Australian Sky & Telescope - April 2018

http://www.skyandtelescope.com.au 17

WANDERING POLE: SOURCE: J. GLEN / R. COE / J. LIDDICOAT /

JOURNAL OF GEOPHYSICAL RESEARCH

; PREHISTORIC REVERSAL: YOHAN GUYODO & JEAN-PIERRE VALET /

NATURE

locations, including at mid-latitudes and the equator. As Peter

Driscoll (Carnegie Institution for Science) says, “You’d see

lots of South Atlantic Anomalies all over the place.” After a

few thousand years, the dipole field would eventually recover

its strength and turn back on with the opposite polarity.

Despite sensationalised media reports that our home

planet could be heading for a magnetic reversal ‘soon,’

researchers have no such short-term concerns. “The soonest

one might begin is probably a couple thousand years from

now,” says Gary Glatzmaier (University of California, Santa

Cruz), “and if it does, then the reversal itself would likely last

about 5,000 years. But it’s just as likely that one won’t occur

that soon.”

It’s possible, however, that we could instead be heading

toward a period of significantly reduced field intensity. The last

of these ‘excursions’ was the Laschamp event of 41,000 years

ago, when the magnetic poles moved substantially but didn’t

actually switch polarity. “The field strength has fluctuated by

10% many times in the past without resulting in a reversal,”

Finlay says, “and the presently observed decrease in the dipole

strength does not seem to be unusually large”.

In fact, Earth’s magnetic field is so chaotic that whether

it’s strengthening or weakening depends on your

time frame. “If you back out to a 10,000-year time

scale, we’re near an uptick,” says Driscoll.

Although the details of what triggers field

reversals remain unknown, Driscoll and other

geophysicists have successfully produced them

in computer simulations of fluid motions in the

outer core. As he explains, “Your model can be

a stable dipole fluctuating happily in its initial

orientation for a long time, and then it reverses

randomly, and then it will be happy in its new

orientation for a while.”

A geomagnetic reversal would be very

bad news, but it would not spell the end of

humankind. There is little or no correlation in the

fossil record between reversals and major extinctions of

life, so apparently the biosphere can withstand these flip-flops

without too much disruption.

But the negative impact on our society would probably be

huge. “People don’t realise how much of our technological

advances rely on the Earth’s magnetic field,” says Stanley.

“Our magnetic field lines protect all of our electronics from

high-energy particles from the Sun.”

With that natural shield greatly reduced, satellites would

be exposed to intense particle bombardment. Worse, we’d lose

the magnetic field’s ability to fend off solar outbursts. Even

a relatively modest solar flare or coronal mass ejection could

pump enough electrical energy into the lower atmosphere

to blow transmission lines over wide regions of the planet,

leading to long-lasting power outages affecting hundreds of

millions of people.

Our planetary neighbours

Although our field is considerably weaker than those of the

gas giants, Earth stands alone as the Solar System’s only

terrestrial planet that can currently muster a strong global

magnetic field.

Mercury has a very weak dipolar field with an average

surface strength only about 1% that of Earth’s. Venus has

W WANDERING POLE Lakebed sediments

reveal that Earth’s virtual geomagnetic pole

wandered all over the planet during the

Gauss-Matuyama field reversal, 2.58 million

years ago. The initial and final stages appear

in orange, rapid oscillations in dark grey, and

the main reversing event — including two

wide swings in latitude — in blue.

X PREHISTORIC REVERSAL Earth’s last field flip, the

Brunhes-Matuyama reversal, occurred 780,000 years ago.

Since then, the field strength has varied wildly. Grey denotes

measurement uncertainty; dots at far right show recent data

from volcanic flows.

2

4

6

8

10

12

Relative field strength

800,000 600,000 400,000 200,000 0

Time before present (years)

Standard error

Volcanic data

0

Paleomagnetic Excursions

Laschamp

Blake

Calabrian Ridge 2

West Eifel

Big Lost

Emperor

Delta La Palma

Brunhes-

Matuyama Jamaica/Pringle Falls

START

B

C

A

B

C

* * A