The most potent magnetic events in the solar system, solar flares have the ability to unleash an enormous amount of energy exceeding $10^{-39}$ J within a matter of minutes. They emit radiation spanning the entire electromagnetic spectrum from radio waves to $\gamma$-rays. They are closely associated with the acceleration of particles into interplanetary space and the ejection of coronal mass. A flare occurs when previously stored energy in the form of inductive magnetic fields, resulting from electric currents flowing into the corona, is rapidly released. The total energy released during a flare aligns with the quantity of magnetic ‘free’ energy available in the magnetic active regions, typically found in the coronal connections of sunspot groups where most flares occur. Evaluating the magnetic free energy from observations is challenging due to its dependence on the magnetic vector field. However, in a few instances where it has been possible, researchers have found that the free energy is comparable to that of large flares. Additionally, alternative energy sources within the corona or chromosphere cannot adequately explain the energy budget of flares. Consequently, it can be concluded that the conversion of stored magnetic energy plays a central role in the flare process.
A ‘flare’ specifically refers to the electromagnetic radiation emitted during an event. This radiation accounts for a significant portion of the total energy released. The amount of energy released varies from one event to another, with a greater number of small events compared to large ones. The distribution of flares, categorized by their peak energy, total energy, or duration, follows a power law pattern. Understanding the gradient of this power law is crucial in determining the contribution of flare-like heating events to the overall energy budget of the solar corona. To assess the importance of a flare, it is commonly classified based on its soft X-ray (SXR) flux at 0.1–0.8 nm, measured by instruments like GOES. Flares are classified as X, M, C, B, or A, with X denoting the highest flux exceeding $10^{-4}$ W m$^{-2}$ at Earth. Subsequent classifications decrease in order of magnitude. The table below shows the emission measure and H$\alpha$ area for each GOES class.
| GOES class | Emission measure EM (cm$^{-3}$) | H$\alpha$ class | H$\alpha$ area deg$^2$ |
|---|---|---|---|
| X10 | $10^{51}$ | 4 | 24.7 |
| X | $10^{50}$ | 3 | 12.4 |
| M | $10^{49}$ | 2 | 5.1 |
| C | $10^{48}$ | 1 | 2.0 |
| B | $10^{47}$ | S | <2.0 |
| A | $10^{46}$ | S | <2.0 |