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Solar Prominences/Filaments

Solar prominences/fiiaments are injections of gases from the chromosphere into the corona. They appear as great clouds of gas, sometimes resting on the Sun’s surface and at other times floating free with no visible connection. When viewed against the solar disk, they appear as long dark ribbons and are called filaments. When viewed against the solar limb (the dark outer edge of the solar disk), they appear bright and are called prominences. (See fig. 1-2-2.) They display a variety of shapes, sizes, and activity that defy general description. They have a fibrous structure and appear to resist solar gravity. They may

Figure 1-2-2.—Features of the solar disk.

 extend 18,500 to 125,000 miles (30,000 to 200,000 km) above the chromosphere. The more active types have temperatures of 10,000°K or more and appear hotter than the surrounding atmosphere.


Sunspots are regions of strong localized magnetic fields and indicate relatively cool areas in the photosphere. They appear darker than their surroundings and may appear singly or in more complicated groups dominated by larger spots near the center. (See fig. 1-2-2.)

Sunspots begin as small dark areas known as pores. These pores develop into full-fledged spots in a few days, with maximum development occurring in about 1 to 2 weeks. When sunspots decay the spot shrinks in size and its magnetic field also decreases in size. This life cycle may consist of a few days for small spots to near 100 days for larger groups. The larger spots normally measure about 94,500 miles (120,000 km) across. Sunspots appear to have cyclic variations in intensity, varying through a period of about 8 to 17 years. Variation in number and size occurs throughout the sunspot cycle. As a cycle commences, a few spots are observed at high latitudes of both solar hemispheres, and the spots increase in size and number. They gradually drift equatorward as the cycle progresses, and the intensity of the spots reach a maximum in about 4 years. After this period, decay sets in and near the end of the cycle only a few spots are left in the lower latitudes (50 to 100).


Plages are large irregular bright patches that surround sunspot groups. (See fig. 1-2-2.) They normally appear in conjunction with solar promi-nences or filaments and may be systematically arranged in radial or spiral patterns. Plages are features of the lower chromosphere and often completely or partially obscure an underlying sunspot.


Solar flares are perhaps the most spectacular of the eruptive features associated with solar activity. (See fig. 1-2-2.) They look like flecks of light that suddenly appear near activity centers and come on instantaneously as though a switch were thrown. They rise sharply to peak brightness in a few minutes, then decline more gradually. The number of flares may increase rapidly over an area of activity. Small flarelike brightening are always in progress during the more active phase of activity centers. In some instances flares may take the form of prominences, violently ejecting material into the solar atmosphere and breaking into smaller high-speed blobs or clots. Flare activity appears to vary widely between solar activity centers. The greatest flare productivity seems to be during the week or 10 days when sunspot activity is at its maximum.

Flares are classified according to size and brightness. In general, the higher the importance classification, the stronger the geophysical effects. Some phenomena associated with solar flares have immediate effects; others have delayed effects (15 minutes to 72 hours after flare). 

Solar flare activity produces significant disruptions and phenomena within Earth’s atmosphere. During solar flare activity, solar particle streams (solar winds) are emitted and often intercept Earth. These solar particles are composed of electromagnetic radiation which interacts with Earth’s ionosphere. This results in several reactions such as: increased ionization (electrically charging neutral particles), photo chemical changes (absorption of radiation), atmospheric heating, electrically charged particle motions, and an influx of radiation in a variety

Figure 1-2-3.—Rotation of Earth about its axis (during equinoxes).

of wavelengths and frequencies which include radio and radar frequencies.

Some of the resulting phenomena include the disruption of radio communications and radar detection. This is due to ionization, incoming radio waves, and the motion of charged particles. Satellite orbits can be affected by the atmospheric heating and satellite transmissions may be affected by all of the reactions previously mentioned. Geomagnetic disturbances like the aurora borealis and aurora australis result primarily from the motion of electrically charged particles within the ionosphere.

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