What is the Sun?
The Sun is a huge ball of gas, consisting mostly of hydrogen, which is undergoing a continous thermonuclear reaction at its centre. Hydrogen isotopes in the core, heated to millions of degrees, combine via nuclear fusion to form helium, releasing energy in the process.
Image: Schematic of the interior of the sun. Courtesy of SOHO (ESA & NASA)
The Sun's internal structure can be divided into several distinct layers. The most interior and hottest region, where nuclear fusion occurs, is the core, where the temperature is around 16 MK. This is surrounded by a thick radiative zone, which transports the energy generated in the core to the outer regions of the sun and the solar system beyond via photons. Closer to the surface lies the convection zone. In this zone the bulk motions of the Sun's plasma become the dominant method of transporting the energy from the core outwards. Above the convection zone are the Sun's surface layers; the photosphere, cromosphere, and the corona. These are the regions which can be directly observed by satellites and Earth-based instruments.
In fact, observations of these surface layers have given rise to one of the most prominent unsolved problems in solar physics. A variety of methods have shown that the temperature of the photosphere is around 6000 K. This is consistent with the Sun's layers becoming cooler further away from the core. However the solar corona, which lies above the photosphere, has a mean temperature of around 1 MK! How can the corona be so hot, when the photosphere is so cool? This is known as the coronal heating problem.
What are solar flares?
Solar flares are powerful, unpredictable releases of energy frequently observed at the surface of the Sun. A solar flare can release up to 1032 ergs in just a few minutes, and are often associated with even more powerful coronal mass ejections (CMEs), the most powerful eruptions in the solar system. The causes of flares are not entirely understood, but are believed to involve release of energy stored in the magnetic fields present in the solar corona. Consequently, flares are closely associated with sunspots, which occur where the magnetic field protrudes from the Sun's surface.
The frequency of flares follows the well known 11 year solar activity cycle. At the maximum of this cycle the Sun is very active, resulting in frequent X-class flares, regular CMEs and a large number of sunspots. Contrastingly, at solar minimum the Sun is relatively placid, with few flares and sunspots observed. In 2008, the Sun is at a minimum in its activity cycle.
How are solar flares observed and studied?
Flares can be observed from Earth precisely because they emit large amounts of energy. Relatively few flares can be observed in the visible part of the electromagnetic spectrum however. Instead, because flaring plasma is so hot, most flares are observed at X-ray and gamma-ray wavelengths. Since X-rays do not penetrate the Earth's atmosphere, spacecraft are used to make observations.
One of the main mechanisms whereby flares emit energy is known as thermal brehmsstrahlung. This occurs when free electrons in the hot plasma interact with ions. This interaction causes the electron to deflect from its course, due to the attraction of the ion. The result is that the electron loses energy, which is emitted in the form of an X-ray. This is usually referred to as thermal emission. Thermal emission has a characteristic exponentional decay with increasing energy, meaning it can be well modelled using spectroscopy. This also means that the temperature of the emitting plasma can be estimated from the emission profile. In a typical flare the energies dominated by thermal emission are in the region 3 keV - 20 keV.
Image: Illustration of the brehmsstrahlung process. An electron is deflected from its course by an ion. The resulting deflection causes the electron to lose energy and emit radiation.
A significant portion of a flaring energy release is in the form of non-thermal emission. Here some particles in the plasma have been accelerated to energies much higher than the mean thermal energy of the hot plasma. These accelerated particles also interact with ions and release energy, however in this case studies have shown that the emission follows a power law decay with increasing energy. Indeed, the energy spectrum can extend up to the gamma-ray range ( > 20 MeV) in exceptionally large flares. This type of energy release is often referred to as non-thermal brehmsstrahlung.
At even higher energies gamma-rays can be produced by the interactions of the heavier ions and protons accelerated by the flare, instead of the lighter electrons. This process often manifests itself in terms of specific emission lines, corresponding to the trace heavier elements involved in the ion-proton collisions. This allows us to study the composition of the Sun's outer layers.
Solar and stellar activity also leads to emission in the microwave range. Although the mechanisms described above contribute to radio emission, the bulk of microwave radiation in solar flares is generated by the gyrosynchotron mechanism. This is very similar to cyclotron emission: electrons in a magnetic field with a given velocity will gyrate around the field line at a frequency dependent only on the magnetic field strength. This results in an emission line at a specific frequency. The only difference between gyrosynchotron emission and cyclotron emission is that in the former case the particles are mildly relativistic. This causes a broadening of the emission lines and the generation of more harmonics, so that the resulting energy spectrum resembles a continuum. As microwaves are not blocked by the Earth's atmosphere, they can be observed with ground-based equipment, such as the Nobeyama Radioheliograph in Japan.