Exploring origin of aurora borealis


Last week’s brief history of the aurora borealis might lead us to conclude that humans have seen aurora in far southern and northern latitudes since we were humans. However, we were utterly clueless about its causes. The definitive explanation would have to wait until 1967, as we shall see.

The first clue about an aurora’s origin came from the work of British scientist William Gilbert during the latter part of the 16th century.

Gilbert attempted to solve one of the great mysteries of the time. Why did a suspended, magnetized needle always point to the north? Others had speculated that the needles naturally pointed toward Polaris, the North Star, although the exact mechanics were unclear. Others speculated that a large magnetic island existed in the unexplored far north.

Gilbert suspected that Earth itself was a magnet. He invented the terrella, a model of Earth with a magnetic field to model our planet’s magnetic characteristics.

To do so, he carved a lodestone, a naturally occurring hunk of magnetite, into a sphere resembling Earth. He then passed a magnetic compass over the surface of the terrella and discovered that it always pointed toward the north magnetic pole of his model.

Gilbert published his discovery in 1600 under the daunting title “De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure,” which roughly translated from Latin means “On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth.”

Thus, we have one key element of the path of aurorae — Earth’s magnetic field. The final answer to the aurora’s causes didn’t begin to dawn until the early 20th century, over 300 years later.

Norwegian astronomer Kristian Birkeland was obsessed with aurorae. He noted that displays happened during periods when there were a lot of sunspots. He thus assumed that the particles causing auroral displays emanated from the sun. But why did those displays center in rings around the north and south poles?

Birkeland found an answer by resurrecting Gilbert’s terrella. By shooting a beam of electrons, then called cathode rays, at a terrella, Birkeland was able to reproduce the auroral ring effect.

The task then became to detect the lines of magnetism flowing from the poles and see if they matched actual auroral displays.

Birkeland began a lifelong task of mapping the lines of magnetic force. To do so, he instigated a series of expeditions to the mountains of northern Norway.

Those expeditions were expensive, and Birkeland’s efforts were out of the mainstream. Consequently, funding for his research was difficult to get.

Birkeland looked for commercial uses for his cathode-ray technology to fund his research. He founded a successful business that used cathode rays to manufacture artificial fertilizer.

His theory had its flaws. In 1616, a year before his death, he finally admitted that the sun’s emanations contained protons in addition to electrons.

As with most groundbreaking research, Birkeland’s work had its detractors in the astronomical community. Among them was eminent British geophysicist Sydney Chapman. He argued that electric currents could not cross the vacuum of space because the electrons need a medium to flow through. Therefore, Birkeland’s currents had to be generated on or by the Earth.

Chapman’s argument met with wide approval. After all, didn’t the light from the sun require a medium, called the aether, to travel through to get to Earth?

Mainstream astronomers continued to disparage Birkeland’s work after he died in 1917. However, as astronomers realized that a medium like the aether wasn’t necessary to transmit light, Birkeland’s ideas began to gain widespread acceptance.

Finally, in 1963, the U. S. Navy satellite 1963-38C provided definitive proof. It carried a magnetometer into space above the highest level of Earth’s atmosphere, the ionosphere.

The satellite observed magnetic disturbances on nearly every pass over Earth’s polar regions. At first, astronomers were confused by what they were, but by 1967, they realized that the disturbances were aligned with Earth’s magnetic field.

Birkeland’s currents are everywhere in our solar system where a strong magnetic field combines with an atmosphere — from Jupiter’s and Saturn’s atmospheres to the sun’s hot plasma.

Over the next year, we can expect an increase in sunspots as the sun reaches the peak of its 11-year sunspot cycle. On a night during an auroral display, look for the complex striations in the aurora’s magnificent light, and remember that Birkeland died long before astronomers eventually named them Birkeland currents in his honor.

Tom Burns is the former director of the Perkins Observatory in Delaware.

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