Lift Off For UK Solar Science.
While ground-based astronomers prepare for the first total solar eclipse on the UK mainland for more than half a century, an armada of satellites will also be observing the Sun and its effects on near-Earth space. Many of these space missions involve scientists and engineers from UK universities and research establishments. While it is not possible to include all of these in one information note, a brief summary of the projects is given below.
Over the past three and a half years, 12 instruments on board the European Space Agency / NASA Solar and Heliospheric Observatory (SOHO) spacecraft have revolutionised scientists' knowledge of our nearest star. Principal investigator on one these instruments, the Coronal Diagnostic Spectrometer, is Richard Harrison of Rutherford Appleton Laboratory. Other groups studying SOHO data are based at Birmingham, Mullard Space Science Laboratory (MSSL) in Surrey, Cambridge and St. Andrews.
From its stable viewpoint 1.5 million km sunward of the Earth, SOHO can observe the Sun continuously at wavelengths not visible from ground based observatories, monitoring changes in different layers of its atmosphere and detecting vast clouds of material ejected into interplanetary space.
SOHO's many breakthroughs include: · The first observations of solar tornadoes and small Earth-sized explosions known as blinkers. · The first observations of ripples caused by shock waves spreading out from solar flares. · Detailed observations and advance warning of coronal mass ejections, vast clouds of gas containing up to 1 billion tonnes of material which are regularly blasted into space - sometimes towards the Earth. · Three dimensional views which reveal the depth and velocity of the hot gas (plasma) flows beneath the Sun's churning surface. · Detailed views of the so-called 'magnetic carpet' which shows the solar surface as a patchwork of positively and negatively charged gas. · Numerous sungrazing comets which pass too close and are pulled into the solar furnace. · Shadows on the night sky caused by solar flares and comet tails.
SOHO is one of several satellites which are revolutionising our understanding of why the temperature of the corona exceeds 1 million degrees Celsius when the temperature of the Sun's surface is a mere 6000C.
UK scientists are also using data from the American TRACE (TRANSITION REGION AND CORONAL EXPLORER) spacecraft to look in unprecedented detail at the fine structure on the Sun's surface. For example, Clare Parnell of St. Andrews University has recorded millions of small magnetic explosions occurring every hour on the Sun. Despite their relatively small size (about 640 km across), each of these events releases roughly the same amount of energy as a nuclear bomb and may help to explain the heating mechanism for the multi-million degree corona.
Valery Nakariakov and Bernard Roberts, also from St. Andrews, have used TRACE to observe waves or oscillations in magnetic loops of the Sun's coronal atmosphere. This new science, known as coronal seismology, allows scientists to determine the properties of the Sun's atmosphere from measurement of these oscillations. It may also provide more clues to help solve the perplexing puzzle of coronal heating.
The Japanese YOHKOH is also shedding new light on the processes taking place in the Sun's turbulent atmosphere. YOHKOH carries instruments to study solar flares and the hottest coronal gases at x-ray and gamma-ray wavelengths. The lead institute for YOHKOH research is the MSSL in Surrey, which is also the site of the UK YOHKOH Data Archive Centre.
YOHKOH has discovered a "dimming" in the corona which is related to coronal mass ejections (CMEs). Large patches of the corona show marked reductions in X-ray intensity following the ejection of vast amounts of solar material. YOHKOH observations also show that "sigmoids" or S-shaped magnetic structures in the corona are frequently associated with the onset of CMEs. This discovery may allow CMEs to be forecast and perhaps allow prediction of the arrival of dangerous magnetic clouds at the Earth.
Although the highly successful YOHKOH is still operating after eight years in space, a more capable replacement is already on the drawing board. The Japanese SOLAR-B spacecraft is scheduled for launch in February 2004. Under the leadership of Leonard Culhane, MSSL is providing one of its three major instruments, an Extreme Ultraviolet Imaging Spectrometer.
SOLAR-B will study the interaction between the Sun's magnetic field and its corona in an effort to improve our understanding of why solar magnetism varies, how this variability influences the total solar output and creates the driving force behind space weather.
THE SUN-EARTH CONNECTION.
One unique investigation of the solar wind is being undertaken by a joint ESA-NASA mission known as ULYSSES as it travels in a giant loop which carries it out over the Sun's polar regions and out to the orbit of Jupiter, nearly 800 million km away. Launched in 1990, ULYSSES is the only spacecraft ever to have travelled over the poles of the Sun.
During its highly successful exploration during a quiet period of solar activity in 1995, scientists discovered that the solar wind blew much faster from holes in the corona located near the solar poles. These holes are visible as 'empty' patches in the corona during an eclipse. ULYSSES is now returning for a second look at the Sun during its period of peak activity in 2000-2001.
Andre Balogh of Imperial College is principal investigator on one of the instruments which studies the interplanetary magnetic field embedded in the solar wind. The universities of Birmingham and Kent are also involved in the ULYSSES mission.
Closer to home, the POLAR spacecraft is one of two NASA satellites which are participating in the International Solar Terrestrial Physics Programme. POLAR's primary objective is to find out how much of the plasma in the Earth's magnetosphere comes from the solar wind and how much is drawn up from the Earth's upper atmosphere.
From a highly elliptical polar orbit, the spacecraft carries nine instruments to measure particles, magnetic and electric fields, and three instruments to make pictures of the aurorae (Northern and Southern Lights) in the visible, ultraviolet and X-ray spectral regions. UK groups including MSSL and RAL participate in three of the nine investigations.
Further breakthroughs in our knowledge of the Sun-Earth connection will come with the launch of four CLUSTER II spacecraft in mid-2000. This unique European Space Agency concept involves formation flying by four spacecraft. By obtaining the first three dimensional observations of near-Earth space, they will reveal in intricate detail how the solar wind interacts with our planet's magnetic field.
The UK is heavily involved in the CLUSTER II project. Andrew Fazakerley of MSSL is the principal investigator for PEACE (Plasma Electron and Current Experiment) instrument, while Andre Balogh at Imperial College is principal investigator for the dual magnetometer instrument and Hugo Alleyne at Sheffield University heads the Digital Wave Processing experiment. Other groups looking forward to participation in CLUSTER II include RAL, British Antarctic Survey, Queen Mary & Westfield College, London and the Universities of Sussex and Warwick.
SOHO - Dr. Richard Harrison, RAL, Chilton, Didcot, OXON OX11 0QX.
SOLAR-B and YOHKOH - Professor Leonard Culhane, MSSL (University College London), Holmbury St. Mary, Dorking, Surrey RH5 6NT.
TRACE - Professor Bernard Roberts, University of St. Andrews, Fyfe, KY16 4SS.
ULYSSES - Professor Andre Balogh, The Blackett Laboratory, Imperial College, London SW7 2BZ.
POLAR - Dr. Manuel Grande, RAL, Chilton, Didcot, OXON OX11 0QX.
CLUSTER II - Professor Andre Balogh (see above)
IMAGES AND FURTHER INFORMATION:
Further information on the UK solar missions can be found at:
The latest SOHO images and movies of the Sun can be found on the Worldwide Web at:
Spectacular solar images from TRACE can be found at:
Peter Bond, RAS Press Officer (Space Science).
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