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70 ASTRONOMY • OCTOBER 2019
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
OUR MAGNETIC POLES SHIFTED
IN THE PAST. ARE WE OVERDUE
FOR ANOTHER SHIFT, AND HOW WILL
THAT AFFECT OUR ELECTRONICS, THE
CONTINENTS, AND THE SUN’S EFFECT
ON US WHILE THE POLES SHIFT?
Alan L. Strzemieczny
Riverside, California
AI
The geomagnetic field is generated by f luid
convection in Earth’s core. How this dynamo
works is complicated, and the field that the core produces
is not symmetric. This has been something of a nuisance
to navigators for centuries — a compass needle does not
point exactly north almost everywhere. In the Arctic
Ocean, at a location known as the north magnetic dip
pole, the needle of a compass held in the horizontal plane
has no preferred orientation. At this location, if a com-
pass needle were allowed complete freedom of orienta-
tion, it would point straight down. Correspondingly, off
the coast of Antarctica, at the south magnetic dip pole, a
freely orienting compass needle would point straight up.
Will Earth’s
magnetic
poles flip?
Earth’s magnetic field changes over time. In particu-
lar, over thousands of years, the locations of the mag-
netic dip poles tend to wander around in the vicinity of
the geographic north and south poles. Occasionally, the
field’s intensity will diminish, and the dip poles will
stray far from the geographic poles. Sometimes the dip
poles will even swap hemispheres — in which case the
polarity of Earth’s magnetic field will have reversed.
This does not happen periodically; in some ways, rever-
sals occur randomly, the result of chaotic dynamics in
Earth’s core. The last time Earth’s geomagnetic polarity
reversed was about 780,000 years ago.
For a geomagnetic polarity that is reversed compared
with the present polarity, compasses would tend
(roughly) to point south!
Lately, scientists have noted that the north dip pole,
presently located off the coast of northern Canada, has
been migrating toward Siberia at about 34 miles
(55 kilometers) per year. Compared with the rate at
which the north dip pole has moved in the previous
couple hundred years, the present rate is relatively fast.
To top it off, the intensity of the field has decreased by
about 10 percent over the past 150 years. This has led to
speculation that Earth’s magnetic field is about to
reverse its polarity.
But let’s step back and consider a few more facts.
While the north dip pole is, for now, moving rather
rapidly, the south dip pole is not. And although geo-
magnetic intensity has been decreasing lately, we know
from paleomagnetism (magnetization preserved in
rocks) that the present intensity is about equal to the
long-term average.
The observed
locations of the north
(left) and south dip
poles have been
measured only
sporadically.
Measurements
showing their
positions are marked
with yellow squares.
The modeled
locations of the poles
from 1590 through
2020 are shown as
colored lines and
circles progressing
from blue (1590)
to yellow (2020).
Although the north
dip pole appears to
be moving relatively
rapidly, a complete
magnetic pole
reversal does not
seem imminent.
ASTRONOMY: ROEN KELLY,
AFTER NOAA
WANDERING
POLES