As we have seen in the previous sections, the main process responsible for producing a flare is the dissipation of electric current in the corona. The magnetic energy stored by coronal field is first released, followed by various dynamic events such as mass ejection and wave propagation. It should be noticed that the low density in the corona makes these accompanying events so dynamic, which is why we feel that flares are dynamic phenomena as well.
Since the electric conductivity is generally high in the corona, the dissipation of electric current there is only efficient in a current sheet where a large amount of electric current flows. The high electric conductivity of the corona therefore contributes to locally concentrating electric current (this means to amplify free magnetic energy), which in fact makes the dissipation of electric current explosive. This gives an explosive character to a flare, which is quite different from the dissipation of electric current in a resistive medium (conductivity is low) where electric current is dissipated easily and less dynamically without amplifying free magnetic energy.
Here, we summarize the evolution of emerging magnetic field toward the onset of a flare (see also Figure 22). Initially, a magnetic structure is formed on the Sun via flux emergence accompanied by shear and/or converging flows at the surface. Outer marginal part of the structure expands by dominant magnetic pressure force, while inner central part keeps the quasi-static (or slow rise) state where either field-aligned current is dominant (force-free state) or the upward Lorentz force is balanced by the gravitational force exerted on a massive object formed at the inner part (e.g., filament). In either case the free magnetic energy is stored mainly at the inner part, while the outer part continuously loses magnetic energy via expansion. If the force balance at the inner part is lost by an instability or loss-of-equilibrium process, part of the free energy is converted into the kinetic energy, producing mass ejection such as filament eruption and CME. This leads to the formation of a current sheet below an ejecting part of the structure which may assume a flux-rope. Magnetic reconnection then occurs in this current sheet to convert remaining free energy into non-magnetic energy, which is observed as a flare. Also the electric field associated with magnetic reconnection produces high-energy particles. Figure 47 shows possible physical processes for producing a flare and accompanying events.
Finally, we should mention that the solar activity including flares potentially has a significant impact on the Earth. This research field investigating the Sun-Earth environment has been developing as the space weather. Flares can produce high-energy particles and CMEs, which sometimes damage telecommunications and power supplies on the Earth. A big flare known as a proton flare produces high-energy protons (> 10 MeV), and these high-energy particles travel through the interplanetary space to the Earth, having a huge impact on the polar region of the Earth (polar cap absorption, PCA). Predicting the occurrence of flares therefore becomes of great importance nowadays when human activity extends to the space. This requires the detailed investigations into the mechanism of such magnetically driven solar activity, and the nature of magnetic field transported via flux emergence into the solar atmosphere is a key to a better understanding of the Sun-Earth system.
Living Rev. Solar Phys. 8, (2011), 6
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