ÐÓ°ÉÔ­´´

PHOTOGRAPHS of Earth rising above the Moon’s horizon, taken by Apollo
11, emphasise the loneliness and isolation of our planet in space. Yet that
seemingly empty space is the objective of a project planned for the middle
of this decade. In 1995, the European Space Agency will launch Cluster,
four spacecraft grouped together in a cluster of oval orbits that will reach
nearly halfway to the Moon. Why is it worthwhile to send satellites to study
a black void? And why use four spacecraft? That black void is not empty;
gas from the Sun swirls around the Earth and its magnetic field, triggering
the spectacular displays of the aurora. Because this gas, made up of charged
particles such as electrons, is constantly changing and moving, a single
spacecraft cannot determine its structure in three dimensions.

To investigate these structures, researchers need to use at least four
spacecraft flying in formation, ideally at the corners of a regular tetrahedron.
That is how the Cluster mission will use its four craft. They will travel
on matched orbits, a few hundred kilometres apart at the start, and will
spread out as the mission progresses. During the two years of their life
in this orbit, the Cluster satellites should investigate many of the puzzling
aspects of space around the Earth.

Physicists want to study space near the Earth because this ionised gas
is more accessible there than throughout the rest of the Universe, where
it exists between the stars and, for example, in explosions of supernovae.
It is much easier to make direct measurements close to the Earth than to
try to reach inside a star. Results from this mission could also help research
into nuclear fusion, which takes place in the same unusual sort of matter.

Interplanetary space is permeated by a gas with as few as 10 particles
per cubic centimetre, far fewer than the emptiest vacuum ever produced on
Earth. If the gas in space was like the air we breathe, it would not be
very important. What makes this tenuous gas so different is that it is made
of charged particles. It is called a plasma. We can see plasma in flames,
lightning, fluorescent lights, the Sun, and the explosions of hydrogen bombs.
More than 99 per cent of the mass of the Universe is in the form of plasma,
but it is mostly difficult to reach, in stars, supernovae, or interstellar
space. One of the easiest places to study this unusual gas is in the space
around the Earth. Although most plasma, such as that between the Earth and
the Moon, is invisible, it is usually continually moving, changing speed,
density and pressure.

The Earth moves through plasma of the Sun’s atmosphere. The upper layers
of the Sun are so hot that they boil off more than a million tonnes of plasma
into space every second. This creates a wind which blows through the Solar
System at a speed of a million miles per hour. Occasionally, flares in sunspots
shoot even denser, hotter and faster bursts of plasma into space. This gusty
solar wind cools as it expands into space, but it is still at 50 000 degrees
when it reaches and engulfs the Earth.

Although we do not have to worry about the solar wind burning up our
planet – the amount of heat it carries is only a tiny fraction of the heat
in sunlight – some interesting effects result because of the Earth’s magnetic
field. This forms a shield, deflecting the flow of plasma from the Sun.
The space within our magnetic field, called the magnetosphere, is isolated
from the solar wind because of some peculiar properties of plasma.

Plasma behaves differently from a normal gas. Lenz’s law says that if
you try to change the magnetic field through a conductor you induce an electric
current with a magnetic field that opposes the change. That is the effect
which makes dynamos work. The better the conductor, the more nearly the
two fields cancel out each other. Plasma is such a good conductor of electricity
that the magnetic fields cancel exactly. As a result, magnetic field lines
appear to be frozen into the plasma. They flow with the plasma, making it
easier to visualise the movement. The field lines behave like bundles of
spaghetti which can be looped, stretched, or squeezed but which stay stringy.
A very important consequence of frozen-in field lines is that, unlike normal
gases, plasmas do not mix. Their magnetic fields keep them apart.

What you see in a plasma depends on how closely you look at it. Charged
particles follow spiral paths around the direction of the magnetic field.
If you zoom in on a length of the spiral much smaller than its radius, its
curvature would not be obvious. You might even think that the particles
were moving in straight lines, ignoring both the magnetic field and all
the other particles. If you step back and view the plasma from much farther
away, the particles would appear to follow the magnetic field lines and
you would see the plasma behaving like hot, wet, slippery spaghetti. In
the solar wind near the Earth the particle spirals are roughly 100 kilometres
across. That is a long way to walk but no more than a gnat’s whisker in
space, so when physicists look at the plasma in space they usually see the
spaghetti, not the pasta. The scale of many structures in plasma in space
is linked to these particle spirals. If researchers want to understand how
structures such as the magnetosphere are formed, and what makes them move,
then they must also find out what is happening on the scale of the particle
spirals. They must put the pasta within a single strand of spaghetti under
a microscope.

An imaginary slice along the meridian marking noon and midnight reveals
that magnetic field lines that start on the Earth stay inside the magnetosphere;
those in interplanetary space, which start on the Sun, remain outside. This
is because the magnetic field lines frozen into both plasmas stop them mixing
with each other. Typically, the distance from the Earth’s surface to the
magnetopause, the boundary of the magnetosphere, is approximately 58 000
kilometres, nine times the radius of the Earth. The edge of the magnetosphere
does not have a fixed position; it changes as the solar wind varies, becoming
smaller when the wind blows harder. The boundary is just a few hundred kilometres
across, about as thin as the film of soap would be around a bubble the size
of the magnetosphere. Although researchers cannot see the magnetopause,
they suspect that it wobbles and ripples in the solar wind like a soap bubble
in the air, because they have noticed the boundary moving back wards and
forwards as satellites fly past.

A swirl of cosmic spaghetti

Neither soap bubbles nor spaghetti help to predict the shock wave which
forms upstream from the magnetosphere. It is just like the shock wave around
a supersonic aircraft, or the bow wave around a boat. It forms because the
solar wind is flowing faster than the speed of the plasma equivalent of
sound, magnetic sound waves. The thickness of a shock wave in air or water
depends on collisions between molecules. The bow shock formed around the
Earth cannot form by collision: it depends on electric and magnetic forces.
Researchers know that its thickness is also several hundred kilometres,
related to the radius of the particle spirals, but its structure is a mystery
which the Cluster mission may help clear up.

Despite the fact that the two bundles of cosmic spaghetti should not
become entangled there is no doubt that they do; the solar wind has significant
effects within the magnetosphere. Magnetic storms, the auroras, the Earth’s
radiation belts and almost anything affected by the 11 year solar cycle
are caused by the impact of the solar wind on the Earth. Energy from the
solar wind strikes the front surface of the magnetosphere at a rate roughly
equal to the average power consumption of the US. A little of this, approximately
10**11 watts, is dissipated in the magnetosphere during a magnetic storm.

But physicists do not know how the magnetosphere gets this power from
the solar wind. It ought to be quite a simple process, because the highly
conducting plasma of the solar wind, with its frozen-in magnetic field,
behaves like an electric dynamo as it flows past the Earth; it is a conductor
moving through a magnetic field. But no power can come to the aurora from
this dynamo unless an electric current flows from the solar wind into the
magnetosphere and back again, in the same way that no power can be drawn
from a torch battery unless the bulb is switched on.

It is not easy to find out how the magnetosphere switch works, because
an interesting property of this type of plasma is that an electric field
cannot drive an electric current across the magnetic field lines. Particles
with positive or negative charges both simply spiral around the direction
of the magnetic field and drift sideways together. As long as geomagnetic
field lines stay confined inside the magnetosphere, no current can flow
into it, and no power can be drawn from the solar wind. In the switch, geomagnetic
field lines must become connected to interplanetary field lines by some
means. Physicists call this process reconnection.

Magnetic field lines running in opposite directions inside and outside
the magnetosphere are separated by a layer of electric current, induced
according to Lenz’s law. Reconnection joins field lines across this electric
current layer. The field lines are frozen in because the plasma has no resistance,
but reconnection happens only if there is some electrical resistance in
the plasma after all. Although it can sometimes be helpful to talk in terms
of frozen-in field lines, we have to be careful not to take the image too
far.

During reconnection, the magnetic fields diffuse into one another, and
field lines on one side appear to become connected to field lines on the
other. This forms a configuration with a neutral point, where the magnetic
field is zero. If you imagine that the magnetic field lines are stretched
like elastic bands, then you can see that the V-shape of the field lines
on either side forms a catapult which will shoot the plasma away from the
neutral point in both directions. The Cluster mission spacecraft will look
for plasma moving more quickly than normal, as an indicator of reconnection.

There are two places in the magnetosphere where the field lines might
reconnect; on the front surface and in the magnetic tail. At the former,
interplanetary magnetic field lines link to geomagnetic field lines; at
the latter, the two halves of the Earth’s field lines join once more. Overall,
there is a balance which keeps a proportion of geomagnetic field lines open
to interplanetary space all the time.

Reconnection is most effective in current layers a few particle spirals
thick, but no one understands what controls the rate at which it proceeds.
To find out, researchers must examine the structure of the current layer
in detail. The switch is clearly not as simple as flipping a toggle and
turning on the aurora. Once the field lines between the solar wind and the
magnetosphere are connected, however, energy, momentum and particles can
cross from one to the other.

These invisible events produce their most spectacular effect at the
edge of space where the atmosphere begins. The auroras, the northern and
southern lights, are caused by showers of electrons cascading down magnetic
field lines and striking the upper atmosphere hard enough to make the atoms
glow. It is like a TV tube with the upper atmosphere replacing the screen
and the plasma dynamics of the solar wind and the magnetosphere providing
the programme. This is one phenomenon where all you need to appreciate the
motion and structure of plasma in space is your eyes.

The delicate features of the aurora can be recorded in photographs,
but still pictures cannot capture the awesome spectacle of these shimmering
forms sweeping across the sky. It is not taking much of a chance to guess
that the invisible processes in the magnetosphere are just as complicated
as the aurora we can see. The lesson is that, despite the large scale of
the phenomenon, the fine structure of the aurora, with its narrow filaments
and thin curtains, both wavy and fast moving, is an essential part of its
nature. The size of the aurora is related once more to the radius of the
particle spirals, as is the boundary of the magnetosphere and the bow shock.

Researchers have built up their three-dimensional picture of the magnetosphere
from observations made by spacecraft orbiting the Earth. The earliest craft,
30 years ago, carried the simplest instruments, magnetometers and geiger
counters, but it was soon obvious that they were not telling the whole story.
ÐÓ°ÉÔ­´´s realised that the electric field was as important as the magnetic
field and they had to develop techniques to measure that as well. The Geiger
counters could detect particles with more than 40 000 electron volts (eV)
of energy, but most of the particles must have had much less. Now particle
detectors can record the distribution of particles over the whole range
from less than 1 eV to 100 million eV within a few seconds. Early researchers
thought that the positive particles were hydrogen nuclei (protons) from
the solar wind, but it turned out that quite a lot of them are oxygen atoms
from the atmosphere.

All this time the accuracy, speed and completeness of the measurements
has been steadily improving. Plasma physicists now have an excellent picture
of the general structures in space, that is, the spaghetti; the magnetic
field has been mapped, they know where its boundaries are, and what particle
distributions to expect throughout the magnetosphere. What they do not understand
are the processes that control these structures because they usually depend
on the thin layers of current that separate different volumes of plasma.
When physicists try to study these thin layers, the shortcomings of their
present methods stand out.

They have to find out how fast the boundaries are moving, and their
shape and orientation in order to calculate their thickness. Without the
thickness, they cannot measure the rate of change of the magnetic field
or the particle pressure. Without this information, they cannot apply the
equations of electromagnetic theory.

Present observations are ambiguous, because a single spacecraft makes
measurements only along the track of its orbit. Any changes it records might
have been either a time variation in the plasma or the result of the spacecraft
crossing some structure in space. Variations in space and time are equally
likely, and they occur on the same timescales when seen from the spacecraft.
Researchers must also remember the pictures of the fine detail in the aurora
when studying the data from a spacecraft and ask – what if the craft were
moving through a structure as complicated as that? The ambiguity between
variations in space and time can only be resolved by making measurements
simultaneously from more than one spacecraft. There have now been two missions
using two spacecraft together in the outer magnetosphere, the International
Sun Earth Explorer, and the Active Magnetospheric Particle Trace Explorer.
They could estimate the thickness or speed of structures in plasma along
the line between the spacecraft. That would have been enough if the magnetosphere
was like most drawings, in two dimensions on a flat piece of paper: unfortunately,
it is three-dimensional.

The four Cluster satellites will fly in a tetrahedron formation, giving
researchers a chance to track structures in plasma in three dimensions .
The spacing between the craft determines the size of structures which the
mission can investigate. They will initially orbit a few hundred kilometres
apart, about the scale of the particle spirals; they will separate as the
mission progresses. On the daylight side of the Earth, the orbit passes
through the magnetopause, the bow shock and goes out into the solar wind.
On the night side it travels down the geomagnetic tail to the region where
the TV programme of the auroral displays is put into the system.

Space plasma physics has been studied for 30 years in space and for
more than 400 years from the ground, with the aid of magnetic observatories
and records of the aurora. One of the most persistent lessons of the space
era has been that measurements made in the plasma itself reveal important
features that could not be detected from the ground and that nobody had
had the imagination to predict. The invisible structures of interplanetary
plasma almost certainly exist throughout the Universe and are just as invisible
at these greater distances. The only way to find out what might be going
on in these remote areas is to try to understand the processes in the only
region where physicists can make in situ observations, around the Earth,
well enough to extend their theoretical understanding to other astrophysical
events.

There is every reason to expect that the processes which Cluster is
setting out to investigate, such as particle acceleration, reconnection,
and the formation of shock waves without collisions between molecules, occur
in many other astrophysical situations. There are also advantages in studying
plasma in space rather than in the laboratory. In space there are no vacuum
chamber walls to worry about and there is so much plasma that the instruments
do not disturb it significantly. The study of plasma in space has its own
special part to play in the understanding of phenomena which range from
fusion reactors through the aurora to solar flares and supernovae.

* * *

TOOLS TO TRACK DOWN SPEEDY STRUCTURES IN THE PLASMA

CLUSTER consists of four spacecraft to be launched together in September
1995, on Ariane 5, the latest rocket in the Ariane series. The orbit is
planned to reach a maximum height of 130 000 kilometres, 21 times the radius
of the Earth. All four spacecraft are identical, each weighing 500 kilograms,
and carry instruments to measure the magnetic field, the distribution of
positive ions and electrons with energies between one and a million electronvolts,
the electric field and waves in the plasma up to frequencies of 250 kilohertz.

Although the design of the four spacecraft is relatively conventional,
there will be some technical challenges for the mission because no one has
operated four identical spacecraft together before. The spacecraft must
also make especially fast and accurate measurements. They must be fast,
because if the spacecraft are 500 kilometres apart, and a structure in the
plasma is moving at 100 kilometres per second, it crosses the distance between
the spacecraft in 5 seconds. Accuracy is important, because the differences
between readings from each spacecraft might be small compared with the actual
values.

Cluster must also send large amounts of information back to the ground
from each craft. This stretches the capability of the transmitters on-board
and the receiving stations on the ground.

Furthermore, to determine the gradients in the density of the particles
or the magnetic field, scientists on Earth must know the distance between
the spacecraft and the direction of the line between them at all times.
This has never been tried with four spacecraft; it is going to require particular
care. Cluster will have an experienced team of engineers from the European
Space Agency and the aerospace industry and physicists from space science
institutes in Europe and the US, many of whom worked together to make a
success of Giotto, the mission that intercepted Halley’s Comet in 1986.

Cluster carries two novel instruments. One fires a beam of electrons
away from the spacecraft and detects them after they have made a complete
circle around the direction of the magnetic field. It measures the electric
and magnetic fields, which influence the path and speed of the electrons,
by finding out in which direction it needs to fire the particles in order
to hit itself in the back of the head, and by measuring the time before
it feels the sting. Since the circle is more than 2 kilometres across, measurements
of the fields are not affected by the spacecraft itself.

The second instrument will stop the spacecraft charging up by emitting
a beam of indium ions from a group of fine-pointed needles. When the metal
of the spacecraft is in sunlight, it gives out electrons because of the
photoelectric effect, and so builds up an electric charge. The ions emitted
by the new device will dissipate that charge, and hold the electric potential
of the spacecraft at less than 1 volt different from the plasma. This will
make it possible to detect electrons with very low energies.

The project is part of an international programme to study space from
the Sun to the Earth, which includes the European satellite SOHO, the Solar
Heliospheric Observatory. The American space agency, NASA, will place one
spacecraft in the solar wind and one in a polar orbit around Earth inside
the magnetosphere. Japan will place one in the distant regions of the Earth’s
magnetic tail. The Soviet Union is likely to have a satellite in an equatorial
orbit and one in a similar orbit to the four Cluster craft but at a distance
of a few times the radius of the Earth from it. This latter spacecraft will
make it possible to monitor plasma structures at two scales, corresponding
to the distances between Cluster and the Soviet spacecraft as well as the
spacing of the Cluster group.

Alan Johnstone lectures in physics and astronomy at University College,
London, and is a principal investigator on the Cluster mission.