What is a Solar Flare?
- Source: Goddard Space Flight Center
Solar Flare Theory
What is a Solar Flare?
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.
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
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.
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