Solar flares and coronal mass ejections (CMEs), the most catastrophic eruptions in our solar system, have been known to affect terrestrial environments and infrastructure. However, because their triggering mechanism is still not sufficiently understood, our capacity to predict the occurrence of solar eruptions and to forecast space weather is substantially hindered. Even though various models have been proposed to determine the onset of solar eruptions, the types of magnetic structures capable of triggering these eruptions are still unclear. In this study, we solved this problem by systematically surveying the nonlinear dynamics caused by a wide variety of magnetic structures in terms of three-dimensional magnetohydrodynamic simulations. As a result, we determined that two different types of small magnetic structures favor the onset of solar eruptions. These structures, which should appear near the magnetic polarity inversion line (PIL), include magnetic fluxes reversed to the potential component or the nonpotential component of major field on the PIL. In addition, we analyzed two large flares, the X-class flare on December 13, 2006 and the M-class flare on February 13, 2011, using imaging data provided by the Hinode satellite, and we demonstrated that they conform to the simulation predictions. These results suggest that forecasting of solar eruptions is possible with sophisticated observation of a solar magnetic field, although the lead time must be limited by the time scale of changes in the small magnetic structures.

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.

We present a detailed examination of downflow motions above flare loops observed in the 2002 July 23 flare. The extreme-ultraviolet images obtained with the Transition Region and Coronal Explorer show dark downflow motions (sunward motions) above the postflare loops, not only in the decay phase but also in the impulsive and main phases. We also found that the times when the downflow motions start to be seen correspond to the times when bursts of nonthermal emissions in hard X-rays and microwaves are emitted. This result implies that the downflow motions occurred when strong magnetic energy was released and that they are, or are correlated with, reconnection outflows.

Recently, many superflares on solar-type stars have been discovered as white-light flares (WLFs). The statistical study found a correlation between their energies (E) and durations (τ ): τ ∝ E 0.39 (Maehara et al. 2015, EP&S, 67, 59), similar to those of solar hard/soft X-ray flares: τ ∝ E 0.2−0.33 . This indicates a universal mechanism of energy release on solar and stellar flares, i.e., magnetic reconnection. We here carried

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.

We consider the relationship between two flare-associated waves, a chromospheric Moreton wave and a coronal EIT wave, based on an analysis of an X-class flare event in AR 8100 on 1997 November 4. A Moreton wave was observed in $\mathrm{H}\alpha$, $\mathrm{H}\alpha {+} 0.8\,$$Å$, and $\mathrm{H}\alpha-0.8\,$$Å$ with the Flare-Monitoring Telescope (FMT) at the Hida Observatory. An EIT wave was observed in EUV with the Extreme ultraviolet Imaging Telescope (EIT) on board SOHO. The propagation speeds of the Moreton wave and the EIT wave were approximately $715 \,\mathrm{km} \,\mathrm{s}^{-1}$ and $202 \,\mathrm{km} \,\mathrm{s}^{-1}$, respectively. The times of visibility for the Moreton wave did not overlap those of the EIT wave, but the continuation of the former is indicated by a filament oscillation. Data on the speed and location clearly show that the Moreton wave differed physically from the EIT wave in this case. The Moreton wave preceded the EIT wave, which is inconsistent with an identification of the EIT wave with a fast-mode MHD shock.

We report on the first simultaneous observation of an Hα Moreton wave, the corresponding EUV fast coronal waves, and a slow and bright EUV wave (typical EIT wave). Associated with an X6.9 flare that occurred on 2011 August 9 at the active region NOAA 11263, we observed a Moreton wave in the Hα images taken by the Solar Magnetic Activity Research Telescope (SMART) at Hida Observatory of Kyoto University. In the EUV images obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) we found not only the corresponding EUV fast "bright" coronal wave, but also the EUV fast "faint" wave that is not associated with the Hα Moreton wave. We also found a slow EUV wave, which corresponds to a typical EIT wave. Furthermore, we observed, for the first time, the oscillations of a prominence and a filament, simultaneously, both in the Hα and EUV images. To trigger the oscillations by the flare-associated coronal disturbance, we expect a coronal wave as fast as the fast-mode MHD wave with the velocity of about 570 -800 km s −1 . These velocities are consistent with those of the observed Moreton wave and the EUV fast coronal wave.

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