25T2
Magnetohydrodynamic Explosive Phenomenain the Solar and Astrophysical Plasmas
太陽・天体における電磁流体爆発現象
Kazunari Shibata
柴田一成
Kwasan and Hida Observatories, Kyoto University, Yamashina, Kyoto 607-8471, Japan
京都大学理学研究科附属天文台 〒607-8471 京都市山科区
Modern view of explosive phenomena in the universe such as flares and jets in active galactic nuclei andyoung stellar object is reviewed with an emphasis of possibility of magnetohydrodynamic (MHD)interpretation based on solar flare MHD model. The unified reconnection model for solar flares has been
proposed and applied to stellar flares and magnetar flares. Finally, we discuss recent discovery of manysuperflares on solar type stars.

1. Introduction
Recent astronomical observations haverevealed that our universe is full of explosions.
The biggest explosions after the Big Bang of ouruniverse itself are the explosion of nuclei ofgalaxies, called AGN (active galactic nuclei).
AGN releases huge energy (~ 10 62 erg in 108years) through all electromagnetic spectrum withhigh time variability (called flares or bursts),and produce well collimated relativistic jets.
AGN probably contain supermassive black holes
at their center, but their true properties are still
very puzzling.
The miniature versions of AGN have been
discovered in various objects, such as close
binary systems and young stellar objects. Thesephenomena also show many flares/bursts as wellas jets or mass ejections. Although the progress ofobservations and theoretical understanding ofthese objects is rapid, the basic mechanism of
formation of jets and flares has not been wellunderstood.
The nearest example of miniature of AGN isthe explosion in the solar atmosphere, called solarflares. The typical solar flare releases total energyof 1029~1032 erg and expels large amount of mass
~1015 g into interplanetary space in the largest
event. When such mass (called coronal massejections or CMEs) with southward magnetic
field hits the Earth, geomagnetic storm occurs,
eventually leading to aurora-substorm near thenorth and south poles.
As for the origin of solar flares, it has been
established in mid-twenty centuries that thesource of energy of solar flares is the magnetic
energy stored near sunspots. After that, variousmagnetohydrodynamic (MHD) mechanism has
been proposed to explain solar flares. Hencemany astrophysicists applied MHD models ofsolar flares to various cosmic jets and flares,
including AGN jets. One such example is theMHD model of astrophysical jets proposed by
Uchida and Shibata [1,2].
It has also been found that many normal stars
show flares which are very similar to solar flares.
Of course, the three dimensional structure ofthese stellar flares have never been observed, so
their origin have not yet been solved. However,there are increasing evidence that these stellar
flares are quite similar to solar flares, so that it
has been believed that MHD mechanism is thepromising mechanism for the origin of stellar
flares.
Hereafter, we will discuss the present status ofunderstanding of the origin of solar flares, andthen discuss application of solar flare model tostellar flares. We also discuss newly discovered
superflares on solar type stars. Finally, we will
discuss application of solar flare model to giant
flares in magnetars.
2. Solar Flares
A solar flare was first discovered by Carrington
in 1859 during visual observations of sunspot.
Since then, its mechanism has been puzzling.
However, Japanese solar satellite, Yohkoh,
revealed various evidence of magnetic reconnection
in soft X-ray observations in 1992-1995 [3,4,5] andestablished at least phenomenologically that thesolar flare is caused by magnetic reconnection
mechanism [6].
Figure 1 shows the unified model of solar flares
which are developed on the basis of Yohkoh soft
X-ray observations and MHD theories ofreconnection [6,7, 8]. It should be noted here that
there is a huge scale gap between microscopic
plasma scale such as ion Larmor radius or ion skindepth (~ 10 cm) and flare size scale (~ 109 cm).

Hence the basic mechanism of fast reconnection
suitable for solar flares (and cosmic flares as well)cannot be solved from microscopic plasma physics
alone. Instead, we have to consider the effective
way to connect micro and macro scales to explain
fast reconnection in solar and cosmic flares, which
may be realized by plasmoid-induced reconnection
and fractal reconnection [8, 9].
3. Stellar Flares
Shibata and Yokoyama (1999, 2002) noted that
there is a remarkable empirical correlation between
emission measure and temperature of solar flares,solar microflares, stellar flares and young stars’flares.
Then they successfully explained thecorrelation using the magnetic reconnection theory
developed by Yokoyama and Shibata (1998). Thismeans that this correlation can be indirect evidenceof magnetic reconnection in stellar flares.
Recently, Maehara et al. (2011) [13] discoveredin Kepler satellite data that there are manysuperflares (with energy 1034~1035 erg, which is100~1000 times that of the largest solar flareobserved so far) on solar type stars whose rotational
velocity is similar to that of the Sun. This is asurprising result because this suggest that suchsuperflares may occur on our present Sun and if sothey will damage our civilization heavily. Why andhow such superflares occur on our Sun at present
will be an urgent issue from space weather point ofview.
4. Magnetar Flares
Magnetars are neutron stars with super strong
magnetic field ~ 1015 G, and sometimes produce
giant flares. Masada et al. (2010) [14] applied solar
flare model to such giant flares in magnetars. It has
been thought that the huge amount of magnetic
energy are stored in the interior of magnetars.
Sometimes, such magnetic energy has been releasedby fast reconnection mechanism occurring in themagnetosphere of magnetars. One interesting future
subject to be studied in detail is the coupling
between magnetic reconnection and QED effect.

Fig.1. Unified reconnection model of solar flares [6]Acknowledgments
I would like to thankH. Maehara, T.
Shibayama, S. Notsu, Y. Notsu, T. Nagao, S.
Kusaba, S. Honda, and D. Nogami for exciting
collaboration on superflares on solar type stars.
References[1] K. Shibata and Y. Uchida: Publ. Astron. Soc. Japan37 (1985) 31[2] Y. Uchida and K. Shibata: Publ. Astron. Soc. Japan37 (1985) 515[3] S. Tsuneta et al.: Publ. Astron. Soc. Japan 44 (1992)
L63.
[4] K. Shibata et al.: Publ. Astron. Soc. Japan 44 (1992)
L173.
[5] S. Masuda et al.: Nature 371 (1994) 495.
[6] K. Shibata et al.: ApJ 451 (1995) L83.
[7] K. Shibata: Ap Sp Sci 264 (1999) 129.
[8] T. Tajima and K. Shibata: Plasma Astrophysics,(Addison-Wesley, 1997)[9] K. Shibata and S. Tanuma: Earth, Space, and Planet
53 (2001) 473.
[10] K. Shibata and T. Yokoyama: ApJ 526 (1999) L49.
[11] K. Shibata and T. Yokoyama: ApJ 577 (2002) 422.
[12] T. Yokoyama and K. Shibata: ApJ 494 (1998) L113.
[13] H. Maehara et al.: Nature (2011) submitted.
[14] Y. Masada et al.: Publ. Astron. Soc. Japan 62(2010) 1093.