Solar Flares

Solar Flares Flare Characteristics 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.

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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 XClass flares. M-Class flares have a tenth the energy and CClass 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. Observations for the last few days are available at NOAA's website for Today's Space Weather.

Flare Observations

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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

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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 image 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.

The Sunspot Cycle (Updated 2008/10/03)

Sunspot Numbers In 1610, shortly after viewing the sun with his new telescope, Galileo Galilei made the first European observations of Sunspots. Daily observations were started at the Zurich Observatory in 1749 and with the addition of other observatories continuous observations were obtained starting in 1849. The sunspot number is calculated by first counting the Click on image for larger version. number of sunspot groups and then the number of individual sunspots. The "sunspot number" is then given by the sum of the number of individual sunspots and ten times the number of groups. Since most sunspot groups have, on average, about ten spots, this formula for counting sunspots gives reliable numbers even when the observing conditions are less than ideal and small spots are hard to see. Monthly averages (updated monthly) of the sunspot numbers (25 kb GIF image), (30 kb pdffile), (62 kb text file)show that the number of sunspots visible on the sun waxes and wanes with an approximate 11-year cycle. (Note: there are actually at least two "official" sunspot numbers reported. The International Sunspot Number is compiled by the Sunspot Index Data Center in Belgium. The NOAA sunspot number is compiled by the US National Oceanic and Atmospheric Administration. The numbers tabulated in spot_num.txt are the monthly averages (SSN) and standard deviation (DEV) derived from the International Sunspot

Numbers)

The Maunder Minimum Early records of sunspots indicate that the Sun went through a period of inactivity in the late 17th century. Very few sunspots were seen on the Sun from about 1645 to 1715 (38 kb JPEG image). Although the observations were not as extensive as in later years, the Sun was in fact well observed during this time and this lack of sunspots is well documented. This period of solar inactivity also corresponds to a climatic period called the "Little Ice Age" when rivers that are normally ice-free froze and snow fields remained year-round at lower altitudes. There is evidence that the Sun has had similar periods of inactivity in the more distant past. The connection between solar activity and terrestrial climate is an area of on-going research.

The Butterfly Diagram

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Detailed observations of sunspots have been obtained by the Royal Greenwich Observatory since 1874. These observations include information on the sizes and positions of sunspots as well as their numbers. These data show that sunspots do not appear at random over the surface of the sun but are concentrated in two latitude bands on either side of the equator. A butterfly diagram (142 kb GIF image) (184 kb pdf-file) (updated monthly) showing the positions of the spots for each rotation of the sun since May 1874 shows that these bands first form at mid-latitudes, widen, and then move toward the equator as each cycle progresses.

The Greenwich Sunspot Data The Royal Greenwich Observatory data has been appended with data obtained by the US Air Force Solar Optical Observing Network since 1977. This newer data has been reformatted to conform to the older Greenwich data and both are available in a local directory of ASCII files. Each file contains records for a given year with individual records providing information on the daily observations of active regions.

Sunspot Cycle Predictions

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MSFC Solar Physics Branch members Wilson, Hathaway, and Reichmann have studied the sunspot record for characteristic behavior that might help in predicting future sunspot activity. Our current predictions of solar activity for the next few years can be found at this link. Although sunspots themselves produce only minor effects on solar emissions, the magnetic activity that accompanies the sunspots can produce dramatic changes in the ultraviolet and soft x-ray emission levels. These changes over the solar cycle have important consequences for the Earth's upper atmosphere.

Why We Study The Sun The Climate Connection

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The Sun is a source of light and heat for life on Earth. Our ancestors realized that their lives depended upon the Sun and they held the Sun in reverent awe. We still recognize the importance of the Sun and find the Sun to be awe inspiring. In addition we seek to understand how it works, why it changes, and how these changes influence us here on planet Earth. The Sun was much dimmer in its youth and yet the Earth was not frozen. The quantity and quality of light from the Sun varies on time scales from milli-seconds to billions of years. During recent sunspot cycles the total solar irradiance has changed by about 0.1% with the sun being brighter at sunspot maximum. Some of these variations most certainly affect our climate but in uncertain ways.

Space Weather

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The Sun is the source of the solar wind; a flow of gases from the Sun that streams past the Earth at speeds of more than 500 km per second (a million miles per hour). Disturbances in the solar wind shake the Earth's magnetic field and pump energy into the radiation belts. Regions on the surface of the Sun often flare and give off ultraviolet light and x-rays that heat up the Earth's upper atmosphere. This "Space Weather" can change the orbits of satellites and shorten mission lifetimes. The excess radiation can physically damage satellites and pose a threat to astronauts. Shaking the Earth's magnetic field can also cause current surges in power lines that destroy equipment and knock out power over large areas. As we become more dependent upon satellites in space we will increasingly feel the effects of space weather and need topredict it.

The Sun as a Star

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The Sun also serves an important role in helping us to understand the rest of the astronomical universe. It is the only star close enough to us to reveal details about itssurface. Without the Sun we would not have easily guessed that other stars also have spots and hot outer atmospheres. The Sun is the key to understanding other stars. We know the Sun's age, radius, mass, and luminosity (brightness) and we have also learned detailed information about its interior and atmosphere. This information is crucial for our understanding of other stars and how they evolve. Many physical processes that occur elsewhere in the universe can be examined in detail on the Sun. In this way solar astronomy teaches us much about stars, planetary systems, galaxies, and the universe itself.

The Sun as a Physical Laboratory The Sun produces its energy by nuclear fusion - four hydrogen nuclei are fused to form single helium nuclei deep within the Sun's core. We have worked for decades to reproduce this process (in a controlled manner) here on Earth. Most of these efforts involve extremely hot plasmas in strong magnetic fields. (This plasma is not the blood product but rather a mixture of ions and electrons produced at high temperatures.) Much of solar astronomy involves observing and understanding plasmas under similar conditions. There continues to be much interaction between solar astronomers and scientific researchers in this and many other areas.