Chromosphere
The chromosphere (literally, "sphere of color") is the second of the three main layers in the Sun's atmosphere and is roughly 3,000 to 5,000 kilometers deep. The chromosphere's rosy red color is only apparent during eclipses. The Chromosphere sits just above the photosphere and below the solar transition region. The layer of the chromosphere atop the photosphere is homogeneous. A forest of hairy appearing spicules rise from the homogeneous layer some of which extend 10,000 km into the corona above.
The density of the chromosphere is only 10−4 times that of the photosphere, the layer beneath, and 10−8 times that of the atmosphere of Earth at sea level. This makes the chromosphere normally invisible and it can be seen only during a total eclipse, where its reddish color is revealed. The color hues are anywhere between pink and red. However, without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere beneath.
The density of the chromosphere decreases with distance from the center of the Sun. This decreases logarithmically from 1017 particles per cubic centimeter, or approximately 2×10−4 kg/m3 to under 1.6×10−11 kg/m3 at the outer boundary. The temperature decreases from the inner boundary at about 6,000 K to a minimum of approximately 3,800 K, before increasing to upwards of 35,000 K at the outer boundary with the transition layer of the corona.
Chromospheres have been observed also in stars other than the Sun.
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.
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