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What is a Solar Flare?

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  • Mark A. LeCuyer
    Source: Goddard Space Flight Center Solar Flare Theory http://hesperia.gsfc.nasa.gov/sftheory/ What is a Solar Flare? A flare is defined as a sudden, rapid,
    Message 1 of 1 , Feb 29, 2000
      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.
      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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