Author name code: sakurai
ADS astronomy entries on 2022-09-14
author:"Sakurai, Takashi"
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Title: Poleward Moving Auroral Arcs and Pc5 Oscillations
Authors: Sakurai, T.; Wright, A. N.; Takahashi, K.; Elsden, T.;
Ebihara, Y.; Sato, N.; Kadokura, A.; Tanaka, Y.; Hori, T.
Bibcode: 2022JGRA..12730362S
Altcode:
We present an example of one-to-one correspondence between poleward
moving auroral arcs (PMAAs) and Pc5 oscillations observed at the Time
History of Events and Macroscale Interactions during Substorms (THEMIS)
Ground Based Observatory station Gillam. The PMAAs consisted of four
successive intensifications (named PMAA1, PMAA2, PMAA3 and PMAA4) with
a period of 3∼4 min over the magnetic latitudes from 68° to 70° in
the auroral oval and varied coherently with the H-component of magnetic
field Pc5 oscillations. PMAA1 and PMAA2 appeared clearly at the magnetic
latitude ∼69°, and the following two PMAAs, which were dimmer,
appeared at the magnetic latitude ∼68°. PMAA1 and PMAA2 exhibited
features of field-line resonances with the maximum luminosity at the
magnetic latitude ∼69.5° and ∼69.4°, respectively. The ground
Pc5 oscillations were concurrent with toroidal mode Pc5 oscillation
observed at the THEMIS-D, -E, and -A satellites at ∼4 MLT in the
outer magnetosphere. The magnetic and electric field oscillations
at THEMIS were synchronized with the PMAAs. The magnetic energy of
the THEMIS Pc5 oscillations is estimated using a numerical model of
damped toroidal oscillations and compared with the kinetic energy
of precipitating electrons associated with the field aligned current
carried by the toroidal oscillations. The result reveals that the Pc5
magnetic energy is much larger than the kinetic energy, implying the
magnetic energy is important for producing auroral emissions in the
ionosphere. We also perform a simulation of the relationship between
PMAAs and toroidal mode Pc5 oscillations. The simulation explains the
observed spatial and temporal structures of the PMAAs.
Title: PSTEP: project for solar-terrestrial environment prediction
Authors: Kusano, Kanya; Ichimoto, Kiyoshi; Ishii, Mamoru; Miyoshi,
Yoshizumi; Yoden, Shigeo; Akiyoshi, Hideharu; Asai, Ayumi; Ebihara,
Yusuke; Fujiwara, Hitoshi; Goto, Tada-Nori; Hanaoka, Yoichiro;
Hayakawa, Hisashi; Hosokawa, Keisuke; Hotta, Hideyuki; Hozumi,
Kornyanat; Imada, Shinsuke; Iwai, Kazumasa; Iyemori, Toshihiko; Jin,
Hidekatsu; Kataoka, Ryuho; Katoh, Yuto; Kikuchi, Takashi; Kubo, Yûki;
Kurita, Satoshi; Matsumoto, Haruhisa; Mitani, Takefumi; Miyahara,
Hiroko; Miyoshi, Yasunobu; Nagatsuma, Tsutomu; Nakamizo, Aoi; Nakamura,
Satoko; Nakata, Hiroyuki; Nishizuka, Naoto; Otsuka, Yuichi; Saito,
Shinji; Saito, Susumu; Sakurai, Takashi; Sato, Tatsuhiko; Shimizu,
Toshifumi; Shinagawa, Hiroyuki; Shiokawa, Kazuo; Shiota, Daikou;
Takashima, Takeshi; Tao, Chihiro; Toriumi, Shin; Ueno, Satoru;
Watanabe, Kyoko; Watari, Shinichi; Yashiro, Seiji; Yoshida, Kohei;
Yoshikawa, Akimasa
Bibcode: 2021EP&S...73..159K
Altcode:
Although solar activity may significantly impact the global environment
and socioeconomic systems, the mechanisms for solar eruptions and
the subsequent processes have not yet been fully understood. Thus,
modern society supported by advanced information systems is at risk
from severe space weather disturbances. Project for solar-terrestrial
environment prediction (PSTEP) was launched to improve this situation
through synergy between basic science research and operational
forecast. The PSTEP is a nationwide research collaboration in Japan
and was conducted from April 2015 to March 2020, supported by a
Grant-in-Aid for Scientific Research on Innovative Areas from the
Ministry of Education, Culture, Sports, Science and Technology of
Japan. By this project, we sought to answer the fundamental questions
concerning the solar-terrestrial environment and aimed to build a
next-generation space weather forecast system to prepare for severe
space weather disasters. The PSTEP consists of four research groups and
proposal-based research units. It has made a significant progress in
space weather research and operational forecasts, publishing over 500
refereed journal papers and organizing four international symposiums,
various workshops and seminars, and summer school for graduate students
at Rikubetsu in 2017. This paper is a summary report of the PSTEP and
describes the major research achievements it produced.
Title: Solar force-free magnetic fields
Authors: Wiegelmann, Thomas; Sakurai, Takashi
Bibcode: 2021LRSP...18....1W
Altcode: 2012arXiv1208.4693W
The structure and dynamics of the solar corona is dominated by the
magnetic field. In most areas in the corona magnetic forces are so
dominant that all non-magnetic forces such as plasma pressure gradients
and gravity can be neglected in the lowest order. This model assumption
is called the force-free field assumption, as the Lorentz force
vanishes. This can be obtained by either vanishing electric currents
(leading to potential fields) or the currents are co-aligned with the
magnetic field lines. First we discuss a mathematically simpler approach
that the magnetic field and currents are proportional with one global
constant, the so-called linear force-free field approximation. In the
generic case, however, the relationship between magnetic fields and
electric currents is nonlinear and analytic solutions have been only
found for special cases, like 1D or 2D configurations. For constructing
realistic nonlinear force-free coronal magnetic field models in
3D, sophisticated numerical computations are required and boundary
conditions must be obtained from measurements of the magnetic field
vector in the solar photosphere. This approach is currently a large
area of research, as accurate measurements of the photospheric field
are available from ground-based observatories such as the Synoptic
Optical Long-term Investigations of the Sun and the Daniel K. Inouye
Solar Telescope (DKIST) and space-born, e.g., from Hinode and the Solar
Dynamics Observatory. If we can obtain accurate force-free coronal
magnetic field models we can calculate the free magnetic energy in
the corona, a quantity which is important for the prediction of flares
and coronal mass ejections. Knowledge of the 3D structure of magnetic
field lines also help us to interpret other coronal observations,
e.g., EUV images of the radiating coronal plasma.
Title: Determination of Large Scale Plasma Properties of Solar Corona
Using the X-Ray Telescope onboard Hinode: IV. Detailed Calibration
of the Off-Axis Data
Authors: Shin, J.; Kano, R.; Sakurai, T.; Kim, Y.; Moon, Y.
Bibcode: 2021AAS...23831307S
Altcode:
Coronal hole is important not only in the field of solar physics but
also of space weather because it is known as the origin of the fast
solar wind. However, the detailed mechanism of coronal heating leading
to the acceleration of solar wind is still unknown to us. Theoretical
modeling of the three-dimensional coronal structures is sensitive
to the values of plasma properties at the base of solar corona and
thus requires in advance accurate empirical description of those
properties. Therefore, the study on the physical conditions of solar
plasma in the coronal holes and near the off-limb areas will give us an
essential clue to understand the boundary conditions on the mechanism
of heating the coronal plasma. The X-Ray Telescope (XRT) onboard Hinode
equips the modified grazing incidence (GI) mirror whose FOV is 34 x
34 arcmin covering the full solar disk and its neighboring regions,
and as a result, the targets near the limb area are usually placed
far from the center. For this reason, the imaging artefacts shown in
the off-axis region should be calibrated carefully in order for the
observed data to be properly interpreted. We will introduce in
this presentation the methods of how to calibrate the data taken near
the periphery of Hinode/XRT full FOV, which include the restoration
of the scattered light and the reduction of vignetting effect. We
have analyzed over-exposed in-flight images during the solar flare
to estimate the amount of scattered light inherent in the observed
data. It is revealed the light scattered due to the roughness of GI
mirror surface has a power-law distribution and its amount depends on
the energy considered, which allows us to complete a PSF profile from
the core to the scattering wing. We have also evaluated the vignetting
effect in Hinode/XRT by analyzing the 2D distribution of effective area
in the FOV taken from pre-launch experiments. Many interesting results
on the optical characteristics will be introduced in our presentation.
Title: Detailed Calibration of the Off-Axis Optical Characteristics
for the X-Ray Telescope onboard Hinode
Authors: Shin, Junho; Kano, Ryouhei; Sakurai, Takashi; Kim, Yeon-Han;
Moon, Yong-Jae
Bibcode: 2021EGUGA..2313004S
Altcode:
The X-Ray Telescope (XRT) onboard the Hinode satellite has a
specially designed Wolter type grazing-incidence (GI) optics with a
paraboloid-hyperboloid mirror assembly to measure the solar coronal
plasma of temperatures up to 10 MK with a resolution of about one
arc sec. One of the main purposes of this scientific mission is to
investigate the detailed mechanism of energy transfer processes from
the photosphere to the upper coronal region leading to its heating and
the solar wind acceleration. An astronomical telescope is in general
designed such that the best-focused image of an object is achieved at or
very close to the optical axis, and inevitably the optical performance
deteriorates away from the on-axis position. The Sun is, however, a
large astronomical object and thus targets near the limb of full-disk
images are placed at the outskirt of the field of view. The design of a
solar telescope should thus consider the uniformity of imaging quality
over a wide FOV, and it is particularly so for X-ray telescopes whose
targets can be in the corona high above the limb. We will explain in
this presentation the importance of detailed calibration of the off-axis
optical characteristics for Hinode/XRT. It have been revealed that
the scattered light caused by the GI mirror surface has a power-law
distribution and shows an energy dependence. We will also introduce
the basic scheme of how the level of scattering wing is determined and
connected to the core from the analysis of highly saturated in-flight
data. Vignetting is another important optical characteristics for
describing the telescope's performance, which reflects the ability to
collect incoming light at different locations and photon energies. We
have evaluated the vignetting effect in Hinode/XRT by analyzing
the ground experimental data and found that the degree of vignetting
varies linearly from the optical center and its pattern shows an energy
dependence. Many interesting results on the calibration of Hinode/XRT
optical characteristics will be introduced and discussed thoroughly.
Title: Determination of Large Scale Plasma Properties of Solar
Corona Using the X-Ray Telescope onboard Hinode: III. PSF and Image
Calibration
Authors: Shin, Junho; Sakurai, Takashi; Kano, Ryouhei; Kim, Yeon-Han;
Moon, Yong-Jae
Bibcode: 2021cosp...43E1049S
Altcode:
To theoretically model the three dimensional coronal structures is
sensitive to the values of plasma properties at the base of solar
corona and thus requires beforehand accurate empirical description of
those properties. Detailed studies on the physical state of coronal
plasma near the off-limb areas will therefore lead us to understand
the mechanism of energy transfer processes from the photosphere to
the upper coronal region leading to its heating and the solar wind
acceleration. The Hinode X-Ray Telescope (XRT) has contributed to
the progress in our understanding of coronal physics for more than a
decade. Since the telescope was designed to observe the Sun in the wide
field of view covering full solar disk, targets near the limb area are
placed at the outskirt of FOV. The optical performance of the instrument
gradually deteriorates as it goes away from the optical center and thus
the off-axis characteristics of Hinode/XRT should be examined with care
in order to precisely interpret the coronal plasma properties near
the solar limb area. This presentation will explain the importance
of accurate calibration of the optical characteristics, especially
for the data taken in the off-axis region. Our previous research has
shown that the light scattered by the XRT mirror surface roughness has
a power-law distribution and also shows an energy dependence, with
which the PSF profile from the core to the scattering wing has been
completed. We will introduce in this study how the level of scattering
wing can be determined quantitatively for each focal plane filter
from in-flight data analysis. We have also evaluated the vignetting
effect in Hinode/XRT by analyzing the distribution of effective area
taken from MSFC/XRCF pre-launch experiment. More interesting results
on the calibration of Hinode/XRT scattered light and the correction
of vignetting effect will be introduced and discussed thoroughly.
Title: Internetwork Magnetic Fields Seen in Fe I 1564.8 nm Full-disk
Images
Authors: Hanaoka, Yoichiro; Sakurai, Takashi
Bibcode: 2020ApJ...904...63H
Altcode: 2020arXiv200912751H
We studied the properties of internetwork magnetic fields in the solar
photosphere, taking advantage of full-disk Stokes V/I maps of the Fe
I 1564.8 nm line, which were obtained during 2010-2019. In contrast to
most previous studies, we used data with moderate spatial and spectral
resolutions. Nonetheless, we were able to distinguish the internetwork
field components and the active region/network boundary components
using large Zeeman splitting of the Fe I 1564.8 nm line. Thus,
our analysis provides a point of view quite different from that of
previous studies. We analyzed the data statistically without ordinary
inversions, yet we successfully derived some properties of internetwork
fields; the internetwork is filled with small-scale magnetic fields,
their strength is within the weak field regime of the Fe I 1564.8
nm line (300-400 G or less), and the internetwork fields are highly
inclined. Although the results were obtained from the analysis performed
from a different perspective, they are consistent with the majority of
previous findings. In addition, no notable variation in the properties
of the internetwork fields was found during the period covering most
of solar cycle 24.
Title: Synoptic solar observations of the Solar Flare Telescope
focusing on space weather
Authors: Hanaoka, Yoichiro; Sakurai, Takashi; Otsuji, Ken'ichi;
Suzuki, Isao; Morita, Satoshi
Bibcode: 2020JSWSC..10...41H
Altcode: 2020arXiv200714054H
The solar group at the National Astronomical Observatory of Japan
is conducting synoptic solar observation with the Solar Flare
Telescope. While it is a part of a long-term solar monitoring,
contributing to the study of solar dynamo governing solar activity
cycles, it is also an attempt at contributing to space weather
research. The observations include imaging with filters for Hα, Ca K,
G-band, and continuum, and spectropolarimetry at the wavelength bands
including the He I 1083.0 nm/Si I 1082.7 nm and the Fe I 1564.8 nm
lines. Data for the brightness, Doppler signal, and magnetic field
information of the photosphere and the chromosphere are obtained. In
addition to monitoring dynamic phenomena like flares and filament
eruptions, we can track the evolution of the magnetic fields that
drive them on the basis of these data. Furthermore, the magnetic field
in solar filaments, which develops into a part of the interplanetary
magnetic cloud after their eruption and occasionally hits the Earth,
can be inferred in its pre-eruption configuration. Such observations
beyond mere classical monitoring of the Sun will hereafter become
crucially important from the viewpoint of the prediction of space
weather phenomena. The current synoptic observations with the Solar
Flare Telescope is considered to be a pioneering one for future synoptic
observations of the Sun with advanced instruments.
Title: Analysis of full-disc Ca II K spectroheliograms. III. Plage
area composite series covering 1892-2019
Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova,
Natalie A.; Solanki, Sami K.; Banerjee, Dipankar; Barata, Teresa;
Belik, Marcel; Gafeira, Ricardo; Garcia, Adriana; Hanaoka, Yoichiro;
Hegde, Manjunath; Klimeš, Jan; Korokhin, Viktor V.; Lourenço, Ana;
Malherbe, Jean-Marie; Marchenko, Gennady P.; Peixinho, Nuno; Sakurai,
Takashi; Tlatov, Andrey G.
Bibcode: 2020A&A...639A..88C
Altcode: 2020arXiv200501435C
Context. Studies of long-term solar activity and variability require
knowledge of the past evolution of the solar surface magnetism. The
archives of full-disc Ca II K observations that have been performed
more or less regularly at various sites since 1892 can serve as an
important source of such information.
Aims: We derive the plage
area evolution over the last 12 solar cycles by employing data from all
Ca II K archives that are publicly available in digital form, including
several as-yet-unexplored Ca II K archives.
Methods: We analysed
more than 290 000 full-disc Ca II K observations from 43 datasets
spanning the period between 1892-2019. All images were consistently
processed with an automatic procedure that performs the photometric
calibration (if needed) and the limb-darkening compensation. The
processing also accounts for artefacts affecting many of the images,
including some very specific artefacts, such as bright arcs found
in Kyoto and Yerkes data. Our employed methods have previously been
tested and evaluated on synthetic data and found to be more accurate
than other methods used in the literature to treat a subset of the data
analysed here.
Results: We produced a plage area time-series
from each analysed dataset. We found that the differences between the
plage areas derived from individual archives are mainly due to the
differences in the central wavelength and the bandpass used to acquire
the data at the various sites. We empirically cross-calibrated and
combined the results obtained from each dataset to produce a composite
series of plage areas. The 'backbone' approach was used to bridge
the series together. We have also shown that the selection of the
backbone series has little effect on the final composite of the plage
area. We quantified the uncertainty of determining the plage areas
with our processing due to shifts in the central wavelength and found
it to be less than 0.01 in fraction of the solar disc for the average
conditions found on historical data. We also found the variable seeing
conditions during the observations to slightly increase the plage
areas during the activity maxima.
Conclusions: We provide the
most complete so far time series of plage areas based on corrected
and calibrated historical and modern Ca II K images. Consistent
plage areas are now available on 88% of all days from 1892 onwards
and on 98% from 1907 onwards. The whole series described in
the paper are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A88
Title: Detailed Calibration of the Off-Axis Optical Characteristics
for the X-Ray Telescope onboard Hinode
Authors: Shin, Junho; Sakurai, Takashi; Kano, Ryouhei; Moon, Yong-Jae;
Kim, Yeon-Han
Bibcode: 2020EGUGA..2210225S
Altcode:
The X-Ray Telescope (XRT) onboard the Hinode satellite has a
specially designed Wolter type grazing-incidence (GI) optics with a
paraboloid-hyperboloid mirror assembly to measure the solar coronal
plasma of temperatures up to 10 MK with a resolution of about one
arcsec. One of the main purposes of this scientific mission is to
investigate the detailed mechanism of energy transfer processes from the
photosphere to the upper coronal region leading to its heating and the
solar wind acceleration. To theoretically model the three-dimensional
coronal structures is sensitive to the values of plasma properties at
the base of solar corona and thus requires beforehand accurate empirical
description of those properties. Though the telescope has provided
unprecedented observations of solar corona for more than a decade,
due to a wide field of view of 34 x 34 arcmin covering the full Sun,
the optical performance of the instrument gradually deteriorates as
it goes away from the optical center. For this reason, the off-axis
characteristics of Hinode/XRT should be examined with care in order
to precisely interpret the coronal plasma properties near the solar
limb area.This presentation will explain the importance of accurate
calibration of the optical characteristics, especially for the data
taken in the off-axis region. Our previous study has shown that
the scattered light caused by the XRT mirror surface roughness has
a power-law distribution and also shows an energy dependence, with
which the PSF profile from the core to the scattering wing has been
completed. We will introduce in this study how the level of scattering
wing can be determined quantitatively for each focal plane filter from
in-flight data analysis. We have also evaluated the vignetting effect
in Hinode/XRT by analyzing the 2D distribution of effective area in
the field of view taken from MSFC/XRCF pre-launch experiment. It is
revealed that, unlike the case of Yohkoh/SXT, the degree of offset of
an optical center is not serious and thus shows little deviation from
rotational symmetry. Also important is that the vignetting pattern
in XRT shows an energy dependence, which has never been considered
before for the analyses of XRT data. More interesting results on
the calibration of Hinode/XRT scattered light and the correction of
vignetting effect will be introduced and discussed thoroughly.
Title: VizieR Online Data Catalog: Plage area composite series
(Chatzistergos+, 2020)
Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki,
S. K.; Banerjee, D.; Barata, T.; Belik, M.; Gafeira, R.; Garcia,
A.; Hanaoka, Y.; Hegde, M.; Klimes, J.; Korokhin, V. V.; Lourenco,
A.; Malherbe, J. -M.; Marchenko, G. P.; Peixinho, N.; Sakurai, T.;
Tlatov, A. G.
Bibcode: 2020yCat..36390088C
Altcode:
Annual, monthly, and daily values of the plage area composite series. The archives included are those from the Arcetri (Ar), Baikal (Ba),
Brussels (Br), Calern (CL), Catania (CT), Coimbra (Co), Kanzelhoehe
(Ka), Kenwood (Ke), Kharkiv (Kh), Kislovodsk (Ki), Kodaikanal taken with
the spectroheliograph (Ko), Kodaikanal taken with the Twin telescope
(KT), Kodaikanal taken with the White-Light Active Region Monitor (WARM)
telescope (KW), Kyoto (Ky), Manila (Ma), Mauna Loa (ML) taken with the
Precision Solar Photometric Telescope (PSPT), McMath-Hulbert (MM), Mees
(MS), Meudon taken with the spectroheliograph (MD1), Meudon taken with
an interference filter (MD2), Mitaka taken with the spectroheliograph
(Mi1), Mitaka taken with the Solar Flare Telescope with an interference
filter (Mi2), Mt Wilson (MW), Pic du Midi (PM), SOlar Diameter Imager
and Surface Mapper (SODISM) telescope on board the PICARD spacecraft
(PS), Rome taken with the equatorial bar at Monte Mario (Ro), Rome taken
with the PSPT (RP1), Rome taken with the PSPT with narrow bandwidth
(RP2), Sacramento Peak (SP), San Fernando taken with the Cartesian
Full-Disk Telescope (CFDT) 1 (SF1), San Fernando taken with the CFDT2
(SF2), Schauinsland (Sc), Teide (Te) taken with the Chromospheric
Telescope (ChroTel), Upice (UP), Valasske Mezirici (VM), Wendelstein
(WS), and Yerkes (YR) sites (3 data files).
Title: Large-Scale Solar Magnetic Fields Observed with the Infrared
Spectro-Polarimeter IRmag at the National Astronomical Observatory
of Japan: Comparison of Measurements Made in Different Spectral
Lines and Observatories
Authors: Demidov, M. L.; Hanaoka, Y.; Sakurai, T.; Wang, X. F.
Bibcode: 2020SoPh..295...54D
Altcode:
This study presents the results of a quantitative analysis of IRmag
(infrared Stokes spectro-polarimeter, National Astronomical Observatory
of Japan, (NAOJ)/Mitaka) observations in three spectral lines Fe I
1564.8 nm, Si I 1082.72 nm, He I 1083.0 nm, and comparison between IRmag
magnetograms and ones from the Solar Dynamic Observatory/Helioseismic
and Magnetic Imager (SDO/HMI), Global Oscillation Network Group (GONG),
Wilcox Solar Observatory (WSO), and Solar Telescope for Operative
Predictions (STOP). It is shown that there is a good correlation between
observations in all three IRmag spectral lines, but with significant
differences in the measured field strengths caused by different depths
of formation and, probably, by calibration issues. The correlation
of IRmag magnetograms with ones from other observatories, and between
different observatories among each other, is as good as 0.70 - 0.95,
depending on the combination, but nevertheless there are significant
systematic differences (linear-regression coefficients deviating
from unity) between different data sets (up to a factor of three or
even more).
Title: Achievements of Hinode in the first eleven years
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick;
Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks,
David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio;
Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra,
Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.;
Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao,
Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota,
Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu,
Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin;
Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young,
Peter R.
Bibcode: 2019PASJ...71R...1H
Altcode:
Hinode is Japan's third solar mission following Hinotori (1981-1982)
and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in
operation currently. Hinode carries three instruments: the Solar Optical
Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These
instruments were built under international collaboration with the
National Aeronautics and Space Administration and the UK Science and
Technology Facilities Council, and its operation has been contributed
to by the European Space Agency and the Norwegian Space Center. After
describing the satellite operations and giving a performance evaluation
of the three instruments, reviews are presented on major scientific
discoveries by Hinode in the first eleven years (one solar cycle long)
of its operation. This review article concludes with future prospects
for solar physics research based on the achievements of Hinode.
Title: Large-scale solar magnetic fields from observations in the
visible and infrared spectral lines and some space weather issues.
Authors: Demidov, M.; Hanaoka, Y.; Sakurai, T.
Bibcode: 2019spw..confE...2D
Altcode:
No abstract at ADS
Title: Statistical Study of the Magnetic Field in Solar Filaments
Authors: Hanaoka, Y.; Sakurai, T.; IRMag Group
Bibcode: 2019ASPC..526..275H
Altcode:
We carried out a statistical study of the magnetic field orientation
in solar filaments based on our daily full-Sun, full-Stokes
spectropolarimetric observations using the He I 10830 line. The analysis
of more than 400 filaments revealed that the sense of rotation of the
magnetic field direction in filaments with respect to their axes shows
strong hemispheric dependence, and that the angle of rotation of the
magnetic field concentrates between 10-30 degrees. The hemispheric
pattern of the filament magnetic field is consistent with that known
in the fine structure of filaments and the magnetic field of limb
prominences.
Title: Determination of Large Scale Plasma Properties of Solar Corona
Using the X-Ray Telescope onboard Hinode: II. Correction for the
Scattered Lights
Authors: Shin, Junho; Kano, Ryouhei; Sakurai, Takashi; Kim, Yeon-Han;
Moon, Yong-Jae
Bibcode: 2019EGUGA..21.9726S
Altcode:
The X-Ray Telescope (XRT) onboard Hinode, which was designed to
observed a variety of coronal structures with temperature between 1
and 10 MK in the range of 34x34 arc min field of view (FOV) covering
the full solar disk, has provided solar X-ray images for more than a
decade and contributed to the progress in our understanding of coronal
physics. In particular, long-term observation of coronal hole regions
covering almost one solar cycle has an important meaning not only in
the field of solar physics but also in relation to the space weather
because the coronal hole is known as the source of solar winds. Detailed
study on the physical conditions of solar plasma in the coronal hole and
also the off-limb area will give us a clue to understand the boundary
conditions and constraints on the theoretical mechanism of heating
the coronal plasma. An astronomical telescope is in general designed
such that the best-focused image of an object is achieved at or very
close to the optical axis, and inevitably the optical performance
deteriorates away from the on-axis position. The Sun is, however, a
large astronomical object and thus targets near the limb of full-disk
images are placed at the outskirt of the field of view. Therefore,
the optical design of a solar telescope should consider with care
the uniformity of imaging quality over a wide FOV. Even after such
a design effort, the off-axis performance of the solar telescopes
should be characterized very carefully in order for the data away from
the center to be properly interpreted. We have evaluated the amount
of scattered light inherent in the Hinode/XRT data by analyzing the
in-flight images highly saturated during the solar flare events. It is
revealed that, like the case of Yohkoh/SXT, the light scattered due
to the roughness of mirror surface has a power-law distribution of
r^-2 and also shows clear energy dependence, which has enabled us to
complete a full description of XRT PSF profile from the core to the
scattering wing. A successful restoration of the scattered lights in
the observed XRT images will provide us with more precise information
on the physical quantities of solar coronal plasma in the off-limb
regions. Many interesting results on the correction for Hinode/XRT
scattered lights will be introduced and discussed thoroughly.
Title: Infrared spectro-polarimeter on the Solar Flare Telescope
at NAOJ/Mitaka
Authors: Sakurai, Takashi; Hanaoka, Yoichiro; Arai, Takehiko; Hagino,
Masaoki; Kawate, Tomoko; Kitagawa, Naomasa; Kobiki, Toshihiko;
Miyashita, Masakuni; Morita, Satoshi; Otsuji, Ken'ichi; Shinoda,
Kazuya; Suzuki, Isao; Yaji, Kentaro; Yamasaki, Takayuki; Fukuda,
Takeo; Noguchi, Motokazu; Takeyama, Norihide; Kanai, Yoshikazu;
Yamamuro, Tomoyasu
Bibcode: 2018PASJ...70...58S
Altcode: 2018PASJ..tmp...63S; 2018PASJ..tmp...82S
An infrared spectro-polarimeter installed on the Solar Flare Telescope
at the Mitaka headquarters of the National Astronomical Observatory of
Japan is described. The new spectro-polarimeter observes the full Sun
via slit scans performed at two wavelength bands, one near 1565 nm for a
Zeeman-sensitive spectral line of Fe I and the other near 1083 nm for He
I and Si I lines. The full Stokes profiles are recorded; the Fe I and Si
I lines give information on photospheric vector magnetic fields, and the
helium line is suitable for deriving chromospheric magnetic fields. The
infrared detector we are using is an InGaAs camera with 640 × 512
pixels and a read-out speed of 90 frames s-1. The solar
disk is covered by two swaths (the northern and southern hemispheres)
of 640 pixels each. The final magnetic maps are made of 1200 × 1200
pixels with a pixel size of 1{^''.}8. We have been carrying
out regular observations since 2010 April, and have provided full-disk,
full-Stokes maps, at the rate of a few maps per day, on the internet.
Title: Image enhancement for the observation of the solar corona
Authors: Zhao, M. Y.; Liu, Y.; Song, T. F.; Zhang, X. F.; Hagino,
M.; Sakurai, T.
Bibcode: 2018SPIE10701E..29Z
Altcode:
In this work, we introduce an image enhancement method ideally suited
for the visualization of coronal intensity images. The steep radial
gradient of the coronal brightness is adjusted by normalising the
coronal image with the Fourier approximation of its local average. A
method based on deconvolution and localised normalising of the data
at many different spatial scales is used to further enhance the fine
structures, and a wavelet shrinkage denoising method is used for noise
suppression. The effectiveness of this method is demonstrated on a
series of images observed by various instruments including spacial
and earth-based coronagraphs as well as photos during total solar
eclipse. This method is very helpful for qualitative analysis of solar
coronal structures that are mostly invisible on original images.
Title: Super-strong Magnetic Field in Sunspots
Authors: Okamoto, Takenori J.; Sakurai, Takashi
Bibcode: 2018ApJ...852L..16O
Altcode: 2017arXiv171208700O
Sunspots are the most notable structure on the solar surface with
strong magnetic fields. The field is generally strongest in a dark area
(umbra), but sometimes stronger fields are found in non-dark regions,
such as a penumbra and a light bridge. The formation mechanism of such
strong fields outside umbrae is still puzzling. Here we report clear
evidence of the magnetic field of 6250 G, which is the strongest field
among Stokes I profiles with clear Zeeman splitting ever observed on
the Sun. The field was almost parallel to the solar surface and located
in a bright region sandwiched by two opposite-polarity umbrae. Using
a time series of spectral data sets, we discuss the formation process
of the super-strong field and suggest that this strong field region
was generated as a result of compression of one umbra pushed by the
horizontal flow from the other umbra, such as the subduction of the
Earth’s crust in plate tectonics.
Title: Hinode's Contributions to Solar Physics
Authors: Sakurai, Takashi
Bibcode: 2018ASSL..449...19S
Altcode:
No abstract at ADS
Title: Statistical Study of the Magnetic Field Orientation in Solar
Filaments
Authors: Hanaoka, Yoichiro; Sakurai, Takashi
Bibcode: 2017ApJ...851..130H
Altcode: 2017arXiv171107735H
We have carried out a statistical study of the average orientation
of the magnetic field in solar filaments with respect to their axes
for more than 400 samples, based on data taken with daily full-Sun,
full-Stokes spectropolarimetric observations using the He I 1083.0
nm line. The major part of the samples are the filaments in the quiet
areas, but those in the active areas are included as well. The average
orientation of the magnetic field in filaments shows a systematic
property depending on the hemisphere; the direction of the magnetic
field in filaments in the northern (southern) hemisphere mostly
deviates clockwise (counterclockwise) from their axes, which run
along the magnetic polarity inversion line. The deviation angles
of the magnetic field from the axes are concentrated between 10°
and 30°. This hemispheric pattern is consistent with that revealed
for chirality of filament barbs, filament channels, and for other
solar features found to possess chirality. For some filaments, it was
confirmed that their magnetic field direction is locally parallel to
their structure seen in Hα images. Our results for the first time
confirmed this hemispheric pattern with the direct observation of the
magnetic field in filaments. Interestingly, the filaments which show
the opposite magnetic field deviation to the hemispheric pattern, are
in many cases found above the polarity inversion line whose ambient
photospheric magnetic field has the polarity alignment being opposite
to that of active regions following the Hale-Nicholson law.
Title: The Strongest Magnetic Field in Sunspots
Authors: Okamoto, J.; Sakurai, T.
Bibcode: 2017AGUFMSH51C2496O
Altcode:
Sunspots are concentrations of magnetic fields on the solar
surface. Generally, the strongest magnetic field in each sunspot is
located in the dark umbra in most cases. A typical field strength in
sunspots is around 3,000 G. On the other hand, some exceptions also
have been found in complex sunspots with bright regions such as light
bridges that separate opposite polarity umbrae, for instance with a
strength of 4,300 G. However, the formation mechanism of such strong
fields outside umbrae is still puzzling. Here we report an extremely
strong magnetic field in a sunspot, which was located in a bright region
sandwiched by two opposite-polarity umbrae. The strength is 6,250 G,
which is the largest ever observed since the discovery of magnetic field
on the Sun in 1908 by Hale. We obtained 31 scanned maps of the active
region observed by Hinode/SOT/SP with a cadence of 3 hours over 5 days
(February 1-6, 2014). Considering the spatial and temporal evolution
of the vector magnetic field and the Doppler velocity in the bright
region, we suggested that this strong field region was generated as
a result of compression of one umbra pushed by the outward flow from
the other umbra (Evershed flow), like the subduction of the Earth's
crust in plate tectonics.
Title: Heating mechanisms of the solar corona
Authors: Sakurai, Takashi
Bibcode: 2017PJAB...93...87S
Altcode:
No abstract at ADS
Title: On the Origin of Differences in Helicity Parameters Derived
from Data of Two Solar Magnetographs
Authors: Xu, Haiqing; Zhang, Hongqi; Kuzanyan, K.; Sakurai, T.
Bibcode: 2016SoPh..291.2253X
Altcode: 2016SoPh..tmp..137X
We analyzed how sensitivity and accuracy in solar magnetic field
measurements may affect the values of mean current helicity density
hc and twist parameter αav by comparing these
values obtained from two magnetographs (SMFT at Beijing and SFT at
Mitaka, Tokyo). When we computed the helicity parameters from the
SFT data, we replaced the values of the longitudinal field component,
transverse field strength, and transverse field azimuth angle with those
from the SMFT data and examined the differences. The results show that
the correlation coefficient and the fraction of the data that agree in
signs of hc or αav increase when an SFT parameter
is substituted by the corresponding SMFT parameter because one source
of discrepancy is removed. The increase in correlation coefficient
is largest when the azimuthal angles and transverse field strengths
are set identical in the two instruments; the correlation coefficient
of hc (αav) increases from 0.74 (0.56) to 0.86
(0.78), respectively, indicating that the differences in the transverse
field strength and its azimuthal angle are the largest source of
discrepancy in the values of hc or αav. We found
a nonlinear relationship in the components of the magnetic field between
the two instruments for some data samples; we conclude that this is
due to the discrepancy in the calibration procedure between the two
instruments. This nonlinearity can be another source of difference in
determining helical parameters between the two instruments.
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2016SoPh..291..337L
Altcode: 2016SoPh..tmp...10L
No abstract at ADS
Title: The Vignetting Effect of the Soft X-Ray Telescope Onboard
Yohkoh: II. Pre-Launch Data Analysis
Authors: Shin, J.; Sakurai, T.
Bibcode: 2016SoPh..291..705S
Altcode: 2016SoPh..tmp...15S
The vignetting effect in the Soft X-ray Telescope (SXT) onboard Yohkoh
is studied using a two-dimensional distribution of the effective
area in the field of view taken from a pre-launch experiment. Our
initial estimation of the vignetting in SXT was carried out by fitting
the data with a second-order polynomial function. The results show
that a two-dimensional vignetting effect in SXT is not rotationally
symmetric, particularly in the y - (north-south) direction, which is
probably due to the offset of SXT's optical center from the location
of its CCD center. We adopted a combined functional form consisting
of a second-order polynomial function and a Gaussian function to take
this asymmetric distribution of the effective area into account. We
also considered the steep gradient of the effective area variation at
the region near the vignetting center for the case of higher photon
energy with this form. We completed a two-dimensional description
of the vignetting effect in SXT by a spline surface fit using the
"cleaned" effective area data whose noise was reduced satisfactorily
by the fitting of our combined function.
Title: Solar Full-Disk Polarization Measurement with the Fe I 15648
Å Line
Authors: Hanaoka, Y.; Sakurai, T.; IRMag Group
Bibcode: 2015IAUS..305...92H
Altcode:
The near-infrared absorption line Fe I 15648 Å, which has a Landé
g-factor of 3, shows a particularly large Zeeman splitting. We regularly
take full-disk polarization maps of the Sun in the Fe I 15648 Å line
(as well as the He I 10830 Å line) with an infrared spectropolarimeter
installed at the Solar Flare Telescope of the National Astronomical
Observatory of Japan (NAOJ). It is known that weak, mostly horizontal
magnetic fields are ubiquitously distributed in the quiet regions
of the Sun, while the strong magnetic fields are concentrated in
active regions and network boundaries. The weak horizontal field
has not been sufficiently investigated due to the difficulty of such
observations. The polarization maps in Fe I 15648 Å show the magnetic
field strength at each pixel, regardless of the filling factor, so
we can easily isolate the weak horizontal field signals from strong
magnetic field ones using the Stokes V profiles of the Fe I 15648 Å
line. Here we present instrumental aspects and observational results
of solar near-infrared full-disk polarimetry. We highlight the weak
horizontal field inferred from Fe I 15648 Å.
Title: TA Anisotropy Summary
Authors: Tinyakov, P.; Sagawa, H.; Troitsky, S.; Thomson, G.; Kawata,
K.; Okuda, T.; Sakurai, T.; Ivanov, D.; Tkachev, I.; Nonaka, T.;
Rubtsov, G.
Bibcode: 2015ICRC...34..326T
Altcode: 2015PoS...236..326T
No abstract at ADS
Title: Vignetting Effect in the Soft X-Ray Telescope Onboard Yohkoh:
I. Numerical Simulation
Authors: Shin, J.; Sakurai, T.
Bibcode: 2015SoPh..290.1531S
Altcode: 2013SoPh..tmp..143S
Using a numerical simulation method, we examine the vignetting
effect in the Soft X-ray Telescope (SXT) onboard Yohkoh. The off-axis
variation of the effective area in the field-of-view shows that the
vignetting in SXT cannot be described properly with a one-dimensional
axisymmetric model. Our model assumes a response function of the X-ray
mirror that is symmetric about the optical center, and an effect of
occultation due to other telescope structures that is symmetric about
the geometrical center; the vignetting is the result of these two
contributions. We found that a rotationally non-symmetric distribution
of the SXT effective area is mostly due to the offset of the optical
center from the geometrical center of the telescope. The deviation
from rotational symmetry due to the offset is most noticeable at the
outskirts of the field-of-view, which results in the dispersion of
effective area when considered as a one-dimensional distribution. The
model cannot completely describe the vignetting in SXT because the
fitting errors are larger than the measurement errors. We ultimately
need a fully two-dimensional model for the vignetting in SXT.
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2015SoPh..290..657L
Altcode: 2015SoPh..tmp...24L
No abstract at ADS
Title: A 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
Authors: Otsuji, Kenichi; Sakurai, Takashi; Kuzanyan, Kirill
Bibcode: 2015PASJ...67....6O
Altcode: 2014arXiv1410.7532O; 2014PASJ..tmp..137O
Current helicity and twist of solar magnetic fields are important in
characterizing 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
the Hinode Solar Optical Telescope (SOT). We used SOT-SP data of 558
vector magnetograms of a total of 80 active regions obtained during
the period 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 "hemispheric sign rule"
when 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 smoothing
of 2{^''.}0 (modeled on ground-based observations) is
applied. Furthermore, we found a tendency that weak and inclined
fields conform to the hemispheric sign rule and strong and vertical
fields violate it. These different properties of helicity through
the strong and weak magnetic field components give important clues in
understanding the solar dynamo as well as the mechanism of formation
and evolution of solar active regions.
Title: Editors' Note
Authors: Sakurai, T.
Bibcode: 2014SoPh..289.4423S
Altcode: 2014SoPh..tmp..124S
No abstract at ADS
Title: The Effect of the CCD Gate Structure in the Determination of
the Undersampled Point Spread Function
Authors: Shin, J.; Sakurai, T.
Bibcode: 2014SoPh..289.2803S
Altcode: 2014SoPh..tmp...33S
Using numerical simulations, we evaluated the gate structure effect of
the front-illuminated (FI) charge-coupled device (CCD) on determining
the point spread function (PSF) from the analysis of undersampled
experimental data. The PSF of the Soft X-ray Telescope onboard Yohkoh
was studied as a model case. Its full-width at half-maximum is about
one pixel size, and the FI CCD was equipped in the telescope as
a detector. Of the three emission lines used during the pre-launch
experiment, the data from the lowest-energy line (carbon K-line, 0.28
keV) were significantly influenced by the gate structure. The results
from previous studies regarding the analysis of pre-launch data were
examined and compared with the results from our simulation, and the
expected error ranges of the gate structure effect are discussed. We
found that the error caused by the effect of the gate structure is
significant; at an energy of 0.28 keV, the error may lead to a too
sharp PSF by about a few tens of percent.
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2014SoPh..289.1455L
Altcode: 2014SoPh..tmp....6L
No abstract at ADS
Title: Infrared Stokes Polarimeter at NAOJ/Mitaka
Authors: Hanaoka, Yoichiro; Sakurai, Takashi
Bibcode: 2014IAUS..300..515H
Altcode:
We have been operating an infrared Stokes spectro-polarimeter, whose
observing wavelength bands include the He i 10830 Å and Fe i 15648 Å
lines. A couple of full-Sun, full-Stokes maps in both wavelength bands
are taken on a daily basis, with the polarization sensitivity better
than 10-3. With this sensitivity, the helium polarization
maps clearly show the atomic and Hanlé polarizations besides the
Zeeman polarization, particularly in prominences/filaments. On
these polarization maps, we can track the magnetic field signals
of the prominences/filaments during their passages on the solar
disk. Therefore, this instrument works as a `synoptic prominence
magnetograph'. Our preliminary study of the linear polarizations seen
in quiescent filaments suggests that the magnetic field producing the
polarization signals is located at the bottom part of a flux-rope,
which supports a filament.
Title: Statistical distribution of current helicity in solar active
regions over the magnetic cycle
Authors: Gao, Y.; Sakurai, T.; Zhang, H.; Kuzanyan, K. M.; Sokoloff, D.
Bibcode: 2013MNRAS.433.1648G
Altcode: 2013arXiv1305.4528G; 2013MNRAS.tmp.1516G
The current helicity in solar active regions derived from vector
magnetograph observations for more than 20 years indicates the so-called
hemispheric sign rule; the helicity is predominantly negative in the
Northern hemisphere and positive in the Southern hemisphere. In this
paper, we revisit this property and compare the statistical distribution
of current helicity with Gaussian distribution using the method of
normal probability paper. The data sample comprises 6630 independent
magnetograms obtained at Huairou Solar Observing Station, China, over
1988-2005 which correspond to 983 solar active regions. We found the
following. (1) For the most of cases in time-hemisphere domains the
distribution of helicity is close to Gaussian. (2) At some domains (some
years and hemispheres) we can clearly observe significant departure
of the distribution from a single Gaussian, in the form of two- or
multicomponent distribution. (3) For the most non-single-Gaussian
parts of the data set we see coexistence of two or more components,
one of which (often predominant) has a mean value very close to zero,
which does not contribute much to the hemispheric sign rule. The other
component has relatively large value of helicity that often determines
agreement or disagreement with the hemispheric sign rule in accord
with the global structure of helicity reported by Zhang et al.
Title: Magnetic helicity as a probe of magnetic flux-tube dynamics
in the solar interior
Authors: Sakurai, Takashi; Gao, Yu; Kuzanyan, Kirill
Bibcode: 2013IAUS..294..301S
Altcode:
Magnetic helicity (volume integral of the product of the magnetic
field vector B and the vector potential A ), or its proxy, the current
helicity at the surface (surface integral of B . J or B z
J z ), is an important quantity which characterizes the
helical nature of solar magnetic fields. The current helicity on the
Sun shows a tendency, though with large dispersion, that it is positive
in the southern hemisphere and negative in the northern hemisphere
(the helicity sign rule). However, there are indications that the
helicity sign rule may be reversed at activity minimum periods. We
will discuss the significance of this property by focusing on the
statistical distributions of helicity whether its dispersion follows
Gaussian distribution or not.
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2013SoPh..283....1L
Altcode:
No abstract at ADS
Title: Detection of Low-Level Activities in Solar-Analog Stars from
Emission Strengths of the Ca II 3934 Line
Authors: Takeda, Yoichi; Tajitsu, Akito; Honda, Satoshi; Kawanomoto,
Satoshi; Ando, Hiroyasu; Sakurai, Takashi
Bibcode: 2012PASJ...64..130T
Altcode:
Activity studies of solar-type stars, especially with reference to the
status of our current Sun among them, have exposed the importance of (1)
homogeneously selecting the sample stars and (2) reliably evaluating
their activities down to a considerably low level. Motivated by these
requirements, we conducted an extensive study on the activities of 118
solar-analog stars (of sufficiently similar properties to each other)
by measuring the emission strength at the core of Ca II 3933.663 line (K
line) on the high-dispersion spectrogram obtained by Subaru/HDS, where
special attention was paid to correctly detecting the chromospheric
emission by removing the wing-fitted photospheric profile calculated
from the classical solar model atmosphere. This enabled us to detect
low-level activities down to logR' ∼ -5.4 (R' is the ratio of the
chromospheric core emission flux to the total bolometric flux), by which
we could detect subtle activity differences that were indiscernible
in previous studies. Regarding the Sun, we found logR'⊙
= -5.33 near to the low end of the distribution, which means that it
belongs to the distinctly low-activity group among solar analogs. This
excludes the once-suggested possibility for the high frequency of
Maunder-minimum stars showing appreciably lower activities than the
minimum-Sun.
Title: Solar Force-free Magnetic Fields
Authors: Wiegelmann, Thomas; Sakurai, Takashi
Bibcode: 2012LRSP....9....5W
Altcode:
The structure and dynamics of the solar corona is dominated by the
magnetic field. In most areas in the corona magnetic forces are so
dominant that all non-magnetic forces like plasma pressure gradient and
gravity can be neglected in the lowest order. This model assumption
is called the force-free field assumption, as the Lorentz force
vanishes. This can be obtained by either vanishing electric currents
(leading to potential fields) or the currents are co-aligned with the
magnetic field lines. First we discuss a mathematically simpler approach
that the magnetic field and currents are proportional with one global
constant, the so-called linear force-free field approximation. In
the generic case, however, the relation between magnetic fields and
electric currents is nonlinear and analytic solutions have been
only found for special cases, like 1D or 2D configurations. For
constructing realistic nonlinear force-free coronal magnetic field
models in 3D, sophisticated numerical computations are required and
boundary conditions must be obtained from measurements of the magnetic
field vector in the solar photosphere. This approach is currently of
large interests, as accurate measurements of the photospheric field
become available from ground-based (for example SOLIS) and space-born
(for example Hinode and SDO) instruments. If we can obtain accurate
force-free coronal magnetic field models we can calculate the free
magnetic energy in the corona, a quantity which is important for the
prediction of flares and coronal mass ejections. Knowledge of the
3D structure of magnetic field lines also help us to interpret other
coronal observations, e.g., EUV images of the radiating coronal plasma.
Title: Helioseismology, Dynamo, and Magnetic Helicity
Authors: Sakurai, T.
Bibcode: 2012ASPC..462..247S
Altcode:
A brief review of the solar activity cycle and its dynamo models is
given, with input from helioseismology. Recent new constraints on
the dynamo models by the observations of torsional oscillations and
magnetic helicity are discussed.
Title: Sixty Years of Norikura Solar Observatory
Authors: Sakurai, T.
Bibcode: 2012ASPC..454..439S
Altcode:
This article describes the history of Norikura Solar Observatory
over sixty years, and introduces its main instruments and research
highlights. The observatory was closed in 2009, and utilization of
the facility and the site in future is being sought.
Title: The 3rd Hinode Science Meeting
Authors: Sekii, T.; Watanabe, T.; Sakurai, T.
Bibcode: 2012ASPC..454.....S
Altcode:
No abstract at ADS
Title: Magnetic Helicity of Solar Active Regions as Revealed by
Vector Magnetograms and Coronal X-Ray Images
Authors: Xu, Haiqing; Gao, Yu; Zhang, Hongqi; Sakurai, Takashi;
Hagino, Masaoki; Sokoloff, Dmitry; Pevtsov, Alexei A.
Bibcode: 2012PASJ...64...54X
Altcode:
We have used photospheric vector magnetograms of 15 different
solar active regions to calculate the current helicity parameter,
αav, and the linear force-free field (LFFF) parameter,
αbest, that fits best the observed transverse field. The
data were obtained with the Solar Magnetic Field Telescope at the
Huairou Solar Observing Station, the National Astronomical Observatories
of China, the Solar Flare Telescope of the National Astronomical
Observatory of Japan, and the Haleakala Stokes Polarimeter at the
Mees Solar Observatory, University of Hawaii, from 1997 to 2000. The
agreement in sign of αav between three vector magnetographs
is better than 90%. For αbest, the agreement is 80%-90%. The
line-of-sight magnetograms observed with the Michelson-Doppler Imager
(MDI) on SOHO and coronal X-ray images observed with the Soft X-ray
Telescope (SXT) on Yohkoh have been used to determine the constant
αc of the LFFF in the corona. The value of αc
corresponds to the extrapolated coronal field whose field lines best
match, by visual inspection, the structure of coronal loops in X-ray
images. It is found that the sign agreement between photospheric
αav or αbest and coronal αc
is lower (60%-85%). We consider the differences in measurements,
observing conditions, data reduction methods, and limitation in LFFF
extrapolation, and discuss their contributions to the dispersions in
the hemispheric sign rule of helicity.
Title: Detection of Low-Level Activities in Solar-Analog Stars from
the Emission Strengths of Ca II 3934 Line
Authors: Takeda, Yoichi; Tajitsu, Akito; Honda, Satoshi; Kawanomoto,
Satoshi; Ando, Hiroyasu; Sakurai, Takashi
Bibcode: 2012arXiv1207.0176T
Altcode:
Activity studies of solar-type stars, especially with reference to the
status of our current Sun among them, have exposed the importance of (1)
homogeneously selecting the sample stars and (2) reliably evaluating
their activities down to a considerably low level. Motivated by these
requirements, we conducted an extensive study on the activities of 118
solar-analog stars (of sufficiently similar properties to each other)
by measuring the emission strength at the core of Ca II 3933.663 line (K
line) on the high-dispersion spectrogram obtained by Subaru/HDS, where
special attention was paid to correctly detecting the chromospheric
emission by removing the wing-fitted photospheric profile calculated
from the classical solar model atmosphere. This enabled us to detect
low-level activities down to log R' ~ -5.4 (R' is the ratio of the
chromospheric core emission flux to the total bolometric flux), by which
we could detect subtle activity differences which were indiscernible
in previous studies. Regarding the Sun, we found log R'sun = -5.33 near
to the low end of the distribution, which means that it belongs to the
distinctly low activity group among solar analogs. This excludes the
once-suggested possibility for the high frequency of Maunder-minimum
stars showing appreciably lower activities than the minimum-Sun.
Title: Preface
Authors: Green, L. M.; Sakurai, T.; van Driel-Gesztelyi, L.
Bibcode: 2012SoPh..278....1G
Altcode:
No abstract at ADS
Title: Chemical compositions of soluble particles around the
Termination 1 in the Dome Fuji ice core
Authors: Oyabu, I.; Iizuka, Y.; Sakurai, T.; Suzuki, T.; Miyake, T.;
Hirabayashi, M.; Motoyama, H.; Hondoh, T.
Bibcode: 2012EGUGA..14.6741O
Altcode:
Micro sized particles preserved in Antarctic ice cores are useful
proxies for reconstructing past climate and environmental changes. The
recent studies on chemical compounds of the particles by using the Dome
Fuji ice core showed that sulfate salts were main soluble particles,
and chemical compositions of primary sulfate salt were calcium sulfate
during the glacial maximum and sodium sulfate during the Holocene
#1. However, it is still unknown that how chemical compositions of
sulfate particles have changed on millennial time scale. In this study,
we focused on sulfate salts and sea salts (precursor of sulfate salts)
and measured constituent element of non-volatile particles in the
Dome Fuji ice core around the Termination 1(9-25 kyr BP). A total
of 48 samples were distributed from Dome Fuji ice core section from
298.900m to 582.590m (Holocene to Last Glacial Maximum: LGM, 9-25 kyr
BP), with a time resolution of about 320 year. Non-volatile particles
were extracted from the samples by sublimation system #2. Constituent
elements and diameter of each non-volatile particle were measured
by scanning electron micro scope (SEM) and energy dispersive X-ray
spectroscopy (EDS). By using a method in our recent paper #3, we
made a classification of non-volatile particles into insoluble dust,
soluble sulfate salts and soluble chloride salts. Also we assumed
that particles containing Ca and S are calcium sulfate, particles
containing Na and S are sodium sulfate and particles containing Na
and Cl are sodium chloride. We found several fluctuations of calcium
sulfate, sodium sulfate, and sodium chloride around the Termination 1,
and these fluctuations are associated with changes in terrestrial as
well as marine environments. Main sulfate salts changed from calcium
sulfate to sodium sulfate after 16.5 kyr BP. A plausible explanation
is that sulfuric acid in atmosphere became to react with sodium
chloride instead of dusts (calcium carbonate) after 16.5 kyr BP,
because atmospheric dust concentrations decreased to Holocene level
around 16.5 kyr BP #4. Mass ratio of sodium sulfate+ sodium chloride
(soluble sodium salt) to total particles showed 3 peaks at 16.5,
13 and 10 kyr BP. These peak ages were consistent with those of sea
ice expansion in southern Atlantic Ocean which was reconstructed
from the diatoms abundance of sea ice indicator in a marine sediment
core #5. Therefore the mass fraction of the sodium salt probably
reflects the sea ice expansion in southern Atlantic Ocean. #1 Ohno, H.,
M. Igarashi and T. Hondoh, Characteristics of salt inclusions in polar
ice from Dome Fuji, East Antarctica, Geophysical Research Letters, 33,
L08501, doi: 10.1029/2006GL025774, 2006. #2 Iizuka, Y. and 6 others,
Constituent elements of insoluble and non-volatile particles during the
Last Glacial Maximum exhibited in the Dome Fuji (Antarctica) ice core,
J. Glaciol., 55(191),58-64, 2009. #3 Iizuka, Y. and 9 others, The rates
of sea salt sulfatization in the atmosphere and surface snow of inland
Antarctica. J. Geophys. Res. In press #4 Delmas, R.J. and 6 others,
Linking Antarctic glaciochemical records to past climatic conditions,
Mem. Natl Inst. Polar Res., Special Issue 57, 105-120, 2003. #5 Bianchi,
C., R. Gersonde, Climate evolution at the last deglaciation: the role
of the Southern Ocean. Earth Planet. Sci. Lett., 228, 407-424, 2004.
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2012SoPh..276....1L
Altcode:
No abstract at ADS
Title: Chromospheric Magnetic Field of Solar Active Regions
Authors: Choudhary, D. P.; Sakurai, T.
Bibcode: 2011AGUFMSH31A1987C
Altcode:
The three dimensional magnetic field of solar active regions is studied
by comparing the observed and computed chromospheric magnetograms
obtained at National Solar Observatory with SOLIS and NASA/NSO
Spectromagnetograph. The model chromospheric field is obtained by
extrapolating the observed photospheric field in to chromosphere
with a potential (current-free) magnetic field model in Cartesian
geometry. The long lived active regions display the strong-field
non-potentiality during their initial phase and converse to a potential
field configuration later.
Title: Beryllium Abundances of Solar-Analog Stars
Authors: Takeda, Yoichi; Tajitsu, Akito; Honda, Satoshi; Kawanomoto,
Satoshi; Ando, Hiroyasu; Sakurai, Takashi
Bibcode: 2011PASJ...63..697T
Altcode: 2011arXiv1103.5275T
An extensive Be abundance analysis was conducted for 118 solar analogs
(along with 87 FGK standard stars) by applying the spectrum synthesis
technique to the near-UV region comprising the Be II line at 3131.066
Å, in an attempt to investigate whether Be suffers any depletion,
such as in the case of Li showing a large diversity. We found that,
while most of these Sun-like stars are superficially similar in terms
of their A(Be) (Be abundances) around the solar value within ∼ ±0.2
dex, 4 out of 118 samples turned out to be strikingly Be-deficient
(by more than ∼2 dex), and these 4 stars belong to the group of
lowest ve sin i (projected rotation velocity). Moreover,
even for the other majority showing an apparent similarity in Be,
we can recognize a tendency that A(Be) gradually increases with an
increase in ve sin i. These observational facts suggest
that any solar analog star (including the Sun) generally suffers some
kind of Be depletion during their lives, where the rotational velocity
(or the angular momentum) plays an important role in the sense that
the depletion tends to be enhanced by slower rotation. Hence, our
findings require that the occasionally stated view ``G-type dwarfs
with Teff ≲ 6000 K are essentially homogeneous in Be with
their original composition retained'' should be revised. Also, our
analysis indicates that the difference of ∼0.2 dex in A(Be) between
the solar photosphere and the meteorite really exists, implying that
the ``UV missing opacity'' is irrelevant at least for this Be II line.
Title: Infrared Stokes Spectro-Polarimeter at the National
Astronomical Observatory of Japan
Authors: Hanaoka, Y.; Sakurai, T.; Shinoda, K.; Noguchi, M.; Miyashita,
M.; Fukuda, T.; Suzuki, I.; Hagino, M.; Arai, T.; Yamasaki, T.;
Takeyama, N.
Bibcode: 2011ASPC..437..371H
Altcode:
We are now constructing an infrared spectro-polarimeter for the Solar
Flare Telescope of NAOJ. It observes the full Sun in two wavelength
bands, one near 1.56 μm for highly Zeeman-sensitive spectral lines of
Fe I and the other near 10830 Å for He I and Si I lines. The instrument
records full Stokes profiles, and a Stokes inversion process will
give information on the strength and orientation of the magnetic field
vector for both of the photosphere and the chromosphere. The infrared
detector we are using is an InGaAs camera manufactured by a Belgian
company Xenics. Its format is 640×512 pixels and its read-out speed is
90 frames s-1. The solar disk will be covered by two swaths
(the northern and southern hemispheres) of 640 pixels each. The final
magnetic maps will be made of 1200×1200 pixels with a pixel size of
1.8 arcsec. Now we are operating regular observations and generate
full-disk, full-Stokes maps (a few maps per day). Our ultimate goal is
to derive the distribution of magnetic helicity over the whole surface
of the Sun, not only in sunspots and active regions.
Title: Quantitative Comparison between the Polarization Data
Taken with the Solar Flare Telescope and with the Hinode SOT
Spectro-Polarimeter
Authors: Hagino, M.; Hanaoka, Y.; Sakurai, T.; Ichimoto, K.
Bibcode: 2011ASPC..437..359H
Altcode:
The aim of this study is to establish the method to derive correct
vector magnetic fields from imaging polarimetry data taken with the
Solar Flare Telescope of the National Astronomical Observatory of
Japan. We compared our imaging polarimetry data taken during 2006
December with the spectro-polarimetry data taken with the Hinode
Solar Optical Telescope. While the polarization signals obtained with
the two instruments are basically consistent to each other, we found
some systematic differences between them, particularly in transverse
magnetic field vectors.
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2011SoPh..269....1L
Altcode: 2011SoPh..tmp...21L
No abstract at ADS
Title: Conference summary: Asia-Pacific region in the world and
in astronomy
Authors: Sakurai, T.
Bibcode: 2011ASInC...2..411S
Altcode:
I will discuss how the Asia-Pacific region is represented in the
world by using some statistical data which are population, GDP, IAU
membership, and Solar Physics authorship. Although the Asia-Pacific
region is under-represented in solar physics and astronomy in general,
the situation is improving with the economical rise of China and India.
Title: VizieR Online Data Catalog: Li abundances in solar-analog
stars. II. (Takeda+, 2010)
Authors: Takeda, Y.; Honda, S.; Kawanomoto, S.; Ando, H.; Sakurai, T.
Bibcode: 2010yCat..35150093T
Altcode: 2010yCat..35159093T
To acquire data for studying stellar activities from Ca II near-IR
triplet, the observations of 118 solar-analog stars (the same sample
as in Paper I, Cat. J/A+A/468/663) were carried out in five different
months (2007 February and April; 2008 May, August, and December) by
using the HIgh-Dispersion Echelle Spectrograph (HIDES; Izumiura 1999)
at the coude focus of the 188cm reflector of Okayama Astrophysical
Observatory (OAO). (1 data file).
Title: Behavior of Li abundances in solar-analog stars. II. Evidence
of the connection with rotation and stellar activity
Authors: Takeda, Y.; Honda, S.; Kawanomoto, S.; Ando, H.; Sakurai, T.
Bibcode: 2010A&A...515A..93T
Altcode: 2010arXiv1003.1564T
Context. We previously attempted to ascertain why the Li i 6708
line-strengths of Sun-like stars differ so significantly despite
the superficial similarities of stellar parameters. We carried out
a comprehensive analysis of 118 solar analogs and reported that a
close connection exists between the Li abundance (ALi)
and the line-broadening width (vr+m; mainly contributed
by rotational effect), which led us to conclude that stellar rotation
may be the primary control of the surface Li content.
Aims: To
examine our claim in more detail, we study whether the degree of stellar
activity exhibits a similar correlation with the Li abundance, which
is expected because of the widely believed close connection between
rotation and activity.
Methods: We measured the residual flux
at the line center of the strong Ca ii 8542 line, r0(8542),
known to be a useful index of stellar activity, for all sample stars
using newly acquired spectra in this near-IR region. The projected
rotational velocity (ve sin i) was estimated by subtracting
the macroturbulence contribution from vr+m that we had
already established.
Results: A remarkable (positive) correlation
was found in the ALi versus (vs.) r0(8542)
diagram as well as in both the r0(8542) vs. ve
sin i and ALi vs. ve sin i diagrams, as had been
expected. With the confirmation of rotation-dependent stellar activity,
this clearly shows that the surface Li abundances of these solar
analogs progressively decrease as the rotation rate decreases.
Conclusions: Given this observational evidence, we conclude that
the depletion of surface Li in solar-type stars, probably caused
by effective envelope mixing, operates more efficiently as stellar
rotation decelerates. It may be promising to attribute the low-Li
tendency of planet-host G dwarfs to their different nature in the
stellar angular momentum. Based on observations carried out at
Okayama Astrophysical Observatory (Okayama, Japan).
Title: Thermal and Magnetic Parameters in Solar Flares Derived from
GOES X-Ray Light Curves
Authors: Yamamoto, Tetsuya T.; Sakurai, Takashi
Bibcode: 2010PASJ...62..755Y
Altcode:
The purpose of this study is to reproduce the GOES X-ray (1-8 Å) light
curves of the impulsive phase of 20 solar flares and to estimate the
thermal and magnetic parameters in flare loops. The expected X-ray
light curves are calculated from the values of the coronal field
strength and inflow velocity under some assumptions. We used the
magnetic reconnection heating model of Shibata and Yokoyama (2002,
ApJ, 577, 422), photospheric vector magnetic field data, and a simple
1-D flare loop model. It is found the maximum inflow velocities are
∼ 107cm s-1, and that the maximum magnetic
reconnection rates are 0.006-0.9, and further that the characteristic
coronal field strengths are 6-100 G. Using the thermal and magnetic
parameters derived, we found that geometrically our calculated flare
loops are higher than the potential fields that model the postflare
loops, and are also higher than the preflare loops containing magnetic
free energy that is large enough to provide the thermal energy of a
flare. Hence, we conclude that the plasma β value would be near unity
in the flare loops we studied, and the magnetic field lines shrunk
during the decay phase. The downward velocities of the field-line
shrinkage are estimated to be 106-107cm
s-1 except one event. The height of the reconnection
point was roughly estimated to be from 1010cm to 4 ×
1010cm. We propose a new interpretation of the Neupert
effect, and also discuss a temporal relationship between the X-ray
flux and the flare loop heating.
Title: Solar Observation Target Identification Convention for use
in Solar Physics
Authors: Leibacher, John; Sakurai, Takashi; Schrijver, Carolus J.;
van Driel-Gesztelyi, Lidia
Bibcode: 2010SoPh..263....1L
Altcode: 2010SoPh..263....1.; 2010SoPh..tmp...71.
We strongly encourage the use of a standardized target identification
to be included in pub- lications on solar events. The primary purpose
is to enable the automated identification of publications on the same
event, or on other related events, in the on-line literature by search
engines such as the Astrophysics Data System (ADS). The convention does
not aim to categorize or classify events, but is limited specifically
to the identification of regions in space and intervals in time within
which events occur.
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2010SoPh..262....1L
Altcode: 2010SoPh..tmp...18L
No abstract at ADS
Title: A new dynamo pattern revealed by solar helical magnetic fields
Authors: Zhang, Hongqi; Sakurai, T.; Pevtsov, A.; Gao, Yu; Xu, Haiqing;
Sokoloff, D. D.; Kuzanyan, K.
Bibcode: 2010MNRAS.402L..30Z
Altcode: 2009arXiv0911.5713Z; 2010MNRAS.tmpL...1Z
A previously unobservable mirror asymmetry of the solar magnetic field -
a key ingredient of the dynamo mechanism which is believed to drive the
11-year activity cycle - has recently been measured. This was achieved
through systematic monitoring of solar active regions carried out for
more than 20 years at observatories in Mees, Huairou and Mitaka. In this
Letter we report on detailed analysis of vector magnetic field data,
obtained at Huairou Solar Observing Station in China. Electric current
helicity (the product of current and magnetic field components in the
same direction) was estimated from the data and a latitude-time plot of
solar helicity during the last two solar cycles has been produced. We
find that like sunspots helicity patterns propagate equatorwards,
but unlike sunspot polarity helicity in each solar hemisphere does not
change sign from cycle to cycle, thus confirming the theory. There are,
however, two significant time-latitudinal domains in each cycle when
the sign briefly inverts. Our findings shed new light on stellar and
planetary dynamos and are yet to be included in the theory.
Title: Solar Physics Publication Ethics Policies
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2009SoPh..260....1L
Altcode:
No abstract at ADS
Title: Alfvénic plasma velocity variations observed at the inner
edge of the low-latitude boundary layer induced by the magnetosheath
mirror mode waves: A THEMIS observation
Authors: Nowada, M.; Shue, J. -H.; Lin, C. -H.; Sakurai, T.; Sibeck,
D. G.; Angelopoulos, V.; Carlson, C. W.; Auster, H. -U.
Bibcode: 2009JGRA..114.7208N
Altcode: 2009JGRA..11407208N
With unique simultaneous observations both in the magnetosheath
and magnetosphere by the THEMIS probes, Alfvénic variations in
the plasma velocity are observed at the inner edge of low-latitude
boundary layer (LLBL) and are induced by the mirror mode waves in the
magnetosheath near the subsolar magnetopause on 31 July 2007. These
Alfvénic variations appeared as the wavy perturbations in the
Vx and Vy components observed by THEMIS C, D,
and E, which had the same periodicity as associated magnetic field
variations. Simultaneously, THEMIS B observed the mirror mode waves in
the magnetosheath. The periodicities of the magnetic and plasma pressure
variations of mirror modes in the magnetosheath were consistent with
those of the Alfvénic wavy variations in the LLBL. Therefore, the
mirror mode waves can induce the magnetopause undulations, launching
Alfvén waves, and resultant Alfvénic variations are observed in the
LLBL. Also, in the succeeding magnetosheath interval by THEMIS B,
we examined whether the mirror mode waves occurred and associated
Alfvénic variations were observed in the LLBL. However, no clear
evidence for an existence of the mirror mode waves was obtained, and
THEMIS C, D, and E do not also observe associated magnetic field and
plasma Alfvénic responses in the LLBL. These results suggest that the
Alfvénic variations in the LLBL are strongly related to the mirror
mode waves in the magnetosheath. On the basis of these results, we
emphasize that the magnetosheath energy is transmitted and transported
into the magnetosphere via magnetopause surface waves.
Title: Comparison of the Vector Magnetograms taken with the SFT/MTK
and the SST/KSW
Authors: Hagino, M.; Nakatani, Y.; Ishii, T. T.; Hanaoka, Y.; Sakurai,
T.; Hiei, E.; Suzuki, D.
Bibcode: 2009ASPC..405..393H
Altcode:
We carried out a detailed comparison between the vector magnetograms
obtained with the Solar Flare Telescope (SFT/MTK) of the National
Astronomical Observatory of Japan and those obtained with the
Sextuple Solar Telescope (SST/KSW) of the Kawaguchi Science Museum
(the former name is the Kawaguchi Science World). We investigated
various characteristics of the errors in the Stokes parameters for each
instrument. The level of noise due to seeing effects in the SST/KSW
magnetograms is higher than in the SFT/MTK magnetograms. On the other
hand, we found that about 60% of the data pixels have the same sign
of current helicity α in these magnetograms. We will discuss the
possibility and limitations of the magnetic field studies using these
magnetograms.
Title: Local Twist and Current Helicity Distributions of Active
Region NOAA 10930
Authors: Su, J. T.; Sakurai, T.; Suematsu, Y.; Hagino, M.; Liu, Yu
Bibcode: 2009ApJ...697L.103S
Altcode:
Hinode high-quality vector magnetograms and G-band data are utilized
to study the distributions of local twist α z and current
helicity hc on the active region of NOAA 10930. The new
findings are as follows. (1) The patches of positive and negative
helicities were intermixed showing a mesh pattern in the umbra and
a thread pattern in the penumbra. (2) For its main stable sunspot
(MSS), there was a positive-helicity patch accounting for ~43% of the
umbra area surrounding the inner umbra, which had a predominantly
negative helicity. For its minor rotating sunspot (MRS), there
was a negative-helicity patch appearing in the umbra. (3) The fine
distributions of α z and hc on a penumbral
filament indicated that it may be possible for the two opposite
helicities to coexist in a filament and their magnitudes were nearly
equivalent.
Title: Time Variation and Statistical Studies of Magnetic Helicity
Injection in Solar Magnetic Regions
Authors: Yamamoto, Tetsuya T.; Sakurai, T.
Bibcode: 2009ApJ...698..928Y
Altcode:
In this study, we analyzed the helicity injection in solar active
regions statistically with the method proposed by Kusano et al. We
investigated the variation in helicity injection as an active region
evolves and the statistical distribution of helicity injection against
the magnetic flux (Φ) and the latitude. The temporal evolution of
active regions is divided into three phases ("emergence," "active," and
"decay" phases). We studied one emergence-phase region, 13 active-phase
regions, and five decay-phase regions. The total number of helicity
injection rates we obtained is 1476. We found the following results. The
dispersions (standard deviations) in the helicity injection rates are
proportional to Φ-0.47 in the active-phase regions and to
Φ-0.34 in the decay-phase regions. The dispersion for the
emergence-phase region, 0.046 day-1, is larger than those
for the active-phase regions, 0.004-0.03 day-1, and those
for the decay-phase regions, 0.004-0.01 day-1. The average
helicity injection rates of the active-phase regions show the latitude
dependence, which is consistent with the "hemispheric rule" of helical
structures in the solar atmosphere, while those of the decay-phase
regions are inconsistent with the hemispheric rule. The average
helicity injection rates for the active-phase regions, 0.003-0.03
day-1, are larger than that for the emergence-phase region,
0.0045 day-1, and those for the decay-phase regions,
<0.002 day-1. These dependences of the helicity injection
rates give us clues on the generation mechanism of helicity and the
evolution of the magnetic field in the solar convection zone.
Title: Forecasting maximum solar flare magnitudes from photospheric
magnetograms
Authors: Yamamoto, Tetsuya T.; Sakurai, T.
Bibcode: 2009SpWea...7.4007Y
Altcode:
The purpose of this paper is to forecast the maximum solar flare
magnitude in an active region and its uncertainty from photospheric
magnetic field data. We analyzed 21 flare samples covering X-ray flare
magnitudes from A5.0 to X17.0. Photospheric magnetic parameters are
obtained from vector and line of sight magnetograms observed with
the Solar Flare Telescope and the Michelson Doppler Imager. Magnetic
parameters are averaged axial field strength, averaged longitudinal
field strength, and characteristic scale length. These parameters show
linear relations with the X-ray flare magnitudes. This enables us to
forecast the maximum flare magnitude an active region will produce. The
uncertainty in the forecast is estimated with a simultaneous tolerance
interval analysis under 95% confidence level and probability. The
minimum uncertainty is factor 3.7 with the data from high-shear regions
(shear angles larger than 60°). Therefore, log F X of
the regionally largest flare is forecasted with an uncertainty of log
(3.7) = 0.57. For example, an X7.0 flare can be predicted with an
uncertainty between X1.9 and X25.9.
Title: Preface to the Proceedings of the European General Assembly
and the United Nations Workshop
Authors: Gopalswamy, N.; Eichhorn, G.; Sakurai, T.; Haubold, H. J.
Bibcode: 2009EM&P..104..139G
Altcode: 2009EM&P..tmp....4G
No abstract at ADS
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2009SoPh..255....1L
Altcode:
No abstract at ADS
Title: Erratum: Optical Characteristics of the Soft X-Ray Telescope
Aboard Yohkoh . I. Interpretation of the Undersampled Point Spread
Function
Authors: Shin, J.; Sakurai, T.
Bibcode: 2009SoPh..254..385S
Altcode:
No abstract at ADS
Title: Optical Characteristics of the Soft X-Ray Telescope Aboard
Yohkoh . I. Interpretation of the Undersampled Point Spread Function
Authors: Shin, J.; Sakurai, T.
Bibcode: 2009SoPh..254..357S
Altcode: 2008SoPh..tmp..195S
We have studied the point spread function (PSF) of the Yohkoh Soft
X-ray Telescope (SXT) using the pre-launch calibration data. It
is revealed from our study that both a careful consideration on the
undersampling effect and a proper choice of statistics are indispensable
for determining the best fit analytical function for the SXT PSF. We
present the results of numerical simulations supporting our approach
toward proper handling of the undersampled PSF. Examples of the derived
SXT PSF using a limited number of pre-launch calibration data are
shown as a case study.
Title: Correlations between Flare Parameters and Magnetic Parameters
in Solar Flares
Authors: Yamamoto, Tetsuya T.; Sakurai, Takashi
Bibcode: 2009PASJ...61...75Y
Altcode:
We investigated correlations between flare parameters and magnetic
parameters by picking up the largest and smaller flares in each sample
region. The flare parameters that we used were the X-ray peak flux,
characteristic duration time, and X-ray energy. These values were
derived from Geostationary Operational Environmental Satellites(GOES)
X-ray light curves (1--8Å). The magnetic parameters used were the
averaged longitudinal field strength, averaged shear field strength,
and characteristic length of flare ribbons. These magnetic parameters
were derived from flaring areas. We used vector magnetograms obtained
with the Solar Flare Telescope (National Astronomical Observatory
of Japan) and SoHO/MDI (Michelson Doppler Imager) magnetograms. In
this paper the regionally largest flare is defined as the flare
whose X-ray peak flux exceeds one-fifth of the X-ray peak flux of
the most intense flare in each region, and all the other flares are
defined as smaller flares. From 1997 to 2003, we selected 20 flares
as the 0.9675[1]regionally largest flares and 15 flares as smaller
flares. We found that the X-ray peak flux of the regionally largest
flares has definite correlations with the magnetic parameters, and
the characteristic duration time has a definite correlation only with
the characteristic length of flare ribbons. The regionally largest
flares and smaller flares show different relations between the flare
parameters and the magnetic parameters. They show, on the other hand,
a single relation between the X-ray energy and the magnetic free energy
index derived from the photospheric magnetic parameters.
Title: Studies on the imaging characteristics of Yohkoh Soft X-ray
Telescope: Optical aberration and scattering
Authors: Shin, Junho; Sakurai, Takashi
Bibcode: 2009AdSpR..43..101S
Altcode:
In order to better understand the characteristics of Yohkoh Soft X-ray
Telescope (SXT) mirror, we have analyzed the in-flight overexposed image
(the starburst image) obtained during the solar flare observation. It
has been revealed from our study that the intensity distribution inside
the shadows shown in the scattering difference image contains little
of the scattered component of the PSF and matches almost correctly the
extension of the PSF core profile. Also it is found that the scattering
wing of the SXT PSF is connected smoothly to the PSF core within
the distance of about 100 200 arcsec from the peak. With numerical
simulations we have shown that an increase in energy affects not only
the level of scattering wing, but also both the shape and the absolute
level of the PSF core. The results have revealed, however, that the
energy dependence for the SXT PSF cannot be easily estimated with
the data obtained from one filter alone, which implies that the data
analysis using multiple filters will enable us to determine the absolute
amount of scattered component as well as the energy dependence of the
SXT PSF. Details on the analysis of starburst image and the results
from numerical simulations will be introduced and discussed thoroughly.
Title: Radio Bursts Associated with Flare and Ejecta in the 13 July
2004 Event
Authors: Pohjolainen, S.; Hori, K.; Sakurai, T.
Bibcode: 2008SoPh..253..291P
Altcode: 2008arXiv0809.3896P; 2008SoPh..tmp..164P
We investigate coronal transients associated with a GOES M6.7 class
flare and a coronal mass ejection (CME) on 13 July 2004. During the
rising phase of the flare, a filament eruption, loop expansion, a
Moreton wave, and an ejecta were observed. An EIT wave was detected
later on. The main features in the radio dynamic spectrum were a
frequency-drifting continuum and two type II bursts. Our analysis
shows that if the first type II burst was formed in the low corona,
the burst heights and speed are close to the projected distances and
speed of the Moreton wave (a chromospheric shock wave signature). The
frequency-drifting radio continuum, starting above 1 GHz, was formed
almost two minutes prior to any shock features becoming visible, and
a fast-expanding piston (visible as the continuum) could have launched
another shock wave. A possible scenario is that a flare blast overtook
the earlier transient and ignited the first type II burst. The second
type II burst may have been formed by the same shock, but only if the
shock was propagating at a constant speed. This interpretation also
requires that the shock-producing regions were located at different
parts of the propagating structure or that the shock was passing
through regions with highly different atmospheric densities. This
complex event, with a multitude of radio features and transients at
other wavelengths, presents evidence for both blast-wave-related and
CME-related radio emissions.
Title: A Test of Three Optical Flow Techniques—LCT, DAVE, and NAVE
Authors: Chae, Jongchul; Sakurai, Takashi
Bibcode: 2008ApJ...689..593C
Altcode:
A time sequence of high-quality images currently produced by
high-resolution observations either from the ground or in space may
be utilized to determine the transverse flow field on the plane of
the sky with the help of optical flow techniques. We have examined the
performance of three different methods—a well-known technique called
local correlation tracking (LCT), a recently developed technique called
the differential affine velocity estimator (DAVE), and a new technique
called the nonlinear affine velocity estimator (NAVE)—using three
kinds of image data: mapping-based synthetic images, a set of MHD
simulation data, and real images (magnetograms) taken by the Solar
Optical Telescope on board Hinode. We have generalized the model
equation of image evolution by adding to the continuity equation a
source term that is proportional to the image value. Synthetic images
were constructed based on the analytical solution of this equation
with different velocity profiles: uniform, affine, or nonaffine. The
tests with the synthetic data indicated that NAVE is very good at
detecting subpixel motions, superpixel motions, and nonuniform
motions, while LCT is not good at detecting nonuniform motions,
especially around critical points, and the performance of DAVE is
degraded in the presence of superpixel motions. In all the methods,
the performance became worse as the velocity field deviated more from
an affine one. We also found that the MHD simulation data we used
are not quite suited for discriminating between the three methods,
maybe because the data do not contain enough structural information
to be used for tracing. In contrast, the determination of velocity
fields from the real image data was somewhat sensitive to the technique
adopted. The technique of NAVE with the source term produced velocity
fields that are the most consistent with the data.
Title: Persistent Horizontal Flows and Magnetic Support of Vertical
Threads in a Quiescent Prominence
Authors: Chae, Jongchul; Ahn, Kwangsoo; Lim, Eun-Kyung; Choe, G. S.;
Sakurai, Takashi
Bibcode: 2008ApJ...689L..73C
Altcode:
There has been some controversy as to whether the magnetic fields
of vertical threads seen in quiescent prominences are predominantly
vertical or horizontal. We report finding special patterns of flow in
a quiescent prominence observed by the Solar Optical Telescope aboard
Hinode. This prominence is a small hedgerow prominence composed of many
vertical threads. To one side of it, we found a pattern of persistent
horizontal flows of Hα-emitting plasma. These flows originated from a
region in the chromosphere, rose to coronal heights, and then extended
horizontally for a long distance until they reached the main body of
the prominence. In the higher altitudes the flows either moved across
vertical threads or lifted them up, while in the lower altitudes
they often formed bright blobs of plasma and shed them, resulting in
a sudden change of flow direction from horizontal to vertical. The
observed persistent horizontal flows support a configuration of
initially horizontal magnetic fields, and our results appear to
be consistent with the traditional theory that vertical threads in
quiescent prominences are stacks of plasma supported against gravity
by the sagging of initially horizontal magnetic field lines.
Title: THEMIS Observations of the Wavy Variations in the Plasma
Velocity at the inner edge of the Low-Latitude Boundary Layer
Authors: Nowada, M.; Shue, J.; Lin, C.; Sakurai, T.; Sibeck, D. G.;
Lyu, L.; Angelopoulos, V.; McFadden, J. P.; Carlson, C. W.; Auster, H.
Bibcode: 2008AGUFMSM23A1685N
Altcode:
The THEMIS spacecraft provides opportunities to make unique simultaneous
observations in the magnetosheath, the magnetospheric boundary layers
and the magnetosphere by a string-of-pearls configuration. We have
examined how the variations caused by the surface waves on the dayside
magnetopause can propagate into the magnetosphere. On July 31, 2007,
THEMIS-C and -D observed the wavy variations in the plasma velocity
at the inner edge of Low-Latitude Boundary Layer (LLBL) under the
northward Interplanetary Magnetic Field (IMF) and quiet solar wind
conditions. These wavy velocity variations appeared in the Vx and
Vy components. The associated magnetic field had microscopic wavy
fluctuations, but no periodic variations were found in simultaneous
plasma density and temperature. The hodograms of the velocities
perpendicular to the magnetic field line had clear vortex-like
structures. From these results, observed plasma velocity variations
are Alfvén waves. On the other hand, THEMIS-E also observed the
wavy velocity variations at the same region as well as THEMIS-C
and -D, but the velocity hodogram did not present the vortex-like
structures. Therefore, these wavy velocity variations are local
phenomena. THEMIS-B simultaneously observed the diamagnetic-like
waves due to the magnetopause undulations in the magnetosheath. This
is because the correlation between the magnetic and plasma pressures
was clearly out-of-phase. During the interval of these waves, the
periodicities of these pressure variations were between 0.5 minute and 1
minute, and consistent with those of the wavy velocity variations. From
these results, the fast-mode waves are induced by the magnetopause
diamagnetic-like waves, and can propagate into the magnetosphere.
Title: Three-Dimensional Structure Analysis of Coronal Magnetic
Field in AR NOAA 10930 Based on Vector Magnetogram Observations
with Hinode/SOT
Authors: Inoue, S.; Kusano, K.; Masuda, S.; Miyoshi, T.; Yamamoto,
T.; Magara, T.; Tsuneta, T.; Sakurai, T.; Yokoyama, T.
Bibcode: 2008ASPC..397..110I
Altcode:
Active region NOAA 10930 produced the first X-class flare observed by
the Hinode satellite. Fortunately, the data covers the time span before
and after the X-class flare arising on December 13, 2006 in this active
region. During this event, the Hinode satellite clearly observed the
typical features of a flare: two ribbon and post flare loop structures
seen by SOT, sigmoidal structure before the onset, and subsequent cusp
loop structure seen by XRT. We analyzed the three-dimensional magnetic
structure of the region before and after the flare on Dec.13, 2006 using
the Non-Linear Force-Free (NLFF) extrapolation method based on extended
magnetofrictional method. As a result of the NLFF extrapolation,
we found that, before the flare onset, strong sheared structures were
formed on the neutral line. Furthermore, we revealed that a sigmoidal
structure was not formed of a single loop, but was composed of strong
multiple sheared field. On other the hand, after the flare, elongated
magnetic flux is partially formed and a part of magnetic shear was
released. The result indicates that the part of NLFF relaxes toward
the potential like field in this event.
Title: Determination of Magnetic Diffusivity from High-Resolution
Solar Magnetograms
Authors: Chae, Jongchul; Litvinenko, Yuri E.; Sakurai, Takashi
Bibcode: 2008ApJ...683.1153C
Altcode:
The magnetic diffusivity in the solar photosphere is determined by
applying a new method to the magnetic induction equation. The magnetic
field evolution is specified by a time sequence of high-resolution
magnetograms of plage regions, taken by Hinode/SOT and SOHO/MDI. The
mean value of magnetic diffusivity determined from SOT magnetograms with
the smallest pixel size of 116 km is about 0.87 +/- 0.08 km2
s-1. This is the smallest value that has been empirically
determined so far. High-resolution and full-disk MDI magnetograms with
the pixel sizes of 440 and 1400 km yielded larger values of 4.4 +/-
0.4 and 18 +/- 7.4 km2 s-1, respectively. The
measured diffusivity values at different length scales are consistent
with a turbulent cascade that ends at a resistive dissipation scale of
about 30 km. The results suggest that turbulent magnetic diffusivity
should be taken into account in the analysis of the observed rate of
magnetic flux cancellation in the photosphere.
Title: Preface: A Topical Issue on the Hinode Mission
Authors: Sakurai, Takashi
Bibcode: 2008SoPh..249..165S
Altcode: 2008SoPh..tmp...82S
No abstract at ADS
Title: On the Solar Cycle Variation of the Hemispheric Helicity Rule
Authors: Pevtsov, A. A.; Canfield, Richard C.; Sakurai, T.; Hagino, M.
Bibcode: 2008ApJ...677..719P
Altcode:
We study the statistical significance of observed temporal variations
of the solar active-region hemispheric helicity rule, as measured by the
latitudinal gradient of the best-fit linear force-free-field parameter,
dα/dvarphi . Using data from four different vector magnetographs,
we compute and compare average annual dα/dvarphi values for these
instruments for 19 years from solar cycles 21, 22, and 23. We find that
although every instrument shows the "wrong" sign for the hemispheric
rule in some years, there is no agreement among the instruments on
which years are abnormal. None of the four data sets shows annual
values of dα/dvarphi departing from the hemispheric helicity rule
by more than 3 σ. We conclude that because the hemispheric helicity
rule is a weak tendency with significant scatter, an annual subset of
active regions is likely to produce statistically unreliable results.
Title: Suppression of convection around small magnetic concentrations
Authors: Morinaga, S.; Sakurai, T.; Ichimoto, K.; Yokoyama, T.;
Shimojo, M.; Katsukawa, Y.
Bibcode: 2008A&A...481L..29M
Altcode:
Aims: It is well known that convective motions in the photosphere are
suppressed by magnetic fields. However, it has been difficult to study
the interaction between convection and small magnetic features, such
as G-band bright points (GBPs) or pores with polarimetric measurements,
because of the available spatial resolution (~1´´). This situation is
changed by the advent of the Spectro-Polarimeter (SP) aboard the Hinode
satellite, which has 0.3 arcsec spatial resolution.
Methods:
We analyzed the pore and its surrounding region in NOAA 10940 near the
disk center. We obtained the field strength and filling factor through
the Milne-Eddington inversion of the Stokes profiles. We also derived
the line-of-sight velocity by the shift of the line core. Using these
physical parameters, we investigated the physical conditions needed to
suppress the convection.
Results: We found that the convection
is suppressed, not by the strength of the magnetic field itself,
but by high concentration of magnetic flux tubes. We also found that
GBPs and pores are distinguished in terms of the filling factor (f);
f ≃ 0.6 for GBPs and f=0.8-0.9 for pores.
Title: Editorial Appreciation
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2008SoPh..247....1L
Altcode: 2008SoPh..tmp....5L
No abstract at ADS
Title: Determination of Magnetic Diffusivity from High Resolution
Solar Magnetograms
Authors: Chae, Jongchul; Litvinenko, Yuri E.; Sakurai, Takashi
Bibcode: 2008cosp...37..482C
Altcode: 2008cosp.meet..482C
The magnetic diffusivity in the solar photosphere is determined by
applying a model of magnetic induction to high resolution magnetograms
of plage regions, taken by HINODE/SOT and SOHO/MDI. The mean value
of magnetic diffusivity determined from SOT magnetograms with the
smallest pixel size of 116 km is about 0.84±0.34 km2 s-1 . This is
the smallest value that has been empirically determined so far. High
resolution and full-disk MDI magnetograms with the pixel sizes of 440
and 1400 km yielded larger values of 4.5±1.4 km2 s-1 and 13±10 km2
s-1 , respectively. The measured diffusivity values at different length
scales are consistent with a turbulent cascade that ends at a resistive
dissipation scale of about 25 km. The results suggest that turbulent
magnetic diffusivity should be taken into account in the analysis of
the observed rate of magnetic flux cancellation in the photosphere.
Title: Non-Linear Force-Free Modeling of AR NOAA 10930 Based on
Vector Magnetogram Observation with Hinode/SOT
Authors: Inoue, S.; Kusano, K.; Masuda, S.; Miyoshi, T.; Magara, T.;
Yamamoto, T.; Sakurai, T.; Tsuneta, S.; Yokoyama, T.
Bibcode: 2007AGUFMSH53A1048I
Altcode:
Since the detection of three-dimensional (3D) magnetic field in active
regions is crucially important to understand the trigger mechanism
of solar flares, the methodologies to reconstruct the 3D magnetic
field from magnetgram observation is quickly developed recently. In
this study, we have developed a new Non-linear Force-Free (NLFF)
field extrapolation method, based on the extended magnetofrictional
model and the divergence field cleaning technique, and applied it onto
the magnetograms of Active Region NOAA10930, which were observed by
Hinode/SOT. The data covers the time span before and after the X-class
flare arising on December 13, 2006 in this active region, and Hinode
satellite clearly observed the typical features of flare; two ribbon
and post flare loop structures by SOT, and sigmoidal structure before
the onset and the cusp loop structures after that by XRT. As a result
of the NLFF extrapolation, we found that, before the onset of flare,
strong sheared structures were formed on the neutral line, whereas the
field overlying magnetic neutral line was potential-like. Furthermore,
we revealed that a sigmoid structure was not formed of a single sheared
loop, but was composed of strong multiple sheared fields. It is also
detected that, after the flare, elongated magnetic flux is partially
formed but a part of magnetic shear was released. The results indicate
that the NLFF relaxes towards the potential field on average during
the flaring phase, although some sigmoidal structure remains. Also,
the long term evolution of the active region and the storage process
of magnetic energy and helicity in the active region are investigated.
Title: Magnetic nature of coronal loops
Authors: Singh, Jagdev; Muneer, S.; Sakurai, Takashi; Ichimoto, Kiyoshi
Bibcode: 2007BASI...35..437S
Altcode:
It is generally believed that the magnetic pressure is much higher than
the gas pressure in the coronal loops and these loops are isothermal
in nature. We made systematic observations of four strong coronal
emission lines in the visible and near infrared part of the spectrum
for about 8 years. Two emission lines were observed at a time, making
raster scans of a steady coronal region. We studied the variation
of line widths of these lines and intensity ratios as a function
of height. The relationship between the widths of these lines and
intensity ratios indicates that the steady coronal loops are not
magnetically isolated. These findings put restrictions on coronal loop
models and indicate that the magnetic pressure in coronal loops may be
much less than assumed. These results strongly suggest that magnetic
field strength in the corona needs to be measured accurately.
Title: Fe XIV green/Fe XIII infrared line ratio diagnostics
Authors: Srivastava, A. K.; Singh, Jagdev; Dwivedi, B. N.; Muneer,
S.; Sakurai, T.; Ichimoto, K.
Bibcode: 2007BASI...35..457S
Altcode:
We consider the first 27-level atomic model of Fe XIII (5.9 K <
log T_{e} < 6.4 K) to estimate its ground level populations,
taking account of electron as well as proton collisional excitations
and de-excitations, radiative cascades, radiative excitations and
de-excitations. Radiative cascade is important but the effect of
dilution factor is negligible at higher electron densities. The
^{3}P_{1}-^{3}P_{0} and ^{3}P_{2}-^{3}P_{1} transitions in the
ground configuration 3s^{2} 3p^{2} of Fe XIII result in two forbidden
coronal emission lines in the infrared region, namely 10747 and 10798
Å. While the 5303 Å green line is formed in the 3s^{2} 3p ground
configuration of Fe XIV as a ^2P_{3/2}-^{2}P_{1/2} magnetic dipole
transition. The line-widths of simultaneously observed Fe XIV green
and Fe XIII infrared forbidden coronal emission lines can be a useful
diagnostic tool to deduce temperature and nonthermal velocity in the
largescale coronal structures using intensity ratios of the lines as
the temperature signature, instead of assuming ion temperature to be
equal to the electron temperature. Since the line intensity ratios
I_{G5303}/I_{IR10747} and I_{G5303}/I_{IR10798} have very weak density
dependence, they are ideal monitors of temperature mapping in the
solar corona. The computed ratios will be compared with the recently
obtained observations in our next paper.
Title: Small-scale X-ray/EUV Jets seen in Hinode XRT and TRACE
Authors: Kim, Y.; Moon, Y.; Cho, K.; Bong, S.; Park, Y.; Sakurai,
T.; Chae, J.
Bibcode: 2007AGUFMSH53A1061K
Altcode:
In this study, we present the morphological and kinematic
characteristics of three small-scale X-ray/EUV jets that are
simultaneously observed by the X-ray telescope (XRT) onboard Hinode
(Solar-B) and the TRACE. For this study, we examined all XRT movies
with a thin Al/Poly filter from 2006 October 20 to April 8 to look
for small-scale X-ray eruptions and then found 34 eruptions. Next we
took a look at the corresponding TRACE 171/195 {Å} images associated
with these X-ray eruptions. As a result, we found three simultaneous
X-ray and EUV jets and their major characteristics can be summarized
as follows : (1) They all have no association with major flares. (2)
From the comparison between XRT and TRACE observations, we found that
they have similar characteristics in terms of projected speed, lifetime,
and size. (3) Their sizes range from 4 × 105 to 5 × 105 km. (4) Their
projected speeds are estimated to be 180-310 km s-1 with an average
speed of about 250 km s- 1 (5) Their lifetimes lie in the range of 100
to 600 s. (6) From the comparison between the XRT images with the SOHO
MDI maps for two events, all they are associated with the parasitic
polarity region. These characteristics are similar to the previous
observations for X-ray jets by the Yohkoh SXT but quite different
from EUV jets associated with Hα surges. In addition, from the EIS
four spectral lines for the last event, we found blueshift (up to -64
km s-1) and redshift (up to 20 km s-1) motions as well as nonthermal
velocities ranging from 57 to 106 km s-1 at the jet footpoint. We
note a tendency : the hotter the maximum ionization temperature is,
the larger the area of blueshift region is. These characteristics
are consistent with a typical jet model that includes the magnetic
reconnection between open coronal fields and emerging magnetic fields.
Title: Center-to-Limb Variation of Stokes V Asymmetries in Solar
Pores Observed with the Hinode Spectro-Polarimeter
Authors: Morinaga, Shuji; Nagata, Shin'ichi; Ichimoto, Kiyoshi;
Suematsu, Yoshinori; Tsuneta, Saku; Katsukawa, Yukio; Shimizu,
Toshifumi; Shine, Richard A.; Tarbell, Theodore D.; Title, Alan M.;
Lites, Bruce; Kubo, Masahito; Sakurai, Takashi
Bibcode: 2007PASJ...59S.613M
Altcode:
Here we present spectro-polarimetric measurements of several pores
and the surrounding regions taken with the Solar Optical Telescope
aboard Hinode at various viewing angles. We analyzed the Stokes V
area asymmetry, and confirmed that it is depressed at the center of
the pores, while it shows large positive values (a blue lobe larger
than a red lobe) in the surrounding area; this is consistent with a
previous report. In addition to this ring of positive asymmetry, we
found regions of alternating positive and negative area asymmetries
when weak V regions were observed near the solar limb. The positive
asymmetry occurs on the disk-center side and the negative asymmetry
on the limb side of the magnetic concentrations. These center-to-limb
variations of the Stokes V area asymmetry can be interpreted as being
a systematic inflow of plasma into the magnetic concentrations from
their surroundings.
Title: Initial Results on Line-of-Sight Field Calibrations of SP/NFI
Data Taken by SOT/Hinode
Authors: Chae, Jongchul; Moon, Yong-Jae; Park, Young-Deuk; Ichimoto,
Kiyoshi; Sakurai, Takashi; Suematsu, Yoshinori; Tsuneta, Saku;
Katsukawa, Yukio; Shimizu, Toshifumi; Shine, Richard A.; Tarbell,
Theodore D.; Title, Alan M.; Lites, Bruce; Kubo, Masahito; Nagata,
Shin'ichi; Yokoyama, Takaaki
Bibcode: 2007PASJ...59S.619C
Altcode:
We present initial results on the line-of-sight field calibration
of the two kinds of Stokes I and V data taken by the Solar Optical
Telescope on the satellite Hinode: spectral profiles of Stokes I and V
parameters recorded on the Spectro-polarimeter (SP), and monochromatic
images of the same parameters recorded on the Narrow-band Filter Imager
(NFI). By applying the center-of-gravity method to the SP data of
AR10930 taken on 2006 December 11, we determined the line-of-sight field
at every location in the active region. As a result, we found that the
line-of-sight field strength ranges up to 2kG in plages, even without
taking into account the filling factor, and up to 3.5kG or higher values
inside the umbra of the major sunspot. We calibrated the NFI data in
reference to the field determined from the SP data. In regions outside
the sunspots and the penumbral regions, we adopted a linear relation,
B|| = βV / I, between the circular polarization, V / I,
and the line-of-sight field strength, B||, and obtained β =
23.5kG in regions outside the sunspots, and β = 12.0kG in penumbral
regions. In umbral regions of sunspots, a first-order polynomial was
adopted to model the reversal of the polarization signal over the
field strength.
Title: Response of the Solar Atmosphere to Magnetic Flux Emergence
from Hinode Observations
Authors: Li, Hui; Sakurai, Takashi; Ichimito, Kiyoshi; Suematsu,
Yoshinori; Tsuneta, Saku; Katsukawa, Yukio; Shimizu, Toshifumi; Shine,
Richard A.; Tarbell, Theodore D.; Title, Alan M.; Lites, Bruce; Kubo,
Masahito; Nagata, Shin'ichi; Kotoku, Jun; Shibasaki, Kiyoto; Saar,
Steven H.; Bobra, Monica
Bibcode: 2007PASJ...59S.643L
Altcode:
No abstract at ADS
Title: Diagnostics of Radio Fine Structures around 3 GHz with
Hinode Data in the Impulsive Phase of an X3.4/4B Flare Event on 2006
December 13
Authors: Yan, Yihua; Huang, Jing; Chen, Bin; Sakurai, Takashi
Bibcode: 2007PASJ...59S.815Y
Altcode:
On 2006 December 13 during the solar minimum, the superactive region
NOAA 10930 at the S05W33 disk location produced an X3.4/4B flare
at 02:40UT. Fine structures were observed in the radio spectra,
which included spikes, reverse slope-type III bursts, type-U burst,
V-shaped burst, pulsations, zebra patterns, and firstly discovered
sub-second spiky zebra-like structures, superimposed on the 2.6-3.8GHz
type IV bursts. The radio fine structures during the impulsive phase
of the flare may be closely associated with coronal structures during
the magnetic-reconnection process, as revealed by Hinode soft X-ray
images. Thus, these microwave fine structure observations may provide
very useful diagnostics at the primary energy release sites when they
occur in the impulsive flare phase. For this flare event, the estimated
coronal magnetic field is about 50-170G in the rising phase of the flare
with a source density of about 1 × 1011cm-3. The
field strength and plasma density are about 90-200G and 1.27 ×
1011cm-3 around the flare maximum.
Title: Feasibility study on the potential of satellite altimetry
for detecting seismic geoid changes due to the 2004 Sumatra-Andaman
earthquake
Authors: Hayashi, Y.; Hirata, K.; Kuragano, T.; Sakurai, T.; Takayama,
H.; Hasegawa, Y.; Hamada, N.
Bibcode: 2007EP&S...59.1149H
Altcode:
Sea surface height data obtained by satellite altimetry from Jason-1
and TOPEX/Poseidon were analyzed to explore the possibility of a seismic
geoid change due to the 2004 Sumatra-Andaman earthquake. This analysis
identified a weakly positive geoid change in a region between the trench
and outer arc. A subsequent investigation of the characteristics of
this coseismic geoid change based on the dislocation theory revealed
that a positive peak should indicate the upper edge of the high-slipped
area or asperity and that a negative peak is responsible for the lower
edge of the earthquake fault. An attempt at modeling the difference
in the sea level anomaly failed to explain the observation since a
large scatter in the original dataset obstructed its confirmation.
Title: Two-Step Reconnections in a C3.3 Flare and Its Preflare
Activity Observed by Hinode XRT
Authors: Kim, Sujin; Moon, Young-Jae; Kim, Khan-Hyuk; Kim, Yeon-Han;
Sakurai, Takashi; Chae, Jongchul; Kim, Kap-Sung
Bibcode: 2007PASJ...59S.831K
Altcode:
We investigated the evolution of a C3.3 impulsive flare and its
preflare activity, which occurred in NOAA Active Region 10923 on 2006
November 12, using Hinode X-Ray Telescope (XRT) data. For an extensive
investigation, we also used GOES X-ray flux, TRACE 171Å, and SOHO MDI
data. Examining the time-series of the XRT and TRACE images, we can
identify the following evolutionary sequences: (1) There were three
bundles of loops along the sheared polarity inversion line forming a
sigmoidal structure during the preflare phase. (2) Preflare brightening
occurred between two upper-loop bundles, and they consequently formed
one larger bundle. (3) The main flare occurred near the location
where this new loop bundle and the third bundle met together. (4) As a
result, a single stacked loop structure was formed. This morphological
evolution of the X-ray loops is quite consistent with a tether-cutting
model involving a single-bipole explosion. Our result shows that the
preflare and the main flare in this event are a two-step reconnection
process, which strongly suggests that the preflare activity plays an
important role in triggering the main flare.
Title: Hinode SP Vector Magnetogram of AR10930 and Its
Cross-Comparison with MDI
Authors: Moon, Yong-Jae; Kim, Yeon-Han; Park, Young-Deuk; Ichimoto,
Kiyoshi; Sakurai, Takashi; Chae, Jongchul; Cho, Kyung Suk; Bong,
Suchan; Suematsu, Yoshinori; Tsuneta, Saku; Katsukawa, Yukio; Shimojo,
Masumi; Shimizu, Toshifumi; Shine, Richard A.; Tarbell, Theodore D.;
Title, Alan M.; Lites, Bruce; Kubo, Masahito; Nagata, Shin'ichi;
Yokoyama, Takaaki
Bibcode: 2007PASJ...59S.625M
Altcode:
We present one Hinode Spectropolarimeter (SP) magnetogram of AR 10930
that produced several major flares. The inversion from Stokes profiles
to magnetic field vectors was made using the standard Milne-Eddington
code. We successfully applied the Uniform Shear Method for resolving
the 180° ambiguity to the magnetogram. The inversion gave very strong
magnetic field strengths (near 4500 gauss) for a small portion of area
in the umbra. Considering that the observed V-profile of 6301.5Å was
well-fitted as well as a direct estimation of the Zeeman splitting
results in 4300-4600 gauss, we think that the field strengths
should not be far from the actual value. A cross-comparison of the
Hinode SP and SOHO MDI high resolution flux densities shows that the
MDI flux density could be significantly underestimated by about a
factor of two. In addition, it has a serious negative correlation
(the so-called Zeeman saturation effect) with the Hinode SP flux
density for umbral regions. Finally, we could successfully obtain
a recalibrated MDI magnetogram that has been corrected for the
Zeeman saturation effect using not only a pair of MDI intensity and
magnetogram data simultaneously observed, but also the relationship
from the cross-comparison between the Hinode SP and MDI flux densities.
Title: Small-Scale X-Ray/EUV Jets Seen in Hinode XRT and TRACE
Authors: Kim, Yeon-Han; Moon, Young-Jae; Park, Young-Deuk; Sakurai,
Takashi; Chae, Jongchul; Cho, Kyung Suk; Bong, Su-Chan
Bibcode: 2007PASJ...59S.763K
Altcode:
We present the morphological and kinematic characteristics of three
small-scale X-ray/EUV jets simultaneously observed by the Hinode
XRT and the TRACE. For this, we examined all XRT movies with a thin
Al/Poly filter from 2006 October 20 to 2007 April 8, and then found
34 small-scale eruptions. We next took a look at the corresponding
TRACE 171/195Å images associated with the eruptions. As a result,
we found three simultaneous X-ray and EUV jets. For two different
bands, they have similar characteristics in terms of the projected
speed (90-310kms-1), lifetime (100-2000s), and size
(1.1-5×105km). These characteristics are similar to the
previous results by the Yohkoh SXT, but are quite different from EUV
jets associated with Hα surges. From the EUV Imaging Spectrometer,
four spectral lines for the last event, we found blueshift (up to
-64kms-1) and redshift (up to 20kms-1) motions as
well as nonthermal velocities ranging from 57 to 106kms-1
at the jet footpoint. We note a tendency: the hotter the maximum
ionization temperature is, the smaller the area of blueshift region
is. These characteristics are consistent with a typical jet model
that includes the magnetic reconnection between open coronal fields
and emerging magnetic fields.
Title: Helicity Injection in Regions of Various Magnetic Fluxes
Authors: Yamamoto, T. T.; Sakurai, T.; Kusano, K.; Yokoyama, T.
Bibcode: 2007ASPC..369..179Y
Altcode:
In this study, we investigated magnetic helicity injection rates
among active regions of various sizes. We analyzed 78 active regions
(more than 600 magnetograms), using the vector magnetograms obtained
with the Solar Flare TelescopeSolar Flare Telescope of NAOJ and
SoHO/MDISOHOMDI magnetogram magnetograms. We used a method proposed
by tet{yamamoto:ksn02, yamamoto:ksn04} to evaluate the helicity
injection. Magnetic fluxes of analyzed regions are from 2×10^{12}
Wb to 4×10^{14} Wb. Unsigned magnetic helicity injection rates are
from 10^{19} Wb^2 s^{-1} to 10^{23} Wb^2 s^{-1}. From a scatter plot
of the magnetic flux and the unsigned magnetic helicity injection
rate, we found that the magnetic helicity injection rate has an upper
limit for a given value of the magnetic flux, and this upper limit is
inversely proportional to the magnetic flux. We discuss these results
with a model of the helicity injection due to helical turbulence
tep*[Σ-Effect;][]{yamamoto:lgc98}.
Title: Observation of a Coronal Mass Ejection and its Source Region
with NOrikura Green-line Imaging System (NOGIS)
Authors: Suzuki, I.; Sakurai, T.; Ichimoto, K.
Bibcode: 2007ASPC..369..543S
Altcode:
NOrikura Green-line Imaging System (NOGIS), with its unique capability
of Doppler imaging, was used to study a CME and its source region on
1999 May 7. The source region at the north-east limb consisted of two
loop systems. Prior to the CME, one of the two loops moved toward
the neighboring other loop in the plane of the sky. Then, the loop
apparently touched and destabilized the other loop, resulting in the
CME with a red-shifted motion. In the NOGIS field-of-view, the CME
propagated non-radially in the plane of the sky. These observations
indicate that the direction of mass ejection was determined by the
magnetic field configuration around the source region and the location
of the initial energy release in the magnetic field structure.
Title: Skew Angle and Magnetic Helicity in Solar Active Regions
Authors: Hagino, M.; Moon, Y. -J.; Sakurai, T.
Bibcode: 2007ASPC..369..163H
Altcode:
The skew angle and magnetic helicity are important quantities in
understanding the magnetic structures in solar active regions. Using
Yohkoh/SXT images and vector magnetograms from the Solar Flare
Telescope at Mitaka for 106 active regions, we have made the first
attempt to examine the relationship between the skew angle and magnetic
helicity. The skew angle is defined as the angle between the coronal
loop and the line perpendicular to the polarity inversion line. We
found that an active region having a large skew angle tends to have a
large helicity value. This result implies that while a coronal loop with
strong twist is nearly parallel to the axis of the polarity inversion
line, a coronal loop with weak twist is perpendicular to the polarity
inversion line, like a potential field structure.
Title: New Solar Physics with Solar-B Mission
Authors: Shibata, K.; Nagata, S.; Sakurai, T.
Bibcode: 2007ASPC..369.....S
Altcode:
No abstract at ADS
Title: Three-Dimensional Filament Eruption Driven by an Emerging Flux
Authors: Notoya, S.; Yokoyama, T.; Kusano, K.; Sakurai, T.; Miyagoshi,
T.; Isobe, H.; Yamamoto, T.
Bibcode: 2007ASPC..369..381N
Altcode:
Some important observations have shown that the strong correlation
between emerging flux and eruptions of quiescent filaments
tep{notoya:Feynman, notoya:Wang}. tet{notoya:Chen} performed
two-dimensional simulations including a flux rope in the corona, and
their results suggest that the eruption process is triggered by the
emerging flux through the reconnection. Our purpose in this paper is
to investigate that ``how a filament is produced and how an eruption
process can be initiated by the emerging flux and what effects of
three-dimensionality appear in the process of eruption". For that
purpose, we performed three-dimensional numerical simulations of the
emerging flux model. From our results, a filamentary structure is
produced from the coronal arcade field by the reconnection process,
and when the reconnection process proceeds effectively, the produced
structure is ejected by the magnetic force. These processes can thought
to be a new mechanism of the eruption which is different from, for
example, the one in tet{notoya:Fan}.
Title: Flare-Associated Oscillations in Coronal Multiple-Loops
Observed with the Norikura Green-Line Imaging System
Authors: Hori, K.; Ichimoto, K.; Sakurai, T.
Bibcode: 2007ASPC..369..213H
Altcode:
We report the first detection of a magnetohydrodynamic kink
oscillation in the coronal green line emission (Fe XIV 5303A,
2MK) by the two-dimensional Doppler coronagraph NOGIS (Norikura
Green-Line Imaging System) at the Norikura Solar Observatory, National
Astronomical Observatory of Japan. It was an impulsively triggered
damping oscillation in Doppler shift in a bundle of face-on coronal
loops on the west limb. In the individual loops, both transverse
(fast kink) and longitudinal modes were observed simultaneously. The
longitudinal mode can be slow wave reflecting at both ends of the
loop. The kink mode had larger amplitude than the longitudinal mode,
and lasted up to ∼7 wave periods (> 100 min). Oscillations in
different loops were not synchronized in phase; the phase velocity
increased smoothly with the loop length. Our preliminary results
support the resonant mode conversion as a possible damping mechanism
of the kink oscillations in a certain part of the observed loops.
Title: Two Fundamental MHD Problems in Solar Physics
Authors: Sakurai, T.
Bibcode: 2007ASPC..369..587S
Altcode:
Two fundamental MHD problems in solar physics are discussed. The first
one is the so-called Parker problem, namely the behavior of magnetic
fields when their fieldline footpoints are moved around by flows. The
expected incapability of settling the field into static equilibrium is
a theoretical basis for the microflare model of coronal heating. The
second one is the so-called Aly-Sturrock conjecture, stating that the
magnetic field stressed by footpoint motions attains energy but may
never exceed the energy of the open field. This is thought to be an
anti-CME theorem.
Title: A solar neutron telescope in Tibet and its capability examined
by the 1998 November 28th event
Authors: Muraki, Y.; Tsuchiya, H.; Fujiki, K.; Masuda, S.; Matsubara,
Y.; Menjyo, H.; Sako, T.; Watanabe, K.; Ohnishi, M.; Shiomi, A.;
Takita, M.; Yuda, T.; Katayose, Y.; Hotta, N.; Ozawa, S.; Sakurai,
T.; Tan, Y. H.; Zhang, J. L.
Bibcode: 2007APh....28..119M
Altcode:
A new solar neutron telescope has been constructed at Yangbajing,
Tibet (600 g/cm 2) and has been successfully operated for
8 years. During this time several interesting events were observed in
coincidence with large solar flares. In this paper the authors discuss
a directional count rate enhancement 1998 November 28 as to whether it
is related to solar phenomena or not. We use this event to evaluate the
sensitivity of the solar neutron telescope and discuss the detection
limit for solar neutrons and high energy solar protons of this detector.
Title: Behavior of Li abundances in solar-analog stars. Evidence
for line-width dependence
Authors: Takeda, Y.; Kawanomoto, S.; Honda, S.; Ando, H.; Sakurai, T.
Bibcode: 2007A&A...468..663T
Altcode:
Context: It is known that the surface lithium abundances of field
solar-analog G dwarfs show a large dispersion of ⪆2 dex (among which
our Sun is located at the lower end) despite the similarity of stellar
parameters, and planet-host stars tend to show comparatively lower Li
abundances in the narrow T_eff range.
Aims: To investigate the
reason for these phenomena, an extensive study of Li abundances and
their dependence on stellar parameters was carried out for a homogeneous
sample of 118 selected solar analogs based on high-dispersion spectra
obtained at Okayama Astrophysical Observatory.
Methods: The
atmospheric parameters were spectroscopically determined by using
the equivalent widths of Fe i and Fe ii lines, the ages/masses
were estimated from stellar evolutionary tracks, and the width of
the macrobroadening (rotation plus macroturbulence) function as
well as Li abundances (A_Li) were established by spectrum-fitting
analyses.
Results: The resulting A_Li vs. T_eff relation revealed
a characteristic inverse-triangle-like distribution enclosed by two
clear-cut boundaries (the slanted one running from ~5900 K to ~5800
K and the vertical one at ~5700 K), while the Sun is located around
its lowest apex. More significantly, A_Li in this region of large
dispersion was found to closely correlate with the macrobroadening width
(v_r+m), which is considered to be the most important parameter.
Conclusions: With a reasonable assumption that the difference of
rotational velocity is mainly responsible for the variety of v_r+m,
we may conclude that the stellar angular momentum plays the decisive
role in determining the surface Li abundances of solar-analog stars
in the T_eff range of ~5900-5700 K. The low-Li tendency of planet-host
stars may thus be interpreted in terms of rotational characteristics. Based on observations carried out at Okayama Astrophysical Observatory
(Okayama, Japan). Tables 2-6 are only available in electronic form
at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)
or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/468/663
Title: VizieR Online Data Catalog: Li abundances in solar-analog stars
(Takeda+, 2007)
Authors: Takeda, Y.; Kawanomoto, S.; Honda, S.; Ando, H.; Sakurai, T.
Bibcode: 2007yCat..34680663T
Altcode:
The atmospheric parameters were spectroscopically determined by using
the equivalent widths of FeI and FeII lines, the ages/masses were
estimated from stellar evolutionary tracks, and the width of the
macrobroadening (rotation plus macroturbulence) function as well as
Li abundances (ALi) were established by spectrum-fitting
analyses. (5 data files).
Title: Helicity comparison among three magnetographs
Authors: Xu, Haiqing; Gao, Yu; Zhang, Hongqi; Sakurai, T.; Pevtsov,
A. A.; Sokoloff, D.
Bibcode: 2007AdSpR..39.1715X
Altcode:
We compare vector magnetograms of 228 active regions observed by Solar
Magnetic Field Telescope (SMFT) at Huairou (HR) Solar Observing Station
and the Solar Flare Telescope (SFT) at Mitaka (MTK) of the National
Astronomical Observatory of Japan from 1992 to 2005 and 55 active
regions observed by SFT and Haleakala Stokes Polarimeter (HSP) at Mees
Solar Observatory, University of Hawaii from 1997 to 2000. Two helicity
parameters, current helicity density hc and αff
coefficient of linear force free field are calculated. From this
comparison we conclude: (1) the mean azimuthal angle differences of
transverse fields between HR and MTK data are systematic smaller than
that between MTK and Mees data; (2) there are 83.8% of hc
and 78.1% of αff for 228 active regions observed at HR and
MTK agree in sign, and the Pearson linear correlation coefficient
between these two data sets is 0.72 for hc and 0.56
for αff. There are 61.8% of hc and 58.2% of
αff for 55 active regions observed at MTK and Mees agree
in sign, and the Pearson linear correlation coefficient between these
two data sets is 0.34 for hc and 0.31 for αff;
(3) there is a basic agreement on time variation of helicity parameters
in active regions observed at HR, Mees, and MTK.
Title: Observation of CME Source Regions by Coronal Emission-Line
Dopplergrams
Authors: Sakurai, T.; Hori, K.; Suzuki, I.; Ichimoto, K.
Bibcode: 2006ihy..workE..35S
Altcode:
Although observations with SOHO/LASCO show the behavior of CMEs beyond
2.5 solar radii, connection between LASCO CMEs and their source
regions in the lower corona observed with SOHO/EIT or Yohkoh/SXT
is not trivial. One way to fill the gap would be to supplement the
Doppler shift information of the moving CME mass. Such an instrument
was built and has been operated since 1997 July at the Norikura Solar
Observatory (2876 m above sea level) of NAOJ. The instrument we call
NOGIS (NOrikura Green-line Imaging System) is made of a 10 cm-aperture
coronagraph and a tunable birefringent filter. NOGIS can provide both
intensity and Doppler velocity images of 2 MK plasmas using the coronal
green-line emission at 5303 Angstrom of Fe XIV. An intensity image is
made by subtracting the sky background (taken at far wings) from the
line-center image. A Doppler image is constructed by subtracting a
blue-wing image from a red-wing image. The line-of-sight velocity up
to 25 km/s can be obtained with an accuracy of about 0.6 km/s. NOGIS
covers a field of view of 1.03 - 1.33 solar radii in a full frame mode,
or a local small area in a partial frame mode with higher cadence of
about 1 minute. So far we have analyzed two CME events which showed
favorable orientations of the regions against the plane of the sky
(1999 May 7 and 2003 June 2). In both events, interaction between two
magnetic flux systems (loops in the case of 1999 May 7 and arcades in
the case of 2003 June 2) was observed.
Title: Spectroscopic Studies of Solar Corona VI: Trend in Line-width
Variation of Coronal Emission Lines with Height Independent of the
Structure of Coronal Loops
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Muneer, S.
Bibcode: 2006JApA...27..115S
Altcode:
We have obtained spectroscopic observations in coronal emission lines
by choosing two lines simultaneously, one [Fe x] 6374Å and the other
[Fe xi] 7892Å or [Fe xiii] 10747Å or [Fe xiv] 5303 Å. We found that
in 95 per cent of the coronal loops observed in 6374 Å, the FWHM of
the emission line increases with height above the limb irrespective
of the size, shape and orientation of the loop and that in case of
5303Å line decreases with height in about 89 per cent of the coronal
loops. TheFWHMof 7892Å and 10747Å emission lines show intermediate
behavior. The increase in the FWHM of 6374Å line with height is the
steepest among these four lines.We have also studied the intensity ratio
and ratio of FWHM of these lines with respect to those of 6374Å as a
function height above the limb. We found that the intensity ratio of
7892Å and 10747Å lines with respect to 6374Åline increases with
height and that of 5303Å to 6374Å decreases with height above the
limb. This implies that temperature in coronal loops will appear
to increase with height in the intensity ratio plots of 7892Å and
6374 Å; and 10747Å and 6374Å whereas it will appear to decrease
with height in intensity ratio of 5303Å to 6374Å line versus height
plot. These findings are up to a height of about 200 arcsec above the
limb. The varying ratios with height indicate that relatively hotter
and colder plasma in coronal loops interact with each other. Therefore,
the observed increase in FWHM with height above the limb of coronal
emission lines associated with plasma at about 1MK may not be due to
increase in non-thermal motions caused by coronal waves but due to
interaction with the relatively hotter plasma. These findings also do
not support the existing coronal loop models, which predict an increase
in temperature of the loop with height above the limb.
Title: Study of Microflares through SOXS Mission
Authors: Jain, Rajmal; Joshi, Vishal; Hanaoka, Yoichiro; Sakurai,
T.; Upadhyay, Nipa
Bibcode: 2006JApA...27..339J
Altcode:
We present a study of 10 microflares observed in 4-30 keV by SOXS
mission simultaneously with Hα observations made at NAOJ, Japan during
the interval between February and August 2004. The X-ray and Hα
light curves showed that the lifetime of microflares varies between
4 and 25 min. We found that the X-ray emission in all microflares
under study in the dynamic energy range of 4-30 keV can be fitted by
thermal plus non-thermal components. The thermal spectrum appeared to
start from almost 4 keV, low level discriminator (LLD) of both Si and
CZT detectors, however it ends below 8 keV. We also observed the Fe
line complex features at 6.7 keV in some microflares and attempted to
fit this line by isothermal temperature assumption. The temperature
of isothermal plasma of microflares varies in the range between 8.6
and 10.1MK while emission measure between 0.5 and 2×1049
cm-3. Non-thermal (NT) emission appeared in the energy range
7-15 keV with exponent -6.8 ≤ γ ≤ -4.8. Our study of microflares
that had occurred on 25 February 2004 showed that sometimes a given
active region produces recurrent microflare activity of a similar
nature.We concluded from X-ray and simultaneous Hα observations that
the microflares are perhaps the result of the interaction of low
lying loops. It appears that the electrons that accelerated during
reconnection heat the ambient coronal plasma as well as interact with
material while moving down along the loops and thereby produce Hα
bright kernels.
Title: Spectroscopic Studies of Solar Corona VIII. Temperature and
Non-Thermal Variations in Steady Coronal Structures
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Muneer,
S.; Raveendran, A. V.
Bibcode: 2006SoPh..236..245S
Altcode:
With a view to investigate variations in parameters of coronal emission
lines over a large range of radial distance from the limb, raster
scans were made with sufficiently long exposure times on several days
during September - October 2003. An analysis of the data shows that (i)
in most of the coronal structures, the FWHM of the Fe XIV 5303 Å line
decreases up to 300″±50″, (ii) the FWHM of the Fe X 6374 Å line
increases up to about 200″ and then remains unchanged up to about
500″, and (iii) the FWHMs of the Fe XI 7892 Å and Fe XIII 10747 Å
lines show an intermediate behaviour with height. The analysis of the
data also shows that the ratio of FWHM of 6374 Å to that of 5303 Å
increases from 0.93 at the limb to 1.18 at 200″ above the limb. From
this and the ratio of intensities of the two lines we infer that the
plasma in steady coronal structures at a height of about 200″ has
a temperature of about 1.5 MK and a non-thermal velocity around 17
km s−1. The observations also show that non-homogeneous
temperatures and non-thermal velocities largely exist in the lower
corona up to about 300″±100″ above the limb. Amplitudes of
variations in FWHM of different emission lines with height in the
coronal loops are similar to those in the diffuse plasma around the
coronal loops.
Title: Do the Line Widths of Coronal Emission Lines Increase with
Height above the Limb?
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi
Bibcode: 2006ApJ...639..475S
Altcode:
In our earlier studies we obtained off-the-limb spectroscopic
observations in a number of forbidden emission lines ([Fe X-XIV]) to
study the physical properties and their temporal variations in steady
coronal structures. Short exposure times adopted in those observations
permitted us to study the variation in line widths up to about 150"
above the limb. With a view to investigating the variations in the
parameters of coronal emission lines up to about 500", we made raster
scans with exposure times that are longer than the earlier exposure
times by a factor of about 10. We find that the FWHM of the [Fe XIV]
5303 Å line decreases up to 300''+/-50'' and
then remains more or less the same up to 500", while that of the [Fe
X] 6374 Å line increases up to about 250" and subsequently remains
unchanged. The FWHMs of the [Fe XI] 7892 Å and [Fe XIII] 10747 Å
lines show an intermediate behavior. Furthermore, the ratio of the
FWHM of 6374 to 5303 Å increases from 0.93 at the limb to 1.18 at 200"
above the limb. The nonvariability in the FWHM of emission lines after
about 300" above the limb in steady coronal structures does not support
the prevailing view that the nonthermal velocity increases with height
due to either the coronal waves or the high-velocity solar wind. The
present results indicate the inadequacy of the earlier coronal loop
models. The observed variations in FWHM of the coronal emission lines
with height above the limb can be explained by assuming the recent
model of coronal loops proposed by Akiyama et al.
Title: Geomagnetic effects of high-density plasma with southward
magnetic field in the interplanetary coronal mass ejection observed
on May 2-3, 1998
Authors: Adachi, H.; Sakurai, T.; Marubashi, K.
Bibcode: 2006EP&S...58..315A
Altcode:
This paper aims to clarify the effect of high-density plasma in
interplanetary coronal mass ejection (ICME) observed during the May
2-3, 1998 geomagnetic storm. The examination is performed based on
the estimation of Dst index, which is calculated with the observed
solar wind parameters of the ICME. The estimated Dst index variation
is compared with Dst index variation provided by the World Data Center
for Geomagnetism, Kyoto (WDC, Kyoto). From this examination, we find
that the trend of the estimated Dst is in good agreement with that of
the provided Dst when the thresholds are taken into account for both
the solar wind plasma density and the dawn-to-dusk solar wind electric
field, as 30 #/cc and 0.49 mV/m, respectively. From the result, we can
conclude that the effect of high-density plasma is important on the
enhancement of geomagnetic storm as well as the effect of the other
solar wind parameters, such as the interplanetary magnetic field (IMF)
Bz and solar wind velocity. On the other hand, the solar source of the
magnetic field of this ICME is examined. The magnetic field structure
of the ICME is examined by fitting the flux rope model to the observed
magnetic field and solar wind speed. The results are compared with the
magnetic structure of the bases of coronal helmet streamers. From this
comparison we can find that the magnetic structure of the interplanetary
flux rope is in good agreement with that of the neutral line of the
base of coronal helmet streamers. The result suggests that if we look
for the causes of geomagnetic storm we should take into account both
the plasma structure and the magnetic structure of the base of coronal
helmet streamers.
Title: Preprocessing of Vector Magnetograph Data for a Nonlinear
Force-Free Magnetic Field Reconstruction
Authors: Wiegelmann, T.; Inhester, B.; Sakurai, T.
Bibcode: 2006SoPh..233..215W
Altcode: 2006astro.ph.12641W
Knowledge regarding the coronal magnetic field is important for
the understanding of many phenomena, like flares and coronal mass
ejections. Because of the low plasma beta in the solar corona,
the coronal magnetic field is often assumed to be force-free and
we use photospheric vector magnetograph data to extrapolate the
magnetic field into the corona with the help of a nonlinear force-free
optimization code. Unfortunately, the measurements of the photospheric
magnetic field contain inconsistencies and noise. In particular,
the transversal components (say Bx and By) of
current vector magnetographs have their uncertainties. Furthermore,
the magnetic field in the photosphere is not necessarily force free
and often not consistent with the assumption of a force-free field
above the magnetogram. We develop a preprocessing procedure to drive
the observed non-force-free data towards suitable boundary conditions
for a force-free extrapolation. As a result, we get a data set which
is as close as possible to the measured data and consistent with the
force-free assumption.
Title: Three-Dimensional Motion of Plasmas Associated with a Coronal
Mass Ejection Observed with NOrikura Green-Line Imaging System (NOGIS)
Authors: Suzuki, Isao; Sakurai, Takashi; Ichimoto, Kiyoshi
Bibcode: 2006PASJ...58..165S
Altcode:
In order to investigate the structure and the driving mechanism of
Coronal Mass Ejections (CMEs), it is important to examine in detail
the magnetic field structure in the low corona. NOrikura Green-line
Imaging System, with its unique capability of Doppler imaging, was
used to study a CME and its source region on 1999 May 7. Prior to the
CME, a small loop at the north-east limb moved toward a neighboring
larger loop in the plane of the sky. Then, the small loop apparently
destabilized the large loop, resulting in the CME with a red-shifted
motion. The CME propagated non-radially in the plane of the sky. These
observations indicate that two loop systems were involved in this CME,
and the direction of mass ejection was determined by the magnetic field
configuration around the source region and the location of the initial
energy release in the magnetic field structure. A brief discussion is
given on the loop oscillation phenomenon observed during this event.
Title: Helicity injections in various regions
Authors: Yamamoto, T.; Sakurai, T.; Kusano, K.; Yokoyama, T.; Inoue,
S.; Notoya, S.
Bibcode: 2006cosp...36.1175Y
Altcode: 2006cosp.meet.1175Y
In this study we investigate the amount of magnetic helicity injection
rate hereafter helicity flux among active regions having different
magnetic fluxes We analyzed 78 active regions more than 600 magnetograms
using the vector magnetograms obtained with the Solar Flare Telescope
of NAOJ and SOHO MDI magnetograms These data are analyzed with a
method proposed by Kusano et al 2002 Ten active regions are tracked
for several days while other regions are studied based on single-day
observation The time cadence of data is 96 minutes Magnetic fluxes of
these regions ranges from 2 e 12 Wb to 4 e 14 Wb and Unsigned helicity
fluxes are from 1 e 17 Wb 2 s to 2 e 22 Wb 2 s From a scatter plot of
the magnetic flux and the helicity flux we found that the helicity
flux has an upper limit for a given value of the magnetic flux and
the upper limit is nearly proportional to the magnetic flux We can
interpret these results with the model of helicity injection due to
helical turbulence Sigma-Effect Longcope et al 1998
Title: Upward velocities of the reconnection points and coronal
magnetic field strengths in flaring regions derived from the GOES
X-ray light curves
Authors: Yamamoto, Tetsuya T.; Sakurai, T.
Bibcode: 2006IAUS..233..128Y
Altcode:
From Yohkoh and SoHO observations, the magnetic reconnection is
considered as a main energy release mechanism of the solar active
phenomena. In this study, we try to reproduce GOES X-ray light curves
(1-8 Å) of solar flares using a model incorporating the radiative
and conductive cooling and the magnetic reconnection heating.
Title: Editorial Appreciation
Authors: Engvold, Oddbjørn; Harvey, Jack; Leibacher, John; Sakurai,
Takashi; Švestka, Zdeněk; van Driel-Gesztelyi, Lidia; Solar Physics
editors
Bibcode: 2006SoPh..233....1E
Altcode:
No abstract at ADS
Title: Solar Magnetic Fields: From the Interior to the Surface
and Beyond
Authors: Sakurai, T.
Bibcode: 2006cosp...36.3509S
Altcode: 2006cosp.meet.3509S
The mechanism of generating and periodically reversing the magnetic
fields of the sun is one of the most fundamental problems in
astrophysics Compared to other fundamental problems like the mechanisms
for flare energy release and coronal heating this dynamo mechanism is
a more difficult one because it works in the invisible solar interior
its time scale is as long as tens of years and the sun only provides
one specific value of its rotation speed Therefore a combination of
helioseismology numerical simulations and observations of stellar
activity cycle is crucial in order to understand the dynamo process in
addition to the observations of magnetic fields and rotation flows on
the solar surface Recent progress in the study of magnetic helicity has
been giving a new diagnostic tool on the behavior of magnetic fields
in the interior The Solar-B satellite to be launched in 2006 September
will also contribute to the understanding of transport diffusion and
amplification processes of surface magnetic fields
Title: Relationship between the mass and the acceleration of CMEs
Authors: Suzuki, I.; Sakurai, T.
Bibcode: 2006cosp...36.2826S
Altcode: 2006cosp.meet.2826S
It has been known that the CME associated with a filament eruption
proceeds with an acceleration while the CME associated with a flare
shows a constant velocity or a deceleration It is also well known
that the CME with a core is associated with a filament eruption These
correlations indicate some relationship between the existence of CME
core and the acceleration of CME We examined the relationship between
the acceleration of CMEs and the variation of their mass distribution
We found that the mass of the accelerating CMEs with a core continued
to increase as they developed while the mass of the CMEs with constant
or decelerating velocities saturated at an asymptotic value Next
we approximated a CME geometrically by a sector and divided it into
concentric layers with equal areas and investigated the variation of the
mass contained in them In the layers closer to the sun the accelerating
CMEs showed an increase in mass with time but the non-accelerating
CMEs did not show the increase From these results we suggest that the
CMEs show different development of their mass distribution depending
on whether they have a core or not and the acceleration of CMEs is
also related to the existence of the core
Title: Outlook for Studies of Magnetic Fields with Solar-B
Authors: Sakurai, T.
Bibcode: 2006cosp...36.3508S
Altcode: 2006cosp.meet.3508S
The Solar-B satellite the successor of Yohkoh Solar-A will be
launched in 2006 September Solar-B is a Japan-US-UK joint project with
contributions for downlink connections from ESA The mission will focus
on high resolution optical imaging with magnetography coupled with
X-ray imaging and XUV spectroscopy The optical telescope on Solar-B is
a 50cm-diameter reflector with a resolution of 0 2 and for the first
time we will be able to study the emergence and evolution of magnetic
fields in the finest scale ever observed Physics of elementary flux
tubes as well as micro-structures in larger flux tubes sunspots are
among the top-priority research targets of Solar-B In order to carry
out these studies analysis methods e g Stokes profile inversion will
have to be improved because elementary flux tubes are nearly but not
completely resolved and the introduction of a simple magnetic filling
factor may not be adequate
Title: Radio and Hard X-ray Quasi-Periodic Pulsations during the
2004 July 13 Flare
Authors: Hori, K.; Pohjolainen, S.; Sakurai, T.
Bibcode: 2006cosp...36..720H
Altcode: 2006cosp.meet..720H
Quasi-periodic pulsations QPPs in radio and hard X-ray emissions have
been observed in association with solar flares Typical pulsation periods
of QPPs range from a few seconds to several minutes Possible causes
of the intensity variations in radio emission are i fluctuation in the
number of particles created by the acceleration process ii modulation in
the whole structure in which the radiation takes place MHD oscillations
and iii modulation in spatial and energy distributions of the radiating
particles Trottet et al 1979 Among them the fast sausage-mode MHD
oscillation has been preferably interpreted as a cause of QPPs which
is manifested in coronal loops by symmetric cross-sectional variations
and plasma compression This compressive mode is nearly transversal
and the perturbations of plasma velocity in the radial direction
are stronger than those along the field Nakariakov and Verwichte 2006
Recently Asai et al 2001 detected spatially resolved microwave QPPs for
the first time with the Nobeyama Radioheliograph NoRH NoRH can provide
microwave images with a time resolution up to 0 1 sec and thus suitable
for the study of spatially resolved QPPs Asai et al 2001 investigated
the oscillation with a periodicity of 6 6s in the context of the fast
kink-mode rather than the fast sausage-mode In their interpretation
the QPPs can be due to the modulation of the electron acceleration by
a global kink oscillation of the flaring loop or by an interaction of
the flaring loops with another loop which performs kink
Title: On the Imaging Characteristics of Yohkoh Soft X-Ray Telescope
Authors: Shin, J.; Sakurai, T.
Bibcode: 2006cosp...36.2747S
Altcode: 2006cosp.meet.2747S
The point spread function PSF of an optical system generally consists of
two major components One is the geometrical component due to aberration
or defocusing which does not have a wavelength dependence Another is the
component due to scattering on the mirror surface which has a wavelength
dependence We have studied the PSF of the soft X-ray telescope SXT
aboard Yohkoh satellite In this paper we combined several sources of
information obtained from both pre-launch calibration and the in-flight
data and estimated the characteristics of the PSF of Yohkoh SXT
Title: Feedback Excitation of Double Reconnection as a Mechanism of
the Impulsive Onset of Solar Flares
Authors: Kusano, K.; Maeshiro, T.; Yokoyama, T.; Sakurai, T.
Bibcode: 2005AGUFMSM13C..02K
Altcode:
The impulsive onset mechanism of solar flares is one of the
long-standing problems, which are not yet well understood. In order to
shed light on it, we study the nonlinear dynamics of the resistive
tearing mode instability growing on a thin current sheet, where
the magnetic shear of coronal magnetic arcade is steeply reversed,
using the high-resolution three-dimensional magnetohydrodynamic
simulations. The simulation indicates that the eruption of magnetic
arcade suddenly arises in the nonlinear phase of the instability. The
mechanism of the impulsive onset of the eruption can be explained
by the feedback excitation of double reconnection, one of which
corresponds to the original tearing reconnection but the another
is driven by the collapsing of magnetic arcade caused by the former
reconnection. Based on the results, a new model of solar flare onset
is proposed. The consistency between the model and the observation is
examined by the correlation analysis between the vector magnetograms
and the ultra-violet image of solar flares observed by the TRACE
satellite. The underlying physics of the double reconnection is
also discussed from the view point of resemblance with the nonlinear
excitation of double tearing mode instability in tokamak plasma.
Title: Simulation Study on the Self-Organization of Sigmoidal
Structure and the Onset of Solar Flares
Authors: Kusano, K.; Inoue, S.; Yamamoto, T.; Yokoyama, T.; Sakurai, T.
Bibcode: 2005ESASP.596E..30K
Altcode: 2005ccmf.confE..30K
No abstract at ADS
Title: Complex Variations in Line-Intensity Ratio of Coronal Emission
Lines with Height Above the Limb
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Watanabe,
Tetsuya
Bibcode: 2005BASI...33..362S
Altcode:
No abstract at ADS
Title: Solar-Cycle Variation of Magnetic Helicity in Active Regions
Authors: Hagino, Masaoki; Sakurai, Takashi
Bibcode: 2005PASJ...57..481H
Altcode:
The hemispheric sign rule of helicity and its long-term variation
were studied. The data were obtained from the Solar Flare Telescope
at Mitaka and the 65-cm solar telescope at Okayama. The data covered
the period of 1983-2001, from the declining phase of solar cycle 21 to
the rising phase of cycle 23. Although the hemispheric sign rule of
helicity generally holds, we also found significant time variations
in the yearly values of helicity during the observation period. The
hemispheric sign rule of helicity is satisfied in the solar maximum
phase, but may not be so in the solar minimum phase.
Title: Editorial
Authors: Leibacher, John; Sakurai, Takashi; van Driel-Gesztelyi, Lidia
Bibcode: 2005SoPh..229....3L
Altcode:
No abstract at ADS
Title: VizieR Online Data Catalog: Atlas and Catalog of Dark Clouds
(Dobashi+, 2005)
Authors: Dobashi, K.; Uehara, H.; Kandori, R.; Sakurai, T.; Kaiden,
M.; Umemoto, T.; Sato, F.
Bibcode: 2005yCat.7244....0D
Altcode:
Here we release the first version of the atlas and catalog of dark
clouds derived by using the optical database Digitized Sky Survey I
(DSS). Applying a traditional star-count technique to 1043 plates
contained in DSS, we have produced an Av map covering the entire region
in the galactic latitude range |b|=<40{deg}. The map was drawn at
two different angular resolutions of 6' and 18', and is released in
FITS format. Based on the Av map, we identified 2448 dark clouds
and 2841 clumps located inside them. Physical parameters, such as the
position, extent, and optical extinction, were measured for each of the
clouds and clumps. We also searched for counterparts among already known
dark clouds in the literature. The catalog of dark clouds presented here
lists the cloud parameters as well as the counterparts. Printed
version of the atlas and catalog was published in a special issue of
Publications of the Astronomical Society of Japan (PASJ) in February
2005 (Dobashi et al., 2005, PASJ, 57, pp.S1-S386). (4 data files).
Title: Magnetic Helicity Injection and Sigmoidal Coronal Loops
Authors: Yamamoto, Tetsuya T.; Kusano, K.; Maeshiro, T.; Yokoyama,
T.; Sakurai, T.
Bibcode: 2005ApJ...624.1072Y
Altcode:
We studied the relationship between magnetic helicity injection and
the formation of sigmoidal loops. We analyzed seven active regions:
three regions showed coronal loops similar to the potential field,
and four regions showed the sigmoidal loops. The magnetic helicity
injection rate was evaluated using the method proposed by Kusano et
al. In order to compare the helicity of regions of various sizes, we
defined the normalized helicity injection rate as the magnetic helicity
injection rate divided by the magnetic flux squared. We found that the
sigmoidal regions and nonsigmoidal regions have comparable normalized
helicity injection rates. Next, we calculated the magnetic helicity
content of the sigmoidal loops by using the magnetic flux tube model
(Longcope & Welsch) and compared it with the magnetic helicity
injected from around the footpoints of three sigmoidal loops. For two
sigmoidal loops, it is found that these values are comparable. Another
loop showed significant disagreement between helicity injection rate
and its magnetic helicity content. Excluding this region on the basis
of its complexity (perhaps multiple loops forming a sigmoidal loop), we
can conclude that geometric twist of the sigmoidal loops is consistent
with the magnetic helicity injected from around the footpoints of the
sigmoidal loops.
Title: A Statistical Study of the Correlation between Magnetic
Helicity Injection and Soft X-Ray Activity in Solar Active Regions
Authors: Maeshiro, T.; Kusano, K.; Yokoyama, T.; Sakurai, T.
Bibcode: 2005ApJ...620.1069M
Altcode:
The correlation between magnetic helicity injection across
the photosphere and soft X-ray activity in the solar corona is
statistically investigated for seven active regions appearing in the
years 1997-2000. The magnetic helicity flux into the solar corona is
analyzed by the induction equation method, using magnetograms observed
by the Michelson Doppler Imager on SOHO and by the vector magnetograph
at the National Astronomical Observatory of Japan. Soft X-ray activity
is evaluated from the data observed by the Yohkoh soft X-ray telescope
(SXT). Soft X-ray activity of active regions in nonflare phases is
found to correlate better with unsigned magnetic helicity flux than
with the simple integration of the magnetic helicity flow. In addition,
several magnetic variables, e.g., magnetic flux and electric current
flux, are investigated, and it is confirmed that any fluxes given by
the area integration of magnetic variables are well correlated with
soft X-ray activity. However, for the magnetic helicity flow, not only
the whole area flux but also the local intensity correlates well with
the soft X-ray intensity. The relation between the spatial structure
of the magnetic shear and soft X-ray activity is also investigated,
and it is revealed that structural complexity in the magnetic shear
tends to increase the efficiency of energy liberation in the solar
corona. These results indicate that the magnetic helicity injected
from the photosphere is relevant to the heating process in the solar
corona, although several magnetic variables, not only magnetic helicity,
could be related to that.
Title: Spectroscopic Studies of Solar Corona VII. Formation of a
Coronal Loop by Evaporation
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Suzuki,
Isao; Hagino, Masaoki
Bibcode: 2005SoPh..226..201S
Altcode:
We obtained time-sequence spectroscopic observations in (Fe X) 6374 Å
and (Fe XIV) 5303 Å lines successively with the 25-cm coronagraph,
and narrow-band and Doppler images in 5303 Å line by the 2-D 10-cm
Doppler coronagraph "NOGIS" at the Norikura Solar Observatory, of
a coronal region for about 7 h on 9 19-20, 2001. The raster scans
were obtained with a quasi-periodicity of about 14 min and "NOGIS"
obtained the images with an interval of about 1 min. The coronal region
observed showed the formation of a coronal loop by a high-speed surge
in the 6374 Å line rising from one of the footpoints of the loop. Off
the limb spectroscopic observations in the 6374 Å line showed large
velocities along the line of sight and vertical to the solar limb
at the time of formation of the loop. The 5303 Å line observations
showed negligible line-of-sight velocities and low vertical velocities
when compared to those in the 6374 Å line. A hump in the intensity
plots in 5303 Å with height appears to move up with respect to the
solar limb with an average velocity of 4km s−1. The FWHM
of the 6374 Å showed a much smaller value of about 0.7 Å near the
foot point as compared to a value of 1.2 Å at larger heights at the
beginning of observations. Later as the loop developed, the FWHM of
6374 Å line showed a gradual decrease along the loop up to 70″ from
the limb, reached a minimum value of about 0.5 Å and then increased
with height during the formation of the loop; this trend lasted for
about 2 h. About 3 h after the beginning of the formation of the loop,
the FWHM of 6374 Å emission line showed normal values and normal rate
of increase with height with some fluctuations. The FWHM of the 5303
Å line did not show such variations along the loop and showed normal
decrease in FWHM with height found earlier (Singh et al., 2003a). These
observations suggest that a relatively cooler plasma at a temperature
of about 0.7 MK or less (corresponding to minimum value of FWHM of 0.5
Å) was ejected from the transition region with a large velocity of
about 48km s−1, heated up in the corona by some process
and formed a coronal loop with a height of about 200″ above the limb
that had lifetime greater than 4 h. It appears that the plasma moved
from one of the footpoints and the loop was formed by evaporation of
chromospheric plasma. No large-scale brightening and Hα flare were
observed in this region during the observational period of 7 h.
Title: Hemispheric sign rule of magnetic helicity on the Sun
Authors: Hagino, Masaoki; Sakurai, Takashi
Bibcode: 2005ARAOJ...7...49H
Altcode:
No abstract at ADS
Title: On a Cyclic Variation of the Hemispheric Helicity Rule
Authors: Pevtsov, A. A.; Hagyard, M. J.; Blehm, Z.; Smith, J. E.;
Canfield, R. C.; Sakurai, T.; Hagino, M.
Bibcode: 2005HiA....13..140P
Altcode:
We report the result of a study of magnetic helicity in solar active
regions during 1980-2000 (cycles 21-23). Using the vector magnetograms
from four different instruments (Haleakala Stokes Polarimeter Marshall
Space Flight Center Mitaka Solar Flare Telescope and Okayama Observatory
Solar Telescope) we calculated the force-free parameter alpha as
in Pevtsov et al. (1995). We use alpha as the proxy for current
helicity. For each instrument we computed a gradient dalpha/dL as
the linear fit of alpha versus latitude L using annual subsets of
data. The hemispheric helicity rule (negative/positive helicity in
northern/southern hemisphere) can be expressed in terms of this gradient
as dalpha/dL < 0. We find that each instrument exhibits change in
sign of this gradient for some years which implies that the hemispheric
helicity rule may not hold in some phases of a solar cycle (see Hagino
and Sakurai 2002). However we do not see consistency between different
instruments in regards to years disobeying the rule. The disagreement
may be due to difference in observations and/or insufficient number of
magnetograms in some years. We conclude that the present data sets do
not allow to make statistically significant inference about possible
cyclic variation of the hemispheric helicity rule.
Title: Dayside Outer Magnetosphere ULF Waves Observed by Geotail
Authors: Sakurai, T.; Tonegawa, Y.; Shinkai, Y.; Nowada, M.
Bibcode: 2005fmpp.conf...71S
Altcode:
No abstract at ADS
Title: On Ion Properties within the Subsolar Magnetopause Current
Layer under the Northward and Southward IMF
Authors: Nowada, M.; Sakurai, T.; Mukai, T.
Bibcode: 2005fmpp.conf...43N
Altcode:
No abstract at ADS
Title: Flare-induced coronal disturbances observed with Norikura
"NOGIS" coronagraph
Authors: Hori, K.; Ichimoto, K.; Sakurai, T.; Sano, I.; Nishino, Y.
Bibcode: 2005IAUS..226...36H
Altcode:
A 2-dimensional Doppler coronagraph "NOGIS" (NOrikura Green-line
Imaging System) at the Norikura Solar Observatory, NAOJ, is a unique
imaging system that can provide both intensity and Doppler velocity
of 2 MK plasma from the green coronal line emission λ5303 Å of
Fe xiv. We present the first detection of a CME onset by NOGIS. The
event was originally induced by a C9.1 confined flare that occurred
on 2003 June 1 at an active region NOAA #10365 near the limb. This
flare triggered a filament eruption in AR 10365, which later evolved
into a partial halo CME as well as an M6.5 flare at the same AR 10365
on 2003 June 2. The CME originated in a complex of two neighboring
magnetic flux systems across the solar equator: AR 10365 and a bundle
of face-on tall coronal loops. NOGIS observed i) a density enhancement
in between the two flux systems in the early phase, ii) a blue-shifted
bubble and jet that later appeared as (a part of) the CME, and iii)
a red-shifted wave that triggered a periodic fluctuations in Doppler
shifts in the face-on loops. These features are crucial to understand
unsolved problems on a CME initiation (e.g., mass supply, magnetic
configuration, and trigger mechanism) and on coronal loop oscillations
(e.g., trigger and damping mechanisms). We stress a possibility that
interaction between separatrices of the two flux systems played a key
role on our event.
Title: Flare-associated Coronal Disturbances Observed with the
Norikura Green-Line Imaging System. I. A Coronal Mass Ejection Onset
Authors: Hori, K.; Ichimoto, K.; Sakurai, T.; Sano, I.; Nishino, Y.
Bibcode: 2005ApJ...618.1001H
Altcode:
We present the first detection of an onset of a coronal mass ejection
(CME) in the coronal green-line emission (Fe XIV λ5303, 2 MK) by
the two-dimensional Doppler coronagraph NOGIS (Norikura Green-Line
Imaging System) at the Norikura Solar Observatory, National Astronomical
Observatory of Japan. On 2003 June 1-2, NOGIS continuously observed the
birthplace of a CME that originated in a complex of two neighboring
magnetic flux systems across the solar equator: a flare-productive
NOAA Active Region 10365, and a bundle of face-on coronal loops
overlaying a quiescent filament. An early precursor of the event
was a density enhancement of a 2 MK plasma in between the two flux
systems. Following a filament eruption from NOAA AR 10365 that was
observed by the Transition Region and Coronal Explorer (TRACE) in the
195 Å passband (1.6 MK), NOGIS observed a blueshifted bubble and a
redshifted wave that almost simultaneously expanded from the boundary
of NOAA AR 10365 and the overlying dense region. The redshifted wave
propagated toward the face-on loop system and triggered a damping
oscillation in Doppler shifts among the adjacent loops within the
system. The blueshifted bubble propagated both inward and upward. The
inward motion triggered an M6.5 flare in AR 10365, while the upward
motion evolved into a partial halo CME that had an angular extent
covering the latitudinal range of the two flux systems. Differing from
typical CME disturbances that evolve within a single flux system with
a bipolar arcade on its center, our event proceeded via interaction,
which was a presumably magnetic reconnection between separatrices
of the two flux systems. These observational properties may suggest
the existence of ``CME corridors'' in multiple complex flux systems,
from which huge CMEs can be launched.
Title: Highest Energy Neutrons Detected by a Solar Neutron Telescope
in Association with the November 28th 1998, Solar Flare
Authors: Muraki, Y.; Tsuchiya, H.; Evenson, P.; Fujiki, K.; Matsubara,
Y.; Menjo, H.; Masuda, S.; Sako, T.; Watanabe, K.; Ohnishi, S.; Yuda,
T.; Katayose, Y.; Hotta, N.; Sakurai, T.; Sakai, T.; Tan, Y. H.
Bibcode: 2005ICRC....1...25M
Altcode: 2005ICRC...29a..25M
No abstract at ADS
Title: Atlas and Catalog of Dark Clouds Based on Digitized Sky
Survey I
Authors: Dobashi, K.; Uehara, H.; Kandori, R.; Sakurai, T.; Kaiden,
M.; Umemoto, T.; Sato, F.
Bibcode: 2005prpl.conf.8137D
Altcode: 2005LPICo1286.8137D
No abstract at ADS
Title: Oscillations in the coronal green-line intensity observed at
Lomnický Stít and Norikura nearly simultaneously
Authors: Minarovjech, Milan; Rušin, Vojtech; Rybanský, Milan;
Sakurai, Takashi; Ichimoto, Kiyoshi
Bibcode: 2005naoj.book...36M
Altcode:
No abstract at ADS
Title: A CME onset observed with Norikura NOGIS coronagraph
Authors: Hori, Kuniko; Ichimoto, Kiyoshi; Sakurai, Takashi; Sano,
Issei; Nishino, Yohei
Bibcode: 2005ARAOJ...7...51H
Altcode:
No abstract at ADS
Title: Some coronal loops have cooler loop-tops
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi
Bibcode: 2005ARAOJ...7...50S
Altcode:
No abstract at ADS
Title: Molecular Cloud Core MCLD 123.5+24.9 in Polaris Cirrus
Authors: Sakurai, T.; Dobashi, K.; Kaiden, M.; Nishiura, S.; Takano,
S.; Kawara, K.; Oyabu, S.; Kozasa, T.; Fukuhara, K.
Bibcode: 2005prpl.conf.8138S
Altcode: 2005LPICo1286.8138S
No abstract at ADS
Title: Study of Magnetic Helicity in the Solar Corona
Authors: Kusano, K.; Maeshiro, T.; Yokoyama, T.; Sakurai, T.
Bibcode: 2004ASPC..325..175K
Altcode:
Several new methodologies to detect the magnetic helicity injection
activity across the photosphere into the solar corona have been
developed recently. We briefly review the basic principle of the typical
two methods, the so-called local correlation tracking (LCT) method and
the induction equation (IE) method. The reliability of them is examined
using a three-dimensional model field. The benchmark examination
indicates that, if the vector magnetic field data are available,
the IE method can greatly improve the accuracy of the helicity flux
measurement. The physical implication of the recent results of the
magnetic helicity measurement is also discussed. In particular, we
emphasize the possibility that not only the intensity but also the
complexity of the magnetic helicity injection from the photosphere
could activate the energy liberation process in the solar corona.
Title: Macrospicules, Coronal Heating, and SolarB
Authors: Yamauchi, Y.; Moore, R. L.; Suess, S. T.; Wang, H.;
Sakurai, T.
Bibcode: 2004ASPC..325..301Y
Altcode:
We investigated the magnetic structures of macrospicules in polar
coronal holes using Hα images taken at Big Bear Solar Observatory. We
found a total of 35 macrospicules. Half of the events were in the form
of an erupting loop while the rest were in the form of a single-column
spiked jet. These erupting-loop and spiked-jet macrospicules are
considered to support models in which the coronal heating and solar
wind acceleration in coronal holes are driven by explosive reconnection
events seated in the network. We believe that the vector magnetograph
on the forthcoming SolarB mission will provide critical clues to the
mechanisms of coronal heating and solar wind acceleration by detecting
magnetic activities at the base of macrospicules in the network and
spicules rooted in the edges of the network flux clumps. These results
are also presented in Astrophysical Journal (Yamauchi et al. 2004).
Title: Magnetic Helicity Injection and Sigmoidal Coronal Loops
Authors: Yamamoto, T. T.; Sakurai, T.; Kusano, K.; Maeshiro, T.;
Yokoyama, T.
Bibcode: 2004ASPC..325..145Y
Altcode:
We analyzed the magnetic helicity injection in NOAA Active Region
8011 with the method proposed by Kusano et al. (2002). It was found
that the helical geometry of a sigmoidal loop in this region is
consistent with the magnetic helicity injected from the foot points of
the loop. We also made a statistical study of the correlation between
X-ray intensity and magnetic helicity injection. We found that the sum
of the absolute values of the magnetic helicity injection shows good
correlation with the X-ray luminosity. The estimated flux of magnetic
free energy injection is not highly correlated with the X-ray flux,
because the former quantity is overestimated in our simple method.
Title: Spectroscopic Studies of Steady Coronal Structures -- Line
Width Variations with Height of Ion{Fe{X-XIV}} Emission Lines
Authors: Singh, J.; Sakurai, T.; Ichimoto, K.; Watanabe, T.
Bibcode: 2004ASPC..325..235S
Altcode:
We have obtained spectrographic observations of several steady coronal
structures at the limb overlying the sunspot regions on several
days in ion{Fe{x-xiv}} emission lines. The line-width measurements
indicate that in steady coronal structures the FWHM of the 6374 AA line
increases with height above the limb with an average value of 1.02
mAA per arcsec. Whereas the FWHM of the 5303 AA line decreases with
an average value of -0.66 mAA per arcsec. The FWHM of the 7892 AA and
10747 AA lines increases with values of 0.55 and 0.29 mAA per arcsec,
respectively. We find that FWHM of emission lines in coronal structures
increases with height if the associated ionization temperature is less
than 1.6 MK, with gradient depending upon the ionization temperature
of the line, while the FWHM decreases with height for lines whose
ionization temperature is greater than 1.6 MK. It implies that it
may not always be possible to interpret the observed increase in FWHM
with height in terms of an increase in the non-thermal velocity. To
investigate further, we propose simultaneous observations in number of
EUV lines with EIS onboard SolarB and in ion{Fe{x-xiv}} in the visible
wavelengths with the 25-cm coronagraph at Norikura.
Title: The Solar-B Mission and the Forefront of Solar Physics
Authors: Sakurai, T.; Sekii, T.
Bibcode: 2004ASPC..325.....S
Altcode:
No abstract at ADS
Title: Complex Variations in the Line-Intensity Ratio of Coronal
Emission Lines with Height above the Limb
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Watanabe,
Tetsuya
Bibcode: 2004ApJ...617L..81S
Altcode:
We obtained spectroscopic observations simultaneously in pairs of
coronal emission lines, one line being [Fe X] λ6374 and the other
line being [Fe XI] λ7892, [Fe XIII] λ10747, or [Fe XIV] λ5303, and
we studied the variations in the intensity and FWHM ratios of these
lines with respect to those of 6374 Å as a function of height above
the limb. We find that the intensity ratio of the 7892 and 10747 Å
line with respect to the 6374 Å line increases with height and that
the intensity ratio of 5303 Å to 6374 Å decreases with height above
the limb. This implies that the temperature in coronal loops will
appear to increase with height if we consider the intensity ratio
of 7892 Å to 6374 Å a negligible variation in temperature in the
case of the 10747 and 6374 Å line pair, while the temperature will
appear to decrease with height if we consider the intensity ratio of
5303 Å to 6374 Å. The normalized FWHM (with respect to wavelength)
ratio of 6374 Å to all the other coronal lines observed increases
with height. The FWHM ratio at the limb depends on the pair of emission
lines chosen; it is about 1 in the case of the 6374 and 7892 Å emission
lines, indicating a common temperature and nonthermal velocity in the
coronal loops near the limb, and it is about 0.7 at the limb in the
case of the 6374 and 5303 Å lines and becomes about 1 at a height
of 120". The varying FWHM ratios with height indicate that hotter
and colder plasmas in coronal loops mix with each other. Therefore,
the observed increase in the FWHM of coronal emission lines, which
are associated with plasma at about 1 MK with height, may not be due
to an increase in nonthermal motions caused by coronal waves but may
be due to an interaction with relatively hotter plasma.
Title: Flare-Induced Coronal Disturbances Observed with Norikura
"NOGIS" Coronagraph: A CME Onset
Authors: Hori, K.; Ichimoto, K.; Sakurai, T.; Nishino, Y.; Nogis Team
Bibcode: 2004ASPC..325..415H
Altcode:
We present the first detection of a CME onset in the coronal green
line emission (ion{Fe{xiv}} 5303 AA, 2 MK) by a 2D Doppler coronagraph
``NOGIS'' at the Norikura Solar Observatory, NAOJ. Different from a
typical CME that evolves within a single magnetic flux system with a
single magnetic neutral line, NOGIS observed a CME that evolved across
two neighboring magnetic flux systems; a flare-productive active region
and an aggregate of face-on coronal loops with a streamer on its top. An
early precursor of the event was a density enhancement of 2 MK plasma in
the space between the two magnetic flux systems. In association with an
M6.5 flare that occurred at the active region, a blue-shifted upward jet
and expansion appeared from a site where the legs of the two systems
apparently connected each other. This expansion later evolved into
a partial halo CME with an angular extent covering the latitudinal
range of the two systems. These observational properties suggest
the existence of sl a CME corridor that is consisted of neighboring
multiple magnetic-flux systems through which a huge CME launches off.
Title: Phase Relationship between the Activity Cycles of Sunspots
and Polar Faculae
Authors: Hagino, M.; Sakurai, T.; Miyazawa, A.
Bibcode: 2004ASPC..325..157H
Altcode:
The eleven-year activity cycles of polar faculae and sunspots are out
of phase by half a cycle. We studied the phase relationship between
the cycles of sunspots and polar faculae, by using the data of sunspot
and facular numbers obtained at Mitaka in the period of 1952--1998 (47
years, four sunspot cycles). We applied the cross-correlation analysis
and wavelet phase differences, and found that the phase relationship
was not persistent. The polar faculae were better correlated with the
sunspots of the previous cycle over two sunspot cycles of 1964--1987,
while the reverse was the case if the data of the whole four cycles
were used.
Title: Latitude Variation of Helicity in Solar Active Regions
Authors: Hagino, Masaoki; Sakurai, Takashi
Bibcode: 2004PASJ...56..831H
Altcode:
We studied the current helicity of 230 active regions on the Sun
during the period of 1992-2001. Vector magnetograms used were obtained
with the Solar Flare Telescope, located at the Mitaka campus of
the National Astronomical Observatory. The latitude distribution of
helicity shows a negative slope; namely, the regions in the northern
(southern) hemisphere tend to show a negative (positive) helicity,
respectively, in agreement with previous studies. The scatter seen in
the helicity is significantly larger than expected from the measurement
errors, implying that the process generating the helicity is of random,
turbulent nature. Therefore, convective motion must play an essential
role in generating the helicity of active regions.
Title: GEOTAIL observation of tilted X-line formation during flux
transfer events (FTEs) in the dayside magnetospheric boundary layers
Authors: Nowada, M.; Sakurai, T.; Mukai, T.
Bibcode: 2004AnGeo..22.2907N
Altcode:
Available from http://www.copernicus.org/site/EGU/annales/22/8/2907.htm?FrameEngine=false;
Title: The Trigger Mechanism of Solar Flares in a Coronal Arcade
with Reversed Magnetic Shear
Authors: Kusano, K.; Maeshiro, T.; Yokoyama, T.; Sakurai, T.
Bibcode: 2004ApJ...610..537K
Altcode:
We have investigated the possibility that magnetic reconnection between
oppositely sheared magnetic loops works as a trigger mechanism of
solar flares, based on three-dimensional numerical simulations. The
simulations were carried out by applying a slow footpoint motion,
which reverses a preloaded magnetic shear, in the vicinity of the
magnetic neutral line. The simulation results clearly indicated that
the reversal of magnetic shear can cause a large-scale eruption
of the magnetic arcade through a series of two different kinds of
magnetic reconnections. The first reconnection is initiated by the
resistive-tearing mode instability growing on the magnetic shear
inversion layer and annihilates the sheared magnetic fluxes, which
are oppositely directed along the magnetic neutral line. As a result
of this, the magnetic arcade collapses into the reconnection point,
and a new current sheet is generated above and below the shear
inversion layer. The generation of new current sheets is followed
by another magnetic reconnection, which drives the eruption of the
sheared magnetic arcade. Mutual excitation of the two reconnections
may explain the explosive property of the flare onset.
Title: Existence of Nanoparticle Dust Grains in the Inner Solar
Corona?
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Hagino,
Masaoki; Yamamoto, Tetsuya T.
Bibcode: 2004ApJ...608L..69S
Altcode:
Motivated by the recent paper by Habbal et al., we have made
spectroscopic observations in the wavelength range of 1072.8-1079.0
nm of the solar corona above the coronal hole region on several days
using a coronagraph. We made raster scans above the coronal hole
region as well as other coronal regions for comparison. The exposure
time of 200 s at a single location permitted us to detect signals of
the order of 10-7 of the solar disk brightness. We did not
find any indication of emission around 1074.7 nm due to fluorescence
from silicon nanoparticle dust grains in the coronal hole region in the
inner corona proposed by Habbal et al. This may be due to the absence
of silicon nanoparticle dust grains in the coronal hole region or to
our detection limit.
Title: The Magnetic Structure of Hα Macrospicules in Solar Coronal
Holes
Authors: Yamauchi, Y.; Moore, R. L.; Suess, S. T.; Wang, H.;
Sakurai, T.
Bibcode: 2004ApJ...605..511Y
Altcode:
Measurements by Ulysses in the high-speed polar solar wind have shown
the wind to carry some fine-scale structures in which the magnetic
field reverses direction by having a switchback fold in it. The
lateral span of these magnetic switchbacks, translated back to the
Sun, is of the scale of the lanes and cells of the magnetic network
in which the open magnetic field of the polar coronal hole and polar
solar wind are rooted. This suggests that the magnetic switchbacks
might be formed from network-scale magnetic loops that erupt into
the corona and then undergo reconnection with the open field. This
possibility motivated us to undertake the study reported here of the
structure of Hα macrospicules observed at the limb in polar coronal
holes, to determine whether a significant fraction of these eruptions
appear to be erupting loops. From a search of the polar coronal holes
in 6 days of image-processed full-disk Hα movies from Big Bear Solar
Observatory, we found a total of 35 macrospicules. Nearly all of these
(32) were of one or the other of two different forms: 15 were in the
form of an erupting loop, and 17 were in the form of a single-column
spiked jet. The erupting-loop macrospicules are appropriate for
producing the magnetic switchbacks in the polar wind. The spiked-jet
macrospicules show the appropriate structure and evolution to be driven
by reconnection between network-scale closed field (a network bipole)
and the open field rooted against the closed field. This evidence for
reconnection in a large fraction of our macrospicules (1) suggests that
many spicules may be generated by similar but smaller reconnection
events and (2) supports the view that coronal heating and solar wind
acceleration in coronal holes and in quiet regions are driven by
explosive reconnection events in the magnetic network.
Title: Differential velocity between solar wind protons and alpha
particles in pressure balance structures
Authors: Yamauchi, Yohei; Suess, Steven T.; Steinberg, John T.;
Sakurai, Takashi
Bibcode: 2004JGRA..109.3104Y
Altcode:
Pressure balance structures (PBSs) are a common high-plasma beta
feature in high-latitude, high-speed solar wind. They have been
proposed as remnants of coronal plumes. If true, they should reflect
the observation that plumes are rooted in unipolar magnetic flux
concentrations in the photosphere and are heated as oppositely directed
flux is advected into and reconnects with the flux concentration. A
minimum variance analysis (MVA) of magnetic discontinuities in PBSs
showed there is a larger proportion of tangential discontinuities than
in the surrounding high-speed wind, supporting the hypothesis that
plasmoids or extended current sheets are formed during reconnection
at the base of plumes. To further evaluate the character of magnetic
field discontinuities in PBSs, differential streaming between alpha
particles and protons is analyzed here for the same sample of PBSs
used in the MVA. Alpha particles in high-speed wind generally have a
higher radial flow speed than protons. However, if the magnetic field
is folded back on itself, as in a large-amplitude Alfvén wave, alpha
particles will locally have a radial flow speed less than protons. This
characteristic is used here to distinguish between folded back magnetic
fields (which would contain rotational discontinuities) and tangential
discontinuities using Ulysses high-latitude, high-speed solar wind
data. The analysis indicates that almost all reversals in the radial
magnetic field in PBSs are folded back field lines. This is found to
also be true outside PBSs, supporting existing results for typical
high-speed, high-latitude wind. There remains a small number of cases
that appear not to be folds in the magnetic field and which may be
flux tubes with both ends rooted in the Sun. The distinct difference
in MVA results inside and outside PBSs remains unexplained.
Title: High-cadence Hα imaging of solar flares
Authors: Hanaoka, Y.; Sakurai, T.; Noguchi, M.; Ichimoto, K.
Bibcode: 2004AdSpR..34.2753H
Altcode:
The Hα observation is a powerful tool to study the high-energy
aspect of solar flares. Spiky brightenings of flare kernels at the
Hα center reflect the rapid fluctuation in particle acceleration;
linear polarization of Hα emission might be evidence of accelerated
protons; red-shifts of the Hα line are caused by the chromospheric
evaporation. To study the spiky brightenings of flare kernels with
high-cadence imaging at the Hα center, a high-speed Hα camera for the
Solar Flare Telescope at Mitaka, NAOJ, had been developed and it started
the regular observation in 2001 July. However, the polarimetry and the
Dopplermetry are also important and they are required to be carried
out in parallel with the high-cadence imaging at the Hα center. Then,
we upgraded the original high-speed Hα camera to a new Hα camera
system for the multi-aspect Hα observations, which performs all
of the high-cadence imaging, the linear polarization measurements,
and the off-band imaging for velocity measurements. The new system
started the observation in 2002 July. In this paper, the multi-aspect
Hα imaging system is described and sample Hα images are presented.
Title: Spectroscopic studies of the solar corona using Fe X, XIII,
XIV lines
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Suematsu,
Yoshinori; Takeda, Aki
Bibcode: 2004naoj.book...46S
Altcode:
No abstract at ADS
Title: Spectroscopic observation of coronal waves
Authors: Sakurai, Takashi; Ichimoto, Kiyoshi; Raju, K. P.; Singh,
Jagdev
Bibcode: 2004naoj.book...45S
Altcode:
No abstract at ADS
Title: Solar-cycle variation of near-sun sky brightness observed
with coronagraphs
Authors: Sakurai, T.; Rusin, V.; Minarovjech, M.
Bibcode: 2004AdSpR..34..297S
Altcode:
When the intensity of emission lines in the solar corona is measured
with a coronagraph, the background sky brightness caused by aerosol in
the earth atmosphere is also measured. Observational records of sky
brightness at 50″ above solar limb have been accumulated for many
years at coronagraph stations. The data obtained at Norikura, Japan,
during the period of 1951-1997 show an 11-year periodicity of solar
cycle. This suggests that the density and/or size distributions of
aerosol particles might be modulated by solar activity (possibly by
UV radiation). Similar data obtained at Lomnický Štít, Slovakia,
in the period of 1980-2001 do not show this 11-year periodicity. This
may be because the latter data set is not long enough, or could be
because of different meteorological conditions in Europe and in Japan.
Title: Relation between pressure balance structures and polar plumes
from Ulysses high latitude solar wind observations
Authors: Yamauchi, Yohei; Suess, S. T.; Sakurai, Takashi
Bibcode: 2004naoj.book...49Y
Altcode:
No abstract at ADS
Title: Magnetic helicity injection and sigmoidal coronal loops
Authors: Yamamoto, T.; Sakurai, T.; Kusano, K.; Yokoyama, T.;
Maeshiro, T.
Bibcode: 2004cosp...35.1951Y
Altcode: 2004cosp.meet.1951Y
Sigmoidal loops are believed to be closely associated with flares,
coronal dimmings and coronal mass ejections. These sigmoidal loops may
have more free energy and cause explosive events, because they look
more twisted than ordinary coronal loops. Kusano et al. (2002) proposed
how to quantitatively evaluate the magnetic helicity injection. We
used their method and studied the evolution of sigmoidal active
regions with emphasis on the magnetic helicity injection. Vector
magnetograms obtained with the Solar Flare Telescope of NAOJ and
SOHO/MDI magnetograms were used. We selected four sigmoidal and three
ordinary active regions, which had good data coverage, from the list
of Canfield et al. (1999). We got two results. First, we found larger
(but not impulsive) helicity injection in the sigmoidal active regions
than in the ordinary active regions. Here the helicity injection
was normalized by the magnetic flux in order to compare the active
regions having differnt sizes. This normalized helicity injection
may be an indicator distinguishing the sigmoidal regions from the
ordinary regions. Second, we found that amounts of magnetic helicity
injection around foot points of the sigmoidal loops are large enough
for helical deformation of the sigmoidal loops. There is no clear event
showing formation of the sigmoidal loops in time histories of magnetic
helicity injection. We therefore investigated local magnetic helictiy
injection with time resolution of 96 minutes which is the cadence of
the MDI magnetograms. Significant magnetic helicity injection around
the foot points is consistent with the geometry of the sigmoidal loops.
Title: Measurement of magnetic helicity injection and free energy
loading into the solar corona
Authors: Kusano, Kanya; Maeshiro, Tomohiro; Yokoyama, Takaaki;
Sakurai, Takashi
Bibcode: 2004naoj.book...47K
Altcode:
No abstract at ADS
Title: Dst prediction for a period of high-density plasmas in
magnetic clouds
Authors: Adachi, H.; Sakurai, T.
Bibcode: 2004cosp...35.3271A
Altcode: 2004cosp.meet.3271A
We examine geomagnetic effects for high-density plasmas in magnetic
clouds and their relationship to solar sources. It is well known that
Bz component of interplanetary magnetic field plays an important
role for estimation of Dst from solar wind parameters (Burton et
al.1975). However, magnetic clouds frequently carry high-density
plasmas, which are interpreted as the remnants of filament. In order to
clarify their geomagnetic effects, we try to estimate Dst by adopting
different methods introducing effects of solar wind parameters. In
our estimation the most important point is laid on the sense of Dst
variation rather than its magnitude. The most suitable estimation is
obtained by setting up a threshold for plasma density, in which for
a case of plasma density greater than 20 /cc the Fenrich and Luhmann
(1998)'s formula should be used, while in the other cases the Burton's
formula are adopted. In both estimations the O'Brien and McPherron
(2000)'s ring current decay time is employed. Furthermore, we examine
the solar origin corresponding to the magnetic clouds and then compared
characteristic signatures of the magnetic cloud with those observed on
the solar surface. As a result, we confirm that the magnetic structure
of interplanetary flux rope is in good agreement with the structures of
the magnetic neutral line near disappearing filaments and heliospheric
current sheet (HCS). On the basis of these studies, we suggest that
for the geomagnetic disturbance forecast, the effect of high-density
plasmas carried with magnetic clouds should be taken into account of
as well as that of interplanetary magnetic field.
Title: Flare-induced mhd kink oscillation in coronal multiple loops
Authors: Hori, K.; Ichimoto, K.; Sakurai, T.
Bibcode: 2004cosp...35.1302H
Altcode: 2004cosp.meet.1302H
We provide the first detection of an MHD kink oscillation in the coronal
green line emission (Fe XIV 5303A, 2MK) by a 2D Doppler coronagraph
"NOGIS" at the Norikura Solar Observatory, NAOJ. It was a damping
oscillation in Doppler shifts among an aggregate of face-on coronal
loops that was standing at the solar limb with a streamer on its
top. The oscillation was induced by a mass expansion, which started in
association with an M6.5 flare and later evolved into a partial halo
CME. The oscillation had a period of about 10 min in the inner loops
while about 14 min in the outer loops, and lasted over 100 minutes. We
discuss the trigger and the damping mechanisms of the oscillation. An
application to the Solar-B and STEREO observation will be proposed
for the similar coronal disturbances.
Title: Observation of CMEs with NOrikura Green-line Imaging System
(NOGIS)
Authors: Suzuki, I.; Sakurai, T.; Ichimoto, K.
Bibcode: 2004cosp...35.1948S
Altcode: 2004cosp.meet.1948S
Since 1980s, it has been discussed that CMEs are either planar
loop-like (two-dimensional) or bubble-like (three-dimensional)
structures. Although loop-like configurations were sometimes mentioned,
the majority of opinion is in favor of bubble-like geometry. However,
our view is that this issue has not yet been settled. For investigating
the three-dimensional structure of CMEs, it is important to examine
their driving mechanism and the magnetic field structure around their
associated active regions near the solar surface. NOGIS (NOrikura
Green-line Imaging System) at the Norikura Solar Observatory (NAOJ)
consists of a 10-cm coronagraph and a tunable birefringent filter,
and observes the coronal green-line emission (Fe XIV 5303Å), which
is also one of the lines observed by LASCO C1. This coronagraph has
a field of view of 1.05 -- 1.5R⊙, and can investigate
the appearance of CME onset. Additionally, it can obtain the Doppler
shift images of the green-line corona. We examined all data from 1997,
which is the beginning of observation, to 2003 and found some events
associated with CMEs. And from the Doppler shift images, we found the
loop oscillations by the flare associated with CMEs. We show these
events observed by NOGIS.
Title: Loading of magnetic helicity and flare activity
Authors: Hagino, M.; Yamamoto, T.; Sakurai, T.
Bibcode: 2004cosp...35.1955H
Altcode: 2004cosp.meet.1955H
Studies of twisted flux tubes are important in understanding the
mechanisms of solar flares and coronal mass ejection. In this study,
we have made a comparison between the helicity injection rate
Hin and the force-free parameter α. The latter is an
indicator of accumulated helicity. The magnetograms we used were
obtained with SOHO/MDI and with the Solar Flare Telescope (SFT)
of NAOJ in Mitaka, Tokyo. SFT can take vector magnetograms every
3 minutes. To measure the helicity injection, we have employed
Kusano's method (Kusano et al. 2002) which can derive the helicity
injection due to both shearing motion and emerging motion. We studied
several active regions which produced major flares (for instance, NOAA
9661). We estimated the values of Hin and α in two ways;
(1) by averaging over the whole region and (2) by isolating the flare
area. In the case of NOAA 9661, a GOES X1.7 class flare was observed
on 2001 October 19. The value of α averaged over the whole region was
negative, which agreed with the hemispheric helicity sign rule because
this region appeared in the northern hemisphere. If the area of 100'' ×
50'' near the flare neutral line was selected, we found the following
results. (1) The temporal changes in α were consistent with the
variations in Hin. (2) No emerging flux activity was seen,
and the contributions to Hin from the emerging motion were
small. (3) The pixel to pixel correlation between Hin and
α in the area showed a positive correlation before the flare. After
the flare the correlation weakened. Property (3) may imply that the
magnetic helicity is being loaded prior to the flare. Therefore, the
correlation between Hin and α can be used as an indicator
of energy build-up process.
Title: The source of magnetic field twist in solar active regions
Authors: Bao, Shudong; Sakurai, Takashi; Suematsu, Yoshinori
Bibcode: 2004naoj.book...48B
Altcode:
No abstract at ADS
Title: Relation Between Polar Plumes and Fine Structure in the Solar
Wind from Ulysses High-Latitude Observations
Authors: Yamauchi, Yohei; Suess, Steven T.; Sakurai, Takashi
Bibcode: 2003AIPC..679..255Y
Altcode:
Ulysses observations showed that pressure balance structures (PBSs)
are a common feature in the high-latitude and high-speed solar winds
near the solar minimum. PBSs have been hypothesized to be remnants of
coronal plumes and to be related to network activity such as magnetic
reconnection in the photosphere. This suggests that information on
the magnetic structure of PBSs would help to study the relation
between PBSs and polar plumes. We have investigated the magnetic
structures of the 104 PBSs by applying a minimum variance analysis to
Ulysses/Magnetometer data and by examining the pitch-angle distribution
of energetic electrons measured with Ulysses/SWOOPS. We found that PBSs
have relatively more tangential discontinuities rather than rotational
from the minimum variance analysis and there is no difference between
PBSs observed in north and south polar regions. From the analysis of
energetic electron data, most PBSs also show local bi-directional
electron flux or isotropic pitch-angle distribution expected in
plasmoids or, less often, the distribution expected in association
with current-sheet structures. This suggests the hypothesis that PBSs
are generated due by network activity such as magnetic reconnection
at the base of polar plumes.
Title: The flare of 1991 June 4 (importance 3B) and the associated
Moreton wave
Authors: Yamaguchi, Kisuke; Sakurai, Takashi; Irie, Makoto; Kumagai,
Kazuyoshi; Hagino, Masaoki; Miyashita, Masakuni; Shiomi, Yasuhiko;
Hiei, Eijiro
Bibcode: 2003RNAOJ...6..101Y
Altcode:
We observed a Moreton wave associated with a flare of importance 3B
in the NOAA region 6659 on 1991 June 4. The Moreton wave was emitted
from a flare bright point and initially showed the form of a loop. The
speed of the Moreton wave was initially 1500 km s-1 and
was later accelerated to 1800 km s-1. This acceleration
may be due to the propagation of the wave into a coronal hole (with
low density and high Alfvén velocity.
Title: Point Spread Function of the Soft X-Ray Telescope Aboard YOHKOH
Authors: Shin, Junho; Sakurai, Takashi
Bibcode: 2003JKAS...36S.117S
Altcode:
No abstract at ADS
Title: Magnetic Helicity of Solar Active Regions and its Implications
Authors: Sakurai, T.; Hagino, M.
Bibcode: 2003JKAS...36S...7S
Altcode:
No abstract at ADS
Title: Observation of Two Forms of Macrospicules in Coronal Holes:
Spikes and Loops
Authors: Yamauchi, Y.; Moore, R. L.; Suess, S. T.; Wang, H.;
Sakurai, T.
Bibcode: 2003SPD....34.0411Y
Altcode: 2003BAAS...35..812Y
Ulysses high-latitude observations show the existence of small
structures in the high-speed solar wind that contain magnetic field
reversals. These reversals sometimes appear to be associated with
plasmoids or current sheets. We have proposed that the reversals
are created by activity low in the magnetic network in coronal holes
[Yamauchi et al., 2002, GRL, v29(10)]. Here we present solar evidence
favoring this hypothesis. Since photospheric magnetic flux observations
have shown that there is a small fraction of opposite polarity in
coronal holes [e.g., Deforest et al., 1997, Sol. Phys., v175(2),
393-410], there should be local magnetic loops in the network. If
one of these loops were to erupt into the corona, it could create
a magnetic field reversal by reconnection with the surrounding open
magnetic field. Tanaka [1972, Report of BBSO, No. 125] observed that
some H-alpha mottles (spicules) in the network show a double-strand
structure. The two strands might be the legs of an erupted network
loop. Any coronal hole macrospicule that showed a bipolar erupting loop
structure rather than a unipolar, jet-like spike, would be a candidate
for such an event. From sequences of full-disk H-alpha images from
Big Bear Solar Observatory, we have found 35 macrospicules in polar
coronal holes. About half of these appear to be erupting loops, while
the rest look more like unipolar spikes. Thus, we have found evidence
that network-scale erupting magnetic loops are a common occurrence in
coronal holes. This strengthens the possibility that such events are
the source of the fine-scale field reversals in the high-speed wind.
Title: Oscillations in the Coronal Green-Line Intensity Observed at
Lominický Štít and Norikura Nearly Simultaneously.
Authors: Minarovjech, Milan; Rušin, Vojtech; Rybanský, Milan;
Sakurai, Takashi; Ichimoto, Kiyoshi
Bibcode: 2003SoPh..213..269M
Altcode:
We studied intensity oscillations of the coronal green line ([Fe xiv]
530.3 nm) observed with two coronagraphs at Lomnický Štít and
Norikura nearly simultaneously. In the spectroscopic data obtained
at Norikura, we have detected and confirmed the earlier detection
of 5-minute oscillations in photoelectric photometer observations
made at Lomnický Štít. Quasi-periodic structures in the green-line
intensity with a tangential speed up to 400 km s−1 have
been detected for the first time. We briefly discuss the implications
of these oscillations on the coronal heating mechanisms.
Title: Analysis of Vector Magnetic Fields in Solar Active Regions
by Huairou, Mees and Mitaka Vector Magnetographs
Authors: Zhang, H.; Labonte, B.; Li, J.; Sakurai, T.
Bibcode: 2003SoPh..213...87Z
Altcode:
We analyze the vector magnetograms in several well-developed active
regions obtained at Huairou Solar Observing Station, National
Astronomical Observatories of China, at Mees Solar Observatory,
University of Hawaii, and at National Astronomical Observatory
of Japan. It is found that there is a basic agreement on the
transversal fields among these magnetographs. The observational
error (mutual difference) for the transversal magnetic fields is
estimated. In addition to comparison of transversal fields among
different instruments, we used the morphological configurations of
sunspot penumbrae in white-light and EUV 171 Å images obtained by
the TRACE satellite as a reference of the orientation of transversal
magnetic fields.
Title: Spectroscopic Studies of the Solar Corona. IV. Physical
Properties of Coronal Structure
Authors: Singh, Jagdev; Ichimoto, Kiyoshi; Sakurai, Takashi; Muneer, S.
Bibcode: 2003ApJ...585..516S
Altcode:
We obtained spectrographic observations of several coronal structures
at the limb overlying the sunspot regions, simultaneously in the Fe
XIV (5303 Å) and Fe X (6374 Å) emission lines on several days, and
simultaneously in three coronal emission lines 6374 Å (Fe X), 10747 Å
(Fe XIII), and 10798 Å (Fe XIII) on some other days. The slit width of
160 μm provided a spatial resolution of 4" and a spectral resolution
of 77 mÅ (Fe XIV), 128 mÅ (Fe X), and 291 mÅ (Fe XIII). The width
and intensity of all these lines were computed using Gaussian fits to
the observed line profiles. The FWHM of the emission lines increases
at an average rate of 1.24 mÅ arcsec-1 for Fe X, 0.29 mÅ
arcsec-1 for Fe XIII, and -0.66 mÅ arcsec-1 for
Fe XIV. These values are inversely correlated with the corresponding
ionization temperature for these emission lines. We speculate that the
FWHM of emission lines in coronal structures increases with height if
the associated ionization temperature is less than 1.6×106
K, with the gradient depending upon the ionization temperature of
the line, while it decreases with height for lines whose ionization
temperature is greater than 1.6×106 K. It implies that it
may not always be possible to interpret the observed increase in FWHM
with height in terms of an increase in nonthermal velocity.
Title: High-Speed Hα Camera and the Real-Time Image Processing
System for Solar Observations
Authors: Hanaoka, Yoichiro; Noguchi, Motokazu; Sakurai, Takashi;
Ichimoto, Kiyoshi
Bibcode: 2003SPIE.4853..576H
Altcode:
We have developed a new digital imaging system for the Hα imager
of the Solar Flare Telescope at Mitaka, NAOJ, for high-cadence
observations of solar flares. To resolve individual spikes elementary
bursts) of impulsive solar flares requires a time resolution within
1 s and a spatial resolution of about 1", and the high-speed Hα
camera realized them. Such high-resolution observations produce huge
amount of data, and it has been the major difficulty to construct
a high-cadence system. Generally the amount of data from solar
optical observations is huge, because they are multi-dimensional
(in space/time/wavelength/polarization status). Efficient real-time
processing of observational data is essentially important to extract
meaningful information from the raw data. Recent advances in computer
technology have made possible to handle vast data with a small
computer. Therefore, firstly we have developed a PC-based flexible
real-time image processing system, which is applicable to various
real-time data processings required for solar optical observations. The
high-speed Hα camera is developed based on this system. In this paper,
the real-time image processing system and the high-speed Hα camera
system are described as well as the actual operation of the Hα camera.
Title: Spectroscopic Studies of the Solar Corona - V. Physical
Properties of Coronal Structures
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Muneer, S.
Bibcode: 2003SoPh..212..343S
Altcode:
Spectra around the 6374 Å [Fe x] and 7892 Å [Fe xi] emission lines
were obtained simultaneously with the 25-cm coronagraph at Norikura
Observatory covering an area of 200' '×500' '
of the solar corona. The line width, peak intensity and line-of-sight
velocity for both the lines were computed using Gaussian fits to the
observed line profiles at each location (4' '×4'
') of the observed coronal region. The line-width measurements
show that in steady coronal structures the FWHM of the 6374 Å emission
line increases with height above the limb with an average value of
1.02 mÅ arc sec−1. The FWHM of the 7892 Å line also
increases with height but at a smaller average value of 0.55 mÅ arc
sec−1. These observations agree well with our earlier
results obtained from observations of the red, green, and infrared
emission lines that variation of the FWHM of the coronal emission lines
with height in steady coronal structures depends on plasma temperatures
they represent. The FWHM gradient is negative for high-temperature
emission lines, positive for relatively low-temperature lines and
smaller for emission lines in the intermediate temperature range. Such
a behaviour in the variation of the FWHM of coronal emission lines with
height above the limb suggests that it may not always be possible to
interpret an increase in the FWHM of emission line with height as an
increase in the nonthermal velocity, and hence rules out the existence
of waves in steady coronal structures.
Title: On a Cyclic Variation of the Hemispheric Helicity Rule.
Authors: Pevtsov, Alexei A.; Hagyard, Mona J.; Blehm, Zachary; Smith,
James E.; Canfield, Richard C.; Sakurai, Takashi; Hagino, Masaoki
Bibcode: 2003IAUJD...3E..35P
Altcode:
We report the result of a study of magnetic helicity in solar active
regions during 1980-2000 (cycles 21-23). Using the vector magnetograms
from four different instruments (Haleakala Stokes Polarimeter Marshall
Space Flight Center Mitaka Solar Flare Telescope and Okayama Observatory
Solar Telescope) we calculated the force-free parameter alpha as
in Pevtsov et al. (1995). We use alpha as the proxy for current
helicity. For each instrument we computed a gradient dalpha/dL as
the linear fit of alpha versus latitude L using annual subsets of
data. The hemispheric helicity rule (negative/positive helicity in
northern/southern hemisphere) can be expressed in terms of this gradient
as dalpha/dL < 0. We find that each instrument exhibits change in
sign of this gradient for some years which implies that the hemispheric
helicity rule may not hold in some phases of a solar cycle (see Hagino
and Sakurai 2002). However we do not see consistency between different
instruments in regards to years disobeying the rule. The disagreement
may be due to difference in observations and/or insufficient number of
magnetograms in some years. We conclude that the present data sets do
not allow to make statistically significant inference about possible
cyclic variation of the hemispheric helicity rule.
Title: High-speed Hα camera for solar flare observations
Authors: Hanaoka, Yoichiro; Noguchi, Motokazu; Ichimoto, Kiyoshi;
Sakurai, Takashi
Bibcode: 2003naoj.book...19H
Altcode:
No abstract at ADS
Title: Interpolation of one- and two-dimensional images with pixelwise
photon number conservation
Authors: Sakurai, Takashi; Shin, Junho
Bibcode: 2003naoj.book...36S
Altcode:
No abstract at ADS
Title: Chromospheric magnetic field of solar active regions
Authors: Choudhary, Debi Prasad; Sakurai, Takashi
Bibcode: 2003naoj.book...37C
Altcode:
No abstract at ADS
Title: Sunspot magnetic fields observed with a large-format infrared
array
Authors: Sakurai, Takashi; Yanagisawa, Kenshi; Kobiki, Toshihiko;
Kasahara, Shouichi; Nakakubo, Kayoko
Bibcode: 2003naoj.book...34S
Altcode:
No abstract at ADS
Title: Annihilation of magnetic helicity: A new model for solar
flare onset
Authors: Kusano, K.; Yokoyama, T.; Maeshiro, T.; Sakurai, T.
Bibcode: 2003AdSpR..32.1931K
Altcode:
Although magnetic reconnection is believed to be a key process for
solar flares, the onset mechanism of flares remains as a long-standing
problem. In this paper, we propose a new model for solar flares, in
which magnetic reconnection converts oppositely-sheared field into the
shear-free field. This process can be understood as the annihilation of
magnetic helicities with different signs, and it is consistent with the
recent results of the helicity observations. Numerical simulations were
carried, which show that, if the helicity is sharply reversed within a
magnetic arcade, reconnection quickly grows in the helicity inversion
layer, driving explosive dynamics. The explosive process appears as a
result of nonlinear instability, which grows faster than exponentially
with time. Based on the results, we predict that the coexistence of
the positive and negative helicity is crucial for the onset of flares.
Title: Relation between magnetic helicity injection and flare
activities in active region NOAA 8100
Authors: Yokoyama, T.; Kusano, K.; Maeshiro, T.; Sakurai, T.
Bibcode: 2003AdSpR..32.1949Y
Altcode:
The analysis of the magnetic helicity injection into active region
NOAA 8100 is shown. This region is known as a source of many coronal
activities such as flares. It is found that most of the flare events
occurred about half a day after the helicity injection rate changed
its sign. And the positions of Hα emission in flares well correspond
to the helicity inversion lines in space. These results suggest that
the sign-reversal of the helicity injection rate is a key signature
of flare activities.
Title: Measurement of magnetic helicity flux into the solar corona
Authors: Kusano, K.; Maeshiro, T.; Yokoyama, T.; Sakurai, T.
Bibcode: 2003AdSpR..32.1917K
Altcode:
We developed a new methodology which can determine magnetic helicity
flux across the photosphere based on the magnetograph observation, In
order to derive the helicity flux, first the velocity tangential to
the solar surface is constructed by applying a correlation tracking
technique on the magnetic observation, and secondly the velocity
component across the photosphere is derived from the condition that
the magnetic evolution must be consistent with the induction equation,
Through this procedure, we can determine the helicity flow across the
photosphere as a function of time and space, Based on this new method,
we analyzed magnetic helicity of the active regions NOAA 9026 and
9077. using the data taken by SOHO/MDI and the vector magnetograph at
NAOJ/Tokyo. As a result, it was revealed that positive and negative
helicity is supplied in each region simultaneously. In particular,
temporary activation of the helicity injection of the both signs was
observed prior to X-class flare events in the GOES classification. The
investigation on the helicity distribution suggests some relationship
between flare onset and the increase of complexity in the helicity
structure.
Title: Magnetic helicity and flare activity
Authors: Sakurai, T.; Hagino, M.
Bibcode: 2003AdSpR..32.1943S
Altcode:
We studied two active regions appeared in 2001 (NOAA 9415 and 9661),
both of which have produced X-class flares. The time evolutions of
magnetic flux and magnetic helicity were derived based on vector
magnetograph observations. These regions satisfied the so-called
hemispheric rule of magnetic helicity, namely regions in the northern
(southern) hemisphere tend to have negative (positive) helicity. The
magnetic helicity integrated over the regions evolved slowly and
did not show abrupt changes at the time of the flares, although the
distributions of magnetic helicity changed significantly over a few
days in the regions. The appearance of localized regions whose helicity
was opposite to that expected from the hemispheric rule was seen in
both cases.
Title: Generation and Annihilation of Helicity in Active Regions
Authors: Kusano, Kanya; Maeshiro, Tomohiro; Miike, Haruka; Yokoyama,
Takaaki; Sakurai, Takashi
Bibcode: 2003IAUJD...3E..32K
Altcode:
Magnetic helicity in active regions is investigated based on the vector
magnetograph observations and the three-dimensional magnetohydrodynamic
simulations. First we measured the helicity flux through the photosphere
into active regions using the magnetic data and the numerical technique
to solve the induction equation inversely and found that the helicity
flux forms a complicated structure in which the sign of helicity is
easily changed within an active region. Secondly from the statistical
analyses for various active regions it was shown that the absolute
value of helicity flux rather than the net flux well correlate the
coronal activity. Thirdly we revealed that for many flares the initial
brightening in H-alpha and Trace 1600 A image located at a region where
the helicity sign was sharply changed on the photosphere. Finally using
the high resolution simulation we demonstrated that the explosive
nonlinear reconnection can arise at the helicity inversion layer
when the counter helicity is gradually injected due the photospheric
motion. Based on the all results we conclude that the annihilation
of both signs of magnetic helicity could be a key mechanism to drive
coronal activity
Title: Flexible prism used as an image stabilizer
Authors: Sakurai, Takashi; Noguchi, Motokazu; Shinoda, Kazuya;
Tanaka, Nobuyuki
Bibcode: 2003naoj.book...35S
Altcode:
No abstract at ADS
Title: Eleven-year solar cycle periodicity in sky brightness observed
at Mt. Norikura
Authors: Sakurai, Takashi
Bibcode: 2003naoj.book...12S
Altcode:
No abstract at ADS
Title: Spectroscopic Studies of the Solar Corona III. Density
Diagnostics Using the Infrared Lines of Fe XIII
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Takeda,
Aki
Bibcode: 2002PASJ...54..807S
Altcode:
We have obtained spectrographic observations of several coronal
regions in three emission lines (6374Å [Fe <FONT SIZE="-2">X
], 10747Å [Fe <FONT SIZE="-2">XIII ], and 10798Å [Fe <FONT
SIZE="-2">XIII ]) simultaneously. The 25-cm coronagraph at Norikura
Observatory and two large-format CCD cameras were used. The peak
intensity, velocity, and line-width values were derived from Gaussian
fits to the observed line profile at each location of the observed
region. The ratio of the intensities of the 10747Å and 10798Å emission
lines in the individual coronal structures range between 1.0 and 2.5
at 10" above the limb, which corresponds to a density range of 9.8 ×
109 - 2.4 × 108 cm-3. The scale-height
temperature values, derived from the variations of the intensity ratio
with height above the limb for all individual coronal structures,
range between 0.6 × 106K and 8.3 × 106K with
a most frequent value around 1.8 × 106K. The large values
of the scale-height temperature for 70% of the structures indicate
that these structures may not be in hydrostatic equilibrium nor be
isothermal in nature.
Title: Spectroscopic Observation of Coronal Waves
Authors: Sakurai, T.; Ichimoto, K.; Raju, K. P.; Singh, J.
Bibcode: 2002SoPh..209..265S
Altcode:
A time sequence over 80 min of coronal green-line spectra was obtained
with a corona- graph at the Norikura Solar Observatory. Doppler
velocities, line intensities, and line widths were derived through
fitting a single Gaussian to the observed line profiles. Coronal waves
have been clearly detected in the Doppler velocity data. The Fourier
analysis shows powers in a 1-3 mHz range, and in higher frequencies
(5-7 mHz) at localized regions. The propagation speed of the waves
was estimated by correlation analysis. The line intensity and line
width did not show clear oscillations, but their phase relationship
with the Doppler velocity indicates propagating waves rather than
standing waves. The existence of Alfvén waves whose speed is 500 km
s−1 or faster is possible but inconclusive, while the
existence of slower waves (of the order of 100 km s−1,
possibly sound waves) is evident. The energy carried by the detected
sound waves is far smaller than the required heat input rate to the
quiet corona.
Title: Spectroscopic Studies of the Solar Corona II. Properties of
Green and Red Emission Lines in Open and Closed Coronal Structures
Authors: Singh, Jagdev; Sakurai, Takashi; Ichimoto, Kiyoshi; Suematsu,
Yoshinori; Takeda, Aki
Bibcode: 2002PASJ...54..793S
Altcode:
We obtained profiles of the green (Fe <FONT SIZE="-2">XIV
5303Å) and red (Fe <FONT SIZE="-2">X 6374Å) emission lines
simultaneously on 1998 July18-19 for about 4hours with a cadence
of 10minutes, covering an area of about 200'' × 500'' in the
solar corona. The line width (FWHM), intensity, and Doppler shift
for both lines were computed using Gaussian fits to the observed
line profiles. We then studied any systematic differences in these
parameters between closed and open field structures, and their time
variations. The derived line widths indicate that the FWHM of the red
line increases with height above the limb at a rate of 0.5 - 2.6 mÅ
arcsec-1 and the green-line width decreases with height
at a rate of 1.2 - 3.4 mÅ arcsec-1. The difference in
the time-averaged values of the widths of the green line in open and
closed coronal structures at a given height above the limb is small,
whereas the width of the red line in open structures is substantially
larger than that in the closed loop-like coronal structures.
Title: Measurement of Magnetic Helicity Injection and Free Energy
Loading into the Solar Corona
Authors: Kusano, K.; Maeshiro, T.; Yokoyama, T.; Sakurai, T.
Bibcode: 2002ApJ...577..501K
Altcode:
We develop a new methodology that can determine magnetic helicity flux
as well as Poynting flux across the photosphere based on magnetograph
observation. By applying this method, we study the injection mechanism
of magnetic helicity and magnetic free energy into the solar corona. In
order to derive the helicity and energy fluxes, first the velocity
tangential to the solar surface is constructed by applying a correlation
tracking technique on the magnetic observation, and second, the velocity
component normal to the photosphere is derived from the condition
that the magnetic evolution must be consistent with the induction
equation. Through this procedure, we can determine the helicity and
energy fluxes separately for the shear motion effect and for the flux
emergence effect. Based on this new method, NOAA Active Region 8100 was
analyzed from 1997 November 1 to 5 using data observed by the Solar and
Heliospheric Observatory Michelson Doppler Interferometer and the vector
magnetograph at the National Astronomical Observatory of Japan (NAOJ)
in Tokyo. The results indicate that the photospheric shear motion and
the flux emergence process have equally contributed to the helicity
injection and have supplied magnetic helicity of opposite signs into
this active region.
Title: The Sources of Magnetic Field Twist in Solar Active Regions
Authors: Bao, S. D.; Sakurai, T.; Suematsu, Y.
Bibcode: 2002ApJ...573..445B
Altcode:
Observations have revealed that a hemispheric preference of magnetic
chirality (handedness) exists throughout the solar atmosphere. For
example, the current helicity of active regions is predominantly
negative (left-handed twist) in the northern hemisphere and
positive (right-handed twist) in the southern. The explanation
of this hemispheric tendency is still open to question. In this
paper we first review several possible mechanisms and clarify some
misunderstandings. In our views, in the photosphere, the differential
rotation acting on already emerged sunspot magnetic fields will lead to
negative current helicity in the northern hemisphere and positive in the
southern, but the same effect caused by the Coriolis force is opposite
in sign. In the turbulent convection zone, the Coriolis force acting on
the rising magnetic flux tubes will result in negative/positive helicity
in the northern/southern hemisphere, but the corresponding action by the
differential rotation will give rise to a reversed result. Moreover,
in this region the α-effect will produce the wrong sign to explain
the observed sense of magnetic twist. It should be noteworthy that
the two current helicities generated by the α-effect, that in the
mean field and that in the fluctuations, have opposite signs, and the
former is positive/negative in the northern/southern hemisphere while
the latter is negative/positive in the northern/southern hemisphere. In
the overshoot region at the base of the convection zone, the current
helicity created by the α-effect has the sign needed. Finally,
we suggest that some surface flows (e.g., converging flows that can
lead to cancellation of opposite-polarity flux in the photosphere)
and magnetic reconnection are also important to the redistribution
(or regeneration) of magnetic twist in active regions.
Title: Relation between Pressure Balance Structures and polar plumes
from Ulysses high latitude observations
Authors: Yamauchi, Yohei; Suess, Steven T.; Sakurai, Takashi
Bibcode: 2002GeoRL..29.1383Y
Altcode: 2002GeoRL..29j..21Y
Ulysses observations have shown that pressure balance structures
(PBSs) are a common feature in high-latitude, fast solar wind
near solar minimum. Previous studies of Ulysses/SWOOPS plasma data
suggest these PBSs may be remnants of coronal polar plumes. Here
we find support for this suggestion in an analysis of PBS magnetic
structure. We used Ulysses magnetometer data and applied a minimum
variance analysis to magnetic discontinuities in PBSs. We found that
PBSs preferentially contain tangential discontinuities, as opposed
to rotational discontinuities and to non-PBS regions in the solar
wind. This suggests that PBSs contain structures like current sheets
or plasmoids that may be associated with network activity at the base
of plumes.
Title: Eleven-year solar cycle periodicity in sky brightness observed
at Norikura, Japan
Authors: Sakurai, T.
Bibcode: 2002EP&S...54..153S
Altcode:
We analyzed the brightness of the sky background observed with
a coronagraph at Norikura, Japan, in the period of 1951-1997. We
discovered that the power spectrum shows a clear eleven-year periodicity
of solar activity cycle, in addition to strong annual variations. The
peaks in the eleven-year component are found in the declining phase of
activity, 2-4 years after the sunspot number maximum. The brightness
of the continuum corona is far fainter than the observed amplitude
of the eleven-year component and cannot account for the observed
phenomenon. A possible interpretation is that the solar activity
modulates the contents of aerosols in the upper atmosphere of the earth,
thus producing variations in scattering of sunlight. Why this effect
is largest in the declining phase of solar activity is still unknown.
Title: Hemispheric Helicity Asymmetry in Active Regions for Solar
Cycle 21-23
Authors: Hagino, M.; Sakurai, T.
Bibcode: 2002mwoc.conf..147H
Altcode:
Magnetic helicity observed at the surface carries information on the
invisible, sub-surface processes such as internal rotation and the
behavior of magnetic flux tubes in the convection zone. It has been
recognized that magnetic helicity shows a hemispheric rule; the northern
(southern) hemisphere tends to show negative (positive) helicity. It
is also known that this rule does not change with solar cycle. The
determination of the helicity, requiring vector magnetograms, is
sensitive to various noises and instrumental effects, and is therefore
not an easy task. Considering the importance of magnetic helicity,
here we present the results based on our data set and compare them with
results from other observatories. We analyzed vector magnetograms in
the period of 1992-2000 obtained with the Solar Flare Telescope at the
National Astronomical Observatory of Japan. The current helicity was
determined by two methods for 180 active regions. The first method
calculates the electric currents over active regions by a direct
differentiation and then evaluates the average helicity, alpha = frac
sum (abla times vec {B})zcdot {m sign} (Bz)
+ Bz. The second method is the fitting of the linear
force-free field vec{B}m cal (alpha) to the observed
transverse field vec{B}m obs and finds the best-fit alpha
which minimizes frac sum [vec{B}m cal(alpha) - vec{B}m
obs]2 sum Bm obs2. The data
points with transverse fields larger than 150 G (noise level) and with
longitudinal fields weaker than 500 G were used. The latter condition
was to minimize the effect of Faraday rotation that may change the
azimuth of the transverse field. We plotted the helicity against solar
latitude and calculated a linear fit to the data. The slopes of the
fit obtained from the first and the second methods are (-1.08 ± 0.51)
times 10-10 mm-1 m deg-1 and (-3.30
± 1.14) times 10-10 mm-1 m deg}-1,
respectively. Our results agree with the previous studies and confirm
the hemispheric rule.
Title: Useful Aspects of Chromospheric Magnetic Field Data
Authors: Sakurai, T.; Choudhary, D. P.; Venkatakrishnan, P.
Bibcode: 2002stma.conf...37S
Altcode:
No abstract at ADS
Title: JOSO national report 2000-2001 - Japan
Authors: Sakurai, T.
Bibcode: 2002joso.book...78S
Altcode:
Solar research in Japan is conducted at national laboratories (National
Astronomical Observatory (NAOJ), Institute of Space and Astronautical
Science (ISAS), Communications Research Laboratory (CRL)), universities,
and at an inter-university institute (Solar-Terrestrial Environment
Laboratory (STELAB) of Nagoya University).
Title: Observational studies of the solar cycle at the National
Astronomical Observatory of Japan
Authors: Sakurai, T.; Suematsu, Y.
Bibcode: 2002AdSpR..29.1565S
Altcode:
Instrumentation for observational studies of the solar activity cycle,
using ground-based optical telescopes at the National Astronomical
Observatory of Japan, is described. A newly-built 10-cm telescope for
sunspot observations is explained in detail. A future instrumentation
plan for long-term studies of the sun, called the Solar Cycle Telescope,
is presented. This will be composed of a 30-cm Makustov telescope
equipped with an infrared Stokes polarimeter, and a 15-cm refractor
for precise Doppler-shift measurements.
Title: Magnetic Helicity Injection, Free Energy Loading, and Solar
Flares
Authors: Sakurai, T.; Yokoyama, T.; Kusano, K.; Maeshiro, T.
Bibcode: 2002aprm.conf..459S
Altcode:
We have developed a new scheme which can evaluate the influxes of
magnetic helicity and magnetic energy through the photosphere based
on magnetograph observations. In order to derive the helicity and
energy fluxes, first the velocity tangential to the solar surface
is constructed by applying a correlation tracking technique on the
magnetograms, and secondly the velocity component across the photosphere
is derived from the induction equation. Through this procedure, we
can determine the contributions to the helicity and energy fluxes
coming from the shear motion and the flux emergence separately. The
application of this method to actual magnetograph data of NOAA region
8100 in 1997 revealed that the contributions to the helicity injection
from the shear motion and the flux emergence were similar in magnitude,
but they tend to have opposite signs. Interestingly, a series of flares
occurred after the helicity injection showed the change in its sign.
Title: Solar cycle variation of near-sun sky brightness observed
with coronagraphs
Authors: Sakurai, T.; Rusin, V.; Minarovjech, M.
Bibcode: 2002cosp...34E2075S
Altcode: 2002cosp.meetE2075S
When the intensity of emission lines in the solar corona is measured
with a coronagraph, the background sky brightness caused by aerosol in
the earth atmosphere is also measured. Observational records of sky
brightnes s at 50" above solar limb have been accumulated for many
years at coronagraph stations. The data obtained at Norikura, Japan,
during the period of 1951-1997 show an eleven-year periodicity of
solar cycle. This suggests that the density and/or size distributions
of aerosol particles might be modulated by solar activity (possibly
by UV radiation). Similar data obtained at Lomnicky Stit, Slovakia, in
the period of 1980-2001 do not show this eleven-year periodicity. This
indicates that the solar effects on aero sol properties also reflect
local atmospheric conditions.
Title: High-cadence H-alpha imaging of solar flares
Authors: Hanaoka, Y.; Sakurai, T.; Noguchi, M.; Ichimoto, K.
Bibcode: 2002cosp...34E2277H
Altcode: 2002cosp.meetE2277H
The H-alpha observation is a powerful tool to study the high-energy
aspects of the solar flares. Spiky brightenings of flare kernels at
H-alpha center reflect a rapid fluctuation in particle accelerations;
redshifts of the H-alpha line are caused by chromospheric evaporation;
polarization of H-alpha emission might be evidence of accelerated
protons. Due to the impulsive, rapidly changing nature of the
flares, highcadence image acquisition is required for the H-alpha
observations. We are developing a new digital imaging system for the
H-alpha imager of the Flare Telescope at Mitaka, NAOJ, to carry out
high-cadence observation of solar flares in the H-alpha line. The
problem in the high-cadence observations has been the handling of
huge amount of data produced by such a type of observations, but
the recent advance of the computer technology enables us to handle
vast data with a small computer. We have already started the regular
observation of high-cadence imaging at the H-alpha center with a time
resolution of 1 sec in July 2001 (now it is 0.5 sec), and since then,
we have collaborated with the Yohkoh satellite and RHESSI. Now, the
development of a polarimeter/Dopplermeter system, which will work
in parallel with the H-alpha center imagings, is under way. In the
conference, we will present the system and some observational results.
Title: Annihilation of magnetic helicity: a new model of solar flare
onset mechanism
Authors: Kusano, K.; Yokoyama, T.; Maeshiro, T.; Sakurai, T.
Bibcode: 2002cosp...34E1888K
Altcode: 2002cosp.meetE1888K
Although various studies suggested that magnetic helicity is
related to solar flares, the onset mechanism of flares has not
yet been understood. Recently, we developed a new methodology to
measure the magnetic helicity flux into the solar corona based on
the vector magnetograph observation, and found that flares tend to
occur when the magnetic helicity whose sign is opposite to that of
the preloaded helicity is injected into the solar corona. It implies
that the co-existence of positive and negative magnetic helicities
is necessary for solar flares unlike any previous theories. Numerical
simulations clearly indicate that the injection of reversed helicity
due to the photospheric motion can lead to explosive energy liberation
at the helicity inversion layer. The explosive process is triggered
by nonlinear instability, which can grow faster than the exponential
function of time. Based on the results, we propose that solar flares
are caused by the annihilation of magnetic helicity, which converts
oppositely-sheared fields into the potential field. The new model is
consistent with various observations.
Title: Multiple arcs and Pc5 pulsations in the postnoon sector:
an inter- hemispheric conjugate observation
Authors: Yang, H.; Moen, J.; Sato, N.; Hu, H.; Liu, R.; Kikuchi, M.;
Zhou, X.; Sakurai, T.; Makita, K.
Bibcode: 2002cosp...34E1019Y
Altcode: 2002cosp.meetE1019Y
Massive statistics with satellite observations shown that there exists
an auroral occurrence peak around 1500MLT, which has thus been named
as '15MLT hot spot'. Chinese Zhongshan Station (69.37°S, 76.38°E,
invariant latitude -74.49° in Antarctica locates on the passage of this
'hot spot' region and its magnetically conjugate point falls around
Svalbard in the northern hemisphere. All sky TV observations there have
confirmed this auroral peak and found that m ltiple arcu aurora is one
of well appearing auroral forms in this region, of which the occurring
mechanism is still in many debates. An inter-hemispheric study of a
multiple arc aurora event happened on 27 May, 1997, is presented in
this paper, where ground observations of Zhongshan are compared with
simultaneous ground observations of the IMAGE magnetometer network and
ones onboard satellites close its conjugate location. Pc5 pulsations in
IMAGE magnetometers and the Geotail satellite magnetometer observations
appeared simu ltaneously with multiple arc aurora in Zhongshan's all
sky TV observation in the postnoon sector. `Bright spots' auroral
activity in the POLAR UV images covering the northern hemisphere
can be mapped to the conjugate counterpart of the southern multiple
arc activity. Brightenings on multiple auroral arcs were found to
swap westward/noonward in the Zhongshan all sky image sequence. This
observation seems to offer a new fact favor the kinetic Alfven wave
hypothesis for the multiple arc formation, in which MHD surface waves
caused by intensive K-H instability are attributed to generate kinetic
Alfven waves which can propagate across field lines, activate arcs on
different field lines and form multiple arc aurora.
Title: Measurement of magnetic helicity flux into the solar corona
Authors: Kusano, K.; Maeshiro, T.; Yokoyama, T.; Sakurai, T.
Bibcode: 2002cosp...34E2235K
Altcode: 2002cosp.meetE2235K
We developed a new methodology which can determine magnetic helicity
flux as well as Poynting flux across the photosphere based on the
magnetograph observation. By applying this method, we studied the
injection mechanism of magnetic helicity and magnetic free energy into
the solar corona. In order to derive the helicity and energy fluxes,
first the velocity tangential to the solar surface is constructed by
applying a correlation tracking technique on the magnetic observation,
and secondly the velocity component across the photosphere is derived
from the condition that the magnetic evolution must be consistent with
the induction equation. Through this procedure, we can determine the
helicity and energy fluxes separately for the shear motion effect and
for the flux emerging effect. Based on this new method, we analyzed
magnetic helicity of nine active regions, which were observed from 1997
to 2001, using the data taken by SOHO/MDI and the vector magnetograph
at NAOJ/Tokyo. The results indicate a tendency that the photospheric
shear motion and the flux emerging process supply magnetic helicity
of the opposite sign in the active regions.
Title: A Flexible Prism used as an Image Stabilizer
Authors: Sakurai, T.; Noguchi, M.; Shinoda, K.; Tanaka, N.; Aoki,
K.; Hagino, M.; Mochizuki, H.
Bibcode: 2002SoPh..205..201S
Altcode:
An image stabilizer was constructed by making use of a flexible prism,
which is widely used as an anti-vibration mechanism in handheld video
cameras. The prism responds to frequencies up to 20 Hz, and can be
simply inserted into existing optics. The performance of the prism
has been confirmed by observing the Sun through it.
Title: A High-Speed Hα Camera for Solar Flare Observations
Authors: Hanaoka, Y.; Noguchi, M.; Ichimoto, K.; Sakurai, T.
Bibcode: 2002mwoc.conf..427H
Altcode:
A single 11-year solar activity cycle is going to be covered by the
Yohkoh observations that have started since 1991. In this paper
the changes of coronal structures over the solar activity cycle,
based upon the Yohkoh SXT observations, are reviewed, referring to
previous observations which lead to important concepts for the origin
of the solar activity and comparing with recent observations of the
photosphere and corona by the other satellites.
Title: The Point Spread Function of the Yohkoh Soft X-Ray Telescope
Authors: Shin, J.; Sakurai, T.
Bibcode: 2002mwoc.conf..421S
Altcode:
No abstract at ADS
Title: Spectroscopic Observation of Coronal Oscillations
Authors: Sakurai, T.; Ichimoto, K.; Raju, K. P.; Singh, J.
Bibcode: 2002mwoc.conf...25S
Altcode:
No abstract at ADS
Title: Magnetic helicity and flare activity
Authors: Sakurai, T.; Hagino, M.
Bibcode: 2002cosp...34E2093S
Altcode: 2002cosp.meetE2093S
We studied two active regions appeared in 2001 (NOAA 9415 and 9661),
both of which have produced X-class flares. Time evolutions of magnetic
flux and magnetic helicity were derived based on vector magnetograph
observations. These regions obey the so-called hemispheric rule
of magnetic helicity, namely regions in the northern (southern)
hemisphere tend to have negative (positive) helicity. The magnetic
helicity showed slow evolution but did not show abrupt changes at
the time of flares. Flare activity was correlated with changes in the
distributions of magnetic helicity that were driven by horizontal flows.
Title: Helicity Injection into the Solar Corona
Authors: Kusano, K.; Maeshiro, T.; Yokoyama, T.; Sakurai, T.
Bibcode: 2002mwoc.conf..151K
Altcode:
The aims of this paper are to consider the relation between the
MHD stability and the magnetic shear in solar corona, as well as
to develop the methodology to measure the helicity injection from
magnetogram observations. We first numerically analyzed the resistive
MHD stability of the magnetic arcades which have a variety of magnetic
shear distribution. As a result of that, it is found that, while the
total magnetic helicity is conserved, the typical structure of the most
unstable mode is changed from the down-stream type to the up-stream
type as the magnetic shear is localized in the center of the arcade. It
implies the possibility that the filament eruption could be triggered
by the localization of the magnetic shear. Secondly, we developed
the methodology to measure the magnetic helicity injection due to the
horizontal motion based on the magnetogram observation. The helicity
injection can be calculated by dH/dt = int vec Vcdot vecA Bn
dS, where vec V, vec A, and Bn are the horizontal velocity,
the vector potential, and the magnetic field normal on the solar
surface. Using the correlation tracking method of the magnetic field,
the velocity vec V is constructed. The helicity injection in the
active region NOAA 8100 from 1997 Nov. 2 to 3 is analyzed using the
magnetic data observed by SOHO/MDI. It indicates that the evolution
of the helicity injection has a certain correlation with the X-ray
activity of the solar corona.
Title: Relation between magnetic helicity injection and flare
activities
Authors: Yokoyama, T.; Kusano, K.; Maeshiro, T.; Sakurai, T.
Bibcode: 2002cosp...34E2324Y
Altcode: 2002cosp.meetE2324Y
Recent studies suggest that the magnetic helicity is a good indecater
of the stored magnetic free energy in an active region. This stored
energy is released as flares, but the physical relationship between the
helicity and the flares is still unknown. In this paper, we show our
analysis of the magnetic helicity/energy injection into the several
active regions, including NOAA 8100 which is known as a source of
many coronal activities such as flares and coronal mass ejections. To
measure the helicity/energy injection, we used our new method which
can derive the injections not only by the shear motion but also by the
emerging motion based on the magnetograph observations. The data sets
we used were taken by SOHO/MDI and by the Solar Flare Telescope (SFT)
at NAOJ in Mitaka, Tokyo. In AR 8100, Eleven major flares greater than
GOES X-ray class C8.0 were observed in this region. Five of the events
were concentrated in the eastern part of the active region where strong
shear motion was seen. It is found that these five events coccured from
20:00 UT on Nov. 3 to 4:00 UT on Nov. 4 about half a day after the
helicity injection rate (sum of the emergence and shear injections)
changed its sign from positive to negative at 8:00 UT on Nov 3. This
tendency is also true for other three events which occurred in the
western part, but the helicity injection was mainly due to the emerging
flux activities. We found that this relationship between the helicity
injection and the flares was also true for the other analysed active
regions. This result suggests that the sign-reversal of the helicity
injection rate is a key signature of flare activities.
Title: Electron Heat Flux in Pressure Balance Structures Observed
by Ulysses
Authors: Yamauchi, Y.; Suess, S. T.; Sakurai, T.
Bibcode: 2001AGUFMSH32A0733Y
Altcode:
Pressure balance structures (PBSs) are a common feature in the
high-latitude solar wind near solar minimum. From previous studies,
PBSs are believed to be remnants of coronal plumes and be related to
network activity such as magnetic reconnection in the photosphere. We
investigated the magnetic structures of the PBSs, applying a minimum
variance analysis to Ulysses/Magnetometer data. At 2001 AGU Spring
meeting, we reported that PBSs have structures like current sheets or
plasmoids, and suggested that they are associated with network activity
at the base of polar plumes. In this paper, we have analyzed high-energy
electron data at Ulysses/SWOOPS to see whether bi-directional electron
flow exists and confirm the conclusions more precisely. As a result,
although most events show a typical flux directed away from the Sun,
some PBSs show bi-directional electron flux and others show an isotropic
distribution of electron pitch angles. Therefore, we can conclude that
plasmoids are flowing away from the Sun, changing their flow direction
dynamically in a way not caused by Alfvén waves. From this, we may
conclude that PBSs are generated due to network activity at the base
of polar plumes and their magnetic structures are like current sheets
or plasmoids.
Title: Sunspot Magnetic Fields Observed with a Large-Format Infrared
Array
Authors: Sakurai, Takashi; Yanagisawa, Kenshi; Kobiki, Toshihiko;
Kasahara, Shouichi; Nakakubo, Kayoko
Bibcode: 2001PASJ...53..923S
Altcode:
Spectro-polarimetric observations of sunspots were carried out by using
a PtSi near-infrared camera (KONIC) of Kiso Observatory, The University
of Tokyo, in the period of 1999 November 3-5. The camera was attached
to a spectrograph of the Domeless Solar Telescope at Hida Observatory,
Kyoto University. We observed the absorption lines of FeI at 15648.5Å
(Lande factor gL = 1.53) and 15652.9Å (gL eff
= 3), and derived the distribution of the magnetic field strengths
and inclination angles across the sunspots. The ratio of the Zeeman
splitting of the two lines was derived to be 0.39 +/- 0.01, while
the ratio of the Lande factors is 0.51. Our data indicates that the
effective Lande factor of the 15652.9Å line is 1.17 +/- 0.03..
Title: Multi-Wavelength Observation of the 3n/x3.3 Flare of 28
November 1998
Authors: You, Jianqi; Li, Hui; Fan, Zhongyu; Sakurai, Takashi
Bibcode: 2001SoPh..203..107Y
Altcode:
The 3N/X3.3 flare of 28 November 1998 was observed in multiple
wavelength simultaneously. The available data include Hα images,
spectra in He i 1083 nm and Ca ii 854.2 nm from Purple Mountain
Observatory (PMO), soft X-ray (SXR) and hard X-ray (HXR) images and
flux from Yohkoh. Morphological relationship investigation and spectral
analysis of these data show: (1) The sudden brightening at loop top
above the active region and the steep increase of SXR flux before
flare onset suggest that the corona there had already been heated to
some extent in the preflare phase. (2) The scales of the Ca ii 854.2
nm emission areas are very similar to those of the Hα line, but the
emission profiles look like those of the Ca ii K line. Most of the He i
1083 nm emissions exist in the bright Hα kernels and can last to the
decay phase. (3) Flare spectra show that line shift and asymmetry are
very common in this flare not only in the impulsive phase but also in
the decay phase. The difference in the line shifts or asymmetry between
Ca ii 854.2 nm and He i 1083 nm, as well as the difference between
the line center and wings of Ca ii 854.2 nm imply the existence of a
velocity gradient in the line-of-sight direction. (4) Post-flare loops
with very deep absorption (≈ 70%) and very-high-velocity red shifts
(30-90 km s−1) were observed in He i 1083 nm during the
decay phase. However, only a slight dip can be found in the Ca ii
854.2 nm profile.
Title: Chromospheric Magnetic Field of Solar Active Regions
Authors: Choudhary, Debi Prasad; Sakurai, Takashi; Venkatakrishnan, P.
Bibcode: 2001ApJ...560..439C
Altcode:
The three-dimensional magnetic field structure of 137 solar active
regions is studied by comparing the observed and computed chromospheric
magnetograms. The model chromospheric field is obtained by extrapolating
the observed photospheric field into the chromosphere with a potential
(current-free) magnetic-field model in Cartesian geometry. The
best correlation between the observed and the model chromospheric
magnetograms is found at the height of 800 km, which also corresponds
to the height of the line formation for Ca II 854.2 nm. In the weak
field range, within +/-300 G, most of the observed field is close to
the potential field. However, departures of about 50 G are observed
in a few active regions. For field values greater than +/-500 G, the
observed field does not always match the model. Whereas a part of this
could be due to the magnetogram calibration, it might also originate
from the ``nonpotentiality'' of the chromospheric field. In the case
of long-lived active regions, which make multiple disk passages, the
strong-field nonpotentiality is observed during their initial phase,
and converges to a potential field configuration later.
Title: On the disappearance of H-alpha filaments and soft X-ray
enhancements as seen from Yohkoh SXT
Authors: Singh, Jagdev; Sakurai, T.; Ichimoto, K.
Bibcode: 2001BASI...29..193S
Altcode:
We have studied H-alpha images of the sun taken at Kodaikanal and
Mitaka observatories and soft X-ray images obtained by SXT onboard
Yohkoh for the years 1993 and 94. From events of disappearing quiescent
H-alpha filaments and associated soft X-ray brightenings we find that
in some cases H-alpha filament appears broken and diffused about a day
earlier than beginning of the soft X-ray transient brightening which
lasted for 7 - 10 hours. Further, enhancements began along filament
channels and then moved along arches which were inclined to the
filament direction. From these observations we postulate that heating
of plasma in the filament begins when it appears broken and less dark
and continues till it triggers some physical process, responsible to
begin soft X-ray enhancement. It is not possible to say that if the
H-alpha filament disappears earlier than the beginning of soft X-ray
enhancement or vice versa as there is time difference between H-alpha
and soft X-ray observations.
Title: Poynting vectors of Pc 5 pulsations observed by the GEOTAIL
satellite in the dayside outer magnetosphere
Authors: Sakurai, T.; Tonegawa, Y.; Shinkai, Y.; Yumoto, K.; Kokubun,
S.; Tsuruda, K.; Mukai, T.
Bibcode: 2001EP&S...53..843S
Altcode:
Poynting vectors of Pc 5 pulsations observed in the dayside outer
magnetosphere are examined with the magnetic and electric field data
simultaneously observed by the GEOTAIL satellite. Poynting energy can
be estimated as about 10-8-10-6 W/m2
in both directions across and along the magnetic field-line. However,
the energy propagating across the magnetic field-line is larger
than the energy propagating along the magnetic field-line. From this
result we can estimate the Pc 5 wave energy flowing into the inner
magnetosphere during an hour as 1010-1013
J, which is one or two orders less than the substorm energy
(about 1014-1015 J). However, by taking into
account of the continuous activation of Pc 5 pulsations in the outer
magnetosphere, Pc 5 waves should play an important role for energetics
in the magnetosphere.
Title: Topology of Magnetic Field and Coronal Heating in Solar
Active Regions
Authors: Wang, Huaning; Yan, Yihua; Sakurai, Takashi
Bibcode: 2001SoPh..201..323W
Altcode:
Force-free magnetic fields can be computed by making use of a new
numerical technique, in which the fields are represented by a boundary
integral equation based on a specific Green's function. Vector magnetic
fields observed on the photospheric surface can be taken as the
boundary conditions of this equation. In this numerical computation,
the following two points are emphasized: (1) A new method for data
reduction is proposed, for removing uncertainties in boundary data
and determining the parameter in this Green's function, which is
important for solving the boundary integral equation. In this method,
the transverse components of the observed boundary field are calibrated
with a linear force-free field model without changing their azimuth. (2)
The computed 3-D fields satisfy the divergence-free and force-free
conditions with high precision. The alignment of these field lines
is mostly in agreement with structures in Hα and Yohkoh soft X-ray
images. Since the boundary data are calibrated with a linear force-free
field model, the computed 3-D magnetic field can be regarded as a
quasi-linear force-free field approximation. The reconstruction of 3-D
magnetic field in active region NOAA 7321 was taken as an example to
quantitatively exhibit the capability of our new numerical technique.
Title: Determination of the magnetic loop configuration of a solar
flare
Authors: Hanaoka, Y.; Sakurai, T.
Bibcode: 2001EP&S...53..593H
Altcode:
Observationally derived three-dimensional magnetic loop structures
of solar flares tell us what type of magnetic configurations is
actually responsible for the reconnection/energy release in solar
flares. So far, the loop configuration has been discussed on a rather
schematic basis, because of insufficient data coverage. However,
now we have data obtained with various advanced instruments, which
are essentially important to define the three-dimensional magnetic
configuration of flare loops. In this paper, we briefly present how
the loop configuration of the solar flares can be determined based on
recent advanced observations. We focus on the flares caused by emerging
flux activity in region NOAA 8100, which appeared in October-November
1997. We present here the presumed quadruple structure of the magnetic
loops of the analyzed flares in three dimensions, and suggest that
the emergence of small loops can give rise to large-scale phenomena
like jets and CMEs.
Title: Magnetic Field Structure of Pressure Balanced Structures from
Ulysses High Latitudes Observations
Authors: Yamauchi, Y.; Suess, S. T.; Sakurai, T.
Bibcode: 2001AGUSM..SH41B14Y
Altcode:
Ulysses observations showed that pressure balance structures (PBSs)
are a common feature in the high-latitude solar winds near the solar
minimum. On the other hand, coronal plumes are common in polar coronal
holes near the solar minimum. It is therefore considered that the PBSs
would be remnants of plumes. Several detailed studies of the PBSs have
been made from Ulysses/SWOOPS observations, but study of their magnetic
structures has not yet been done. The study of the magnetic structure
is important because previous observations and theoretical models of
plumes indicate that they are related to the network activity such
as magnetic reconnection on the photosphere. We have investigated the
magnetic structures of the PBSs with Ulysses magnetometer and SWOOPS
data. We have found that magnetic reversals in radial magnetic field
take place while the spacecraft passes through most of the PBSs. These
magnetic reversals have been interpreted as large amplitude Alfvénic
fluctuations but our results suggest that Ulysses is also traversing
current sheets or plasmoids associated with network activity at the
base of plumes.
Title: Interpolation of One- and Two-Dimensional Images with Pixelwise
Photon Number Conservation
Authors: Sakurai, Takashi; Shin, Junho
Bibcode: 2001PASJ...53..361S
Altcode:
The interpolation of one- and two-dimensional images is considered. The
usual linear or cubic spline interpolations try to connect given
data points. On the other hand, image data give area integrals of
the radiation intensity (i.e., photon counts) over pixels. Based
on variational principles, we have developed schemes of quadratic
and fourth-order polynomial interpolation which reproduce the given
pixelwise photon counts exactly. These methods were tested on some
simple examples and showed satisfactory results.
Title: Spatial Structure of Simple Spiky Bursts at
Microwave/Millimeter Wavelengths
Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Sakurai, T.;
Grechnev, V. V.
Bibcode: 2001ApJ...547.1090K
Altcode:
We present the results of a study of spatial structure of sources
of microwave and millimeter bursts with simple spiky time profiles
at 17 and 34 GHz, similar to those found to be common at 3 mm
wavelength. These bursts are of short duration, with fast 2-4 s rise
time to peak, followed by a rapid exponential decay. When mapped at
high spatial resolution with the Nobeyama Radio Heliograph (NoRH), the
radio images show direct evidence that the radio sources are compact
bipolar loops: source sizes are less than 5" and three of the five
events studied show closely spaced oppositely polarized components in
the circular polarization maps. All five events are located directly
over magnetic neutral lines in the photosphere. The soft X-ray behavior
is not entirely consistent with the Neupert effect in these events,
since all five events show a rise in the soft X-ray flux well before
any nonthermal electrons are present in the corona and the ratio of
peak soft X-ray flux to peak radio 17 GHz flux may vary by many orders
of magnitude from one event to the next. The abrupt time profiles
of these events and their physical properties are consistent with a
single-loop scenario in which magnetic energy release and acceleration
of nonthermal electrons are confined to a compact localized region.
Title: Magnetic Field Reconstruction in a Solar Active Region
Authors: Wang, H.; Yan, Y.; Sakurai, T.
Bibcode: 2001IAUS..203..328W
Altcode:
It is widely believed that coronal structures delineate magnetic
field lines anchored on the photospheric surface. Supposing coronal
magnetic fields are in a force-free state from the chromosphere to
the height of two solar radii, we reconstruct 3D force-free magnetic
fields by making use of a new numerical technique, in which the fields
are represented by a boundary integral equation based on a specific
Green's function. Vector magnetic fields observed on the photospheric
surface can be taken as the boundary conditions of this equation. In
this numerical computation, the following two points are emphasized: (1)
A new method for data reduction is proposed, for removing uncertainties
in boundary data and determining the parameter in this Green's function,
which is important for solving the boundary integral equation. In this
method, the transverse components of the observed boundary field are
calibrated with a linear force-free field model without changing their
azimuth. (2) The computed 3-D fields satisfy the divergence-free and
force-free conditions with high precision. The alignment of these field
lines are in agreement with structures in Hα and Yohkoh
soft X-ray images. Since the boundary data are calibrated with a linear
force-free field model, the computed 3D magnetic filed can be regarded
as a quasi-linear force-free field approximation. The reconstruction
of 3D magnetic field in active region NOAA 7321 was taken as an example
to quantitatively exhibit the capability of our new numerical technique.
Title: Stokes Profile Inversion without Resort to Specific Atmospheric
Models
Authors: Sakurai, T.
Bibcode: 2001ASPC..236..535S
Altcode: 2001aspt.conf..535S
No abstract at ADS
Title: A Spectroscopic Study of the Solar Corona from Norikura and
SOHO data
Authors: Raju, K. P.; Sakurai, T.; Ichimoto, K.
Bibcode: 2001IAUS..203..488R
Altcode:
We report the results from a spectroscopic study of the solar corona,
wherein, we examine some of the current problems in the corona, such
as the plume-interplume differences in coronal holes, coronal loops in
active regions and wave propagation in the corona. The distribution of
emission line intensities, Doppler velocities and line widths in the
corona were obtained from the spectroscopic observations made in the
coronal emission lines from Norikura Coronagraph. The coronal images
in Fe IX,X 171 Å and Fe XII 195 Å from SOHO EIT were used to get
the temperature map of the corona. Combining both, the nonthermal
velocities in the coronal region are obtained without the usual
assumption of a uniform ion temperature. Following results are obtained
from the study. (1) The Doppler velocities show excess blue-shifts
over red-shifts in coronal holes with differences in plume-interplume
regions. (2) The nonthermal velocities show a pronounced difference
between the coronal hole and closed-field regions which points to the
important role of nonthermal broadening mechanism in the acceleration
of fast solar wind. (3) The nonthermal velocities are larger by about
20% at the interplume regions as compared to plumes. This supports
the view that the interplume regions are the source regions of the
fast solar wind. (4) The preliminary analysis of the time sequence
data shows signatures of wave propagation in the corona.
Title: Preliminary study of the evolution of solar magnetic structures
and photospheric horizontal velocity fields
Authors: UeNo, S.; Kitai, R.; Ichimoto, K.; Sakurai, T.; Soltau, D.;
Brandt, P. N.
Bibcode: 2001AdSpR..26.1793U
Altcode:
In October 1997, we made a coordinated observation of the solar
photosphere and chromosphere at Hida observatory (Kyoto university,
Japan) and at Teide observatory (Tenerife) over a 10 day. We obtained
imaging data series continuously during 6 hr 45 min in G-band (4308 Å)
observed with the Domeless Solar Telescope (DST) at Hida on 24th October
(effective FOV; 96″×99″). Additionally, in this observation,
we simultaneously observed image series of the chromosphere during
the latter 4 hr 10 min in H α line center and H α ±0.6 Å. From
these data set, we could detect that emerging flux tubes crossed
the photosphere to the chromosphere and that 'convective collapse'
phenomena appeared at the stage of the spot formation. Moreover,
we confirmed that the lifetime of mesogranulation was about 4000 sec
(70 min) from the temporal evolution of velocity patterns. We show
here only a summary of these observations.
Title: Propagation and Resonance Characteristics of Pc 5 Waves
Revealed in Coordinated Space and Ground Observations (invited)
Authors: Tonegawa, Y.; Sakurai, T.; Sato, N.
Bibcode: 2001aprs.conf..221T
Altcode:
No abstract at ADS
Title: Topology of Magnetic Field and Coronal Heating in Solar Active
Regions - II. The Role of Quasi-Separatrix Layers
Authors: Wang, Huaning; Yan, Yihua; Sakurai, Takashi; Zhang, Mei
Bibcode: 2000SoPh..197..263W
Altcode:
The photospheric vector magnetic fields, Hα and soft X-ray images of
AR 7321 were simultaneously observed with the Solar Flare Telescope
at Mitaka and the Soft X-ray Telescope of Yohkoh on October 26,
1992, when there was no important activity in this region. Taking
the observed photospheric vector magnetic fields as the boundary
condition, 3D magnetic fields above the photosphere were computed with
a new numerical technique. Then quasi-separatrix layers (QSLs), i.e.,
regions where 3D magnetic reconnection takes place, were determined
in the computed 3D magnetic fields. Since Yohkoh data and Mitaka
data were obtained in well-arranged time sequences during the day,
the evolution of 3D fields, Hα features and soft X-ray features in
this region can be studied in detail. Through a comparison among the 3D
magnetic fields, Hα features and soft X-ray features, the following
results have been obtained: (a) Hα plages are associated with the
portions of QSLs in the chromosphere; (b) diffuse coronal features
(DCFs) and bright coronal features (BCFs) are morphologically confined
by the coronal linkage of the field lines related to the QSLs; (c)
BCFs are associated with a part of the magnetic field lines related to
the QSLs. These results suggest that as the likely places where energy
release may occur by 3D magnetic reconnection, QSLs play an important
role in the chromospheric and coronal heating in this active region.
Title: Nonthermal Flare Emission from MEV-Energy Electrons at 17,
34, and 86 GHZ
Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Sakurai, T.
Bibcode: 2000ApJ...545.1084K
Altcode:
We present analyses of two solar flares observed with high spatial
resolution at 86 GHz with the BIMA millimeter-wavelength telescope
and at 17 and 34 GHz with the Nobeyama radioheliograph. The flares
were observed on 1998 November 24 and 1999 May 1. At millimeter
wavelengths these are impulsive events, and therefore they must be
produced by MeV-energy electrons. The present study using simultaneous
observations of two flares at 86, 34, and 17 GHz provides an excellent
opportunity to study high-energy electrons with high spatial resolution
observations at three optically thin frequencies. The morphology of
millimeter emission can reveal both the properties of the MeV-energy
electrons and the nature of the coronal magnetic field lines where they
radiate. One of the two events we present is the first clear case of a
λ=3 mm source in which both footpoints of a loop are detected. In the
second event the polarization image at 17 GHz also suggests a bipolar
or looplike morphology. Such morphological observations can be used
to constrain the nature of the magnetic field in the solar corona.
Title: The Physical Conditions in a Polar Coronal Hole and Nearby
Regions from Norikura and SOHO Observations
Authors: Raju, K. P.; Sakurai, T.; Ichimoto, K.; Singh, Jagdev
Bibcode: 2000ApJ...543.1044R
Altcode:
The distribution of emission-line intensities, Doppler velocities, and
line widths in a polar coronal hole and nearby regions are obtained
from the spectroscopic observations carried out on 1998 November 3
at the Norikura Solar Observatory, Japan. The coronal red line [Fe X]
λ6374 that is prominent at coronal hole temperatures is used for the
study. The coronal images in Fe IX and Fe X 171 Å and Fe XII 195 Å
from the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and
Heliospheric Observatory (SOHO) are used to get the temperature map of
the corona at the time of observation. Combining both, we have obtained
the nonthermal velocities in the region without the usual assumption
of a uniform ion temperature. Several plume structures are identified
within the coronal hole, and it is found that line widths are smaller
in plumes than in the interplume regions, which is also reported from
recent SOHO observations. The line-of-sight Doppler velocities in
the coronal hole are larger than those in the quiet region, probably
because of the excess outflow in the coronal hole. A rough negative
correlation between intensity and Doppler velocity, similar to that
between intensity and line width, is observed in the coronal hole. The
typical nonthermal velocity in coronal holes is 24 km s-1
while that in quiet regions is 15 km s-1. The enhanced
nonthermal velocity in the coronal hole is suggestive of the important
role of the nonthermal broadening mechanism in the acceleration of fast
solar wind. Also, the nonthermal velocities are larger (up to 27%) at
the interplume regions as compared to plumes. The findings generally
support the prevailing view that the interplume regions are the source
regions of the fast solar wind.
Title: Three Dimensional Numerical Study of Solar Coronal Magnetic
Field Based on the Magnetograph Observations
Authors: Kusano, Kanya; Maeshiro, Tomohiro; Yokoyama, Takaaki; Sakurai,
Takashi; Kageyama, Akira
Bibcode: 2000APS..DPPYP1021K
Altcode:
Magnetohydrodynamic (MHD) properties of active regions in solar corona
is investigated by combining the numerical analysis technique and the
vector magnetograph observations. First, we numerically construct the
potential magnetic field of several major active regions, and compare
them with the tangential component of the magnetic field observed by
Solar Flare Telescope in National Astronomical Observatory of Japan. As
a result, we found that the magnetic free energy, which is formed by
the the gap between the observed field and the potential field, as well
as the current helicity, which is produced by the electric current and
the magnetic field across the photosphere, is reduced in some big flare
events. It suggests that the solar flares are processes to reduce the
magnetic helicity contained in the coronal magnetic field. Secondly,
we develop the numerical model to construct the three dimensional
(3D) nonlinear equilibria in active regions using the vector magnetic
field observed by magnetograph. Furthermore, the MHD stability of
the practical solar corona will be discussed based on the numerical
calculations using the observed data.
Title: Comments on I. Okamoto's Article 'Ending of One Paradigm for
Astrophysical Jets'
Authors: Sakurai, Takashi
Bibcode: 2000AstHe..93..447S
Altcode:
I. Okamoto claimed in his recent article in the Astronomical Herald that
the widely accepted view of the collimation of outflows from rotating
magnetized objects is a misconception. Here the author argues in favour
of the magnetized collimation, elucidating its physical mechanism. The
current sheet in the solution which Okamoto rejected as artificial
will form, as a witness in the solar wind.
Title: New Boundary Integral Equation Representation for Finite
Energy Force-Free Magnetic Fields in Open Space above the Sun
Authors: Yan, Yihua; Sakurai, Takashi
Bibcode: 2000SoPh..195...89Y
Altcode:
A boundary integral equation to describe a force-free magnetic field
with finite energy content in the open space above the solar surface
is found. This is a new representation for a 3-D nonlinear force-free
field in terms of the boundary field and its normal gradient at the
boundary. Therefore the magnetic field observed on the solar surface
can be incorporated into the formulation directly and a standard
numerical technique, the boundary element method, can be applied to
solve the field. A numerical test case demonstrates the power of the
method by recovering the analytical solution to the desired accuracy
and its application to practical solar magnetic field problems is
straightforward and promising.
Title: Magnetic Field Evolution Leading to Solar Flares II. Cases
with High Magnetic Shear and Flare-Related Shear Change
Authors: Li, Hui; Sakurai, Takashi; Ichimoto, Kiyoshi; UeNo, Satoru
Bibcode: 2000PASJ...52..483L
Altcode:
Following Paper I in which we considered five solar flares, we have
selected another three solar flares greater than GOES X-ray class
M/Hα importance 1. The three active regions discussed here are
characterized by high magnetic shear. We investigated the spatial
relationships among Hα flare ribbons, soft X-ray (SXR) flare
loops, and magnetic configurations for the three flares produced
in these active regions. Our results show that only one of these
three flares satisfies the sufficient conditions for a flare to
occur proposed by Hagyard (1990, AAA 052.075.047). We also discuss
the magnetic shear changes around the flaring time only along the
neutral lines associated with the studied flares and over the whole
flaring area. The flare-related changes on the neutral line are small
(2deg-4deg) and the association of these changes
with the flares is not conclusive. The average shear in the flaring
areas of the flares associated with high shear decreases significantly
after the flares and it may be a better parameter to characterize the
flare-related shear changes in such cases.
Title: Cyclical Variation of the Quiet Corona and Coronal Holes
Authors: Sakurai, T.
Bibcode: 2000JApA...21..389S
Altcode:
No abstract at ADS
Title: Magnetic Field Evolution Leading to Solar Flares I. Cases
with Low Magnetic Shear and Flux Emergence
Authors: Li, Hui; Sakurai, Takashi; Ichimoto, Kiyoshi; UeNo, Satoru
Bibcode: 2000PASJ...52..465L
Altcode:
We picked up five solar flares larger than GOES X-ray class M/Hα
importance 1, and studied the magnetic evolution of the corresponding
active regions. In particular, we investigated the spatial relationships
among Hα flare ribbons, soft X-ray (SXR) flare loops, and magnetic
configurations. Our results show that all of these flares are not
associated with high magnetic shear, but with emerging flux, indicating
that flux emergence is a common phenomenon in this kind of solar
flare with low magnetic shear. The maximum values of the transverse
magnetic fields on the neutral lines responsible for these flares are
300-800 G. All of these flares but one have multiple (more than two) Hα
ribbons, and almost all of these ribbons are located at the footpoints
of SXR flare loops. The flare-related shear changes, which can be both
positive and negative, may be determined by the balance of the energy
carried by the emerging flux and that required to power the flare.
Title: Hα and Soft X-Ray Brightening Events Caused by Emerging Flux
Authors: Tang, Y. H.; Li, Y. N.; Fang, C.; Aulanier, G.; Schmieder,
B.; Demoulin, P.; Sakurai, T.
Bibcode: 2000ApJ...534..482T
Altcode:
By using Yohkoh soft X-ray images, vector magnetograms, and Hα
filtergrams, the brightening event that occurred on 1994 May 18 has
been studied in detail. It occurred in a nearly potential large-scale
magnetic configuration as shown by the comparison between the magnetic
extrapolation (linear force-free field) and the large-scale soft
X-ray loops. This event is related to the emergence of a new magnetic
flux of about 3×1020 Mx. The impulsive enhancement of
the emerging flux occurs about 20 minutes before the peaks of the
Hα and soft X-ray brightening and lasts for about 10 minutes. This
brightening may be a signature of reconnection taking place between the
preexisting nonpotential loops and the new emerging small loops. The
magnetic energy provided by reconnection is likely to be stored in
the nonpotential loops and the emerging flux as implied by the vector
magnetograms. By using the electron temperature and the electron density
of the brightening event derived from the analysis of the Yohkoh data,
an energy budget has been estimated. The result indicates that the
energy needed can be reasonably provided by magnetic reconnection.
Title: Soft X-ray flares and magnetic configuration in a solar active
region in February 1992
Authors: Zhang, H. Q.; Sakurai, T.; Shibata, K.; Shimojo, M.;
Kurokawa, H.
Bibcode: 2000A&A...357..725Z
Altcode:
In this paper, we examine the observational soft X-ray flares and
the relationship with photospheric vector magnetograms in the active
region (NOAA 7070). We analyze the soft X-ray flare on Feb. 24-25,
1992, especially the pre-flare and the relationship with the highly
sheared photospheric vector magnetic field near the photospheric
magnetic neutral line. We find that the initial reconnection of the
magnetic field in the flare on Feb. 24-25, 1992 probably occurs near
the magnetic neutral line in the lower atmosphere of the active region,
where the highly sheared magnetic flux erupts up and triggers the
reconnection of the large-scale magnetic field. The possible process
of the magnetic reconnection of the limb flare on Feb. 20-21, 1992
in this active region is proposed also based on the analogy with the
flare on Feb. 24-25 near the center of the solar disk.
Title: Microwave/Millimeter Wavelength Bursts with Simple Spiky
Time Profiles
Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Sakurai, T.
Bibcode: 2000SPD....31.0242K
Altcode: 2000BAAS...32..818K
We report the detection at 17 and 34 GHz of microwave and millimeter
bursts which have simple spiky time profiles similar to those found to
be common at λ = 3 mm. These bursts are of short duration, with fast
2 - 4 sec rise time to peak, followed by an exponential decay. These
bursts can be of any intensity, from 1 sfu to 10's of sfu; they are
very strongly polarized (> 50%), and they have similar properties
regardless of the nature of the active region in which the bursts
originate. The bursts seem to originate in compact sources which are
generally unresolved with 15" and 7" resolution of the Nobeyama Radio
Heliograph at 17 and 34 GHz respectively. We provide both direct and
indirect evidence that these compact sources are low-lying bipolar
loops. The direct evidence follows from the physical appearance of
the loop as well as from the bipolar nature of the loop. The indirect
evidence follows from the offset in position of the footpoint emission
in microwaves and hard X-rays, implying a compact asymmetric loop with
microwaves originating from the stronger magnetic field foot point
and the hard X-rays originating from the weaker field foot point.
Title: On the formation of a helmet streamer on January 24, 1992 at
the south-west limb
Authors: Singh, Jagdev; Sakurai, T.; Ichimoto, K.; Hiei, E.
Bibcode: 2000BASI...28...33S
Altcode:
We have analysed the soft X-ray images of the sun obtained with
the YOHKOH satellite, white light coronal images observed at Mauna
Loa and H-alpha pictures of the sun taken at Mitaka and Kodaikanal
to study helmet streamers. We find that heating of a filament and
subsequent brightening in X-rays, and eruption in the region lead to
the formation of the helmet streamer on January 24, 1992. In another
event of February 24, 1993, only the brightening and eruption-like
expansion of the brightening in soft X-rays lead to the formation of
the streamer. No H-alpha filament was seen in this region before and
after the event of brightening in soft X-rays and formation of streamer
on February 24, 1993. We, therefore, postulate that the plasma in the
streamer comes from the solar surface during the X-ray eruption, and
magnetic field of the region perhaps helps in containing the plasma
and the formation of the streamer.
Title: Brightening Event in Hα and Soft X-Ray on May 18, 1994
Authors: Tang, Y. H.; Li, Y. N.; Schmieder, B.; Aulanier, G.; Demoulin,
P.; Fang, C.; Sakurai, T.
Bibcode: 2000AdSpR..25.1829T
Altcode:
By using Yohkoh soft X-ray images, vector magnetograms and Hα
filtergrams, a brightening event that occurred on May 18, 1994 has
been studied in detail. It occurred in a nearly potential large-scale
magnetic configuration as shown by the comparisons between the magnetic
extrapolations (linear force-free field) and the large-scale soft
X-ray loops. This brightening event observed in Hα line and soft
X-ray seems to be related to new emerging magnetic flux with an
amount of photospheric magnetic flux of about 3× 1020
Mx. The emerging flux increases obviously about 20 minutes before the
Hα and soft X-ray brightening, and lasts for about 103
s. This brightening may be a signature of reconnection taking place
between the pre-existing loop and the new emerging small loops
Title: Nonthermal Flare Emission from MeV-Energy Electrons at 17,
34, and 86, GHz
Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Sakurai, T.
Bibcode: 2000ASPC..206..307K
Altcode: 2000hesp.conf..307K
No abstract at ADS
Title: Enhanced Coronal Heating and 3d Solar Magnetic Fields in
AR 7321
Authors: Wang, Huaning; Sakurai, Takashi; Yan, Yihua
Bibcode: 2000AdSpR..25.1769W
Altcode:
The photospheric vector magnetic fields, Ho and soft X-ray images of
AR 7321 were simultaneously observed with the Solar Flare Telescope at
Mitaka and the Soft X-ray Telescope of Yohkoh on 26 October 1992, when
there were no drastic activities in this region. Taking the observed
photospheric vector magnetic fields as the boundary condition, the
3D magnetic fields above the photosphere were computed with a new
extrapolation method. Through a comparison among 3D magnetic fields,
Hα features, and soft X-ray features, the following results have
been obtained: (a) Magnetic separatrices or quasi-separatrix layers
play an important role in chromospheric and coronal heating, but they
do not seem to be dominant in the enhanced coronal heating. (b) The
enhanced coronal heating might be due, not only to the magnetic shear,
but also to the emerging magnetic flux
Title: Restoration and Interpretation of Images from the Yohkoh Soft
X-Ray Telescope
Authors: Shin, J.; Sakurai, T.
Bibcode: 2000AdSpR..25.1765S
Altcode:
Due to the finite width of the point spread function (PSF) of the Yohkoh
Soft X-Ray Telescope (SXT) mirror, a certain amount of blurring effect
is inevitable in the observed soft X-ray images. Thus it is necessary
to subtract its effect from the observed images for both morphological
and photometric purposes. Because of undersampling due to a large pixel
size of the CCD camera of SXT, however, the application of general
algorithms to image restoration has so far been unsatisfactory for the
SXT images. Therefore, we have developed a new method for increasing
the sampling density of Yohkoh, SXT images under the condition of photon
number conservation in each pixel. The correction for the undersampling
effect was also necessary to obtain a good estimate of the PSF from
the ground experimental data. Based on this best-fit PSF, we apply
the maximum entropy method algorithm to the observed SXT images. The
restored images succeed in showing fine details in coronal loops,
but there still remains the problem of insufficient noise suppression
Title: Determination of the Point Spread Function of the Yohkoh Soft
X-Ray Telescope
Authors: Sakurai, T.; Shin, J.
Bibcode: 2000AdSpR..25.1761S
Altcode:
We re-evaluated the point spread function (PSF) of the Yohkoh Soft
X-Ray Telescope (SXT). The best-fit model to the ground experimental
data can be obtained by considering the undersampling effect at the
central area of the PSF. Our result shows that the core of the Moffat
function which models the PSF begins to be smoothly connected to the
scattering wing at the distance of about 15" from the peak. Although
the available experimental data are limited, first-order vector
perturbation theory helps us to consider the wavelength dependence
of the scattering wing of the PSF. The autocovariance function of
the SXT mirror surface roughness is expressed in our study by the
product of exponential function and the modified Bessel function. The
calculated angular distribution of the scattering component is compared
satisfactorily with the result of in-flight data analysis. The results
of the wavelength dependence of the PSF are discussed in detail
Title: Magnetic Field Configuration in Impulsive Solar Flares Revealed
with Yohkoh and Nobeyama Radioheliograph
Authors: Nishio, M.; Kosugi, T.; Yaji, K.; Nakajima, H.; Sakurai, T.
Bibcode: 2000AdSpR..25.1791N
Altcode:
The results of an analysis of 25 solar impulsive flares, observed
simultaneously with microwave and X-ray imaging instruments, are
presented. From the relative locations of microwave, hard X-ray, and
soft X-ray sources, we have found that, for fourteen out of the 25
cases, at least two loops are involved. Typical sizes of the two loops
are different from each other; one is typically <= 20" and the other
30"-80". In seven out of remaining thirteen cases, circular polarization
images in microwaves show the bipolar structure at the location of a HXR
source, indicating the existence of a small loop. These observations
suggest that in the majority of impulsive flare events a system of
two or more loops (or bipoles) interacts with each other, releasing
magnetic energy and producing energetic electrons. Detailed analysis
of two out of the 25 events supports this result
Title: Magnetic Separatrix and Coronal Loop Heating in an Active
Region
Authors: Sakurai, T.; Wang, H.
Bibcode: 1999spro.proc...77S
Altcode:
Although coronal heating is basically due to magnetic fields, it has
been unknown why some field lines show up as coronal loops and the
others do not. Based on the study of magnetic fields and coronal loops
in active region NOAA 7321, we propose that only a subset of field
lines constituting the magnetic separatrices are illuminated. Some
coronal loops are much brighter than the others, indicating that there
is enhanced heating in a limited subset of coronal loops. Such enhanced
heating is ascribed to the magnetic shear at the loop foot-points or
to the emerging magnetic flux.
Title: Dayside magnetopause Pc 3 and Pc 5 ULF waves observed by the
GEOTAIL Satellite
Authors: Sakurai, T.; Tonegawa, Y.; Kitagawa, T.; Yumoto, K.; Yamamoto,
T.; Kokubun, S.; Mukai, T.; Tsuruda, K.
Bibcode: 1999EP&S...51..965S
Altcode:
This study is intended to reveal the oscillation and propagation
characteristics of ULF waves observed near the dayside magnetopause. A
comparative study of such ULF wave phenomena is made by using the data
obtained from the successive skimmings of the dayside magnetopause
by the GEOTAIL Satellite performed on three successive days of 9,
18, and 27 December 1994. The instrumentations of GEOTAIL are good
for such a task, since the satellite simultaneously measured the
magnetic and electric fields, and low energy plasma data. Based the
observed data, we studied the dynamic spectral characteristics of
both magnetic and electric fields. The results revealed that the
dominant ULF waves appearing in the dynamic spectra were Pc 3 and Pc
5 oscillations. The Pc 3 oscillations were observed during a limited
local time around noon without an extension to the dawn and dusk
flanks, and dominantly appeared at a frequency of ~25 mHz in the
azimuthal component of the magnetic field. This frequency component
showed clear resonant oscillation characteristics. In addition to
this, a fast mode earthward propagation characteristic was also
recognized. The Poynting flux of the Pc 3 signals showed that the
energy flux was about 10 nJ/m2s on average, the strongest
being along the magnetic field line. Pc 5 oscillations showed another
dominant spectral power which appeared in the radial component of the
electric field, suggesting that the resonance oscillations were well
established. However, the Pc 5 oscillations in the subsolar region
did not show any earthward propagation characteristics.
Title: Shift-and-add reconstruction of solar granulation images
Authors: Baba, N.; Miura, N.; Sakurai, T.; Ichimoto, K.; Soltau, D.;
Brandt, P.
Bibcode: 1999SoPh..188...41B
Altcode:
To restore an atmospherically degraded image of solar granulation the
shift-and-add (SAA) method is applied to its specklegrams. It is the
first time, to the best of our knowledge, that such a technique has
been used for image reconstruction of solar granulation, a largely
extended target. SAA, therefore, enables us to monitor restored images
of solar granulation in a simple and fast way.
Title: Application of shift-and-add method to reconstruction of
solar granulation
Authors: Baba, Naoshi; Miura, Noriaki; Sakurai, Takashi
Bibcode: 1999SPIE.3749..168B
Altcode:
The shift-and-add (SAA) method is applied to specklegrams of solar
granulation to reconstruct its high-resolution image. The SAA recovers
solar granulation from the atmospherically degraded images. From our
reconstruction result it is shown that SAA method is useful for a
largely extended object.
Title: Resolution Improvement of Solar Images
Authors: Miura, Noriaki; Baba, Naoshi; Sakurai, Takashi; Ichimoto,
Kiyoshi; Soltau, Dirk; Brandt, Peter
Bibcode: 1999SoPh..187..347M
Altcode:
A method for the improvement of resolution in an observed solar image
is proposed. A blind deconvolution method is used for restoration of an
atmospherically-degraded solar image, and a super-resolution method is
applied to its restored image to improve the resolution. It is confirmed
that a blind deconvolution process can restore fine structures that are
blurred in an observed image, and that the super-resolution process
can make a cutoff frequency in a blind-deconvolved image higher. A
time series of super-resolved images of a sunspot observed with the
70-cm Vacuum Tower Telescope at Teide Observatory is presented.
Title: Superresolution applied to solar granulation images
Authors: Miura, Noriaki; Baba, Naoshi; Sakurai, Takashi
Bibcode: 1999SPIE.3749..116M
Altcode:
Superresolution is attained for solar granulation images observed from
the ground. The images are preprocessed with a blind deconvolution
method, and then are superresolved. The resultant images show features
with resolution higher than the diffraction limit.
Title: A New Imaging System of the Corona at Norikura
Authors: Ichimoto, Kiyoshi; Noguchi, Motokazu; Tanaka, Nobuyuki;
Kumagai, Kazuyoshi; Shinoda, Kazuya; Nishino, Tetsuo; Fukuda, Takeo;
Sakurai, Takashi; Takeyama, Norihide
Bibcode: 1999PASJ...51..383I
Altcode:
A new imaging system of the coronal green line (Fe XIV 5303
Angstroms) was constructed at the Norikura Solar Observatory. The
system consists of a 10-cm coronagraph, a tunable Lyot filter, and
a cooled CCD camera. The transmission curve of the Lyot filter can
be modulated by two liquid-crystal variable retarders. This scheme
provides quick wavelength tuning and efficient subtraction of sky
background. Two-dimensional distributions of the intensity and
Doppler shift of the coronal green line can be obtained within 30
seconds with accuracies of better than 10-6I_Solar and 1 km
s-1. Regular operation was started in 1997 September. The
aim of the new system is to investigate plasma motions associated with
the magnetic field reconnection and waves in the solar corona.
Title: Spectroscopic Studies of the Solar Corona I. Spatial Variations
in Line Parameters of Green and Red Coronal Lines
Authors: Singh, Jagdev; Ichimoto, Kiyoshi; Imai, Hideki; Sakurai,
Takashi; Takeda, Aki
Bibcode: 1999PASJ...51..269S
Altcode:
We obtained simultaneously profiles of the coronal green line (Fe
XIV 5303 { Angstroms}) and red line ([Fe X] 6374 { Angstroms}) on a
number of days at several regions covering an area of about 200''times
500'' in the solar corona. The intensity, velocity, and width for
both of these lines were computed by making a Gaussian fit to the
observed line profile. We find that in coronal structures the spatial
variations in the red and green line intensities are correlated. The
ratio of green- to red-line intensities varies between 0.6--9.2 for
different coronal structures. The value of the intensity ratio in an
individual coronal structure also varies with height above the solar
limb along the structure. The range of values of the intensity ratio
observed implies that most of the structures under investigation had
a temperature in the range of 1.2--1.6times 106 K. Also,
we find that in coronal structures the width of the red line increases
with height above the limb, whereas the green-line width in the same
region decreases with height. This behavior of the line widths can be
explained if we assume the mixing of plasma in the middle and higher
portions of the coronal structures by microturbulence or traveling
waves, which have been detected recently. No Hα loops and activity
were seen in these regions during the observation periods.
Title: Analysis Of The Disappearing Filament And Flare Of 7 May 1992
Authors: Tang, Y. H.; Mouradian, Z.; Schmieder, B.; Fang, C.;
Sakurai, T.
Bibcode: 1999SoPh..185..143T
Altcode:
By using Yohkoh soft X-ray data, Hα filtergrams, and radio data, the
activation of the disappearing filament and the flare eruption on 7
May 1992 have been studied. Main conclusions are as follows: (1) the
emergence of new magnetic flux tends to affect the pre-existing X-ray
loops, which usually appear in arcades spanning Hα filament, changing
the magnetic environment of the filament, and then enhance the current
in the filament. Therefore newly emerging flux plays a fundamental
role in the destabilization of this filament. (2) According to the Hα
data and the rising motion of the filament, the corresponding current
variation in the filament has been calculated. It seems that the current
interruption may be a possible trigger mechanism for this filament
disappearance. (3) The magnetic field strength and the energy flux
of energetic electrons in the source region of microwave bursts have
been estimated by using the microwave spectrum. During the main phase,
the mean magnetic strength and the energy flux of energetic electrons
are about 300-400 G and 1×1011 erg cm−2 s −1, respectively. (4)
The energy provided by reconnection of the current sheet and the total
energy of the current filament are estimated and we show that there
is enough energy stored in the filament to feed the 7 May, 1992 flare.
Title: On the relation between incident and emergent light beams of
optical fibers. II. Single-mode fibers.
Authors: Imai, H.; Sakurai, T.
Bibcode: 1999RNAOJ...4..101I
Altcode: 1999RNOAJ...4..101I
The change in F-ratio when a beam of light goes through an optical fiber
is examined for a single-mode fiber (core radius = 9 μm). A previous
study (Makita and Imai 1988) showed that, for multi-mode fibers,
the F-ratio of the output beam is smaller (i.e. the beam spreads)
than the F-ratio of the input beam. Such degradation in F-ratio is
not seen in a single-mode fiber because the output beam's F-ratio is
fixed by the mode propagating in the fiber.
Title: Double-frequency oscillations of low energy plasma associated
with transverse Pc 5 pulsations: GEOTAIL satellite observations
Authors: Sakurai, T.; Tonegawa, Y.; Kitagawa, T.; Nowada, M.; Yamawaki,
A.; Mukai, T.; Kokubun, S.; Yamamoto, T.; Tsuruda, K.
Bibcode: 1999EP&S...51...43S
Altcode:
The GEOTAIL satellite observed an interesting oscillation phenomenon
of low energy plasma (LEP) in the dawnside outer magnetosphere. The
oscillation was taking place with a frequency double that of the
transverse oscillation of Pc 5 pulsations. The double-frequency
oscillation appeared in the plasma density and temperature, clearly
showing an out-of-phase relationship between them. However, this
phenomenon is revealed to be an instrumental effect of the LEP detector,
which has a low energy threshold of measuring an ion population at
32 eV/Q. The imbedded ion population is found to be composed of cold
ions with an energy of less than the threshold. They are convected
past the LEP detector by the Pc 5 wave and enter the detector energy
window twice per wave period. Plasma bulk parameters calculated using
the detected ions produce an oscillation that has a frequency exactly
double that of the Pc 5 wave. However, it should be noted that this
phenomenon is observed with a large amplitude electric field oscillation
only in intervals when the satellite passes through the dawnside outer
magnetosphere under very quiet magnetic conditions, i.e., periods of
the northward interplanetary magnetic field.
Title: Emission line intensities of the solar corona and sky
brightness observed at Norikura: 1950 - 1997.
Authors: Sakurai, T.; Irie, M.; Imai, H.; Miyazaki, H.; Sykora, J.
Bibcode: 1999PNAOJ...5..121S
Altcode:
The coronal green line (Fe XIV 5303 Å) intensities of the Sun
have been measured at the Norikura Solar Observatory since 1950. The
instrument used and the method of measurement are described in detail,
and the observed variation of coronal green line intensities is
presented. Comparison with respect to the reference data compiled
from world-wide coronagraph stations indicates that the intensity
scale of Norikura in the period 1955 - 1964 is different from the other
period. A correlation between the intensity scale and the sky brightness
is found in the period 1955 - 1964. An empirical re-scaling of the
data is then made in order to compensate for such correlation. The
sky brightness shows, in addition to strong one-year periodicity,
an 11-year solar cycle periodicity. It is also weakly correlated with
certain meteorological parameters. An oscillatory variation of 2 - 3
year periodicity is seen in the sky brightness data, but its relation
to meteorological parameters is not conclusive.
Title: Stray-Light Effect on Magnetograph Observations
Authors: Chae, Jongchul; Yun, Hong Sik; Sakurai, Takashi; Ichimoto,
Kiyoshi
Bibcode: 1998SoPh..183..229C
Altcode:
To examine the stray-light effect in magnetograph observations, we
have determined the point spread functions of the vector magnetograph
mounted on the Japanese Solar Flare Telescope based on two indirect
methods, one analyzing the solar limb intensity profile, and the
other using the Fourier power spectra of photospheric intensity
distributions. Point spread functions consist of two parts: a blurring
part which describes seeing and small-spread-angle stray light, and
a scattering part which describes large-spread-angle stray light. The
FWHM spatial resolution is typically 3.0'', and the amount of scattered
light is about 15% on clear days. We find that the blurring part is
well described by a Moffat function whose Fourier transform is given
by an exponential function. Our results indicate that polarization
measurements of low-intensity magnetic elements like sunspots may be
significantly underestimated due to the large-spread-angle stray light,
and polarization measurements of magnetic elements which are smaller
than 5-7'' appear to be disturbed by small-spread-angle stray light.
Title: Stray-light correction in magnetograph observations using
the maximum entropy method
Authors: Chae, Jongchul; Yun, Hong Sik; Sakurai, Takashi; Ichimoto,
Kiyoshi
Bibcode: 1998SoPh..183..245C
Altcode:
We have developed a method of stray-light correction which is applicable
to filter-based magnetograph observations. Stray-light-corrected Stokes
images are obtained by performing the deconvolution of observed Stokes
images by the point spread function which is determined from the Stokes
I image. For image deconvolution, the maximum entropy principle is
used to guarantee that intensity should be positive and polarization
degrees should be less than unity. We present an iterative algorithm
for the maximum entropy method, which seeks the solution in Fourier
space and thus accomplishes fast convergence. We find that our method
is effective in correcting stray light which has a spread angle greater
than the full width at half maximum of the point spread function. We
also discuss the effect of stray light on magnetograph calibration.
Title: Solar atmospheric heating and topology of solar magnetic field.
Authors: Wang, Huaning; Sakurai, T.; Yan, Yihua
Bibcode: 1998ArBeS...4...54W
Altcode:
The photospheric vector magnetic fields, Hα and soft X-ray images of
AR 7321 were simultaneously observed with the Solar Flare Telescope at
Mitaka and the Soft X-ray Telescope of Yohkoh on October 26, 1992,
when there were no drastic activities in this region. Taking the
observed photospheric vector magnetic fields as the boundary condition,
3D magnetic fields above the photosphere were computed with a new
extrapolation method. Since Yohkoh data and Mitaka data were obtained
in well arranged time sequences during the day, the evolution of 3D
fields, Hα features and soft X-ray features in this region can be
studied in detail.
Title: Multi-Wavelength Analysis of the Flare on 2 October 1993
Authors: Fang, C.; Tang, Y. H.; Hénoux, J. -C.; Huang, Y. R.; Ding,
M. D.; Sakurai, T.
Bibcode: 1998SoPh..182..163F
Altcode:
By use of Yohkoh hard X-ray flux and soft X-ray images, and of vector
magnetograms and 2D spectral observations, a 1N/C6.5 flare observed on
2 October 1993 is analysed in detail. Evidence is provided not only
morphologically but also quantitatively that the dynamics at kernels
A and C of the flare in the impulsive phase were controlled mainly by
electron beam bombardment, while the heating of kernel B is mainly due
to heat conduction. By plotting the energy gradient of the electron
energy flux as a function of energy for the various spectral indexes
observed during the flare, the acceleration mechanism is found to be
such that there is a constant energy E0, close to 20 keV,
for which the electron flux d F1/dE is constant. It is shown
that such a conclusion can be reached more directly by using the photon
flux, which in that case must be constant for E=E0, whatever
the value of the power index. This result implies also that the electron
spectrum is represented by a power law and that the X-ray photons are
produced in a thick target. Instantaneous momentum balance is shown to
exist between the upflowing soft X-ray-emitting and the downflowing
Hα- emitting plasma at the kernels of the flare. The observed Hα
red asymmetry is well reproduced by the non-LTE computation, with the
down-moving condensation included. The observation of the magnetic
field suggests that the flare was triggered probably by magnetic
flux emergence.
Title: Cross-Section Variations of Coronal Magnetic Loops
Authors: Wang, Huaning; Sakurai, Takashi
Bibcode: 1998PASJ...50..111W
Altcode:
Axisymmetric nonlinear force-free field models were employed to
study the variation of the cross-section of coronal loops. In order
to simulate these loops, we placed the point source of the fields
under the photosphere and made the symmetric axis parallel to the
photospheric surface. Based upon qualitative theoretical analyses and
numerical simulations on the dipoles in these models, we compared
the simulated magnetic loops with similar arclength and symmetric
distributions of magnetic strength, and obtained the following results:
The cross-section expansion of the current-free loops is fairly large
and nearly isotropic, and that of highly sheared force-free loops is
small and anisotropic. The anisotropy of the cross-section expansion,
however, tends to vanish in extremely sheared loops. Magnetic loops
with constant thickness, as an extreme case in the dipole nonlinear
force-free fields, can be reached asymptotically when the magnetic
shear increases. They tend to appear in the vicinity of the plane,
vertically intersecting the symmetric axis at the point source, and
their footpoints are not exactly located in the peak sites of vertical
electric current density on the photospheric surface.
Title: Loop-Loop Interaction in Impulsive Solar Flares Inferred from
Microwave and X-Ray Images
Authors: Nishio, M.; Yaji, K.; Kosugi, T.; Nakajima, H.; Sakurai, T.
Bibcode: 1998ASSL..229..207N
Altcode: 1998opaf.conf..207N
No abstract at ADS
Title: Magnetic Reconnection in the Active Region Inferred by
Homologous Soft X-ray Flares in February 1992
Authors: Zhang, H. Q.; Sakurai, T.; Shibata, K.; Shimojo, M.; Kurokawa,
H.; Morita, S.; Uchida, Y.
Bibcode: 1998ASSL..229..391Z
Altcode: 1998opaf.conf..391Z
No abstract at ADS
Title: Long-Term Monitoring Studies of the Sun at the National
Astronomical Observatory of Japan
Authors: Sakurai, Takashi
Bibcode: 1998ASPC..140..483S
Altcode: 1998ssp..conf..483S
No abstract at ADS
Title: Non-Linear Evolution of Erupting Coronal Magnetic Fields
Authors: Setiahadi, B.; Anwar, B.; Akioka, M.; Sakurai, T.
Bibcode: 1998ASSL..229..353S
Altcode: 1998opaf.conf..353S
No abstract at ADS
Title: Coronal Loops and Magnetic Field Modeling - a New Boundary
Integral Representation for Force-free Magnetic Fields (invited)
Authors: Sakurai, Takashi; Yan, Yihua
Bibcode: 1998asct.conf....1S
Altcode:
No abstract at ADS
Title: The Activation Characteristics of a Disappearing Filament
Authors: Tang, Y. H.; Sakurai, T.
Bibcode: 1998asct.conf...49T
Altcode:
No abstract at ADS
Title: Maximum Energy of Force-Free Magnetic Fields
Authors: Sakurai, T.
Bibcode: 1998ASSL..229..345S
Altcode: 1998opaf.conf..345S
No abstract at ADS
Title: Deconvolution of YOHKOH Soft X-Ray Images
Authors: Shin, J.; Sakurai, T.; Miura, N.
Bibcode: 1998ASSL..229..361S
Altcode: 1998opaf.conf..361S
No abstract at ADS
Title: Summary of Session A: Coronal Heating and Solar Wind
Acceleration
Authors: Sakurai, T.
Bibcode: 1998HiA....11..869S
Altcode:
No abstract at ADS
Title: Gradual Opening of Solar/Stellar Magnetic Fields Driven by
Surface Shear Flows
Authors: Sakurai, Takashi
Bibcode: 1998ASPC..154.1154S
Altcode: 1998csss...10.1154S
Force-free magnetic fields attain more and more energy as the footpoints
of field lines are driven by flows on the surface. However, the energy
is never able to exceed an upper limit which is set by the field with
all the field lines open to the outer space. This property of the
force-free fields has so far been discussed based on analytic studies
using integral relationships (virial relations, Aly 1984) about the
magnetic field, and also based on numerical simulations. In this study,
we use a simple solution for the force-free fields derived by Low and
Lou (1990) and show the above-mentioned property most directly.
Title: Coronal Loops Above a Sunspot Region
Authors: Fang, C.; Tang, Y. H.; Ding, M. D.; Zhao, J.; Sakurai, T.;
Hiei, E.
Bibcode: 1997SoPh..176..267F
Altcode:
By analysing the data of Yohkoh soft X-ray images, vector magnetograms
and 2D spectral observations, coronal loops above a large sunspot on
16-19 May 1994 have been studied. It is shown that the loops follow
generally the alignment of concentrated magnetic flux. The results
indicate that the soft X-ray emission is low just above the sunspot,
while some loops connecting regions with opposite magnetic polarities
show strong soft X-ray emission. Especially, the part of the loops
near the weaker magnetic field region tends to be brighter than the
one near the stronger magnetic field. The temperature around the top
of the loops is typically ∼3 × 106 K, which is higher
than that at the legs of the loops by a factor of 1.5-2.0. The density
near the top of the loops is about 5 x 109 cm-3,
which is higher than that of the leg parts of the loops. These loops
represent probably the sites where strong magnetic flux and/or current
are concentrated.
Title: On the correlation between line width and line depth of the
solar HeI 1083 NM line
Authors: Venkatakrishnan, P.; Sakurai, T.; Suematsu, Y.; Ichimoto, K.
Bibcode: 1997BASI...25..527V
Altcode:
No abstract at ADS
Title: Magnetic Field Configuration in Impulsive Solar Flares Inferred
from Coaligned Microwave/X-Ray Images
Authors: Nishio, Masanori; Yaji, Kentaro; Kosugi, Takeo; Nakajima,
Hiroshi; Sakurai, Takashi
Bibcode: 1997ApJ...489..976N
Altcode:
The results of an analysis of 14 impulsive solar flares, observed
simultaneously with microwave and X-ray imaging instruments, are
presented. From the relative locations of microwave, hard X-ray,
and soft X-ray sources, we have found that in 10 of the 14, at least
two loops are involved. Typical sizes of the two loops differ; one
is typically <=20" and the other 30"-80". Microwave emission is
detected from both loops, while hard X-ray emission is preferentially
radiated from the shorter of the two loops. The shorter loop is brighter
than the longer loop in soft X-rays. However, intensity variations of
microwaves from the two loops are similar. These observations, together
with comparisons of the microwave/X-ray images with photospheric
magnetograms, suggest that in the majority of impulsive flare events two
loops interact with each other, releasing magnetic energy and producing
energetic electrons. The observations are therefore consistent with
the emerging flux model of flares.
Title: Analysis of it YOHKOH SXT Coronal Loops and Calculated
Force-Free Magnetic Field Lines from Vector Magnetograms
Authors: Yan, Yihua; Sakurai, Takashi
Bibcode: 1997SoPh..174...65Y
Altcode:
In this paper some soft X-ray loops observed by Yohkoh/SXT and
extrapolated magnetic fields from the vector magnetogram data observed
at the Mitaka Flare Telescope are compared and analyzed. The computed
field lines generally agree with Yohkoh/SXT coronal loops. Typical
examples of the magnetic field intensities, loop widths, and loop
heights along field lines are presented.
Title: Measurement of the coronal electron temperature at the total
solar eclipse on 1994 November 3.
Authors: Ichimoto, K.; Kumagai, K.; Sano, I.; Kobiki, T.; Sakurai,
T.; Munoz, A.
Bibcode: 1997ASIC..494...31I
Altcode: 1997topr.conf...31I
The coronal electron temperature was determined for the first time from
the shape of weak depressions at 3900 Å and 4300 Å in the continuous
coronal spectra. It was found that the coronal streamer has overall
temperatures of about 1.5 - 1.7 MK, while the coronal hole has evidently
lower temperature of 0.9 - 1.1 MK. No significant height variation of
the temperature was found in the streamer, while the coronal hole shows
evidence of increasing temperature with height. An acceleration of the
expanding motion between 1.1 Rsun and 2.0 Rsun
by ≡80 km s-1 was found in the streamer.
Title: Observations of field-aligned density microstructure near
the Sun
Authors: Grall, R. R.; Coles, W. A.; Spangler, S. R.; Sakurai, T.;
Harmon, J. K.
Bibcode: 1997JGR...102..263G
Altcode:
Radio scattering observations made with multiple antennas provide a
direct measure of the two-dimensional microstructure of the solar
wind. Previous multiple antenna observations have shown that the
microstructure at scales of the order of 10 km becomes highly
field-aligned inside of 6 RS [e.g., Armstrong et al.,
1990]. Single antenna observations, which can measure only a radial cut
through the microstructure, have shown that scales larger than 1000 km
have a Kolmogorov spectrum, whereas the smaller-scale structure has
a flatter spectrum and is considerably enhanced above the Kolmogorov
``background'' [e.g., Coles et al., 1991]. Here we present new multiple
antenna ``angular broadening'' observations made in 1990 and 1992. These
confirm that the microstructure is highly field-aligned near the Sun,
they show that it has elliptical symmetry, and they show that the axial
ratio changes quite abruptly near 6 RS. We also present
simultaneous measurements at 9 RS of the anisotropy on scales
of 1 to 30 km and on scales of 200 to 3000 km. Significant anisotropy
was seen on the smaller scales but not on the larger scales. This
suggests that the process responsible for the anisotropic microstructure
is distinct from the larger-scale, more isotropic structure.
Title: Soft X-ray, Microwave and He I Measurements of Coronal Holes
Authors: Brajsa, R.; Ruzdjak, V.; Vrsnak, B.; Pohjolainen, S.; Urpo,
S.; Sakurai, T.; Wohl, H.
Bibcode: 1997IAUJD..19E..15B
Altcode:
The Poster presents and describes microwave signatures of three
coronal holes that were recorded on May 27, 1993. Differences in the
brightness temperatures between an equatorial and two polar coronal
holes were found. The measurements in the He I 10830 AA absorption
line for that day were also analyzed and compared with the microwave
and soft X-ray data.
Title: Coronal Loops and Magnetic Field Structure in the Sun
Authors: Sakurai, T.; Yan, Y.; Wang, H. N.
Bibcode: 1997IAUJD..19E..40S
Altcode:
Coronal X-ray loops are in general believed to trace the magnetic
field lines. We computed magnetic field lines in the corona based
on the observed magnetic fields on the photosphere, and compared
the results with coronal loop structures observed by the Yohkoh
satellite. The overall structures of coronal loops are satisfactorily
reproduced. Computations show that the diameter of flux tubes should
expand typically by a factor of 3--4 from the foot-points to the top. On
the contrary, the observed loops only expand in diameter by a factor of
1.5--2 (Klimchuk et al., PASJ 44, L181, 1992). This apparent conflict
has been explained in terms of the pinch effect of the field-aligned
electric currents (McClymont and Mikic, ApJ 422, 899, 1994). We look
into this topic more in detail by using analytic as well as numerical
models of magnetic fields.
Title: 2-D MHD Simulation on Helmet Streamer Formation
Authors: Setiahadi, B.; Anwar, B.; Akioka, M.; Sakurai, T.
Bibcode: 1997IAUJD..19E..43S
Altcode:
We performed two-dimensional MHD simulation on formation of the helmet
streamer in the solar corona. The solar corona is assumed initially
in hydrostatic balance and isothermal. In our scheme an arcade type
magnetic field permeates the corona from below as a result of an
evolving old active region. The result of our simulation shows the
corona evolves to a new dynamical equilibrium and forms a loop type
arcade in the lower part and surrounded by the cusp type arcade. Along
the helmet borders, regions of relatively high solar wind are formed.
Title: The Solar Flare of 1992 August 17 23:58 UT} %
Authors: Takahashi, Masaaki; Watanabe, Tetsuya; Sakai, Jun-Ichi;
Sakao, Taro; Kosugi, Takeo; Sakurai, Takashi; Enome, Shinzo; Hudson,
Hugh S.; Hashimoto, Shizuyo; Nitta, Nariaki
Bibcode: 1996PASJ...48..857T
Altcode:
A small flare (C4.3 in the GOES X-ray class) was well observed by
all of the instruments on board Yohkoh. The X-ray light curves have
double peaks which are about 5 min apart. Until the first peak from
flare onset, four compact areas brighten up in the soft X-ray region,
which are aligned almost on one straight line. We regard them as being
footpoints of two sets of loops, which are identifiable in soft X-ray
images, since their locations match those of hard X-ray sources. Indeed,
after the second peak, the temporal behavior of the temperature and
emission measure at each point is consistent with the existence of
two such loops. Comparing our results with recent MHD simulations,
we propose a possible scenario for this flare that is based on the
coalescence of two loops.
Title: Book Review: Proceedings of the third China-Japan seminar on
solar physics / International Academic Publishers, Beijing, 1995
Authors: Wang, J. X.; Ai, G. X.; Sakurai, T.; Hirayama, T.
Bibcode: 1996SoPh..169..225W
Altcode:
No abstract at ADS
Title: Radio interferometer observations of turbulence in the inner
solar wind
Authors: Spangler, S. R.; Sakurai, T.; Coles, W. A.; Grall, R. R.;
Harmon, J. K.
Bibcode: 1996AIPC..382..265S
Altcode:
Measurements can be made of Very Long Baseline Interferometer
(VLBI) phase scintillations due to plasma turbulence in the solar
corona and solar wind. These measurements provide information on
the spectrum and intensity of density fluctuations with scale
sizes of a few hundred to several thousand kilometers. If we
model the spatial power spectrum of the density fluctuations
as Pδn(q)=CN2q-α,
where q is the spatial wavenumber, these observations yield both
α and the path-integrated value of CN2. The
recently completed Very Long Baseline Array (VLBA) is capable
of making such measurements over the heliocentric distance range
from a few solar radii to 60 solar radii and beyond. This permits
the determination, with the same technique and instrument, of the
radial evolution of turbulent characteristics, as well as their
dependence on solar wind transients, sector structure, etc. In
this paper we present measurements of 16 sources observed at a
wide range of solar elongations and at different times. These
observations show that the coefficient CN2
depends on heliocentric distance as approximately
CN2~(R/Rsolar)-3.7. The
radio derived power spectral characteristics are in agreement
with in situ measurements by the Helios spacecraft for regions of
slow solar wind, but fast solar wind does not have large enough
density fluctuations to account for the magnitude of the observed
scintillations. The observed radial dependence is consistent with a
Wentzel-Kramers-Brillouin (WKB)-type evolution of the turbulence with
heliocentric distance.
Title: Anisotropic microstructure near the sun
Authors: Coles, W. A.; Grall, R. R.; Spangler, S. R.; Sakurai, T.;
Harmon, J. K.
Bibcode: 1996AIPC..382..264C
Altcode:
Radio scattering observations provide a means of measuring a
two-dimensional projection of the three-dimensional spatial spectrum
of electron density, i.e., in the plane perpendicular to the line
of sight. Earlier observations have shown that the microstructure
at scales of the order of 10 km becomes highly field-aligned inside
of 10 Rsolar [Armstrong et al., 1990]. Earlier work has
also shown that density fluctuations at scales larger than 1000 km
have a Kolmogorov spectrum, whereas the smaller scale structure has
a flatter spectrum and is considerably enhanced above the Kolmogorov
``background'' [Coles et al., 1991]. Here we present new observations
made during 1990 and 1992. These confirm the earlier work, which was
restricted to one source on a few days, but they suggest that the
anisotropy changes abruptly near 6 Rsolar which was not
clear in the earlier data. The axial ratio measurements are shown on
Figure 1 below. The new observations were made with a more uniform
sampling of the spatial plane. They show that contours of constant
correlation are elliptical. This is apparently inconsistent with the
spatial correlation of the ISEE-3 magnetic field which shows a ``Maltese
Cross'' shape [Matthaeus et al., 1990]. However this inconsistency may
be only apparent: the magnetic field and density correlations need not
have the same shape; the scale of the magnetic field correlations is at
least 4 orders of magnitude larger; they are much further from the sun;
and they are point measurements whereas ours are path-integrated. We
also made two simultaneous measurements, at 10 Rsolar, of
the anisotropy on scales of 200 to 4000 km. Significant anisotropy
was seen on the smaller scales, but the larger scale structure was
essentially isotropic. This suggests that the process responsible
for the anisotropic microstructure is independent of the larger scale
isotropic turbulence. It is then tempting to speculate that the damping
of this anisotropic process inside of 6 Rsolar contributes
to the acceleration of the solar wind.
Title: Measurement of the Coronal Electron Temperature at the Total
Solar Eclipse on 1994 November 3
Authors: Ichimoto, Kiyoshi; Kumagai, Kazuyoshi; Sano, Issei; Kobiki,
Toshihiko; Munoz, Alberto; Sakurai, Takashi
Bibcode: 1996PASJ...48..545I
Altcode:
A spectroscopic observation was carried out at the total solar eclipse
on 1994 November 3 in Putre, Chile, in order to investigate the electron
temperature and the expanding motion of the corona. The shape of weak
depressions at 3900 Angstroms and 4300 Angstroms in the continuous
coronal spectra was compared with the theoretical electron-scattered
coronal spectra; thus, the magnitude of the electron thermal motion was
determined directly. It was found that a coronal streamer on the eastern
limb and a coronal hole on the south-pole have overall temperatures of
about 1.5--1.7 MK and 0.9--1.1 MK, respectively. No significant height
variation in the temperature was found in the streamer, while the
coronal hole shows evidence of increasing temperature with height. An
acceleration of the expanding motion between 1.1 RO and 2.0 RO by ~
80 km s(-1) was found in the streamer.
Title: New observational facts about solar flares from ground-based
observations
Authors: Sakurai, T.; Hiei, E.
Bibcode: 1996AdSpR..17d..91S
Altcode: 1996AdSpR..17Q..91S
Recent observational studies on solar flares by using ground-based
instruments are reviewed. Magnetic field measurements are being carried
out in many observatories, and conclusive evidence on the storage of
magnetic energy and its release at flares has accumulated. On the other
hand it is interesting that some controversial results were reported
in regard to the change in magnetic fields associated with flares.
Title: Enhanced He{I} Absorption at the Feet of Solar X-Ray Loops
Authors: Venkatakrishnan, P.; Sakurai, Takashi; Suematsu, Yoshinori;
Ichimoto, Kiyoshi
Bibcode: 1996PASJ...48L...1V
Altcode:
A comparison of He{I} spectroheliograms and Yohkoh soft X-ray images
of active regions indicates that He{I} absorption is enhanced at
the feet of hot X-ray loops. It is suggested that the conduction of
heat from the loops into the transition region at their feet would
produce enhanced transition-region emission around 50 eV that would
in turn cause enhanced excitation of He{I} leading to the excess He{I}
absorption that is observed.
Title: Chromospheric Evaporation in Four Solar Flares Observed
by YOHKOH
Authors: Ding, M. D.; Watanabe, T.; Shibata, K.; Sakurai, T.; Kosugi,
T.; Fang, C.
Bibcode: 1996ApJ...458..391D
Altcode:
We present an analysis of the Ca XIX soft X-ray spectra of four
flares observed by the Yohkoh spacecraft. These spectra show evident
blueshifted signatures. Using a two-component fitting technique,
we obtain the time variations of the fitting parameters. The
peak line-of-sight upflow velocities range from 180 to 320 km
s-1. The durations of upflows with observable velocities
(≥ 100 km s-1) are found to be 4-10 minutes. Particularly,
for two of the four flares, the emission measure of the blueshifted
component is larger than that of the stationary component at early
times. This fact agrees qualitatively with the predictions of numerical
simulations of chromospheric evaporation. There is a rough correlation
between the nonthermal turbulent velocity and the upflow velocity,
consistent with the results of Fludra et al. We also find that most Ca
xix resonance profiles show no significant shift of the profile peak,
regardless of the blue asymmetry which sometimes appears. The possible
causes of this phenomenon are discussed.
Title: Helium 10830 Å measurements of the Sun
Authors: Brajša, R.; Pohjolainen, S.; Ruždjak, V.; Sakurai, T.;
Urpo, S.; Vršnak, B.; Wöhl, H.
Bibcode: 1996SoPh..163...79B
Altcode:
Measurements of the Sun in the near-infrared He I 10830 Å absorption
line were performed using the echelle spectrograph with a dispersion
of 6.71 mÅ per pixel at the Vacuum Tower Telescope (German Solar
Telescopes, Teide Observatory, Izaña, Tenerife, Spain) on May 26,
1993. These measurements were compared with full-disc soft X-ray images
of the Sun (Japanese solar satellite Yohkoh), full-disc solar images
in Hα (Big Bear Solar Observatory), full-disc solar images in the
He I 10830 Å line (National Solar Observatory, Kitt Peak) and with
full-disc microwave solar maps at 37 GHz (Metsähovi Radio Research
Station). In the He 10830 Å line the Sun displays a limb darkening
similar to that in the visible part of the spectrum. Active regions
and Hα filaments show a strong absorption in the He 10830 Å line,
whereas the absorption is weak in coronal holes.
Title: Heating Mechanisms of the Solar Corona
Authors: Sakurai, T.
Bibcode: 1996mpsa.conf...21S
Altcode: 1996IAUCo.153...21S
No abstract at ADS
Title: Maximum Energy of Force-Free Magnetic Fields
Authors: Sakurai, T.; Chae, J. C.
Bibcode: 1996mpsa.conf..579S
Altcode: 1996IAUCo.153..579S
No abstract at ADS
Title: X-ray and Magnetic Features of H alpha Surges
Authors: Okubo, A.; Matsumoto, R.; Miyaji, S.; Akioka, M.; Zhang,
H.; Shimojo, M.; Nishino, Y.; Ichimoto, K.; Shibata, K.; Sakurai, T.
Bibcode: 1996mpsa.conf..437O
Altcode: 1996IAUCo.153..437O
No abstract at ADS
Title: Measurement of the Coronal Electron Temperature at the Total
Solar Eclipse on 3rd Nov. 1994
Authors: Ichimoto, K.; Kumagai, K.; Sano, I.; Kobiki, T.; Sakurai,
T.; Munoz, A.
Bibcode: 1996mpsa.conf..413I
Altcode: 1996IAUCo.153..413I
No abstract at ADS
Title: Solar Optical Instruments at the National Astronomical
Observatory of Japan
Authors: Sakurai, T.
Bibcode: 1996gbaa.conf..418S
Altcode:
No abstract at ADS
Title: Microwave and Hard X-Ray Observations of Footpoint Emission
from Solar Flares
Authors: Kundu, M. R.; Nitta, N.; White, S. M.; Shibasaki, K.; Enome,
S.; Sakao, T.; Kosugi, T.; Sakurai, T.
Bibcode: 1995ApJ...454..522K
Altcode:
We investigate radio and X-ray imaging data for two solar flares in
order to test the idea that asymmetric precipitation of nonthermal
electrons at the two ends of a magnetic loop is consistent with the
magnetic mirroring explanation. The events we present were observed in
1993 May by the HXT and SXT X-ray telescopes on the Yohkoh spacecraft
and by the Nobeyama 17 GHz radioheliograph. The hard X-ray images in
one case show two well-separated sources; the radio images indicate
circularly polarized, nonthermal radio emission with opposite polarities
from these two sources, indicating oppositely directed fields and
consistent with a single-loop model. In the second event there are
several sources in the HXT images which appear to be connected by
soft X-ray loops. The strongest hard X-ray source has unpolarized
radio emission, whereas the strongest radio emission lies over strong
magnetic fields and is polarized. In both events the strongest radio
emission is highly polarized and not coincident with the strongest
hard X-ray emission. This is consistent with asymmetric loops in
which the bulk of the precipitation (and hence the X-ray emission)
occurs at the weaker field footpoint.
Title: Comparative study of the accuracy of various sun-tracking
schemes.
Authors: Sakurai, T.
Bibcode: 1995RNAOJ...2..669S
Altcode: 1995RNOAJ...2..669S
Various schemes of sun-trackers for solar telescopes are discussed,
and the achievable accuracies are estimated. With an application to
space-borne telescopes in mind, only photon noise and solar brightness
inhomogeneities are considered as sources of disturbance to the
tracking, and no seeing effects are considered.
Title: Coordinated Observation of the Solar Corona Using the Norikura
Coronagraph and the YOHKOH Soft X-Ray Telescope
Authors: Ichimoto, K.; Hara, H.; Takeda, A.; Kumagai, K.; Sakurai,
T.; Shimizu, T.; Hudson, H. S.
Bibcode: 1995ApJ...445..978I
Altcode:
Spectroscopic observations of coronal emission lines were carried
out at the Norikura Solar Observatory in cooperation with the soft
X-ray telescope on board the Yohkoh satellite to study the plasma
distributions at different temperatures. Intensity and velocity
distributions in Fe XIV wavelength 5303 (green), Fe X wavelength
6374 (red), and Ca XV wavelength 5694 (yellow) lines are compared
with the soft X-ray images. It is found that the soft X-ray images
closely resemble those of the yellow line that represents a rather
high temperature component of the corona. On the other hand the
low-temperature component seen in the green and the red lines shows
quite a different distribution from that of the high-temperature
component; the low-temperature component consists of many thin loops
or streaks, while the high-temperature component is more diffuse. We
find that the active elements of the cool component, i.e., complex
loop systems, rapid changes of small structures, and localized large
plasma motions, all tend to be cospatial with the hot component.
Title: Anisotropic microturbulence near the Sun
Authors: Coles, William A.; Grall, R. R.; Spangler, S. R.; Sakurai,
T.; Harmon, J. K.
Bibcode: 1995sowi.conf...40C
Altcode:
Interplanetary scintillation observations which sample the spatial
spectrum of electron density at scales between 10 and 100 km show
power-law spectra which are flatter than the Kolmogorov spectra
observed at larger scales by spacecraft and also, indirectly, by
phase scintillation of coherent radio signals. Furthermore, angular
broadening observations of compact radio sources have shown that the
microscale density fluctuations are field-aligned and become more
anisotropic as R decreases inside 10 solar radius. We present angular
broadening observations taken in October of 1992 which were recorded
nearly simultaneously on the VLA and VLBA arrays. The VLA samples
structure at scales between 3 and 30 km, whereas the VLBA samples
scales between 200 and 4000 km. The small scale VLA measurements of
the south polar source 1246-075 showed lower turbulence than those
of the equatorial source 1256-057, consistent with previous work
showing that the density delta N2e is a factor of
10-15 lower in coronal holes. The VLA observations inside of 10 solar
radius were anisotropic, as expected. We were not able to measure the
equatorial source with the VLBA inside of 10 solar radius because the
scattering was too strong, however we did observe the polar source
just inside this distance with both the VLA and the VLBA. Significant
anisotropy was seen on the smaller scales, but the larger scales were
essentially isotropic. This suggests that the process responsible
for the anisotropic microturbulence is distinct from the larger scale
isotropic turbulence.
Title: Radio interferometer measurements of turbulence in the inner
solar wind
Authors: Spangler, S. R.; Sakurai, T.; Coles, William A.; Grall,
R. R.; Harmon, J. K.
Bibcode: 1995sowi.conf...40S
Altcode:
Measurements can be made of Very Long Baseline Interferometer
(VLBI) phase scintillations due to plasma turbulence in the solar
corona and solar wind. These measurements provide information on the
spectrum and intensity of density fluctuations with scale sizes of a
few hundred to several thousand kilometers. If we model the spatial
power spectrum of the density fluctuations as Pdelta n(q)
= C2N q-alpha, where q is the spatial
wavenumber, these observations yield both alpha and the path-integrated
value of C2N. The recently completed Very Long
Baseline Array (VLBA) is capable of making such measurements over the
heliocentric distance range from a few solar radii to 60 solar radii
and beyond. This permits the determination with the same technique and
instrument of the radial evolution of turbulent characteristics, as well
as their dependence on solar wind transients, sector structure, etc. In
this paper we present measurements of 13 sources observed at a wide
range of solar elongations, and at different times. These observations
show that the coefficient C2N, depends on
heliocentric distance as approximately C2N
varies as (R/Solar Radius)-3.7. The radio derived power
spectral characteristics are in agreement with in situ measurements
by the Helios spacecraft for regions of slow solar wind, but fast
solar wind does not have large enough density fluctuations to account
for the magnitude of the observed scintillations. The observed radial
dependence is consistent with a WKB-type evolution of the turbulence
with heliocentric distance. Our data also show indication of turbulence
enhancement associated with solar wind transients.
Title: The Solar-B Mission
Authors: Sakurai, T.
Bibcode: 1995SPD....26..601S
Altcode: 1995BAAS...27..960S
No abstract at ADS
Title: Solar Flare Telescope at Mitaka
Authors: Sakurai, Takashi; Ichimoto, Kiyoshi; Nishino, Yohei; Shinoda,
Kazuya; Noguchi, Motokazu; Hiei, Eijiro; Li, Ting; He, Fengbao; Mao,
Weijun; Lu, Haitian; Ai, Guoxiang; Zhao, Zhaowang; Kawakami, Singo;
Chae, Jong-Chul
Bibcode: 1995PASJ...47...81S
Altcode:
A new solar telescope constructed recently at Mitaka, Tokyo, is
described. This instrument, called the Solar Flare Telescope, comprises
four telescopes which respectively perform: (1) magnetic-field vector
measurements, (2) Doppler-velocity measurements, (3) Hα observations,
and (4) continuum observations. The instrument has been operated as
a ground-based support for the X-ray satellite Yohkoh launched in
1991 August.
Title: Analytical model of the phase mixing phenomena by the use of
the wkb method of solution
Authors: Sakurai, T.; Kitayama, O.; Ma, J.
Bibcode: 1995GApFD..79..277S
Altcode:
A typical time evolution of the phase mixing phenomena is clarified for
an idealized model from the standpoint of the local analysis. The model
equations are taken from those of the Alfvén type standiing waves in
a square region subject to a uni-directional magnetic field and to the
effect of small diffusivity in which the Alfvén velocity changes with
the magnetic lines offeree. These equations are solved by the use of
the WKB method of solution combined with a small perturbation method
with respect to the diffusivity. The results gives the time evolution
(1) in the initial period in which mutually independent standing waves
along each magnetic lines of forces are modulated by the phase mixing
effect and (2) in the final period in which these modulated waves are
damped off to become asymptotic steady state.
Title: The Sun's magnetic activity.
Authors: Sakurai, T.
Bibcode: 1995AstHe..88...61S
Altcode:
The history of the observations of the Sun's magnetic field is reviewed,
and recent development and future prospects are presented.
Title: Measurement of the Coronal Electron Temperature at the Total
Solar Eclipse on November 3, 1994
Authors: Ichimoto, K.; Kumagai, K.; Sano, Y.; Kobiki, T.; Sakurai,
T.; Munos, A.
Bibcode: 1995pist.conf...72I
Altcode:
No abstract at ADS
Title: He I λ1083nm Observations and Chromospheric and Coronal
Activities
Authors: Suematsu, Y.; Ichimoto, K.; Sakurai, T.
Bibcode: 1995itsa.conf..413S
Altcode:
No abstract at ADS
Title: Comparison of Synoptic Maps of Solar Soft X-Ray Features,
Photospheric Magnetic Fields, and Helium 1083 NM
Authors: Harvey, J.; Slater, G.; Nitta, N.; Shibata, K.; Tsuneta,
S.; Sakurai, T.; Hara, H.
Bibcode: 1994AAS...18512308H
Altcode: 1994BAAS...26Q1523H
We studied the wealth of structural features visible in Yohkoh/SXT
and NSO/KP synoptic maps of the Sun that cover Carrington rotations
1847 through 1879. In order to do this comparison, various methods to
reduce soft X-ray maps to simple structural elements were explored. In
the end, the best way of comparing the various data sets turned out to
be to filter the X-ray maps to emphasize high-spatial frequencies and
then to either simply blink the various images or to make colorized
composite maps that distinctively assign different colors to various
quantities. Among the results are: 1. Active regions exhibit normal or
"anemone" (fountain-like) X-ray loop structure tendency depending on
whether the surrounding large-scale unipolarity of the magnetic field
is small or large. 2. There is a systematic twist of the coronal loops
around magnetic concentrations in the southern hemisphere and vice-versa
in the north. The sense is the same as one would expect from the action
of differential rotation. 3. Dark lanes in the X-ray images are centered
over large-scale polarity patterns of one sign or the other. 4. The
X-ray loops at the boundaries between large-scale opposite polarity
patterns are frequently strongly sheared. The presence or absence of
a filament in these locations may be related in a complicated way to
the amount of shear. 5. At the resolution of the synoptic maps, the
footpoints of X-ray loops are almost always rooted in locally strong
magnetic concentrations and also in extra-dark 1083 nm elements.
Title: The Magnetic Evolution of the Activity Complex AR:7260 -
a Roadmap
Authors: Leka, K. D.; Canfield, R. C.; Mickey, D. L.; van
Driel-Gesztelyi, L.; Nitta, N.; Sakurai, T.; Ichimoto, K.
Bibcode: 1994SoPh..155..301L
Altcode:
The active region NOAA 7260 rotated onto the north solar hemisphere
as a mature bipole: a dominant negative-polarity sunspot with trailing
plage and scattered small spots in attendance. The dominantp spot itself
had strong magnetic fields and covered almost 400 × 10−6
of a solar hemisphere. For a period of seven days beginning 14 August,
1992 this active region displayed rapid and drastic evolution: no fewer
than 50 magnetic bipoles emerged in the area trailing the large sunspot,
increasing the region's magnetic flux by more than 1022
Mx. This new group of sunspots formed a complexβγδ configuration
with twoδ spots and a high degree of magnetic shear.
Title: Peculiar magnetic field evolution of active region NOAA 7562
in August 1993 - results from campaign observation with Yohkoh
Authors: Sakurai, T.; Suematsu, Y.; Ichimoto, K.; Kurokawa, H.; Kitai,
R.; Akioka, M.; Tohmura, I.; Soltau, D.; Mickey, D. L.; Zhang, H.;
Li, W.; Zirin, H.; Tang, F.
Bibcode: 1994ESASP.373..337S
Altcode: 1994soho....3..337S
No abstract at ADS
Title: Preflare State
Authors: Rust, David M.; Sakurai, Takashi; Gaizauskas, Victor; Hofmann,
Axel; Martin, Sara F.; Priest, Eric R.; Wang, Jing-Xiu
Bibcode: 1994SoPh..153....1R
Altcode:
Discussion on the preflare state held at the Ottawa Flares 22
Workshop focused on the interpretation of solar magnetograms and
of Hα filament activity. Magnetograms from several observatories
provided evidence of significant build up of electric currents in
flaring regions. Images of X-ray emitting structures provided a clear
example of magnetic relaxation in the course of a flare. Emerging
and cancelling magnetic fields appear to be important for triggering
flares and for the formation of filaments, which are associated with
eruptive flares. Filaments may become unstable by the build up of
electric current helicity. Examples of heliform eruptive filaments
were presented at the Workshop. Theoretical models linking filaments
and flares are briefly reviewed.
Title: Yohkoh observations of the creation of high-temperature plasma
in the flare of 16 December 1991
Authors: Culhane, J. L.; Phillips, A. T.; Inda-Koide, M.; Kosugi,
T.; Fludra, A.; Kurokawa, H.; Makishima, K.; Pike, C. D.; Sakao, T.;
Sakurai, T.; Doschek, G. A.; Bentley, R. D.
Bibcode: 1994SoPh..153..307C
Altcode:
Yohkoh observations of an impulsive solar flare which occurred on 16
December, 1991 are presented. This flare was a GOES M2.7 class event
with a simple morphology indicative of a single flaring loop. X-ray
images were taken with the Hard X-ray Telescope (HXT) and soft X-ray
spectra were obtained with the Bragg Crystal Spectrometer (BCS)
on board the satellite. The spectrometer observations were made at
high sensivity from the earliest stages of the flare, are continued
throughout the rise and decay phases, and indicate extremely strong
blueshifts, which account for the majority of emission in CaXIX during
the initial phase of the flare. The data are compared with observations
from other space and ground-based instruments. A balance calculation
is performed which indicates that the energy contained in non-thermal
electrons is sufficient to explain the high temperature plasma which
fills the loop. The cooling of this plasma by thermal conduction
is independently verified in a manner which indicates that the loop
filling factor is close to 100%. The production of `superhot' plasma
in impulsive events is shown to differ in detail from the morphology
and mechanisms appropriate for more gradual events.
Title: Spectroscopic Observations of Coronal Emission Lines and
their Relation to Soft X-ray Images
Authors: Ichimoto, K.; Kumagai, K.; Sakurai, T.; Hara, H.; Takeda,
A.; YOHKOH SXT Team
Bibcode: 1994kofu.symp..113I
Altcode:
Spectroscopic observations of coronal emission lines were carried out
to obtain the plasma distribution at different temperatures. Images
in FeXIV5303A(green), FeX6374A(red) and CaXV5694A(yellow) are compared
with the soft X-ray images taken with the Soft X-ray Telescope on board
Yohkoh. It is found that the distributions of green and red lines are
quit different from that of the soft X-rays, while the yellow line
shows quite similar distribution.
Title: A Potential Field Model for Open Field Lines in the Active
Region Corona
Authors: Sakurai, T.
Bibcode: 1994kofu.symp..363S
Altcode:
An extention of the conventional source surface model is constructed by
using the Green's function method for potential magnetic fields. The
method is applied to simple configurations such as dipole fields to
examine the performance of the proposed method. Then the method is
applied to a real magnetogram and the result is compared with observed
structures in the corona.
Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux
Authors: Nitta, N.; Driel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.;
Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K.
Bibcode: 1994kofu.symp..385N
Altcode:
Active region NOAA 7260 exhibited remarkable flare activity as an
emerging flux region appeared in the following part and evolved into
the delta configuration. While it is difficult to associate an emerging
bipole with a flare both temporally and spatially, there is an overall
correlation of the total darkness integrated over of the sunspot area,
as measured in the Yohkoh/SXT white-light images, with the soft X-ray
flux and flare occurrence. It appears that the flares in the emerging
flux region occurred preferentially at locations close to the spot of
preceding polarity that emerged in the earliest evolution of the region.
Title: Electron precipitation and mass motion in the 1991 June 9
white-light flare
Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Metcalf, T. R.;
Hiei, E.; Sakurai, T.; Ichimoto, K.
Bibcode: 1994SoPh..151..389D
Altcode:
We use Hα line profiles as a diagnostic of mass motion and
nonthermal electron precipitation in the white-light flare (WLF) of
1991 June 9 01:34 UT. We find only weak downflow velocities (≈10 km
s−1) at the site of white-light emission, and comparable
velocities elsewhere.We also find that electron precipitation is
strongest at the WLF site. We conclude that continuum emission in this
flare was probably caused by nonthermal electrons and not by dynamical
energy transport via a chromospheric condensation.
Title: Behavior of Accelerated Electrons in a Small Impulsive Solar
Flare on 1992 August 12
Authors: Takano, Toshiaki; Enome, Shinzo; Nakajima, Hiroshi; Shibasaki,
Kiyoto; Nishio, Masanori; Hanaoka, Yoichiro; Torii, Chikayoshi;
Sekiguchi, Hideaki; Bushimata, Takeshi; Kawashima, Susumu; Shinohara,
Noriyuki; Irimajiri, Yoshihisa; Koshiishi, Hideki; Kosugi, Takeo;
Shiomi, Yasuhiko; Sakurai, Takashi; Ichimoto, Kiyoshi
Bibcode: 1994PASJ...46L..21T
Altcode:
A GOES C1.0-class impulsive flare was observed by the new Nobeyama
Radioheliograph on 1992 August 12 with 1 s temporal and 10('') spatial
resolutions at 17 GHz. The radio flare consisted of an impulsive phase
of ~ 20 s and a decay phase of ~ 90 s. Radio images showed double
sources in the impulsive phase, whereas in the decay phase a single
elongated source appeared which connected the double sources. Soft
X-ray images with Yohkoh/SXT show that the radio double sources
correspond to foot points of newly appearing coronal loops and the
single source was located at the top of one of the loops. The radio
emission for both phases can be explained by gyrosynchrotron radiation
from accelerated electrons. These loops began to brighten at their
intersecting point ~ 1 min before the radio flare. All of these facts
suggest that the reconnection of magnetic fields heated up the coronal
loops and produced accelerated electrons, which ran through the loops,
precipitated onto the foot points, and caused the radio flare. Less
than 1% of the electrons were mirrored at the foot points and trapped
at the top of the loop. The lack of radio emission in the loop top area
during the impulsive phase implies that the accelerated electrons were
highly beamed.
Title: Integrated Force-Balance Equations for the Magnetic Field in
Spherical Geometry
Authors: Sakurai, Takashi
Bibcode: 1994ASPC...68..307S
Altcode: 1994sare.conf..307S
No abstract at ADS
Title: Optical and SXT Observations of the x9 Flare of Nov. 1992
Authors: Ichimoto, K.; Sakurai, T.; Nishino, Y.; Noguchi, M.; Shinoda,
K.; Yamaguchi, A.; Kumagai, K.; Hirayama, T.; Tsuneta, S.; Acton, L.
Bibcode: 1994xspy.conf..259I
Altcode:
No abstract at ADS
Title: The Achievements of YOHKOH and the Next Solar Mission
Authors: Sakurai, T.
Bibcode: 1994xspy.conf..231S
Altcode:
No abstract at ADS
Title: Time Variation of Hard X-Ray Image in the Early Phase of
Solar Impulsive Bursts
Authors: Takakura, T.; Inda, M.; Makishima, K.; Masuda, S.; Kosugi,
T.; Sakao, T.; Sakurai, T.; Ogawara, Y.
Bibcode: 1994xspy.conf...71T
Altcode:
No abstract at ADS
Title: Coronal activities and photospheric magnetic field changes
Authors: Sakurai, T.
Bibcode: 1994smf..conf..387S
Altcode:
No abstract at ADS
Title: Flares in Active Region NOAA 7260
Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Sakurai,
T.; Shibata, K.; Ichimoto, K.; Canfield, R. C.; Wülser, J. -P.;
Metcalf, T. R.; Mickey, D. L.
Bibcode: 1994xspy.conf..111N
Altcode:
No abstract at ADS
Title: X-Ray Activity in Coronal Loops and its
Photospheric/chromospheric Signatures
Authors: Sakurai, T.; Shibata, K.; Ichimoto, K.; Takata, M.
Bibcode: 1994xspy.conf...37S
Altcode:
No abstract at ADS
Title: Quadrupole Magnetic Field in Arcade-Type Flares-A
Model of "dark Filament in Neutral Sheet" and "interchange
Collapse-Reconnection Model for Arcade-Type -
Authors: Uchida, Y.; McAllister, A.; Khan, J.; Sakurai, T.; Jockers, K.
Bibcode: 1994xspy.conf..161U
Altcode:
No abstract at ADS
Title: Solar faculae observed at Mitaka during the period 1951 - 1991.
Authors: Irie, M.; Obara, S.; Wakabayashi, A.; Okashita, N.; Ohtani,
R.; Sakon, R.; Sakurai, T.
Bibcode: 1993RNAOJ...2..403I
Altcode: 1993RNOAJ...2..403I
The authors analysed solar faculae during the period 1951 - 1991, which
were recorded on daily sketches of sunspots by using a 20 cm Zeiss
refractor at Mitaka. Faculae in the polar regions (polar faculae) were
counted individually, and the authors derived monthly counts of polar
faculae. For faculae in lower latitudes (active region faculae), they
measured the area covered by the faculae. The authors the constructed
a butterfly diagram similar to those for sunspots. It is found that
polar faculae appear in the region of low coronal brightness.
Title: Construction of magneto-otical filters and their application
to solar observations.
Authors: Miyazaki, H.; Sakurai, T.; Okamoto, T.; Ichimoto, K.;
Miyashita, M.; Koyama, K.; Sakata, A.; Wada, S.
Bibcode: 1993RNAOJ...2..417M
Altcode: 1993RNOAJ...2..417M
The magneto-optical filter (MOF) is made of a transparent cell placed in
a strong magnetic field. The cell contains the vapor of material like
sodium. By utilizing the magneto-optical effect near the wavelength
of a spectral line, MOF provides a very narrow and stable passband,
which is particularly suitable for helioseimological observations. The
cell becomes less transparent as it ages, and can only be used in
observations for several months. The authors' aim was to construct a
filter that can be continuously used for several years without losing
the sodium vapor at the cell wall. This was achieved by heating the
whole cell to about 200°C.
Title: Time Variation of the Hard X-Ray Image during the Early Phase
of Solar Impulsive Bursts
Authors: Takakura, Tatsuo; Inda, Mika; Makishima, Kazuo; Kosugi, Takeo;
Sakao, Taro; Masuda, Satoshi; Sakurai, Takashi; Ogawara, Yoshiaki
Bibcode: 1993PASJ...45..737T
Altcode:
The time variations of hard X-ray images of four impulsive bursts with
simple source structures were investigated in a comparison with the
magnetic structure. Two of them are limb bursts. Common variations
during the early phase are as follows: i) The hard X-ray brightening
seems to start at the top of a single coronal loop. ii) The X-ray
source spreads during the increasing phase of the burst in both
directions along the loop, and both ends become brighter, especially
at higher energies with generally unequal brightness. The loop top is
still bright, especially at lower energies, to show three peaks. The
speed of the expansion of the X-ray source amounts to about 10(4) km
s(-1) in three cases. iii) At and after the peak of the X-ray flux,
the source tends to be a single source at the loop top, especially
at lower energies. iv) The effective temperature for quasi-thermal
electrons and their number density during the early phase in the
vicinity of the loop top are (4--6)times 10(7) K and (5--2)times 10(9)
cm(-3) , respectively, so that the electron mean free path is greater
than three-times the local temperature scale height. These observations
are consistent with the idea that anomalous resistivity, which triggers
impulsive bursts, is caused by electron plasma waves generated in the
process of heat conduction.
Title: Magnetic field structures and flares
Authors: Sakurai, Takashi
Bibcode: 1993AdSpR..13i.109S
Altcode: 1993AdSpR..13..109S
Recent studies on the relation between flares and magnetic field
structures are reviewed. Observations of electric currents in the solar
atmosphere are summarized, and a theoretical framework to interpret
the observations is presented.
Title: Detection of electronic-shell structure in divalent-metal
clusters (Hg)n
Authors: Ito, H.; Sakurai, T.; Matsuo, T.; Ichihara, T.; Katakuse, I.
Bibcode: 1993PhRvB..48.4741I
Altcode:
The electronic-shell structure in divalent-metal clusters
(Hg)n was observed from the size distribution of
mercury-cesium clusters, (Hg)nCs+. The cluster
ions (Hg)nCs+ were produced by Cs+ ion
bombardment of an amalgam of mercury and silver, and the mass analysis
was performed using a grand-scale sector mass spectrometer. For the
cluster size n>40, the spectral behavior exhibited consecutive
hills and valleys, which was interpreted in terms of electronic-shell
structure of pure mercury clusters (Hg)n. The evidence for
electronic-shell structure of clusters containing up to 1500 valence
electrons was clearly shown. The presumptive evidence for the phase
shift of magic numbers theoretically expected from a quantum beating
effect were found at the region of the cluster size around n=500.
Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux
and Reconnection
Authors: Nitta, N.; Drel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.;
Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K.
Bibcode: 1993BAAS...25.1223N
Altcode:
No abstract at ADS
Title: Solar Flare Telescope project.
Authors: Ichimoto, K.; Sakurai, T.; Hiei, E.; Nishino, Y.; Shinoda, K.;
Noguchi, M.; Imai, H.; Irie, M.; Miyashita, M.; Tanaka, N.; Kumagai,
K.; Sano, I.; Suematsu, Y.; Okamoto, T.; Miyazaki, H.; Fukushima,
H.; Yose, Y.; Zhao, Z. W.
Bibcode: 1993RNAOJ...1..375I
Altcode: 1993RNOAJ...1..375I
The Solar Flare Telescope was built at Mitaka in 1989. This instrument
comprises four telescopes which observe (1) Hα images, (2) continuum
images, (3) vector magnetic fields, and (4) Doppler velocities,
respectively. The instrument aims at the study of energy build-up
and energy release in solar flares, in cooperation with the Solar-A
satellite (Yohkoh). The Solar Flare Telescope has been in operation
since 1990 December.
Title: Magnetic Field Observation with the Solar Flare Telescope
Authors: Ichimoto, K.; Sakurai, T.; Nishino, Y.; Shinoda, K.; Noguchi,
M.; Kumagai, K.; Imai, H.; Irie, M.; Miyashita, M.; Tanaka, N.; Sano,
I.; Suematsu, Y.; Hiei, E.
Bibcode: 1993ASPC...46..166I
Altcode: 1993IAUCo.141..166I; 1993mvfs.conf..166I
No abstract at ADS
Title: A Simulation Study of the Solar Wind Including the Solar
Rotation Effect
Authors: Washimi, H.; Sakurai, T.
Bibcode: 1993SoPh..143..173W
Altcode:
An axisymmetric solar wind structure including the solar rotation
effect is studied by the method of MHD computer simulation. For the
case of the radial magnetic field configuration, the simulation result
is fairly well coincident with the steady-state solution. For the
case of the dipole magnetic field configuration, the properties of
the solution depend on the ratio of the gas pressure to the magnetic
pressureβ-ratio) in the model. If theβ-ratio is small, a clearly
defined stagnation region appears in the wind, in which the flow speed
is very small and the azimuthal magnetic field is very weak because
of the corotation of the plasma. If theβ-ratio is greater than 1,
the plasma is not effectively trapped by the magnetic field so that
the stagnation region is not clearly defined in the solution.
Title: Observations of X-ray Jets Using YOHKOH Soft X-Ray Telescope
Authors: Shibata, K.; Ishido, Y.; Acton, L.; Strong, K.; Hirayama,
T.; Uchida, Y.; McAllister, A.; Matsumoto, R.; Tsuneta, S.; Shimizu,
T.; Hara, H.; Sakurai, T.; Ichimoto, K.; Nishino, Y.; Ogawara, Y.
Bibcode: 1993ASPC...46..343S
Altcode: 1993IAUCo.141..343S; 1993mvfs.conf..343S
No abstract at ADS
Title: Computational Modeling of Solar Magnetic Fields (Invited)
Authors: Sakurai, T.
Bibcode: 1993ASPC...46...91S
Altcode: 1993IAUCo.141...91S; 1993mvfs.conf...91S
No abstract at ADS
Title: Ten Years of the Okayama Vector Magnetograph
Authors: Makita, M.; Sakurai, T.; Shibasaki, K.; Koyano, H.
Bibcode: 1993ASPC...46..180M
Altcode: 1993IAUCo.141..180M; 1993mvfs.conf..180M
No abstract at ADS
Title: Correlation between X-ray Temporal Variability and Magnetic
Environment in Solar Flares
Authors: Nitta, N.; Harvey, K.; Hudson, H.; Ichimoto, K.; Metcalf,
T.; Mickey, D.; Sakai, J. -I.; Sakao, T.; Sakurai, T.; Takahashi, M.
Bibcode: 1992AAS...181.5503N
Altcode: 1992BAAS...24.1211N
The X-ray time history of a solar flare can reflect basic processes of
heating and/or acceleration, which in turn may depend on the magnetic
environment of the site. Some flares show a simple rise and fall
temporal behavior, whereas others show more than one peak. Comparisons
of images taken by the Soft X-ray Telescope (SXT) aboard the Yohkoh
spacecraft with ground-based magnetic data (Hawaii, Kitt Peak and
Mitaka) reveal that, at least for a flare-productive active region
(NOAA 7260), flares with double-peaked and single-peaked time profiles
occurred at systematically different locations within the region. We
discuss this result in terms of theoretical models, especially those
of coalescence of two current loops.
Title: Flare-Related Relaxation of Magnetic Shear as Observed with
the Soft X-Ray Telescope of YOHKOH and with Vector Magnetographs
Authors: Sakurai, Takashi; Shibata, Kazunari; Ichimoto, Kiyoshi;
Tsuneta, Saku; Acton, Loren W.
Bibcode: 1992PASJ...44L.123S
Altcode:
The soft X-ray Telescope of Yohkoh observed an M-class flare on 1992
February 6. As the flare progressed, an initially sheared coronal loop
structure was seen to evolve toward a relaxed magnetic configuration. In
association with this evolution, the vector magnetograph observations
detected a decrease in the electric currents.
Title: Observations of X-Ray Jets with the YOHKOH Soft X-Ray Telescope
Authors: Shibata, Kazunari; Ishido, Yoshinori; Acton, Loren W.; Strong,
Keith T.; Hirayama, Tadashi; Uchida, Yutaka; McAllister, Alan H.;
Matsumoto, Ryoji; Tsuneta, Saku; Shimizu, Toshifumi; Hara, Hirohisa;
Sakurai, Takashi; Ichimoto, Kiyoshi; Nishino, Yohei; Ogawara, Yoshiaki
Bibcode: 1992PASJ...44L.173S
Altcode:
Time series of Soft X-ray Telescope images have revealed many X-ray
jets in the solar corona. The typical size of a jet is 5 times 10(3)
--4 times 10(5) km, the translational velocity is 30--300 km s(-1) ,
and the corresponding kinetic energy is estimated to be 10(25) --10(28)
erg. Many of the jets are associated with flares in X-ray bright points,
emerging flux regions, or active regions. They sometimes occur several
times from the same X-ray feature. In some cases, a dark void appears
after ejection at the footpoint of the jet. The void seems to be
the result of a change in the topology of the X-ray emitting plasma,
perhaps due to magnetic reconnection. Some jets show a structure which
suggests a helical magnetic field configuration along the jet. One
of the jets associated with a flaring bright point was identified as
being an Hα surge. In this case, the X-ray bright point is situated
just on the Hα bright point at the footpoint of the surge. The top
of the surge is not bright in X-rays. We briefly discuss the origin
of these newly discovered X-ray jets.
Title: Book-Review - Dynamics and Structure of Quiescent Solar
Prominences
Authors: Priest, E. R.; Sakurai, T.
Bibcode: 1992SSRv...61..430P
Altcode:
No abstract at ADS
Title: The X10 Flare of 1991 June 9: White Light, H-alpha, Magnetic
Fields, and Electric Currents
Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Metcalf, T. R.;
Hiei, E.; Sakurai, T.; Ichimoto, K.
Bibcode: 1992AAS...180.4108D
Altcode: 1992BAAS...24..795D
We present observations by several instruments of the white-light
flare (WLF) of 1991 June 9 01:34 UT. A white-light image from the
National Astronomical Observatory of Japan indicates the location
of the WLF within the active region (NOAA 6659). Stokes polarimetry
from Mees Solar Observatory (University of Hawai`i) yields a vector
magnetogram and a map of the vertical electric currents. Also from
Mees comes Hα imaging spectroscopy, which is an indicator of such
specific chromospheric processes as nonthermal electron precipitation,
high coronal pressure, and mass motion. Both Mees instruments provide
continuum images, allowing coregistration of the various datasets. The
white-light emission arises from an extended area including both
sunspot umbra and penumbra. The active region magnetic field is strongly
nonpotential and has numerous vertical electric current channels. The
WLF is situated in an area of relatively low current density at the
edge of the strongest current in the active region, and the nearby
magnetic neutral line is highly sheared. The WLF site shows electron
precipitation, but only in its penumbral portion; stronger magnetic
mirroring in the umbra may inhibit precipitation there. Also, the
precipitation is not especially strong (relative to that observed
elsewhere in this flare), suggesting that the electron-beam model
for WLFs is not appropriate in this case. Also, the lack of strong
redshifts argues against a dynamical energy transport model.
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
Flux Tubes - Part Three
Authors: Goossens, Marcel; Hollweg, Joseph V.; Sakurai, Takashi
Bibcode: 1992SoPh..138..233G
Altcode:
The resonances that appear in the linear compressible MHD formulation
of waves are studied for equilibrium states with flow. The conservation
laws and the jump conditions across the resonance point are determined
for 1D cylindrical plasmas. For equilibrium states with straight
magnetic field lines and flow along the field lines the conserved
quantity is the Eulerian perturbation of total pressure. Curvature
of the magnetic field lines and/or velocity field lines leads to more
complicated conservation laws. Rewritten in terms of the displacement
components in the magnetic surfaces parallel and perpendicular to the
magnetic field lines, the conservation laws simply state that the waves
are dominated by the parallel motions for the modified slow resonance
and by the perpendicular motions for the modified Alfvén resonance.
Title: White-Light Flares of 1991 June in the NOAA Region 6659
Authors: Sakurai, Takashi; Ichimoto, Kiyoshi; Hiei, Eijiro; Irie,
Makoto; Kumagai, Kazuyoshi; Miyashita, Masakuni; Nishino, Yohei;
Yamaguchi, Kisuke; Fang, Geng; Kambry, Maspul Aini; Zhao, Zhaowang;
Shinoda, Kazuya
Bibcode: 1992PASJ...44L...7S
Altcode:
We report on observations of flare activities in an active region NOAA
6659, which appeared on the sun in 1991 June. Among six X-class flares
in this region, we observed three flares (June 4, 9, and 11), all of
which were white-light flares. A detailed discussion is given concerning
a particularly interesting white-light flare which occurred on June 11.
Title: Fine Structures of Chromospheric Magnetic Field and Material
Flow in a Solar Active Region
Authors: Zhang, Hongqi; Ai, Guoxiang; Sakurai, T.; Kurokawa, H.
Bibcode: 1991SoPh..136..269Z
Altcode:
In this paper, we analyze the relations between photospheric vector
magnetic fields, chromospheric longitudinal magnetic fields and velocity
fields in a solar active region. Agreements between the photospheric and
chromospheric magnetograms can be found in large-scale structures or in
the stronger magnetic structures, but differences also can be found in
the fine structures or in other places, which reflect the variation of
the magnetic force lines from the photosphere to the chromosphere. The
chromospheric superpenumbral magnetic field, measured by the Hβline,
presents a spoke-like structure. It consists of thick magnetic fibrils
which are different from photospheric penumbral magnetic fibrils. The
outer superpenumbral magnetic field is almost horizontal. The direction
of the chromospheric magnetic fibrils is generally parallel to the
transverse components of the photospheric vector magnetic fields. The
chromospheric material flow is coupled with the magnetic field
structure. The structures of the Hβ chromospheric magnetic fibrils
in the network are similar to Hβ dark fibrils, and the feet of the
magnetic fibrils are located at the photospheric magnetic elements.
Title: Katsuo Tanaka (1943 - 1990).
Authors: Hiei, E.; Sakurai, T.
Bibcode: 1991SoPh..136.....H
Altcode:
No abstract at ADS
Title: Obituary
Authors: Hiei, E.; Sakurai, T.
Bibcode: 1991SoPh..136D...4H
Altcode:
No abstract at ADS
Title: Observations from the Hinotori Mission
Authors: Sakurai, Takashi
Bibcode: 1991RSPTA.336..339S
Altcode:
The satellite Hinotori was launched in 1981 by the Institute of Space
and Astronautical Science of Japan. Two major experiments on board the
Hinotori satellite were a hard X-ray imaging telescope with modulation
collimators, and a high dispersion soft X-ray crystal spectrometer
utilizing the Bragg diffraction of X-rays on quartz crystals. These
two instruments have revealed for the first time that solar flares show
varying characteristics depending on the environment of flaring regions,
and that flares produce plasmas as hot as 3-4 × 107 K.
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
Flux Tubes - Part One
Authors: Sakurai, Takashi; Goossens, Marcel; Hollweg, Joseph V.
Bibcode: 1991SoPh..133..227S
Altcode:
A basic procedure is presented for dealing with the resonance problems
that appear in MHD of which resonant absorption of waves at the
Alfvén resonance point is the best known example in solar physics. The
procedure avoids solving the full fourth-order differential equation
of dissipative MHD by using connection formulae across the dissipation
layer.
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
Flux Tubes II. Absorption of Sound Waves by Sunspots
Authors: Sakurai, Takashi; Goossens, Marcel; Hollweg, Joseph V.
Bibcode: 1991SoPh..133..247S
Altcode:
The absorption of solar five-min oscillations by sunspots is interpreted
as the resonant absorption of sound waves by a magnetic cylinder. The
absorption coefficient is calculated both analytically under
certain simplifying assumptions, and numerically under more general
conditions. The observed magnitude of the absorption coefficient,
which is up to 0.5 or even more, can be explained for suitable ranges
of parameters. Limitations in the present model are also discussed.
Title: VLBI Angular Broadening Studies of Plasma Turbulence in The
Solar Wind
Authors: Sakurai, T.; Spangler, S. R.; Mutel, R. L.; Armstrong, J.
Bibcode: 1991BAAS...23..939S
Altcode:
No abstract at ADS
Title: Solar Meriodional Motions Derived from Sunspot Observations
Authors: Kambry, M. A.; Nishikawa, J.; Sakurai, T.; Ichimoto, K.;
Hiei, E.
Bibcode: 1991SoPh..132...41K
Altcode:
Sunspot drawings obtained at the National Astronomical Observatory of
Japan during the years 1954-1986 were used to determine meridional
motions of the Sun. A meridional flow of a few ms−1
was found, which is equatorward in the latitude range from -20°
to +15° and is poleward at higher latitudes in both hemispheres. A
northward flow of 0.01° day−1 or 1.4 ms−1
at mid-latitudes (between 10° and 20°) was also detected. From our
limited data-set of three solar cycles, an indication of solar-cycle
dependence of meridional motions was found.
Title: Observations from the Hinotori mission.
Authors: Sakurai, T.
Bibcode: 1991psf..conf..339S
Altcode:
The satellite Hinotori was launched in 1981 by the Institute of Space
and Astronautical Science of Japan. Two major experiments on board the
Hinotori satellite were a hard X-ray imaging telescope with modulation
collimators, and a high dispersion soft X-ray crystal spectrometer
utilizing the Bragg diffraction of X-rays on quartz crystals. These
two instruments have revealed for the first time that solar flares show
varying characteristics depending on the environment of flaring regions,
and that flares produce plasmas as hot as 3 - 4×107K.
Title: Coronal observations with Solar-A satellite
Authors: Sakurai, Takashi
Bibcode: 1991AdSpR..11a.349S
Altcode: 1991AdSpR..11..349S
The Solar-A satellite carries the soft X-ray telescope (SXT), which is a
modernized version of Skylab's soft X-ray telescopes. The SXT instrument
is expected to make significant contributions to the research into
the physics of the solar corona. This review discusses coronal heating
mechanisms, the physics of coronal loops, X-ray bright points, coronal
holes, and coronal mass ejections, together with the observational
approach to these topics to be taken by the SXT instrument.
Title: Active regions, sunspots and their magnetic fields.
Authors: Semel, Meir; Mouradian, Zadig; Soru-Escaut, Irina; Maltby,
Per; Rees, David; Makita, Mitsugu; Sakurai, Takashi
Bibcode: 1991sia..book..844S
Altcode:
Surface magnetism is the progenitor of active regions, sunspots, and
all related phenomena. This cause and effect is reversible so that,
using well-established empirical laws, the presence and morphology of
photospheric magnetic fields can be deduced from active-region light
emission structure. In the (simplifying) case of sunspots, MHD and
thermodynamic theory find some success in the interpretation of the
interaction of magnetic fields and solar plasma. Coronal magnetic fields
also appear to be predictable by extrapolation techniques starting
from the photospheric conditions. Alternatively, surface magnetism can
be observed "directly" by means of the spectroscopic Zeeman effect
and Stokes polarimetry. Eventually these empirical, theoretical and
direct-measurement techniques must converge to identical results as
we better understand the physics of active regions.
Title: Magnetic Structures in the Corona
Authors: Sakurai, T.
Bibcode: 1991LNP...387..245S
Altcode: 1991LNP...387..243S; 1991fpsa.conf..245S
The research into the physics of the solar corona by using the solar-A
satellite is reviewed. Discussions are given on coronal heating
mechanisms, the physics of coronal loops, coronal holes, coronal mass
ejections, and the observational approach to these topics to be taken
by the Solar-A satellite.
Title: Solar Flare Telescope and 10-cm New Coronagraph
Authors: Ichimoto, K.; Sakurai, T.; Yamaguchi, A.; Kumagai, K.;
Nishino, Y.; Suematsu, Y.; Hiei, E.; Hirayama, T.
Bibcode: 1991LNP...387..320I
Altcode: 1991fpsa.conf..320I
Two new telescopes were built at the National Astronomical
Observatory of Japan, i.e. the Solar Flare Telescope and the 10-cm New
Coronagraph. The Solar Flare Telescope was constructed at Mitaka to make
observations of photospheric velocity fields, vector magnetic fields,
and H and continuum images of active regions simultaneously. The whole
system will be completed in 1991. The 10-cm new coronagraph, which
was developed to make precise measurements of the coronal intensity in
several wavelengths, has already been in operation for one year at the
Norikura Solar Observatory. At present the accuracy of about 10-6 of
the solar disk intensity is achieved in continuum light, but further
improvement in the photometric accuracy remains to be done.
Title: Helioseismology observations by Solar-A satellite
Authors: Sakurai, Takashi
Bibcode: 1991AdSpR..11d..89S
Altcode: 1991AdSpR..11...89S
The observations of brightness oscillations of the Sun which will
be made by using the aspect telescope on board the Solar-A satellite
are discussed.
Title: In Memoriam - Katsuo Tanaka
Authors: Zirin, Harold; Sakurai, Takashi
Bibcode: 1991LNP...387....5Z
Altcode: 1991fpsa.conf....5Z
No abstract at ADS
Title: Large sunspot group of March 1989 (NOAA 5395).
Authors: Irie, M.; Yamaguchi, K.; Sakurai, T.
Bibcode: 1991RNAOJ...1..193I
Altcode: 1991RNOAJ...1..193I
A large sunspot group (NOAA region 5395) was observed in March
1989. This sunspot group is one of the largest ever observed, and its
area reached 3600×10-6 of the solar hemisphere. This region
showed many peculiarities, namely (1) it appeared at a high latitude
(33°N), (2) it was composed of a single huge penumbra in which
several umbrae with different polarities were embedded (the so-called
δ-configuration), and (3) its magnetic field is predominantly made
of the following polarity. The evolution of this region is traced
for three solar rotations. Flare and surge activities are studied in
relation to the magnetic shear and sunspot motions in this region.
Title: New perspectives in flare models and theories
Authors: Sakurai, Takashi
Bibcode: 1991AdSpR..11e..17S
Altcode: 1991AdSpR..11...17S
Solar flares are understood as a process of explosive liberation of
magnetic energy, coming after a slow phase of energy build up. This
review first discusses the slow evolution and the instabilities of
magnetic equilibria. Then the energy release mechanisms, particularly
the magnetic field reconnection, are discussed. Lastly the flare models
proposed so far are reviewed.
Title: Multiple ground-based and satellite observations of global
Pi 2 magnetic pulsations
Authors: Yumoto, K.; Takahashi, K.; Sakurai, T.; Sutcliffe, P. R.;
Kokubun, S.; Lühr, H.; Saito, T.; Kuwashima, M.; Sato, N.
Bibcode: 1990JGR....9515175Y
Altcode:
Four Pi 2 magnetic pulsations, observed on the ground at L=1.2-6.9
in the interval from 2300 UT on May 22 to 0300 UT on May 23, 1985,
provide new evidence of a global nature of Pi 2 pulsations in the inner
(L<~7) region of the magnetosphere bounded by the plasma sheet
during quiet geomagnetic conditions. In the present study, magnetic
data have been collected from stations distributed widely both in
local time and in latitude, including conjugate stations, and from the
AMPTE/CCE spacecraft located in the magnetotail. On the basis of high
time resolution magnetic field data, the following characteristics of
Pi 2 have been established: horizontal components, H and D, of the Pi 2
oscillate nearly antiphase and in-phase, respectively, between the high-
and low-altitude stations in the midnight southern hemisphere. Both
the H and D components of the Pi 2 have nearly in-phase relationships
between the nightside and the dayside stations at low latitude. The
Pi 2 amplitude is larger at the high-latitude station and decreases
toward lower latitudes. The dominant periods of the Pi 2 are nearly
identical at all stations. Although a direct coincidence between
spacecraft-observed and ground-based global Pi 2 events does not exist
for these events, the Pi 2 events are believed to be a forced field
line oscillation of global scale, coupled with the magnetospheric
cavity resonance wave in the inner magnetosphere during the substorm
expansive phase.
Title: Magnetohydrodynamic solar/stellar wind models.
Authors: Sakurai, T.
Bibcode: 1990CoPhR..12..247S
Altcode:
This paper reviews a method to calculate steady, axisymmetric wind
models with frozen-in magnetic fields. The wind solution along
the magnetic field is given by an algebraic equation (the Bernoulli
equation) for the density. There appear two critical points, the slow
mode and the fast mode critical points. The shape of the magnetic field
should be determined in such a way that the force-balance across the
field is satisfied. This requirement leads to a second-order partial
differential equation for the magnetic stream function. This equation
is similar at the Alfvén point, and an additional constraint is
introduced there to obtain a regular solution. A numerical scheme is
developed following this basic formulation, and examples of solutions
are presented. The basic feature of the solution is the poleward
deflection of the flow due to the build-up of toroidal magnetic field
in the wind. The magnetic winds from rotating objects are therefore
collimated along the rotation axis.
Title: Construction of Long-Life Magneto-optical Filters for
Helioseismology Observations
Authors: Sakurai, T.; Tanaka, K.; Miyazaki, H.; Ichimoto, K.; Sakata,
A.; Wada, S.
Bibcode: 1990LNP...367..277S
Altcode: 1990psss.conf..277S
A design of magneto-optical filters we are developing is described. By
heating the cell to about 200°C, a lifetime more than a year has
been achieved.
Title: A comparison between progressive extension method (PEM)
and iterative method (IM) for magnetic field extrapolations in the
solar atmosphere
Authors: Wu, S. T.; Sun, M. T.; Sakurai, Takashi
Bibcode: 1990MmSAI..61..477W
Altcode:
This paper presents a comparison between two numerical methods for
the extrapolation of nonlinear force-free magnetic fields, viz the
Iterative Method (IM) and the Progressive Extension Method (PEM). The
advantages and disadvantages of these two methods are summarized,
and the accuracy and numerical instability are discussed. On the basis
of this investigation, it is claimed that the two methods do resemble
each other qualitatively.
Title: CCD data acquisition system installed on the spectrograph at
the Norikura Solar Observatory.
Authors: Hamana, S.; Kumagai, K.; Ichimoto, K.; Sakurai, T.; Hiei, E.
Bibcode: 1990RNAOJ...1...13H
Altcode: 1990RNOAJ...1...13H
A new data acquisition system is installed on the 25-cm coudé-type
coronagraph at the Norikura Solar Observatory. Three CCD cameras (2/3
inch, 512×480 pixels) are mounted on the spectrograph; one at the short
focal length exit window and the other two at the long focal length exit
window. Fields of view along the entrance slit of the spectrograph are
9'15″ 2'22″and 3'00″ respectively. Spectral images are digitized
and integrated on the image processing unit. The new system has made
possible (1) the observations in the near-infrared wavelength, (2)
high accuracy in data compared to photographic observations, and (3)
the analyses of large amounts of spectral data.
Title: Phase Relation between Velocities and Temperature Fluctuations
of the Solar 5-Minute Oscillation
Authors: Ichimoto, K.; Hamana, S.; Kumagai, K.; Sakurai, T.; Hiei, E.
Bibcode: 1990LNP...367..205I
Altcode: 1990psss.conf..205I
Phase relations between the velocities and temperature oscillations in
the solar photosphere are investigated on the - diagram. Distributions
of the phase differences on the - plane are roughly reproduced
by a simple analytical model, but the detailed fitting is not
satisfactory. In the 5-minute band, temperature reaches its peak when
the atmosphere is moving downward. The amount of the phase difference
between temperature and velocity suggests the radiative damping time
of 1-40 s. Identification of the g-mode oscillation is not clear.
Title: Helioseismology Observations by Solar-A Satellite
Authors: Sakurai, Takashi
Bibcode: 1990LNP...367..253S
Altcode: 1990psss.conf..253S
Helioseismological observations that can be made by using the aspect
telescope on board the Solar-A satellite are discussed.
Title: Computational Modeling of Magnetic Fields in Solar Active
Regions
Authors: Sakurai, Takashi
Bibcode: 1989SSRv...51...11S
Altcode:
The magnetic field plays an important role in various solar
activities. This paper reviews techniques for computational modeling of
magnetic fields in solar active regions. The input data are photospheric
magnetic fields supplied by magnetograph observations. The field above
the photosphere is computed by assuming an equation for the magnetic
field. Three classes of magnetic fields, namely current-free fields,
constant-α force-free fields, and general force-free fields are
considered. Their physical/mathematical significances and computational
procedures are systematically presented.
Title: Magnetic Equilibria and Instabilities
Authors: Sakurai, Takashi
Bibcode: 1989SoPh..121..347S
Altcode: 1989IAUCo.104..347S
Solar flares are understood as a process of explosive liberation of
magnetic energy, coming after a slow phase of energy build-up. The slow
evolution of magnetic equilibria may end up with (a) the termination of
an equilibrium sequence, or (b) an instability. The distinction between
the two can be made by drawing schematic potential curves. Case (a)
has been extensively studied in two-dimensional models. The appearance
of multiple solutions, or disappearance of a solution takes place as
the system evolves away from the current-free configuration. Case (b)
can be discussed in terms of ideal MHD or resistive MHD instabilities. A
possible route to explosive energy release is suggested by combining
these two cases.
Title: Structure of the Solar Corona
Authors: Sakurai, T.; Hiei, E.
Bibcode: 1989HiA.....8..513S
Altcode:
No abstract at ADS
Title: Largescale Magnetic Field Phenomena
Authors: Harrison, R. A.; Bentley, R. D.; Brosius, J.; Dwivedi,
B. N.; Jardine, M.; Klimchuk, J. A.; Kundu, M. R.; Pearce, G.; Saba,
J.; Sakurai, T.; Schmahl, E. J.; Schmelz, J.; Sime, D. G.; Steele,
C. D. C.; Sun, M. T.; Tappin, S. J.; Waljeski, K.; Wang, A. H.; Wu,
S. T.
Bibcode: 1989tnti.conf....1H
Altcode:
No abstract at ADS
Title: AMPTE/CCE observations of substorm-associated standing Alfvén
waves in the midnight sector
Authors: Takahashi, K.; Kokubun, S.; Sakurai, T.; McEntire, R. W.;
Potemra, T. A.; Lopez, R. E.
Bibcode: 1988GeoRL..15.1287T
Altcode:
Magnetic field and medium energy particle data acquired with the
AMPTE/CCE spacecraft have been used to study substorm-associated ULF
pulsations occurring in the midnight sector at a radial distance of 8
to 9 RE. The particle data are used not only to identify ion
injections but also to detect the electric field of ULF waves. A case
study of events observed on 23 May 1985 (day 143) shows that the waves
have the properties of a fundamental-mode standing Alfvén wave. The
waves started nearly simultaneously with the Pi 2 pulsations observed
at the Syowa station (L = 6.1) near midnight local time. However,
different frequencies are observed on the ground and in space, which
is consistent with the view that the Alfvén waves are excited at the
local Alfvén resonance frequency. This CCE observation gives the first
unambiguous evidence of substorm-associated standing Alfvén waves in
the nightside magnetosphere.
Title: Physical Characteristics of Lobes of Luminous Radio Galaxies
Authors: Sakurai, T.; Spangler, S. R.
Bibcode: 1988BAAS...20..734S
Altcode:
No abstract at ADS
Title: Crystal structure of perryite.
Authors: Okada, A.; Ito, T.; Kobayashi, K.; Sakurai, T.
Bibcode: 1987anme...12...76O
Altcode:
No abstract at ADS
Title: A study of magnetic energy build-up based on vector
magnetograms
Authors: Sakurai, T.
Bibcode: 1987SoPh..113..137S
Altcode: 1982SoPh..113..137S
Vector magnetograms taken at the Okayama Astrophysical Observatory
are studied. A sequence of procedures applied to the data for analysis
are explained, and their validity is studied by examining the global
magnetic force balance.
Title: Collimation of Stellar Winds by the Magnetic Field
Authors: Sakurai, T.
Bibcode: 1987IAUS..115..383S
Altcode:
No abstract at ADS
Title: Measurement of the magnetic vector on the sun at the Okayama
Astrophysical Observatory.
Authors: Makita, Mitsugu; Hamana, Shigeo; Sakurai, Takashi; Shimizu,
Minoru
Bibcode: 1987LIACo..27..185M
Altcode: 1987oahp.proc..185M
Observations taken in Japan beginning in 1982 have been used to
obtain magnetic vectors on the sun. Investigation of drift curves
of the measured polarization shows that the residual instrumental
polarization can be represented by trigonometric functions of twice
the sun's hour angle. Peak-to-peak amplitudes of fluctuations in the
line wing (5250 A) and in the neighboring continuum (5282-5297 A) are
0.0041 and 0.0001 respectively. The observed radial velocity drifts
linearly at the rate of about 0.2 to 1 km/s/hr. It is suggested that
corrections could give the accuracy of 10 m/s.
Title: Magnetically collimated winds from accretion disks
Authors: Sakurai, Takashi
Bibcode: 1987PASJ...39..821S
Altcode:
A steady, axisymmetric wind from a magnetized accretion disk is
studied by applying a numerical scheme developed for the stellar
wind modeling (Sakurai, 1985). As in the magnetic stellar winds,
the collimation of the wind toward the rotation axis is seen in
the wind from a disk. The driver of the collimation is the toroidal
magnetic field which develops in the wind due to the rotation of the
disk. Magnetic winds from rotating disks can therefore naturally lead
to the formation of collimated jets, which are found in star-forming
regions and in extragalactic radio sources.
Title: Coronal heating in closely-packed flux tubes: a
Taylor-Heyvaerts relaxation theory.
Authors: Browning, P. K.; Sakurai, T.; Priest, E. R.
Bibcode: 1986A&A...158..217B
Altcode:
The aim of this paper is to take a more quantitative and detailed look
at dissipation in an array of closely-packed flux tubes. An initially
potential coronal loop is investigated, whose footpoints are twisted
up by cellular photospheric motions, forming a network of twisted flux
tubes. The motions are assumed to be slow compared with the reconnection
time-scale, so that the stressed field reconnects and dissipates some
of its energy as heat. The generalised Taylor's hypothesis is used
to investigate the effects of reconnection on the flux tubes and to
determine the efficiency of the dissipation. A basic mathematical
model is set up and the procedure for calculating the evolution is
outlined. The authors investigate the response of the field to the
footpoint motions and evaluate the heating produced. The results are
discussed, applications to the coronal heating problem are considered,
and the predictions are compared with the known heating requirements
of the corona.
Title: VLA Observations of the Radio Galaxies 3C109, 3C223.1,
and 3C379.1
Authors: Sakurai, T.; Spangler, S. R.
Bibcode: 1986BAAS...18Q.689S
Altcode: 1986BAAS...18..689S
No abstract at ADS
Title: Heating Mechanisms of the Solar Corona
Authors: Sakurai, T.
Bibcode: 1986hmps.conf...17S
Altcode:
Several mechanisms proposed for the heating of the solar corona
are reviewed. After briefly going through the classical acoustic
heating theory, magnetic heating mechanisms are discussed. These are
divided into two categories depending on the ratio between the two
time scales: τA (the magnetic response time of coronal
loops) and τc (the time scale of convective motion on the
photosphere). A special attention is paid to the heating by Alfvén
waves undergoing a phase mixing.
Title: Observation of Magnetic Field Vector in Solar Active Regions
Authors: Sakurai, T.; Makita, M.
Bibcode: 1986hmps.conf...53S
Altcode:
No abstract at ADS
Title: Solar Vector Magnetograms of the Okayama Astrophysical
Observatory
Authors: Makita, Mitsugu; Hamana, Shigeo; Nishi, Keizo; Shimizu,
Minoru; Sakurai, Takashi; Shibasaki, Kiyoto
Bibcode: 1986Ap&SS.118..163M
Altcode:
The title instrument is mounted on the 65 cm solar Coudé telescope
at the Okayama observatory. Observation is usually of the Fe i 5250
Å line. The data obtained are briefly described.
Title: Magnetic stellar winds: a 2-D generalization of the Weber-Davis
model.
Authors: Sakurai, T.
Bibcode: 1985A&A...152..121S
Altcode:
A numerical method is presented to calculate steady, axisymmetric
wind models with frozen-in magnetic fields. As a straigthforward
generalization of the model of Weber and Davis (1967), the wind solution
along the magnetic field is obtained by an algebraic Bernoulli equation
for the density. The important feature found in the numerical solution
is the poleward deflection of the wind flow due to the magnetic force
of spiraling field lines. Asymptotic behaviour of the solution at large
distances shows that the flow does not become radial but is collimated
in the direction of the rotation axis.
Title: Limits on thermal plasma in the lobes of the radio galaxies
3C 79 and3C 379.1
Authors: Spangler, S. R.; Sakurai, T.
Bibcode: 1985ApJ...297...84S
Altcode:
Dual-frequency VLA polarimetric observations of the radio galaxies
3C 79 and 3C 379.1 show remarkably small Faraday rotation gradients
across most or all of both sources. The measured rms difference between
the 6 and 20 cm polarization position angles is as low as 2° for
portions of 3C 379.1. These measurements are used to place limits on
internal Faraday rotation and thus on the thermal plasma density in
the radio-emitting lobes. The upper limits to the thermal density are
≡4×10-5cm-3 for both 3C 79 and 3C 379.1.
Title: Observation of Magnetic Field Vector in Solar Active Regions
Authors: Sakurai, T.; Makita, M.; Shibasaki, K.
Bibcode: 1985tphr.conf..313S
Altcode:
The vector magnetograph at Okayama Astrophysical Observatory has been
in operation since 1982. The conversion from the observed polarization
degrees to the magnetic field components is made by using Unno's formula
(1956) applied to a homogeneous magnetic field.
Title: Phase Mixing of Alfven Waves
Authors: Sakurai, T.
Bibcode: 1985tphr.conf..263S
Altcode:
In the theory of coronal heating by means of Alfvén waves, a process
called "the phase mixing" gives a very natural and efficient way of
dissipating the wave energy. The author describes briefly the basic
scheme of phase mixing and its consequences.
Title: Comparisons of simultaneous vector magnetograms
Authors: Makita, M.; Nishi, K.; Shimizu, M.; Hamana, S.; Sakurai,
T.; Grigoryev, V. M.; Kuklin, G. V.; Selivanov, V. L.
Bibcode: 1985svmf.nasa..399M
Altcode:
Simultaneous vector magnetograms were obtained with the different
magnetographs of the Okayama Astrophysical Observatory and the Sayan
Mountain Observatory in October 1983. The data obtained show a good
correspondence in general. The comparison makes clear something on the
measuring accuracy of each magnetograph. Good correspondence is shown
in circular and linear polarizations, and, in and near sunspots, of the
velocity field. The azimuth of the magnetic fields show good agreement.
Title: Stereoscopic determination of the three-dimensional geometry
of coronal magnetic loops
Authors: Berton, R.; Sakurai, T.
Bibcode: 1985SoPh...96...93B
Altcode:
The three-dimensional shape of coronal magnetic loops is restored
from extreme ultraviolet (XUV) images of the Sun (Skylab mission 3,
1973) by using the perspective effect due to the solar rotation. An
original method is developed which only depends on the assumption that
the magnetic structures under consideration are (at least geometrically)
stable within the time interval used for restoration. Large scale loops
interconnecting different active regions are studied by applying this
method. They are found to lie approximately in planes inclined from
the local vertical. Generally these loops are asymmetric, i.e. their
apices are shifted toward one of the footpoints. This tendency is also
confirmed by the computation of coronal magnetic fields based on the
photospheric magnetic data.
Title: Rocket Observation of the Extreme Ultraviolet Images of a
Solar Flare and Active Regions
Authors: Hirayama, T.; Tanaka, K.; Watanabe, T.; Akita, K.; Sakurai,
T.; Nishi, K.
Bibcode: 1985SoPh...95..281H
Altcode:
Images of a flare and active regions were obtained in the extreme
ultraviolet emission lines such as CIII 977 Å, Ne VIII 770 Å, and HI
Lβ, and hydrogen Lyman continua with a spatial resolution of less than
ten seconds of arc together with one-dimensional scanning at 1650 Å. A
microchannel plate was used as a detector, and pointing accuracy was,
for about half of the observation time, around 0.5 arc sec.
Title: Magnetic Field Structures of Hard X-Ray Flares Observed by
HINOTORI Spacecraft
Authors: Sakurai, T.
Bibcode: 1985SoPh...95..311S
Altcode:
The magnetic field structure of five flares observed by HINOTORI
spacecraft is studied. The double source structure of impulsive
flares seems to indicate hard X-ray emission from the two footpoints
of a flaring loop, but the potential field computation does not
reproduce a loop connecting the two sources. Therefore the magnetic
field could be in a sheared configuration and the force-free field
modeling would be the next step to examine. On the other hand gradual
flares are characterized by hard X-ray sources located in the corona,
2-4 x 104 km above the photosphere. The potential field
modeling is found to give a reasonable fitting in this type of flares,
and the hard X-ray sources are located at the top of the magnetic loop
or arcade. This configuration is consistent with the thick-target trap
model of the hard X-ray bursts.
Title: Magnetodynamical processes in interacting magnetospheres of
RS CVn binaries
Authors: Uchida, Y.; Sakurai, T.
Bibcode: 1985IAUS..107..281U
Altcode:
Magnetodynamical processes in RS CVn binaries are discussed in the
scheme of the active longitude belt picture (Uchida and Sakurai, 1983)
in which the photometric wave is due to a number of spot repairs which
emerge, drift across, and are submerged in the active longitudinal belt
on the K-star. The formation of the corona and the origin of flares in
these close binary systems having starspots are interpreted in terms of
the reconnections of the magnetic flux tubes of the companion star with
the emerging and submerging pairs of spots on the K star. The injection
of the hot plasma into the large-scale pole-to-spot connections is
required to explain the extended corona with large emission measure, and
this is attributed to the sweeping-pinch mechanism (Uchida and Shibata,
1984) associated with the relaxation of the toroidal component in the
twisted magnetic flux tubes which emerge and reconnect with the flux
tube connecting pole and spots.
Title: Magnetic stellar winds. A 2-D generalization of Weber-Davis
model.
Authors: Sakurai, T.
Bibcode: 1985MPARp.174.....S
Altcode:
No abstract at ADS
Title: Observations by the solar vector magnetograph of the Okayama
Astrophysical Observatory.
Authors: Makita, M.; Hamana, S.; Nishi, K.; Shimizu, M.; Koyano, H.;
Sakurai, T.; Komatsu, H.
Bibcode: 1985PASJ...37..561M
Altcode: 1985PASJ...37..561H
The vector magnetograph of the Okayama Astrophysical Observatory has
been in operation since the fall of 1982. The instrument, operation,
and data reduction are described briefly. Some observations of the
development of a small active region, a flare, and azimuths of the
magnetic field relative to the Hα fine structure are reported. The
accuracy of the measurement is limited by the incident photon number
and as good as 0.1 % in the degree of polarization.
Title: Phase mixing of Alfvén waves.
Authors: Sakurai, T.
Bibcode: 1985MPARp.212..263S
Altcode:
In the theory of coronal heating by means of Alfvén waves, a process
called "the phase mixing" gives a very natural and efficient way of
dissipating the wave energy. The author describes briefly the basic
scheme of phase mixing and its consequences.
Title: Magnetic Field Structures of Hard X-Ray Flares Observed
by Hinotori
Authors: Sakurai, T.
Bibcode: 1985spit.conf..819S
Altcode:
No abstract at ADS
Title: Coronal heating in closely packed flux tubes: a
Taylor-Heyvaerts relaxation theory.
Authors: Browning, P. K.; Sakurai, T.; Priest, E. R.
Bibcode: 1985MPARp.181.....B
Altcode:
No abstract at ADS
Title: Observation of magnetic field vector in solar active regions.
Authors: Sakurai, T.; Makita, M.; Shibasaki, K.
Bibcode: 1985MPARp.212..312S
Altcode:
The vector magnetograph at Okayama Astrophysical Observatory has been
in operation since 1982. The conversion from the observed polarization
degrees to the magnetic field components is made by using Unno's formula
(1956) applied to a homogeneous magnetic field.
Title: Spectral characteristics of Pc 3 and Pc 4/5 magnetic pulsation
bands observed near L=6
Authors: Tonegawa, Y.; Fukunishi, H.; Hirasawa, T.; McPherron, R. L.;
Sakurai, T.; Kato, Y.
Bibcode: 1984JGR....89.9720T
Altcode:
From dynamic spectral analysis of magnetic pulsations observed
at auroral zone stations near L=6, it has been found that there
are two predominant spectral bands throughout the daytime, one in
the Pc 3 range with frequencies of ~20-80 mHz and the other in the
Pc 4/5 range with frequencies of ~3-10 mHz. At synchronous orbit,
corresponding spectral bands were seen in the azimuthal component
with a harmonic structure when the satellite was located at the
off-magnetic equator. The result suggests that the 3- to 10-mHz band
is the fundamental mode of standing shear Alfven wves and the 20- to
80-mHz band is the higher harmonics. However, both bands were not always
enhanced simultaneously. Switching of activity between both bands was
frequently observed on magnetically quiet days. This result indicates
that the Pc 3 and Pc 4/5 bands are excited by different external driving
sources. It is likely that when the frequency range of the external
driving source is in the Pc 3 range, this driving source excites the
higher harmonics of standing Alfvén waves near L=6, while when the
frequency range is in the Pc 4/5 range, the external driving source
excites the fundamental mode.
Title: Magnetic field structures of hard X-ray flares observed by
HINOTORI spacecraft.
Authors: Sakurai, T.
Bibcode: 1984ESASP.220..283S
Altcode: 1984ESPM....4..283S
The magnetic field structure of five flares observed by HINOTORI
spacecraft is studied. The double source structure of impulsive flares
indicates hard X-ray emission from the two footpoints of a flaring
loop. The potential field computation, however, does not reproduce
a loop connecting the two sources, implying that the magnetic field
could be in a highly sheared configuration. Gradual flares are
characterized by hard X-ray sources located in the corona, 20,000
to 40,000 km above the photosphere. The potential field modeling is
found to give a reasonable fitting in this type of flare, and the hard
X-ray sources are located at the top of the magnetic loop or arcade,
as is expected in the thick-target trap model of hard X-ray bursts.
Title: Hard X-ray imaging of a solar gradual hard X-ray burst on
April 1, 1981
Authors: Takakura, T.; Ohki, K.; Sakurai, T.; Wang, J. L.; Xuan,
J. Y.; Li, S. C.; Zhao, R. Y.
Bibcode: 1984SoPh...94..359T
Altcode:
An intense solar X-ray burst occurred on April 1, 1981. X-ray images
of this gradual hard X-ray burst were observed with the hard X-ray
telescope aboard the Hinotori satellite for the initial ten minutes
of rise and maximum phases of the burst. The hard X-ray images (13-29
keV) look like a large loop without considerable time variation of an
elongated main source during the whole observation period. The main
X-ray source seems to lie along a ridge of a long coronal arcade 2 ×
104 km above a neutral line, while a tangue-like sub-source
may be another large coronal loop although the whole structure of the
X-ray source looks like a large semi-circular loop. Both nonthermal
and hot thermal (3-4 × 107 K) electrons are contributing
to the source image. The ratio of these components changed in a wide
range from 2.3 to 0.4 during the observation, while the image was rather
steady. It suggests that both heating and accelerations of electrons
are occurring simultaneously in a common source. Energetic electrons
of 15-30 keV would be collisionally trapped in the coronal magnetic
loops with density of the order of 1011 cm−3.
Title: Generation of coronal electric currents due to convective
motions on the photosphere. II - Resonance and phase mixing of
Alfven waves
Authors: Sakurai, T.; Granik, A.
Bibcode: 1984ApJ...277..404S
Altcode:
A study is made on the resonance of Alfven waves in a coronal loop
excited at its footpoints by the photospheric convective motion. The
driving velocity field at the footpoints is modeled by random wave
trains. The nonuniformity of the Alfven velocity in the loop is taken
ito account so that the Alfven surface wave resonance and the phase
mixing of Alfven body waves take place. The derived dissipation
rate of wave energy, which turns out to be sufficient to heat the
corona, depends neither on the values of resistivity and viscosity, in
agreement with Ionson (1978, 1982) nor on whether the Alfven velocity
is uniform or not. The velocity amplitude in the loop, however,
depends on the resistivity and the viscosity as well as on the degree
of nonuniformity. When the phase mixing is in operation, the expected
velocity amplitude is smaller than in the case of uniform medium and
lies within the range of the linear treatment of waves.
Title: Satellite observations of Pi 2 activity at synchronous orbit
Authors: Sakurai, T.; McPherron, R. L.
Bibcode: 1983JGR....88.7015S
Altcode:
Pi 2 magnetic pulsations are frequently observed at synchronous orbit
by the UCLA fluxgate magnetometer on ATS 6. Events that occurred in
September 1974 have been studied by using digital power spectra and
coherency analysis to determine wave characteristics. From examination
of wave form and application of spectral analysis, these Pi 2 events
can be divided into three types. The first is a superposition of
a 100-s oscillation and a large-amplitude, higher-frequency Pi 1
activity. The second is a 100-s wave unaccompanied by Pi 1. Both
types have a significant compressional component. The third is a pure
transverse wave in the azimuthal component. The pure transverse waves
are quite rare, while the compressional waves occur during almost
every substorm. In general, if the satellite is in the local time
sector 1900-0300 LT, a Pi 2 burst accompanies every onset. In addition,
onsets at the satellite are associated with ground Pi 2 bursts. The peak
occurrence time of the satellite Pi 2 is 2100 LT. An examination of Pi
2 polarization at the satellite suggests that a polarization reversal
occurs around midnight during quiet magnetic conditions (Kp<=3+),
left-handed premidnight and right-handed postmidnight. This result
is similar to that obtained from ground-based studies at stations
equatorward of the auroral electrojet. The initial perturbation in
the azimuthal component of a Pi 2 event is in the same sense as the
perturbations caused by substorm-associated, field-aligned currents,
positive (eastward) premidnight, negative (westward) postmidnight. This
observation indicates that there may be a very close association
between their causative mechanisms.
Title: Computed Magnetic Field Structure of the Flares Observed by
HINOTORI Hard X-Ray Telescope
Authors: Sakurai, T.
Bibcode: 1983SoPh...86..339S
Altcode:
Potential field computations have been carried out to study the
location of hard X-ray sources observed by the HINOTORI hard X-ray
imaging instrument, SXT. Of the two flares studied, the X-ray source
of the 1981 May 13 event, a very unusual gradual flare, appears
to lie at the top of an arcade of field lines. In the other event,
the 1981 October 15 flare, the observed double source structure is
not explained in the present computation, implying the existence of
non-negligible electric currents in the flare region.
Title: Dynamical interpretation of the very hot region appearing at
the top of the loop
Authors: Shibata, K.; Uchida, Y.; Sakurai, T.
Bibcode: 1983SoPh...86..345S
Altcode:
In order to explain the appearance of a hard X-ray source at the
top of a loop, we present a model in which the dynamical effects
of the dark filament mass infallng along the loop in association
with the "disparition brusque" plays an important role. The crash
of the infalling mass produces high temperature regions in the low
corona above the two footpoints of the loop, and the up-going shocks,
created in the crash and strengthened in propagating upwards along the
steep density gradient in the tail of the infalling mass, produce a
very high temperature (108 K) region upon colliding with
each other near the top of the loop. Successive occurrence of this
process in successively higher loops in magnetic arcade may account
for the sources of gradual hard X-ray bursts appearing at the top of
the loop-like structure.
Title: Analytical method of characteristics applied to the azimuthally
dependent solar wind
Authors: Sakurai, T.
Bibcode: 1983MNRAS.203.1187S
Altcode:
The application of the analytical method of characteristics to
the hypersonic part of a corotating quasi-steady configuration
on the ecliptic plane of the azimuthally dependent solar wind is
discussed. A QRH approximation has been improved at large heliocentric
distances. The approximate equations are made exactly linear by a
variable transformation to the characteristics coordinates. The initial
value problem of the transformed equations is solved analytically by
Riemann's method. Weak shock conditions are shown to be incorporated
easily in our method of solution. A typical example is given which
shows the appearance of a pair of weak shock waves in the flowfield.
Title: Local time asymmetry in the characteristics of Pc5 magnetic
pulsations
Authors: Yumoto, K.; Saito, T.; Sakurai, T.
Bibcode: 1983P&SS...31..459Y
Altcode:
The wave characteristics of Pc5 magnetic pulsations are analyzed
with data of OGO-5, ISEE-1 and -2 satellites. The toroidal modes
( δBD > δBH) of Pc5 pulsations are
observed at a higher magnetic latitude in the dawnside outer
magnetosphere. The compressional and poloidal modes ( δBz. dfnc;
∼ δBH > δBD) of Pc5 pulsations
are mostly observed near the magnetic equator in the duskside outer
magnetosphere. This L.T. asymmetry in the occurrence of dominant modes
of Pc5's in space can be explained by the velocity shear instability
(Yumoto and Saito, 1980) in the magnetospheric boundary layer, where
Alfvénic signals in the IMF medium are assumed to penetrate into
the magnetospheric boundary layer along the Archimedean spiral. The
asymmetrical behaviour of Pc5 pulsation activity on the ground across
the noon meridian can be also explained by the ionospheric screening
effect on the compressional Pc5 magnetic pulsations. The compressional
modes with a large horizontal wave number in the duskside magnetosphere
are expected to be suppressed on the ground throughout the ionosphere
and atmosphere.
Title: Interacting magnetospheres in RS CVn binaries - Coronal
heating and flares
Authors: Uchida, Y.; Sakurai, T.
Bibcode: 1983ASSL..102..629U
Altcode: 1983ards.proc..629U; 1983IAUCo..71..629U
Coronae and flares in RS CVn systems are interpreted as due to
gradual and sudden releases of magnetic free energy built up by
the interaction of stellar magnetic fields in these close binary
systems. It is proposed that the photometric wave in the light curve
which is the basis of the starspot hypothesis may correspond to the
stellar analog, in an extreme form, of an 'active longitude belt,
(ALB) on the sun in which active spot groups are seen to emerge,
drift across, and disappear. This notion eliminates the assumption
of a gigantic spot or aggregate of spots staying almost fixed on the
surface of a rapidly rotating star. The magnetic field of the system is
calculated and the ALB concept is used as a mechanism for energizing
the magnetic field. The possible role of ALBs in the magnetic field
reconnection concept of flare causation is suggested.
Title: Formation of a counter current region near a gravitating
sphere in a steady Stokes flow of a viscous ideal gas
Authors: Sakurai, T.
Bibcode: 1983GApFD..24..225S
Altcode:
The steady axisymmetric Stokes flow of a viscous ideal gas past a
gravitating sphere is studied. A simple form for the viscosity and
thermal conductivity of the gas is assumed and the flow is taken as
a small perturbation on the equilibrium state at rest. The method of
separation of variables is used to reduce the basic equations to an
ordinary fourth order differential equation. Drag coefficients, flow
configuations and velocity distributions are given. In particular,
the formation of a counter current region near the sphere is clarified
for cases with strong gravity.
Title: Green's Function Methods for Potential Magnetic Fields
Authors: Sakurai, Takashi
Bibcode: 1982SoPh...76..301S
Altcode:
The Green's function method to calculate potential magnetic field on
the Sun, which was first established by Schmidt (1964) in the case
that the field component normal to a flat boundary plane is specified,
is extended to the following three cases: (a) The field component along
the line of sight, which is not generally normal to the flat boundary
plane, is specified; (b) the line of sight component on a spherical
boundary surface is specified; (c) the normal component on a spherical
surface is specified, together with the condition that the field becomes
approximately radial on an outer spherical surface (the so-called source
surface). Properties of these Green's functions are examined, and the
applicability of these methods to solar magnetic data is discussed.
Title: Solar corona: the magnetosphere of the sun.
Authors: Sakurai, T.
Bibcode: 1982mpcb.conf...43S
Altcode:
No abstract at ADS
Title: RS CVn binaries - interaction of magnetospheres.
Authors: Uchida, Y.; Sakurai, T.
Bibcode: 1982mpcb.conf...77U
Altcode:
No abstract at ADS
Title: Generation of coronal electric currents due to convective
motions on the photosphere
Authors: Sakurai, T.; Levine, R. H.
Bibcode: 1981ApJ...248..817S
Altcode:
Generation of electric currents in a magnetized plasma overlying a
dense convective layer is studied, assuming that the magnetic field
perturbation is small and satisfies the force-free equation. Currents
are produced by rotational motions on the boundary in the case of
a uniform equilibrium field. In a simple two-dimensional bipolar
configuration, however, both irrotational and incompressible motions
give rise to currents, and the current density has a peak at the
magnetic neutral line. Scaling laws for the current density as well
as for the stored magnetic energy are derived, and the possibility of
heating the solar corona through the dissipation of coronal currents
generated in this way is discussed.
Title: X-ray, EUV, and centimetric observations of solar active
regions - an empirical model for bright radio sources
Authors: Pallavicini, R.; Sakurai, T.; Vaiana, G. S.
Bibcode: 1981A&A....98..316P
Altcode:
Coordinated observations of solar active regions in X-rays, EUV, and
2.8 cm radio waves, and current-free extrapolations of the photospheric
magnetic field to coronal levels are used to determine the emission
mechanism of bright radio sources. High spatial resolution X-ray and
EUV observations of the active regions show that the temperature and
density structure of the atmosphere above the sunspots differs greatly
from that above the plages. Extended low brightness temperature
radio sources, associated with plage areas, are consistent with
thermal free-free emission from the transition region and the corona
above the active centers. High brightness temperature radio sources,
associated with sunspots, cannot be explained by thermal free-free
emissions either in hot, dense loops or in the cooler, lower pressure
loops, observed emanating from sunspot umbrae. There is evidence that
thermal gyroresonance absorption at the second and third harmonics of
the gyrofrequency can produce sunspots associated with bright radio
sources. The empirical models of the coronal loops, based on energy
balance, and the effects of electric currents flowing in the corona
above the sunspots are discussed.
Title: Calculation of force-free magnetic field with non-constant α
Authors: Sakurai, Takashi
Bibcode: 1981SoPh...69..343S
Altcode:
A numerical method is developed for solving the force-free magnetic
field equation, ▽ × B = αB, with spatially-varying α. The boundary
conditions required are the distribution of Bn (viz. normal
component of the field on the photosphere) as well as the value of
α in the region of positive (or negative) Bn. Examples
of calculations are presented for a simple model of a solar bipolar
magnetic region. It is found that the field configuration and the
energy stored in the field depend crucially on the distribution of
α. The present method can be applied to a more complex configuration
observed on the Sun by making use of actual magnetic field measurements.
Title: Magnetic Field Extrapolations, XUV Observations and the Nature
of Microwave Emission from Sunspots
Authors: Pallavicini, R.; Sakurai, T.; Vaiana, G. S.
Bibcode: 1980BAAS...12..896P
Altcode:
No abstract at ADS
Title: Generation of Coronal Electric Currents due to Convective
Motions on the Photosphere
Authors: Sakurai, T.; Levine, R. H.
Bibcode: 1980BAAS...12..909S
Altcode:
No abstract at ADS
Title: Energy storage and instability in magnetic flux tubes
Authors: Sakurai, T.
Bibcode: 1980IAUS...91..291S
Altcode:
A method for calculating the force-free field is proposed. It is
shown that the motion of the footpoint of the field line feeds the
energy into the flux tube. The instability of an over-twisted tube
is also studied. The calculation could explain the motion of eruptive
prominences. The energy build-up and instability in the magnetic field
can be important processes in solar flares.
Title: Examination of the resonance theory on Pcs by means of an
analysis of magnetic fluctuations in the magnetosheath and the
magnetosphere
Authors: Saito, T.; Takahashi, K.; Sakurai, T.
Bibcode: 1979P&SS...27..809S
Altcode:
Magnetic fluctuations observed in the magnetosheath and the outer
magnetosphere with Ogo-5 during 6 months from November 1968, are
analyzed to examine the resonance theory that monochromatic waves
excited outside the magnetosphere are transmitted in the compressional
mode into the magnetosphere, being transmitted further along the
closed field lines in the torsional mode and are finally observed as
long-period pcs on the Earth's surface. Ten observed results on the
wave characteristics of the fluctuations including variance, spectrum,
relation to the plasma stream, integrated power, longitudinal dependence
are obtained and summarized. The fluctuations in the magnetosheath
are found to be dominantly Alfvénic. Several pieces of evidence to
support the resonance theory are found.
Title: A New Approach to the Force-Free Field and Its Application
to the Mag- netic Field of Solar Active Regions
Authors: Sakurai, T.
Bibcode: 1979PASJ...31..209S
Altcode:
We study the force-free magnetic field which defines the equilibrium of
a magnetoplasma through the equation rot B = α(x)B when the magnetic
force dominates other forces. The implication of current-free and
force-free magnetic fields is re-discussed in terms of the variational
principles, and a correct variational problem suitable for the
force-free field is set up and solved by a direct method known as the
finite-element method. Examples of calculation show that the motion
of the footpoints of the field lines feeds the energy into the tube
of force. The stored amount of energy, however, depends on whether or
not the magnetic field is strong enough at its weakest part along the
tube so that the work done against the magnetic force by the motion
of footpoints can become large. The value of α(x) calculated in
our solution varies in space and even changes its sign from place to
place even in a simple geometry. The conventional constant-α model is
thus found to be a very particular model which is very unlikely to be
realized in an active region. The possibility of applying the present
method to the energy build-up process of solar flares is discussed.
Title: Solar terrestrial activity chart, hissagrams for 1969 and
1970 - Dynamic spectrograms of ULF waves observed at Onagawa
Authors: Saito, T.; Sakurai, T.
Bibcode: 1978STERJ...2..128S
Altcode:
No abstract at ADS
Title: The earth's palaeomagnetosphere as the third type of planetary
magnetosphere
Authors: Saito, T.; Sakurai, T.; Yumoto, K.
Bibcode: 1978P&SS...26..413S
Altcode:
From the viewpoint of dynamical topology, planetary magnetospheres
are classified into three: Types 1, 2 and 3. When the rotation
vector and dipole moment of a planet and the velocity vector of the
solar wind are denoted as Ω, M, and V, respectively, the planetary
magnetosphere with Ω∥ M⊥ V is called Type 1. The magnetospheres
of the present Earth, Jupiter, and Uranus at its equinoctial points
belong to this type. The magnetosphere with Ω∥ M∥ V is called
Type 2, which includes the Uranian magnetosphere at its solstitial
points. The magnetosphere with Ω⊥ M and Ω⊥ V is called Type 3. The
Earth's palaeomagnetosphere is considered to have experienced Type
3 during excursions and transition stages of palaeomagnetic polarity
reversals. In the Type 3 magnetosphere, drastic diurnal variations are
expected in configurations of the dayside cusps, tail axis, neutral
sheet, polar caps, and so on. A possible relation between the Type
3 palaeomagnetosphere and palaeoclimate of the Earth during polarity
reversals and geomagnetic excursions is suggested. It is also suggested
that the heliomagnetosphere during polarity reversals of the general
field of the Sun exhibits a drastic configuration change similar to
the Type 3 palaeomagnetosphere of the Earth. A relation between the
perpendicular condition Ω⊥ M and magnetic variable stars and pulsars
is briefly discussed.
Title: Magnetic field and current sheets in the corona above active
regions.
Authors: Sakurai, T.; Uchida, Y.
Bibcode: 1977SoPh...52..397S
Altcode:
A new method for the calculation of coronal magnetic field is proposed
and it is shown to reproduce the EUV features in the corona as observed
by Skylab experiments satisfactorily well. One of the remarkable points
is that it reproduces the loopy threads in the active region corona
and also the large scale field lines connecting active regions. The
existence of coronal current is expected wherever the present
coronal-current-free model fails to represent the feature. A method
of calculating the coronal sheet-current is also developed with the
purpose of knowing the shape of the current sheet and the amount of
magnetic stress energy stored due the the presence of it by comparing
the calculated field configuration with the observed local distortion
of the EUV threads. This may be used in pinning down the possible site
of the flare and in discussing the flare occurrence in terms of the
energy stored there.
Title: Heating and reconnection of the emerging magnetic flux-tubes
and the role of the interchange instability.
Authors: Uchida, Y.; Sakurai, T.
Bibcode: 1977SoPh...51..413U
Altcode:
We propose in the present paper that the basic behaviors of
newly-emerged magnetic regions (NEMR) as seen in EUV and soft X-rays
from space are interpreted by the interchange instability of the
magnetic field of NEMR in the global situation surrounding it.
Title: Observation of a Gamma-Ray Burst at Baloon Altitude
Authors: Nishimura, J.; Fujii, M.; Tawara, T.; Miyamoto, S.; Oda,
M.; Ogawara, Y.; Yamagami, T.; Kajiwara, M.; Murakami, H.; Yoshimori,
M.; Nakagawa, M.; Sakurai, T.
Bibcode: 1977ICRC...11...23N
Altcode: 1977ICRC...15k..23N; 1978ICRC...11...23N
During the search for gamma-ray bursts with balloon flights of about 120
hr duration, a significant increase of X-ray counting rate was observed
in three independent counters. The event started at 10h 06m 31s UT
September 23, 1975 and lasted for about 30 sec. The peak intensity is
estimated to be approximately 0.000001 erg/cm sec and the total flux is
0.000006 erg/cm. No obvious solar-terrestrial disturbances are reported
during this period, and the event is concluded to be a small gamma-ray
burst. From the analysis of the modulated X-ray flux observed by the
rotating-cross-modulation-collimator used in this experiment, the
most likely celestial position of the burst is determined at alpha =
19h 50m + or - 1m, delta = 45 deg 06 min + or - 10 min. This is the
most accurate determination of the position of the gamma-ray burst
thus far obtained.
Title: Generation of a Deeply Penetrating Gravity-Like Oscillation
by a Non- Resonant Second-Order Interaction of Waves
Authors: Sakurai, T.
Bibcode: 1977PASJ...29..543S
Altcode:
We study a non-resonant second-order interaction between an inner
gravity oscillation and an unstable convection in a rotating two-layered
Boussinesq fluid, the lower and the upper part of which are stably and
unstably stratified, respectively. A growing gravity-like oscillation
generated by this interaction has an oscillation period equal to that
of its mother gravity oscillation. Its growth time is equal to that
of its mother convection. The generated oscillation can penetrate
deeply into the lower stable region, although its forcing effect is
rapidly damped with the depth. Related solar problems are discussed
briefly. Key words: g-like mode generation; g-mode; Non-resonant
secondorder interaction; Sun.
Title: Observation of a gamma-ray burst at altitude.
Authors: Nishimura, J.; Fujii, M.; Tawara, Y.; Miyamoto, S.; Oda, M.;
Yamagami, T.; Kajiwara, M.; Murakami, H.; Yoshimori, M.; Nakagawa,
M.; Sakurai, T.
Bibcode: 1977BISAT..13..857N
Altcode:
No abstract at ADS
Title: Mechanism of association between Pi 2 pulsation and
magnetospheric substorm.
Authors: Saito, T.; Sakurai, T.; Koyama, Y.
Bibcode: 1976JATP...38.1265S
Altcode:
Based on the analysis of the data obtained at the ground and by
satellites, the following morphological characteristics of Pi2 are
revealed: (1) Pi2 shows a peak amplitude at the auroral oval and a
secondary maximum inside the plasmapause latitude, (2) a phase reversal
of the NS-component of Pi2 exhibits both at the auroral oval and near
the plasmapause latitude, (3) Pi2 occurs associated with a release
of the magnetic energy in the magnetotail lobe, (4) Pi2 propagates
in a compressional mode in the magnetosphere and is detected as a
torsional mode on the ground. Our result obtained above leads us to a
Pi2 model that the substorm-associated compressional mode of HM waves
excited in the magnetotail is converted to the torsional mode of HM
waves propagating along the field-line anchoring at the northern and
southern auroral ovals and is observed as magnetic pulsation Pi2 on
the dark hemisphere of the earth.
Title: Magnetic pulsation Pi2 and substorm onset
Authors: Sakurai, T.; Saito, T.
Bibcode: 1976P&SS...24..573S
Altcode:
Coincidence between the onset of sudden brightening of the auroral arc
in the auroral oval and the onset of Pi2 magnetic pulsation in low
latitudes is examined based on the auroral data obtained at a chain
of stations in Alaska and the Pi2 data obtained at the low-latitude
station Onagawa. The result shows that the low-latitude Pi2 occurs
almost simultaneously with the sudden brightening of the auroral arc,
i.e. the onset of an auroral substorm ( T = 0). It is concluded that
the onset of substorms can be identified quite well with the onset of
the low-latitude Pi2.
Title: Balloon observations on the lunar occultation of the hard
X-ray source in the Crab Nebula.
Authors: Fukada, Y.; Hayakawa, S.; Kasahara, I.; Makino, F.; Akiyama,
H.; Matsuoka, M.; Nishimura, J.; Oda, M.; Tanaka, Y.; Nakagawa, M.;
Sakurai, T.; Iyengar, V. S.; Manchanda, R. K.; Kunte, P. K.
Bibcode: 1976BISAT..12..647F
Altcode:
No abstract at ADS
Title: Magnetohydrodynamic interpretation of the motion of
prominences.
Authors: Sakurai, T.
Bibcode: 1976PASJ...28..177S
Altcode:
To clarify the relationship between the motion and the magnetic field
in eruptions of chromosphere filaments (prominences), three types of
prominence eruptions: arch type, loop type, and gigantic arch type,
identified on the basis of their shapes, are analyzed. The mechanism
by which the filament, regarded as a magnetic flux tube, is set in
motion is studied by examining the stability of a magneto-fluid column
(pinch). The growth rates of the instability are evaluated by solving
numerically the eigenvalue problem, the onset of the ascending motion
being ascribed to the screw-mode instability. In analyzing the causes
of the existence of different types of motion, a method is proposed to
solve numerically the three-dimensional MHD equation of motion, using
Ritz's variational technique. It is shown that the characteristic motion
of the arch-, loop-, and gigantic arch-type eruptions may be reproduced
by perturbing a model sequence with decreasing pitch angles of the
unperturbed helical magnetic field lines. A good agreement is found
between the model-predicted type of motion and the observed values of
the pitch angles of the filamentary structure in eruptive prominences.
Title: Variations of magnetospheric convection electric fields during
substorms as inferred from pc1 hydromagnetic waves
Authors: Sakurai, T.
Bibcode: 1975P&SS...23..611S
Altcode:
The convection electric field in the vicinity of the plasmapause in
the midnight sector during magnetospheric substorms has been obtained
on the basis of spectral analysis of Pc1 hydromagnetic (HM) waves
observed at the low latitude station, Onagawa ( Φ = 28.°3, Λ =
206.°8). Variations of the field are consistent for four independent
substorm events studied. The calculation implies that the convection
electric field increases westwards up to ~1.0 mV/m during the expansion
phase of the substorms, changes polarity near the end of the expansion
phase, and then points eastwards during the recovery phase.
Title: The evolution of the solar inner rotation.
Authors: Sakurai, T.
Bibcode: 1975MNRAS.171...35S
Altcode:
The evolution of the axisymmetric rotation of the solar radiative
interior is analyzed on the basis of the Eddington-Sweet perturbation
theory. The effect of the molecular weight gradient is neglected,
but the effects of eddy viscosity and solar wind torque are taken into
account. The initial boundary value problem is reduced to a system of
the gravity potential equation and the higher-order nonlinear diffusion
equations, and a reasonably stable method is presented for solving the
system of equations. It is shown that this formulation can resolve the
difficulty of the Eddington-Sweet theory whereby the meridional velocity
becomes indefinitely large on the interface between the radiative and
convective regions. Evolution of the inner rotation is calculated for
several hypothetical values of the eddy viscosity.
Title: Computer Simulations of Gas Flow around Close Binary Systems
Authors: Sorensen, S. -A.; Matsuda, T.; Sakurai, T.
Bibcode: 1975Ap&SS..33..465S
Altcode:
A gas-dynamical model of gas streams around close binary systems is
given. The velocity feld and the density distribution are determined
for different parameter ranges. The results succeed in explaining the
formation of a ring and a disk around the accreting component. The
models furthermore reveal the existence of a tongue of matter extending
from the inner Lagrangian point and a jet perpendicular to the system
axis.
Title: Oscillations in sunspot umbrae due to trapped Alfvén waves
excited by overstability.
Authors: Uchida, Y.; Sakurai, T.
Bibcode: 1975PASJ...27..259U
Altcode:
Oscillations detected in sunspot umbras are considered to be
a vertical atmospheric motion caused by a standing Alfven wave
trapped in the region between an overstable subphotospheric layer
and the chromosphere-corona transition layer. It is suggested that
the wave motion is excited by overstable convection at the base of
the oscillating region. The fundamental eigenfrequency is found to
correspond to a period of 140 to 180 sec for oscillations in the umbra
of a typical sunspot, and it is shown that the eigenfrequencies depend
primarily on the Alfven velocity at the base of the oscillating
layer. Previous interpretations of this phenomenon are briefly
discussed.
Title: Dynamic spectral analysis of cosmic ray anisotropy by means
of high-speed spectral analysis method.
Authors: Kanno, T.; Saito, T.; Sakurai, T.; Yumoto, K.; Ishida, Y.
Bibcode: 1975RISRJ..29..118K
Altcode:
Hourly digital values of cosmic ray neutron intensity observed at Deep
River, Canada, from January to June, 1972 have been converted into
analog signal and transcribed on a magnetic tape. Time variations in
cosmic ray anisotropies having periods ranging from infinity to 1/6
days have been obtained from the MT signal by means of the high-speed
spectrum analysis (hissa) method. The diurnal, the semidiurnal,
and the terdiurnal components frequently exhibit nonharmonic time
variations. Cosmic ray storms are manifested on the hissagrams as a
characteristic type of dynamic spectral pattern repeating with solar
rotation period. The diurnal anisotropy is found to be enhanced on
days when the interplanetary magnetic field is directed toward the sun.
Title: Flow in Close Binary Systems
Authors: Sorensen, S.; Matsuda, T.; Sakurai, T.
Bibcode: 1974PThPh..52..333S
Altcode:
No abstract at ADS
Title: The Evolution of the Solar Inner Rotation by the
Eddington-Sweet Type Circulation under the Influence of the Solar
Wind Torque, II. Proposal of a New Method of Solution
Authors: Sakurai, T.
Bibcode: 1973PASJ...25..563S
Altcode:
We formulate an initial boundary value problem of a system of partial
differential equations from the basic equations of our previous paper
under the same title (SAxuRAi 1972a). We clarify that the difficulty,
that the meridional velocity diverges on the interface between the
radiative and the convective regions, is resolved by the introduction
of the unsteadiness of the angular velocity. Preliminary results in
which the solar interior is more spun down than the solar surface are
described. Key words: Eddington-Sweet circulation; Solar inner rotation;
Unsteady flow approach.
Title: The Evolution of the Solar Inner Rotation by the
Eddington-Sweet Type Circulation under the Influence of the Solar
Wind Torque
Authors: Sakurai, T.
Bibcode: 1972PASJ...24..153S
Altcode:
The evolution of the axisymmetric rotation of the radiative interior
of the sun is investigated. The basic equations are derived within the
framework of the Eddington-Sweet theory of perturbation. The effect of
the molecular weight gradient built up by the proton-proton reaction
and the effect of the solar wind torque are taken into account. The
initial state of the rotation is assumed to be rigid body rotation
and to have an angular velocity about 20 to 65 times larger than
the present surface value. The numerical calculations are performed
by the two-term approximation of the Legendre expansion. The results
show that the Eddington-Sweet circulation brings the angular momentum
inwards to partially spin up (not spin down) the deep interior. The
convection zone suffers a double extraction of the angular momentum:
one from the above inner transport by the Eddington-Sweet circulation
and the second from the solar wind torque. Thus, the present decline of
the surface angular velocity is faster than that for the case in which
only the convection zone is spun down. The time the surface angular
velocity takes to decline to the present value is estimated to be
on the order of 10 to 108 years. This clearly shows the necessity of
some mechanism which brings the angular momentum out from the solar
interior. Key words: Angular momentum transfer; Eddington-Swee t
circulation; Solar wind torque.
Title: Spin down of radiation-penetrated, opaque, compressible fluid
in a circular cylinder.
Authors: Sakurai, T.
Bibcode: 1972PhFl...15..555S
Altcode:
The linear impulsive spin down from a rotational equilibrium of rigid
body rotation of an opaque, compressible fluid in a circular cylinder
under the effect of radiation is investigated. The opacity of the fluid
is assumed to result from the electron scattering, and the radiative
Prandtl number PR is assumed to be proportional to the
square root of the Ekman number E where P = ν/κR, E =
ν/R02Ω, and ν is the kinematic viscosity, κR is
the radiative thermal diffusivity, R0 is the radius of the
cylinder, and Ω is the unperturbed angular velocity, respectively. The
treatment is also restricted to the case for which the ratio of
the thermal energy to the rotational kinetic energy (H1
= RT0/Ro2Ω2, where T0 and R are
the temperature and the universal gas constant, respectively) is very
large corresponding to typical conditions in stellar spin down. As a new
aspect of the linear spin down of the non-Boussinesq compressible fluid,
the quasisteady asymptotic state is shown to be rotating nonuniformly
despite the fact that such an asymptotic state in Boussinesq fluid is
characterized by rigid body rotation. This nonuniform asymptotic state
is one in which each horizontal fluid layer rotates like a rigid body
with a perturbation angular velocity proportional to the unperturbed
temperature. It is also shown that the spin-down time is of the order
of H1τc, where τc is the thermal
diffusion time. Because H1 can be rephrased to be equal to the
square ratio of the Brunt-Väisälä frequency to Ω, we may say that
our spin-down process is similar to that of the Boussinesq fluid case
except for the above quasistatic asymptotic state.
Title: Dynamics of the azimuthally dependent solar wind.
Authors: Matsuda, T.; Sakurai, T.
Bibcode: 1972CosEl...3...97M
Altcode:
No abstract at ADS
Title: Quasi-radial hypervelocity approximation of the azimuthally
dependent solar wind.
Authors: Sakurai, T.
Bibcode: 1971CosEl...1..460S
Altcode:
No abstract at ADS
Title: Spin Down of Boussinesq Fluid in the Circular Cylinder,
as a Simulation of the Solar Spin-Down Procedure
Authors: Sakurai, T.
Bibcode: 1970stro.coll..329S
Altcode: 1970IAUCo...4..329S
No abstract at ADS
Title: The Negative Viscosity Phenomenon Associated with the Rossby
Wave on the Differentially-Rotating Solar Photosphere
Authors: Sakurai, T.
Bibcode: 1970PASJ...22..177S
Altcode:
The Rossby wave on the solar photosphere is investigated from the view
point that the photosphere can be treated as a differentially-rotating
isothermal spherical shell of non-conducting inviscid ideal gas and that
the gravitational effect can be approximated by a Roche-type model. The
differentially-rotating equilibrium state in which the temperature
depends only on the distance from the center of the sphere is obtained,
and the linearized local equations for the superposed small disturbances
which are applicable to a strongly-stratified solar atmosphere are
formulated. The solution clarifies the negative viscosity phenomenon
by which the kinetic energy of the disturbance is transferred to that
of the main zonal motion of the differential rotation at the cost of
damping of the disturbance. Key words: Solar differential rotation;
Rossby wave; Negative viscosity.
Title: Spin down problem of rotating stratified fluid in thermally
insulated circular cylinders.
Authors: Sakurai, T.
Bibcode: 1969JFM....37..689S
Altcode:
No abstract at ADS
Title: Hydrodynamical Investigation of the Solar Differential Rotation
Authors: Sakurai, T.
Bibcode: 1966PASJ...18..174S
Altcode:
The differential rotation of the sun is investigated from Biermann's
viewpoint which ascribes its cause to the non-isotropy of the eddy
viscosity. The hydrogen convection layer is considered to be a thin
layer rotating independently of the inner stable region, and its
rotational motion is investigated on the basis of 's generalized mixing
length theory and the fact that the adiabatic relation is a very good
approximation in the layer. The mixing length is assumed to be equal to
the scale height and the root mean square of the turbulent component of
the velocity, as well as the ratio of the horizontal component of the
eddy viscosity to the vertical component (= S) to be constant all over
the layer. The basic equations are solved by an approximation method
based on the expansion with respect to the ratio of the layer thickness
to the solar radius. The distribution of the angular velocity as well
as the meridional current is discussed in detail. The theoretical
result on the surface angular velocity distribution agrees quite
well with observation up to about 40 degrees latitude and the largest
discrepancy (which appears at the pole) is within 12 per cent for the
case with 5=1.2.