Solar Flare
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| Solar Flare |
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 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 2011, give or take one year.
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.
Source: https://www.facebook.com/photo.php?fbid=122654647882937&set=a.120985114716557.25487.113077625507306&type=1&theater
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Astronomers find clues to decades-long coronal heating
mystery
Scientists found evidence
that magnetic waves in a polar coronal hole contain enough energy to heat the
Sun’s corona.
By Columbia
University , New York
RELATED TOPICS: SOLAR SYSTEM / SUN
Solar corona
This is one of the highest-resolution images ever taken of
the solar corona, or outer atmosphere. It was captured by NASA's High
Resolution Coronal Imager (Hi-C) in the ultraviolet wavelength of 19.3 nanometers. NASA
Michael Hahn and Daniel Wolf Savin from Columbia University ’s
Astrophysics Laboratory in New York
found evidence that magnetic waves in a polar coronal hole contain enough
energy to heat the corona and that they also deposit most of their energy at
sufficiently low heights for the heat to spread throughout the corona. The
observations help to answer a 70-year-old solar physics conundrum about the
unexplained extreme temperature of the Sun’s corona — known as the coronal
heating problem.
Hahn and Savin analyzed data from the Extreme Ultraviolet
Imaging Spectrometer aboard the Japanese satellite Hinode. They used
observations of a polar coronal hole, a region of the Sun where the magnetic
fields lines stretch from the solar surface far into interplanetary space.
To understand the coronal heating problem, imagine a flame
coming out of an ice cube. A similar effect occurs on the surface of the Sun.
Nuclear fusion in the center of the Sun heats the solar core to 15 million
degrees. Moving away from this furnace, by the time one arrives at the surface
of the Sun, the gas has cooled to a relatively refreshing 10,800° Fahrenheit
(6,000° Celsius). But the temperature of the gas in the corona, above the solar
surface, soars back up to over 1.8 million degrees F (1 million degrees C).
What causes this unexpected temperature increase has puzzled scientists since
1939.
Two dominant theories exist to explain this mystery. One
attributes the heating to the loops of the magnetic field, which stretch across
the solar surface and can snap and release energy. Another ascribes the heating
to waves emanating from below the solar surface, which carry magnetic energy
and deposit it in the corona. Observations show both of these processes
continually occur on the Sun. Until now, scientists have been unable to
determine if either one of these mechanisms releases sufficient energy to heat
the corona to such high temperatures.
Hahn and Savin’s recent observations show that magnetic
waves are the answer. The advance opens up a realm of further questions, chief
among them is what causes the waves to damp. Hahn and Savin are planning new
observations to try to address this issue.
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