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Author name code: watanabe-hiroko
ADS astronomy entries on 2022-09-14
=author:"Watanabe, Hiroko"
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Title: Observations of umbral dots and their physical models
Authors: Watanabe, Hiroko
2014PASJ...66S...1W Altcode: 2014arXiv1407.4581W; 2014PASJ..tmp..111W
The Hinode satellite opens a new era in sunspot research, because of its
high spatial resolution and temporal stability. Fine-scale structures
in sunspots, called umbral dots (UDs), have become one of the hottest
topics in terms of close observations of magnetoconvection. In this
paper, a brief review of the observed properties of UDs is given
based on recent literature. UDs born in the periphery of the umbra
exhibit inward migration, and their speeds are positively correlated
with the magnetic field inclination. Longer-lasting UDs tend to be
larger and brighter, while the lifetimes of UDs show no relation
to their background magnetic field strength. UDs tend to disappear,
or stop their proper motion by colliding with a locally strong field
region. The spatial distribution of UDs is not uniform over an umbra,
but is rather located at the boundaries of cellular patterns. From our
two-dimensional correlation analysis, we measured the characteristic
width of the cell boundaries (≈ 0{^”<SUB>.</SUB>}5) and the size of
the cells (≈ 6″). We then performed a simplified analysis to obtain
statistics of how the UD distribution is random or clustered using
Hinode blue continuum images. We have found a hint that the UDs become
less dense and more clustered for later-phase sunspots. These results
may be related to the evolutional change of the subsurface structure
of a sunspot. Based on these observational results, we discuss their
physical models by means of numerical simulations of magnetoconvection.
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Title: Formation and Decay of Rudimentary Penumbra around a Pore
Authors: Watanabe, Hiroko; Kitai, Reizaburo; Otsuji, Kenichi
2014ApJ...796...77W Altcode:
We analyze the evolution of a pore in the active region NOAA 10940
using the data obtained by the Hinode satellite on 2007 February
3. The pore we analyzed showed the formation of a rudimentary penumbra
structure, succeeded by an abrupt disappearance after about 5 hr. The
pore had an approximate radius of 3.5 Mm and a total magnetic flux
of 3.0 × 10<SUP>19</SUP> Mx, which is a little smaller than the
necessary magnetic flux for penumbral formation supposed by Rucklidge
et al. (1-1.5 × 10<SUP>20</SUP> Mx). Our observation describes
a rare phenomenon which was in the unstable phase between a pore
and a sunspot. The area of the dark umbra gradually decreased when
the rudimentary penumbral filaments formed the penumbral structure,
meaning that the penumbra develops at the expense of the umbral magnetic
flux. This statement was confirmed by a rough estimation of the magnetic
flux variation observed by the Hinode Fe I magnetogram. Five hours after
the formation phase, the decay phase began. In this decaying phase,
multiple opposite polarity patches are found to appear in the exterior
of the pore (a different location from the penumbra formation site). We
interpret these opposite polarities as signatures of the horizontal
magnetic field, which preferably appears in the course of the unstable
reconfiguration of the magnetic field structure. During the course of
the disappearance of the penumbra, the horizontal penumbral field seems
to become vertical because of the dark umbral area that recovered by
about 10%.
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Title: Morphological study of penumbral formation
Authors: Kitai, Reizaburo; Watanabe, Hiroko; Otsuji, Ken'ichi
2014PASJ...66S..11K Altcode: 2014PASJ..tmp...96K; 2014arXiv1407.4573K
Penumbrae are known to be areas of mainly horizontal magnetic field
surrounding umbrae of relatively large and mature sunspots. In this
paper, we observationally studied the formation of penumbrae in NOAA
10978, where several penumbral formations were observed in G-band
images of the Solar Optical Telescope on board Hinode. Thanks to the
continuous observation by Hinode, we could morphologically follow the
evolution of sunspots and found that there are several paths to the
penumbral formation: (1) active accumulation of magnetic flux, (2) rapid
emergence of magnetic field, and (3) appearance of twisted or rotating
magnetic tubes. In all of these cases, magnetic fields are expected
to sustain high inclination at the edges of flux tube concentration
longer than the characteristic growth time of downward magnetic pumping.
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Title: Temporal Evolution of Velocity and Magnetic Field in and
around Umbral Dots
Authors: Watanabe, Hiroko; Bellot Rubio, Luis R.; de la Cruz
Rodríguez, Jaime; Rouppe van der Voort, Luc
2012ApJ...757...49W Altcode: 2012arXiv1207.6006W
We study the temporal evolution of umbral dots (UDs) using measurements
from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar
Telescope. Scans of the magnetically sensitive 630 nm iron lines
were performed under stable atmospheric conditions for 71 minutes
with a cadence of 63 s. These observations allow us to investigate
the magnetic field and velocity in and around UDs at a resolution
approaching 0farcs13. From the analysis of 339 UDs, we draw the
following conclusions: (1) UDs show clear hints of upflows, as predicted
by magnetohydrodynamic simulations. By contrast, we could not find
systematic downflow signals. Only in very deep layers, we detect
localized downflows around UDs, but they do not persist in time. (2)
We confirm that UDs exhibit weaker and more inclined fields than their
surroundings, as reported previously. However, UDs that have strong
fields above 2000 G or are in the decay phase show enhanced and more
vertical fields. (3) There are enhanced fields at the migration front
of UDs detached from penumbral grains, as if their motion were impeded
by the ambient field. (4) Long-lived UDs travel longer distances with
slower proper motions. Our results appear to confirm some aspects of
recent numerical simulations of magnetoconvection in the umbra (e.g.,
the existence of upflows in UDs), but not others (e.g., the systematic
weakening of the magnetic field at the position of UDs).
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Title: Overview of Chromospheric Lyman-Alpha SpectroPolarimeter
(CLASP)
Authors: Narukage, Noriyuki; Tsuneta, Saku; Bando, Takamasa; Kano,
Ryouhei; Kubo, Masahito; Ishikawa, Ryoko; Hara, Hirohisa; Suematsu,
Yoshinori; Katsukawa, Yukio; Watanabe, Hiroko; Ichimoto, Kiyoshi;
Sakao, Taro; Shimizu, Toshifumi; Kobayashi, Ken; Robinson, Brian; Kim,
Tony; Winebarger, Amy; West, Edward; Cirtain, Jonathan; De Pontieu,
Bart; Casini, Roberto; Trujillo Bueno, Javier; Stepan, Jiri; Manso
Sainz, Rafael; Belluzzi, Luca; Asensio Ramos, Andres; Carlsson, Mats
2011SPIE.8148E..0HN Altcode: 2011SPIE.8148E..16N
The solar chromosphere is an important boundary, through which all of
the plasma, magnetic fields and energy in the corona and solar wind
are supplied. Since the Zeeman splitting is typically smaller than
the Doppler line broadening in the chromosphere and transition region,
it is not effective to explore weak magnetic fields. However, this is
not the case for the Hanle effect, when we have an instrument with
high polarization sensitivity (~ 0.1%). "Chromospheric Lyman- Alpha
SpectroPolarimeter (CLASP)" is the sounding rocket experiment to detect
linear polarization produced by the Hanle effect in Lyman-alpha line
(121.567 nm) and to make the first direct measurement of magnetic
fields in the upper chromosphere and lower transition region. To
achieve the high sensitivity of ~ 0.1% within a rocket flight (5
minutes) in Lyman-alpha line, which is easily absorbed by materials,
we design the optical system mainly with reflections. The CLASP
consists of a classical Cassegrain telescope, a polarimeter and a
spectrometer. The polarimeter consists of a rotating 1/2-wave plate
and two reflecting polarization analyzers. One of the analyzer also
works as a polarization beam splitter to give us two orthogonal linear
polarizations simultaneously. The CLASP is planned to be launched in
2014 summer.
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Title: Ellerman Bombs at High Resolution. I. Morphological Evidence
for Photospheric Reconnection
Authors: Watanabe, Hiroko; Vissers, Gregal; Kitai, Reizaburo; Rouppe
van der Voort, Luc; Rutten, Robert J.
2011ApJ...736...71W Altcode: 2011arXiv1105.4008W
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<SUP>-1</SUP>. 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.
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Title: Observations of Plasma Blob Ejection from a Quiescent
Prominence by Hinode Solar Optical Telescope
Authors: Hillier, Andrew; Isobe, Hiroaki; Watanabe, Hiroko
2011PASJ...63L..19H Altcode: 2011arXiv1103.3750H
We report findings from 0”.2 resolution observations of the
2007 October 03 quiescent prominence observed with the Solar
Optical Telescope on the Hinode satellite. The observations show
clear ejections from the top of the quiescent prominence of plasma
blobs. The ejections, originating from the top of prominence threads,
are impulsively accelerated to approximately Alfvén velocities and
then undergo ballistic motion. The ejections have a characteristic
size between ∼ 1000-2000 km. These characteristics are similar
to downwardly propagating knots (typical size ∼ 700 km) that have
been observed in prominence threads, we suggest that the plasma blob
ejections could be the upward moving counterpart to the downwardly
propagating knots. We discuss the tearing instability as a possible
mechanism to explain the ejections.
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Title: Temporal Evolution of a Rapidly-Moving Umbral Dot
Authors: Watanabe, Hiroko; Tritschler, Alexandra; Kitai, Reizaburo;
Ichimoto, Kiyoshi
2010SoPh..266....5W Altcode: 2010SoPh..tmp..147W
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<SUP>−1</SUP>)
when compared to typical UDs. The lifetime and size of the UD was 8.7
min 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<SUP>−1</SUP>.
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Title: Internal Fine Structure of Ellerman Bombs
Authors: Hashimoto, Yuki; Kitai, Reizaburo; Ichimoto, Kiyoshi; Ueno,
Satoru; Nagata, Shin'ichi; Ishii, Takako T.; Hagino, Masaoki; Komori,
Hiroyuki; Nishida, Keisuke; Matsumoto, Takuma; Otsuji, Kenichi;
Nakamura, Tahei; Kawate, Tomoko; Watanabe, Hiroko; Shibata, Kazunari
2010PASJ...62..879H Altcode:
We conducted coordinated observations of Ellerman bombs (EBs) between
Hinode Satellite and Hida Observatory (HOP12). CaII H broad-band
filter images of NOAA 10966 on 2007 August 9 and 10 were obtained
with the Solar Optical Telescope (SOT) aboard the Hinode Satellite,
and many bright points were observed. We identified a total of 4
bright points as EBs, and studied the temporal variation of their
morphological fine structures and spectroscopic characteristics. With
high-resolution CaII H images of SOT, we found that the EBs, thus far
thought of as single bright features, are composed of a few of fine
subcomponents. Also, by using Stokes I/V filtergrams with Hinode/SOT,
and CaII H spectroheliograms with Hida/Domeless Solar Telescope (DST),
our observation showed: (1) The mean duration, the mean width, the
mean length, and the mean aspect ratio of the subcomponents were
390 s, 170 km, 450 km, and 2.7, respectively. (2) Subcomponents
started to appear on the magnetic neutral lines, and extended their
lengths from the original locations. (3) When the CaII H line of EBs
showed the characteristic blue asymmetry, they are associated with the
appearance or re-brightening of subcomponents. Summarizing our results,
we obtained an observational view that elementary magnetic reconnections
take place one by one successively and intermittently in EBs, and that
their manifestation is the fine subcomponents of the EB phenomena.
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Title: Spicule Dynamics over a Plage Region
Authors: Anan, Tetsu; Kitai, Reizaburo; Kawate, Tomoko; Matsumoto,
Takuma; Ichimoto, Kiyoshi; Shibata, Kazunari; Hillier, Andrew; Otsuji,
Kenichi; Watanabe, Hiroko; Ueno, Satoru; Nagata, Shin'ichi; Ishii,
Takako T.; Komori, Hiroyuki; Nishida, Keisuke; Nakamura, Tahei; Isobe,
Hiroaki; Hagino, Masaoki
2010PASJ...62..871A Altcode: 2010arXiv1002.2288A
We studied spicular jets over a plage area and derived their
dynamic characteristics using Hinode Solar Optical Telescope (SOT)
high-resolution images. A target plage region was near to the west limb
of the solar disk. This location permitted us to study the dynamics
of spicular jets without any overlapping effect of spicular structures
along the line of sight. In this work, to increase the ease with which
we could identify spicules on the disk, we applied the image processing
method `MadMax' developed by Koutchmy et al. (1989). It enhances fine,
slender structures (like jets), over a diffuse background. We identified
169 spicules over the target plage. This sample permited us to derive
statistically reliable results regarding spicular dynamics. The
properties of plage spicules can be summarized as follows: (1) In a
plage area, we clearly identified spicular jet features. (2) They were
shorter in length than the quiet region limb spicules, and followed a
ballistic motion under constant deceleration. (3) The majority (80%)
of the plage spicules showed a cycle of rise and retreat, while 10% of
them faded out without a complete retreat phase. (4) The deceleration
of the spicule was proportional to the velocity of ejection (i.e.,
the initial velocity).
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Title: Characteristic Dependence of Umbral Dots on thier Magnetic
Structure
Authors: Watanabe, Hiroko; Kitai, Reizaburo; Ichimoto, Kiyoshi
2010cosp...38.2933W Altcode: 2010cosp.meet.2933W
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 hr 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.
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Title: Parameters of the Magnetic Flux inside Coronal Holes
Authors: Abramenko, Valentyna; Yurchyshyn, Vasyl; Watanabe, Hiroko
2009SoPh..260...43A Altcode: 2009arXiv0908.2460A
The parameters of the magnetic flux distribution inside low-latitude
coronal holes (CHs) were analyzed. A statistical study of 44 CHs
based on Solar and Heliospheric Observatory (SOHO)/MDI full disk
magnetograms and SOHO/EIT 284 Å images showed that the density of
the net magnetic flux, B<SUB>net</SUB>, does not correlate with the
associated solar wind speeds, V<SUB>x</SUB>. Both the area and net flux
of CHs correlate with the solar wind speed and the corresponding spatial
Pearson correlation coefficients are 0.75 and 0.71, respectively. A
possible explanation for the low correlation between B<SUB>net</SUB>
and V<SUB>x</SUB> is proposed. The observed non-correlation might
be rooted in the structural complexity of the magnetic field. As a
measure of the complexity of the magnetic field, the filling factor,
f(r), was calculated as a function of spatial scales. In CHs, f(r)
was found to be nearly constant at scales above 2 Mm, which indicates a
monofractal structural organization and smooth temporal evolution. The
magnitude of the filling factor is 0.04 from the Hinode SOT/SP data
and 0.07 from the MDI/HR data. The Hinode data show that at scales
smaller than 2 Mm, the filling factor decreases rapidly, which means
a multifractal structure and highly intermittent, burst-like energy
release regime. The absence of the necessary complexity in CH magnetic
fields at scales above 2 Mm seems to be the most plausible reason
why the net magnetic flux density does not seem to be related to the
solar wind speed: the energy release dynamics, needed for solar wind
acceleration, appears to occur at small scales below 1 Mm.
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Title: Magnetic Structure of Umbral Dots Observed with the Hinode
Solar Optical Telescope
Authors: Watanabe, Hiroko; Kitai, Reizaburo; Ichimoto, Kiyoshi;
Katsukawa, Yukio
2009PASJ...61..193W Altcode: 2008arXiv0811.1074W
A high-resolution, seeing-free spectroscopic observation of a decaying
sunspot was made with the Solar Optical Telescope aboard the Hinode
satellite. The target was NOAA 10944, located in the west side of the
solar surface from 2007 March 2 to March 4. The umbra included many
umbral dots (UDs) with a size of ∼300 km in continuum light. We report
on the magnetic structures and Doppler velocity fields around UDs, based
on the Milne-Eddington inversions of the two iron absorption lines at
6302Å. Histograms of the magnetic field strength (B), inclination angle
(i), and Doppler velocity (v) of UDs showed a center-to-limb variation;
observed at the disk center, the UDs had (i) slightly smaller field
strength (ΔB = -17Gauss) and (ii) relative blue shifts (Δv = 28m
s<SUP>-1</SUP>) 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 the
oscillatory light curves with multiple peaks separated around 10min,
which may indicate the presence of the oscillatory convection. We
discuss our results within the frameworks of two theoretical models:
the monolithic model (Schüssler & Vögler 2006, ApJ, 641, L73)
and the field-free intrusion model (Spruit & Scharmer 2006, A&A,
447, 343).
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Title: Cooperative Observation of Ellerman Bombs between the Solar
Optical Telescope aboard Hinode and Hida/Domeless Solar Telescope
Authors: Matsumoto, Takuma; Kitai, Reizaburo; Shibata, Kazunari;
Nagata, Shin'ichi; Otsuji, Kenichi; Nakamura, Tahei; Watanabe, Hiroko;
Tsuneta, Saku; Suematsu, Yoshinori; Ichimoto, Kiyoshi; Shimizu,
Toshifumi; Katsukawa, Yukio; Tarbell, Theodore D.; Lites, Bruce W.;
Shine, Richard A.; Title, Alan M.
2008PASJ...60..577M Altcode:
High-resolution CaIIH broad-band filter images of NOAA10933 on 2007
January 5 were obtained by the Solar Optical Telescope aboard the Hinode
satellite. Many small-scale (∼1") bright points were observed outside
the sunspot and inside the emerging flux region. We identified some of
these bright points with Ellerman bombs (EBs) by using Hα images taken
by the Domeless Solar Telescope at Hida observatory. The sub-arcsec
structures of two EBs seen in CaIIH were studied in detail. Our
observation showed the following two aspects: (1) The CaIIH bright
points identified with EBs were associated with the bipolar magnetic
field structures, as reported by previous studies. (2)The structure
of the CaIIH bright points turned out to consist of the following two
parts: a central elongated bright core (0.7" × 0.5") located along
the magnetic neutral line and a diffuse halo (1.2"×1.8").
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Title: Chromospheric Anemone Jets as Evidence of Ubiquitous
Reconnection
Authors: Shibata, Kazunari; Nakamura, Tahei; Matsumoto, Takuma; Otsuji,
Kenichi; Okamoto, Takenori J.; Nishizuka, Naoto; Kawate, Tomoko;
Watanabe, Hiroko; Nagata, Shin'ichi; UeNo, Satoru; Kitai, Reizaburo;
Nozawa, Satoshi; Tsuneta, Saku; Suematsu, Yoshinori; Ichimoto, Kiyoshi;
Shimizu, Toshifumi; Katsukawa, Yukio; Tarbell, Theodore D.; Berger,
Thomas E.; Lites, Bruce W.; Shine, Richard A.; Title, Alan M.
2007Sci...318.1591S Altcode: 2008arXiv0810.3974S
The heating of the solar chromosphere and corona is a long-standing
puzzle in solar physics. Hinode observations show the ubiquitous
presence of chromospheric anemone jets outside sunspots in active
regions. They are typically 3 to 7 arc seconds = 2000 to 5000 kilometers
long and 0.2 to 0.4 arc second = 150 to 300 kilometers wide, and their
velocity is 10 to 20 kilometers per second. These small jets have an
inverted Y-shape, similar to the shape of x-ray anemone jets in the
corona. These features imply that magnetic reconnection similar to that
in the corona is occurring at a much smaller spatial scale throughout
the chromosphere and suggest that the heating of the solar chromosphere
and corona may be related to small-scale ubiquitous reconnection.
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Title: Small-Scale Magnetic-Flux Emergence Observed with Hinode
Solar Optical Telescope
Authors: Otsuji, Kenichi; Shibata, Kazunari; Kitai, Reizaburo; Ueno,
Satoru; Nagata, Shin'ichi; Matsumoto, Takuma; Nakamura, Tahei;
Watanabe, Hiroko; Tsuneta, Saku; Suematsu, Yoshinori; Ichimoto,
Kiyoshi; Shimizu, Toshifumi; Katsukawa, Yukio; Tarbell, Theodore D.;
Lites, Bruce; Shine, Richard A.; Title Alan M.
2007PASJ...59S.649O Altcode: 2007arXiv0709.3207O
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 at wavelengths of Fe 6302Å, G band,
and CaII H. In Stokes I images of Fe 6302Å, emerging magnetic flux was
recognized as dark lanes. In the G band, they showed to be their shapes
almost the same as in Stokes I images. These magnetic fluxes appeared
as dark filaments in CaII H images. Stokes V images of Fe 6302Å showed
pairs of opposite polarities at footpoints of each filament. These
magnetic concentrations were identified to correspond to bright points
in G band/CaII H images. From an analysis of time-sliced diagrams, we
derived the following properties of emerging flux, which are consistent
with those of previous studies: (1) Two footpoints separate each other
at a speed of 4.2kms<SUP>-1</SUP> during the initial phase of evolution,
and decrease to about 1kms<SUP>-1</SUP> 10minutes later. (2) CaII H
filaments appear almost simultaneously with the formation of dark lanes
in Stokes I in an observational cadence of 2minutes. (3) The lifetime
of the dark lanes in the Stokes I and G band is 8minutes, while that
of Ca filament is 12minutes. An interesting phenomena was observed,
that an emerging flux tube expanded laterally in the photosphere with a
speed of 3.8kms<SUP>-1</SUP>. A discussion on the horizontal expansion
of the flux tube is given with refernce to previous simulation studies.
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Title: Umbral Fine Structures in Sunspots Observed with Hinode Solar
Optical Telescope
Authors: Kitai, Reizaburo; Watanabe, Hiroko; Nakamura, Tahei; Otsuji,
Ken-ichi; Matsumoto, Takuma; UeNo, Satoru; Nagata, Shin'ichi; Shibata,
Kazunari; Muller, Richard; Ichimoto, Kiyoshi; Tsuneta, Saku; Suematsu,
Yoshinori; Katsukawa, Yukio; Shimizu, Toshifumi; Tarbell, Theodore D.;
Shine, Richard A.; Title, Alan M.; Lites, Bruce
2007PASJ...59S.585K Altcode: 2007arXiv0711.3266K
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.
