Chromosphere

The Sun observed through a telescope with an H-alpha filter

Figure 1. Skylab measured the temperature (solid curve) and density (dashed curve) of the chromosphere between the thinner transition region and the lower photosphere (darker orange).

Photo taken by Luc Viatour during the total eclipse of 1999

The chromosphere (literally, "sphere of colour") is the second of the three main layers in the Sun's atmosphere and is roughly 2 000-kilometers deep. It sits just above the photosphere and just below the solar transition region.

The density of the chromosphere is very small, it being only 10 ^{− 4} times that of the photosphere, the layer just below it, and 10⁸ times that of the atmosphere of Earth. This makes the chromosphere normally invisible and it can only be seen during a total eclipse, where its reddish colour is revealed. However, without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere.

Although relatively thin, the density of the chromosphere increases over almost seven orders of magnitude (five million times), from a low at its outer boundary with the solar transition region to a high at its inner boundary with the photosphere (that is, from $1.0 \times 10^{-11} \text{ kg}/\text{m}^3$ to $2.0 \times 10^{-4} \text{ kg}/\text{m}^3$). See Figure 1. Skylab's solar atmosphere results chart for details of how its temperature and density vary with height.

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 H_α 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 4400 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.

Features

Many interesting phenomena can be observed in the chromosophere:

• Filaments (and prominences, which are filaments viewed from the side) underlie many coronal mass ejections and hence are important to the prediction of space weather. Solar prominences rise up through the chromosphere from the photosphere, sometimes reaching altitudes of 150 000 km. These gigantic plumes of gas are the most spectacular of solar phenomena, aside from the less frequent solar flares.
• The most common feature is the presence of spicules, long thin fingers of luminous gas which appear like the blades of a huge field of fiery grass growing upwards from the photosphere below. Spicules rise to the top of the chromosphere and then sink back down again over the course of about 10 minutes. Similarly, there are horizontal wisps of gas called fibrils, which last about twice as long as spicules.

See the flash spectrum of the solar chromosphere (Eclipse of March 7, 1970).

See also

• Plage (astronomy)
• Orders of magnitude (density)
• Moreton wave

External links

• Animated explanation of the Chromosphere (and Transition Region) (University of Glamorgan).
• Animated explanation of the temperature of the Chromosphere (and Transition Region) (University of Glamorgan).