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March 6 (Fri)
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Dr. Satoshi Inoue (Nagoya University)
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Magnetohydrodynamic Simulation of the X2.2 Solar Flare On 2011 February 15
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In this study, we perform a Magnetohydrodynamic (MHD) simulation combined with photospheric magnetic field to clarify the dynamics of an X2.2 solar flare taking place on February 15 2011. Because the observations provide three components of the magnetic field only at the photosphere, we first extrapolate the three-dimensional (3D) coronal magnetic field under a nonlinear force-free field (NLFFF) approximation from the photospheric magnetic field, and then perform an MHD simulation by using the NLFFF prior to the flare as an initial condition. Photospheric magnetic field was taken from the observation of the Soar Dynamics Observatory (SDO) at 00:00 UT on February 15, which is 2 hours approximately before the X2.2-class flare. As a result, we found that the NLFFF never shows the drastic dynamics seen in observations, i.e., it is in stable state against not only ideal MHD instabilities but also any small perturbations. On the other hand, the MHD simulation showed that when the strongly twisted lines are formed close to the neutral line, which are produced via tether-cut reconnection in the twisted lines of the NLFFF, consequently they can erupt away from the solar surface via the complicated reconnection. This result supports the argument that the strongly twisted lines formed in NLFFF via tether-cut reconnection are responsible for breaking the force balance condition of the magnetic fields in the lower solar corona. We further found that the eruptive flux tube merges with the ambient weakly twisted lines through a reconnection, resulting in the formation of larger flux tube. Eventually, because the large flux tube can successfully exceed a critical height of the torus instability, it would grow into the CME. In addition to show these dynamics, we compare our simulation results with observations, in order to confirm the reliability of our simulation.
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Jan 5 (Mon)
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Shunya Kono (University of Tokyo)
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| Chromospheric heating mechanism by Alfven waves
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abstract will be informed.
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December 11 (Thu)
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Dr. Carlos Quite Noda (IAC/Univ of La Lalaguna, Spain)
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Analysis of Gregor/GRIS NIR data at He I 10830
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December 04 (Thu)
| Dr. Joten Okamoto (JAXA/ISAS)
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| Wave dissipation and associated heating in Hinode-IRIS observations of a solar prominence
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Coronal heating and the acceleration of the solar wind are unsolved problems in solar physics. The propagation of Alfvenic waves along magnetic field lines is one of the candidate mechanisms to carry energy to large distances from the surface and heat the coronal plasma. Such waves can be observed in fine structures of prominences. In particular, Hinode observations have directly resolved small-scale transverse oscillations of field lines as a result of Alfvenic waves. More recently, IRIS provides spectral information of fine strucutures to investigate the detailed property of these waves. With collaborative observations of IRIS and Hinode, we found the following interesting features associated with waves and possible heating:
(1) The velocity amplitude of the oscillations tends to be larger at higher altitudes.
(2) The horizontal threads have smaller length and shorter lifetime at higher altitudes.
(3) As the moving threads in the Ca II line (10 kK) fade away, co-spatial threads appear
in the hotter Si IV line (80 kK) with similar horizontal speeds.
(4) Characteristic phase differences more than 90 up to 180 degrees between transverse motions in the plane of the sky and line-of-sight velocities of the oscillating threads were detected, and this phenomenon supports a scenario in which resonant absorption takes place on the surface of oscillating prominence flux tubes in the corona.
In this talk, I will show these observational results and discuss propagation and dissipation of waves.
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November 19 (Wed)
| Dr. Takashi Sekii (NAOJ)
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A brief introduction to helio- and asteroseismology
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Seismic study of the Sun and stars permits us to peer into the interior of these objects. Basic wave physics behind the techniques are reviewed, and why and how such inferences are possible is discussed, together with some recent results.
[Upon request, this seminar is primarily intended for students]
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July 08 (Tue)
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Hwanhee Lee (Kyung Hee Univ.)
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Magnetic configurations related to coronal heating and solar winds derived from MHD simulations
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Solar winds composed of heated coronal plasmas originate from magnetic structures formed on the Sun. To investigate a physical mechanism for producing them, we derive two key parameters describing the magnetic configurations of those structures from flux-emergence magnetohydrodynamic simulations: twist and expansion profiles of coronal loops. These parameters reveal the characteristics of the magnetic configuration responsible for the coronal heating and the generation of solar winds, which we discuss in detail during the talk. Also one of our ongoing projects, a three-dimensional modeling of a solar flare is briefly explained.
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July 08 (Tue)
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Dr. Tetsuya Magara (Kyung Hee Univ.)
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MHD simulations for a solar magnetic field that emerges, organizes, dissipates and erupts toward the interplanetary space
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Since it was discovered that a solar magnetic field has very dynamic behavior such as emerging below the solar surface, forming magnetic structures, dissipating to produce flares and jets and eventually erupting into the interplanetary space, numerical simulations have been a useful and powerful tool to investigate the time-dependent processes of a solar magnetic field. In this talk I summarize research activities in our group for investigating and reproducing various time-dependent solar processes. The topics I will discuss are:
1) Self-consistent modeling of a CME
2) Nonlinear force-free field modeling (validity & application)
3) Coronal loop properties (expansion & twist profiles... given in detail by Lee Hwanhee)
4) Energetics of an emerging flux region (scaling laws of free magnetic energy)
5) 3D modeling of a solar flare... given partially by Lee Hwanhee
6) ICME orientation and its effect on solar wind properties
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June 10 (Tue)
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Dr. Tetsu Anan (Kyoto Univ.)
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PPT
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Open a New Window of Plasma Diagnostics in the Solar Physics with Spectropolarimetric Observation
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Magnetic fields govern the plasma dynamics in the outer layers of the solar atmosphere, and electric fields acting on neutral atoms that move across the magnetic field enable us to study the dynamical coupling between neutrals and ions in the plasma. In order to measure the magnetic and electric fields, we developed a new universal spectropolarimeter on the Domeless Solar Telescope at Hida observatory to realize precise spectropolarimetric observation in a wide range of wavelength in visible and near infrared, and we observed the full Stokes spectra of the Paschen series of neutral hydrogen in a surge and in some active region jets that took place at the solar limb on 2012 May 5. First, we inverted the Stokes spectra taking into account only the effect of magnetic fields on the energy structure and polarization of the hydrogen levels. Having found no definitive evidence of the effects of electric fields in the observed Stokes profiles, we then estimated an upper bound for electric fields by calculating the polarization degree under the magnetic field configuration derived in the first step, with the additional presence of a perpendicular electric field of varying strength. The inferred direction of the magnetic field on the plane of the sky approximately aligns to the active region jets and the surge, with magnetic field strengths in the range 10G < B < 640G for the surge. Because the velocity of neutral atoms of hydrogen moving across the magnetic field derived from these upper limits of the electric field is far below the bulk velocity of the plasma perpendicular to the magnetic field as measured by the Doppler shift, we conclude that the neutral atoms must be highly frozen to the magnetic field in the surge.
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June 4 (Wed)
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Yumi Bamba (Nagoya Univ., ISAS)
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Study on the Triggering Process of Solar Flares based on Hinode and SDO Observations
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Understanding the mechanism that produces solar flares is important not only from the scientific point of view but also for improving space weather predictability. There are numerous observational and computational studies, which attempted to reveal the onset mechanism of solar flares. However, the underlying mechanism of flare onset remains elusive. To elucidate the flare trigger mechanism, we have analyzed several flare events which were observed by Hinode/SOT (Bamba+2013). The observed signatures strongly support the ideas of the flare trigger mechanism which was presented by Kusano+2012. They proposed that the solar flares can be triggered by the interaction between the global magnetic field of the active region and one of the two types of small magnetic disturbances. However, only four events in the Hinode data sets have been utilizable because of the limitation of SOT FOV. Therefore, increasing the number of events is required for evaluating the flare trigger models. SDO observes the full disk of the sun and all flares, although its spatial resolution is lower than that of Hinode/SOT. Then we evaluated whether the data sets obtained by SDO are usable for the detailed analysis for our flare trigger study. We analyzed the M6.6 flare which occurred on 13 February 2011 as a sample event, as it was observed both by Hinode and SDO. In this seminar, I report our results so far, and talk about the future plans of our flare trigger study with Hinode and SDO.
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May 28 (Wed)
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Dr. Kyoko Watanabe (ISAS)
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RHESSI 13 and White-Light Flare Observations by the Hinode/SOT
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In this seminar, I will show you some topics discussed at 13th RHESSI workshop. And I\81fll also show you my presentation about white-light flares.
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May 15 (Thu) | Dr. Han Uitenbroek (National Solar Observatory) |
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| Non-LTE line formation with coherent scattering and the MgII h&k lines
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In this seminar I will give a brief introduction to Non-LTE line formation and outline an iterative procedure that can be used to iteratively solve radiative transfer problems with scattering. In addition, I will show the importance of partially coherent scattering for formation the strongest spectral lines in the solar spectrum, like the Lyman lines, the Ca II H and K lines, and in particular the Mg II h&k lines. The latter spectral lines have come under a lot of attention lately with the launch of the IRIS mission. I will discuss why these lines may provide new insight into the upper layers of the solar chromosphere, and what atmospheric properties can be determined from them.
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May 14 (Wed) 15:30 - 17:00
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Shinsuke Takasao (Kyoto University)
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Dynamical energy build-up process during the formation of flare-productive active regions
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It has been known that the large fraction of the flare-productive active regions show strong magnetic shear above the polarity inversion line at the photospheric level. The magnetic shear, i.e. the electric current, is the main source of the free energy, therefore the main energy source of the flares. To understand the energy build-up processes, we have carried out 3D MHD simulations. Considering that the magnetic shear was increased by sunspot motions (rotation and translation motion) in many flare-productive active regions, we simulated that a twisted flux tube emerges from the upper convection zone to the corona in a 3D domain. We found that the electric current is injected by the torsional motion of the magnetic concentrations (spots) from the convection zone to the upper layers. We also found that this process can be affected by the dynamical plasma motions above the photosphere, like strong downflows of the lifted material to the photospheric spots. In this talk, I will talk about the dynamical coupling among the multi-layers in the solar atmosphere, and compare our model with the previous theoretical model by Longcope and Welsch (2000).
We will also briefly introduce other dynamical phenomena associated with the emergence, like the magnetic field evolution at different heights, surge-like jets and shocks in downflows. This might help us understand observations. |
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April 16 (Wed) 15:00 - 16:30
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Takuma Matsumoto (Hinode project team, ISAS/JAXA)
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PDF
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Connecting the Sun and the solar wind: the self-consistent transition of heating mechanisms
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We have performed a 2.5-dimensional magnetohydrodynamic simulation that resolves the propagation and dissipation of Alfven waves in the solar atmosphere. Alfvenic fluctuations are introduced on the bottom boundary of the extremely large simulation box that ranges from the photosphere to far above the solar wind acceleration region. Our model is ab initio in the sense that no corona and no wind are assumed initially. The numerical experiment reveals the quasi-steady solution that has the transition from the cool to the hot atmosphere and the emergence of the high speed wind. The global structure of the resulting hot wind solution fairly well agrees with the coronal and the solar wind structure inferred from observations. The purpose of this study is to complement the previous paper by Matsumoto & Suzuki and describe the more detailed results and the analysis method. These results include the dynamics of the transition region and the more precisely measured heating rate in the atmosphere. Particularly, the spatial distribution of the heating rate helps us to interpret the actual heating mechanisms in the numerical simulation. Our estimation method of heating rate turned out to be a good measure for dissipation of Alfven waves and low beta fast waves.
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April 8 (Tue) 13:30 - 15:00
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Viggo Hansteen (Institute for theoretical astrophysics, University of Oslo)
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PDF
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Numerical modeling of the outer solar atmosphere, with some examples from IRIS observations
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With the advent of massively parallel computers It is now possible to model the outer solar atmosphere with some degree of realism. A net result of this exercise is a model of the sun that spans the convection zone to the corona. In this talk I will go through the methods and assumptions used and the results so far and, to a certain extent, discuss how IRIS observations are being used to constrain and correct the numerical models.
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