A NEW VIEW OF MASS EJECTIONS:
It seems that the behaviour of the Sun in some ways resembles that of humans with a dry throat: the first spasm leads to a further series of coughs in a kind of chain reaction. In the case of the Sun, huge clouds of hot gas are ejected during each spasm in the form of Coronal Mass Ejections. This, and other new results from the LASCO (Large Angle Spectrometric Coronagraph) instrument on board the European Space Agency/NASA spacecraft SOHO (Solar and Heliospheric Observatory), will be presented at the UK National Astronomy Meeting at the University of St Andrews by Dr Mark Lyons from the University of Birmingham.
What Are CMEs?
Astronomers have known since the early 1970s that Coronal Mass Ejections (CMEs) are regularly thrown out into space by the Sun. These huge clouds of gas consist of electrically charged particles (protons and electrons) and have a typical mass of hundreds of millions of tonnes. In some cases they are directed towards the Earth and they travel so rapidly that they usually cross the 150 million km gap within three days of their launch from the Sun. Scientists would very much like to be able to forecast these events since, on arrival, they interact with the Earth's magnetic field, producing geomagnetic disturbances which can disrupt electricity supplies and cause damage to satellites.
In addition, understanding how CMEs are produced is crucial to understanding the overall workings of the Sun. They are a dominant feature of the solar corona (the white halo seen around the Sun during solar eclipses) and may play a major role in the behaviour of the solar magnetic field.
The latest tool being used to advance the research into CMEs is the LASCO instrument carried on board the SOHO satellite. LASCO is a joint project between the University of Birmingham, the Naval Research Laboratory (Washington DC), the Max- Planck-Institut fur Aeronomie (Germany) and the Laboratoire d'Astronomie Spatiale (France). It consists of three telescopes (known as coronagraphs) which are capable of blocking out the bright disk of the Sun and allowing the fainter light from the corona to be observed.
Combined, the three coronagraphs of LASCO give images of the solar corona from 1.1 to 30 solar radii (from just above the visible surface to a distance of about 20 million km from the Sun). This wide angle view and its high sensitivity give LASCO a tremendous advantage over previous instruments.
In general, a CME is thought to occur when closed magnetic configurations in the solar corona are destabilised by some trigger. This destabilisation then leads to the expulsion of matter from the solar atmosphere. The latest research at Birmingham is revealing that the entire Sun can be affected by CMEs. This is displayed most strikingly by events observed by LASCO where an initial mass ejection is closely followed by a series of others. In some cases CMEs occur at widely separated points almost simultaneously. For the first time LASCO is showing us that the corona behaves as a single unit, capable of storing large amounts of magnetic energy which can be released from more than one point by some initial triggering mechanism.
Further evidence for a global reaction of the corona was provided by an event observed on the 23rd February 1997. LASCO C1 images (covering a region from 1.1 to 3 solar radii) showed the expansion of a CME in the lower corona moving with a speed of 880 km/s from the north-east limb of the Sun. This quickly destabilised a sequence of much larger magnetic loop structures to the south which then became the dominant feature of the CME. This sequence of events implies that a higher magnetic loop system spans the solar equator to physically connect regions in opposite hemispheres.
The Solar Wind
For a long time the Sun has been known to produce a 'wind' of charged particles. Mass ejections from the Sun are known to contribute a significant fraction of the total material of solar wind. LASCO observations are now providing new information about the flow of the solar wind nearer to solar surface where its properties have not been closely studied.
A study of CME events carried out by Professor George Simnett at Birmingham University has shown that they begin to undergo an acceleration at a distance of about 6 solar radii. So the LASCO observations indicate that this is probably where the solar wind begins.
Notes for Editors
The SOHO satellite was launched in December 1995. It orbits a stable point (the Lagrangian L1 point), situated approximately 1.5 million km from Earth towards the Sun at which the gravitational pull on the satellite from the Earth and the Sun are equal. This position ensures that LASCO has an uninterrupted view of the Sun.
LASCO observations will continue as solar activity moves towards a maximum in its 11 year cycle by the end of the century. Dr Mark Lyons is a research fellow working on the LASCO project at the University of Birmingham. The solar group at the University of Birmingham, headed by Prof. George Simnett, is using images from LASCO to provide new insights into the CME process.
More information about the LASCO research carried out by the solar group at the University of Birmingham and about the SOHO project, including images of CMEs, can be found on the following Web sites.
Press room at the National Astronomy Meeting, University of St Andrews(8.30 - 18.00 Tue 31 March to Thur 2 April; 9.00 - 12.00 Fri 3 April): Phone: 01334-462168 and 01334-462169 Fax: 01334-463130