From: nasanews
Date: Dec 7 18:40 UTC
Subject: Ulysses Results
Scientists from NASA's Jet Propulsion Laboratory and the European Space
Agency are reporting the first observations ever made in the Sun's polar
regions at today's meeting of the American Geophysical Union in San
Francisco.
A number of longstanding questions have been answered in exploring this
region of the Sun, but, as usual, there have been major surprises, said
Dr. Edward J. Smith, NASA project scientist at the Jet Propulsion
Laboratory, who is participating in today's press conference, along with
Dr. Richard Marsden, ESA project scientist, and two of the Ulysses
science experiment principal investigators.
The gas being continuously carried by the Sun's solar wind has been
found to be a very fast and relatively smooth flow, Smith said in
characterizing the polar environment from Ulysses data. Surprisingly,
the strength of the Sun's magnetic field over the poles is the same as
it is near the equator. Large amplitude magnetic waves are continuously
present in the polar regions. And the intensity of cosmic ray
particles arriving from the galaxy is only slightly larger than near
the Sun's equator.
These and other findings were made possible by the Ulysses mission, an
international collaboration between the American and European space
agencies. The Ulysses spacecraft finally reached the Sun's south pole
this past summer after a journey of almost four years. It took this
long because no existing rocket is powerful enough to break free of
Earth's gravity and send a spacecraft directly over the Sun's poles. The
spacecraft, which was launched on October 6, 1990, had first to travel
outward to Jupiter, arriving on February 8, 1992, and use Jupiters
gravity field to redirect its flight path toward the Sun's poles. The
Ulysses trajectory is such that, at the time of the polar passage, the
spacecraft was more than twice as far from the Sun as the average
distance from the Sun to the Earth. Consequently, although Ulysses is
the first spacecraft to probe the Sun's polar regions, it does not
travel near the Sun.
The team of scientists led by Dr. John Phillips of Los Alamos National
Laboratory reported that near the pole the solar wind is flowing away
from the Sun at nearly twice the speed that is typically observed near
the Sun's equator and that is continuously arriving at Earth. The solar
wind flow is also much smoother than at low latitudes, causing a marked
reduction in the space weather that typically exists near the equator
and which affects the Earth by causing magnetic storms and the Aurora
Borealis, or northern lights.
The solar wind comes from the Sun's outermost atmospheric region, the
corona, Phillips said. The corona is so hot that it consists only of
negatively charged electrons and the positively charged atoms (ions)
from which they have been removed. The speed of these hot electrons and
ions is so great that even the strong gravity field of the Sun cannot
prevent their escape into space.
As they leave the Sun, however, the heat energy of these electrons and
ions is converted into a high speed flow by a process that is analogous
to that which takes place inside a rocket engine. Scientists now
expect to take advantage of the simplicity of the solar wind flow in
the polar regions to better understand the physical conditions under
which the solar wind originates, Phillips said. Such knowledge should
also clarify the high speed emission of gas from other stars, known as
the stellar winds, which is thought to be commonly occurring throughout
the universe.
Scientists had expected to see the magnetic fields embedded in the
solar wind increase in strength as Ulysses traveled toward the pole,
but there were more surprises awaiting them. The solar wind fields
bear the imprint of magnetic fields originating on the Sun, which
increase in strength toward the poles, just like the magnetic field of
the Earth. Surprisingly, however, Ulysses' observations have not
revealed the expected increase from the equator to the poles.
The scientific team, headed by Dr. Andre Balogh of Imperial College,
London, interpreted this finding to mean that the magnetic fields in
the Sun's polar region are pushing the solar wind toward the equator to
produce a field whose strength does not depend on latitude.
Consequently, the magnetic field must be exerting a much stronger
influence on the solar wind near the Sun than had previously been
thought. The mathematical models used by scientists to relate the Sun's
fields to those carried off by the solar wind must now be
reconsidered.
Another major result involving magnetic fields is the continual
presence in the polar cap of very strong waves. The waves cause
changes in the magnetic field that are comparable to the field strength
so that the field direction varies continuously through large angles.
The waves are considered to be an important aspect of the solar wind
flow and may contribute to the heating and speeding up of the solar
wind. In addition, the waves, which are traveling outward from the
Sun, are thought to be opposing the entry of the electrically charged
cosmic ray particles into the polar regions. The waves are similar to
those that can be produced on Earth by wiggling the free end of a rope
that is attached to a post at the other end. A possible explanation of
the magnetic waves is that the ends of the magnetic line of force
attached to the Sun are being subjected to churning motions of the Sun's
surface; the motions themselves resemble those seen in a bowl of
porridge on a hot stove.
Waves of this kind have previously been observed sporadically by
spacecraft observing the solar wind in the vicinity of Earth, Smith
said. The waves accompany high speed solar wind, which can also be
present for several days at a time. The presence of the waves has been
shown to produce a high intensity, long-lasting polar aurora.
As a result of Ulysses' observations, it is now believed that during
such episodes the Earth is temporarily immersed in solar wind from the
Sun's polar regions, which is also characterized by high speed and the
presence of waves. Thus, Ulysses has found a direct physical
connection between the polar caps of the Sun and the Earth.
Scientists had speculated for many years that the Sun's poles form a
funnel that allows easy access to the inner solar system of cosmic ray
particles. If this were true, a large increase in the particle
intensity should have been recorded as Ulysses traveled poleward.
However, measurements reported by Professor J.A. Simpson of the
University of Chicago, who heads the cosmic ray science team, show only
a slight increase in intensity. Consequently, the funnel is
non-existent.
Cosmic rays are the nuclei of atoms from which all electrons have been
removed, Marsden, ESA project scientist, said. They are created in
cataclysmic events occurring elsewhere in the galaxy and reach the
vicinity of the Sun traveling at nearly the speed of light. They tend
to travel along lines of magnetic force. It was anticipated that they
could travel along radial solar wind magnetic fields into the polar
region. But, in fact, the supposed funnel is caused by the diverging
shape of the Sun's polar cap magnetic fields.
However, the strong waves being reported by the Ulysses magnetic field
investigators are now thought to be deflecting the cosmic rays back
into space, thereby acting as a baffle that is overcoming the focusing
effect of this so-called funnel, Marsden added.
Ulysses is now leaving the south polar region of the Sun and is heading
back toward the Sun's equator and the ecliptic plane in which Earth
orbits the Sun. The spacecraft will then begin to travel northward
along its trajectory to begin exploring the Sun's north polar cap
beginning in June 1995.
Ulysses is managed jointly by NASA and the European Space Agency to
study the regions above the Sun's poles. The Jet Propulsion Laboratory
manages the U.S. portion of the mission for NASA's Office of Space
Science, Washington, D.C.
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- T o d d E. V a n H o o s e a r -
``'''vanhoose@lalaland.cl.msu.edu - vanhoose@msu.edu - vanhoose@lalaland.cl.msu.edu
(._.) Michigan State University - East Lansing, MI USA
(_) Computer Laboratory - Department of Communication
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