Source:
The International Herald Tribune, 15 July 2004, By William J. Broad, NYT
Earths Magnetosphere NASA Graphic
The collapse of the earth’s
magnetic field, which both guards the planet and guides many of its creatures,
appears to have started in earnest about 150 years ago. The field’s strength has
waned 10 percent to 15 percent so far and this deterioration has accelerated of
late, increasing debate over whether it portends a reversal of the lines of
magnetic force that normally envelop the earth.
All magnetic objects produce
invisible lines of force that extend between the poles of the object. An
easy way to visualize this is to spread iron filings on a sheet of paper
and place a bar magnet under the paper. The iron filings will arrange
themselves around the magnet and along the magnetic field lines.
In the simplest terms, Earth can
be thought of as a dipole (2-pole) magnet.
Magnetic field lines radiate between Earth’s north and south magnetic poles just
as they do between the poles of a bar magnet. Charged particles become trapped
on these field lines (just as the iron filings are trapped), forming the
magnetosphere. Source NASA
During a reversal, the main field
weakens, almost vanishes, and then reappears with opposite polarity. Afterward,
compass needles that normally point north would point south, and during the
thousands of years of transition much in the heavens and earth would go askew. .
A reversal could knock out power grids, hurt astronauts and satellites, widen
atmospheric ozone holes, send polar auroras flashing to the equator and confuse
birds, fish and migratory animals that rely on the steadiness of the magnetic
field as a navigation aid. But experts said the repercussions would fall short
of catastrophic, despite a few proclamations of doom and sketchy evidence of
past links between field reversals and species extinctions.
Although a total flip may be
hundreds or thousands of years away, the rapid decline in magnetic strength is
already damaging satellites. Last month, the European Space Agency approved the
world’s largest effort at tracking the field’s shifts. A trio of new satellites,
called Swarm, are to monitor the collapsing field with far greater precision
than before and help scientists forecast its prospective state.
“We want to get some idea of how
this would evolve in the near future, just like people trying to predict the
weather,” said Gauthier Hulot, a French geophysicist working on the satellite
plan. “I’m personally quite convinced we should be able to work out the first
predictions by the end of the mission.” . The discipline is one of a number –
like high-energy physics and aspects of space science – where Europeans have
come from behind to seize the initiative, dismaying some American experts.
No matter what the new findings,
the public has no reason to panic, scientists say. Even if a flip is imminent,
it might take 2,000 years to mature. The last one took place 780,000 years ago,
when Homo erectus was still learning how to make stone tools. Some experts
suggest a reversal is overdue. “The fact that it’s dropping so rapidly gives you
pause,” said John Tarduno, a professor of geophysics at the University of
Rochester. “It looks like things we see in computer models of a reversal.” In an
interview, Tarduno put the odds of an impending flip at more likely than not,
adding that some of his colleagues were placing informal bets on the possibility
but realized they would probably be long gone by the time the picture
clarified.
Deep inside the earth, the
magnetic field arises as the fluid core oozes with hot currents of molten iron
and this mechanical energy gets converted into electromagnetism. It is known as
the geodynamo. In a car’s generator, the same principle turns mechanical energy
into electricity. No one knows precisely why the field periodically reverses but
scientists say the responsibility probably lies with changes in the turbulent
flows of molten iron, which they envision as similar to the churning gases that
make up the clouds of Jupiter. . In theory, a reversal could have major effects
because over the ages many aspects of nature and society have come to rely on
the field’s steadiness. When baby loggerhead turtles embark on an 8,000-mile, or
12,800-kilometer, trek around the Atlantic, they use invisible magnetic clues to
check their bearings. So do salmon and whales, honeybees and homing pigeons,
frogs and Zambian mole rats, scientists have found. . On a planetary scale, the
magnetic field helps shield the earth from solar winds and storms of deadly
particles. Its so-called magnetosphere extends out 37,000 miles from the earth’s
sunlit side and much farther behind the planet, forming a cometlike tail. Among
other things, the field’s collapse, scientists say, could let in bursts of
radiation, causing a variety of disruptions. . Charles Jackman, an atmospheric
scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has
worked with European colleagues on a computer model that mimics the
repercussions. A weak field, they reported in December, could let solar storms
pummel the atmosphere with enough radiation to destroy significant amounts of
the ozone that protects the earth from harmful ultraviolet light.
Ultraviolet radiation, the
short, invisible rays from the sun, can harm some life forms, depress crop
yields and raise cancer rates, causing skin cancer and cataracts in humans.
Jackman said that the ozone damage from any one solar storm could heal naturally
in two to three years but that the protective layer would stay vulnerable to new
bursts of radiation as long as the earth’s magnetic field remained weak.
