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Ellerman Bombs at High Resolution. I. Morphological Evidence for Photospheric Reconnection  

Hiroko Watanabe   Submitted: 2011-07-06 20:45

High-resolution imaging-spectroscopy movies of solar active region NOAA 10998 obtained with the Crisp Imaging Spectropolarimeter at the Swedish 1-m Solar Telescope show very bright, rapidly flickering, flame-like features that appear intermittently in the wings of the Balmer Hα line in a region with moat flows and likely some flux emergence. They show up at regular Hα blue-wing bright points that outline the magnetic network, but flare upward with much larger brightness and distinct 'jet' morphology seen from aside in the limbward view of these movies. We classify these features as Ellerman bombs and present a morphological study of their appearance at the unprecedented spatial, temporal, and spectral resolution of these observations. The bombs appear along the magnetic network with footpoint extents up to 900 km. They show apparent travel away from the spot along the pre-existing network at speeds of about 1 km s–1. The bombs flare repetitively with much rapid variation at timescales of seconds only, in the form of upward jet-shaped brightness features. These reach heights of 600-1200 km and tend to show blueshifts; some show bi-directional Doppler signature and some seem accompanied with an Hα surge. They are not seen in the core of Hα due to shielding by overlying chromospheric fibrils. The network where they originate has normal properties. The morphology of these jets strongly supports deep-seated photospheric reconnection of emergent or moat-driven magnetic flux with pre-existing strong vertical network fields as the mechanism underlying the Ellerman bomb phenomenon.


Authors: Hiroko Watanabe, Gregal Vissers, Reizaburo Kitai, Luc Rouppe van der Voort, Robert J. Rutten
Projects: None

Publication Status: Published in ApJ, 736, 71
Last Modified: 2011-07-07 14:25
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Temporal Evolution of a Rapidly-Moving Umbral Dot  

Hiroko Watanabe   Submitted: 2010-09-24 00:13

We performed two-dimensional spectroscopic observations of the preceding sunspot of NOAA 10905 located off disk center (S8 E36, μ ≈ 0.81) by using the Interferometric BI-dimensional Spectrometer (IBIS) operated at the Dunn Solar Telescope (DST) of the National Solar Observatory, New Mexico. The magnetically insensitive Fe I line at 709.04 nm was scanned in wavelength repetitively at an interval of 37 s to calculate sequences of maps of the line-wing and line-core intensity, and the line-of-sight Doppler velocity at different line depths (3% to 80%). Visual inspection of movies based on speckle reconstructions computed from simultaneous broadband data and the local continuum intensity at 709.04 nm revealed an umbral dot (UD) intruding rapidly from the umbral boundary to the center of the umbra. The apparent motion of this object was particularly fast (1.3 km s-1) when compared to typical UDs. The lifetime and size of the UD was 8.7min and 240 km, respectively. The rapid UD was visible even in the line-core intensity map of Fe I 709.04 nm and was accompanied by a persistent blueshift of about 0.06 km s-1.

Authors: Hiroko Watanabe ? Alexandra Tritschler ? Reizaburo Kitai ? Kiyoshi Ichimoto
Projects: National Solar Observatory (Sac Peak)

Publication Status: Published in Solar Physics Journal, Volume 266, Number 1 / September 2010
Last Modified: 2010-09-24 06:58
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Characteristic Dependence of Umbral Dots on their Magnetic Structure  

Hiroko Watanabe   Submitted: 2009-07-15 23:25

Umbral dots (UDs) were observed in a stable sunspot in NOAA 10944 by the Hinode Solar Optical Telescope on 2007 March 1. The observation program consisted of blue continuum images and spectropolarimetric profiles of Fe I 630 nm line. An automatic detection algorithm for UDs was applied to the 2-hour continuous blue continuum images, and using the obtained data, the lifetime, size, and proper motion of UDs were calculated. The magnetic structure of the sunspot was derived through the inversion of the spectropolarimetric profiles. We calculated the correlations between UD's parameters (size, lifetime, occurrence rate, proper motion) and magnetic fields (field strength, inclination, azimuth), and obtained the following results: (1) Both the lifetime and size of UDs are almost constant regardless of the magnetic field strength at their emergence site. (2) The speed of UDs increases as the field inclination angle at their emergence site gets larger. (3) The direction of movement of UDs is nearly parallel to the direction of the horizontal component of magnetic field in the region with strongly inclined field, while UDs in the region with weakly inclined field show virtually no proper motion. Our results describe the basic properties of magnetoconvection in sunspots. We will discuss our results in comparison to recent MHD simulations by Schussler & Vogler (2006) and Rempel et al. (2009).

Authors: Hiroko Watanabe, Reizaburo Kitai, Kiyoshi Ichimoto
Projects: Hinode/SOT

Publication Status: accepted for publication in ApJ
Last Modified: 2009-07-16 08:34
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Magnetic Structure of Umbral Dots Observed with Hinode Solar Optical Telescope  

Hiroko Watanabe   Submitted: 2008-11-06 20:49

High resolution and seeing-free spectroscopic observation of a decaying sunspot was done with the Solar Optical Telescope aboard Hinode satellite. The target was NOAA 10944 located in the west side of the solar surface from March 2 to March 4, 2007. The umbra included many umbral dots (UDs) with size of ~300 km in continuum light. We report the magnetic structures and Doppler velocity fields around UDs, based on the Milne-Eddington inversion of the two iron absorption lines at 6302 angstrom. The histograms of magnetic field strength(B), inclination angle(i), and Doppler velocity(v) of UDs showed a center-to-limb variation. Observed at disk center, UDs had (1)slightly smaller field strength (Delta B=-17 Gauss) and (2)relative blue shifts (Delta v=28 m s-1) compared to their surroundings. When the sunspot got close to the limb, UDs and their surroundings showed almost no difference in the magnetic and Doppler values. This center-to-limb variation can be understood by the formation height difference in a cusp-shaped magnetized atmosphere around UDs, due to the weakly magnetized hot gas intrusion. In addition, some UDs showed oscillatory light curves with multiple peaks around 10 min, which may indicate the presence of the oscillatory convection. We discuss our results in the frameworks of two theoretical models, the monolithic model (Schussler & Vogler 2006) and the field-free intrusion model (Spruit & Scharmer 2006).

Authors: Hiroko Watanabe, Reizaburo Kitai, Kiyoshi Ichimoto, Yukio Katsukawa
Projects: Hinode/SOT

Publication Status: accepted for publication in PASJ Vol.61 No.2, astro-ph/0811.1074
Last Modified: 2008-11-07 11:22
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Subject will be restored when possible  

Hiroko Watanabe   Submitted: 2008-05-28 22:03

High spatial resolution observation of an emerging flux region (EFR) was done using a vector magnetograph and a Hα Lyot filtergraph with Domeless Solar Telescope at Hida Observatory on October 22, 2006. In Hα wing images, we could see many Ellerman bombs (EBs) in the EFR. Two observation modes, slit scan and slit fixed, were performed with the vector magnetograph, along with Hα filtergraph. Using Hα wing images, we detected 12 EBs during slit scan observation period and 9 EBs during slit fixed observation period. With slit scan observation, we found that all the EBs were distributed in the area where the spatial gradient of vertical field intensity was large, which indicates the possibility of rapid topological change in magnetic field in the area of EBs. With slit fixed observation, we found that EBs were distributed in the areas of undulatory magnetic fields, both in vertical and horizontal components. This paper is the first to report the undulatory pattern in horizontal components of magnetic field, which is also evidence for emerging magnetic flux by Parker instability. These results allow us to confirm the association between EBs and emerging flux tubes. Three triggering mechanism of EBs is discussed with respect to emerging flux tubes: 9 out of 21 EBs occurred at the footpoints of emerging flux tubes, 8 out of 21 EBs occurred at the top of emerging flux tubes, and 4 out of 21 EBs occurred at unipolar region. Each case can be explained by magnetic reconnection in the low chromosphere.

Authors: H. Watanabe, R. Kitai, K. Okamoto, K. Nishida, J. Kiyohara, S. UeNo, M. Hagino, T. T. Ishii, and K. Shibata
Projects: None

Publication Status: accepted for publication in ApJ
Last Modified: 2008-09-23 21:01
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Subject will be restored when possible  

Hiroko Watanabe   Submitted: 2008-04-06 23:39

A high resolution imaging observation of a sunspot umbra was made with the Hinode Solar Optical Telescope. Filtergrams at wavelengths of the blue and green continua were taken during three consecutive days. The umbra consisted of a dark core region, several diffuse components, and numerous umbral dots. We derived basic properties of umbral dots (UDs), especially their temperatures, lifetimes, proper motions, spatial distribution, and morphological evolution. The brightness of UDs is confirmed to depend on the brightness of their surrounding background. Several UDs show fission and fusion. Thanks to the stable condition of the space observation, we could for the first time follow the temporal behavior of these events. The derived properties of the internal structure of the umbra are discussed from the viewpoint of magnetoconvection in a strong magnetic field.

Authors: Reizaburo Kitai, Hiroko Watanabe, Tahei Nakamura, Ken-ichi Otsuji, Takuma Matsumoto, Satoru UeNo, Shin-ichi Nagata, Kazunari Shibata, Richard Muller, Kiyoshi Ichimoto, Saku Tsuneta, Yoshinori Suematsu, Yukio Katsukawa, Toshifumi Shimizu, Theodore D. Tarbell, Richard A. Shine, Alan M. Title, Bruce W. Lites
Projects: Hinode/SOT

Publication Status: Published in PASJ 59 No.SP3 (Hinode Special Issue)
Last Modified: 2008-09-23 21:11
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Hiroko Watanabe   Submitted: 2008-04-06 23:38

A high resolution imaging observation of a sunspot umbra was made with the Hinode Solar Optical Telescope. Filtergrams at wavelengths of the blue and green continua were taken during three consecutive days. The umbra consisted of a dark core region, several diffuse components, and numerous umbral dots. We derived basic properties of umbral dots (UDs), especially their temperatures, lifetimes, proper motions, spatial distribution, and morphological evolution. The brightness of UDs is confirmed to depend on the brightness of their surrounding background. Several UDs show fission and fusion. Thanks to the stable condition of the space observation, we could for the first time follow the temporal behavior of these events. The derived properties of the internal structure of the umbra are discussed from the viewpoint of magnetoconvection in a strong magnetic field.

Authors: Reizaburo Kitai, Hiroko Watanabe, Tahei Nakamura, Ken-ichi Otsuji, Takuma Matsumoto, Satoru UeNo, Shin-ichi Nagata, Kazunari Shibata, Richard Muller, Kiyoshi Ichimoto, Saku Tsuneta, Yoshinori Suematsu, Yukio Katsukawa, Toshifumi Shimizu, Theodore D. Tarbell, Richard A. Shine, Alan M. Title, Bruce W. Lites
Projects: Hinode/SOT

Publication Status: Published in PASJ 59 (Hinode Special Issue), astro-ph/0711.3266
Last Modified: 2008-09-23 21:11
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Hiroko Watanabe   Submitted: 2007-11-21 00:59

High resolution imaging observation of a sunspot umbra was done with Hinode Solar Optical Telescope (SOT). Filtergrams in wavelengths of blue and green continuum were taken during three consecutive days. The umbra consisted of a dark core region, several diffuse components and numerous umbral dots. We derived basic properties of umbral dots (UDs), especially their temperatures, lifetimes, proper motions, spatial distribution and morphological evolution. Brightness of UDs is confirmed to depend on the bright- ness of their surrounding background. Several UDs show fission and fusion. Thanks to the stable condition of space observation, we could first follow the temporal behavior of these events. The derived properties of internal structure of the umbra are discussed in viewpoint of magnetoconvection in a strong magnetic field.

Authors: Reizaburo Kitai, Hiroko Watanabe, Tahei Nakamura, Ken-ichi Otsuji, Takuma Matsumoto, Satoru UeNo, Shin-ichi Nagata, Kazunari Shibata, Richard Muller, Kiyoshi Ichimoto, Saku Tsuneta, Yoshinori Suematsu, Yukio Katsukawa, Toshifumi Shimizu, Theodore D. Tarbell, Richard A. Shine, Alan M. Title, Bruce W. Lites
Projects: Hinode

Publication Status: ccepted for publication in PASJ (Hinode Special Issue), astro-ph/0711.3266
Last Modified: 2007-11-21 11:20
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Hiroko Watanabe   Submitted: 2007-11-21 00:59

High resolution imaging observation of a sunspot umbra was done with Hinode Solar Optical Telescope (SOT). Filtergrams in wavelengths of blue and green continuum were taken during three consecutive days. The umbra consisted of a dark core region, several diffuse components and numerous umbral dots. We derived basic properties of umbral dots (UDs), especially their temperatures, lifetimes, proper motions, spatial distribution and morphological evolution. Brightness of UDs is confirmed to depend on the bright- ness of their surrounding background. Several UDs show fission and fusion. Thanks to the stable condition of space observation, we could first follow the temporal behavior of these events. The derived properties of internal structure of the umbra are discussed in viewpoint of magnetoconvection in a strong magnetic field.

Authors: Reizaburo Kitai, Hiroko Watanabe, Tahei Nakamura, Ken-ichi Otsuji, Takuma Matsumoto, Satoru UeNo, Shin-ichi Nagata, Kazunari Shibata, Richard Muller, Kiyoshi Ichimoto, Saku Tsuneta, Yoshinori Suematsu, Yukio Katsukawa, Toshifumi Shimizu, Theodore D. Tarbell, Richard A. Shine, Alan M. Title, Bruce W. Lites
Projects: Hinode

Publication Status: Published in PASJ 59 (Hinode Special Issue), astro-ph/0711.3266
Last Modified: 2008-01-07 03:14
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Abstracts by Author
Ellerman Bombs at High Resolution. I. Morphological Evidence for Photospheric Reconnection
Temporal Evolution of a Rapidly-Moving Umbral Dot
Characteristic Dependence of Umbral Dots on their Magnetic Structure
Magnetic Structure of Umbral Dots Observed with Hinode Solar Optical Telescope
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