Originally published 5 March 1984
On the evening of September 30, 1961, observers in the northeastern United States were treated to a spectacular display of northern lights — the aurora borealis. It was the first I witnessed (I grew up in the south) and remains the best show of the lights I have seen.
The aurora that night went on for hours. The northwestern horizon was hung with shimmering green curtains. The luminous drapery wavered and folded upon itself as if moved by a gentle wind. Near the zenith a 4th of July fireworks show was under way. Streamers and starbursts of red and green light streaked the sky.
The source of these pyrotechnics was the sun. Two days earlier a massive flare was recorded on the face of the sun by observers at solar observatories. Within minutes, a dark filament embedded in the sun’s surface exploded into blazing sheets of flame, looping up into the solar atmosphere and tearing away into space. The flare blasted into space an enormous flux of charged particles — protons and electrons mostly — with velocities over a million miles per hour. When those particles collided with atoms and molecules in the Earth’s upper atmosphere, they caused the air to glow.
Particularly violent solar flares excite the most spectacular displays of auroral lights. But even in less violent moments the sun throws off a stream of subatomic particles called the solar wind. Life on the surface of the Earth is sheltered from this potentially harmful breeze by the Earth’s magnetic field.
Most of the wind of charged particles from the sun is deflected by the Earth’s magnetic field and flows harmlessly around the planet like water around a rock in a stream. Some of the more energetic particles penetrate the magnetic shield and spiral in along lines of magnetic force to collide with the atmosphere in high northern and southern latitudes. Almost all of these successful intruders are stopped by the atmosphere, igniting the auroral lights. Only the most energetic particles reach the surface of the Earth, and most of these come to the ground in polar regions where life is sparse.
Reversal of the field
Several decades ago Earth scientists became aware of an unexpected feature of the Earth’s magnetic field: at unpredictable intervals, typically several times every million years, the field reverses. North magnetic pole becomes south pole and vice versa. During a reversal, a compass needle would swing around and point the other way.
In fact, University of Minnesota geophysicists Subir Banerjee and Donald Sprowl have recently reported that the Earth’s magnetic field has decreased in strength more than 50 percent in the last 4,000 years — a decrease that could indicate the beginning of a reversal of the Earth’s magnetic poles.
While geophysicists do not yet understand what causes the bizarre flip-flops of the Earth’s field, geologists studying the magnetic orientation of rock in the sea floor have discovered as many as 12 field reversals in the last 9 million years, the last one occurring 710,000 years ago.
The turnabouts are not instantaneous. The field slowly collapses and builds up again with the opposite polarity. A reversal may take ten or twenty thousand years. For a period of several thousand years the planet is effectively without a magnetic field!
If there are long intervals when the Earth is magnetically naked before the solar wind, might not plants and animals feel the brunt of the celestial bombardment?
The fossil record holds tantalizing hints that polarity reversals might affect the course of evolution. In one study, J.D. Hays of the Lamont-Doherty Earth Observatory examined radiolaria in 28 drill samples of sea-floor sediments brought up from the floor of the Pacific. Radiolaria are one-celled marine organisms that secrete delicate and beautifully filigreed skeletons of silica. Hays found eight species of radiolaria that became extinct within the past 2.5 million years. Of the eight, six species disappeared from the sediments at the time of polarity reversals. Other studies of microorganisms in sea-floor sediments have tended to confirm Hays’ work.
How living things are affected
At least five mechanisms have been proposed to explain how the collapse of the Earth’s magnetic umbrella could have consequences for life.
The most obvious danger is the direct bombardment of exposed organisms by the solar wind (and by cosmic rays from space). The bombardment might produce genetic damage that leads to the extinction of species. It turns out that this is unlikely. Even in the absence of the magnetic field, the Earth’s atmosphere should provide a reasonably effective shield against incoming particles.
A second proposal links polarity reversals with geological activity in the Earth’s upper mantle. According to this idea, internal turbulence that triggers spasms of surface volcanic activity is also the trigger for a polarity reversal. In this scenario, it is dust thrown into the atmosphere by massive volcanic eruptions that cause evolutionary chaos. Magnetic reversals are not the cause of the extinctions, only a coincident effect. Some cautious evidence has been offered to show that some past episodes of intense volcanic activity were in fact coincident with polarity reversals.
Polarity flip-flops might cause a change in climate. When the Earth is without its magnetic sheath, incoming charged particles increase ionization (atoms stripped of electrons) in the upper atmosphere. Ionization seems to enhance high altitude cloud cover. Cloud cover modifies climate. Drastic climatic change might lead to the extinction of certain life forms.
A fourth theory relates the collapse of the Earth’s magnetic field to a depletion of the ozone layer. An increase in the level of ionization in the stratosphere encourages chemical reactions that lead to diminishing ozone levels. This in turn exposes the surface of the Earth to potentially harmful ultraviolet radiation from the sun.
A last explanation for extinctions during reversals locates the source of the trouble within the bodies of living creatures. In recent years substances responsive to magnetic fields have been found in animals as diverse as bacteria, molluscs, mud snails, honey bees, butterflies, pigeons and dolphins. These internal “compasses” seem to play a role in navigation. A flop of the field could cause dangerous disorientations. Bacteria, for example, might find themselves swimming toward the surface of a pond rather than the muddy bottom.
Future research will decide the extent to which life is in jeopardy during magnetic polarity reversals, and by what mechanisms the reversals take their toll.
It has now been seven hundred thousand years since the last polarity change, considerably longer than the average interval between reversals. In the not so distant future life may once again find itself exposed before the solar wind.
In the meantime, the beautiful auroral lights are a healthy sign that the planet’s magnetic field and atmosphere are doing their job, protecting life on Earth from a potential danger from the sun.
