Third day brings bonanza of new results
5 October 2000Many new and tantalising results were discussed yesterday, during the third day of the 34th ESLAB symposium on the 3D heliosphere at solar maximum. Here is a selection:
Is the composition of the interstellar cloud through which the heliosphere moves changing? George Gloeckler from the University of Maryland told the symposium that the SWICs instrument on Ulysses has detected a steady increase in the density and temperature of neutral helium in the heliosphere over recent months.
"It's not yet clear that the source of the helium is interstellar. But we've done everything we can to remove sources within the heliosphere and the density's still going up," he said. Matters will become clearer over the next few months as SWICs takes more measurements. But the issue may not be fully resolved until after the solar maximum when a decline in neutral helium would suggest a source within the heliosphere that fluctuates with the solar cycle, but a continued elevation would raise interesting questions about what's going on immediately outside our Sun's sphere of influence.
The SWICs instrument has also been detecting "pick-up" ions coming from a new and unexpected source close to the Sun. Pick-up ions are created when neutral particles become charged after interacting with the solar wind. The pick-up ions in question are distinct from those discovered by Ulysses during its first solar orbit, which come from neutral interstellar gas. The new source includes ionised complex molecules as well as the gas neon, which does not originate outside the heliosphere.
Ingrid Mann from the Max Planck Institute for Aeronomy proposed interplanetary dust as the source. The dust penetrates to within about ten solar radii of the Sun where it becomes impregnated with solar wind ions after millions or even billions of years. When the dust particles eventually collide, they give up their store of ions including some new ones created by chemical interactions between the solar wind ions and molecules within the dust particles.
Ed Roelof from Johns Hopkins University presented some results from Ulysses and the ACE and Voyager spacecraft. Several times during the build up to the present solar maximum, the Voyagers, which are now in the outer reaches of the heliosphere 70AU away from the Sun, have detected a considerable increase in the density of energetic particles. One increase lasted for 75 days and preceded the passage of a major disturbance that originated at the surface of the Sun several months previously.
Roelof suggested that the build up of a particle reservoir was responsible. During high solar activity, material thrown out into the solar wind by the extra activity builds up to form a "dam". If a very large event occurs, the energetic particles thrown out are dammed back to form a "reservoir" of increased density which then proceeds to the edge of the heliosphere.
Another question was whether the event that marks the solar maximum, the reversal of the Sun's magnetic field, has yet occurred. Ground-based observations from Earth and the SOHO spacecraft, which is observing the Sun from near the Earth, suggest that it has. From Ulysses' perspective above high southerly latitudes, however, it hasn't yet. "It's not clear whether the reversal has happened or not. But it looks as though we're close to it," said Andre Balogh from Imperial College, London who is Principal Investigator for the Ulysses magnetometer.
Observations in the run up to solar maximum have revealed a tortuous magnetic field. Balogh recounted his feelings when first seeing the artist's impression of the spiral magnetic field many years ago illustrated above. "I hope it isn't like that," he had said to himself. "But I'm afraid it is!" he told the meeting.
The new discoveries presented throughout the day prompted Lou Lanzerotti from Bell Labs to make a plea for a coordinated and vigorous programme of heliospheric measurements in the next few years over the declining phase of the solar cycle. "We need to understand the global heliosphere which determines the environment around our Earth," he said. With the two Voyager spacecraft in the outer regions of the heliosphere, Cassini at Saturn, ACE and Imp 3 in Earth orbit and Ulysses with its out-of-ecliptic view, there may not be a better opportunity to get a global view of our local environment in space for years to come.