Look down, look up, look out!
http://www.economist.com/science/displaystory.cfm?story_id=9143913

Space weather
Look down, look up, look out!
KANGERLUSSUAQ, GREENLAND

The weather in space is controlled by events at the centre of the Earth.
A pity, then, that the magnetic field generated there may be about to go
into reverse

GREENLAND was discovered without the use of magnetic compasses. The
Viking longboats that arrived here in 982 relied on the stars and the
sun to maintain their orientation. But it was the compass (a Chinese
invention) that enabled later European navigators to bestride the world.
It was the compass, too, which revealed that the Earth itself has a
magnetic field and thus led to the first serious question about the
planet's internal structure: what is down there that generates this
field?
Such a question is not merely academic.

Besides directing compasses, the Earth's magnetic field reaches out into
space to direct the flow of the solar wind around the planet?forming a
structure called the magnetosphere (depicted above). When the wind hits
this field it creates a shock wave known as the bow shock. Most of the
protons and electrons of the wind go round this shock wave, and lots of
dangerous radiation from the sun is thus diverted away from the Earth.
There is, however, a growing body of evidence that the Earth's magnetic
field is about to disappear, at least for a while. The geological record
shows that it flips from time to time, with the south pole becoming the
north, and vice versa. On average, such reversals take place every
500,000 years, but there is no discernible pattern. Flips have happened
as close together as 50,000 years, though the last one was 780,000 years
ago. But, as discussed at the Greenland Space Science Symposium, held in
Kangerlussuaq this week, the signs are that another flip is coming soon.

One of those signs is that the strength of the field has been falling by
5% a century recently. A similar (though more rapid) diminution
accompanies the reversing of the sun's magnetic field, which happens
every 11 years or so. Other evidence comes from old navigation records.
Researchers such as Nils Olsen, of the Danish National Space Centre,
have used such records to chart the growth of patches of abnormal
magnetism. They are able to do so because these records use both compass
bearings and astronomical observations to locate a vessel. The changing
relationship between the two shows that patches of abnormal magnetism
have been growing off south-east Africa and in the South Atlantic.

Just when the magnetic field will flip is impossible to predict from
what is known at the moment; the best guess is that there are still
several centuries to go. Nor is it clear how long its protective shield
will be down. (The record in the rocks is little help, since a
geological eyeblink represents many human lifetimes.) But understanding
how the magnetosphere works now should help to deal with the
consequences if and when it vanishes.

Bright lights
One of the pioneers of the field was Dr Olsen's colleague Eigil
Friis-Christensen. Thirty-five years ago he took a boat north along the
west coast of Greenland. As he travelled, he set up instruments called
magnetometers to measure electric currents in the upper atmosphere.
These magnetometers and their successors have played the role that
barometers did for early weather forecasters. Then, the pattern of
pressure changes the instruments recorded tracked the passage of storms
in the atmosphere. Now, it is storms in the magnetosphere that are
recorded.
Dr Friis-Christensen's own work has focused on a structure known as the
"cusp". This separates the magnetosphere's two main compartments: the
crown, a round projection that stretches sunwards by about five times
the diameter of the Earth, and the tail, which is shaped as its name
suggests and runs far into space on the Earth's night time side.

The cusp is responsible for the famous auroras that grace high
latitudes. This is because it is at the cusp that magnetic field-lines
stream down towards the ground, acting as paths for electrons and
protons that have slipped past the bow shock. When these particles hit
the upper atmosphere they generate light in the same way that electrons
from the cathode of an old-fashioned television set do when they hit the
phosphorescent dots of the screen.
The famous night-time auroras (borealis in the north, australis in the
south) are the result of particles streaming in from the tail. But
particles come in from the crown, as well, forming invisible daytime
auroras that Dr Friis-Christensen was among the first to study.

Another Greenland-based instrument, a few miles from Kangerlussuaq, has
pinned down more details about the cusp. The Sondrestrom Upper
Atmospheric Research Facility has a 32-metre-wide radar dish. It
measures conductivity from an altitude of 60km to 600km and has helped
define the cusp's circuitry. That is important because, although it is
not technically part of the atmosphere, the plasma of charged particles
in the magnetosphere experiences what might be (and indeed often is)
referred to as weather.

Like the weather on Earth, this space weather has consequences. If it
gets nasty, communications satellites may be knocked out and radio
communications within the atmosphere disrupted. In extreme
circumstances, power grids may go down, too. Foul weather in space is
also bad news for astronauts. A bad storm could kill an unshielded
individual. But although the source of such foul space weather is
known?it happens when giant flares on the surface of the sun pour out
more protons and electrons than normal?the details depend on
structures within the magnetosphere that are only now coming under
scrutiny. Sometimes storms drift past the Earth with little impact. On
other occasions they pummel the magnetosphere's crown to about half its
normal distance from the Earth's surface. Furthermore, the
magnetosphere's structure is layered, like an onion. The same type of
particle can take on entirely different characteristics, depending on
which layer it is in. The art of forecasting space weather is in its
infancy.

How much longer it will remain within Dr Friis-Christensen's purview,
though, is moot. Barometers are now curiosities, as satellite-based
forecasting has taken over. The same thing is about to happen to space
meteorology. Five satellites, collectively called THEMIS, that were
launched in mid February, may make his magnetometers as old-fashioned as
the mahogany instrument hanging in a hotel lobby.