Second-phase acceleration versus second-step acceleration in solar flares

Solar hard-X-ray and gamma-ray observations are reviewed that make it possible to distinguish two phases and two steps of energetic electron and ion acceleration in solar flares. A distinction is made between second-phase and second-step acceleration of electrons, protons, and heavy nuclei in flares, and the impulsive acceleration of flare protons is discussed. Hard-X-ray and gamma-ray time profiles of a solar flare are examined in detail. It is proposed that in flares producing impulsive gamma rays, energetic protons and heavy nuclei are accelerated by the second-step mechanism.