4
Connecting Sun to Earth
Some say that the Northern Lights are the glare of the Arctic ice and snow;
And some say that it’s electricity, and nobody seems to know.
But I’ll tell you now—and if I lie, may my lips be stricken dumb—
It’s a mine, a mine of the precious stuff that men call radium.
Robert W. Service, “The Ballad of the Northern Lights”
In the years leading up to Carrington’s flare and Loomis’s auroral show, scientists had slowly become aware that one of the connections between the Sun and Earth might be magnetism. Successive experiments with the invisible force that attracts and repels objects and guides electricity forced scientists to consider that what they could see on the Sun and in the night skies was only half as important as what they could not see.
Around A.D. 1000, Chinese inventors used the mysterious magnetic properties of lodestone to develop the compass, and within a few hundred years, floating lodestones and magnetized needles pointed the way to the north and south for sailors and scientists. But it wasn’t until a personal physician to Queen
Elizabeth I, William Gilbert, examined the subject that anyone made a systematic attempt to explain why a compass worked. In the late sixteenth century, Gilbert compiled the wisdom of his predecessors and began his own series of experiments and demonstrations with magnets, a few of which were eventually performed in front of the Queen. He took a spherical magnet that he called a terrella—little Earth—and moved a small compass around its surface. Wherever he moved it, the needle continued to point to the sphere’s magnetic north pole. This led Gilbert to propose that the Earth itself was a giant magnet, with north and south poles, as if a great bar magnet had been buried inside. He published his ideas in 1600 in a treatise entitled De magnete: Magnus magnes ipse est globus terrestris, or On the Magnet: The Earth Itself Is a Great Magnet. It was the beginning of the modern era of geomagnetism and geophysics.
More than a century after Gilbert’s exposé on magnets, a London watch and scientific instrument maker found that an unseen force was tinkering with his magnetized needles. In 1722, George Graham was measuring the deviation of the compass needle from true north.1 He noticed that on some days the needle made irregular motions that he could not explain. The needle danced even when the compass was kept stationary inside of a glass box. Graham, who was a fellow of London’s Royal Society, published his mysterious observations in the Philosophical Transactions of the Royal Society, but he had no satisfactory explanation.
Seventeen years later Graham (in England) and Anders Celsius (in Sweden) simultaneously detected the same sort of unexplained, irregular deflections of the compass needle. Celsius and his assistant, Olof Hiorter, noted that the deflection seemed to occur when auroras danced in the sky. Though the two men did not have a name for their phenomenon—Alexander von Humboldt would not devise one until the 1830s—Graham and Celsius had made the first direct observation of a magnetic storm.
The scientific understanding of electromagnetism is the great triumph, and the most important event, of the nineteenth century.
The pioneering physics experiments of Charles Augustin de Coulomb, Hans Christian Oersted, André-Marie Ampère, Michael Faraday, Carl Friedrich Gauss, Wilhelm Weber, and Joseph Henry uncovered the laws that link electricity and magnetism. They learned that changing magnetic fields produce electric currents and electric currents produce magnetic fields. The brilliant James Clerk Maxwell (who is buried close to Charles Darwin and Isaac Newton in Westminster Abbey) would develop his theory of electromagnetism, including the crown jewel—that light is an electromagnetic wave. The discovery and control of electricity and magnetism were as important to civilization as the discovery of fire.
Part of the exploration was led by such promoters of science as Alexander von Humboldt and General Edward Sabine, who set up global networks of magnetic observatories. Starting in the 1830s, Sabine, Gauss, Weber, and von Humboldt launched the “magnetic crusade” to tease out the secrets of Earth’s magnetic field. They began charting the daily magnetic disturbances that Graham and Celsius discovered. The worldwide scope of these magnetic storms suggested that the environment was reacting to something grand in the space around Earth.
By 1859 there were enough magnetic observatories and interested observers to allow Loomis and Stewart to chronicle so precisely what we now know was the first great space weather event. In the years following, scientists made the first comprehensive global attempt to fit all the pieces of the space weather puzzle into one cohesive picture. And it was a catalyst for more extensive studies of the physics of the Earth—geophysics—and the space around it—space physics.
Much of the work over the next 40 years was centered on improving the observations, with both better instruments and systematic standardized measurements. For instance, American astronomer George Ellery Hale invented the spectroheliograph in 1892, a device that allowed him to view the Sun in individual wavelengths of light. By July 15 of that year, Hale was able to photo-
graph the evolution of a large solar flare—a flare that preceded a magnetic storm about 19 hours later. He also used his new instrument to determine that sunspots were intensely magnetic.
By the end of the nineteenth century, the evidence was mounting that the Sun was affecting the magnetic field around Earth and stirring up storms and auroras. Many magnetic storms occurred in tandem with reports of solar flares or of large sunspot regions near the Sun’s equator. Other times, as scientist E. W. Maunder discovered, storms recurred in 27-day periods, a span of time equal to the rotation rate of the Sun. Maunder boldly asserted in a paper to Britain’s Royal Astronomical Society that “our magnetic disturbances have their origin in the Sun.” But no one had a satisfactory explanation for the physics of the connection; namely, how and why the Sun should impact Earth in this way. And not all were convinced. The influential president of the Royal Society of London, Lord Kelvin, wrote in 1892 that “the supposed connection between magnetic storms and sunspots is unreal.”
In Norway, physicist Kristian Birkeland used theory, experiment, and observations to try to provide a comprehensive physical explanation for the electromagnetic link between the Earth and the Sun. Birkeland had been a student of Henri Poincaré, one of the leading physicists of his time. He had done noteworthy theoretical work on electromagnetism, producing the first general solution of Maxwell’s equations. In fact, one of Birkeland’s papers—published in the journal Comptes Rendus in 1894—was referenced in the American Journal of Physics as recently as 1982. Nearly 100 years later, Birkeland’s pioneering work was still relevant to physics.2
But Birkeland’s true scientific love was the aurora, and he made it the central focus of much of his research. Reviving William Gilbert’s experiments, Birkeland placed a magnetized sphere representing Earth—his own terrella—inside a vacuum chamber and fired cathode rays at it (see Figure 7). From centuries of reports and firsthand experience, Birkeland knew that auroras were mostly a phenomenon of the polar regions.3 So Birkeland was not necessarily surprised when he fired cathode rays at the terrella and
FIGURE 7. Kristian Birkeland works in his laboratory to simulate the aurora by shooting beams of electrons at his terrella, or “little Earth.” The apparatus could generate up to 20,000 volts. Photo from The Norwegian Aurora Polaris Expedition, 1902–1903, by Birkeland, published in Christiana, Norway, in 1908.
saw that the beam followed the magnet’s field lines and hit the sphere near the poles. He surmised that the Sun must have been shooting beams of corpuscles (what we now call electrons) toward Earth, where the planet’s magnetic field pulled them in near the poles. His young colleague, mathematician Carl Stoermer, added that since those “corpuscles” carried electric current, the magnetic field of Earth would be disturbed, as in a magnetic storm.
Birkeland’s experiments failed to account for one of the most important traits of auroras: they are common around the polar regions but exceedingly rare at the poles themselves. Essentially, auroras form a ring around the poles but do not form over the poles. In a few experimental instances, Birkeland’s beam produced a ring of light and what appeared to be a dark hole near the poles of his terrella. But mostly he observed just a cap of light on his “little Earth.” Stoermer analyzed the motion of the electrons math-
ematically, but he could not find a compelling reason why electrons from the Sun should avoid the poles themselves.
Birkeland’s theory of electron beams from the Sun had other problems. The repulsion between the like-charged electrons would quickly expand the beam; by the time the beam reached Earth it would be too diffuse to cause magnetic storms. Critics also pointed out that if the Sun got rid of many electrons via Birkeland’s beam, it would soon become positively charged, and the negative electrons would not be able to escape. At a time when scientists knew about electrons—but the proton had not yet been discovered— Birkeland could give no satisfactory answer. Yet he refused to give ground on his theories. Birkeland’s observations led him to other remarkable conclusions. In particular, he argued strenuously that electrical currents from space (carried by his electron beams) flowed down along Earth’s magnetic field horizontally for a few hundred kilometers in the atmosphere and then back out to space.
Finally, Birkeland’s theory of electron beams failed to account for the occurrence of magnetic storms close to Earth’s equator. Again, Stoermer searched for an answer. In studying the behavior of electrons in a magnetic field, he found that some particles would reach the terrella near the poles, while others would become trapped around the equator, hovering at some distance from the little Earth. The electrons never actually reached the terrella at the equator, so how could Birkeland’s electron beams from the Sun cause havoc at midlatitudes?4
Birkeland continued to pursue his ideas, but over time the quality of his work declined. Alex Dessler, a pioneering space physicist and Birkeland enthusiast, has suggested that Birkeland suffered from mercury poisoning, a common hazard for laboratory scientists of the time. Birkeland died in Japan in 1917, eight months short of his fiftieth birthday. His untimely death short-circuited the Nobel Prize for which he was being considered.
Just as Birkeland exited the scene, Sydney Chapman entered it. A brilliant theorist, Chapman made several seminal contributions to space physics, but he did not think much of Birkeland’s ideas. He began to investigate magnetic storms following his own instincts
and approaches. In 1930, Sydney Chapman and colleague Vincent Ferraro thought they had the best explanation for how the Sun could stir up the atmosphere and magnetic field of Earth. The two scientists postulated that magnetic storms and auroras were caused by clouds of electrically neutral gas, with equal amounts of negatively and positively charged particles; we now call this mixture of electrons and protons the fourth state of matter, plasma. Chapman and Ferraro argued that these clouds of particles were ejected from the Sun during a solar flare, flew across empty interplanetary space, and enveloped the Earth. These clouds would be excellent conductors of electricity and so would generate currents and distort Earth’s magnetic field. Looking at more than a hundred years of magnetic observatory data, Chapman and Ferraro noted that most magnetic storms began with a sudden increase in the strength of Earth’s magnetic field worldwide. That jump, they proposed, heralded the arrival of the cloud from the Sun.
But the plasma clouds from the Sun weren’t just smacking directly into Earth. Chapman and Ferraro proposed that Earth’s magnetic field carved a bubble or “cavity” out of space, one that would deflect and repel much of the spray from the Sun. And somehow this interaction of the Earth’s magnetic field (dubbed the magnetosphere by geophysicist Thomas Gold in 1959) would cause some particles to be trapped around the Earth in a ring of electric current around the equator, though they could not explain why. Though some of the details were incorrect, the “Chapman-Ferraro cavity” formed the basis for the modern concept of space weather.
Although Chapman was right about many things, he was not necessarily right to dismiss many of Birkeland’s theories. Swedish physicist Hannes Alfvén kept many of Birkeland’s ideas alive, often in defiance of the conventional scientific wisdom. He saw merit in Birkeland’s direct electrical connections between Sun and Earth and he updated many of those theories. Alfvén in his own right made several important theoretical discoveries in plasma physics (the physics of ionized gases). As with Birkeland, Alfvén’s theories were not always accepted at first but once understood became crucial for understanding the physics of plasmas on Earth and in space.
And in the end he saw the scientific community verify Birkeland’s idea of electric currents flowing from space down into the ionosphere and back out again. In 1970, Alfvén won the Nobel Prize for his work.
Pursuing and analyzing Chapman and Ferraro’s theories, other observers pointed out that the Sun wasn’t just ejecting intermittent plasma clouds. In fact, there was a steady breeze. Looking at the tails of comets, German scientists Cuno Hoffmeister and Ludwig Biermann proposed in the 1940s and 1950s that the Sun was constantly emitting something extra, something more than just light and occasional plasma blobs. They noted that comets had two tails—one of dust, one of ions—and only the dust tail could be explained by the pressure of sunlight pushing against the comet. The ion tail was not a steady, constant stream like the dust tail; it wiggled and bent regardless of the direction of the comet. Occasionally, the ions accelerated in a burst, though the speed of the comet had not changed. Hoffmeister and Biermann advanced a theory that the Sun was emitting a steady stream of particles, a “solar corpuscular radiation.” These streams of plasma from the Sun, ebbing fast and slow, must be sloughing ions off of the comets. Of course, like the ring current around Earth, no one could explain why this radiation should exist.
In trying to decipher the structure of the corona of the Sun (its outer atmosphere), Eugene Parker of the University of Chicago developed a new theory about the Sun and its emissions. Logic suggested that the pressure and density of the corona should drop off considerably at great distances from the Sun. But observations and calculations showed that the corona seemed to escape and flow away from the Sun. In a famous paper published in 1958, Parker showed how the corona would not only expand but also speed up until it became supersonic. This “solar wind” would then spiral through the solar system, filling the space between the planets. Parker also showed how the solar wind would carry the solar magnetic field into interplanetary space. His idea that the solar wind could carry its own magnetic field would eventually
become crucial to understanding how solar activity could affect the Earth and its field.
With the launch of the Sputniks by the Soviet Union and the Explorers by the United States during the first International Geophysical Year (1957–1958), centuries of scientific theories, remote observations, and wild speculation were finally confirmed or rejected by firsthand observation.
Equipped with a Geiger counter developed by James Van Allen, George Ludwig, and other scientists at the University of Iowa, the Explorer 1 satellite was launched on January 31, 1958. One of the goals of the flight—besides proving that the United States could orbit a satellite, as the Soviets had done with Sputnik 1 and 2— was to detect high-energy particles in space. Van Allen and his team were expecting to detect radiation from cosmic rays and other astrophysical sources, as researchers had detected with scientific balloon flights to Earth’s upper atmosphere. And, in fact, their Explorer 1 Geiger counter did detect such rays. But during the flight, the high-energy particle counts rose higher than expected, reaching the top of the scale before dropping out completely. Since Explorer 1 did not have a data recorder, Van Allen’s team could receive data only when the satellite passed over a few ground stations. Hence, they didn’t know whether the loss of data was a glitch in the instrument, a new space phenomenon, or a problem with the radio signals.
When Explorer 3 was launched on March 26, 1958—this time with a data recorder—the mysterious readings of the Geiger counter became clear. As the satellite rose up to the apogee of its orbit, the particle counts rose steadily until they reached the highest level, stayed at the maximum for a while, and then abruptly dropped to zero. On the satellite’s way back down to perigee, the reverse process occurred. Van Allen and colleague Carl McIlwain realized that the Geiger counter dropped to zero because the radiation in space had saturated their detector—there were too many particles
for the instrument to count, so it shut down. Earth was surrounded by high-energy particles, prompting University of Iowa scientist Ernie Ray to exclaim: “My God, space is radioactive!” Analysis of the Explorer 1 and 3 data led Van Allen and his team to declare that two doughnut-shaped regions of magnetically trapped electrons and protons surrounded Earth. Colleagues named the feature the “Van Allen radiation belts.”
Actually, the radiation belts could have been named the “Vernov belts” if Cold War politics had not gotten in the way of science. Sputnik 1 did not include any scientific instruments, but Sputnik 2 carried a simple set of spectrophotometers for detecting solar radiation, built by Russian scientist Sergei Vernov. As with the U.S. Explorer 1 satellite, Sputnik 2 could only relay information when the satellite was within radio visibility of the Soviet ground stations. So the data received were spotty and hard to interpret. Vernov’s instrument had detected some increases in radiation levels in space, but there was no way to know if the phenomenon was local or global.
In May 1958, Sputnik 3 carried the first substantial package of scientific instruments on a Soviet flight, including another of Vernov’s Geiger counters. To overcome the problem of radio contact from Russia, engineers built a tape recorder to collect and save the data for later transmission. The tape recorder did not function properly in ground tests, but to the bewilderment of scientists the head of the Soviet rocket program, Sergei Korolev, ordered the launch of Sputnik 3 before the problem could be fixed. As the scientists feared, the tape recorder did not work properly during the flight, and the scientific payoff of the mission was meager.
To make matters worse, Vernov’s instrument did detect some tantalizing and puzzling increases in radiation during the real-time radio passes, but he was never able to retrieve the most useful data. The apogee of the orbit—when it would have passed through the belts—occurred while the satellite was over Australia. Scientists down under tracked Sputnik, but when they asked the Soviets for the key to the radio signals, they were rebuffed. Vernov’s instrument probably collected enough data to fully map the inner and
outer radiation belts, but instead that first scientific prize of the Space Age was left for Van Allen and his Explorer 4 mission in the summer of 1958.
Vernov’s protégé, Konstantin Gringauz, ran into Korolev years later while walking in Moscow’s Gorky Park. Gringauz asked Korolev why he had ordered the launch when everyone knew that the tape recorder would not work. Korolev told him that the order had come directly from Nikita Khrushchev. Thrilled by the successful launches of Sputnik 1 and 2, the premier had asked his scientists for another Soviet spectacle in time to boost the fortunes of the Italian Communist party in that country’s elections.
In 1959, Gringauz and colleagues restored some Russian scientific pride when they used “ion traps” (plasma particle detectors) flying on the Soviet Lunik 2 and 3 spacecraft to detect the first traces of the solar wind flowing into the magnetosphere “cavity” around Earth. Three years later, when the U.S. Mariner II flew outside Earth’s protective magnetosphere toward Venus, it detected a solar wind that flowed constantly. That solar wind fluctuated in fast and slow bursts and had periods of 27 days, as Maunder had suggested 60 years before. Gene Parker and other solar wind theorists were vindicated.
Finally, by the early 1970s, scientists were able to get above the murky atmosphere of Earth and take sophisticated telescopes into space for a better look at the Sun and the space around Earth. In 1971, a science team led by Richard Tousey of the U.S. Naval Research Laboratory in Washington, D.C., detected the first signs of Chapman and Ferraro’s plasma clouds from the Sun with the NASA Orbiting Solar Observatory 7. Their coronagraph—which created a false eclipse by blocking the visible disk of the Sun from the field of view—allowed them to spy on the solar atmosphere. What they saw was a complex cluster of fast-moving clouds of plasma erupting from the Sun.
Three years later researchers from the High Altitude Observatory (HAO) in Boulder, Colorado, built a more sophisticated coronagraph that was sent up to the Skylab space station. Bob MacQueen and his science team at HAO were able to take time-lapse photos
of these blobs of plasma, which would eventually come to be known as coronal mass ejections. Dave Rust and colleagues from American Science and Engineering used their own Skylab instrument—a soft X-ray telescope—to discover holes in the corona of the Sun, where Parker’s solar wind could periodically stream toward Earth. Just as Maunder had suspected 100 years before, Rust and colleagues found that solar wind jetted out of holes in the corona and the streams would sweep past Earth in a cyclical pattern of 27 days (one solar rotation), much like the beacon from a lighthouse rotates its shaft of light in and out of view.
With large numbers of energetic particles trapped in the space around Earth and present throughout the solar system—and pictures of great blobs of plasma peeling off of the Sun—the puzzling circumstantial evidence that led Carrington, Loomis, and Stewart to connect solar activity with earthly fireworks was no longer a matter of speculation. It was now an observed fact.
The Cold War competition to explore space produced a few other surprises, not all of them pleasant. Suspicious of the possible source of the radiation in the Van Allen belts—nuclear weapons exploded by their enemies—and fearful of the effects of exploding weapons in space, the United States and the Soviet Union conducted several nuclear “tests” in the magnetosphere. Military leaders and scientists were partly motivated by physicist Nicholas Christofilos’s prediction that nuclear explosions in near-Earth space could produce artificial radiation belts with significant military effects.
To test Christofilos’s theory, the United States conducted Operation Argus in 1958, the “world’s largest scientific experiment.” From ships in the South Atlantic, about 1,000 miles southwest of South Africa, the Navy launched three modest warheads to 100, 182, and 466 miles in altitude. They studied how the energetic particles interacted with Earth’s magnetic field, with radar and communications devices, and with the electronics of satellites
and ballistic missiles. They found that Christofilos was right, as did the Soviets in similar experiments. Space radiation could indeed disrupt the work of man-made instruments and electronics.
The experimentation reached its absurd, Dr. Strangelove-style climax with the Starfish High Altitude Nuclear Test program in 1962. From an island near the equator in the middle of the Pacific Ocean, the United States launched a 1.4-megaton nuclear bomb about 300 miles into space. The explosion supercharged the Van Allen radiation belts and created artificial belts 100 to 1000 times stronger than normal space radiation levels. The high-energy electrons damaged the solar arrays of several satellites and caused three of them to fail. The electromagnetic pulse generated by the test led to power surges in electrical cables in Hawaii, blowing fuses, streetlights, and circuit breakers. Residual radiation from the experiment lingered in the magnetosphere for nearly seven years.
The Argus and Starfish tests were a stern warning to the nascent satellite industry and to military leaders that space might be a dangerous and difficult place to work.
