Sky and Telescope - March 2018 - 18

Field Flip Ahead?
Dipole offset
from center
by 500 km

Inner
radiation
belt

Magnetic
axis

Outer
radiation
belt

South Atlantic Anomaly

South
Atlantic
Anomaly

20,000

30,000

40,000

50,000

60,000

Field Strength at Surface (nanoteslas)

p CRUCIAL OFFSET Top: Earth's magnetic field is strongly dipolar, as if an enormous bar magnet lay near its center. But the field is
tilted slightly and offset from Earth's center. Above: This snapshot
of the strength of the magnetic field at Earth's surface in June 2014
reveals an especially weak area between South America and Africa,
known as the South Atlantic Anomaly, created by the dipole field's
offset from Earth's center.

Swarm
spacecraft

Short-Term Chaos
Magnetic signatures preserved in ancient rocks show that
Earth's dynamo has been operating at more or less its current
healthy level for almost all of our planet's history. But when
geophysicists zoom in with their equivalent of a microscope,
they see crazy things happening.
For example, they can now track changes in 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
(pictured 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

"Jet stream"
in outer core
p FAST CURRENT ESA's Swarm satellites have discovered a "jet
stream" moving westward in Earth's liquid outer core. The molten
current appears to be gradually speeding up.

M A RCH 2 018 * SK Y & TELESCOPE

Reversals are clearly documented in the paleomagnetic
record, particularly in sequences of seaﬂoor rocks created
along tectonic spreading centers. 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.

CRUCIAL OFFSE T ( TOP): SK Y & TELESCOPE / LE A H TISCIONE; (BOT TO M): ESA / DT U SPACE;
FAST CURREN T: ESA / AOES MEDIA L A B

Rotation
axis

Sky and Telescope - March 2018

Table of Contents for the Digital Edition of Sky and Telescope - March 2018