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Statistical Analysis of Current Helicity and Twist in Solar Active Regions over the Phases of the Solar Cycle Using the Spectro-Polarimeter Data of Hinode  

Kenichi Otsuji   Submitted: 2014-10-29 19:10

Current helicity and twist of solar magnetic fields are important quantities to characterize the dynamo mechanism working in the convection zone of the Sun. We have carried out a statistical study on the current helicity of solar active regions observed with the Spectro-Polarimeter (SP) of Hinode Solar Optical Telescope (SOT). We used SOT-SP data of 558 vector magnetograms of a total of 80 active regions obtained from 2006 to 2012. We have applied spatial smoothing and division of data points into weak and strong field ranges to compare the contributions from different scales and field strengths. We found that the current helicity follows the so-called hemispheric sign rule when the weak magnetic fields (absolute field strength < 300 gauss) are considered and no smoothing is applied. On the other hand, the pattern of current helicity fluctuates and violates the hemispheric sign rule when stronger magnetic fields are considered and the smoothing of 2.0 arcsec (mimicking ground-based observations) is applied. Furthermore, we found a tendency that the weak and inclined fields better conform to and the strong and vertical fields tend to violate the hemispheric sign rule. These different properties of helicity through the strong and weak magnetic field components give important clues to understanding the solar dynamo as well as the mechanism of formation and evolution of solar active regions.

Authors: Kenichi Otsuji, Takashi Sakurai and Kirill Kuzanyan
Projects: Hinode/SOT

Publication Status: accepted
Last Modified: 2014-10-30 09:49
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Statistical Study on the Nature of Solar Flux Emergence  

Kenichi Otsuji   Submitted: 2011-06-12 20:09

We studied 101 flux emergence events ranging from small ephemeral regions tolarge emerging flux regions which were observed with Hinode Solar OpticalTelescope filtergram. We investigated how the total magnetic flux of theemergence event controls the nature of emergence. To determine the modes ofemergences, horizontal velocity fields of global motion of the magnetic patchesin the flux emerging sites were measured by the local correlation tracking.Between two main polarities of the large emerging flux regions with more thanaround 2 imes 1019 Mx, there were the converging flows of anti-polaritymagnetic patches. On the other hand, small ephemeral regions showed noconverging flow but simple diverging pattern. When we looked into the detailedfeatures in the emerging sites, irrespective of the total flux and the spatialsize, all the emergence events were observed to consist of single or multipleelementary emergence unit(s). The typical size of unitary emergence is 4 Mm andconsistent with the simulation results. From the statistical study of the fluxemergence events, the maximum spatial distance between two main polarities, themagnetic flux growth rate and the mean separation speed were found to followthe power-law functions of the total magnetic flux with the indices of 0.27,0.57, and -0.16, respectively. From the discussion on the observed power-lawrelations, we got a physical view of solar flux emergence that emergingmagnetic fields float and evolve balancing to the surrounding turbulentatmosphere. Key words: Sun: magnetic fields - Sun: emerging flux - Sun:photosphere - Sun: chromosphere

Authors: Kenichi Otsuji, Reizaburo Kitai, Kiyoshi Ichimoto, Kazunari Shibata
Projects: Hinode/SOT,SoHO-MDI

Publication Status: Accepted
Last Modified: 2011-06-14 19:51
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Ca II K Spectral Study of an Emerging Flux Region using Domeless Solar Telescope in Hida Observatory  

Kenichi Otsuji   Submitted: 2010-05-12 18:10

A cooperative observation with Hida observatory and Hinode satellite was performed on an emerging flux region. The successive Ca II K spectro-heliograms of the emerging flux region were taken by the Domeless Solar Telescope of Hida observatory. Hinode observed the emerging flux region with Ca II H and Fe I Stokes IQUV filtergrams. In this study, detailed dynamics and temporal evolution of the magnetic flux emergence was studied observationally. The event was first detected in the photospheric magnetic field signals. 3 minutes later, the horizontal expansion of the dark area was detected. And then, 7 minutes later than the horizontal expansion, the emerging loops were detected with the maximal rise speed of 2.1 km s-1 at chromospheric heights. The observed dynamics of emerging magnetic flux from the photosphere to the upper chromosphere is well consistent with the results of previous simulation works. The gradual rising phase of flux tubes with a weak magnetic strength was confirmed by our observation.

Authors: Kenichi Otsuji, Reizaburo Kitai, Takuma Matsumoto, Kiyoshi Ichimoto, Satoru Ueno, Shin'ichi Nagata, Hiroaki Isobe, Kazunari Shibata
Projects: Hinode/SOT

Publication Status: Accepted
Last Modified: 2010-05-13 07:17
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Small Scale Magnetic Flux Emergence Observed with Hinode/Solar Optical Telescope  

Kenichi Otsuji   Submitted: 2007-09-20 06:15

We observed small scale magnetic flux emergence in a sunspot moat region by the Solar Optical Telescope (SOT) aboard the Hinode satellite. We analyzed filtergram images observed in the wavelengths of Fe 6302 angstrom, G-band and Ca II H. In Stokes I images of Fe 6302 angstrom, emerging magnetic flux were recognized as dark lanes. In G-band, they showed their shapes almost the same as in Stokes I images. These magnetic flux appeared as dark filaments in Ca II H images. Stokes V images of Fe 6302 angstrom showed pairs of opposite polarities at footpoints of each filament. These magnetic concentrations are identified to correspond to bright points in G-band/Ca II H images. From the analysis of time-sliced diagrams, we derived following properties of emerging flux, which are consistent with the previous works. (1) Two footpoints separate each other at a speed of 4.2 km s-1 during the initial phase of evolution and decreases to about 1 km s-1 in 10 minutes later. (2) Ca II H filaments appear almost simultaneously with the formation of dark lanes in Stokes I in the observational cadence of 2 minutes. (3) The lifetime of the dark lanes in Stokes I and G-band is 8 minutes, while that of Ca filament is 12 minutes. An interesting phenomena was observed that an emerging flux tube expands laterally in the photosphere with a speed of 3.8 km s-1. Discussion on the horizontal expansion of flux tube will be given with refernce to previous simulation studies.

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

Publication Status: Accepted for publication in PASJ (Hinode Special Issue), astro-ph/0709.3207
Last Modified: 2010-05-13 09:17
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Abstracts by Author
Statistical Analysis of Current Helicity and Twist in Solar Active Regions over the Phases of the Solar Cycle Using the Spectro-Polarimeter Data of Hinode
Statistical Study on the Nature of Solar Flux Emergence
Ca II K Spectral Study of an Emerging Flux Region using Domeless Solar Telescope in Hida Observatory
Small Scale Magnetic Flux Emergence Observed with Hinode/Solar Optical Telescope

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