The lower region of the Sun’s atmosphere is called the chromosphere. Its name comes from the Greek root chroma(meaning color), for it appears bright red when viewed during a solar eclipse.
The chromosphere extends for about 2,000 kilometers (1,200 miles) above the visible surface of the Sun. Compared to the radius of the Sun, which is nearly 700 thousand kilometers (more than 400 thousand miles), the chromosphere is a very thin layer.
The chromosphere is above the photosphere, the visible “surface” of the Sun. It lies below the solar corona, the Sun’s upper atmosphere, which extends many thousands of kilometers above the chromosphere into space.
The plasma (electrically charged gas) in the chromosphere has a very low density. It is about ten thousand times less dense than the underlying photosphere, and more than a million times less dense than Earth’s atmosphere. Because it is such a thin layer and made of such tenuous plasma, the chromosphere is normally hidden from our view… it’s light is drowned out by the brilliance of the underlying photosphere. It is typically only seen during a solar eclipse or through the use of special telescopes on satellites.
The temperature of the chromosphere varies substantially with height above the photosphere. At first, the temperature decreases with height – from roughly 6,000° C (11,000° F) at the photosphere to about 4,000° C (7,200° F) a couple hundred kilometers higher up. Strangely, temperatures begin to climb in the upper reaches of the chromosphere, reaching a few tens of thousands of degrees. Scientists aren’t completely sure why temperature increases with height in the upper chromosphere, though they suspect that magnetic activity in the Sun’s atmosphere supplies the energy behind the heating.
An extremely thin (by the Sun’s standards) layer called the transition region forms the border between the chromosphere and the corona above it. Temperatures rise very quickly in the transition region, from a few tens of thousands of degrees to more than a million degrees.
Solar filaments and prominences are features that rise up through the chromosphere into the corona. H-alpha (short for hydrogen-alpha) images of the Sun – photos taken with a specific wavelength of red light emitted by hydrogen atoms in the Sun’s atmosphere – are particularly good at revealing filaments and other structures in the chromosphere. Some wavelengths of ultraviolet “light” also help us peer into the chromosphere, especially the hotter, higher sections near the transition region.