Research into the Sun’s atmosphere
Submitted by rau on 28 April 2008
A special programme of research investigating the Sun and its influence on the Solar System is currently underway. Initiated by the United Nations, the programme is called International Heliophysical Yearw1 and scientists all across Europe are taking part. One topic of interest is the Sun’s atmosphere; there are many questions about our local star still to be answered.
One consequence of the Sun’s hot corona, along with its high thermal conductivity, is that it is constantly expanding into space. This expansion is called the solar wind and there are two types: the slow wind which travels at about 400 km/s and the fast solar wind which travels at about 800 km/s. Currently, neither the acceleration mechanisms nor the locations of these two types are really understood, but both are being investigated.
Research is also being carried out to see how the solar wind affects planets without a magnetic field. For example, the Venus Express mission is currently in orbit around Venus and is measuring the erosion of the Venusian atmosphere by the solar wind.
The most dramatic form of activity that takes place in the Sun’s atmosphere are huge eruptions of plasma and magnetic field known as coronal mass ejections, or CMEs. Originally discovered in the 1970s, it has since been shown that their frequency varies cyclically (with what is known as the solar cycle): CMEs occur a minimum of once every three days, and a maximum of three to five times per day. These eruptions can be directed towards Earth and, just like with the solar wind, a connection to Earth’s magnetic field can be made. Under these conditions, severe consequences are felt on Earth; heating and expansion of the Earth’s atmosphere leads to changes in satellite orbits. The very real effect of CMEs makes them incredibly interesting to study and there is currently a fleet of spacecraft observing the Sun and Earth to do just this.
The cause of CMEs is known to be related to the Sun’s magnetic fields, which are created by electric currents in what is called the solar dynamo, deep in the interior. Bundles of concentrated fields rise and emerge through the photosphere and extend up into the corona. This magnetic field is continually being injected into the atmosphere and it is thought that CMEs provide a way to remove it and prevent a build-up. Studies are being carried out, with spacecraft such as SOHO, TRACE, STEREO and Hinode, to monitor how the magnetic field structures change over time.
The STEREO mission consists of two spacecraft orbiting the Sun in a way that allows them to move away from Earth in space (one orbit is slightly closer to the Sun than the Earth’s, and one slightly further out). This means that the two spacecraft view the Sun from different positions in space and just as our two eyes give us a sense of depth and perspective, the STEREO spacecraft are giving a 3D view of the erupting magnetic structures (see image). The 3D view is being used to try and work out the physics of the eruption using knowledge of the structure of the magnetic fields. STEREO is also helping predict which CMEs will collide with Earth. This knowledge could be used by satellite operators or organisations that run electricity grids: for example, the orbits of satellites could be particularly closely monitored when it is thought that CMEs will collide with Earth.
The Hinode spacecraft is the equivalent of the Hubble Space Telescope for the Sun, and it is allowing the study of the evolution of the immense atmospheric magnetic structures over time in great detail. It is thought that the only way to get enough energy to expel the billions of tonnes of solar material that make up a CME is by using energy stored in the twisted and distorted magnetic fields. Hinode is making measurements of how twisted the field is and the results are being combined with those found using STEREO. Once we understand why CMEs occur, we can start to predict which magnetic structures will erupt and eventually which ones will have the greatest effect on Earth.