Å®ÉúСÊÓƵ

Jupiter’s magnetosphere – the ‘magnetic bubble’ which surrounds the
planet – is one of the most complex objects in the Solar System and it is
very different from the Earth’s, according to data sent back by the European
space probe Ulysses, which passed the giant planet earlier this month. Furthermore,
it has changed greatly since it was last observed, by the Voyager space
probes in the late 1970s.

The Earth’s magnetosphere is shaped like a teardrop, but Jupiter’s is
much flatter, more like a plaice. The shape is determined by powerful electric
currents that flow through space around the planet, beyond the orbits of
the planet’s major moons.

Ulysses’s mission is to fly under the Sun’s south pole in 1994, looping
over its north pole in 1995. It was sent to Jupiter so that the giant planet’s
gravity could swing it out of the plane in which the planets orbit before
it heads towards the Sun. The craft flew over Jupiter’s north pole on 8
February, passing only 450 000 kilometres (slightly more than six times
the radius of Jupiter) above the the planet’s equator before swinging under
the south pole and heading back towards the Sun.

The rendezvous with Jupiter went precisely as planned, according to
Andre Balogh, whose responsibility is the magne-tometer on Ulysses. The
spacecraft is now travelling out of the plane of the planets’ orbits. ‘Ulysses
has survived brilliantly,’ Balogh says. ‘None of the instruments has suffered
any functional damage from Jupiter’s radiation field.’

Some of these instruments probed Jupiter’s environment – including its
magnetic field, the strongest of any planet. Ulysses was making the first
visit to Jupiter for 12 years. In the 1970s, two Pioneer probes and two
Voyager probes passed the planet within a six-year period. These probes
traversed only the region above Jupiter’s equator, and their measurements
indicated that the planet’s magnetism is an outsized version of the Earth’s.
This is a roughly spherical region, in which the planet’s magnetism is confined
by the solar wind of charged particles from the Sun. A tail stretches away
on the side away from the Sun.

Now, Ulysses has probed the third, north-south dimension. The spacecraft
entered Jupiter’s magnetosphere a couple of days earlier than expected,
on 2 February. After swinging around Jupiter, it left the magnetosphere
on 12 February – again a couple of days earlier than predicted. The results
mean the magnetosphere is squashed. It is stretched to about 50 times Jupiter’s
diameter in the plane of the planet’s orbit, while it reaches less than
half that distance in the vertical direction.

Balogh explains the shape by a vast electrical current that flows through
the tenuous gases surrounding the planet. The current is carried by atoms
of sulphur and oxygen supplied by Jupiter’s volcanic moon Io. The particles
initially follow Io’s orbit, 421 000 out from the planet. Volcanic debris
from Io, dubbed the ‘Io torus’, has been observed all around its orbit.
Ulysses, however, has found that the atoms do not form a continuous ring,
but form separate patches around Io’s orbit.

Energetic particles trapped in Jupiter’s magnetic field ionise these
atoms to form plasma. The plasma spreads out beyond Io’s orbit into a thin
sheet. As the planet spins, the ionised gas is whirled around by the magnetic
field of Jupiter. Ulysses found that this current sheet stretches out to
at least 40 times the radius of Jupiter, which is about twice the size of
the current sheet measured by the previous spacecraft.

According to Balogh, the sheet is carrying an electric current of about
a billion amperes. It generates its own magnetic field, which is independent
of the magnetic field produced inside Jupiter. In the outer parts of the
magnetosphere, the field from the current sheet thus gives rise to a magnetosphere
with a flattened shape. This explains the results from the magnetometer
and the experiments on board Ulysses that made direct measurements of charged
particles and radiation.

According to Ulysses’ results, Jupiter’s magnetosphere at present extends
twice as far in the plane of the planets’ orbit as it did in the 1970s.
‘The magnetosphere has inflated like a balloon, as hot plasma has escaped
from the region of Io,’ says Balogh.

The amount of plasma varies over the years for two reasons. For a start,
the vocanoes on Io erupt intermittently. But this cannot be the whole story,
because although Io is not very active at the moment, the magnetosphere
is burgeoning. The second reason the plasma varies involves the torus around
Io’s orbit, where atoms reside after they leave Io. The outer edge of this
torus is unstable, and every so often,a sudden burst of ions leaves the
torus and spreads out to join the current sheet, puffing up the magnetosphere.

At its closest approach to Jupiter, Ulysses passed just outside the
orbit of Io and it experienced a magnetic field of 2.4 microteslas. The
current sheet provided 10 per cent of this field. The rest was due to Jupiter’s
internal magnetic field. The Pioneers and Voyagers found that the planet
had a field 20 000 times stronger than the Earth’s, with the axis of the
magnetic field inclined by about 10 degrees. Ulysses measured the same
magnetic field to within a few per cent. ‘The magnetometer tells us it’s
the same planet,’ Balogh concludes.

Another experiment on board measured the impact of dust particles on
Ulysses. This is the first dust detector to be carried to another planet.
Between the planets, Ulysses was hit by only one dust particle a week. However,
as it swept past Jupiter, the space probe was struck eight times, by particles
between 1 and 10 micrometres across. ‘Jupiter is a fairly dusty environment,’
says Tony McDonnell of the University of Kent, the leader of the team.

Jupiter’s immense gravity attracts interplanetary dust particles, raising
the concentration of dust near Jupiter to about one hundred times the concentration
in interplanetary space. Two of the eight dust impacts recorded by Ulysses
were probably due to interplanetary particles drawn to Jupiter in this way.

Ulysses hit the other six particles as it passed immediately above the
planet’s equator. They probably form part of a very tenuous ring of dust
around Jupiter. This is the outer region of the faint ring that the cameras
of Voyager 1 and 2 discovered much closer in to Jupiter. The Voyager probes
showed that the brightest part of the ring lies 129 000 kilometres out from
the centre of Jupiter, with a faint extension to at least 200 000 kilometres.
Ulysses has now found that the ring of dust reaches out to twice that dis-tance
or more.

According to McDonnell, the small dust particles have been chipped away
by impacts on rocky objects about the size of sand grains that are orbiting
Jupiter around its equator. The impact may come from another grain or from
a particle of interplanetary dust drawn in by Jupiter’s gravity. Some of
the dust may be spewed out by the active volcanoes on Io. ‘We don’t have
any data on the composition of the dust,’ McDonnell says, ‘so we can’t be
certain of its origin.’