SOHO reveals how sunspots take a stranglehold on the Sun
5 November 2001A sunspot turns out to be a kind of whirlpool, where hot gas near the Suns surface converges and dives into the interior at speeds of up to 4000 kilometres per hour. This is the latest discovery by the ESA-NASA SOHO spacecraft.
Solar physicists have long known that intense magnetic fields in sunspots strangle the normal up-flow of energy from the interior, leaving the sunspot cooler and therefore darker than its surroundings. The converging flows of gas around a spot, found by SOHO, explain why the magnetic fields become concentrated, and how a sunspot can persist for days or weeks.
Bernhard Fleck, ESA's project scientist for SOHO, comments, "The origin and stability of sunspots has been one of the long-standing mysteries in solar physics. I am delighted to see that with SOHO we are beginning to crack also this problem."
The gas flows around and beneath a sunspot have been detected by a team of scientists in the USA, using the Michelson Doppler Imager (MDI) on SOHO. The instrument explores the solar interior by detecting natural sound waves at a million points on the Suns surface.
"After many years of contradictory theories about sunspots, MDI on SOHO is at last telling us what really happens," comments Junwei Zhao of Stanford University, California, lead author of a report published in the Astrophysical Journal. Inflows and down-flows similar to those now detected with SOHO were envisaged in 1974 by Friedrich Meyer of Germanys Max-Planck-Institut f|r Physik und Astrophysik, and his colleagues. A similar expectation figured in a theory of sunspots advanced in 1979 by Eugene Parker of Chicago. "Our observation seems to provide strong evidence for both predictions," Zhao says.
Sunspots have fascinated scientists since Galileos time, 400 years ago, when they shattered a belief that the Sun was divinely free of any blemish. As symptoms of intense magnetic activity, sunspots are often associated with solar flares and mass ejections that affect space weather and the Earth itself. The Suns activity peaks roughly every 11 years, and the latest maximum in the sunspot count occurred in 2000.
Even with huge advances in helioseismology, which deduces layers and flows inside the Sun by analysis of sound waves that travel through it and agitate the surface, seeing behind the scenes in sunspots was never going to be easy. The MDI team refined a method of measuring the travel time of sound waves, invented in 1993 by Thomas Duvall of NASA Goddard, called solar tomography. It is like deducing what obstacles cross-country runners have faced, just by seeing in what order the contestants arrive at the finish. Here the runners are packets of sound waves, and the obstacles are local variations in temperature, magnetic fields and gas flows beneath the Suns surface.
"We needed better mathematical tricks," comments Duvall. "So we put together ideas from classical and quantum physics, and also from a recent advance in seismology on the Earth."
In an earlier application of solar tomography, the team examined in detail the ante-natal events for an important group of sunspots born on 12 January 1998. They found sound waves beginning to travel faster and faster through the region where sunspots were about to form. Less than half a day elapsed between signs of unusual magnetic activity in the Suns interior and the appearance of the dark spots on a previously unblemished surface.
"Sunspots form when intense magnetic fields break through the visible surface," says Alexander Kosovichev of Stanford. ""We could see the magnetic field shooting upwards like a fountain, faster than we expected."
Even late on the previous day there was little hint of anything afoot, either at the surface or in the interior. By midnight (Universal Time) a region of strong magnetic field had risen from a depth of 18000 kilometres and was already half way to the surface, travelling at 4500 kilometres per hour. Sound speeds were increasing above the perturbed zone. By 8:00 a.m. an intense, rope-like magnetic field was in possession of a column of gas 20000 kilometres wide and reaching almost to the visible surface. In the uppermost layer beneath the surface, the magnetic rope divided itself into strands that made the individual sunspots of the group.
Under a large, well-established sunspot, in June 1998, the sound waves revealed a persistent column of hot, magnetized gas rising from deep in the interior. At a depth of 4000 kilometres it spread fingers towards neighbouring parts of the surface where it sustained some smaller sunspots. The magnetic column was not connected to another nearby spot where the magnetic field went in the opposite direction. Immediately below the large spot was a cushion of cooler, less intensely magnetized gas.
A closer look at the gas flows, during the development of that June 1998 sunspot, led to the further findings now reported. The inflows and down-flows in the immediate vicinity of the sunspot reach downwards for only a few thousand kilometres from the surface, which means less than one per cent of the distance to the Suns centre. The discovery therefore depended on MDIs unique ability to explore just below the surface.
The whirlpool of gas is responsible for the persistence of a sunspot. The cooling due to the magnetic field of the sunspot provokes the down-flow, and the gas disappearing downwards is replaced by more gas flowing inwards towards the spot. It brings with it its own associated magnetic field and prevents the strong magnetic field of the sunspot from dissipating. So the cooling and down-flow continue, and the process is self-sustaining.
The down-flow of gas may also help to explain the puzzling fact that the Sun is actually brighter when it is freckled with dark spots. The VIRGO instrument on SOHO, operated by a Swiss-led team, confirmed the observations of earlier solar spacecraft, showing that sunshine is slightly more intense at sunspot maximum. Douglas Gough of Cambridge University, a leading solar theorist, notes that the down-flow of gas seen by MDI on SOHO can redistribute energy bottled up by a sunspot.
"What is interesting from the physical point of view is that, being cool, the descending flow is readily able to extract the heat that accumulates beneath the spot," Gough says. "It then spreads the heat away from the sunspot and eventually brings it to the surface of the Sun far from the spot, from where it is radiated into space."
SOHO is a project is a project of international cooperation between ESA 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.
Note on scientific papers
Investigation of Mass Flows Beneath a Sunspot by Time-Distance Helioseismology by Junwei Zhao and Alexander G. Kosovichev (Stanford) and Thomas L. Duvall Jr. (NASA Goddard) is published in Astrophysical Journal, vol. 557, p. 384, 2001. A related paper, Time-Distance Inversion Methods and Results by Kosovichev, Duvall and Philip H. Scherrer (Stanford) appeared in Solar Physics, vol. 192, p. 159, 2000.
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