ESA

Paris, 24 April 1998

Surprises from SOHO include tornadoes on the Sun

The Sun has tall gyrating storms far larger and faster than tornadoes on the Earth. This unexpected finding is among the latest results from the solar spacecraft SOHO, to be announced at a European Space Agency press briefing on 28 April. British scientists discovered the solar tornadoes in images and data from SOHO's scanning spectrometer CDS. So far they have detected a dozen such events. They occur most frequently near the north and south poles of the Sun and are almost as wide as the Earth.

Steady windspeeds of 15 kilometres per second and gusts ten times faster (which means 500,000 kilometres per hour) occur in the solar tornadoes. For comparison, tornadoes on the Earth blow at 400-500 kilometres per hour. The solar measurements are made by the Doppler effect -- the same principle as that used by police radars to detect speeding motorists. The observed wavelength of emission from hot oxygen atoms changes according to whether the gas is moving towards the detector or away from it, and the CDS instrument is very sensitive to these variations.

One of SOHO's main tasks is to trace the sources of the wind from the Sun that pervades the Solar System. Gusts and shocks in the solar wind buffet the Earth's environment, causing auroras and magnetic storms and endangering satellites and power supplies. The newly discovered tornadoes may contribute to the solar wind, especially to a fast windstream that emanates from relatively cool parts of the solar atmosphere called coronal holes.

"We see the hot gas in the tornadoes spiralling away from the Sun and gathering speed," says David Pike of the Rutherford Appleton Laboratory, UK, who is co-discoverer of the solar tornadoes with Helen Mason of Cambridge University. "These spectacular events in the Sun's atmosphere must have widespread effects. Our next step will be to try to relate the solar tornadoes to observations of the fast solar wind farther out in space, as seen by other instruments in SOHO."

Built in Europe for the European Space Agency, SOHO carries twelve sets of instruments provided by European and American investigators, and it was dispatched into space on 2 December 1995 by a NASA launcher. SOHO is a project of international cooperation between ESA and NASA.

Other news items concerning SOHO, included in what follows, are:

The Sunspot Bug

SOHO's scientists gathered at the Rutherford Appleton Laboratory this week, on the second anniversary of full scientific operations which began in April 1996. They are celebrating the decision by ESA and NASA to extend their mission to 2003. This means that SOHO, having observed the Sun in its quietest state in 1996, will also see it at its most tumultuous, when the count of dark sunspots on the Sun's face rises to a maximum around the year 2000.

During the last sunspot maximum, in 1989-91, solar storms caused power failures in Canada and Sweden and destroyed or damaged several satellites. Some computers crashed as a result of impacts by solar particles. Since then the human species has become more dependent upon satellites and computers, and advanced microchips are more vulnerable to the Sun's electromagnetic effects and particles. To the Millennium Bug, a problem involving software in the transition to the year 2000, one must add the physical threat of the Sunspot Bug.

SOHO is the world's chief watchdog for the Sun. From a special vantage point 1.5 million kilometres out in space, where the Sun never sets, the spacecraft observes solar activity for 24 hours a day. Its images go to the regional warning centres of the International Space Environment Service, which alert engineers responsible for power systems, spacecraft and other technological systems to impending effects on the Earth's environment.

Western Europe is served from Paris-Meudon where forecasters obtain, every day, SOHO's images of the whole Sun at four wavelengths from the extreme ultraviolet imaging telescope EIT. Developed and operated by a French-led consortium, EIT is like a weather satellite for the Sun. Its images reveal the scenes of intense activity in the Sun's atmosphere that can trigger solar flares and mass ejections.

SOHO's visible-light coronagraph LASCO, provided by a US-led team, demonstrated in April 1997 its special ability to spot a mass ejection heading towards the Earth, where it caused a mild storm. CELIAS (currently under Swiss leadership) is the solar-wind instrument on SOHO, which confirms the arrival of a mass ejection 30-60 minutes before it reaches the Earth. By measuring the speed and density of the material ejected from the Sun, CELIAS gives a clear warning of the likely severity of the storm.

As the solar storms increase in ferocity and frequency in the next year or two, so will SOHO's importance in this regard. It is the flagship of a multinational fleet of spacecraft monitoring the Sun and its effects. Other members of the fleet include the ESA-NASA Ulysses solar-polar spacecraft and the forthcoming ESA-NASA flotilla of four satellites, Cluster II, to observe and interpret effects in the Earth's vicinity. In all of this work, the monitoring of the Sun goes hand-in-hand with fundamental research and discovery.

"The scientific surprises announced today illustrate SOHO's special role," says Roger Bonnet, ESA's director of science. "To make sense of the Sun, and if possible to forecast the storms and long-terms changes which affect our technology and our weather on the Earth, are urgent tasks for space research. While SOHO provides early warnings of solar outbursts, it also looks for unknown and basic features of the Sun that may make forecasting better. In SOHO the distinction between "useful" and "fundamental" science is abolished."

The Sun's super-hot and dynamic atmosphere

SOHO has recently solved part of a long-standing mystery about the Sun, according to Eric Priest of St Andrews University, UK, who reviewed its achievements in fundamental studies of the Sun's atmosphere at today's press briefing. For more than half a century, scientists have known that the atmosphere reaches temperatures of millions of degrees C, compared with less than 6000 degrees at the Sun's visible surface. How does the atmosphere become so astoundingly hot?

"SOHO casts doubt on one leading theory of the atmospheric heating, and confirms another," says Priest. "We have checked the idea that magnetic waves might be responsible. We have been able to observe for the first time some of the proposed waves, but they fade out before they reach the hottest part of the atmosphere. On the other hand we now have plenty of evidence that at least part of the heating comes from a clash of magnetic field lines. They tangle like spaghetti in the solar atmosphere and reconnect, causing thousands of explosions every day that release energy into the atmosphere. The beauty and elegance of this theory is that it explains a wide variety of different phenomena in a natural way."

The evidence includes an ever-changing carpet of outgoing and ingoing magnetic fields on the Sun's visible surface. US scientists using SOHO's MDI/SOI instrument have discovered that the pattern of the magnetic carpet changes completely every 40 hours. That implies a continual rearrangement of loops in the atmosphere, by magnetic reconnections. The resulting explosions appear as jets of gas detected by SOHO's SUMER instrument, and as bright spots called blinkers in the CDS instrument.

The newly-found solar tornadoes also figured in Priest's review of SOHO's achievements. They reveal just how dynamic the atmosphere is, and may well be an important cause of the solar wind. One of SOHO's specified tasks is to trace the origins of the solar wind and the accelerators that drive it out into the Solar System in all directions.

The coronal holes from which the fast solar wind emanates are concentrated towards the Sun's poles. In these regions the local magnetic field creates no barrier to hot gas leaving the Sun and attaining a windspeed of 750 kilometres per second. By contrast, the magnetically congested equatorial zone is the source of a relatively slow and variable solar wind of around 400 kilometres per second.

In an early result from SOHO, members of the US-led team for the ultraviolet coronagraph UVCS detected a remarkable acceleration of oxygen atoms leaving the Sun from the relatively cool coronal holes, the source of the fast wind. The team suggests twisting magnetic waves as the driver. These are possibly related to the tornadoes.

As for the slow wind, the visible-light coronagraph LASCO observes many large and small mass ejections leaving the Sun, propelled by major and minor explosions. Several mass ejections can occur almost simultaneously at widely scattered places in the Sun's equatorial zone. LASCO team members in the UK consider that the mass ejections contribute an important fraction of the slow wind, and that the material can undergo acceleration at a wide range of distances from the Sun.

Puzzles of the Sun's interior

While SOHO's MDI/SOI routinely provides the best-ever charts of the Sun's magnetism, its main task is to look inside the Sun, by the technique of helioseismology. At a million points all across the Sun's visible face, the instrument measures vertical motions in the surface due to sound waves reverberating through the Sun. Scientists interpret the rhythms to reveal temperatures and motions in the gas of the Sun's interior, in much the same way as seismologists probe the layers of the Earth's interior by analysing earthquake waves.

In an astonishing series of "firsts" the MDI/SOI team charted flows of gas just beneath the Sun's visible surface, and related these to the upwelling and downflows of large convection eddies down to newly-measured depths. The team also detected a slow flow of sub-surface material from the equator to the poles, and carefully measured the different rotation rates around the Sun's north-south axis of gas at different latitudes and depths.

An important change in rotation rates occurs between the turbulent convection zone, below the visible surface, and the more orderly radiation zone deeper inside the Sun. Here scientists hope that they can locate the source of the magnetic field that causes so much agitation at the surface. At present new results from MDI/SOI are appearing in leading scientific journals at a rate of about one a week.

SOHO gives the helioseismologists a space platform unaffected by weather or sunsets, where their instruments can record solar oscillations without interruption for years on end. That is a requirement for the most exact work, where the scientists are looking for small, tell-tale differences between SOHO's observations and the predictions from their theories of how the Sun works. The accumulated data from other helioseismic instruments on SOHO are, after two years, just reaching the stage where new conclusions may be drawn.

The French-led GOLF consortium and the Swiss-led VIRGO team observe the grosser modes of oscillation of the whole Sun. The GOLF instrument detects vertical motions while VIRGO sees rhythmic variations in the Sun's brightness due to the sound waves. The fine details of oscillations detected by MDI/SOI are unrivalled for probing the turbulent outer regions of the Sun. The oscillations recorded by GOLF and VIRGO are most effective for probing deep into the solar interior, right down to the core, where the thermonuclear reactions occur which power the Sun. When scientists combine the results from SOHO's helioseismic instruments, they detect small but important divergences from what their theories predict for the core.

"I've long suspected that the Sun's core may be lopsided," comments Douglas Gough of Cambridge University, UK, in appraising SOHO's helioseismic results. "That could explain why much of the Sun's core appears to be too cool to explain the output of light from the surface. >From the latest compilations of SOHO data we are now getting strong indications that the Sun's core is not quite as theorists thought it should be. We hope that by 2003 we shall have accumulated enough data to settle these profound and important issues."

The rotation rates of the Sun's outer regions, as measured with MDI/SOI, decline steadily, northwards and southwards from the equator, and then show a small but distinct drop in the rotation speed near the poles. This could be due to a braking action of the Sun's magnetic field, and Gough thinks it may be related to the production of the fast solar wind coming from the polar regions.

When the wind from the Sun meets a breeze from the stars

SOHO's solar-wind instrument CELIAS quickly distinguished itself by detecting many heavy chemical elements among the charged atoms in the solar wind, which were previously unrecorded. The CELIAS team has gone on to study differences between the slow and fast solar winds. These appear both in the proportions of various elements, and in the extent of their ionization (loss of electrons). The information feeds back into the theories of how the solar windstreams are accelerated out of the Sun.

But the Sun is not alone in the Universe, and SOHO is bathed in streams of particles coming both from the Sun and from cosmic space, where the Sun cruises through thin interstellar gas. The solar wind blows a huge bubble called the heliosphere, which deflects charged atoms of alien origin. Neutral atoms, on the other hand, penetrate the heliosphere to appear as an interstellar breeze blowing through the Solar System.

The SWAN instrument on SOHO, provided by a French-Finnish team, sees interstellar hydrogen as a glow of ultraviolet light in the whole sky. The glow is brighter from the incoming breeze, upwind of the Sun. Encounters with the solar wind tear the electrons from the hydrogen atoms and stop their ultraviolet emission, so downwind from the Sun SOHO sees a cavity in the glow. The SWAN team has now analysed two years of data to deduce the speed of the interstellar breeze relative to the Sun, and has pinpointed its source direction more accurately than ever before. The breeze blows from the direction of the constellation Ophiuchus, close to Scorpius (in astronomical coordinates, Right Ascension 16h 36 min, declination minus 13 deg. 39 arcmin).

Hydrogen is not the only element in the interstellar breeze, and early in December 1997, SOHO ran into a focused stream of helium gas from the stars. At that time the breeze was coming almost directly from beyond the Sun, as seen by SOHO. The Sun's gravity deflected the helium atoms towards a broad focus straddling the orbits of the Earth and of SOHO. The ultraviolet coronagraph UVCS registered a big increase in helium in a halo around the Sun.

CELIAS detects interstellar atoms that become charged and accelerated in encounters with the solar wind. These "pick-up ions" are swept back towards the edge of the heliosphere, where a permanent shock wave energizes them even more and returns some of them to the inner Solar System. This heliospheric tennis produces particles seen as fake (anomalous) cosmic rays by the COSTEP and ERNE particle detectors on SOHO, supplied by German-led and Finnish-led teams. The anomalous cosmic rays are mixed with true galactic cosmic rays from exploded stars in the Milky Way. COSTEP and ERNE also register energetic particles coming directly from the Sun, especially after major eruptions in the solar atmosphere.

The lastest success for SOHO concerning the interstellar influx is the detection by CELIAS of energetic neutral hydrogen atoms, which reveals baseline play in Nature's heliospheric tennis. The discoverers, Martin Hilchenbach of MPAe, Germany, and Johnny Hsieh of the University of Arizona, explain them as anomalous cosmic rays that escape to the interstellar medium, recover their lost electrons, and make a return trip into the Solar System. These neutrals, like the anomalous cosmic rays, give scientists clues to disturbances near the boundary of the heliosphere, many billions of kilometres away.

"It's been extremely gratifying that a spacecraft designed to look at the Sun can also tell us so much about the local interstellar neighbourhood the Solar System is travelling through," says Antoinette Galvin of the University of New Hampshire, in assessing the successes of the particle and solar-wind instruments on SOHO. "SOHO is sampling the material from which future stars and planets may be formed."

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