dark penumbral cores. Its darkness has the same cause as the dark lanes in umbral light-bridges, reproduced in numerical simulations by Nordlund and Stein (2005). We predict that the large vertical and horizontal gradients of the magnetic field inclination and azimuth in the potential field model will produce the net circular polarization seen in observations. The model also explains the significant elevation of bright filaments above their surroundings. It predicts that dark areas in the penumbra are of two different kinds: dark filament cores containing the most inclined (horizontal) fields, and regions between bright filaments, containing the least inclined field lines. Comment: submitted to A&A
    Astronomy and Astrophysics 08/2005; · 5.08 Impact Factor
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    ABSTRACT: The penumbra radiates an energy flux that is roughly 75% of the quiet-sun value. One mechanism proposed to bring this flux to the surface is interchange convection of magnetic flux tubes according to which hot flux tubes rise to the surface, cool off their heat by radiation and sink down again. Another way to deposit heat in the penumbral photosphere is by steady upflows along magnetic flux tubes. We discuss these two mechanisms and elaborate on consequences that can be compared with and constrained by observations. We estimate the time scales for variations of the intensity and the magnetic field pattern. By comparing them with the corresponding observed time scales, we find that pure interchange convection is unable to account for the observed penumbral heat flux. Heating the penumbra by steady upflows along magnetic flux tubes, however, turns out to be sufficient to explain the penumbral brightness, under the condition that significant magnetic return flux is present within the penumbra. Associated with the magnetic return flux, downflows within the penumbra should be present, in accordance with recent observational findings of such downflows. Exploring other possible heating mechanisms, we find that dissipation of magnetic energy is negligible, while dissipation of the kinetic energy of the Evershed flow could contribute significantly to the brightness of the penumbra.
    Astronomy and Astrophysics 11/2003; · 5.08 Impact Factor

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