Like the Earth the Sun has an atmosphere, but the two are incredibly different. The Sun’s is thousands of miles thick and has a variety of changeable layers that have different compositions and characteristics.
Solar storms are violent outbursts of activity on the Sun that interfere with the Earth’s magnetic field and inundate our planet with particles. They are the result of outpourings of energy from the Sun, either in the form of a Coronal Mass Ejection (CME) or a solar flare. The former is a release of a large amount of material, mostly plasma, from the Sun while the latter is a sudden release of electromagnetic radiation commonly associated with a sunspot. While no direct connection has been found between CMEs and solar flares, both are responsible for causing solar storms on Earth. The reason why these two events occur is due to the Sun’s atmosphere and its turbulent interior, with all of its components playing a part in bathing our planet in bursts of energy.
The lowest part of the atmosphere, the part directly above the Sun’s radiative zone, is the photosphere. This is the visible part of the Sun that we can see, it is 300-400 kilometres (180-240 miles) thick and has a temperature of about 5,530 degrees Celsius (9,980 degrees Fahrenheit). This produces a white glow although from Earth this usually appears yellow or orange due to our own atmosphere. As you travel through the photosphere away from the Sun’s core the temperature begins to drop and the gases become cooler, in turn emitting less light. This makes the photosphere appear darker at its outer edges and gives the Sun an apparently clearly defined outer boundary, although this is certainly not the case as the atmosphere extends outwards much further.
Once you pass through the photosphere you enter the chromosphere, which is about 2,000 kilometres (1,240 miles) thick. The temperature rises to about 9,730 degrees Celsius (17,540 degrees Fahrenheit), surpassing that of the photosphere. The reason for this is that the convection currents in the underlying photosphere heat the chromosphere above, producing shock waves that heat the surrounding gas and send it flying out of the chromosphere as tiny spikes of supersonic plasma known as spicules.
The final layer of the Sun’s atmosphere is the corona. This huge expanse of material can stretch as far as several million miles outwards from the surface. Oddly, the temperature of the corona averages 2 million degrees Celsius (3.6 million degrees Fahrenheit), far hotter than that of the photosphere and chromosphere. The reason for this is unknown; as far as we are aware, atoms tend to move from high to low temperature and not vice versa, so the process of material moving out of the Sun beyond the photosphere is not understood.
On the photosphere, dark and cool regions known as sunspots appear in pairs as a result of intense magnetic fields. The magnetic fields, caused by gases moving in the Sun’s interior, leave one sunspot and enter another. Sunspot activity rises and falls on an 11-year cycle, as discussed in the next section. Sometimes clouds of gases from the chromosphere will follow these magnetic field lines in and out of a pair of sunspots, forming an arch of gas known as a solar prominence. A prominence can last up to three months and may extend up to 50,000 kilometres (30,000 miles) above the surface. Once they reach their maximum height they break and erupt, in turn sending massive amounts of material racing outwards through the corona, an event known as a coronal mass ejection (CME).
When the sun’s magnetic field is concentrated in sunspot areas, the resultant magnetic field lines can extend and snap, causing a violent explosion on the surface of the Sun called a solar flare. At the moment of eruption vast amounts of radiation are emitted into space, which we call a solar storm when it reaches Earth. The particles within a solar storm often interact with particles in the atmosphere of planets in the Solar System, causing fantastic displays of light at their poles as the gases in the planet’s atmosphere are heated by the particles. On Earth we know these as the aurora borealis in the Northern Hemisphere and the aurora australis in the Southern Hemisphere.
Find out more about the Sun in issue 01 of All About Space.