Earth's Inconstant Magnetic Field
12.29.03
Our planet's magnetic field is in a constant state of change, say researchers who are beginning to understand how it behaves and why.
Every few years, scientist Larry Newitt of the Geological Survey of Canada goes hunting. He grabs his gloves, parka, a fancy compass, hops on a plane and flies out over the Canadian arctic. Not much stirs among the scattered islands and sea ice, but Newitt's prey is there--always moving, shifting, elusive.
His quarry is Earth's north magnetic pole.
At the moment it's located in northern Canada, about 600 km from the nearest town: Resolute Bay, population 300, where a popular T-shirt reads "Resolute Bay isn't the end of the world, but you can see it from here." Newitt stops there for snacks and supplies--and refuge when the weather gets bad. "Which is often," he says.
Right: The movement of Earth's north magnetic pole across the Canadian arctic, 1831--2001. Credit: Geological Survey of Canada. [More]
Scientists have long known that the magnetic pole moves. James Ross located the pole for the first time in 1831 after an exhausting arctic journey during which his ship got stuck in the ice for four years. No one returned until the next century. In 1904, Roald Amundsen found the pole again and discovered that it had moved--at least 50 km since the days of Ross.
과학자들은 자기장의 극이 이동해왔다는 것을 알고 있습니다. 1831년에 처음 북극의 극점을 찾았는데 1904년에 관찰결과 원 지점보다 50km 이동해 온것을 발견했습니다.
The pole kept going during the 20th century, north at an average speed of 10 km per year, lately accelerating "to 40 km per year," says Newitt. At this rate it will exit North America and reach Siberia in a few decades.
극점은 20세기에도 계속 북쪽으로 년당 10km의 속도로 이동해 왔는데, 최근에는 40km의 속도로 가속됐습니다. 앞으로 수십년 안으로 극점은 북미를 떠나서 시베리아로 갈것이라고 합니다.
Keeping track of the north magnetic pole is Newitt's job. "We usually go out and check its location once every few years," he says. "We'll have to make more trips now that it is moving so quickly."
뉴위이 이야기 하길 "우리는 몇년에 한번씩 극점을 찾으러 나가는데 현재는 아주 빨리 변하고 있어서 좀더 자주 나가야 합니다.
Earth's magnetic field is changing in other ways, too: Compass needles in Africa, for instance, are drifting about 1 degree per decade. And globally the magnetic field has weakened 10% since the 19th century. When this was mentioned by researchers at a recent meeting of the American Geophysical Union, many newspapers carried the story. A typical headline: "Is Earth's magnetic field collapsing?"
지구의 자기장도 그 힘이 19세기에 비해 10퍼센트 정도 약해졌다고 합니다.
Probably not. As remarkable as these changes sound, "they're mild compared to what Earth's magnetic field has done in the past," says University of California professor Gary Glatzmaier.
Sometimes the field completely flips. The north and the south poles swap places. Such reversals, recorded in the magnetism of ancient rocks, are unpredictable. They come at irregular intervals averaging about 300,000 years; the last one was 780,000 years ago. Are we overdue for another? No one knows.
때로는 자기장의 남북 극반동이 일어나기도 합니다. 그런 반동은 광물에 새겨진 자기흔을 보면 30만년에 한번씩 일어나는걸로 보이고, 마지막 극 반동은 78만년전에 일어난걸로 보이는데, 다음 반동은 언제 날지? 아무도 모릅니다.
Above: Magnetic stripes around mid-ocean ridges reveal the history of Earth's magnetic field for millions of years. The study of Earth's past magnetism is called paleomagnetism. Image credit: USGS. [More]
According to Glatzmaier, the ongoing 10% decline doesn't mean that a reversal is imminent. "The field is increasing or decreasing all the time," he says. "We know this from studies of the paleomagnetic record." Earth's present-day magnetic field is, in fact, much stronger than normal. The dipole moment, a measure of the intensity of the magnetic field, is now 8 x 1022 amps x m2. That's twice the million-year average of 4 x 1022 amps x m2.
To understand what's happening, says Glatzmaier, we have to take a trip ... to the center of the Earth where the magnetic field is produced.
어떤일이 일어나고 있는지를 이해하기 위해서는 자기장이 만들어 지는 지구 깊숙이 까지 들어가야 합니다.
At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it "the inner core." It's really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2o of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core."
지구의 가운데에선 딱딱한 철의 구형이 있습니다. 태양의 표면만큼 뜨겁습니다. 연구자들이 부르기를 내핵이라고 합니다. 이것은 정말 세상속의 세상입니다. 내핵은 달의 70퍼센트 만큼의 면적이 됩니다. 고유의 속도로 회전하며 바깥층의 지구 속도보다 경도 0.2정도 빠르게 회전하는데 내핵에는 고유의 액상의 바다도 있는데 아주 깊은 층의 액체오 외핵층이라 합니다.
Right: a schematic diagram of Earth's interior. The outer core is the source of the geomagnetic field. [Larger image]
Earth's magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has "hurricanes"--whirlpools powered by the Coriolis forces of Earth's rotation. These complex motions generate our planet's magnetism through a process called the dynamo effect.
지구의 자기장은 지구내부의 철의 액상바다에서 나오는데 전자성을 뛰며 항상성의 운동을 가집니다. 지구의 내핵과 밖의 회전에 따라 소용돌이를 만드는데 다이나모 효과라 합니다.
Using the equations of magnetohydrodynamics, a branch of physics dealing with conducting fluids and magnetic fields, Glatzmaier and colleague Paul Roberts have created a supercomputer model of Earth's interior. Their software heats the inner core, stirs the metallic ocean above it, then calculates the resulting magnetic field. They run their code for hundreds of thousands of simulated years and watch what happens.
슈퍼컴퓨터로 관측을 하는데..
What they see mimics the real Earth: The magnetic field waxes
and wanes, poles drift and, occasionally, flip. Change is normal, they've learned. And no wonder. The source of the field, the outer core, is itself seething, swirling, turbulent. "It's chaotic down there," notes Glatzmaier. The changes we detect on our planet's surface are a sign of that inner chaos.
지구의 자기장은 消長하며 지축은 이동하며. 자기장의 연원인 외핵은 끓으면서 소용돌이 치고 유동하여 아주 카오스적 입니다. 지국표면에 나타나는 변화는 내핵의 신호입니다.
They've also learned what happens during a magnetic flip. Reversals take a few thousand years to complete, and during that time--contrary to popular belief--the magnetic field does not vanish. "It just gets more complicated," says Glatzmaier. Magnetic lines of force near Earth's surface become twisted and tangled, and magnetic poles pop up in unaccustomed places. A south magnetic pole might emerge over Africa, for instance, or a north pole over Tahiti. Weird. But it's still a planetary magnetic field, and it still protects us from space radiation and solar storms.
Above: Supercomputer models of Earth's magnetic field. On the left is a normal dipolar magnetic field, typical of the long years between polarity reversals. On the right is the sort of complicated magnetic field Earth has during the upheaval of a reversal. [
More]
수퍼컴퓨터로 본 자기장의 그래프입니다. 왼쪽은 정상적인 자기장이며, 오른쪽은 자기장 축이 이동할때의 그래프 입니다.
And, as a bonus, Tahiti could be a great place to see the Northern Lights. In such a time, Larry Newitt's job would be different. Instead of shivering in Resolute Bay, he could enjoy the warm South Pacific, hopping from island to island, hunting for magnetic poles while auroras danced overhead.
Sometimes, maybe, a little change can be a good thing.
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