Mysterious clouds of gas falling towards the Sun have been spotted with the Solar and Heliospheric Observatory (SOHO) spacecraft. They go against the fast-moving streams of gas that pour out continuously into space in the solar wind.
In today's issue of Astrophysical Journal Letters, the scientists who found them suggest that the inflows are due to frequent local adjustments to the Sun's magnetic field. The discovery promises a better understanding of the sources of the solar magnetism that envelops the Earth, quarrels with our own planet's field, and to some extent protects us from cosmic rays coming from the stars.
For many years astronomers have watched glowing fountains and arches rise and fall in the Sun's lower atmosphere. The gas clouds now seen begin their descent from far out in the atmosphere. They were first noticed in 1997, in images from the Large Angle and Spectrometric Coronagraph (LASCO) instrument on SOHO. Similar inflows then turned up in re-examined images going back to early in 1996, soon after SOHO's launch, and many others have occurred since.
About 8,000 inflow events have now been logged -- most of them since 1998 while the Sun has been at its most active, as judged by the high count of sunspots. The inflows can start at an altitude of up to 1,700,000 miles (2,700,000 kilometres) above the visible surface, a distance equal to twice the Sun's diameter. Here the accelerating solar wind, leaving the Sun, has reached a speed of about 75 miles per second (120 kilometres per second). Fighting against it, the gas clouds travel in at 31 to 62 miles per second (50-100 kilometres per second). Typically they appear to come to rest about 435,000 miles (700,000 kilometres) out.
"For decades, we've been able to observe cooler gas rising and then falling, under the influence of gravity, lower down in the solar atmosphere," notes Dr. Joe Gurman, NASA's project scientist for SOHO. "Until now, though, the hotter, electrically charged gas higher in the Sun's outer atmosphere, the corona, has only been seen flowing up and away. Now we have to figure out how these downflows relate to the evolution of the magnetic fields that are responsible for both the normal outflow the solar wind and for the violent processes we call space weather."
The LASCO instrument on board SOHO blots out the intense direct light from the Sun's visible surface, to keep the Sun permanently in eclipse so its much fainter outer atmosphere can be seen. Even with this powerful instrument, the inflows are hard to see. Most useful for the purpose is the LASCO C2 component, which shows the region from 435,000 to 2,175,000 miles (700,000 to 3,500,000 kilometres) from the visible surface.
At the Naval Research Laboratory, Dr. Neil Sheeley and Dr. Yi-Ming Wang find the inflows by subtracting one electronic image from the next, taken 20-25 minutes later, and assembling a series of such difference images into a movie. The human eye is then good at spotting unusual, inward movement against the general background of outflowing gas.
Although the gas feels a very strong pull from the Sun's gravity, this is not the decisive force acting on the inflows. The high rate at which they gather speed initially, and their eventual slowdown, suggest instead that they are firmly under the control of a magnetic force. A few inflows are a backwash from explosive mass ejections, which are sporadic events, but the overwhelming majority occur quite regularly within regions of slow solar wind.
A downpour of 20 inflows per day, seen on the left side of the Sun, can be followed after a lull of two weeks by a similar downpour seen on the right side. This means that the occurrences persist in a particular region on the Sun, which takes two weeks to move from left to right as the Sun rotates. The regional association can continue for months, and suggests to Sheeley and Wang how the inflow regions and the magnetic field are related.
Near the Sun's surface the magnetic field is a patchwork, with field lines looping outwards from some places and back in at others. The solar wind drags the longest loops far out into space, creating contrasting sectors where the magnetic field lines are directed in opposite ways (polarities). Before the solar wind has
reached its full speed, opposing field lines can short-circuit at a boundary between sectors to form new magnetic loops. These collapse towards the Sun, carrying with them the inflowing gas clouds now observed.
"We are seeing something opposite to what we expected," says Sheeley. "Normally, when this happens, we initially doubt the observation -- suspecting, for example, that the movie is running backwards. But when we confirm that the observation is really correct, we are forced to change our way of thinking. Such mind-changing discoveries help us past temporary snags in our understanding and inevitably lead to progress."
In this case, one area of progress may concern magnetic fields and their effects on Earth. If Sheeley and Wang are right in thinking that the inflows indicate collapsing magnetic loops, pulling material inwards against the outward drag of the solar wind, then these are showing us how the Sun recycles the magnetic field in its atmosphere. This recycling process regulates the strength of the interplanetary magnetic field that extends outward past Earth and affects space weather.
SOHO's inflows thus provide unexpected clues to what influences solar magnetic activity near the Earth and all across the Solar System. Apart from magnetic storms, which can harm technical systems, there is growing interest in the shield that the interplanetary magnetic field provides against cosmic rays. These energetic particles from the Galaxy can cause genetic mutations in living things and glitches in computers, and some scientists think they are also involved in cloud formation on the Earth. But their variations have puzzling features.
Reductions and increases in cosmic rays reaching the Earth are not in perfect step with rising and falling sunspot activity. And when sunspot cycles were of longer duration, 100 years ago, the interplanetary magnetic field was weaker and the cosmic-ray intensity was on average higher than nowadays. The 'mind-changing' discovery of the inflows near the Sun may lead to better explanations of these variations, and eventually to predictions of cosmic-ray intensities.
SOHO is a project of international cooperation between the European Space Agency and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA. NASA launched SOHO in December 1995, and in 1998 ESA and NASA decided to extend its highly successful operations until 2003.
A multinational scientific team that includes the US Naval Research Laboratory, France's Laboratoire d'Astronomie Spatiale, Germany's Max Planck-Institut für Aeronomie, and the UK's Birmingham University, contributed LASCO to SOHO.
LASCO is already famous for registering explosive mass ejections from the Sun, and discovering many sungrazing comets.
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