Australian Sky & Telescope - April 2018

CRUCIAL OFFSET (TOP):

SKY & TELESCOPE /

LEAH TISCIONE; (BOTTOM): ESA / DTU SPACE; FAST CURRENT: ESA / AOES MEDIALAB

field strength more precisely than ever before. Measurements

taken by the European Space Agency’s three orbiting Swarm

satellites, combined with earlier data from the CHAMP

(Challenging Minisatellite Payload) and Ørsted spacecraft,

show that the field strength rose 2% over Asia and fell 3.5%

over North America from 1999 to 2016. And that broad

intensity dip south of the equator, known as the South

Atlantic Anomaly (see panel at left), is moving 20 km

westward each year and continues to weaken.

These local variations demonstrate that Earth’s magnetic

field is far more complicated than a simple dipole. Chaotic,

turbulent fluid motions in the outer core drag regions of

strong and weak fields around. These movements concentrate

magnetic field lines in some areas — at high latitudes, for

example — and thin them in other areas, such as the South

Atlantic. Swarm researcher Chris Finlay (Technical University

of Denmark) points out that these chaotic motions explain

why an extended region of weaker field is currently being

pulled westward toward the Americas, while regions of

stronger field are converging near Asia.

Swarm data enabled scientists to identify a ‘jet stream’

of molten iron in the outer core that is flowing westward at

about 40 km per year. This unexpected feature lies mostly

under Alaska and Siberia. The stream appears to be moving

patches of strong magnetism at high northern latitudes,

leading to the local changes seen at the surface. It’s uncertain

how much the jet stream is connected to the wandering of

the north magnetic pole, and whether processes at lower

latitudes are also contributing.

And superposed on top of these small-scale changes is the

general weakening trend in the global field over the past 175

years, leading to speculation that we could be heading toward

a reversal.

The ultimate flip-flop

Reversals are clearly documented in the paleomagnetic

record, particularly in sequences of seafloor rocks created

along tectonic spreading centres. These planet-wide records

confirm that Earth’s magnetic field has been flip-flopping

for hundreds of millions of years. But the reversal rate has

changed drastically over very long time scales.

For example, there are no recorded reversals from 120

million to 83 million years ago, one of three such ‘supercrons’

dating back 550 million years. Over the past few tens of

millions of years, the reversals have occurred about once every

250,000 years on average. But they’re inherently unpredictable

— the last major reversal was 780,000 years ago — making it

natural to wonder if our planet is long overdue for a flip-flop.

Scientists have not directly observed a reversal in action,

so it’s not entirely clear what happens. Still, they generally

agree that the field strength doesn’t just drop to zero and then

ramp back up. Instead, the dipolar field will probably break

down into many small-scale localised patches. Bundles of

magnetic field lines should intersect the surface in numerous

SCRUCIAL OFFSETTop:Earth’s magnetic field is strongly

dipolar,asifanenormousbarmagnetlaynearitsmiddle.But

thefieldistiltedslightlyandoffsetfromEarth’scentre.Above:

This snapshot of the strength of the magnetic field at Earth’s

surfaceinJune2014revealsanespeciallyweakareabetween

SouthAmericaandAfrica,knownastheSouthAtlanticAnomaly,

producedbythedipolefield’soffsetfromEarth’scentre.

SFAST CURRENTESA’s Swarm satellites have discovered a ‘jet

stream’movingwestwardinEarth’sliquidoutercore. The molten

current appears to be gradually speeding up.

Rotation

axis

Inner

radiation

belt Outer

radiation

belt

South Atlantic Anomaly

Dipole offset

from centre

by 500 km

Magnetic

axis

S

N

FieldStrengthatSurface(nanoteslas)

20,000 30,000 40,000 50,000 60,

Swarm

spacecraft

‘J e t s t r e a m’

in outer core

South

Atlantic

Anomaly

FIELD FLIP AHEAD?

16 AUSTRALIAN SKY & TELESCOPE April 2018