In the middle of the eighteenth century, little was known about the nature of space weather. The aurora had been observed for centuries, but its nature and cause remained enigmatic. On the other hand, the discovery of magnetic disturbances (which at the time referred to erratic compass needle deflections) was relatively new, and no one knew what to make of this unusual phenomenon. However, a few pioneering scientists made significant strides in understanding the connections between the aurora and magnetic measurements, and their new scientific insights enabled the first space weather prediction to be made. Although a curiosity back then, today predictions of space weather phenomena are critically important as ever more engineered systems that can be affected by space weather processes are employed for commerce, science, exploration, and national security purposes. To truly understand and appreciate this first prediction, it is useful to look at the circumstances that allowed it to happen.
The aurora borealis, or northern lights, has been observed for centuries. Other than cave drawings that might be interpreted as depicting the aurora, the earliest possible mention is in Chinese writings from around 2600 B.C. The first certain written accounts are by Greek philosophers from around 450 B.C. [Eather, 1980]. Aristotle made one of the first attempts to explain the aurora by considering it within the framework of his theory of four elements (earth, water, air, and fire), where he speculated that the aurora was burning air set on fire by rising too high [Siscoe, 1978]. The aurora figured very prominently in the mythology and folklore of early Scandinavian cultures and from Roman through medieval times was often viewed as a harbinger of disaster or war [Brekke and Egeland, 1994]. In the 1600s, Galileo was one of the first to use the term “Aurora Borealis” [Siscoe, 1978]; he also speculated that air would sometimes rise high enough to reflect sunlight and produce the aurora [Eather, 1980].
Much interest was generated in the aurora by the great auroral display of 1716. Edmond Halley (of Halley's Comet fame) published a paper describing the “surprising appearance of lights seen in the air,” where he described his observations and speculation about their cause [Halley, 1716]. He developed his theory by connecting three pieces of information: 1) The auroral forms he saw seemed to be vertical but less perpendicular the farther south they were; 2) the aurora was seen more frequently in most northern lands and never near the equator; and 3) the dipole nature of the magnetic field around a spherical magnet (Figure 1). In his theory, the aurora was a luminous magnetic vapor that somehow seeped out from inside the Earth near the poles and was guided by the magnetic field, making Halley the first to connect the aurora to the Earth's magnetic field. Others gave different explanations; one popular view was that the aurora was a sign from God indicating that judgment was near. Even though there was much speculation, in the mid-1700s, the nature and cause of the aurora was still quite a mystery.
Something else that had been known for centuries was the magnetic nature of the Earth. A landmark in the study of Earth's magnetism was the publication of “De Magnete” by William Gilbert in 1600 [Gilbert, 1600]. Considered by many to mark the beginning of modern science, the book was certainly the beginning of the scientific study of terrestrial magnetism. Gilbert set out to learn all he could about magnetism, from books, experiments, and observations (an approach that marked a change from the “believe the expert” mentality that prevailed in the Middle Ages). One of Gilbert's experiments involved the study of how magnetized needles behaved around a spherical magnet. He noted that they reproduced the behavior of real compass needles, not only pointing toward the poles but also slanting downward at an angle that depended on the distance from the poles (this inclination of balanced compass needles had been discovered by Robert Norman in 1581 [Norman, 1581]). Gilbert's experiments led him to conclude that the Earth itself was a giant magnet, a revolutionary idea at the time as the directionality of the compass needle had previously been attributed to various sources (e.g., the pull of the North Star or the attraction of continental landmasses).
It had been known since the 1400s that compasses did not always point directly north (the difference is what we now call magnetic declination), but in 1635, a somewhat surprising discovery was made. Henry Gellibrand reported that, in fact, the angle between true north and magnetic north changed slowly over time: regarding magnetic declination at London, he wrote, “Hence therefore we may conclude that for the space of 54 years … there hath been a sensible diminution of 7 degrees and better” [Gellibrand, 1635]. Gilbert had just declared the Earth a giant permanent magnet, but now for the first time, the idea was introduced that the Earth's magnetic field had a dynamic nature. Even further, in 1724, George Graham, a British watchmaker, noted rapid changes in magnetic declination, as much as half a degree in a few hours (possibly the first recorded magnetic storm observation) [Graham, 1725]. There was growing evidence that the Earth's magnetic field exhibited “disturbances” on fairly short time scales.
Shortly thereafter, in 1741, a critical discovery was made by Olaf Hiorter, a Swedish astronomer. Working with Anders Celsius on the study of magnetic variations, Hiorter published a paper with the following observation from 1 March 1741:
A motion of the magnetic needle has been found which deserves the attention and wonder of everyone. Who could have thought that the aurora and the magnet would have a connection, and that the aurora, when it reached its highest elevation… could, within a few minutes, cause considerable deviations of a few whole degrees in the magnetic needle? [Hiorter, 1747]
In this paper, two independent lines of study, the aurora and the magnetic variations, were linked together for the first time. Celsius was also involved in coordinating one of the first geographically separate scientific observations with none other than George Graham. The idea was to determine whether the magnetic variations they observed were due to local effects or something larger. Through their cooperation, they were able to confirm that the aurora, which they knew appeared over large areas, was producing magnetic variations over wide areas as well—in at least one case, simultaneously in London and Uppsala, Sweden [Hiorter, 1747].
The stage was then set for the first space weather prediction to be made by Pehr Wargentin (Figure 2). Pehr Wargentin was a Swedish astronomer and mathematician. Born in 1717, Wargentin became interested in astronomy after observing a lunar eclipse at age 12. He attended the University of Uppsala (Olaf Hiorter was one of his instructors) and in 1749 became Secretary of the Royal Swedish Academy of Sciences. He was also the first director of the Stockholm Observatory. In a 1750 publication, Wargentin made the following observation:
On February 28, 1749, at 4 o'clock in the afternoon, I saw that within a few minutes the needle jumped westward a half of a degree. At once I predicted to Mr. Ekström that we should expect aurora that evening; and this actually happened. [Wargentin, 1750]
Wargentin observed a sudden magnetic variation at 4:00 in the afternoon and, based on Hiorter's earlier discovery, predicted that the aurora would appear that night, and in fact it did. Thus, the first space weather prediction (of the occurrence of visible aurora) was made by Pehr Wargentin and was also verified by him. However, even though the two phenomena (aurora and magnetic disturbances) were linked together in the mid-1700s, researchers as yet had no knowledge of electric currents. It would be another 70 years (Hans Christian Oersted's experiment in 1820) until the discovery that electric currents could produce magnetic fields, allowing an understanding of how this connection worked and the realization that the aurora was somehow electrical. That makes Wargentin's prediction that much more impressive.
The first prediction was not spectacular but was enabled by the pioneering efforts of Graham, Celsius, Hiorter, and many others. Theirs was not easy work; since there were no continuous magnetometer traces yet produced, magnetic field studies involved hours and hours of staring at magnetic needles suspended on threads (Figure 3) and recording the minute changes as they were visually observed. The dedication of these researchers, however, led to the linkage of two unrelated (at the time) phenomena. It would be another 100 years before the connection with yet another unrelated field of study, sunspots, would start to truly unlock the secrets of magnetic storms (a term that did not exist at the time, first used by Alexander von Humboldt in 1808), events that disrupted the first electrical telegraphs in the middle of the nineteenth century and today affect numerous engineered systems on the ground and in space. Wargentin's prediction was one important link in the long chain of discoveries and connections that tell the story of space weather, and this fledgling science was well on its way.