Proton acceleration and its energy spectra during the coalescence of two cross current loops

We investigate plasma dynamics during coalescence of two cross current loops by using a resistive three-dimensional MHD simulation code, paying particular attention to finding the most effective electromagnetic fields for producing high-energy protons. Next we investigate the orbit of many protons in the electromagnetic fields obtained from the MHD simulations and find that proton acceleration is more efficient compared with the case of two parallel loops coalescing. It is shown that the maximum proton energy is about 25 MeV, which exceeds the energy (2.223 MeV) of the observed prompt nuclear de-excitation lines of gamma-ray and that the proton energy spectrum is a power-law type with an index of about 2-2.3. The simulation results imply that proton-associated gamma-ray sources are located near the footpoints with magnetic north polarity. We found that strong proton acceleration leading to the observed prompt line gamma-ray emissions can be realized only when both poloidal and axial magnetic fields reconnect completely.