Comparing Chromosphere and Photosphere
Whilst the photosphere has an absorption line spectrum, the chromosphere's spectrum is dominated by emission lines. In particular, one of its strongest lines is the Hα at a wavelength of 656.3 nm; this line is emitted by a hydrogen atom whenever its electron makes a transition from the n=3 to the n=2 energy level. A wavelength of 656.3 nm is in the red part of the spectrum, which causes the chromosphere to have its characteristic reddish colour.
By analysing the spectrum of the chromosphere, it was found that the temperature of this layer of the solar atmosphere increases with increasing height in the chromosphere itself. Its temperature at the top of photosphere is only about 4,400 K, whilst at the top of chromosphere, some 2,000 km higher, it reaches 25,000 K. This is however the opposite of what we find in the photosphere, where the temperature drops with increasing height. It is not yet fully understood what phenomenon causes the temperature of the chromosphere to paradoxically increase as you move away from the Sun's interior. However, it seems likely to be explained, partially or totally, by magnetic reconnection.
Read more about this topic: Chromosphere
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