Image Processing Reference
In-Depth Information
Figure4.23 The Sun imaged with soft x rays. (CourtesyofYohkoh/SXT)
regions (sunspots) and heats gas in the solar atmosphere to several million degrees
by virtue of powerful electric and magnetic fields within the flare. The magnetic
field pops out of the surface owing to the differential rotation of the Sun—the
material at the Sun's equator rotates faster than the material at the poles. The Sun's
magnetic field is locked to the dense solar material, and the differential rotation
stretches and tangles the magnetic field like rubber bands, causing it to pop out of
the surface in loops. The hot gas contained in the loops then glows with x-ray light.
Another interesting feature of the x-ray emission from the Sun is its variability,
which can be as much as a factor of 100, in contrast to the extreme stability of the
Sun's brightness in the visible waveband (it varies by about 0.1% in an 11-year
cycle.). How can this be? This is because we are observing very different aspects
of the Sun in these two regions of the electromagnetic spectrum. When we observe
the Sun in the visible waveband, we are seeing light emitted by the outer layer that
maintains a very steady temperature. In contrast, x-ray emissions from the Sun are
all from solar flares, where there is little stability and large changes can happen
over a period of days or hours.
The soft x-ray telescope on the Yohkoh satellite, which carries two x-ray
imaging sensors aboard it, produced Fig. 4.23. 7 One is sensitive tohardxrays,
in the 20-80-keV range, and the other to soft x rays in the 0.1-4 keV range.
The detectors are silicon devices that convert the x rays into electrical signals. In
this particular design, the x rays are imaged onto the detector array with grazing-
incidence optics. The x rays strike a ring-like metal shape at a grazing incidence
(like stones skipping off the surface of a lake) and are deflected onto the detector
elements. The efficiency of this type of optic is fairly low, since the collection area
is a small fraction of the total aperture.
7 Yohkoh is the Japanese word for “sunbeam.”
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