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Flare
Characteristics
Credit: NASA
Solar flares
are tremendous explosions on the surface of the Sun. In a matter
of just a few minutes they heat material to many millions of
degrees and release as much energy as a billion megatons of TNT.
They occur near sunspots, usually along the dividing line
(neutral line) between areas of oppositely directed magnetic
fields.
Flares release energy in
many forms - electro-magnetic (Gamma rays and X-rays), energetic
particles (protons and electrons), and mass flows. Flares are
characterized by their brightness in X-rays (X-Ray flux). The
biggest flares are X-Class flares. M-Class flares have a tenth
the energy and C-Class flares have a tenth of the X-ray flux seen
in M-Class flares. The National Oceanic and Atmospheric
Administration (NOAA) monitors the X-Ray flux from the Sun with
detectors on some of its satellites.
Flare
Observations
Solar flares are often observed
using filters to isolate the light emitted by hydrogen atoms in
the red region of the solar spectrum (the H-alpha spectral line).
Most solar observatories have H-alpha telescopes and some
observatories monitor the Sun for solar flares by capturing
images of the Sun every few seconds. The images at the left are
from the Big
Bear Solar Observatory. The image at the upper left shows
material erupting from a flare near the limb of the Sun on
October 10th, 1971. The 4.2MB
mpeg movie of this
flare shows how material is blasted off of the Sun within just a
few minutes. The image at the lower left shows a powerful flare
observed on the disk of the Sun on August 7th, 1972. This is an
example of a "two-ribbon" flare in which the flaring
region appear as two bright lines threading through the area
between sunspots within a sunspot group. (See the 2.2MB
mpeg movie.) This particular flare, the "seahorse
flare," produced radiation levels that would have been
harmful to astronauts if a moon mission had been in progress at
the time.
Flares
and Magnetic Shear
The key to understanding and
predicting solar flares is the structure of the magnetic field
around sunspots. If this structure becomes twisted and sheared
then magnetic field lines can cross and reconnect with the
explosive release of energy. In the bottom left, to the left the
blue lines represent the neutral lines between areas of
oppositely directed magnetic fields. Normally the magnetic field
would loop directly across these lines from positive (outward
pointing magnetic field) to negative (inward pointing magnetic
field ) regions. The small line segments show the strength and
direction of the magnetic field measured with the MSFC Vector
Magnetograph. These lines and line segments overlie an image of a
group of sunspots with a flaring region. The flare (the bright
area) lies along a section of a neutral line where the magnetic
field is twisted (or sheared) to point along the neutral line
instead of across it. We have found that this shear is a key
ingredient in the production of solar flares.
Source
/ Credit: NASA
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