| Solar Corona |
| The solar corona is the outermost layer of the sun’s atmosphere--a very hot halo (millions
of degrees), that, in the form of the solar wind, extends well past the Earth’s orbit.
The corona is observed in detail during solar eclipses. In practice, the corona is
observed routinely by ground-based and satellite coronagraphs in the red line Fe X
(634.7nm -- ten times ionized iron line) and the green line Fe XIV (530.3 nm -- 14
times ionized iron line), and occasionally in the high energy yellow line Ca XV (569.4
nm -- 15 times ionized Calcium line). White light observations of the solar corona
are also made routinely, and used for detecting coronal mass ejections (CMEs) that
may affect the Earth. The coronagraph is a special telescope with an occulting disk
to block the direct light of the sun and thus able to measure the brightness of the
fainter and less dense corona. The intensities are recorded in tabular form, historically
in 5-degree increments around the sun’s disk. The data center holds data in analog
form from 12 stations covering the period 1943 to present. Currently, 2 stations send
daily coronal scans to the data center. A coronal activity index from Lomnicky Stit,
Slovak Republic, 1939-present, is also available. Coronal data exhibit the 11 year
solar cycle variation. Solar X-ray measurements since 1991 by the Yohkoh and GOES
Solar X-ray Imager (SXI) satellites allow continuous coverage of the constantly changing
hot solar corona.
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