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Help understanding solar imaging wavelength and coronal temp

by Bob65
Tags: coronal, imaging, solar, temp, wavelength
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Nov14-12, 08:56 AM
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The TRACE website has a solar image that is described as, quoting from the text associated with the image, "This is a false-color, 3-layer composite from the TRACE observatory showing the solar corona: the blue, green, and red channels show the 171A, 195A, and 284A, respectively. These TRACE filters are most sensitive to emission from 1, 1.5, and 2 million degree plasma, thus showing the entire corona and detail of coronal loops in the lower solar atmosphere."

And now that I typed the description above into this post, I think I see where my misunderstanding has arisen. I thought the text stated that the 171A radiation corresponded to the 1 MK plasma, the 195A to the 1.5 MK, and the 284A to the 2 MK plasma. This seemed backward to me, since the shorter wavelength radiation should correspond to the higher temperature plasma (e.g., the 171A radiation corresponds to the 2 MK plasma, etc.) I suspect that is true, and the quoted text description "sorta implies" (but does not state) an incorrect association of shorter wavelengths with lower temperature plasma.

Am I correct? Does the 171A radiation correspond to the 2 MK plasma, etc.?

Thanks for the help.

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The reference you quoted discusses the solar corona so the light being imaged is in emission. The emission is from recombination of electrons and positive ions in the plasma, namely iron. The wavelengths associated with the wavelengths you are asking about correspond to electronic transitions (recombinations of electrons and positively charged iron ions) and the wavelength is entirely dependent on the energy level differences of the various ionized species rather than temperature. The population of the energetic species is temperature dependent.

Wavelength/ Species
171 A/ Fe IX
195 A/ Fe XII
284 A/ Fe XV

You can see that as we go up in wavelength we are viewing progressively more energetic events. That is, the temperature at which we view iron fifteen or Fe15 transitions necessarily needs to be at a higher temperature than FeIX. By the time we are removing the more inner shell electrons to create a place for an ion/electron recombination we are probing much more energic events, ie. higher temperatures.

Interesting reading.

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