The Sun is a restless star. From time to time it emits powerful electromagnetic storms that, when reaching Earth, can disable any electronics-based technology, from satellites to radio communications and GPS. It is not something unknown. On March 13, 1989, a cloud of high-energy protons and electrons from the Sun arrived at our planet. As a result, spectacular auroras appeared in the skies of the polar regions (the most exposed to this type of particles). But the storm left five million Canadians without electricity, damaged many transformers in the United States and interrupted the signal of several satellites.
Three solar cycles later, the sun is once again in a high phase of activity. Hence, electromagnetic storms are expected which, again, could cause technological problems. But the storms will also bring with them the blooming auroras. An aurora is a celestial luminous phenomenon, especially common in high latitude regions, produced by the collision of energetic particles with the gases of the Earth's upper atmosphere. Its observation has generated myths and ideas in an attempt to explain its nature.
The oldest known mention of an aurora appears in a Chinese text from around 2600 BC. C., known as Chu shu chi nien (Chronicles written on bamboo strips). It goes like this: “Fu Pao, mother of the Yellow Emperor, Shuan-Yuan, saw great lightning circulating around the star Su of Bei-Don [the Big Dipper] and illuminating the entire field. Then, she became pregnant.” The impossibility of contemplating the mentioned constellation, located in the northern hemisphere, during a storm, suggests that, in reality, the author is referring to an aurora.
Another relevant fact is the link that was believed to exist between the auroras and the birth of children, a myth shared by the Siberian tribe of the Chuvash. However, the most widespread belief among northern peoples, such as the Inuit, was to assume that the deceased were present at the dawn.
Further south, the Greeks believed that the god Apollo moved every winter to a northern region called Hyperborea and that he manifested himself there in the form of the dawn. Aside from the myth, it is likely that Aristotle (4th century BC) observed a real one in Greece itself, an area of little auroral activity. With a scientific spirit, in his Meteorology, the philosopher tried to give an explanation to the colorful flashes of the auroras. According to him, they were produced by the collision of the vapor caused by the Sun with the fire of this star at the height of the sublunary sphere (the space between the Moon and the Earth).
The wise Plutarch (1st-2nd centuries) was more accurate in his description: “For seventy-five days a burning body of great extension was seen in the sky, like a cloud of fire, which [...] moved with movements intricate and regular [...]. The flames of fire were carried in all directions like falling stars.”
Meanwhile, in neighboring Rome, its inhabitants associated the reddish color of the auroras (the one they usually adopt in Mediterranean latitudes) with celestial military activities. Thus, Seneca says that, during the empire of Tiberius, several cohorts ran towards the city of Ostia believing that it had been burned at night. Even the death of Julius Caesar was related to the figures of knights and infantry that were said to have been seen at dawn in the year 44 BC. C. Some perceptions that Seneca, disseminator of Aristotle's theses, never shared.
In the 16th century the auroras continued to be phenomena that were difficult to explain. Due to their infrequent appearance in central Europe, they were considered a sign of bad omen. Furthermore, its reddish flashes provoked prayers and processions among believers to mitigate possible divine punishment.
In most cases, attempts were made to understand them based on supernatural explanations. For example, a 1560 aurora seen near Bamberg in Germany was interpreted as sparks from the clash of swords in a celestial battle. And another ten years later in Kuttenberg, in what is now the Czech Republic, like a set of torches resting on a starry cloud.
The fanciful models, however, would not last much longer, thanks to progress in scientific knowledge and observation instruments.
Galileo Galilei coined the term aurora in an essay that he published with one of his students, Guiducci, in 1616. In it, after describing the amazing illumination of the northern sky, he concludes: “… thus forming for us this northern lights.” Three years later he offered an erroneous explanation of its nature. For Galileo, the bright lights were the result of the heating of the air around the Earth and the reflection of sunlight on the atmosphere.
Nor was René Descartes correct, around the same time, in his speculations about the process of formation of the auroras. The versatile Frenchman assumed that these were due to a large number of “wind clouds” colliding, placing the air under pressure and exciting it. Although he did not rule out that they were the result of the reflection of sunlight on ice crystals at high altitude, an idea that was maintained for centuries.
From Great Britain, Edmund Halley, famous for having calculated the orbit of the comet that bears his name, also followed the auroras closely. He tells himself that he was dying to see one, although he believed that he would die before that. Luck, however, smiled on him in 1716, when he beheld the most splendid dawn of the century.
In a treatise he presented at the Royal Society, he postulated the existence of a subtle force of nature, which he called “magnetic effluvium,” capable of penetrating all the substances and pores of the Earth, crossing the planet from pole to pole. With a brilliant intuition – although his thesis is not entirely correct – he related the theory of magnetism, formulated a century earlier by his compatriot William Gilbert, with the phenomenon of the auroras. Halley suspected that magnetic particles could sometimes be luminous and alter Earth's atoms to the point of producing light in space.
Upon his death in 1742, the Swedish astronomers Anders Celsius, to whom we owe the centigrade scale that bears his name, and Olof P. Hiorter, his brother-in-law, experimentally demonstrated the conspicuous relationship between auroras and magnetic fields. After making more than six thousand systematic observations between that year and the previous one, they appreciated the remarkable coincidence that occurred when, when an aurora appeared, magnetic disturbances were generated that deflected the compass needles.
Five decades later, the British chemist and physicist Henry Cavendish calculated the altitude of the auroras using the ancient method of triangulation. He deduced that northern light occurs in the upper atmosphere, between 100 and 130 km away from the Earth's surface (according to NASA, it occurs at about 95 km).
Another important contribution to the knowledge of this phenomenon took place at the beginning of the 19th century, when the French physicist Jean B. Biot put an end to the widespread belief that auroras were reflections of sunlight. During his stay on the Shetland Islands in Scotland, he observed the auroras with a polarimeter, an instrument that allows studying optically active substances. If the northern light had been a reflection of the sun's light, its beams would have vibrated in a single plane, since it would have been polarized. But the polarimeter detected no signs of this.
Biot's observation was soon confirmed by the Swedish physicist Anders Jonas Ångström. Using a spectrometer, useful for analyzing the characteristic spectrum of a wave movement, such as that of the light of the auroras, Ångström also demonstrated that these are composed of luminous gases (nitrogen and oxygen), instead of other substances assumed until then, such as water or ice particles.
In 1859 the close relationship between solar activity and auroras was evident. It all started when British astronomer Richard Carrington saw bright fragments of white light around some sunspots. It was the first time that a solar flare, or coronal mass emission, had been observed. About eighteen hours later, the magnetic instruments at the Kew Observatory in London measured important variations in the Earth's magnetic field.
Meanwhile, on the other side of the Atlantic, Elias Loomis, a professor at Yale University, confirmed that the northern lights that illuminated the United States for a week were of unusual brilliance. You could even read the newspaper in the middle of the night. Loomis exchanged information with other scientists and correspondents and found that the aurora had also appeared in Europe, Asia and certain areas of the southern hemisphere.
Half a century later, the Norwegian physicist Kristian Birkeland connected the dots between the Sun, magnetism and the auroras. Birkeland discovered that the magnetic field guides electric particles towards the vicinity of the poles, where they impact. And, in a model that is very close to reality, he imagined that the Sun could also shoot beams of particles towards the Earth, where its magnetic field would carry them near the poles. The auroras would thus be the result of this interaction.
The Anglo-American professor Sydney Chapman and his associate Vicent Ferraro proposed, in the 1930s, a thesis slightly different from Birkeland's. Instead of beams of particles coming from the Sun, they spoke more correctly of something more diffuse: clouds of particles launched from the Sun, electrically charged, that crossed space and enveloped the Earth, producing auroras. Due to their quality as electrical conductors, they could generate currents and distort the Earth's magnetic field. Today we know that these clouds are actually solar wind, composed of a hot mixture of protons and electrons. It is the so-called plasma.
The launch of satellites into space, since the late 1950s, has contributed to studying and understanding the auroras in greater depth. However, there is still much to know. Today, the ability to predict a solar storm, or sudden eruption of particles from the Sun prior to the appearance of auroras, is limited, experts acknowledge. And no one is unaware of the damage it can cause. You only need to go back to 1989.
Until a few years ago, researchers have evaluated the probability of future activity of the Sun by analyzing two-dimensional satellite images of the star and, more recently, real-time 3D images. With this they were able to predict the arrival of a solar flare a few hours in advance, and their objective was to predict large solar eruptions.
Today, some satellites orbiting the Earth can detect alterations in solar activity, but they only allow warning about their impact just 45 minutes in advance. Scientific satellites such as the Solar Orbiter, from ESA in collaboration with NASA, and the Parker Solar Probe, from the US space agency, can extend predictions to more than a day. AI models are also being applied to available solar storm data to help improve warning systems.
This text is part of an article published in number 534 of the magazine Historia y Vida. Do you have something to contribute? Write to us at redaccionhyv@historiayvida.com.
