The Phases of Flares

Solar flares are not instantaneous events; they evolve through distinct temporal phases , each characterized by different types of radiation emissions (from radio waves to gamma rays), particle acceleration , and plasma behaviour . These phases help scientists understand the energy release mechanisms and how energy is transported and distributed throughout the solar atmosphere.

The passage describes these phases using data from observations across multiple wavelengths and particle types, as illustrated in Figure 2.

Let’s break it down phase by phase:

1. Preflare Phase

• What happens :

  Before the main flare eruption, there is a gradual heating of the coronal plasma in the region where the flare will occur.

• Observables :

  • Visible in soft X-rays and EUV (Extreme Ultraviolet) — both are sensitive to hot coronal plasma.
  • This suggests that magnetic energy is already being released , but slowly and steadily.

• Significance :

  • Indicates that something is happening deep in the magnetic structure before the explosive energy release.
  • May be related to magnetic reconnection starting in a small way or the build-up of magnetic stress .

2. Impulsive Phase

• What happens :

  This is the main energy release phase of the flare. A huge number of energetic particles are accelerated very quickly.

• Key features :

  • Electrons and ions are accelerated to high energies:
    • Up to 10³⁸ energetic electrons
    • Ions with similar total energy
  • This phase accounts for most of the energy release in the flare.

• Observables :

  • Hard X-ray footpoint sources appear at chromospheric altitudes (Hoyng et al., 1981). These are regions where energetic electrons collide with denser chromospheric plasma, emitting hard X-rays via bremsstrahlung.
  • Some high-energy particles become trapped in magnetic loops , producing intense radio emissions (especially in the microwave range ).
  • This phase also produces non-thermal radiation across many wavelengths.

• Time frame :

  Typically lasts from a few seconds to several minutes .

• Significance :

  • Marks the onset of magnetic reconnection , where magnetic field lines break and reconnect, releasing vast amounts of stored magnetic energy.
  • This energy accelerates particles and heats the surrounding plasma.

3. Flash Phase (Rise of Hα Emission)

• What happens :

  There is a rapid increase in Hα emission intensity and line width .

• Relation to other phases :

  • Largely coincides with the impulsive phase , though sometimes the peak in Hα occurs slightly later.
  • Reflects the response of the chromosphere to the energy deposited from above (e.g., from accelerated electrons).