explanation blue bibcodes open ADS page with paths to full text
Author name code: takasao
ADS astronomy entries on 2022-09-14
author:"Takasao, Shinsuke"
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Title: Anatomy of Photoevaporation Base: Linking the Property of
the Launched Wind to Irradiation Flux
Authors: Nakatani, Riouhei; Takasao, Shinsuke
2022ApJ...930..124N Altcode:
Ultraviolet and X-rays from radiation sources disperse nearby
gas clumps by driving winds due to heating associated with the
photochemical processes. This dispersal process, photoevaporation,
constrains the lifetimes of the parental bodies of stars and planets. To
understand this process in a universal picture, we develop an analytical
model that describes the fundamental physics in the vicinity of the
wind-launching region. The model explicitly links the density and
velocity of photoevaporative winds at the launch points to the radiation
flux reaching the wind-launching base, using a jump condition. The
model gives a natural boundary condition for the wind-emanating
points. We compare the analytical model with the results of radiation
hydrodynamic simulations, where a protoplanetary disk is irradiated
by the stellar extreme-ultraviolet, and confirm good agreement of
the base density and velocity, and radial profiles of mass-loss
rates. We expect that our analytical model is applicable to other
objects subject to photoevaporation not only by extreme-ultraviolet
but by far-ultraviolet/X-rays with suitable modifications. Future
self-consistent numerical studies can test the applicability.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). II. Flares and Eruptions
Authors: Cheung, Mark C. M.; Martínez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham S.; Reeves, Katharine K.; Fletcher, Lyndsay; Jin,
Meng; Nóbrega-Siverio, Daniel; Danilovic, Sanja; Antolin, Patrick;
Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward;
Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc L.; Boerner, Paul;
Jaeggli, Sarah; Nitta, Nariaki V.; Daw, Adrian; Carlsson, Mats; Golub,
Leon; The
2022ApJ...926...53C Altcode: 2021arXiv210615591C
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (subarcseconds) and temporal (less than a few tens of
seconds) scales of the coronal dynamics of solar flares and eruptive
phenomena. The highest-resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by the Interface Region Imaging
Spectrograph for the low solar atmosphere, we need high-resolution
spectroscopic measurements with simultaneous imaging to understand the
dominant processes. In this paper: (1) we introduce the Multi-slit Solar
Explorer (MUSE), a spaceborne observatory to fill this observational
gap by providing high-cadence (<20 s), subarcsecond-resolution
spectroscopic rasters over an active region size of the solar transition
region and corona; (2) using advanced numerical models, we demonstrate
the unique diagnostic capabilities of MUSE for exploring solar coronal
dynamics and for constraining and discriminating models of solar flares
and eruptions; (3) we discuss the key contributions MUSE would make
in addressing the science objectives of the Next Generation Solar
Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme
Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope
(and other ground-based observatories) can operate as a distributed
implementation of the NGSPM. This is a companion paper to De Pontieu
et al., which focuses on investigating coronal heating with MUSE.
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Title: Spontaneous Formation of Outflows Powered by Rotating
Magnetized Accretion Flows in a Galactic Center
Authors: Takasao, Shinsuke; Shuto, Yuri; Wada, Keiichi
2022ApJ...926...50T Altcode: 2021arXiv211107373T
We investigate how magnetically driven outflows are powered by a
rotating, weakly magnetized accretion flow onto a supermassive black
hole using axisymmetric magnetohydrodynamic simulations. Our proposed
model focuses on the accretion dynamics on an intermediate scale between
the Schwarzschild radius and the galactic scale, which is ~1-100 pc. We
demonstrate that a rotating disk formed on a parsec-scale acquires
poloidal magnetic fields via accretion, and this produces an asymmetric
bipolar outflow at some point. The formation of the outflow was found to
follow the growth of strongly magnetized regions around disk surfaces
(magnetic bubbles). The bipolar outflow grew continuously inside the
expanding bubbles. We theoretically derived the growth condition of
the magnetic bubbles for our model that corresponds to a necessary
condition for outflow growth. We found that the north-south asymmetrical
structure of the bipolar outflow originates from the complex motions
excited by accreting flows around the outer edge of the disk. The
bipolar outflow comprises multiple mini-outflows and downflows (failed
outflows). The mini-outflows emanate from the magnetic concentrations
(magnetic patches). The magnetic patches exhibit inward drifting
motions, thereby making the outflows unsteady. We demonstrate that
the inward drift can be modeled using a simple magnetic patch model
that considers magnetic angular momentum extraction. This study could
be helpful for understanding how asymmetric and nonsteady outflows
with complex substructures are produced around supermassive black
holes without the help of strong radiation from accretion disks or
entrainment by radio jets such as molecular outflows in radio-quiet
active galactic nuclei, e.g., NGC 1377.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE): II. Flares and Eruptions
Authors: Cheung, Chun Ming Mark; Martinez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham; Reeves, Katharine; Fletcher, Lyndsay; Jin,
Meng; Nobrega, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred,
Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope,
Dana; Takasao, Shinsuke; DeRosa, Marc; Boerner, Paul; Jaeggli, Sarah;
Nitta, Nariaki; Daw, Adrian; Carlsson, Mats; Golub, Leon
2021AGUFMSH51A..08C Altcode:
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (sub-arcseconds) and temporal scales (less than a few tens
of seconds) of the coronal dynamics of solar flares and eruptive
phenomena. The highest resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by IRIS for the low solar atmosphere,
we need high-resolution spectroscopic measurements with simultaneous
imaging to understand the dominant processes. In this paper: (1)
we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne
observatory to fill this observational gap by providing high-cadence
(<20 s), sub-arcsecond resolution spectroscopic rasters over an
active region size of the solar transition region and corona; (2)
using advanced numerical models, we demonstrate the unique diagnostic
capabilities of MUSE for exploring solar coronal dynamics, and for
constraining and discriminating models of solar flares and eruptions;
(3) we discuss the key contributions MUSE would make in addressing the
science objectives of the Next Generation Solar Physics Mission (NGSPM),
and how MUSE, the high-throughput EUV Solar Telescope (EUVST) and the
Daniel K Inouye Solar Telescope (and other ground-based observatories)
can operate as a distributed implementation of the NGSPM. This is a
companion paper to De Pontieu et al. (2021, also submitted to SH-17),
which focuses on investigating coronal heating with MUSE.
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Title: Modeling the corona and XUV emission from the Sun and
Sun-like stars
Authors: Shoda, Munehito; Takasao, Shinsuke
2021AGUFM.P55D1960S Altcode:
The evolution of the planetary atmosphere is significantly affected
by the XUV emission from the host star. It is, however, impossible to
directly measure the XUV spectrum of a given star because XUV photons
suffer from significant interstellar extinction. In this presentation,
we propose a model to predict the XUV emission from Sun-like stars,
by extending the self-consistent solar coronal heating model. The
simulations are performed for a range of loop lengths and magnetic
filling factors at the stellar surface. With the solar parameters, our
model reproduces the observed solar XUV spectrum below the Lyman edge,
which validates the capability in predicting the XUV spectra of other
Sun-like stars. The model also reproduces the observed nearly-linear
relation between the unsigned magnetic flux and X-ray luminosity. From
the simulation runs with various loop lengths and filling factors, we
have found a scaling relation of logLEUV = 9.93 + 0.67logLX where LEUV
and LX are the cgs-unit luminosity in the EUV (100 A < < 912 A)
and X-ray (5 A < 100 A) ranges, respectively. By assuming a power-law
relation between the Rossby number and magnetic filling factor, the
widely known relation between the Rossby number and X-ray luminosity
is also reproduced. This study provides the theoretical relation useful
in estimating the hidden stellar EUV luminosity from X-ray observations.
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Title: Corona and XUV emission modelling of the Sun and Sun-like stars
Authors: Shoda, Munehito; Takasao, Shinsuke
2021A&A...656A.111S Altcode: 2021arXiv210608915S
The X-ray and extreme-ultraviolet (EUV) emissions from low-mass stars
significantly affect the evolution of the planetary atmosphere. However,
it is observationally difficult to constrain the stellar high-energy
emission because of the strong interstellar extinction of EUV
photons. In this study, we simulate the XUV (X-ray plus EUV) emission
from Sun-like stars by extending the solar coronal heating model
that self-consistently solves, with sufficiently high resolution, the
surface-to-coronal energy transport, turbulent coronal heating, and
coronal thermal response by conduction and radiation. The simulations
are performed with a range of loop lengths and magnetic filling factors
at the stellar surface. With the solar parameters, the model reproduces
the observed solar XUV spectrum below the Lyman edge, thus validating
its capability of predicting the XUV spectra of other Sun-like
stars. The model also reproduces the observed nearly linear relation
between the unsigned magnetic flux and the X-ray luminosity. From
the simulation runs with various loop lengths and filling factors,
we also find a scaling relation, namely log L<SUB>EUV</SUB> = 9.93
+ 0.67 log L<SUB>X</SUB>, where L<SUB>EUV</SUB> and L<SUB>X</SUB>
are the luminosity in the EUV (100 Å < λ ≤ 912 Å) and X-ray
(5 Å < λ ≤ 100 Å) range, respectively, in cgs. By assuming a
power-law relation between the Rossby number and the magnetic filling
factor, we reproduce the renowned relation between the Rossby number
and the X-ray luminosity. We also propose an analytical description
of the energy injected into the corona, which, in combination with
the conventional Rosner-Tucker-Vaiana scaling law, semi-analytically
explains the simulation results. This study refines the concepts of
solar and stellar coronal heating and derives a theoretical relation for
estimating the hidden stellar EUV luminosity from X-ray observations.
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Title: Hydrodynamic Model of Hα Emission from Accretion Shocks of
a Proto-giant Planet and Circumplanetary Disk
Authors: Takasao, Shinsuke; Aoyama, Yuhiko; Ikoma, Masahiro
2021ApJ...921...10T Altcode: 2021arXiv210616113T
Recent observations have detected excess Hα emission from young stellar
systems with an age of several Myr such as PDS 70. One-dimensional
radiation-hydrodynamic models of shock-heated flows that we developed
previously demonstrate that planetary accretion flows of >a few
ten km s<SUP>-1</SUP> can produce Hα emission. It is, however, a
challenge to understand the accretion process of proto-giant planets
from observations of such shock-originated emission because of a
huge gap in scale between the circumplanetary disk (CPD) and the
microscopic accretion shock. To overcome the scale gap problem, we
combine two-dimensional, high-spatial-resolution global hydrodynamic
simulations and the one-dimensional local radiation-hydrodynamic
model of the shock-heated flow. From such combined simulations for
the protoplanet-CPD system, we find that the Hα emission is mainly
produced in localized areas on the protoplanetary surface. The accretion
shocks above the CPD produce much weaker Hα emission (approximately
one to two orders of magnitude smaller in luminosity). Nevertheless,
the accretion shocks above the CPD significantly affect the accretion
process onto the protoplanet. The accretion occurs at a quasi-steady
rate if averaged on a 10 day timescale, but its rate shows variability
on shorter timescales. The disk surface accretion layers including
the CPD shocks largely fluctuate, which results in the time-variable
accretion rate and Hα luminosity of the protoplanet. We also model
the spectral emission profile of the Hα line and find that the
line profile is less time-variable despite the large variability in
luminosity. High-spectral-resolution spectroscopic observation and
monitoring will be key to revealing the property of the accretion
process.
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Title: A Necessary Condition for Supernova Fallback Invading Newborn
Neutron-star Magnetosphere
Authors: Zhong, Yici; Kashiyama, Kazumi; Shigeyama, Toshikazu;
Takasao, Shinsuke
2021ApJ...917...71Z Altcode: 2021arXiv210309461Z
We numerically investigate the dynamics of a supernova fallback
accretion confronting with a relativistic wind from a newborn
neutron star (NS). The time evolution of the accretion shock in
the radial direction is basically characterized by the encounter
radius of the flow r<SUB>enc</SUB> and a dimensionless parameter
$\zeta \equiv L/{\dot{M}}_{\mathrm{fb}}{c}^{2}$ , where L is the NS
wind luminosity and ${\dot{M}}_{\mathrm{fb}}$ is the fallback mass
accretion rate. We find that the critical condition for the fallback
matter to reach near the NS surface can be simply described as $\zeta
\lt {\zeta }_{\min }\equiv {{GM}}_{* }/{c}^{2}{r}_{\mathrm{enc}}$
or ${r}_{\mathrm{enc}}L/{{GM}}_{* }{\dot{M}}_{\mathrm{fb}}\lt 1$
independent of the wind Lorentz factor, where M<SUB>*</SUB> is the NS
mass. With combining the condition for the fallback matter to bury the
surface magnetic field under the NS crust, we discuss the possibility
that the trifurcation of NSs into rotation-powered pulsars, central
compact objects, and magnetars can be induced by supernova fallback.
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Title: 3D MHD simulations of an accreting young star
Authors: Takasao, Shinsuke; Tomida, Kengo; Iwasaki, Kazunari; Suzuki,
Takeru
2021csss.confE.282T Altcode:
Young stars such as protostars and pre-main-sequence stars evolve via
the interaction with the surrounding accretion disks. It is believed
that stellar and disk magnetic fields play important roles in shaping
the accretion structure and exchanging the angular momentum between
the stars and the disks. However, because of the complexity of gas
dynamics around the stars, the star-disk interaction remains poorly
understood, which makes the construction of the stellar evolution models
difficult. To reveal the interaction processes, we have been performing
3D magnetohydrodynamic simulations of accretion onto a young star with
different stellar magnetic fields. In the case of a weakly magnetized,
magnetosphere-free star, we found that failed disk wind becomes
supersonic, high-latitude accretion flows onto the star (Takasao et
al. 2018). This result may explain the reason why Herbig Ae/Be stars
show fast accretion. In a different model with stronger disk fields,
we showed that the star can produce recurrent explosions via magnetic
reconnection (Takasao et al. 2019). We consider that the mechanism is
relevant to protostellar flares in class-0/I protostars. In addition
to the above two models, we have been investigating the magnetospheric
accretion which is very relevant to classical T-Tauri stars. In this
talk, we will introduce our 3D modeling and discuss how the star-disk
interaction changes depending on the stellar and disk field strengths.
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Title: Transition Region from Turbulent to Dead Zone in Protoplanetary
Disks: Local Shearing Box Simulations
Authors: Pucci, Fulvia; Tomida, Kengo; Stone, James; Takasao, Shinsuke;
Ji, Hantao; Okamura, Shoichi
2021ApJ...907...13P Altcode: 2020arXiv201108219P
The dynamical evolution of protoplanetary disks is of key interest
for building a comprehensive theory of planet formation and to
explain the observational properties of these objects. Using the
magnetohydrodynamics code Athena++, with an isothermal shearing box
setup, we study the boundary between the active and dead zone, where
the accretion rate changes and mass can accumulate. We quantify how
the turbulence level is affected by the presence of a non-uniform
Ohmic resistivity in the radial x direction that leads to a region of
inhibited turbulence (or dead zone). Comparing the turbulent activity
to that of ideal simulations, the turbulence-inhibited area shows
density fluctuations and magnetic activity at its boundaries, driven
by energy injection from the active (ideal) zone boundaries. We find
magnetic dissipation to be significantly stronger in the ideal regions,
and the turbulence penetration through the boundary of the dead zone is
determined by the value of the resistivity itself, through the Ohmic
dissipation process, though the thickness of the transition does not
play a significant role in changing the dissipation. We investigate
the 1D spectra along the shearing direction: magnetic spectra appear
flat at large scales both in ideal as well as resistive simulations,
though a Kolmogorov scaling over more than one decade persists in
the dead zone, suggesting the turbulent cascade is determined by the
hydrodynamics of the system: magnetorotational instability dynamo
action is inhibited where sufficiently high resistivity is present.
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Title: Estimation of Low-energy Cutoff of Non-thermal Electrons from
a Spectro-polarimetric Observation
Authors: Anan, T.; Yoneya, T.; Ichimoto, K.; Ueno, S.; Shiota, D.;
Nozawa, S.; Takasao, S.; Kawate, T.
2020AGUFMSH0430015A Altcode:
Low-energy cutoff of the non-thermal electron energy distribution
is crucial to derive the total non-thermal electron energy. A flare
kernel associated with a C4 class flare was observed in a spectral
window including the He I triplet 1083.0 nm and Si I 1082.7 nm with a
spectro-polarimeter on the Domeless Solar Telescope at Hida Observatory
on 2015 August 9. The observed Stokes profiles of the He I triplet in
the flare kernel are well reproduced through inversions considering
the Zeeman and the Paschen-Back effects with a three-slab model of
the flare kernel, in which two slabs which have upward and downward
velocities produce emissions and one slab produces an absorption. The
magnetic field strength inferred from the emission components of the
He I line is 1400 G, which is significantly stronger than 690 G that
is observed at the same location in the same line 6.5 hr before the
flare. In addition, the photospheric magnetic field vector derived from
the Si I10827 Å is similar to that of the flare kernel. To explain
this result, we suggest that the emission in the He I triplet during
the flare is produced in the deep layer, around which bombardment of
non-thermal electrons leads to the formation of a coronal temperature
plasma. Assuming a hydrogen column density at the location where the He
I emissions are formed, and a power-law index of non-thermal electron
energy distribution, we derived the low-energy cutoff of the non-thermal
electron as 20-30 keV independently from methods using hard X-ray data.
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Title: Investigation of Coronal Properties of X-Ray Bright G-dwarf
Stars Based on the Solar Surface Magnetic Field-Corona Relationship
Authors: Takasao, Shinsuke; Mitsuishi, Ikuyuki; Shimura, Takuma;
Yoshida, Atsushi; Kunitomo, Masanobu; Tanaka, Yuki A.; Ishihara,
Daisuke
2020ApJ...901...70T Altcode: 2020arXiv200804255T
We investigated the coronal properties of G-dwarf stars including
the Sun over a wide range of X-ray luminosity L<SUB>X</SUB> (3 ×
10<SUP>26</SUP> to 2 × 10<SUP>30</SUP> erg s<SUP>-1</SUP>). We analyzed
the archival data of 10 X-ray bright (L<SUB>X</SUB> > 10<SUP>28</SUP>
erg s<SUP>-1</SUP>) G-dwarf stars to derive their emission measure (EM)
and the coronal temperature (T) during the periods when no prominent
stellar flares were observed. We attempted to explain the relation
on the basis of our understanding of the present Sun: a steady corona
model based on the so-called Rosner-Tucker-Vaiana (RTV) scaling laws
and the observed power-law distribution function of surface magnetic
features. We derived a theoretical scaling law of the EM-T relation for
a star with multiple active regions, and applied it to the observations
combined with data in the literature. We found that with the solar
parameters, our scaling law seems to be consistent with the data of
slowly rotating stars. However, more X-ray-bright stars are located
well above the scaling law based on the solar parameter. The scaling
law may explain the observations if those stars show a power-law
distribution function of active regions with the same power-law index
but a 10-100 times larger coefficient. This suggests that X-ray bright
stars show more active regions for a given size than the Sun. Since
our samples include rapidly rotating stars, we infer that the offset
of the X-ray bright stars from the present Sun-based scaling law is
due to the enhancement of the surface magnetic field generation by
their rapid rotation.
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Title: Accretion Properties of PDS 70b with MUSE
Authors: Hashimoto, Jun; Aoyama, Yuhiko; Konishi, Mihoko; Uyama,
Taichi; Takasao, Shinsuke; Ikoma, Masahiro; Tanigawa, Takayuki
2020AJ....159..222H Altcode: 2020arXiv200307922H
We report a new evaluation of the accretion properties of PDS 70b
obtained with the Very Large Telescope/Multi Unit Spectroscopic
Explorer. The main difference from the previous studies of
Haffert et al. and Aoyama & Ikoma is in the mass accretion
rate. Simultaneous multiple line observations, such as Hα and
Hβ, can better constrain the physical properties of an accreting
planet. While we clearly detected Hα emissions from PDS 70b, no
Hβ emissions were detected. We estimate the line flux of Hβ with
a 3σ upper limit to be 2.3 × 10<SUP>-16</SUP> erg s<SUP>-1</SUP>
cm<SUP>-2</SUP>. The flux ratio F<SUB>Hβ</SUB>/F<SUB>Hα</SUB> for
PDS 70b is <0.28. Numerical investigations by Aoyama et al. suggest
that F<SUB>Hβ</SUB>/F<SUB>Hα</SUB> should be close to unity if the
extinction is negligible. We attribute the reduction of the flux ratio
to the extinction, and estimate the extinction of Hα (A<SUB>Hα</SUB>)
for PDS 70b to be >2.0 mag using the interstellar extinction
value. By combining with the Hα linewidth and the dereddening line
luminosity of Hα, we derive the PDS 70b mass accretion rate to be
≳5 × 10<SUP>-7</SUP> M<SUB>Jup</SUB> yr<SUP>-1</SUP>. The PDS 70b
mass accretion rate is an order of magnitude larger than that of PDS
70. We found that the filling factor f<SUB>f</SUB> (the fractional area
of the planetary surface emitting Hα) is ≳0.01, which is similar to
the typical stellar value. The small value of f<SUB>f</SUB> indicates
that the Hα emitting areas are localized at the surface of PDS 70b.
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Title: 3D simulations of accretion onto a star: Fast funnel-wall
accretion
Authors: Takasao, Shinsuke; Tomida, Kengo; Iwasaki, Kazunari; Suzuki,
Takeru K.
2020IAUGA..30..138T Altcode:
We show the results of global 3D magnetohydrodynamics simulations of an
accretion disk with a rotating, weakly magnetized central star (Takasao
et al. 2018). The disk is threaded by a weak large-scale poloidal
magnetic field. The central star has no strong stellar magnetosphere
initially and is only weakly magnetized. We investigate the structure of
the accretion flows from a turbulent accretion disk onto the star. Our
simulations reveal that fast accretion onto the star at high latitudes
is established even without a stellar magnetosphere. We find that the
failed disk wind becomes the fast, high-latitude accretion as a result
of angular momentum exchange mediated by magnetic fields. The rapid
angular momentum exchange occurs well above the disk, where the Lorentz
force that decelerates the rotational motion of gas can be comparable
to the centrifugal force. Unlike the classical magnetospheric accretion
model, fast accretion streams are not guided by magnetic fields of the
stellar magnetosphere. Nevertheless, the accretion velocity reaches
the free-fall velocity at the stellar surface owing to the efficient
angular momentum loss at a distant place from the star. Our model can
be applied to Herbig Ae/Be stars whose magnetic fields are generally
not strong enough to form magnetospheres, and also provides a possible
explanation why Herbig Ae/Be stars show indications of fast accretion.
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Title: Comparative Study of Data-driven Solar Coronal Field Models
Using a Flux Emergence Simulation as a Ground-truth Data Set
Authors: Toriumi, Shin; Takasao, Shinsuke; Cheung, Mark C. M.; Jiang,
Chaowei; Guo, Yang; Hayashi, Keiji; Inoue, Satoshi
2020ApJ...890..103T Altcode: 2020arXiv200103721T
For a better understanding of the magnetic field in the solar corona
and dynamic activities such as flares and coronal mass ejections, it
is crucial to measure the time-evolving coronal field and accurately
estimate the magnetic energy. Recently, a new modeling technique called
the data-driven coronal field model, in which the time evolution of
magnetic field is driven by a sequence of photospheric magnetic and
velocity field maps, has been developed and revealed the dynamics
of flare-productive active regions. Here we report on the first
qualitative and quantitative assessment of different data-driven
models using a magnetic flux emergence simulation as a ground-truth
(GT) data set. We compare the GT field with those reconstructed from
the GT photospheric field by four data-driven algorithms. It is found
that, at minimum, the flux rope structure is reproduced in all coronal
field models. Quantitatively, however, the results show a certain
degree of model dependence. In most cases, the magnetic energies and
relative magnetic helicity are comparable to or at most twice of the GT
values. The reproduced flux ropes have a sigmoidal shape (consistent
with GT) of various sizes, a vertically standing magnetic torus, or
a packed structure. The observed discrepancies can be attributed to
the highly non-force-free input photospheric field, from which the
coronal field is reconstructed, and to the modeling constraints such
as the treatment of background atmosphere, the bottom boundary setting,
and the spatial resolution.
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Title: Comparative Study of Data-driven Coronal Field Models with
a Ground-truth Flux Emergence Simulation
Authors: Toriumi, S.; Takasao, S.; Cheung, C. M. M.; Jiang, C.; Guo,
Y.; Hayashi, K.; Inoue, S.
2019AGUFMSH34B..04T Altcode:
To better understand the dynamic activities in the so lar corona, it is
desirable to follow the temporal evolution of coronal magnetic field and
accurately measure the stored free magnetic energy. Data-driven coronal
field models, in which the coronal field evolves in response to the
sequentially updated photospheric field, have recently been developed
and revealed the dynamics of flare-producing active regions. Here
we report on the first attempt to qualitatively and quantitatively
compare different data-driven models by using a magnetic flux emergence
simulation as a ground-truth data set. We find that, at least, all
models succeed in reproducing the twisted flux rope structure in the
atmosphere. However, they show a certain degree of model dependence in,
for instance, the structure of the flux rope, the rising speed, and
the estimation of magnetic energy and helicity. In the presentation,
we discuss the possible causes of the discrepancies, attributing them
to the highly non-force-free input photospheric field, from which the
coronal field is reconstructed, and the constraints in the data-driven
models.
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Title: Giant Protostellar Flares: Accretion-driven Accumulation and
Reconnection-driven Ejection of Magnetic Flux in Protostars
Authors: Takasao, Shinsuke; Tomida, Kengo; Iwasaki, Kazunari; Suzuki,
Takeru K.
2019ApJ...878L..10T Altcode: 2019arXiv190202007T
Protostellar flares are rapid magnetic energy release events
associated with the formation of hot plasma in protostars. In the
previous models of protostellar flares, the interaction between a
protostellar magnetosphere with the surrounding disk plays crucial
role in building-up and releasing the magnetic energy. However, it
remains unclear if protostars indeed have magnetospheres because
vigorous disk accretion and strong disk magnetic fields in the
protostellar phase may destroy the magnetosphere. Considering
this possibility, we investigate the energy accumulation and
release processes in the absence of a magnetosphere using a
three-dimensional magnetohydrodynamic simulation. Our simulation
reveals that protostellar flares are repeatedly produced even in such
a case. Unlike in the magnetospheric models, the protostar accumulates
magnetic energy by acquiring large-scale magnetic fields from the
disk by accretion. Protostellar flares occur when a portion of the
large-scale magnetic fields are removed from the protostar as a result
of magnetic reconnection. Protostellar flares in the simulation are
consistent with observations; the released magnetic energy (up to ∼3
× 10<SUP>38</SUP> erg) is large enough to drive observed flares,
and the flares produce hot ejecta. The expelled magnetic fields
enhance accretion, and the energy build-up and release processes are
repeated as a result. The magnetic flux removal via reconnection leads
to redistribution of magnetic fields in the inner disk. We therefore
consider that protostellar flares will play an important role in the
evolution of the disk magnetic fields in the vicinity of protostars.
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Title: A New HLLD Riemann Solver with Boris Correction for Reducing
Alfvén Speed
Authors: Matsumoto, Tomoaki; Miyoshi, Takahiro; Takasao, Shinsuke
2019ApJ...874...37M Altcode: 2019arXiv190202810M
A new Riemann solver is presented for the ideal magnetohydrodynamics
(MHD) equations with the so-called Boris correction. The Boris
correction is applied to reduce wave speeds, avoiding an extremely small
timestep in MHD simulations. The proposed Riemann solver, Boris-HLLD,
is based on the HLLD solver. As done by the original HLLD solver,
(1) the Boris-HLLD solver has four intermediate states in the Riemann
fan when left and right states are given, (2) it resolves the contact
discontinuity, Alfvén waves, and fast waves, and (3) it satisfies all
the jump conditions across shock waves and discontinuities except for
slow shock waves. The results of a shock tube problem indicate that
the scheme with the Boris-HLLD solver captures contact discontinuities
sharply, and it exhibits shock waves without any overshoot when using
the minmod limiter. The stability tests show that the scheme is stable
when | u| ≲ 0.5c for a low Alfvén speed ({V}<SUB>A</SUB>≲ c),
where u, c, and V <SUB> A </SUB> denote the gas velocity, speed
of light, and Alfvén speed, respectively. For a high Alfvén speed
({V}<SUB>A</SUB>≳ c), where the plasma beta is relatively low in many
cases, the stable region is large, | u| ≲ (0.6{--}1)c. We discuss the
effect of the Boris correction on physical quantities using several
test problems. The Boris-HLLD scheme can be useful for problems with
supersonic flows in which regions with a very low plasma beta appear
in the computational domain.
---------------------------------------------------------
Title: Measurement of vector magnetic field in a flare kernel with
a spectropolarimetric observation in He I 10830 Å
Authors: Anan, Tetsu; Yoneya, Takurou; Ichimoto, Kiyoshi; UeNo, Satoru;
Shiota, Daikou; Nozawa, Satoshi; Takasao, Shinsuke; Kawate, Tomoko
2018PASJ...70..101A Altcode: 2018arXiv180806821A; 2018PASJ..tmp..113A
A flare kernel associated with a C4 class flare was observed in a
spectral window including the He I triplet 10830 Å and Si I 10827
Å with a spectropolarimeter on the Domeless Solar Telescope at
Hida Observatory on 2015 August 9. The observed Stokes profiles of
the He I triplet in the flare kernel in its post-maximum phase are
well reproduced through inversions considering the Zeeman and the
Paschen-Back effects with a three-slab model of the flare kernel,
in which two slabs which have upward and downward velocities produce
emissions and one slab produces an absorption. The magnetic field
strength inferred from the emission components of the He I line is 1400
G, which is significantly stronger than 690 G that is observed at the
same location in the same line 6.5 hr before the flare. In addition,
the photospheric magnetic field vector derived from the Si I10827 Å is
similar to that of the flare kernel. To explain this result, we suggest
that the emission in the He I triplet during the flare is produced in
the deep layer, around which bombardment of non-thermal electrons leads
to the formation of a coronal temperature plasma. Assuming a hydrogen
column density at the location where the He I emissions are formed,
and a power-law index of non-thermal electron energy distribution, we
derived the low-energy cutoff of the non-thermal electron as 20-30 keV,
which is consistent with that inferred from hard X-ray data obtained
by RHESSI.
---------------------------------------------------------
Title: Fast Accretion into a Weakly Magnetized Star
Authors: Takasao, Shinsuke
2018tcl..confE..52T Altcode:
Generally it has been assumed that the presence of a fast (close to
the escape velocity) accretion is an indication of the magnetospheric
accretion. However, observations indicate that fast accretion also
occurs even in a weakly magnetized stars like Herbig Ae stars, which
poses a question about the picture of accretion we have developed. We
performed 3D MHD simulations by using the Athena++ code, and analyzed
the accretion from an MRI (Magneto-Rotational Instability)-active disk
onto a weakly magnetized star. As a result, we found that fast accretion
to a high-latitude, which is similar to the magnetospheric accretion,
is possible even without the stellar magnetosphere. Our results suggest
a possibility that stars without the magnetosphere can show a violent
accretion behavior associated with X-ray activities. We will discuss
the physics of the accretion on the basis of our simulations.
---------------------------------------------------------
Title: A Three-dimensional Simulation of a Magnetized Accretion Disk:
Fast Funnel Accretion onto a Weakly Magnetized Star
Authors: Takasao, Shinsuke; Tomida, Kengo; Iwasaki, Kazunari; Suzuki,
Takeru K.
2018ApJ...857....4T Altcode: 2018arXiv180107245T
We present the results of a global, three-dimensional
magnetohydrodynamics simulation of an accretion disk with a rotating,
weakly magnetized central star. The disk is threaded by a weak,
large-scale poloidal magnetic field, and the central star has no
strong stellar magnetosphere initially. Our simulation investigates the
structure of the accretion flows from a turbulent accretion disk onto
the star. The simulation reveals that fast accretion onto the star at
high latitudes occurs even without a stellar magnetosphere. We find
that the failed disk wind becomes the fast, high-latitude accretion
as a result of angular momentum exchange mediated by magnetic fields
well above the disk, where the Lorentz force that decelerates the
rotational motion of gas can be comparable to the centrifugal
force. Unlike the classical magnetospheric accretion scenario,
fast accretion streams are not guided by magnetic fields of the
stellar magnetosphere. Nevertheless, the accretion velocity reaches
the free-fall velocity at the stellar surface due to the efficient
angular momentum loss at a distant place from the star. This study
provides a possible explanation why Herbig Ae/Be stars whose magnetic
fields are generally not strong enough to form magnetospheres also
show indications of fast accretion. A magnetically driven jet is not
formed from the disk in our model. The differential rotation cannot
generate sufficiently strong magnetic fields for the jet acceleration
because the Parker instability interrupts the field amplification.
---------------------------------------------------------
Title: Modelling Quasi-Periodic Pulsations in Solar and Stellar Flares
Authors: McLaughlin, J. A.; Nakariakov, V. M.; Dominique, M.; Jelínek,
P.; Takasao, S.
2018SSRv..214...45M Altcode: 2018arXiv180204180M
Solar flare emission is detected in all EM bands and variations in flux
density of solar energetic particles. Often the EM radiation generated
in solar and stellar flares shows a pronounced oscillatory pattern, with
characteristic periods ranging from a fraction of a second to several
minutes. These oscillations are referred to as quasi-periodic pulsations
(QPPs), to emphasise that they often contain apparent amplitude and
period modulation. We review the current understanding of quasi-periodic
pulsations in solar and stellar flares. In particular, we focus on
the possible physical mechanisms, with an emphasis on the underlying
physics that generates the resultant range of periodicities. These
physical mechanisms include MHD oscillations, self-oscillatory
mechanisms, oscillatory reconnection/reconnection reversal, wave-driven
reconnection, two loop coalescence, MHD flow over-stability, the
equivalent LCR-contour mechanism, and thermal-dynamical cycles. We
also provide a histogram of all QPP events published in the literature
at this time. The occurrence of QPPs puts additional constraints on
the interpretation and understanding of the fundamental processes
operating in flares, e.g. magnetic energy liberation and particle
acceleration. Therefore, a full understanding of QPPs is essential in
order to work towards an integrated model of solar and stellar flares.
---------------------------------------------------------
Title: Numerical Modeling of Flare-productive Active Regions of
the Sun
Authors: Toriumi, S.; Takasao, S.
2017AGUFMSH43C..07T Altcode:
It is known that strong flare events on the Sun take place in active
regions (ARs), especially in delta sunspots with closely-packed positive
and negative polarities. The delta spots are produced as a result of
complex magnetic flux emergence and have strong-field, highly-sheared
polarity inversion lines (PILs). Here we report on the numerical
simulations of four types of such flare-productive ARs, namely, (1)
Spot-Spot, a complex AR with AR-sized PIL, (2) Spot-Satellite, in which
a newly-emerging bipole appears next to the pre-existing sunspot,
(3) Quadrupole, where two emerging bipoles collide against each
other, and (4) Inter-AR, the flares occurring between two separated
ARs. We reproduced these four cases by conducting a series of 3D MHD
flux emergence simulations and found, for example, that the sheared
PILs in these ARs are created through the stretching and advection
of horizontal magnetic fields due to relative spot motions. As ARs
develop, free magnetic energy becomes stored in the corona, which could
be eventually released through flare eruptions. In the presentation,
we also mention the relationship between the HMI/SHARP parameters
measured in the photosphere and the free energy stored in the corona,
and discuss why these parameters successfully predict the flares.
---------------------------------------------------------
Title: Numerical Simulations of Flare-productive Active Regions:
δ-sunspots, Sheared Polarity Inversion Lines, Energy Storage,
and Predictions
Authors: Toriumi, Shin; Takasao, Shinsuke
2017ApJ...850...39T Altcode: 2017arXiv171008926T
Solar active regions (ARs) that produce strong flares and coronal mass
ejections (CMEs) are known to have a relatively high non-potentiality
and are characterized by δ-sunspots and sheared magnetic structures. In
this study, we conduct a series of flux emergence simulations from the
convection zone to the corona and model four types of active regions
that have been observationally suggested to cause strong flares, namely
the spot-spot, spot-satellite, quadrupole, and inter-AR cases. As
a result, we confirm that δ-spot formation is due to the complex
geometry and interaction of emerging magnetic fields, and we find that
the strong-field, high-gradient, highly sheared polarity inversion line
(PIL) is created by the combined effect of the advection, stretching,
and compression of magnetic fields. We show that free magnetic energy
builds up in the form of a current sheet above the PIL. It is also
revealed that photospheric magnetic parameters that predict flare
eruptions reflect the stored free energy with high accuracy, while
CME-predicting parameters indicate the magnetic relationship between
flaring zones and entire ARs.
---------------------------------------------------------
Title: A Theoretical Model of X-Ray Jets from Young Stellar Objects
Authors: Takasao, Shinsuke; Suzuki, Takeru K.; Shibata, Kazunari
2017ApJ...847...46T Altcode: 2017arXiv170805388T
There is a subclass of X-ray jets from young stellar objects that are
heated very close to the footpoint of the jets, particularly DG Tau
jets. Previous models have attributed the strong heating to shocks
in the jets. However, the mechanism that localizes the heating at the
footpoint remains puzzling. We presented a different model of such X-ray
jets, in which the disk atmosphere is magnetically heated. Our disk
corona model is based on the so-called nanoflare model for the solar
corona. We show that the magnetic heating near the disks can result in
the formation of a hot corona with a temperature of ≳10<SUP>6</SUP>
K, even if the average field strength in the disk is moderately weak,
≳1 G. We determine the density and the temperature at the jet base
by considering the energy balance between the heating and cooling. We
derive the scaling relations of the mass-loss rate and terminal
velocity of jets. Our model is applied to the DG Tau jets. The
observed temperature and estimated mass-loss rate are consistent
with the prediction of our model in the case of a disk magnetic field
strength of ∼20 G and a heating region of <0.1 au. The derived
scaling relation of the temperature of X-ray jets could be a useful
tool for estimating the magnetic field strength. We also find that the
jet X-ray can have a significant impact on the ionization degree near
the disk surface and the dead zone size.
---------------------------------------------------------
Title: “Dandelion” Filament Eruption and Coronal Waves Associated
with a Solar Flare on 2011 February 16
Authors: Cabezas, Denis P.; Martínez, Lurdes M.; Buleje, Yovanny J.;
Ishitsuka, Mutsumi; Ishitsuka, José K.; Morita, Satoshi; Asai, Ayumi;
UeNo, Satoru; Ishii, Takako T.; Kitai, Reizaburo; Takasao, Shinsuke;
Yoshinaga, Yusuke; Otsuji, Kenichi; Shibata, Kazunari
2017ApJ...836...33C Altcode: 2017arXiv170100308C
Coronal disturbances associated with solar flares, such as Hα Moreton
waves, X-ray waves, and extreme ultraviolet (EUV) coronal waves,
are discussed herein in relation to magnetohydrodynamic fast-mode
waves or shocks in the corona. To understand the mechanism of
coronal disturbances, full-disk solar observations with high spatial
and temporal resolution over multiple wavelengths are of crucial
importance. We observed a filament eruption, whose shape is like a
“dandelion,” associated with the M1.6 flare that occurred on 2011
February 16 in Hα images taken by the Flare Monitoring Telescope at
Ica University, Peru. We derive the three-dimensional velocity field
of the erupting filament. We also identify winking filaments that are
located far from the flare site in the Hα images, whereas no Moreton
wave is observed. By comparing the temporal evolution of the winking
filaments with those of the coronal wave seen in the EUV images data
taken by the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory and by the Extreme Ultraviolet Imager on board the Solar
Terrestrial Relations Observatory-Ahead, we confirm that the winking
filaments were activated by the EUV coronal wave.
---------------------------------------------------------
Title: Observational Evidence of Particle Acceleration Associated
with Plasmoid Motions
Authors: Takasao, Shinsuke; Asai, Ayumi; Isobe, Hiroaki; Shibata,
Kazunari
2016ApJ...828..103T Altcode: 2016arXiv161100108T
We report a strong association between the particle acceleration and
plasma motions found in the 2010 August 18 solar flare. The plasma
motions are tracked in the extreme ultraviolet (EUV) images taken by
the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory and the Extreme UltraViolet Imager (EUVI) on the Solar
Terrestrial Relations Observatory spacecraft Ahead, and the signature of
particle acceleration was investigated by using Nobeyama Radioheliograph
data. In our previous paper, we reported that in EUV images many plasma
blobs appeared in the current sheet above the flare arcade. They were
ejected bidirectionally along the current sheet, and the blobs that
were ejected sunward collided with the flare arcade. Some of them
collided or merged with each other before they were ejected from
the current sheet. We discovered impulsive radio bursts associated
with such plasma motions (ejection, coalescence, and collision with
the post flare loops). The radio bursts are considered to be the
gyrosynchrotron radiation by nonthermal high energy electrons. In
addition, the stereoscopic observation by AIA and EUVI suggests
that plasma blobs had a three-dimensionally elongated structure. We
consider that the plasma blobs were three-dimensional plasmoids (I.e.,
flux ropes) moving in a current sheet. We believe that our observation
provides clear evidence of particle acceleration associated with the
plasmoid motions. We discuss possible acceleration mechanisms on the
basis of our results.
---------------------------------------------------------
Title: Above-the-loop-top Oscillation and Quasi-periodic Coronal
Wave Generation in Solar Flares
Authors: Takasao, Shinsuke; Shibata, Kazunari
2016ApJ...823..150T Altcode: 2016arXiv160609354T
Observations revealed that various kinds of oscillations are excited
in solar flare regions. Quasi-periodic pulsations (QPPs) in flare
emissions are commonly observed in a wide range of wavelengths. Recent
observations have found that fast-mode magnetohydrodynamic (MHD)
waves are quasi-periodically emitted from some flaring sites
(quasi-periodic propagating fast-mode magnetoacoustic waves;
QPFs). Both QPPs and QPFs imply a cyclic disturbance originating
from the flaring sites. However, the physical mechanisms remain
puzzling. By performing a set of two-dimensional MHD simulations of
a solar flare, we discovered the local oscillation above the loops
filled with evaporated plasma (above-the-loop-top region) and the
generation of QPFs from such oscillating regions. Unlike all previous
models for QPFs, our model includes essential physics for solar flares
such as magnetic reconnection, heat conduction, and chromospheric
evaporation. We revealed that QPFs can be spontaneously excited by
the above-the-loop-top oscillation. We found that this oscillation is
controlled by the backflow of the reconnection outflow. The new model
revealed that flare loops and the above-the-loop-top region are full
of shocks and waves, which is different from the previous expectations
based on a standard flare model and previous simulations. In this paper,
we show the QPF generation process based on our new picture of flare
loops and will briefly discuss a possible relationship between QPFs
and QPPs. Our findings will change the current view of solar flares to
a new view in which they are a very dynamic phenomenon full of shocks
and waves.
---------------------------------------------------------
Title: The formation and evolution of reconnection-driven, slow-mode
shocks in a partially ionised plasma
Authors: Hillier, A.; Takasao, S.; Nakamura, N.
2016A&A...591A.112H Altcode: 2016arXiv160201112H
The role of slow-mode magnetohydrodynamic (MHD) shocks in magnetic
reconnection is of great importance for energy conversion and transport,
but in many astrophysical plasmas the plasma is not fully ionised. In
this paper, we use numerical simulations to investigate the role of
collisional coupling between a proton-electron, charge-neutral fluid and
a neutral hydrogen fluid for the one-dimensional (1D) Riemann problem
initiated in a constant pressure and density background state by a
discontinuity in the magnetic field. This system, in the MHD limit,
is characterised by two waves. The first is a fast-mode rarefaction
wave that drives a flow towards a slow-mode MHD shock wave. The
system evolves through four stages: initiation, weak coupling,
intermediate coupling, and a quasi-steady state. The initial stages
are characterised by an over-pressured neutral region that expands
with characteristics of a blast wave. In the later stages, the system
tends towards a self-similar solution where the main drift velocity
is concentrated in the thin region of the shock front. Because of
the nature of the system, the neutral fluid is overpressured by the
shock when compared to a purely hydrodynamic shock, which results in
the neutral fluid expanding to form the shock precursor. Once it has
formed, the thickness of the shock front is proportional to ξ<SUB>
I</SUB><SUP>-1.2</SUP> , which is a smaller exponent than would be
naively expected from simple scaling arguments. One interesting result
is that the shock front is a continuous transition of the physical
variables of subsonic velocity upstream of the shock front (a c-shock)
to a sharp jump in the physical variables followed by a relaxation to
the downstream values for supersonic upstream velocity (a j-shock). The
frictional heating that results from the velocity drift across the
shock front can amount to ~2 per cent of the reference magnetic energy.
---------------------------------------------------------
Title: Fractal Reconnection in Solar and Stellar Environments
Authors: Shibata, K.; Takasao, S.
2016ASSL..427..373S Altcode: 2016arXiv160609401S
Recent space based observations of the Sun revealed that magnetic
reconnection is ubiquitous in the solar atmosphere, ranging from
small scale reconnection (observed as nanoflares) to large scale
one (observed as long duration flares or giant arcades). Often the
magnetic reconnection events are associated with mass ejections or jets,
which seem to be closely related to multiple plasmoid ejections from
fractal current sheet. The bursty radio and hard X-ray emissions from
flares also suggest the fractal reconnection and associated particle
acceleration. We shall discuss recent observations and theories related
to the plasmoid-induced-reconnection and the fractal reconnection in
solar flares, and their implication to reconnection physics and particle
acceleration. Recent findings of many superflares on solar type stars
that has extended the applicability of the fractal reconnection model
of solar flares to much a wider parameter space suitable for stellar
flares are also discussed.
---------------------------------------------------------
Title: Numerical Study on the Emergence of Kinked Flux Tube for
Understanding of Possible Origin of δ-spot Regions
Authors: Takasao, Shinsuke; Fan, Yuhong; Cheung, Mark C. M.; Shibata,
Kazunari
2015ApJ...813..112T Altcode: 2015arXiv151102863T
We carried out an magnetohydrodynamic simulation where a subsurface
twisted kink-unstable flux tube emerges from the solar interior to the
corona. Unlike the previous expectations based on the bodily emergence
of a knotted tube, we found that the kinked tube can spontaneously
form a complex quadrupole structure at the photosphere. Due to the
development of the kink instability before the emergence, the magnetic
twist at the kinked apex of the tube is greatly reduced, although the
other parts of the tube are still strongly twisted. This leads to the
formation of a complex quadrupole structure: a pair of the coherent,
strongly twisted spots and a narrow complex bipolar pair between it. The
quadrupole is formed by the submergence of a portion of emerged magnetic
fields. This result is relevant for understanding the origin of the
complex multipolar δ-spot regions that have a strong magnetic shear
and emerge with polarity orientations not following Hale-Nicholson
and Joy Laws.
---------------------------------------------------------
Title: A Theoretical Model of a Thinning Current Sheet in the
Low-β Plasmas
Authors: Takeshige, Satoshi; Takasao, Shinsuke; Shibata, Kazunari
2015ApJ...807..159T Altcode: 2015arXiv150405677T
Magnetic reconnection is an important physical process in various
explosive phenomena in the universe. In previous studies, it was
found that fast reconnection takes place when the thickness of a
current sheet becomes on the order of a microscopic length such as
the ion Larmor radius or the ion inertial length. In this study, we
investigated the pinching process of a current sheet by the Lorentz
force in a low-β plasma using one-dimensional magnetohydrodynamics
(MHD) simulations. It is known that there is an exact self-similar
solution for this problem that neglects gas pressure. We compared the
non-linear MHD dynamics with the analytic self-similar solution. From
the MHD simulations, we found that with the gas pressure included the
implosion process deviates from the analytic self-similar solution as
t\to {t}<SUB>0</SUB>, where t<SUB>0</SUB> is the explosion time when
the thickness of a current sheet of the analytic solution becomes
0. We also found that a pair of MHD fast-mode shocks is generated
and propagates after the formation of the pinched current sheet as
t\to {t}<SUB>0</SUB>. On the basis of the Rankine-Hugoniot relations,
we derived the scaling law of the physical quantities with respect to
the initial plasma beta in the pinched current sheet. Our study could
help us estimate the physical quantities in the pinched current sheet
formed in a low-β plasma.
---------------------------------------------------------
Title: Magnetohydrodynamic Shocks in and above Post-flare Loops:
Two-dimensional Simulation and a Simplified Model
Authors: Takasao, Shinsuke; Matsumoto, Takuma; Nakamura, Naoki;
Shibata, Kazunari
2015ApJ...805..135T Altcode: 2015arXiv150405700T
Solar flares are an explosive phenomenon where super-sonic flows and
shocks are expected in and above the post-flare loops. To understand
the dynamics of post-flare loops, a two-dimensional magnetohydrodynamic
(2D MHD) simulation of a solar flare has been carried out. We found
new shock structures in and above the post-flare loops, which were not
resolved in the previous work by Yokoyama & Shibata. To study the
dynamics of flows along the reconnected magnetic field, the kinematics
and energetics of the plasma are investigated along selected field
lines. It is found that shocks are crucial to determine the thermal
and flow structures in the post-flare loops. On the basis of the 2D MHD
simulation, we developed a new post-flare loop model, which we defined
as the pseudo-2D MHD model. The model is based on the one-dimensional
(1D) MHD equations, where all variables depend on one space dimension,
and all the three components of the magnetic and velocity fields
are considered. Our pseudo-2D model includes many features of the
multi-dimensional MHD processes related to magnetic reconnection
(particularly MHD shocks), which the previous 1D hydrodynamic models are
not able to include. We compared the shock formation and energetics of
a specific field line in the 2D calculation with those in our pseudo-2D
MHD model, and found that they give similar results. This model will
allow us to study the evolution of the post-flare loops in a wide
parameter space without expensive computational cost or neglecting
important physics associated with magnetic reconnection.
---------------------------------------------------------
Title: Within the International Collaboration CHAIN: a Summary of
Events Observed with Flare Monitoring Telescope (FMT) in Peru
Authors: Ishitsuka, J.; Asai, A.; Morita, S.; Terrazas, R.; Cabezas,
D.; Gutierrez, V.; Martinez, L.; Buleje, Y.; Loayza, R.; Nakamura,
N.; Takasao, S.; Yoshinaga, Y.; Hillier, A.; Otsuji, K.; Shibata, K.;
Ishitsuka, M.; Ueno, S.; Kitai, R.; Ishii, T.; Ichimoto, K.; Nagata,
S.; Narukage, N.
2014SunGe...9...85I Altcode:
In 2008 we inaugurated the new Solar Observatory in collaboration with
Faculty of Sciences of San Luis Gonzaga de Ica National University,
300 km south of Lima. In March of 2010 a Flare Monitoring Telescope
of Hida Observatory of Kyoto University arrived to Ica, part of CHAIN
Project (Continuous H-alpha Imaging Network). In October of the same
year we hosted the First FMT Workshop in Ica, then in July of 2011 the
Second FMT Workshop was opened. Since that we are focused on two events
registered by FMT in Peru to publish results. FMT is a good tool to
introduce young people from universities into scientific knowledge;
it is good also for education in Solar Physics and outreach. Details
of this successful collaboration will be explained in this presentation.
---------------------------------------------------------
Title: Numerical Experiments of Flux Emergence of Kinked Flux Tubes
into Solar Atmosphere
Authors: Takasao, Shinsuke; Fan, Yuhong
2014cosp...40E3280T Altcode:
The so-called “delta-sunspots” are known to be among the most
flare-productive active regions in the solar atmosphere. Observations
show that a fraction of the delta-spots emerge with strong magnetic
shear and with polarity orientations not following the Hale-polarity
rule obeyed by the majority of active regions. To understand the
observed evolution of these active regions and the origin of their
high magnetic activity, we carried out MHD simulations in which a
subsurface twisted flux tube with kinked or knotted geometry emerges
from the convection zone into the solar atmosphere. From the numerical
experiments, we found the followings: 1. Although the initial twist is
strong, the magnetic shear near the apex is reduced due to kinking in
the early phase of the emergence. This leads to a larger growth rate
of the magnetic Rayleigh-Taylor (RT) instability, and therefore many
current sheets are formed due to RT instability. 2. The current carried
by the twisted flux tube in the convection zone, which can be the main
source of the free energy, is transported to the upper atmosphere by
the torsional Alfven wave. 3. Magnetic reconnections take place above
the neutral line at the photosphere. This causes the plasma ejections
in the corona. We will give a detailed picture of the emergence of a
kinked flux tube on the basis of the numerical experiments, and clarify
the difference between the unkinked tube emergence and the kinked
tube emergence. These results could be important for understanding
the magnetic field evolution in flare productive active regions.
---------------------------------------------------------
Title: Numerical Simulations of Solar Chromospheric Jets Associated
with Emerging Flux
Authors: Takasao, Shinsuke; Isobe, Hiroaki; Shibata, Kazunari
2013PASJ...65...62T Altcode: 2013arXiv1301.7325T
We studied the acceleration mechanisms of chromospheric jets associated
with emerging flux using a two-dimensional magnetohydrodynamic (MHD)
simulation. We found that slow-mode shock waves generated by magnetic
reconnection in the chromosphere and the photosphere play key roles
in the acceleration mechanisms of chromospheric jets. An important
parameter is the height of magnetic reconnection. When magnetic
reconnection takes place near the photosphere, the reconnection outflow
collides with the region where the plasma beta is much larger than
unity. Then, the plasma moves along a magnetic field. This motion
generates a slow-mode wave. The slow-mode wave develops to a strong
slow shock as it propagates upward. When the slow shock crosses the
transition region, this region is lifted up. As a result, we obtain
a chromospheric jet as the lifted transition region. When magnetic
reconnection takes place in the upper chromosphere, the chromospheric
plasma is accelerated due to the combination of the Lorentz force
and the whip-like motion of the magnetic field. We found that the
chromospheric plasma is further accelerated through the interaction
between the transition region (steep density gradient) and a slow shock
emanating from the reconnection point. In the process, the magnetic
energy released by magnetic reconnection is efficiently converted into
the kinetic energy of jets. This is an MHD effect that has not been
discussed before.
---------------------------------------------------------
Title: Dynamic Features of Current Sheet Associated with the 2010
August 18 Solar Flare
Authors: Takasao, S.; Asai, A.; Isobe, H.; Shibata, K.
2012ASPC..456..221T Altcode:
We report the observation of the magnetic reconnection site in the 2010
August 18 flare. We simultaneously found both reconnection inflow and
outflow. By using these velocities, we estimated the nondimensional
reconnection rate and found that it varied from 0.20 to 0.055. We
also observed dynamic plasma blobs in the sheet structure. The plasma
blobs collided with the hot loops and radio emissions were found at
this site, which may suggest particle acceleration. We hypothesize
that the sheet structure is the current sheet and that these plasma
blobs are plasmoids, which could be important for understanding the
dynamics of the reconnection region.
---------------------------------------------------------
Title: Observation of Dynamic Features of Current Sheet Associated
with 2010 August 18 Solar Flare
Authors: Takasao, S.; Asai, A.; Isobe, H.; Shibata, K.
2012decs.confE..93T Altcode:
We report the simultaneous extreme-ultraviolet observation of magnetic
reconnection inflow and outflow in a flare on 2010 August 18 observed
by SDO/AIA. We found that during the rise phase of the flare, some
plasma blobs appeared in a sheet structure above hot loops. The
plasma blobs were ejected bidirectionally along the sheet structure
(i.e. reconnection outflow). Simultaneously, bright threads visible
in the extreme-ultraviolet images moved toward the sheet structure
(i.e. reconnection inflow). Using the velocities of the inflow and
outflow, we estimated the non-dimensional reconnection rate and found
it varies during this period from 0.20 to 0.055. We also found that
the plasma blobs in the sheet structure collided and possibly merged
with each other before they were ejected from the sheet structure. From
these observational results, we hypothesize that the sheet structure
is the current sheet and that these plasma blobs are plasmoids or
magnetic islands. This observational report could be important for
understanding the dynamics of the reconnection region.
---------------------------------------------------------
Title: Simultaneous Observation of Reconnection Inflow and Outflow
Associated with the 2010 August 18 Solar Flare
Authors: Takasao, Shinsuke; Asai, Ayumi; Isobe, Hiroaki; Shibata,
Kazunari
2012ApJ...745L...6T Altcode: 2011arXiv1112.1398T
We report the simultaneous extreme-ultraviolet observation of
magnetic reconnection inflow and outflow in a flare on 2010 August
18 observed by the Atmospheric Imaging Assembly on board the Solar
Dynamics Observatory. We found that during the rise phase of the
flare, some plasma blobs appeared in the sheet structure above the
hot loops. The plasma blobs were ejected bidirectionally along the
sheet structure (outflow), at the same time as the threads visible
in extreme-ultraviolet images moved toward the sheet structure
(inflow). The upward and downward ejection velocities are 220-460 km
s<SUP>-1</SUP> and 250-280 km s<SUP>-1</SUP>, respectively. The inflow
speed changed from 90 km s<SUP>-1</SUP> to 12 km s<SUP>-1</SUP> in 5
minutes. By using these velocities, we estimated the nondimensional
reconnection rate, which we found to vary during this period from 0.20
to 0.055. We also found that the plasma blobs in the sheet structure
collided or merged with each other before they were ejected from
the sheet structure. We hypothesize that the sheet structure is the
current sheet and that these plasma blobs are plasmoids or magnetic
islands, which could be important for understanding the dynamics of
the reconnection region.
