Solar and Space Plasma Physics

The Sun, and the universe as a whole, is constantly showing us new and 
wonderful phenomena, and we are given the task of trying to understand 
what we are seeing. Due to the great complexity and nonlinearity of the 
systems under observation, it is necessary perform numerical simulations 
to understand what is happening. From the emergence of magnetic flux 
into the solar atmosphere (e.g. Isobe et al. 2005, 2006) to eruptions 
on the surface of magnetars, neutron stars with extremely strong magnetic 
field (Matsumoto et al. 2011), simulations allow us to investigate the 
dynamics of these systems. The great similarity in the equations used 
to study these different pictures means that something learnt about 
one system can often tell us something about another. 
Our group works heavily on numerical simulations, and their comparison 
to observations, to understand the many wonders of the universe.

 References:
 H. Isobe, T. Miyagoshi, K. Shibata & T. Yokoyama; Nature, Volume 434, 
  Issue 7032, pp. 478-481 (2005).  
 H. Isobe, T. Miyagoshi, K. Shibata & T. Yokoyama; Publications of the
  Astronomical Society of Japan, Vol.58, No.2, pp. 423-438 (2006)
 J. Matsumoto, Y. Masada, E. Asano & K. Shibata;  The Astrophysical Journal, 
  Volume 733, Issue 1, article id. 18, 15 pp. (2011).  


Exploring MHD instabilities in the Solar Atmosphere

MHD instabilities play a fundamental role in many of the observed phenomena 
we see in the solar atmosphere and the stunning observations from the 
Hinode satellite have provided great motivation for our group to pursue 
this area of research. A prime example of this would be the Rayleigh-Taylor 
plumes that are observed in quiescent prominences. These plumes are formed 
at the boundary between low density bubbles and the dense prominence material 
above, making them the perfect example of the magnetic Rayleigh-Taylor 
instability. Through the application of 3D MHD simulations using a prominence 
model, Hillier et al (2011, 2012) numerically investigate the formation and 
evolution of the plumes. Figure 1 shows a 3D rendering of the simulation 
results from the onset of the instability to the full nonlinear development 
of the plumes.

  
   Figure 1: 3D MHD simulation of the magnetic Rayleigh-Taylor instability 
   in a prominence model.
 
 References:
 A. Hillier, H. Isobe, K. Shibata & T. Berger; The Astrophysical Journal 
  Letters, Volume 736, Issue 1, article id. L1, 6 pp. (2011).
 A. Hillier, T. Berger, H. Isobe & K. Shibata; The Astrophysical Journal, 
  Volume 746, Issue 2, article id. 120, 13 pp. (2012).
 

Magnetic Reconnection, Jets and Flares

Magnetic reconnection is the process where the change in connectivity in 
magnetic field lines results in the release of magnetic energy. 
As this process can release energy on very short time scales, it is 
believe to be the cause of many of the explosive energy release phenomena 
in the solar atmosphere, and many other astrophysical systems as well. 
Making the study of magnetic reconnection of huge importance for the 
advancement of Astrophysics

Penumbral microjets, discovered by Hinode, are fast jets aligned with the 
magnetic field of a sunspot penumbra. This leads to an interesting question: 
How can magnetic reconnection drive jets along magnetic field lines? 
Using 3D resistive MHD simulations, Nakamura et al (2012) addressed this 
exact issue. Using initial conditions where a strong, angled magnetic field, 
weak gas pressure region and a weak, horizontal magnetic field, strong gas 
pressure region are separated by a current sheet, they were able to show 
how magnetic reconnection could form gas pressure gradients along the magnetic 
field, driving jets (see Figure 2). Using this model we can understand how 
magnetic reconnection can create magnetic-field aligned jets.

   
   Figure 2: 3D resistive MHD simulation of component asymmetric magnetic
   reconnection. The reconnection creates gas pressure gradients along 
   the magnetic field, driving field-aligned jets.

Hinode telescope also found the presence of a plentitude of chromospheric 
jets (Shibata et al 2007). To study these jets, and their formation through 
magnetic reconnection, Takasao et al (2013) performed a numerical study 
of magnetic reconnection between emerging magnetic flux and the ambient 
magnetic field in a model solar atmosphere. In this simulation, it was 
found that when reconnection occurred in the chromosphere the jet was driven 
by MHD slow shocks directly created by the reconnection, as shown in Figure 3.

   
   Figure 3: Formation of a chromospheric jet by a reconnection generated 
   slow shock in a 2D resistive MHD simulation. The jet is launched by the 
   slow shock passing through the transition region.

Solar flares are probably the most famous example of energy release through 
magnetic reconnection. The huge energy release creates photons and particles 
at incredibly high energy, making flares the most dramatic phenomena in 
solar physics. MHD has been shown to be incredibly effective in describing 
the physics of flares. For a full review of the physics of MHD processes 
in solar flares, please see the review by Shibata & Magara (2011) on 
'Solar flares: MHD processes' published in Living reviews of Solar Physics.

To greater investigate the connection between the eruption of flux rope 
and the flare reconnection, Nishida et al (2013) performed 3D simulations 
of destabilization of a flux rope and investigated the reconnection this 
created. Figure 4 shows the flux rope as it destabilizes creating a current 
sheet beneath it. Investigating evolution of this current sheet showed 
the formation of multiple plasmoids, which are miniature fluxropes, 
resulting in the reconnection becoming highly turbulent.

   
   Figure 4: 3D resistive MHD simulation of the eruption of a flux rope, 
   as a result of the kink instability, and the reconnection in the 
   current sheet created below the erupting flux rope.

 References:
 N. Nakamura, K. Shibata & H. Isobe; The Astrophysical Journal, Volume 761, 
  Issue 2, article id. 87, 10 pp. (2012).
 K. Shibata et al., Science, Volume 318, Issue 5856, pp. 1591- (2007).  
 S. Takasao, H. Isobe & K. Shibata; Publications of the Astronomical Society 
  of Japan, Vol.65, No.3, Article No.62, 22 (2013). 
 K. Shibata & T. Magara; 'Solar Flares: Magnetohydrodynamic Processes',
  Living Reviews in Solar Physics, vol. 8, no. 6 (2011).
 K. Nishida, N. Nishizuka & K. Shibata; The Astrophysical Journal Letters, 
  Volume 775, Issue 2, article id. L39, 6 pp. (2013).


Astrophysical Plasma

From the launching of astrophysical jets through magnetic fields, 
to the study of outflows and reconnection in the atmosphere of magnetars 
and dynamics of molecular clouds in the galactic centre, numerical simulations 
provide a very powerful tool for the investigation of dynamic Astrophysical 
phenomena. Our group works on Newtonian and special relativistic simulations 
of hydrodynamic and magnetohydrodynamic simulations to help our understanding 
of the plasma in our Universe. 

To investigate the acceleration of astrophysical jets post-launching, 
Matsumoto et al (2012) performed simulations of rarefaction acceleration
(a special relativistic effect) inside the jet. Through the repeated formation 
of shocks, the interaction of shocks with the contact discontinuity at the 
edge of the jet and the formation of rarefaction waves as a result of this 
interaction, the jet can undergo multiple accelerations from the rarefaction 
waves. Figure 5 shows the evolution of the jet in both 1D and 2D simulations. 
Though the 1D simulations are simpler than the 2D simulations, the similarity 
between them is remarkable.

   
   Figure 5: 1D and 2D simulations of jet evolution showing the increase
   in Lorentz factor as a result of the rarefaction acceleration of the jet.
   It is remarkable how similar the results are from the 1D and 2D simulations. 

 References:
 J. Matsumoto, Y. Masada & K. Shibata; The Astrophysical Journal, Volume 751, 
  Issue 2, article id. 140, 18 pp. (2012).