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3 years ago in Solar Physics , Spectroscopy By Abhay R

Why is a photospheric spectrum an absorption spectrum while the chromosphere shows emission during eclipses?

In my solar physics course, we learn the photosphere produces a Fraunhofer absorption spectrum. But during totality, the famous "flash spectrum" shows brilliant emission lines like H-alpha. I'm struggling to visualize the geometry and physics. Is the chromosphere simply hot enough to emit, or is there a specific radiative transfer effect caused by the Moon blocking the photospheric background?

 

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By Rachna M Answered 1 year ago

The key is the geometry and the concept of optical depth. The photosphere is dense and hot, producing a continuous blackbody spectrum. As this light passes through the cooler, thinner chromosphere, atoms absorb at specific wavelengths, creating the dark Fraunhofer lines. During totality, the Moon perfectly blocks the intensely bright photospheric continuum. Suddenly, the only light we see is from the chromosphere itself, which is optically thin at its core it can emit its characteristic lines (like H-alpha at 6563 Ã…) without immediately re-absorbing them. The "flash" is that thin layer glowing on its own against the dark sky

 

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