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Observational Evidence of Particle Acceleration associated with Plasmoid Motions  

Shinsuke Takasao   Submitted: 2016-11-14 04:56

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.

Authors: Shinsuke Takasao, Ayumi Asai, Hiroaki Isobe, and Kazunari Shibata
Projects: None

Publication Status: Published in ApJ
Last Modified: 2016-11-16 12:07
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Fractal Reconnection in Solar and Stellar Environments  

Shinsuke Takasao   Submitted: 2016-07-04 19:55

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.

Authors: Kazunari Shibata and Shinsuke Takasao
Projects: None

Publication Status: Published as an invited chapter in "Magnetic Reconnection: Concepts and Applications", Springer-Verlag, W. D. Gonzalez and E. N. Parker, eds. (2016)
Last Modified: 2016-07-06 10:35
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Above-the-loop-top Oscillation and Quasi-periodic Coronal Wave Generation in Solar Flares  

Shinsuke Takasao   Submitted: 2016-07-04 19:51

Observations revealed that various kinds of oscillations are excited in solar flare regions. Quasi-periodic pulsations (QPPs) in the 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 of 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. It was 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 will 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 with full of shocks and waves.

Authors: Shinsuke Takasao and Kazunari Shibata
Projects: None

Publication Status: Published in ApJ
Last Modified: 2016-07-06 10:36
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Numerical Study on Emergence of Kinked Flux Tube for Understanding of Possible Origin of Delta-spot Regions  

Shinsuke Takasao   Submitted: 2015-11-10 18:22

We carried out a magnetohydrodynamics 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 is 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 of the origin of the complex multipolar delta-spot regions that have a strong magnetic shear and emerge with polarity orientations not following Hale-Nicholson and Joy Laws.

Authors: Shinsuke Takasao, Yuhong Fan, Mark C. M. Cheung, and Kazunari Shibata
Projects: None

Publication Status: accepted for publication in ApJ
Last Modified: 2015-11-11 13:01
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Magnetohydrodynamic shocks in and above post-flare loops: two-dimensional simulation and a simplified model  

Shinsuke Takasao   Submitted: 2015-04-23 03:35

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 and Shibata 2001. To study the dynamics of flows along the reconnected magnetic field, 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 have developed a new post-flare loop model which we call the pseudo-2D MHD model. The model is based on the 1D MHD equations, where all the 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 compare the shock formation and energetics of a specific field line in the 2D calculation with those in our pseudo-2D MHD model, and we 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 and without neglecting important physics associated with magnetic reconnection.

Authors: Shinsuke Takasao, Takuma Matsumoto, Naoki Nakamura, and Kazunari Shibata
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2015-04-25 18:49
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Simultaneous Observation of Reconnection Inflow and Outflow Associated with the 2010 August 18 Solar Flare  

Shinsuke Takasao   Submitted: 2011-12-07 21:18

We report the simultaneous extreme ultraviolet observation of magneticreconnection inflow and outflow in a flare on 2010 August 18 observed by theAtmospheric Imaging Assembly on board the {it Solar Dynamics Observatory}. Wefound that during the rise phase of the flare, some plasma blobs appeared inthe sheet structure above the hot loops. The plasma blobs were ejectedbidirectionally along the sheet structure (outflow), at the same time as thethreads visible in extreme ultraviolet images moved toward the sheet structure(inflow). The upward and downward ejection velocities are 220-460 km s-1and 250-280 km s-1, respectively. The inflow speed changed from 90 kms-1 to 12 km s-1 in 5 minutes. By using these velocities, weestimated the nondimensional reconnection rate, which we found to vary duringthis period from 0.20 to 0.055. We also found that the plasma blobs in thesheet structure collided or merged with each other before they were ejectedfrom the sheet structure. We hypothesize that the sheet structure is thecurrent sheet and that these plasma blobs are plasmoids or magnetic islands,which could be important for understanding the dynamics of the reconnectionregion.

Authors: Shinsuke Takasao, Ayumi Asai, Hiroaki Isobe, and Kazunari Shibata
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2011-12-08 12:47
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Abstracts by Author
Observational Evidence of Particle Acceleration associated with Plasmoid Motions
Fractal Reconnection in Solar and Stellar Environments
Above-the-loop-top Oscillation and Quasi-periodic Coronal Wave Generation in Solar Flares
Numerical Study on Emergence of Kinked Flux Tube for Understanding of Possible Origin of Delta-spot Regions
Magnetohydrodynamic shocks in and above post-flare loops: two-dimensional simulation and a simplified model
Simultaneous Observation of Reconnection Inflow and Outflow Associated with the 2010 August 18 Solar Flare

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