active region

A Dictionary of Astronomy

active region An area on the Sun where magnetic fields emerge through the photosphere into the chromosphere and corona. Active regions on the photosphere include sunspots and faculae. Their counterparts higher in the chromosphere are plages. Also in the chromosphere are dark fibrilles and filaments. Active regions in the corona are areas of enhanced density and temperature sometimes called coronal condensations. Other examples of active regions are areas on the photosphere where sunspots have faded, and X-ray bright points in the corona. Flares occur in active regions.

Active regions are allocated numbers by the US government's National Oceanic and Atmospheric Administration (NOAA) in order of appearance. The present numbering system started on 1972January5. Four‐digit numbers are assigned up to 9999, after which the numbering returns to 0000, as happened on 2002June14; the numbers are prefixed by AR for Active Region. Active regions that exist for more than one solar rotation are given a new number each time they reappear.

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© A Dictionary of Astronomy 1997, originally published by Oxford University Press 1997.

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