The Sun In Action

I.The Sunspot Cycle

1. 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 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 of the sunspot numbers 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.
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2.The Butterfly Diagram

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 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.
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3.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. 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.

II.Solar Flares

1.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.

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. Observations for the last few days are available at NOAA's website for Today's Space Weather.
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2. 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 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. 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.
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3.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 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. This shear is a key ingredient in the production of solar flares.
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III.Post Flare Loops

In the hours following a solar flare we often see a series of loops above the surface of the Sun. These loops are best seen when viewed in the light emitted by hydrogen in the red region of the solar spectrum (H-alpha emission). The loops shown to the left formed after an active region (AR 7205) flared on June 26, 1992. Within the magnetic confines of these loops the material is somewhat isolated from the million degree corona and can cool to much lower temperatures. These particular loops are of interest because they include a set of "bent-over" loops that figure prominently in theoretical models for some flares.
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The velocity of the material flowing in these loops can be determined using the "Doppler effect." The light from material moving toward us is shifted toward the blue end of the spectrum while light from material moving away from us is shifted toward the red end. The image at the left shows the Doppler shift of the H-alpha emission. This information can be used with the observed motion of the material to determine the three-dimensional flow of material within these loops. The loops form an arcade, a series placed one after another to form a tunnel-like structure. The "bent-over" loop threads through the arcade with footpoints on opposite sides on the opposite ends.

These observations were obtained with the Swedish Solar Telescope on LaPalma in the Canary Islands by T. Tarbell (Lockheed/Palo Alto) and the data was provided through B. Schmieder (Observatoire de Paris, Meudon). The scientific results can be found in a paper by Moore, Schmieder, Hathaway, and Tarbell, Solar Physics 176, pp 153-169 (1997).
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VI.Coronal Mass Ejections

Coronal mass ejections (or CMEs) are huge bubbles of gas threaded with magnetic field lines that are ejected from the Sun over the course of several hours. Although the Sun's corona has been observed during total eclipses of the Sun for thousands of years, the existence of coronal mass ejections was unrealized until the space age. The earliest evidence of these dynamical events came from observations made with a coronagraph on the 7th Orbiting Solar Observatory (OSO 7) from 1971 to 1973. A coronagraph produces an artificial eclipse of the Sun by placing an "occulting disk" over the image of the Sun. During a natural eclipse of the Sun the corona is only visible for a few minutes at most, too short a period of time to notice any changes in coronal features. With ground based coronagraphs only the innermost corona is visible above the brightness of the sky. From space the corona is visible out to large distances from the Sun and can be viewed continuously.

Coronal mass ejections are often associated with solar flares and prominence eruptions but they can also occur in the absence of either of these processes. The frequency of CMEs varies with the sunspot cycle. At solar minimum we observe about one CME a week. Near solar maximum we observe an average of 2 to 3 CMEs per day.

Coronal Mass Ejections disrupt the flow of the solar wind and produce disturbances that strike the Earth with sometimes catastrophic results. The Large Angle and Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) has observed a large number of CMEs. The event of April 7th, 1997 is shown to the left (click on the image for the animation). It produced a "halo event" in which the entire Sun appeared to be surrounded by the CME. Halo events are produced by CMEs that are directed toward the Earth. As they loom larger and larger they appear to envelope the Sun itself.
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V.Surface Flows

The surface of the sun is in constant motion due to the presence of several velocity components. These components include: rotation, cellular convection, oscillations, and meridional flow. The largest velocity signal is that due to solar rotation with an equatorial velocity of 2000 m/s. Both the oscillations and the convective motions have amplitudes of about 300 m/s. The meridional flow is the weakest at only about 20 m/s. Each of these components plays an important role in helping us understand the sun and how it produces its 11-year cycle of solar activity.

Solar velocity data is available from the Global Oscillation Network Group (GONG) instruments and the Michelson Doppler Imager (MDI) on the SOHO Mission. Both of these investigations determine the flow velocities by measuring the Doppler shift of a spectral line formed by nickel atoms in the cooler layers of the solar atmosphere.

The GONG data presently consists of 256 by 256 pixel intensity images at three different spectral positions within the line. The sum of these raw images gives an intensity image which shows sunspots and limb darkening. The size of the change in intensity at the three spectral positions gives a modulation image that shows roughly where magnetic fields are located. The shift in the position of the spectral line with respect to its laboratory position gives a velocity image which is dominated by solar rotation and the cellular pattern of solar convection called supergranulation. GONG also produces magnetograms, images of the sun's magnetic field.
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The primary purpose of these GONG velocity images is to provide data for analyzing the oscillations of the sun. For studies of the nearly steady flows the oscillatory signal represents a source of noise and needs to be removed from the data. This is done by taking a weighted average of 17 velocity images taken at 1-minute intervals to produce an image of the nearly steady flows.

The SOHO/MDI data consists of 1024 by 1024 pixel intensity images at four different spectral positions within the spectral line. This higher resolution data provides excellent information on the cellular flows. Using the same image analysis program to separate the flow components provides a much clearer image of the supergranulation convection pattern. This instrument also has a high-resolution mode with 3X magnification. This higher magnification reveals even finer details of the sun's convective flow elements.
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