ИСТИНА

Properties of superflares on G dwarf stars registered with the Kepler mission are compared with characteristics of the most powerful solar flares. The aim of this study is to search of possible mechanisms of stellar flares that possess the total energies by several orders of the magnitudes more that those for the largest solar flares. We discuss briefly our results on the evolution of the solar-type activity on low-mass stars. The Kepler data show that the total energy of some flares on main sequence G stars reaches 10^34–10^36 ergs. The maximal number of such events were registered on the younger G stars with the rotational periods of a few days and 13–15 days as well. We suppose two different mechanisms for explanation of superflares on these two groups of stars where the role of the large-scale and local magnetic fields differs. All these stars are strongly spotted. However, activity of the stars rotating with the periods around 14 days is associated with the local magnetic fields, while the large-scale fields govern the active processes, as it happens on the Sun. Herewith the free energy, which is a difference of the magnetic energy of the force-free configuration and the potential energy, deposits in the chromosphere of the activity complex and then the free energy is spent on development of non-stationary processes. This allows us to give the upper estimate of the energy of flares which are able to occur in a given large active region. For the Sun, this value does not exceed 3x10^32 ergs. Because mean longitudinal magnetic fields of G stars are 10 times more than the maximal magnetic field of the Sun as a star, the energy of stellar superflares can only slightly exceed 10^34 ergs. The main distinction between solar and stellar superflares is the large differences in area of a source of the optical continuum and duration of its radiation. For flares with the higher energies on these G stars and especially on faster rotating young G stars, this mechanism is not suitable. We propose to consider here the situation when the large mass of the plasma with the temperatures around 10^4 K appears above the chromosphere. This can happen during the first stage of the global reconstruction of the structure of the whole corona. The source with the large depth in the optical continuum should arise either during non-typical ejection of the dense, large-scale rope or during the response of the chromosphere on the impact of the electron beam of the very high energy (the gentle evaporation). Such a scenario differs strongly from white-light flares on the Sun.