6 October 2000
Meeting concludes on launch anniversary
On 6 October 1990, the Ulysses spacecraft was
launched. Today, ten years later, the international band of scientists
attending the 34th ESLAB symposium are concluding proceedings with their
sense of excitement and eager anticipation undimmed. The meeting has heard
of numerous observations, made over recent months, that are leading to new
insights about the behaviour of the Sun and the heliosphere at solar maximum.
Many speakers have looked forward to further observations over the next four
years as the solar cycle returns to minimum.
Ulysses continues to return results not just about the heliosphere, but also about the
solar system's interstellar environment. This morning, for example, Bruce McKibben from
the University of Chicago, revealed how the COSPIN high energy telescope on Ulysses found
that the composition of cosmic rays originating elsewhere in the galaxy matches the composition
of the building blocks of the solar system. This is a surprise: galactic cosmic rays are much
younger than the solar system, which has undergone more evolution.
The Ulysses mission so far has revealed just how complex the heliosphere is, in particular the
variety of sources of energetic particles within it.
"We always used to think of the Sun as
the only source of particles in the heliosphere. Then we found interstellar particles, and now
it turns out that interplanetary dust is a source of particles, too," said Richard Marsden,
Ulysses project scientist.
Ulysses had a chequered start to its career, having been modified and then delayed for many years.
Peter Wenzel, Head of Solar System Division at ESTEC, however, remarked that this was fortuitous.
"If Ulysses had set off on schedule, its first orbit would have occurred at solar maximum, which
is a complex time. We were lucky enough to encounter solar minimum first, which enabled us to
establish a basic understanding of the Sun, from which we are far better equipped to make sense
of the turmoil at maximum."
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5 October 2000
Third day brings bonanza of new results
Many 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.
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4 October 2000
Ulysses encounters a different solar wind
"It's really exciting how different the solar wind is this time compared with the first orbit,"
David McComas from the Los Alamos National Laboratory told the 34th ESLAB symposium on the
heliosphere this morning. His observation was repeated by many of the speakers: however you
look at the solar wind or corona, there's evidence of far more solar activity now than during Ulysses'
first south polar passage in 1994.
"We're not seeing the fast and slow wind we saw then. Now its intermediate in speed," said McComas.
During the 1994 south polar fly-by, the Sun was at a minimum in its 11- year activity cycle.
Then, the Ulysses spacecraft detected a slow-moving solar wind at low latitudes near the Sun's
equator and a fast wind coming from high latitudes near the pole.
This summer, as the Sun approaches maximum activity, the fast wind has all but disappeared.
"We detected it just once at about 65 degrees south," said McComas. The intermediate-speed wind
could be a mixture of fast and slow winds emanating from the more highly variable solar corona,
he said - or it could be a new type of wind originating from a different source.
The different source idea, however, found little favour with other speakers. By classifying
the solar wind by composition or even temperature, it's possible to distinguish "fast" and
"slow" wind even at solar maximum when both types emanate from all regions of the Sun, said
Rudi von Steiger from the International Space Science Institute, Bern, Switzerland. Fast and
slow wind are composed of the same ions, but in different proportions.
Fast wind originates from coronal holes, which concentrate around the pole at solar minimum,
but appear at all latitudes at maximum. These are cooler areas of the corona where magnetic
fields stretch out away from the Sun. The rest of the Sun's surface is characterised by
closed magnetic loops which re-direct escaping material back to the Sun's surface.
Slow wind is released when these closed field lines are cut, said Len Fisk from the University
of Michigan. He presented theoretical work, which suggested that this can happen when neighbouring
open magnetic fields gradually migrate through closed-loop regions. To thoroughly test this idea,
however, requires a more sophisticated spacecraft than Ulysses or its companions, SOHO and Cluster.
The proposed Solar Orbiter, now undergoing studies at ESA, could do the job: it would have the
ability to distinguish sufficiently small areas on the Sun's surface and monitor their progress.
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3 October 2000
Latest solar maximum findings to be presented at symposium
If the Voyager spacecraft are ever to cross the heliosphere’s boundary, they will probably
do so within the next year or two before the effects of this year’s solar maximum cause the
heliosphere to expand. Ed Stone from the Jet Propulsion Laboratory, California, told an international
gathering of solar scientists yesterday afternoon, that the two spacecraft are now probably closer to
the boundary,which estimates put at 80-115 Astronomical Units (AU) from the Sun, than the 16 AU
distance between them.
The latest findings from the Ulysses spacecraft will be presented on 3-6 October
when more than 100 scientists from 16 countries gather at ESTEC, ESA’s Research
and Technology centre in the Netherlands, for a symposium on “the 3D heliosphere
at solar maximum”. New revelations are expected about how the behaviour of the
Sun at solar maximum affects the heliosphere, the giant magnetic bubble the solar
wind blows out into space.
Dr Stone was giving an introductory keynote address to the 34 ESLAB symposium on
the 3D heliosphere at solar maximum, which began three days of proceedings at ESTEC
in the Netherlands.
The meeting will be discussing new results from Ulysses and other spacecraft as the
Sun reaches solar maximum.
Changes to the structure of the solar corona as the Sun progresses from minimum to maximum activity
were vividly illustrated by Mike Andrews from Computational Physics Inc. at the Naval Research
Laboratory, Washington DC, who showed a series of images taken by the choronograph on the SOHO
spacecraft. At solar minimum, coronal streamers were seen emerging only from around the Sun’s
equator, but in December 1997 a transition took place, marked by the appearance of long-lived
streamers emerging from high latitudes. This is illustrated by the two accompaying figures, both
taken by SOHO's Large Angle and Spectrometric Coronagraph (LASCO) C2 instrument, the first
image was taken around Solar minimum on 13 May 1996, the second on 30 September 2000, near
Solar maximum.
“This meeting is bringing together friends and colleagues from around the world that have
worked together for many, many years,” said Peter Wenzel, Head of Solar System Division at
ESTEC during the welcome address. As well as now sharing a common interest in the structure
and extent of the heliosphere, Ulysess and the Voyager spacecraft also share the distinction
of carrying some of the oldest hardware in space. Klaus Fahlenbock from Dornier (now Astrium),
the prime contractor, reminded delegates that Ulysses was built 18 years ago, even though the
10th anniversary of its launch is being celebrated this week. |
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