IT ALL happened so quickly. During the small hours of a cold Canadian
night, the voltage of Quebec’s power grid began to fluctuate alarmingly.
Seconds later, the lights went out across the entire province. Some 6 million
people were without electricity for nine hours. Within two days, NASA had lost
track of some of its spacecraft and the northern lights were glowing in the
sky south of London. These events, which took place in mid-March 1989, had the
same cause – a monumental storm the fiercest for 30 years.
But this was no terrestrial storm. It was a geomagnetic storm whipped up by
the Sun, which occasionally throws out bursts of high-energy particles and
clouds of ionised gas, or plasma.
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The particles can damage and confuse satellites, while the plasma buffets
the Earth’s magnetic field and makes it shudder. As a result, electric
currents surge around the upper atmosphere wreaking havoc. Power grids break
down as the changing fields induce direct currents in power lines built for
alternating current. Compass needles tremble, and the changing density of the
ionosphere – the charged layer of the atmosphere – disrupts short-wave radio
communications.
While nothing can prevent these storms, many people would be thankful for
some warning that they are on the way. So a network of space weather
forecasting centres has developed around the globe since the 1960s. At the
moment, the forecaster’s predictions are nowhere near as reliable as
terrestrial warnings of winds and rain, but they are certain of one thing: in
four years, the Earth is in for a season of spectacular storms as the Sun
throws an almighty tantrum. Fortunately, by then a host of experimental
satellites should have helped to sharpen up their forecasting powers.
For a variety of reasons, geomagnetic storms pose particular problems for
space-based systems. The accuracy of the Global Positioning System, for
example, which allows people to fix their position on Earth by receiving
signals from satellites, depends on the transmission properties of the
atmosphere. When those properties change unexectedly during a storm,
navigational fixes from the GPS can turn out to be wildly inaccurate.
The energetic electrons that race around high above the Earth during a
storm also cause problems by creating uneven build-ups of charge on
satellites. Eventually, sparks can fly from one part of the satellite to
another and trigger phantom commands.
The Canadian TV satellites Anik E1 and E2 succumbed to this effect on the
same day during a storm in 1994. According to Peg Shea and Don Smart, space
scientists at the US Air Force Phillips Laboratory in Massachusetts, rogue
electric currents commanded the satellites to turn their solar panels away
from the Sun. E1’s backup system held it steady, but E2’s did not: its
batteries drained and it floated out of control for six months, until its
orbit brought the solar panels back into sunlight.
Dragged from orbit
The energy from magnetic storms, along with increases in extreme
ultraviolet light from the Sun, can also have a dramatic impact on spacecraft
by heating up the atmosphere. As the temperature rises, the density of the
rarefied gas increases. For a satellite at an altitude of 400 kilometres, the
increase can be twentyfold, and the added drag is enough to pull the satellite
into a lower orbit. NASA’s ill-fated space station Skylab, which orbited at an
altitude of about 435 kilometres, was one victim of atmospheric heating. It
was dragged back to Earth in July 1979 after a period of high solar
activity.
High-energy particles from the Sun can also confuse spacecraft that
navigate by the stars. According to Shea and Smart, these particles can rip
through the protective glass window in the spacecraft’s star tracker faster
than the speed of light in the glass. As they bump into atoms along their
path, the atoms emit light that concentrates into blue flashes known as
Cherenkov radiation – the optical equivalent of sonic booms. Unless the star
tracker’s software filters out these flashes, they can be mistaken for the
light from distant stars. New constellations of bogus stars twinkle all
around, and as the familiar stellar landscape distorts, the telescope can lose
its bearings.
It is not just machines that are vulnerable to the Sun’s tantrums.
Astronauts are also at risk. Sometimes spacecraft pass through high-latitude
regions where very high-energy solar protons spill into the atmosphere during
storms. Sergei Avdeyev, a Russian cosmonaut who spent several months on the
Mir space station in 1992 and 1995, ranks a period of high solar activity
among his most terrifying experiences. He recalls seeing strange, unexplained
flashes of light.
“You could feel the Sun’s radiation everywhere,” he says. “I felt that the
particles of radiation were walking through my eyes, floating through my brain
and maybe clashing with some nerves.” According to Smart, high-energy proton
radiation can destroy the body’s cells: “In the most extreme cases, it could
be lethal for an unshielded astronaut.”
Help is at hand, however, from the network of space weather forecasting
stations. Today, ten centres worldwide collect data from local observatories
and satellites, and can issue alerts round the clock. Power companies at
threatened latitudes can isolate vulnerable parts of their power grids, GPS
users can switch to other means of navigation, and astronauts can retreat to
the safest area of their craft.
Part of the secret of accurate predictions is to understand solar activity
and its impact on Earth. Forecasters know that our planet’s neighbourhood is
constantly bombarded by the solar wind – plasma that has boiled off the Sun’s
outer atmosphere, or corona. At an altitude of about 60 000 kilometres, the
solar wind reaches the magnetosphere, the region in which the Earth’s magnetic
influence holds sway. The magnetosphere deflects the wind, but in the process
is compressed on the sunny side and dragged out on the night side into a long
tail.
The solar wind also carries with it magnetic field lines drawn out from the
Sun. If the magnetic fields of the Earth and wind have the same polarity, they
repel each other as if they were the like poles of bar magnets. But if the
wind’s polarity happens to be opposite to the Earth’s, the two fields merge.
They may then “reconnect” on the sunny side of the magnetosphere, rearranging
their magnetic field lines.(See Diagram) Stormy
weather: normally the solar wind stretches the Earth’s magnetosphere out into
space (top). But the fiercest geomagnetic storms take place when a coronal
mass ejection arrives with its magnetic field aligned in the opposite
direction to the Earth’s (middle). The two fields merge and may “reconnect” on
the sunny side (bottom). If this happens, the magnetosphere’s outer field
lines are peeled off and dragged towards the tail, which stretches and
inflates. The tail eventually springs back to its original shape, accelerating
charged particles in the magnetosphere towards the poles where they create
spectacular auroras This process peels off the outermost field lines
of the magnetosphere on the sunny side and drags them towards the tail. As
this happens, solar plasma can leak into the magnetosphere.FIG-mg20153501.GIF
On the night side of the Earth, the tail inflates and stretches before
snapping back to its original shape. This process accelerates charged
particles inside the magnetosphere along the Earth’s magnetic field lines,
towards the poles. Here, they collide with atoms in the atmosphere to create
the auroras – the northern and southern lights.
This is pretty average weather in space. But storms can strike at any time.
One source of really wild weather is coronal holes, regions of the solar
surface where the corona is less dense and magnetic field lines sprout
vertically out into space. They appear as dark patches on X-ray pictures and
usually form near the Sun’s poles. Andre´ Balogh of Imperial College,
London, leader of the magnetic experiments on the Ulysses mission, says that
the spacecraft scrutinised these holes in 1994 and 1995. It confirmed that the
wind from these regions streams out at around 800 kilometres per second,
around twice as fast as the ordinary solar wind from equatorial latitudes.
The trouble begins when, occasionally, a coronal hole reaches down to the
Sun’s lower latitudes. The wind from the coronal hole then ploughs into the
slower moving solar wind creating vast disturbances in space. This disturbed
plasma, with its tangled magnetic fields, can strike the Earth’s magnetosphere
much more violently than usual. More energetic particles from the
magnetosphere spill into the Earth’s ionosphere and atmosphere. Spectacular
auroras shimmer in the skies, and the fluctuating magnetic fields set up large
currents on the surface of the magnetosphere and in the ionosphere. It is
these complex currents that can damage the power grids and disturb short-wave
radio communication.
But the fiercest storms take place when the Sun fires a giant blob of
magnetised plasma towards Earth. According to Balogh, plasma blobs come from
regions of the Sun where vigorous convection tangles magnetic field lines. In
these cases fields of opposite polarity are forced so close together that they
reconnect violently, generating a massive shock wave. What happens next is
still up for grabs. Balogh suggests that magnetic reconnection beneath the
corona creates the power for solar flares, which accelerate particles to a
third of the speed of light. But reconnection within the corona can throw out
a mammoth blob of magnetised plasma – a coronal mass ejection, or CME.
These giant magnetic clouds first came to light in the 1970s. “Until
Skylab, we didn’t even know they existed,” says Smart. But now it is clear
that the Sun spits out CMEs several times a month. They have temperatures of
more than 1 million K, and masses of up to a billion tonnes. The largest carry
enough energy to boil a lake several thousand times the size of the Caspian
Sea. CMEs expand to widths of millions of kilometres, and can whip up storms
on Earth around two or three days after they leave the Sun. “The amount of
power that’s distributed to the magnetosphere in one of those major storms is
equivalent to the consumption of North America for a year,” says Smart.
Fortunately, the Sun’s activity gives away some clues that forecasters can
use to warn of impending trouble. Geomagnetic storms are most frequent, for
example, when the number of sunspots is rising to its maximum, or just after
the maximum. This means they tend to follow the mysterious 11-year sunspot
cycle. The storm that knocked out Quebec’s power grid struck close to the last
maximum in 1989, and the forecasters expect similar storms in 2000 and
2001.
Solar flares erupt most regularly close to the sunspot maximum, and
forecasters watch them to warn of particle showers. Flares are relatively easy
to see because they radiate energy right across the electromagnetic spectrum.
In some cases, once a flare has peaked, its payload of fast-moving particles
will reach Earth in less than 20 minutes. According to Ernest Hildner,
director of the Space Environment Center (SEC) in Boulder, Colorado, the
shower can go on for longer than 24 hours.
Rattling plasma
Storms due to coronal holes peak just after the sunspot maximum. Some holes
at lower solar latitudes last for up to a year and return after each solar
rotation. CMEs destined for Earth are, however, less predictable. They are too
faint to be seen against the Sun’s dazzling face, says Hildner, but there are
some clues that a shock wave is climbing through the corona. When the wave
“rattles” the surrounding plasma, it emits energy at a resonant frequency – in
this case, in the radio band – which depends on the density. As the shock wave
emerges through the thinning atmosphere, the radio broadcast drifts to lower
frequencies and reveals how fast the wave is moving. This gives a good guide
to the speed of the CME that the Sun will eventually unleash.
Forecasters at the SEC keep a constant eye on the state of the Sun’s
surface, and every day at noon announce the odds on whether a geomagnetic
storm will strike the next day. “At the present time, we are almost in the
state of an old farmer forecasting the weather,” says Hildner. “He’s lived on
his farm for a few years. He can wet his finger and hold it up to the wind. He
can look at the clouds going by, and he can make a pretty fair forecast.” But
Hildner admits that the forecasts are not very reliable. In fact 60 per cent
of the geomagnetic storm warnings issued by the Boulder centre turn out to be
false alarms. Worse, the centre fails to issue warnings for two-thirds of the
storms that do strike.
The trouble is that even if forecasters know a CME is heading for Earth,
they do not know what weapons it carries. It might tiptoe past and not disturb
anyone, or it might create havoc. According to Balogh, everything depends on
the polarity of the blob’s magnetic field. If the polarity is opposite to the
Earth’s, the two magnetic fields reconnect just as with the solar wind – only
this time a furious storm follows.
Unfortunately, forecasters have no way of knowing the orientation of the
CME’s magnetic field in advance. It might help if they had computer models
that would allow them to predict how storms evolve. But trustworthy models
have never emerged. “The bewildering complexity of things going on all the
time on the Sun is really frightening,” says Balogh.
Because of this complexity, solar physics is littered with unanswered
questions. Take coronal holes, for example. “We don’t really know why they
occur,” says Balogh. “That’s the bottom line at the moment.” Mystery also
surrounds CMEs. It is not clear if they are discrete blobs with their own
magnetic circuits or giant magnetic loops that lash out into space with their
magnetic feet still planted in the Sun. Both might be true.
Forecasters also run up against problems trying to understand how plasma
moves across the Solar System. Hildner says that timing predictions for storms
are inaccurate because some CMEs seem to slow down en route while others do
not – and no one knows why. And there’s still no consensus on exactly what
causes the currents that surge through the magnetosphere.
The answers to some of these questions may not be far from reach, however.
Later this year, the European Space Agency (ESA) plans to launch four
satellites to study the magnetosphere. The Cluster spacecraft, shaped like
giant biscuit tins with insect-like antennas, will fly in formation around the
boundary of the magnetosphere to try to find exactly where solar particles
invade. Balogh, who will lead the analysis of Cluster’s magnetic data, says
the craft should show conclusively that magnetic reconnection allows particles
to invade the magnetosphere.
A host of other spacecraft could also find some answers very soon. The
Solar and Heliospheric Observatory (SOHO), launched by ESA and NASA in
December, will study the outer layers of the Sun and the solar wind. This week
NASA plans to launch POLAR, a satellite that will catch energetic particles
over the Earth’s poles. Last August, the Russian Space Research Institute sent
a pair of spacecraft to the tail of the Earth’s magnetosphere. And later this
year, a second pair of Russian spacecraft is due to head for the regions above
the poles.
Early warning system
“We’re so excited and pleased to have that many probes in the magnetosphere
simultaneously,” says Hildner. “We’ll have the chance to understand the driver
that’s coming in from the Sun to whack the Earth.”
But even with all the information that these missions gather, the space
weather forecasters still need satellites “up wind” of the Earth to predict
what the planet is in for. The WIND spacecraft, launched by NASA in November
1994 to study the solar wind, showed just how this could be done. In October
last year, it caught a CME heading for the Earth at more than 3.3 million
kilometres an hour. The spacecraft probed the plasma blob and beamed back the
bad news: the magnetic field orientation spelt trouble. The SEC issued an
urgent alert and right on cue, magnetic turmoil broke loose on Earth just over
half an hour later. For two days, the northern lights danced as far south as
Denver.
It was just luck, however, that the CME washed over WIND when the
spacecraft was phoning home. WIND had been designed to record its measurements
and beam them back just once a day. Had the CME come a moment after its
communications channels went silent, the forecasters would never have known
what was about to hit them until it was too late.
Ideally, Hildner says, a spacecraft should patrol the region between the
Earth and Sun and report back round the clock. And in less than two years,
that wish could come true. In August 1997, NASA plans to launch the Advanced
Composition Explorer (ACE), a craft that will make observations of all the
solar, interstellar and galactic particles that mingle near the Earth. The
forecasters have funded an extra transmitter onboard the craft to continuously
transmit data back to Earth. ACE will sit near the point between the Earth and
Sun where their gravitational fields balance. “That is about an hour upstream
of Earth,” says Hildner. “When a disturbance washes over that spacecraft, the
signal will come down at the speed of light. So we can immediately infer that
half an hour or an hour later – depending on the speed of the disturbance –
the Earth will start to ring like a bell.”
For many forecasters, however, hitching a lift on a research craft is
second best. Pierre Lantos of the warning centre at Meudon, Paris, says that
these spacecraft tend to take on highly specialised missions. Their tasks are
frequently changed and they are usually only active for a couple of years.
Lantos wants dedicated, long-life spacecraft for forecasting – something that
the major space agencies do not see as a high priority. “NASA and ESA view
their roles as that of exploring and discovering,” says Hildner. “They are
understandably reluctant to tie up scarce resources in routine, boring
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Tedious though monitoring may be, the need for forecasts grows day by day.
Lantos says that the lighter, cheaper satellites now in vogue rely on ever-
smaller electronic components that are much more vulnerable to radiation
damage. Future storms will disrupt the orbits of satellites at a time when
their operators are demanding ever more precise positioning. And the number of
spacecraft patrolling the heavens is growing month by month.
When the Sun celebrates the new millennium with an almighty storm and sets
the northern skies ablaze again, there will be more to lose than ever before.
Weather forecasts from the Space Environment Center are available at
http://www.sel.bldrdoc.gov.