When space-shuttle astronauts repaired Solar Max last April, the solar-flare-observing satellite was returned to full service just in time for the big one. The largest flare since 1978 erupted on April 24.

This heralded the second most intense solar activity ever recorded. It was as though the Sun itself were celebrating the rejuvenation of the first observatory that was adequately equipped to enable scientists to appreciate fully what was going on.

Thus the second phase of the Solar Maximum Mission (SMM) - a phase not in the original plan - literally started with a bang.

Launched Feb. 14, 1980, the SMM satellite was to have spent the next two years observing violent solar outbursts, known as flares. Solar physicists have gained a general knowledge of flare activity from observations made over many decades. But they still don't understand what flares are, what role they play on the Sun, or what causes them.

Now, for the first time, they had an observatory that could give them the detailed information they needed.

And Solar Max did give them this information for nine months.

Then its attitude control system blew its fuses. Instruments that needed to be aimed with precision could no longer be properly oriented. Also, the electronics on two instruments malfunctioned.

Now the satellite is operational again. Its pointing precision is restored. And all but one of its seven instruments are fully serviceable. Only one X-ray sensor - the hard (high energy) X-ray imaging spectrometer - is not working well. This had malfunctioned in 1980 and was not on the astronauts' repair list. SMM scientists are not especially concerned about the loss, according to the US National Aeronautics and Space Administration (NASA).

Although it was not planned that way, SMM project scientist Bruce Woodgate says the 31/2-year hiatus has been a boon. The present roughly 11-year cycle of solar activity was at its peak in 1980. Now the cycle is waning and Solar Max is unexpectedly available to study a quieter Sun.

Also, extensive analysis of the earlier Solar Max data has given the research team insight on how best to use their observatory. ''The analysis that we did between 1980 and 1984 has enabled us to sharpen some of our questions,'' Dr. Woodgate says.

Three major research areas beckon:

* Solar flares remain the satellite's principal target. Now scientists can study them during the declining phase of a solar cycle and see how and to what extent they change character.

* Solar Max has found that sunspots dim the Sun's total energy flux by as much as 0.3 percent. Also, the satellite has detected a general dimming of the Sun by 0.1 percent since 1980.

Such changes in the sunshine reaching Earth could affect weather and climate. Now scientists can continue to monitor these important variations.

* Finally, as an unanticipated bonus, Solar Max should be able to observe Halley's comet. Indeed, it will be the best placed and best equipped observing platform for studying the comet's tail as it rounds the Sun in early 1986.

Viewed in galactic perspective, the Sun is an unpretentious, average star. But for Earth and its life forms, it's the most important star in the universe. It has given our planet light and heat for some 5 billion years and should do so for 5 billion more.

By Earth standards, the Sun is gigantic, with a radius 108 times that of Earth and a mass 328,000 times that of Earth and its Moon combined.

According to present theory, half of that mass is concentrated within only 1. 5 percent of the Sun's volume. This is the core, where 99 percent of the solar energy is generated by the fusion of hydrogen atoms. Energy streams out from this core to set up a roiling convection in the Sun's outer layers. Magnetic fields, tangled by this convection and twisted by the Sun's rotation, underlie the sunspots and solar outbursts that are the objects of Solar Max observations.

The solar disk we see is called the photosphere. This is the Sun's true surface to the extent that a giant gas ball can be said to have a surface. With an average temperature of 6,000 degrees Celsius, it is cool compared with the 15 -million-degree fusion reactor beneath it in the core or the equally hot outer solar atmosphere above it.

Between this outer atmosphere, called the corona, and the photosphere, is the chromosphere with temperatures of tens of thousands of degrees and a thin transition region where the temperature climbs to hundreds of thousands of degrees and which merges into the corona.

One of the main puzzles of solar physics is to find the energy mechanisms that maintain the hot chromosphere and corona. Not only are these regions hot, they produce powerful flares and giant explosions that hurl millions of tons of solar material far into space. Yet they are bounded below by the relatively cool photosphere and above by the cold of interplanetary space.

Moreover, virtually all of the Sun's energy flux passes right on through them unabsorbed.

The action of magnetic forces or powerful shock waves has been suspected. But to find the answer, scientists need the kind of detailed observations that have been impossible before Solar Max.

Different aspects of the Sun's outer layers and their activity are best studied by radiation of wavelengths characteristic of the temperatures that typify the phenomena observed.

Thus flare activity at temperatures of millions of degrees may best be revealed by energetic X-rays. Cooler phenomena at the photoshere can be studied with visible or infrared radiation.

Solar Max has instruments that cover a spectrum which, in terms of temperature, ranges from around a thousand degrees in the infrared to a billion degrees or more for gamma rays.

This ability to make detailed observations simultaneously over a wide range of wavelengths gives Solar Max its unprecedented power.

It enables scientists to get a three-dimensional view of all important aspects of events in the Sun's outer regions.

Moreover, Solar Max does not operate in isolation. Much of its work is done in concert with ground-based observatories around the world and with rocket or other satellite observations. Hundreds of scientists in something like 17 countries have joined the US in the SMM project. This kind of cooperation is rapidly building a new understanding of solar phenomena, Dr. Woodgate notes.

Speeding particles and magnetic fields ejected from the Sun, along with X-rays and other intense flare radiation, can affect Earth's own magnetic field and outer atmosphere.

They set spectacular aurora aglow, disrupt some radio communications, and sometimes send damaging electric currents surging through power lines. Also, energetic protons can endanger astronauts and sensitive electronic equipment on board satellites orbiting Earth.

Thus the study of particle acceleration, magneto-hydrodynamics, and other esoteric physics of solar flares has a practical aim beyond gaining scientific knowledge. If the action of the flares can be better understood, they can be better predicted and timely protective action can be taken at Earth.

This sense of a practical need to know coupled with the desire for scientific understanding also inspires the studies made by the one SMM instrument not dedicated to flares. This is ACRIM (Active Cavity Radiometer Irradiance Monitor) which measures the total energy flux from the Sun.

This instrument with a complex name is essentially a black cone inside a container. It absorbs virtually all visible light plus other radiations (chiefly infrared and ultraviolet) that account for almost all of the energy flux. It's said to be ''active'' because the cavity temperature is regulated by a heater.

From the view point of weather and climate, this is the most important item of Solar Max equipment. It measures the solar energy flux arriving at Earth - the so-called solar constant, which turns out not to be so constant after all.

Meteorologists suspect that small variations in this energy supply influence both weather and climate.

On average, the Sun is sending us some 1,368 watts per square meter as measured at the Solar Max distance of a few hundred miles out from Earth's surface.

In the 147 years since the French physicist Claude Pouillet first measured this quantity and called it the solar ''constant,'' he and his successors have suspected it may have been misnamed.

But the Solar Max ACRIM is the first instrument that has been able to pin the suspected variability down.

Unlike most other Solar Max instruments, ACRIM does not need highly accurate orientation. It has collected data continuously since being turned on in 1980. Every two minutes, it measures the energy flux from the Sun with a precision of about 0.005 percent. This unprecedented precision and continuity of measurements have demonstrated that major sunspot groups can dim the solar constant by up to 0.3 percent.

It also has picked up a long-term decline in the solar energy flux of about 0 .04 percent a year since measurements began in February 1980.

It is too early to know what these changes in solar energy input actually do to weather and climate.

But Dr. Woodgate notes that they are substantial enough to concern meteorologists. Also, solar physicists need to find out what the long-term decline implies for solar physics. Woodgate says the decline probably is tied to the solar cycle and thus will eventually reverse.

Yet it could be a longer-term variation. And that could be a matter for concern. He explains, ''If it were to carry on for a few decades, this (decline in energy flux) would affect the Earth's climate. In fact, if it went on for 3, 000 years . . . it would shut the Sun off. Well, we know that's not going to happen. . . . But it's very important for climatological reasons and for our understanding of the Sun to continue that observation through the rest of the solar cycle.''

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