Satellites Keep Watch On The Sun

Press, Volume CVIII, Issue 31726, 9 July 1968, Page 17

Satellites Keep Watch On The Sun

Sun worship is as old as mankind. All of the ancient civilisations Babylonians, Romans, Aztecs and Incas—have realised the fundamental life-giving role of the Sun. In this coldest winter for years we are sharply aware of the value of its vitalising rays. Without the Sun this Earth would be a sterile, iceencrusted stone and iron globe swinging aimlessly around the Milky Way. The temperature at the equator would be the same as at the poles: approximately four degrees above absolute zero, or just the coldest freeze imaginable. Knowing of the Sun's vital role is a far cry from understanding it. Of course the Sun provides heat and light But the way by which the Sun creates its vast outflow of energy was unknown until scarcely 30 years ago. In 1938 Hans Bethe proposed the now-famous carboncycle whereby stars convert matter into energy by fusing four hydrogen atoms together to form an atom of helium. This explained the source of solar energy but left a great deal unsaid about the transport of this energy to our Earth. The fusion reactions occur deep in the core of the Sun and the released energy, mainly in the form of gamma rays, fights its way up to near the surface of the ' Sun. The surface layers, stirred violently by the gigantic flux of raw energy, boil and bubble at a temperature of 6000 degrees and spill most of the energy into space in the form of heat and light Solar Particles Although the surface, or photosphere, of the Sun sheds the bulk of the solar energy in a rather small band of the spectrum centred on the visible region, there nevertheless remains an appreciable amount of energy radiated over a much wider range of wave-lengths ranging from gamma rays to radio waves. The Sun also emits a stream of solar particles: The most energetic are called solar cosmic rays and the least active, but more numerous, are referred to as the “solar wind.”

All of these radiations give rise to terrestrial effects of one sort or another as they impinge upon the outer layers of the Earth’s atmosphere. The effect which interests us most and is least well understood is their influence on our weather. Fuller knowledge of solar radiation should result in significant advances in the understanding of meteorology and lead to ultimate control over weather.

Since Explorer I, the Americans have launched over 70 satellites and space-probes carrying instruments for examining the Sun or various solar radiations. Of the 200 or so Soviet Cosmos vehicles, a fair percentage have kept an eye on solar activity. Australia's first satellite, W.R.E.S.A.T. 1, was primarily designed to gather data on solar radiation and its effect on the upper atmosphere. Also there have been hundreds of sounding rocket flights from dozens of countries, including Skylark flights from Woomera, which have carried instruments high beyond the blanketing atmosphere for a glimpse of the Sun as it really is. The most sophisticated sunfollowing satellites are the orbiting solar observatories, of which the United States has launched four and has another almost ready for firing into orbit Since 1960 the Solrad series of satellites has provided a series of long-term solar observations. Unlike the larger 0.5.0.'s the Solrads have all been piggy-back satellites launched in conjunction with other space missions. Whereas the heat and light radiated from the Sun remains fairy constant in intensity, measurements from space indicate large variations in the solar ultra-violet radiation and as observations extend into the X-ray region of the spectrum the variability becomes extreme. In a matter of minutes the X-ray emission from a solar flare may increase a thousand-fold and remain intense for hours with very little visible evidence of activity. Yet this enhanced X-ray intensity causes marked changes in the Earth’s upper atmosphere and may influence weather at lower altitudes. Even when solar activity is I

at its maximum, flares on the sun are usually only visible in a very narrow range of colours. Solar flares in white light are so rare that fewer than 20 are known to have occurred during the past century. White light flares last only a matter of minutes but they are accompanied by a burst of solar X-ray and cosmic ray activity which could be lethal to an unshielded astronaut. Origin of Rays The normal X-ray activity of the Sun is one of mild fluctuation which continues throughout the periods between the major flare bursts. These X-rays originate in the solar atmosphere many thousands of miles above the photosphere. Solar microwave radiation originates from the same region, which accounts for the discovery by the 0.5.0. and Solrad satellites of a close association between these two diverse types of radiation. This result is in accord with modern theories of the solar atmosphere and corona. The million degree temperatures in the lower corona generate the X-rays by purely thermal processes whereas the sporadic X-rays emitted whenever a flare occurs are produced by a process very similar to that in an X-ray tube of the type used for radiography in medicine and industry. They are called “characteristic X-rays” because their wavelengths reveal the kind of atom in which they originate. Another region of the spectrum which is being intensively studied is the ultraviolet. Last year, for the first time, a satellite transmitted back to Earth a picture showing the appearance of the sun in the ultra-violet light emitted by magnesium at temperatures in excess of a million degrees. Gradually a more complete understanding of solar processes is emerging, and as the complex interactions between the Sun and Earth become clearer we may hope to discover such things as why this winter is colder than usual and why that tropical storm went off course and headed southwards to give New Zealand an unforgettable battering.