The Solar Atmosphere
The lowest layer of the solar atmosphere is the photosphere, a shallow shell of gas only about 400km deep. It is the optical surface of the sun but it is opaque to visible light. All the visible light emitted from the photosphere is akin to a black body with a peak in the visible spectrum. The temperature of the photosphere is about 5800K, but this decreases as you go out of the photosphere towards the chromosphere.
The most notable feature of the photosphere is he convection cells called granules - they are cells of gas thousands of kilometres in diameter, which control the shifting boiling pattern of the Sun. Sun spots are also a very noticeable feature though only they are only a temporary phenomena. These are areas of the surface where their is intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. This reduced temperature is the reason for the area appearing black with respect to the surrounding surface, although they are still at temperatures of roughly 4,000 K. They also expand and contract as they move across the surface of the sun and can be as large as 80,000 kilometres. The Sunspot population quickly rises and more slowly falls on an irregular cycle of 11 years, the distribution is related to the latitude with a concentration in two latitude bars either side of the equator.
Situated above the photosphere, the chromosphere is the middle layer of the solar atmosphere comprising of a layer of gas approximately 3000km thick. It is transparent to visible light so can't be seen against the bright photosphere, but is visible during a total solar eclipse. While the photosphere spectrum is a black body, the chromosphere has an emision spectrum, created when the electrons change energy state and emit a photon, in this case the photons have a wavelength of 656.3nm, which is the H Alpha Balmer line giving it a reddish colour. The chromosphere, despite being further away from the centre of the Sun, has a higher temperature than the photosphere. It is not known what causes this, but one theory is that it is the results of the dispersion of magnetohydrodynamic waves over the solar surface.
There are several common features within the chromosphere:
- Spicules - they are long thin dynamic jets which go upwards from the photosphere below, and rise to the top of the chromosphere before falling back. They are usually associated with regions of high magnetic flux. If they are seen on the dish, they are known as fibrils.
- Prominences - these rise up through the chromophere and can be seen as a loop on the limb of the sun. These are giant plumes of gas which can reach altitudes of 150,000km. When view on the on the dish of the sun they are known as filaments.
The outermost layer of the solar atmosphere extends from the chromosphere out several million kilometres, where it becomes the solar wind. It can usually only be seen when the photosphere is blocked in some way. It has a temperature range from 1 million Kelvin to about 5 million Kelvin, but is not considered 'hot' as it does not actually contain much thermal energy (few atoms, few collisions).The solar wind is a stream of photons and electrons constantly escaping the Sun at high speed, up to 400km/s. It occurs because the temperature of the corona is so high that the Sun's gravity can't counter it.
Many of the most exciting features are found in the corona:
- Solar fares - is a large explosion in the Sun's atmosphere that can release as much as 6x1025 joules of energy. They are caused by the phenomenon of magnetic reconnection. Along with coronal mass ejections, these can disrupt radio transmission and cause power issues on Earth.
- Coronal Mass Ejections - is an ejection of material from the corona, containing electrons and protons, plus some magnetic field. These can disrupt the Earth's magnetosphere, which can produce particularly strong aurora.
- Coronal loops - highly structured loops which are a direct consequence of the twisted solar magnetic flux within the Sun.