ÐÓ°ÉÔ­´´

At some time in the next year or so, the space probe Ulysses will become
the first man-made object to pass over the poles of the Sun. By 12 October
it had already reached 37degrees south and was 3.95 astronomical units from
the Sun. Ulysses’ visit to the Sun, which involves scientists from 12 countries,
will be the grand finale of the first of two space explorations that have
both astronomers and physicists on the edge of their seats. The second is
much closer to home. At the end of 1995, the European Space Agency will
launch its four Cluster spacecraft, packed with identical instruments, which
will orbit the Earth as a group. They will be at the corners of an imaginary
tetrahedron, some 1000 kilometres apart.

These two missions will investigate some of the strangest places in
the Universe – the plasma that surrounds the Earth and the Sun. In these
regions of space, which are up to 100 or so times the radius of the Earth
or Sun, charged particles (ions and electrons) are accelerated to unheard
of energies, by moving magnetic fields. By investigating how they differ
from each other and from plasmas created in fusion experiments on Earth,
physicists aim to answer some big questions. How are particles accelerated
to such high energies? Is there a better way of controlling plasmas on Earth?

Ulysses’s mission encompasses more than a study of plasmas, however.
Its task is to make a pioneering exploration of the vast region of the Solar
System that lies outside the ecliptic plane – the plane in which the Earth
and planets orbit around the Sun. The spacecraft started its journey with
the launch of the space shuttle Discovery from Cape Canaveral on 6 October
1990. But, fortunately for physicists, to go out of the ecliptic plane it
had to enlist the aid of Jupiter to boost its energy and sling it southwards,
back towards the Sun. Physicists leapt at this chance to investigate Jupiter’s
plasma: the last time they had had such an opportunity was the Voyager missions
of the late 1970s.

Jupiter is the most photogenic of the non-terrestrial planets. One of
its moons, Io, has active volcanoes, and dynamic cloud structures give the
planet a unique appearance, the most famous feature being its Red Spot.
But Jupiter is also particularly interesting to many physicists, for two
other reasons. First, the radius of the planet is 71 398 kilometres, more
than ten times the radius of the Earth. Secondly, it rotates approximately
once every 10 hours. This rapid rotation, and a magnetic field that is 10
000 times that of Earth, make Jupiter a fascinating astronomical laboratory
for the study of plasmas.

Jupiter is the only major astrophysical system that can be studied at
close quarters. It is the most extreme, high temperature plasma environment
that can be measured by visiting spacecraft. This plasma consists of charged
particles such as ions and electrons. Physicists can measure the density
of these particles, work out the composition of their elements, and the
temperature of the various constituents. These turn out to be quite unlike
anything physicists have seen on Earth.

There are obvious differences in scale – a tokamac, a machine used
to create high temperature plasmas for nuclear fusion experiments, measures
around 10 metres across, Jupiter 1010 metres. But the main difference between
the Jovian plasma and that found, say, in a tokamac, is the number of particles
in a given volume. Even in the densest regions, outside the planet itself
and its moons, there are never more than between 10 and 100 electrons per
cubic centimetre, compared with about 1019 per cubic centimetre in a more
down-to-Earth plasma at atmospheric pressure.

This density difference in turn affects the frequency with which particles
collide. Apart from the planet itself and its moons, the Jovian environment
is virtually collisionless. The huge scale of the system means that particles
can be accelerated to extremely high energies – electrons to energies above
20 megaelectronvolts (MeV) and ions up to around 10 MeV per nucleon. By
comparison, the electrons that generate X-rays in hospital machines have
energies of around 100 000 electronvolts. Energetic particles like these
are a feature of many astrophysical systems and a much better understanding
of how particle acceleration works is needed to unravel all their mysteries.
Most of all, though, Jupiter provides a unique opportunity to study the
behaviour of plasma under extreme conditions and test and correct the theories
developed working with the unruly and unreliable plasmas in earthly laboratories.

So, on 8 February 1992, a large group of people gathered at the Jet
Propulsion Laboratory, Pasadena, California. They included scientists,
engineers and the media, and they were there to savour at first hand the
results coming back from the first spacecraft to visit Jupiter since the
Voyager missions in 1979. The radiation from Jupiter would be the most severe
the equipment on board Ulysses would ever encounter. ÐÓ°ÉÔ­´´s were confident
that if it could survive Jupiter, it would survive to complete its mission.
But for the physicists, things did not go entirely smoothly. Our instrument
team, led by Louis Lanzerotti, had like several others gone to Pasadena
armed with modern computer workstations which enabled us to perform sophisticated
analyses of the data within minutes of it being received. We were using
sensitive, solid-state telescopes designed to detect charged particles.
These were protected by covers which could be moved over their apertures
as they passed through what, on the basis of previous Jupiter missions,
was expected to be the most intense radiation zones.

It was a delicate compromise between the thirst for knowledge (covers
open) and instrument safety (covers closed, so no data). Immediately after
Ulysses had negotiated what we thought was the highest radiation zone, radiation
levels were detected that were far too high for safety; we had evidently
opened the covers too soon. Any command sent now would take 45 minutes to
reach the spacecraft and the covers took an hour to close. Horrified, we
simply had to sit it out and hope. To our great relief, the spacecraft and
its instruments survived the encounter unscathed, and even provided some
additional interesting results: the high-particle fluxes measured in this
potentially disastrous exposure confirmed that much of the energetic particle
acceleration occurred near the planet, and probably had something to do
with Io, Jupiter’s innermost moon.

Physicists already have a detailed picture of Jupiter’s magnetosphere
from the Voyager mission. The solar wind compresses Jupiter’s magnetic field
on the side facing the Sun, and drags it out on the dark side into a long
tail. Plasma escapes from the system largely along the tail, as a magnetospheric
wind. This material is constantly being replenished, ionised and energised
by other matter. Thanks to the Voyager and Ulysses missions, physicists
now believe that this additional matter comes from three sources: from the
volcanic activity on Io, from plasma coming from the Jovian ionosphere,
and from the solar wind.

One of the most challenging and long – standing problems in astrophysics
is to understand how low density, high temperature, magnetised plasmas accelerate
charged particles, and, in turn, how these accelerated particles influence
and control their environment. The theory that tells us most about this
knotty problem involves equations developed mainly by the 19th-century Scottish
scientist James Maxwell. When applied to magnetised plasmas, it is called
magnetohydrodynamic (MHD) theory. The way MHD waves travel through plasmas
was explained by the Nobel laureate Hannes Alfven in the 1930s, and since
then wave-particle interactions have been used by many researchers in all
sorts of ingenious ways to try to account for particle acceleration.

The data from Ulysses provide a much more complete set of measurements
than those made by Voyager. The early missions really only identified the
fluxes and energy spectra of the major ions and electrons. Ulysses measured
their angular distribution with respect to the magnetic field as well –
crucial to an understanding of how they are accelerated.

Maxwell’s equations tell us that for charged particles to be accelerated,
an electric field has to be produced, and for this to happen there has to
be a magnetic field that varies over time. The Jovian environment is a perfect
place for this because of the rapidly rotating magnetic field which is offset
to the rotation axis. To the solar wind, Jupiter appears to be a rocking,
magnetic dipole. This causes the disc of current associated with the magnetic
field to warp, so that near the planet it is on the magnetic equator, while
far from the planet it is closer to the rotational equator. Within this
‘current sheet’ the plasma moves around, rotating roughly with the planet.
Some theorists have recently predicted that this rotating current sheet
should be able to energise heavy ions, and armed with the latest measurements
from Ulysses, physicists are busy testing this theory.

Most of Jupiter’s moons also lie within the current sheet. These rotate
according to their distance from the planet, obeying Newton’s laws. So they
move relative to the rotating magnetic field, and electric fields develop
across them. This leads to particle acceleration, particularly around Io,
whose volcanic activity also fills up the magnetosphere with sulphur and
oxygen ions. The detailed measurements of the fluxes and energy spectra
of these ions made by Ulysses tell physicists more about these process.

One of the instruments on the craft, HI-SCALE, measured the radiation
fluxes within the magnetosphere (see below). The energy spectra of ions
and electrons were taken throughout the period of the encounter. Violet/red
represents the highest fluxes and blue/black the lowest. The range of colours
covers a factor of one million in flux. Ulysses entered Jupiter’s magnetosphere
when the picture starts becoming colourful on 2 February, and left it on
14 February. The blank is where we switched off the instrument for a day
around closest approach because of possible radiation damage. These measurements
show that the intensities of the ions and electrons varied over a period
of ten hours.

HI-SCALE also measured the relative behaviour of the electrons, low
energy ions and heavy nuclei. The electrons are travelling at around four-tenths
the speed of light. During its approach to Jupiter, Ulysses discovered that
heavier nuclei, such as carbon, nitrogen and oxygen, are mainly confined
to the inner regions of the plasma sheet. This is a very useful proof of
the theory that acceleration of the heavy ions originating from Io occurs
in the plasma sheet and that these ions do not easily penetrate to high
latitudes (that is, near Jupiter’s ‘poles’). Ulysses also passed through
a region of the high-latitude magnetosphere which had never been crossed
before.

One advantage Ulysses has over Voyager is its ability to monitor the
direction ions and electrons come from. This led Ulysses to make a very
important discovery: frequent bursts of narrow beams of electrons and ions,
moving in opposite directions. These bursts were very closely confined to
the direction of the magnetic field – so much so that they were aligned
with it. This is a crucial piece of evidence for understanding how the particles
are accelerated. Some theories predict the bursts, whereas others predict
a quite different scenario. Beams of ions and electrons going in opposite
directions constitute an electric current, and such currents are consistent
with those believed to be necessary to produce the Jovian aurora – fluctuating
bands of light seen from time to time near the polar regions. Ulysses detected
these currents for the first time.

It is clear that most of the electrons and low energy ions, mostly protons,
are not accelerated in the plasma sheet, otherwise their intensities would
match those of the heavy ions, which come from Io. So there must be at least
two general regions of particle acceleration within the magnetosphere.
Plasma physicists will be able to investigate this in more detail when they
have finished analysing all the data.

Another remarkable feature of the high-latitude region, made by the
detector COSPIN, was bursts of highly relativistic electrons of energy greater
than 16 MeV. After a burst the energy slowly decreases again, until the
next burst ten hours later. Even more dramatically, it has remained in phase
since the first hint of the phenomenon was recorded by Pioneer 10 in 1973.
Jupiter, it seems, accelerates these electrons to velocities near that of
light as regularly as clockwork.

Ulysses also confirmed the existence of a rather unusual energetic particle,
the hydrogen molecule H₃⁺, first detected by Voyager 1, and found its energy.
Using pictures taken with the NASA Infra-Red Telescope in Hawaii just before
Ulysses’s encounter with Jupiter, physicists eventually pinned down the
ion to Jupiter’s auroral zone. This proves that some of the magnetospheric
plasma comes from Jupiter itself, and that it is probably accelerated by
electric fields on the field lines of the auroral zone, at high latitudes.
The discovery of the auroral emission, plus the energetic H3+ ions in the
magnetosphere, give important clues about the origin and acceleration mechanism
of Jupiter’s energetic ions.

Previous missions to Jupiter had shown that the solar wind provides
nearly all the helium ions in the magnetosphere. From this, physicists can
infer what element comes from the solar wind and what from elsewhere, by
comparing its abundance with that of helium throughout the magnetosphere.
This is how they have discovered that most of the carbon in the magnetosphere
comes from the solar wind.

These and other discoveries have told physicists a lot about the plasma
environment of Jupiter, in particular about the processes responsible for
accelerating electrons and ions to high energies, and they are looking eagerly
to the future. One way they picture the Sun and the Solar System is as a
giant equivalent of Jupiter and its moons. The Sun accelerates particles
to much higher energies than Jupiter, around 50 gigaelectronvolts for the
ions, for example. It has a magnetic field, and it is rotating. The vast
region of space dominated by the solar magnetic field, called the heliosphere,
is the solar equivalent of Jupiter’s magnetosphere. The boundary of the
heliosphere is where the solar magnetic field loses control to the interstellar
medium. ÐÓ°ÉÔ­´´s suspect that a bow shock – a boundary in the plasma –
is formed as the magnetic field ploughs throughout the interstellar wind.
They calculate this to be some 50 astronomical units away from the Sun.

So, viewed from afar, the Sun’s heliosphere is just like a giant magnetosphere.
As Ulysses travels from Jupiter around the poles of the Sun, it will be
gathering data that will explain the plasma environment, and acceleration
processes for the heliosphere, just as it did for Jupiter’s magnetosphere.
Cluster will investigate the same phenomena in great detail for the Earth’s
magnetosphere. Physicists have been studying the magneto-sphere since the
first scientific satellites were launched, but these investigations have
never been carried out by more than two coordinated spacecraft, and have
never produced conclusive results about the acceleration processes. Up to
now, if we have observed a change, we have not known if it is a change in
space or in time, since we have not had any spatial resolution in more than
one dimension. Cluster, with its tetrahedral array of spacecraft, will be
able to make measurements in three dimensions. Eventually, it should be
possible to combine information from Cluster with the single-s pacecraft
observations of both the Jovian magnetosphere and the heliosphere made by
Ulysses. The results should be truly spectacular.

* * *

THE CURIOUS TALE OF A COMET’S TAIL

When the European Space Agency’s spacecraft Giotto encountered Halley
and Grigg-Skjellerup (GS), space scientists gained a fascinating insight
into the behaviour of comets – and space plasmas.

One of a comet’s two tails is a plasma; the other is dust. The fact
that the plasma tail of a comet points away from the Sun was the first
clue to the existence of a gusty solar wind – a plasma consisting of ions
and electrons – flowing rapidly through the Solar System away from the Sun.
We now know that the solar wind carries with it a magnetic field, and interacts
with anything in its way, including comets – this is how the plasma tail
is formed. But how does a comet nucleus of only a few kilometres diameter
produce a plasma tail millions of kilometres long?

The cometary nucleus is not magnetised – it is only ice and dust – so
it interacts with the solar wind in a quite different way from magnetised
planets like Earth or Jupiter. Evaporating gas, mainly water, from the warmed
nucleus, drifts away from the comet with a speed of about 1 kilometre per
second. Such a neutral water atom has a good chance of being ionised in
sunlight, though it may take a week. As soon as it is formed, the ion feels
an electric field due to the relative motion between itself and the solar
wind, and it is accelerated. At the same time it spirals around the magnetic
field. The resulting motion of each ion is a cycloid – the path the valve
on a bicycle wheel traces out as it rotates around the axle and simultaneously
‘drifts’ along as the bike moves forward. This ‘ExB’ drift is well known
in plasmas and its speed can be calculated.

The solar wind gives energy to the new ions over a vast region of space,
slowing down near the comet where there are more ions to ‘pick up’. If the
wind slows down suddenly enough a bow shock – a boundary in the plasma
– forms. Comets have the most complex bow shocks in the Solar System. Giotto
identified one a million kilometres from Halley.

Giotto also confirmed the existence of another boundary, 4000 kilometres
from the nucleus of Halley. Inside this region there are huge densities
of ions and the magnetic field of the solar wind is completely excluded,
forming a ‘draped’ pattern near the comet. This draping in the downstream
region forms the plasma tail. At least two unexpected extra boundaries were
seen between these two in Halley, one of which remains unexplained.

At GS we had expected that all these boundaries separating regions of
different plasma flow would be ‘washed out’ by gyrating water ions. But
to our great surprise Giotto saw a remarkably sharp, bow shock structure.
Giotto also detected a mystery boundary like Halley’s – another puzzle for
theorists.

The ion pickup process seen at comets like GS and Halley is also important
for fusion experiments on Earth, where fuel pellets are injected and where
the reaction products, alpha particles, form new ions. The ion pickup process
is also important at other places in the Solar System. Future missions to
Mars (the Russian Mars-94) and Saturn (NASA/ESA’s Cassini) will measure
the process at these other planets. Meanwhile, ESA has ambitious plans to
send a spacecraft, Rosetta, to rendezvous with a comet in 2008 during its
approach towards the Sun. A probe will sample the comet’s surface and instruments
on the main spacecraft will examine the plasma around the comet as the comet
warms. These missions should tell us even more about pick up in plasmas.

Meanwhile, perhaps the most exciting new result for plasma scientists
is the remarkably clear, well-ordered and large waves seen in the ‘sea’
of plasma near to and inside the bow shock at Grigg-Skjellerup. The frequency
of the waves is exactly the frequency at which the new ions gyrate around
the magnetic field (one rotation of the bike wheel). This was different
from Halley, where a whole spectrum of frequencies was seen, and it may
be an effect of the smallness of the comet on the shapes of the particle
distributions. The waves at the gyration frequency may be linked to the
fact that there is effectively an uneven ‘bike wheel’ of ions in the plasma.
This major new result is giving theorists plenty to work on.

Andrew Coates

George Simnett is a researcher in space plasmas at the University of
Birmingham and a member of the HI-SCALE Ulysses team.

Andrew Coates is a researcher in the Mullard Space Science Laboratory
at University College London.