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penumbra


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1 The outer region of the shadow cast in space by a planet or satellite; an observer within the penumbra would witness a partial eclipse. The penumbra surrounds the dark, narrower umbra (1), within which the Sun is completely obscured. The penumbra is darkest immediately adjacent to the umbra. During a lunar eclipse the Moon passes through the penumbra of the Earth's shadow before and after its umbral passage.

2 The lighter, outer region of a sunspot, with a temperature of around 5500 K. Small spots often do not have a penumbra, but in mature sunspots the penumbra is well developed and occupies about 70% of the total spot area. The penumbra consists of filaments radiating from the central umbra. These filaments are brighter than the umbra, but less bright than the surrounding photosphere. They consist of light grains lasting about an hour, which drift in towards the umbra where they become umbral dots, apparently smaller versions of photospheric granules. The horizontal outward flow of gas called the Evershed effect occurs in the penumbra and a little beyond it. The magnetic field intensity is about 0.1 tesla, less than in the umbra, and the field is aligned more nearly horizontally than in the umbra. Outside the penumbra of mature spots is a radial pattern of fibrilles—the superpenumbra—visible in Hα light.

Subjects: Astronomy and Astrophysics.


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