A solar flare is an enormous explosion in the solar atmosphere, involving sudden bursts of particle acceleration, plasma heating, and bulk mass motion. It is believed to result from the sudden release of energy stored in the magnetic fields that thread the solar corona in active regions around sunspots. In the largest flares, 10³² ergs or more can be released in a few minutes to a few tens of minutes. Such large flares only occur a few times within a year or two of the maximum in solar activity that occurs every 11 years or so, with the last maximum in 1989. Many smaller flares occur down to the limits of detectability of modern instruments at about 10²⁷ ergs. These smaller events generally last for shorter times down to a few seconds; their occurrence rate also follows the 11-year cycle, peaking at several tens of flares per day.

Solar flares were first detected from their visible or optical emissions. These so called "white light" flares are rarely reported since they are difficult to detect against the intense and constant optical emission from the photosphere. Such a flare is shown below. The companion full-Sun image in soft X-rays reveals the location on the solar disc of the white light emission. In general, a solar flare produces copious radiation across the full electromagnetic spectrum from the longest wavelength radio waves to the highest energy gamma rays. The contrast over the background (quiet-Sun) emission is much higher at the shorter X-ray and gamma-ray wavelengths that will be observed with HESSI. Furthermore, these high energy radiations carry direct information about the energetically dominant products of the energy release that is not available from emissions at any other wavelength. The X rays result from the interactions of the high energy electrons energized during the flare, and the gamma rays result primarily from nuclear interactions of the high energy protons and other heavier ions.

Solar Flare seen in white light and in X rays

The image in the top left corner (above) shows the full Sun as seen in soft X rays. The location of the flare is shown by the small rectangle that is expanded to show the white light (contours) and hard X-ray (gray scale) images.

During a large solar flare, the X-ray and gamma-ray flux is observed to increase by many orders of magnitude over preflare levels. Indeed, preflare fluxes are not detectable at energies above 10 - 20 keV. The time profile at several different energies for a large flare on 6 March, 1989, is shown below.

Solar flare on 6 March 1989

The following different stages can be recognized from this plot:

1. The preflare stage from about 13:50 to 13:56 UT in which the soft X-ray emission gradually increases but little if any hard X rays or gamma rays are detected above the instrumental background level.
   
2. This is followed by the so-called impulsive phase in which the hard X-ray and gamma-ray emission rises impulsively, often with many short but intense spikes of emission, each lasting a few seconds to tens of seconds. The soft X-ray flux rises more rapidly during this phase with its time profile roughly matching the time integral of the hard X-ray profile in many cases.
   
3. After about 14:06 UT the gradual phase begins, and the hard X-ray and gamma-ray fluxes start to decay away more or less exponentially with a time constant of minutes. The soft X-ray flux continues to rise to a later peak and then it too falls exponentially but with a significantly longer time constant, sometimes as long as several hours.
   
4. In the particular flare shown above, a second phase of hard X-ray and gamma-ray emission occurs after about 14:10 UT in which the fluxes vary more gradually than during the impulsive phase. This later, more gradual phase of high energy emissions is not detected in most flares. Note that the soft X-ray flux continues to fall smoothly during this phase.
   

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 below, 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 10⁷ K (and possibly as high as several times 10⁸ K in some cases).

Composite Solar Flare Spectrum of X rays and Gamma Rays

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 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, the e-folding energy being a measure of the temperature of the plasma. 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 electron-ion 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. As we discuss below, 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 flaring volume and on their impulsive temporal variability.

At even higher energies, we find gamma rays produced, not from the flare electrons, but from nuclear interactions of the protons and heavier ions accelerated in the flare. These high energy 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 relatively abundant 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 and hence also impose severe constraints on the acceleration mechanism itself. Despite the wealth of information believed to be available from observations of these gamma-ray lines, no gamma-ray spectrometer with the resolution necessary to reveal anything other than the intensities of the strongest lines has ever been flown.