Brilliant flares the most dramatic of solar activity

Press, 9 January 1987, Page 22

Brilliant flares the most dramatic of solar activity

The brilliant flares are the most dramatic of all solar activity. They suddenly shine forth in a dazzling burst of light, fading away within a few hours.

Flares usually occur during times of maxima sunspot activity, appearing in the chromosphere above very active sunspot groups. They emit streams of particles that have marked effects on Earth causing auroral displays and radio blackouts. They are due to bursts of magnetic energy over active sunspot groups. The chromosphere is the lower part of the solar atmosphere, stretching 16,000 kilometres above the photosphere. Its temperature increases rapidly with height, and, at its outer boundary with the solar corona, it reaches one million degrees. Chromosphere means “sphere of colour,” an apt description for the varied and beautiful phenomena occurring in this region. It becomes visible during a total solar eclipse as a narrow, reddish band encircling the sun. The colour is caused by light emitted by hydrogen atoms at a particular wavelength, known as the hydrogen alpha line. The chromosphere is homogenous to a height of 3000 kilometres. Above this it has the appearance of a burning forest because spicules darting up from the photosphere eject material into the outer parts of the chromosphere. This material consists of jets of hot gas at temperatures of about 10,000 deg. It is estimated that there are 500,000 Spidles on the Sun at anjt

one time. They are aligned along the magnetic fields of huge cells in the chromosphere, each of which exceeds 30,000 km across.

Spicules change continuously as each jet lasts for less than five minutes. Giant spicules have been detected from space at ultra-violet wave lengths. These appear to have a longer duration, lasting for about half an hour and reaching 35,000 km in length.

Prominences are another feature of the chromosphere. These are huge clouds of mainly hydrogen gas that stretch from the photosphere to the corona. They take one of two forms, quiescent or eruptive. The former are long-lived, lasting for several months and have lengths of several thousand kilometres. They seem to be supported by magnetic fields in the corona. About one third are associated with sunspot groups on which their gas streams down.

Eruptive prominences, by contrast, are forever changing and last no longer than a few hours. They are associated with flares. They take one of two forms. Following a flare they often take what is termed a surge prominence because they appear to be material ejected by a flare at speeds of 1000 km per second at temperatures round 30,000 deg. The second form is that of huge loops and are therefore termed loop prominences. These consist of material falling back on to the sun after a flare. Prominences can be

seen during a total solar eclipse, appearing as large, red tongues of flame arching over the solar surface.

The outer atmosphere of the Sun is the corona, seen during a total eclipse as a pearly-white crown around the Sun. Its boundary with the chromosphere is 16,000 km above the solar surface, while it has no definite outer boundary. It just gradually thins out into space and is detected being carried past Earth by the solar wind. The corona varies in shape according to the amount of solar activity, so that when this is at a minimum the corona appears as huge streamers centred on the equator. The shape becomes more or less circular near solar maxima with streamers in all directions. Between these two extremes the corona has shapes that are intermediate between that seen at maxima or minima solar activity. . There are two parts of the corona. The bright inner portion is called the K corona and is caused by sunlight scattered by electrons. The outer regions form the F corona and is a result of sunlight scattered by dust particles. The gases of the corona are probably heated by shock waves from the Sun to a temperature of about two million degrees.

A continuous stream of protons and electrons flows outwards from the Sun in the form of the solar wind. It is forced away from the Sun in all directions by energy from the photosphere. The rate at which the particles depends on the

amount of solar activity. The average speed is about 400 km a second, but it can attain speeds of 800 km a second when the Sun is very active.

The solar wind affects Earth where the Van Allen belts trap the particles. These in turn affect the magnetic fields of Earth and when there is much solar activity it causes magnetic storms and other phenomena. The solar wind also affects comets accelerating knots in their tails under the pressure of the wind and forcing the straight tails to point directly away from the Sun. These are only some of the forms of solar activity. The study of the Sun not only has many applications to our understanding of Earth but also provides many insights into the nature of the billions of stars similar to the Sun.

My reference last month to the newly discovered nova in Centaurus has aroused a great deal of interest among readers. This was the brightest nova discovered for 11 years. It reached its maximum brightness on November 25 at magnitude 4.7 after having risen very slowly from magnitude .18. It faded one magnitude over the 30 days since maximum arid will probably continue to decline at this rate until it reaches magnitude 8. Then it will pass through a transition stage during which it will oscilate with several peaks.

Later it will continue to fade slowly as it appears to be a slow nova, resembling in many the V

brilliant nova of 1925, known as RR Pictoris, which is still visible at magnitude 12. Already I have received over 100 observations of Nova Centauri and it is likely to be one of the best observed novae.

Venus reaches its greatest elongation west of the Sun on January 16. It will be a brilliant object, magnitude -4.5, in the morning sky, rising about three hours before the Sun throughout January. Jupiter, in Aquarius, will be visible in the early evening, setting at 12.40 a.m. on January 1 and at 10.40 p.m. on January 31, by which time it will be low in the western sky as twilight ends. Mars, in Pisces, will be slightly higher in the sky than Jupiter and more to the north-west. It is now so far away and its disk so small that it will not be of much interest.

Saturn, in Ophiuchus, rises about 4.30 a.m. on January 1 and at 2.30 a.m. on January 31. The remaining bright planet, Mercury, is too close to the Sun' to be visible. It reaches superior conjunction on January 13. • The Moon will provide much of interest to telescopic viewers as on the night of January 10-11 when it passes through the well-known star cluster, the Pleiades. In the space of two hours from 11.50 p.m. some stars will be occulted. On January 12 at 9.43 p.m. a bright star, magnitude 1.8, will disappear behind the Moon to reappear again at 11.11 p.m. when this occultation ends. All times are Daylight (summer) Time.