Abstract
We define the impulsive phase of a flare as its first phase, characterized by: X-ray bursts of short (seconds to tens of seconds) duration, a patchy X-ray morphology, and injection of energy. It lasts some five to ten minutes. The gradual or diffuse phase starts virtually at the same time as the impulsive one and is characterized by a gradually varying X-ray flux from a larger, diffuse, area situated higher than the sources of the impulsive X-ray bursts. The diffuse cloud is initially (during the first five minutes) hotter by a few million degrees than the sources of the impulsive phase bursts and is assumed to be caused by convective motions with upward velocities of a few hundred km s−1. It contains about the same number of energetic electrons as the impulsive burst patches contained initially. It cools gradually down by radiative and conductive losses, a process that may last for about an hour.
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