The phases of flares

1.3 The phases of flares

The timing of the different emissions of the same flare is presented schematically in Figure 2

. In the preflare phase the coronal plasma in the flare region slowly heats up and is visible in soft X-rays and EUV. A large number of energetic electrons (up to 10³⁸) and ions (with similar total energy) is accelerated in the impulsive phase, when most of the energy is released. The appearance of hard X-ray footpoint sources at chromospheric altitude is a characteristic of this phase (Hoyng et al., 1981

). Some high-energy particles are trapped and produce intensive emissions in the radio band. The thermal soft X-ray and H $α$ emissions finally reach their maxima after the impulsive phase, when energy is more gently released, manifest in decimetric pulsations, and further distributed. The rapid increase in H $α$ intensity and line width has been termed flash phase. It coincides largely with the impulsive phase, although H $α$ may peak later. In the decay phase, the coronal plasma returns nearly to its original state, except in the high corona ( $>$ 1.2 $R ⊙$ , where $R ⊙$ is the photospheric radius), where magnetic reconfiguration, plasma ejections and shock waves continue to accelerate particles, causing meter wave radio bursts and interplanetary particle events. In Figure 3

the various phases can be recognized.

Figure 2:

A schematic profile of the flare intensity at several wavelengths. The various phases indicated at the top vary greatly in duration. In a large event, the preflare phase typically lasts a few minutes, the impulsive phase 3 to 10 minutes, the flash phase 5 to 20 minutes, and the decay one to several hours (from Benz, 2002

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Figure 3:

mov-Movie (15436 KB) Flare observed in the Fe xii line at 195 Å (sensitive to 1.5 MK plasma) by the TRACE satellite. Note the different phases, starting with irregular brightnings in the impulsive phase. The luminosity peaks when a sheet-like structures appears above the initial brightening. The diffuse emission at this time is attributed to the presence of the Fe xxiv (192 Å) line within the TRACE passband, emitted by a plasma of 20 MK. The flare proceeds into a long decay phase with post-flare loops growing in height.