Fundamental Magnetohydrodynamic Processes of Solar Flares: Formation of Flare-productive Regions and Evolution of Flare Loops

Solar flares are the most explosive phenomenon in the solar system. The energy source is magnetic energy stored in the corona above sunspot regions. The sunspot regions or active regions are formed as a result of magnetic flux emergence from the interior of the Sun into the solar atmosphere. Solar flares release magnetic energy supplied by flux emergence via magnetic reconnection. In this thesis, flux emergence process and magnetic reconnection process are studied by means of numerical simulations and observations. It has been known for decades that certain photospheric magnetic configurations of active regions are responsible for the high flare activity. The so-called δ-spot regions are known to be among the most flare-productive active regions. Many attempts have been done for understanding their origins. However, the formation mechanism has been puzzling, which makes understanding of the origin of their high flare activity difficult. In this thesis, we will propose a new formation mechanism on the basis of a three-dimensional magnetohydrodynamics (3D MHD) simulation. The numerical simulation revealed that a highly-twisted magnetic flux bundle can spontaneously result in a complex quadrupole magnetic structure at the photosphere which is favorable for eruption. This study clarifies the impact of the subsurface structure of a flux tube on the flare activity. Magnetic reconnection drives super-sonic plasma flows, and therefore can produce various kinds of waves and shock waves. Shocks in and above the post-flare loops are believed to be important for both the non-thermal particle acceleration and the evolution of the thermal structure. However, because of the complexity of the flow structure in the flare regions, the shock formation has not been well understood. To advance our understanding, we performed high-resolution 2D MHD simulations of a solar flare. As a result, many new shocks were discovered. Furthermore, it is found that even the very small above-the-loop-top region is full of shocks and waves. This thesis will show the formation process of newly found shocks in detail. On the basis of the 2D MHD simulation, we developed a 1D new post-flare loop model, which we defined as the pseudo-2D MHD model. This model enables us to study the evolution of the post-flare loops in a wide parameter space without expensive computational cost or neglecting important physics associated with magnetic reconnection. Observations have revealed the oscillatory nature of solar flares. Quasiperiodic pulsations seen in the lightcurves in a wide wavelength range is a common feature. Recent imaging observations also found various kinds of waves generated by flares, implying that these waves are related to quasiperiodic pulsations. Since MHD waves can be used as a powerful tool to diagnose the local coronal plasma condition, many studies have tried to understand the oscillatory features in and around the post-flare loops. However, in spite of many attempts, we have less knowledge about the excitation of waves. This thesis will provide a detailed picture of the excitation of waves by the reconnection outflow. As a result of 2D MHD simulations of a flare, we discovered the local oscillation of the flare loop-top and the generation of coronal waves from such oscillating flare loop-top.