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Detection of Supergranulation Alignment in Polar Regions of the Sun by Helioseismology  

Kaori Nagashima   Submitted: 2010-12-10 11:41

We report on a new phenomenon of `alignment' of supergranulation cells in the polar regions of the Sun. Recent high-resolution datasets obtained by the Solar Optical Telescope (SOT) onboard the Hinode satellite enabled us to investigate supergranular structures in high-latitude regions of the Sun up to 80 degrees latitude. We have carried out a local helioseismology time-distance analysis of the data, and detected acoustic travel-time variations due to the supergranular flows. The supergranulation cells in both the north and south polar regions show systematic alignment patterns in the north-south direction. The south-pole datasets obtained in a month-long Hinode campaign indicate that the supergranulation alignment property may be quite common in the polar regions. We also discuss the latitudinal dependence of the supergranulation cell sizes; the data show that the east-west cell size decreases towards higher latitudes.

Authors: Kaori Nagashima, Junwei Zhao, Alexander G. Kosovichev, Takashi Sekii
Projects: Hinode/SOT

Publication Status: Accepted for publication in ApJ Letters
Last Modified: 2010-12-10 11:48
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Observations of Sunspot Oscillations in G band and Ca II H line with Solar Optical Telescope on Hinode  

Kaori Nagashima   Submitted: 2007-09-07 09:11

Exploiting high-resolution observations made by the Solar Optical Telescope onboard Hinode, we investigate the spatial distribution of power spectral density of oscillatory signal in and around NOAA active region 10935. The G-band data show that in the umbra the oscillatory power is suppressed in all frequency ranges. On the other hand, in Ca II H intensity maps oscillations in the umbra, so-called umbral flashes, are clearly seen with the power peaking around 5.5 mHz. The Ca II H power distribution shows the enhanced elements with the spatial scale of the umbral flashes over most of the umbra but there is a region with suppressed power at the center of the umbra. The origin and property of this node-like feature remain unexplained.

Authors: Kaori Nagashima, Takashi Sekii, Alexander G. Kosovichev, Hiromoto Shibahashi, Saku Tsuneta, Kiyoshi Ichimoto, Yukio Katsukawa, Bruce W. Lites, Shin'ichi Nagata, Toshifumi Shimizu, Richard A. Shine, Yoshinori Suematsu, Theodore D. Tarbell, and Alan M. Title
Projects: Hinode

Publication Status: Accepted for publication in PASJ (Hinode Special Issue), astro-ph/0709.0569
Last Modified: 2007-09-07 09:11
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Triggering Mechanism for the Filament Eruption on 2005 September 13 in Active Region NOAA 10808  

Kaori Nagashima   Submitted: 2007-07-01 22:13

On 2005 September 13 a filament eruption accompanied by a halo CME occurred in the most flare-productive active region NOAA 10808 in Solar Cycle 23. Using multi-wavelength observations before the filament eruption on Sep. 13th, we investigate the processes leading to the catastrophic eruption. We find that the filament slowly ascended at a speed of 0.1km s-1 over two days before the eruption. During slow ascending, many small flares were observed close to the footpoints of the filament, where new magnetic elements were emerging. On the basis of the observational facts we discuss the triggering mechanism leading to the filament eruption. We suggest the process toward the eruption as follows: First, a series of small flares played a role in changing the topology of the loops overlying the filament. Second, the small flares gradually changed the equilibrium state of the filament and caused the filament to ascend slowly over two days. Finally, a C2.9 flare that occurred when the filament was close to the critical point for loss of equilibrium directly led to the catastrophic filament eruption right after itself.

Authors: Kaori Nagashima, Hiroaki Isobe, Takaaki Yokoyama, Takako T. Ishii, Takenori J. Okamoto, and Kazunari Shibata
Projects: TRACE

Publication Status: ApJ (in press), astro-ph/0706.3519
Last Modified: 2007-07-08 23:25
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Statistical Study of the Reconnection Rate in Solar Flares Observed with YOHKOH/SXT  

Kaori Nagashima   Submitted: 2006-07-23 08:29

We report a statistical study of flares observed with the Soft X-Ray Telescope (SXT) onboard Yohkoh in the year 2000. We measure physical parameters of 77 flares, such as the temporal scale, size, and magnetic flux density and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationships among these parameters and find weak correlation between the temporal and spatial scales of the flares. We estimate reconnection inflow velocity, coronal Alfvén velocity, and reconnection rate using the observed values. The inflow velocities are distributed from a few km s-1 to several tens of km s-1, and the Alfvén velocities in the corona are in the range from 103 to 104 km s-1. Hence, the reconnection rate is 10-3 to 10-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within 1 order of magnitude of the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model.

Authors: Kaori Nagashima & Takaaki Yokoyama
Projects: Yohkoh-SXT

Publication Status: ApJ, 647, 654 (astro-ph/0605712)
Last Modified: 2006-08-18 19:47
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Abstracts by Author
Detection of Supergranulation Alignment in Polar Regions of the Sun by Helioseismology
Observations of Sunspot Oscillations in G band and Ca II H line with Solar Optical Telescope on Hinode
Triggering Mechanism for the Filament Eruption on 2005 September 13 in Active Region NOAA 10808
Statistical Study of the Reconnection Rate in Solar Flares Observed with YOHKOH/SXT

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