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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 optical emission to x-rays and gamma rays at the short wavelength end. The amount of energy released is the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time! 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 a large flare in white light.
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As the magnetic energy is being released, particles, including electrons, protons, and heavy nuclei, are heated and accelerated in the solar atmosphere. The energy released during a flare is typically on the order of 1027 ergs per second. Large flares can emit up to 1032 ergs of energy. This energy is ten million times greater than the energy released from a volcanic explosion. On the other hand, it is less than one-tenth of the total energy emitted by the Sun every second.
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 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. The Sun will reach its next maximum in the year 2000 or 2001.
A person cannot view a solar flare by simply staring at the Sun. (NEVER LOOK DIRECTLY AT THE SUN! EYE DAMAGE CAN RESULT.) Flares are in fact difficult to see against the bright emission from the photosphere. Instead, specialized scientific instruments are used to detect the radiation signatures emitted during a flare. The radio and optical emissions from flares can be observed with telescopes on the Earth. Energetic emissions such as x-rays and gamma rays require telescopes located in space, since these emissions do not penetrate the Earth's atmosphere.
Why Study Solar Flares?
Solar flares are the most energetic explosions in the solar system.
Solar flares have a direct effect on the Earth's atmosphere.
The intense radiation from a solar flare travels to Earth in eight minutes. As a result:
The Earth's upper atmosphere becomes more ionized and expands. Long distance radio signals can be disrupted by the resulting change in the Earth's ionosphere. A satellite's orbit around the Earth can be disturbed by the enhanced drag on the satellite from the expanded atmosphere Satellites' electronic components can be damaged.
Energetic particles accelerated in solar flares that escape into interplanetary space are
dangerous to astronauts and to electronic instruments in space.
Similar energy release processes take place in other cosmic events. These events occur on objects that are too far away to study in the detail that solar flares can be studied on the Sun. Understanding solar flares can aid in understanding these events.
These objects include:
Flare Stars
Pulsars
Black holes
Quasars
Solar flares provide an opportunity to study physical processes in nature that are similar to those that occur in laboratory devices designed for the purpose of achieving controlled thermonuclear fusion.
HESSI: A High-Resolution Spectroscopic Imager
 Scientific motivation is only part of what drives a space mission. Equally important are technical feasibility and, of course, cost. Given these considerations, a new mission to study the explosive release of energy during solar flares was proposed to NASA's Small Explorer (SMEX) program. This mission, the High Energy Solar Spectroscopic Imager (HESSI), has been selected for launch in mid 2000. Two important technological advances give us reason to expect substantial progress from the HESSI mission: the ability to obtain high-resolution spectra from soft x-ray to gamma-ray energies, and the ability to image hard x-rays and gamma rays.
We have seen that spectra allow us to deduce the physical properties of the radiating particles in flares. The x-ray spectra that have been available to date have generally been ambiguous: the spectra are not well determined and do not provide much information to allow models to be tested and compared. With HESSI it will be possible to obtain flare spectra that are well determined, both because of higher spectral resolution and the broader range of photon energies that will be covered. The lower energy end of these spectra will allow us to consistently observe the hottest plasma in flares and study its relationship to the accelerated electrons producing nonthermal bremsstrahlung x-rays. The middle and high energy end of the spectra will allow us to consistently observe the energies to which electrons are accelerated and deduce whether more than one acceleration mechanism may be required. The constraints on models will be much tighter.
Spectra from around 400 keV to about 10 MeV also contain gamma-ray lines. HESSI will for the first time resolve many of these spectral lines, providing important information about accelerated protons and other ions (see Gamma-Ray Lines and Accelerated Ions).
Without imaging we can only observe the sum of the x-ray emission from all locations within a flaring region. For a complex flare such a spectrum can be hopeless to interpret. From the Yohkoh observations and our flare model we have seen that even for a simple flare critical information is lost without imaging. Using an imaging method that builds upon that employed on Yohkoh, HESSI will combine hard x-ray imaging with high-resolution spectroscopy to produce a unified spectroscopic imager. This instrument will provide data from which images can be constructed at any x-ray or gamma-ray energy, or spectra can be generated at any location.
Artist's concept of the High Energy Solar Spectroscopic Imager (HESSI)
Time resolution is also an important consideration for our spectroscopic imager, since flare emissions change rapidly with time. HESSI will build up full flare images about once every second. This will allow us to follow the general evolution of hot plasma and accelerated particles throughout the duration of a flare. Flare data will be taken on a time scale as short as one millisecond (0.001 second), however. We note that the time required for a 100 keV electron to travel directly from the top of the cusp of our model flare loop to one of the footpoints is on the order of 0.1 second. With this high-time-resolution data it will be feasible to actually trace the motion of electrons within the flare loop.
*Material on this web page courtesy of NASA
URL: http://hesperia.gsfc.nasa.gov/sftheory/
Web Author: Gordon Holman
Web Author: Sarah Benedict
Responsible NASA Official: Gordon D. Holman
Laboratory for Astronomy and Solar Physics
NASA/Goddard Space Flight Center
Solar Physics Branch/Code 682
Greenbelt, MD 20771, USA
For further information E-mail holman@stars.gsfc.nasa.gov
Last modification: May 9, 2001 |
