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[Gr.,=color sphere], layer of rarefied, transparent gases in the solar atmosphere; it measures 6,000 mi (9,700 km) in thickness and lies between the photosphere (the sun's visible surface) and the corona (its outer atmosphere).
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Spicules are spike-like prominences visible at the limb of the sun during solar eclipses or during observation of the sun in monochromatic light, for example, the light of the hydrogen line Hλ. They extend into the solar corona to heights of 6,000–10,000 km and have diameters of 200–2,000 km. The average lifetime of a spicule is 5–7 min. Spicules move upward at speeds of 20–30 km/sec; the speeds of internal movements are 5–10 km/sec. The temperature of a spicule is approximately 8000°K in the lower part and about 16,000°K in the upper part. The density of spicules varies with height from 2 × 1011 to 3 × 1010 atoms/cm3. Hundreds of thousands of spicules exist on the sun at one time. They arise primarily at the edges of the cells of the chromospheric network.
(1) In certain invertebrates a skeletal element consisting of calcium carbonate or, less commonly, silica (silicon dioxide). Spicules occur in sponges (in the form of one-, three-, four-, and many-rayed needles), octocorallians, aplacophorans, mollusks, holothurians and other echinoderms (in the form of small wheels, anchors, or lattices), and ascidians (in the form of thorny spheres).
(2) In nematodes, a part of the male copulatory organ. In some species the spicules are supplementary formations; they protrude from the cloacal opening of the male and serve to widen the female’s vagina. In other species sulcate spicules come together and conduct the spermatozoa into the vagina of the female.