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Sun gives off light in wavelengths from gamma to radio. Some features that are seen in gamma are not seen in visible or radio wavelengths. Therefore, to fully investigate Sun, we have to view it in all of these different wavelengths.
(The following text and pictures are taken from the SOHO web site.)
These images were taken by SOHO/EIT on the same day. Because very little UV light penatrates the Earth's atmosphere, we have to go into space to take images like these.
It is not possible for the human eye to see ultraviolet light directly.
Thus, the colors of these images are just ways to represent them so we can
see them. To keep the images straight, we assign certain colors to EIT images
taken in particular UV wavelengths, so usually the 195 Å images are
in green, the 304 Å images are in orange, etc.
Sun at 171 Å. Light at this wavelength is chiefly emitted by Fe IX and X (iron ionized 8 or 9 times) at 1 million degrees K
Sun at 195 Å. Light at this wavelength is chiefly emitted by Fe XII (iron ionized 11 times) at 1.5 million degrees K.
Sun at 304 Å. Light at this wavelength is emitted by He II (helium ionized once) at 60,000 - 80,000 degrees K.
Note that you see different features when viewing Sun at these different wavelengths. At a wavelength of 304 Å, we are able to see through the corona to the more cooler chromosphere. The prominence at 304 Å cannot even be seen in the other wavelengths. The reason is that the prominence is at a much lower temperature than the corona and does not give off light at 171 Å or 195 Å.
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