Overview of Solar Flares
Solar flares are one of the main science targets of RHESSI. A flare is defined as a sudden, rapid, and intense variation in brightness. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through visible light to x-rays and gamma rays. The amount of energy released could power the whole world for 10 million years! On the other hand, it is less than one-tenth of the total energy emitted by the Sun every second. The first solar flare recorded in astronomical literature was on September 1, 1859. Two scientists, Richard C. Carrington and Richard Hodgson, were independently observing sunspots at the time, when they viewed what turned out to be one of the largest flares in visible light.
- A sketch of the sunspots Richard Carrington observed on September 1, 1859
Typically, a person cannot view a solar flare by simply staring at the Sun. Flares are in fact difficult to see because the Sun is already so bright. Instead, specialized scientific instruments are used to detect the light emitted during a flare. Radio and optical emission from flares can be observed with telescopes on Earth. Energetic emission such as x-rays and gamma-rays require telescopes located in space, since these emissions, thankfully, do not penetrate Earth's atmosphere.
As the magnetic energy is being released, particles, including electrons, protons, and heavy nuclei, are heated and accelerated in the solar atmosphere (the solar corona).
There are typically three stages to a solar flare. First is the precursor stage, where the release of magnetic energy is triggered. Soft x-ray emission is detected in this stage. In the second or impulsive stage, protons and electrons are accelerated to energies exceeding 1 million electron volts (MeV). During the impulsive stage, radio waves, hard x-rays, and gamma rays are emitted. The gradual build up and decay of soft x-rays can be detected in the third, decay stage. The duration of these stages can be as short as a few seconds or as long as an hour.
Solar flares extend out to the layer of the Sun called the corona. The corona is the outermost atmosphere of the Sun, consisting of highly rarefied gas. This gas normally has a temperature of a few million degrees Kelvin. Inside a flare, the temperature typically reaches 10 or 20 million degrees Kelvin, and can be as high as 100 million degrees Kelvin. The corona is visible in soft x-rays, as in the above image. Notice that the corona is not uniformly bright, but is concentrated around the solar equator in loop-shaped features. These bright loops are located within and connect areas of strong magnetic field called active regions. Sunspots are located within these active regions. Solar flares occur in active regions.
The frequency of flares coincides with the Sun's eleven year cycle. When the solar cycle is at a minimum, active regions are small and rare and few solar flares are detected. These increase in number as the Sun approaches the maximum part of its cycle.
- Composite spectrum of a large flare
The X rays and gamma rays are produced by several different processes with the result that a complex spectrum is produced involving both line and continuum emission. A composite spectrum of a large flare is shown in the figure on the right, where the contributions to the total emission are indicated in the different energy ranges. The longer wavelength or softer X rays from less than 1 keV to several tens of keV are produced by hot plasma with a temperature of at least 10 million Kelvin.
Plasma at such high temperatures emits soft X rays through the interactions of free electrons with the nuclei (primarily protons) of the plasma. This radiation is called bremsstrahlung (from the German word meaning "braking radiation") since the radiation is produced as the electrons are deflected in the Coulomb (e.g. electric) field of the ions. This type of emission from a plasma at a given temperature has a characteristic continuum spectrum that falls off exponentially with increasing energy. In an inhomogeneous plasma with a distribution of temperatures, more complicated spectral shapes are possible. Detailed spectral measurements of this emission can be used to determine the distribution of emission measure over temperature for the plasma in the emitting volume.
Shorter wavelength or harder X rays with energies greater than 10 keV are also believed to be due to bremsstrahlung, but they are produced by electrons that have been accelerated to much higher energies than those found in the ambient plasma. The resulting bremsstrahlung spectrum, in general, will not have the exponential behavior characteristic of a thermal source. Spectral measurements of such hard X rays do indeed show a less steep fall-off than at lower energies, often with a power-law rather than an exponential shape. The bremsstrahlung spectrum can extend up into the gamma-ray range. Indeed, in some of the biggest flares, the spectrum is seen to extend to energies in excess of 100 MeV. The hard X-ray emission contains unique information, not only on the spectrum of the high energy electrons accelerated during the flare, but also on their locations in the flare and on their temporal variability.
At even higher energies, we find gamma rays produced, not from the flare electrons, but from nuclear interactions of the accelerated protons and heavier ions. These particles interact with the nuclei of the different elements in the ambient solar atmosphere to produce a far more complicated emission spectrum than the relatively smooth continuum bremsstrahlung spectrum. Many individual gamma-ray lines from a wide variety of different elements in the solar atmosphere have been detected. They result from the decay of such elements as carbon, nitrogen, oxygen, etc. that are excited to high energy states in the various nuclear interactions. The relative intensities of the various lines provide information about the composition of both the accelerated particles and the target nuclei.
Furthermore, the lines are Doppler broadened and shifted because of the high velocities of the nuclei as they decay and emit the gamma rays. Consequently, the widths and detailed shapes of the lines can reveal the distribution of velocities of the emitting particles.
