Recent observations of solar type stars with the Kepler satellite by Maehara et al. have revealed the existence of superflares (with energy of 10 33 ~ 10 35 erg) on Sun-like stars, which are similar to our Sun in their surface temperature (5600 K ~ 6000 K) and slow rotation (rotational period > 10 days). From the statistical analysis of these superflares, it was found that superflares with energy 10 34 erg occur once in 800 years and superflares with 10 35 erg occur once in 5000 years on Sun-like stars. In this paper, we examine whether superflares with energy of 10 33 ~ 10 35 erg could occur on the present Sun through the use of simple order-of-magnitude estimates based on current ideas relating to the mechanisms of the solar dynamo. If the magnetic flux is generated by the differential rotation at the base of convection zone as assumed in typical dynamo models, it is possible that the present Sun would generate a large sunspot with total magnetic flux ~2 x 10 23 Mx within one solar cycle period, and lead to superflares with energy of 10 34 erg. On the other hand, it would take ~40 years to store total magnetic flux ~ 10 24 Mx for generating 10 35 erg superflares. Many questions remain relating to how to store 10 24 Mx below the base of convection zone and how to erupt a magnetic flux tube in a short time to create a sunspot with 10 24 Mx, which presents a challenge to dynamo theorists. Hot Jupiters, however, which have been often argued to be a necessary ingredient for generation of superflares, do not play any essential role on generation of magnetic flux in the star itself, if we consider only magnetic interaction between the star and the hot Jupiter. This seems to be consistent with Maehara et al.'s finding of 148 superflare-generating solar type stars which do not have a hot Jupiter companion. Altogether, our simple calculations, combined with Maehara et al.'s analysis of superflares on Sun-like stars, show that there is a possibility that superflares of 10 34 erg would occur once in 800 years on our present Sun, while it is premature to conclude whether it is possible for 10 35 erg superflares to occur on our present Sun on the basis of application of current dynamo theory.

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 −1 and 250-280 km s −1 , respectively. The inflow speed changed from 90 km s −1 to 12 km s −1 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.

Magnetic flux emergence from the solar convection zone into the overlying atmosphere is the driver of a diverse range of phenomena associated with solar activity. In this article, we introduce theoretical concepts central to the study of flux emergence and discuss how the inclusion of different physical effects (e.g., magnetic buoyancy, magnetoconvection, reconnection, magnetic twist, interaction with ambient field) in models impact the evolution of the emerging field and plasma.

We present an examination of the multiwavelength observation of a C7.9 flare that occurred on 1998 November 10. This is the first imaging observation of the quasi-periodic pulsations (QPPs). Four bursts were observed with the hard X-ray telescope aboard Yohkoh and the Nobeyama Radioheliograph during the impulsive phase of the flare. In the second burst, the hard X-ray and microwave time profiles clearly showed a QPP. We estimated the Alfvén transit time along the flare loop using the images of the soft X-ray telescope aboard Yohkoh and the photospheric magnetograms and found that the transit time was almost equal to the period of the QPP. We therefore suggest, based on a shock acceleration model, that variations of macroscopic magnetic structures, such as oscillations of coronal loops, affect the efficiency of particle injection/acceleration.

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