Detection of opposite polarities in a sunspot light bridge: evidence of low‐altitude magnetic reconnection

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.52). 04/2007; 376(3):1291 - 1295. DOI: 10.1111/j.1365-2966.2007.11525.x
Source: arXiv

ABSTRACT ABSTRACTA multiwavelength photometric analysis was performed in order to study the sub-structure of a sunspot light bridge in the photosphere and the chromosphere. Active region NOAA 8350 was observed on 1998 October 8. The data consist of a 100 min time series of 2D spectral scans of the lines Fe i 5576 Å, H 6563 Å, Fe i 6302.5 Å, and continuum images at 5571 Å. We recorded line-of-sight magnetograms in 6302.5 Å. The observations were taken at the Dunn Solar Telescope at US National Solar Observatory, Sacramento Peak. We find evidence for plasma ejection from a light bridge followed by Ellerman bombs. Magnetograms of the same region reveal opposite polarity in light bridge with respect to the umbra. These facts support the notion that low-altitude magnetic reconnection can result in the magnetic cancellation as observed in the photosphere.

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    ABSTRACT: We present high-resolution magnetic field measurements of a sunspot light bridge (LB) that produced chromospheric plasma ejections intermittently and recurrently for more than 1 day. The observations were carried out with the Hinode Solar Optical Telescope on 2007 April 29 and 30. The spectro-polarimeter reveals obliquely oriented magnetic fields with vertical electric current density higher than 100 mA m–2 along the LB. The observations suggest that current-carrying highly twisted magnetic flux tubes are trapped below a cusp-shaped magnetic structure along the LB. The presence of trapped current-carrying flux tubes is essential for causing long-lasting chromospheric plasma ejections at the interface with pre-existing vertically oriented umbral fields. A bidirectional jet was clearly detected, suggesting magnetic reconnections occurring at very low altitudes, slightly above the height where the vector magnetic fields are measured. Moreover, we found another strong vertical electric current on the interface between the current-carrying flux tube and pre-existing umbral field, which might be a direct detection of the currents flowing in the current sheet formed at the magnetic reconnection sites.
    The Astrophysical Journal 04/2009; 696(1):L66. · 6.28 Impact Factor
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    ABSTRACT: Context: Pores are among the photospheric features that form when the magnetic field emerges onto the solar surface. In pores or sunspots, light bridges are bright features that separate umbral areas into two or more irregular regions. Aims: We study the structure of a solar pore (AR10812) with a light bridge. Methods: We analyzed both broad-band and narrow-band images acquired with the Interferometric BI-dimensional Spectrometer at the adaptive optics channel of the NSO/Dunn Solar Telescope. Narrow-band images acquired in the photospheric Fe I 709.04 nm line were used to determine the line-of-sight velocity field. Results: The roundish shape of the pore allows us to derive the radial profiles of both intensity and vertical velocity. The pore has a downward velocity, of about -200 m s-1, and is surrounded by an annular downflow structure with an average velocity of about -300 m s-1 with respect to the nearby quiet sun. The light bridge shows a long narrow dark structure running along its axis. Corresponding to this dark lane, we measure a weak upflow of about 70 m s-1, flanked by a downflow of about 150 m s-1 with respect to the pore. The topology of this velocity structure resembles a convective roll. The anticorrelation between continuum intensity and photospheric velocity may be due to the higher gas pressure in a photospheric field-free cusp, above the light bridge, located between two magnetic walls. We present an analytical model capable of reproducing the observations.
    Astronomy and Astrophysics 10/2008; 489(2):747-754. · 4.48 Impact Factor
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    ABSTRACT: An analysis of high-resolution Dopplergrams and continuum images of NOAA AR 8350 is presented. The observations were recorded with the universal birefringent filter attached to the Dunn Solar Telescope at the National Solar Observatory, Sunspot, New Mexico. We find upward velocity of the order of 400 m s-1 within umbral dots, surrounded by downward velocity of the order of 300 m s-1. This observation is compatible with the simulations of three-dimensional radiative magnetoconvection with gray radiative transfer in sunspot umbra by Schüssler & Vögler, which support the idea that umbral dots appear as a result of magnetoconvection.
    The Astrophysical Journal 12/2008; 665(1):L79. · 6.28 Impact Factor


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