by Leonid LAZUTIN, Dr. Sc. (Phys. & Math.), leading researcher, Institute of Nuclear Physics, Lomonosov Moscow State University

Describing the thrilling display of Northern Lights, the Aurora Borealis, was a painful, excruciating experience to me. I've seen these magic lights many a time in the forty years of my life in the Far North. Putting out the electric lights in my observatory, I could gaze at the sky for hours on end-feasting my eyes on the glimmering arches. All that was music to me. But I think it's next to impossible to picture this natural phenomenon in words, give a touch of poetic to it...

Therefore in my search for proper metaphors I chose to turn to the French astronomer Camille Flammarion (1842-1925) who, in his famous work L'Atmosphere, offers an ample collection of descriptions made by many travelers and natural scientists. But try as I did, I could not find what I wanted. So many superlatives and errors in the same breath! What a disappointment...

LOOK AT THE SKY

Let us take, for example, the color of auroras. These are actually the excited lines of atmospheric nitrogen, oxygen and hydrogen that come up against a background of green and red, not counting in the ultraviolet radiation invisible to the naked eye. Well and good, opening L'Atmosphere, we read: "...White; pink; red; golden; golden- red; light yellow; blue; bluish; light blue; purple; orange", among other hues. The author's fertile imagination discovered a fantastic riot of color.

I've written out other passages from this polar vade mecum in which northern lights are compared to most different things and phenomena: "...Waves... curtains... rays... sheaves, or bundles of beams... arches... fans... fiery domes... crowns... flaming dew... blotches... spots... bouquets... spirals... wriggling snakes... huge wings... tints and tinges... fiery showers... rockets... soft fringes... vast fiery columns... glow of a fire... Bengal light... white-hot fireplace..."

If you visualize all that in flux and interplay in the pitch-dark night sky, the above stock of epithets and metaphors will be well enough. I might as well add here that words like beams, arches, folds, crown, and pulsating spots have entered into our scientific terminology for good.

But here is how the Pomors, the aboriginal population of the Russian Far North, describe auroral displays as the writer Pavel Melnikov (1818-1883) recounts it in his book In the Woods:

"There came bleaches in the sky, and then the lights of dawn were at play... the beams lit up, the poles went asighing and walked on the sky. There were flashes even... That's why the missus fooled around - because auroras were loose up in the sky...

"It's always that way: you see no bleaches yet, but the needle starts quivering, to and fro, it goes hay-

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wire... You see the divine force in that, don't you?"

I think this description of the northern lights is without peer. Indeed, "the poles went asighing"... This metaphor will leave no one cold. It's a pity that the old Russian word pazori ("auroras") is out of use today. The English call this natural phenomenon just northern lights, often capitalized, or use the Latin equivalent aurora borealis, in contrast to aurora australis, or the southern lights in the Antarctic. Verbally, the Russian word pazori means "polar dawns", or "polar lights". However, the Russian translation of the German das Nordlicht,introduced by the Russian savant of the eighteenth century Mikhailo Lomonosov, has caught on and is in common usage now. Incidentally, Lomonosov was the first - or one of the first - scientists to suggest the electrical nature of northern lights.

MAGNETIC STORMS AND SUBSTORMS

As to the "divine force", that is the tie-in between the northern lights and the quiverings of a magnetic needle, there is nothing mysterious about it any longer. The "divine force" has long since been known as the Lorentz force, now used to provide traction for electric locomotives. Here, I feel, our readers should digest a bit of scientific information. Now, don't be surprised: an aurora similar to northern lights is something that you can always observe at home - that is the glow of your television screen. With the aurora borealis, however, we have the atmosphere of the earth instead of the screen. Receiving a beam of very fast particles, the electrons, it causes them to slow down; and while slowing down, the electrons produce luminescence. Like it is in your television set, the magnetic

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and electric fields control a flux of these particles. There is also an electron gun, or charged particle accelerator. As to where the terrestrial particle accelerator is situated and how it operates - well, that's a big enigma known to the Almighty alone so far. The greater part of the energy of elementary particles intruding into the atmosphere is expended on ionization, a process whereby molecules of the air lose electrons and give rise to charged particles which, in turn, produce an electric current that, flowing at an altitude of 80-120 km, distorts the magnetic field of the earth and causes the magnetic needle to run riot.

The readership of the Science in Russia magazine might well remember an article by Dr. Mikhail Panasyuk published in 2000.* The solar plasma flux and solar wind, the author says, mar the magnetic field of our planet; the earth - rather, its magnetosphere - is a welcome trap for energized particles and plasma. In contrast to other traps in the universe, mind you, the terrestrial magnetosphere is quite nearby, just above our heads.

The most wonderful thing about it all is that the interaction of the magnetosphere with the solar wind is eruptive, explosive as it is - unlike, say, the measured pace of a flour mill's wheel that picks up part of the energy it needs for work. Rather, biological analogies invite themselves here - for instance, the leaps and bounds of a kangaroo or of a squid, a large sea animal, or mollusk, with ten arms (hence another name, decapod ). The slow accumulation of energy by a magnetic field and the gradual change of its configuration gives way to an outbreak, outburst of energy, to a drive of electromagnetic waves. We call that a substorm, a term coined by the American scientist Xiu-Ishu-Akasofu in the 1960s, who for many years headed Alaska's Institute of Geophysics. Elementary substorms alternate with quiet periods, but they occur oftener in periods of enhanced solar activity, like it is now. The external envelope of the terrestrial atmosphere is in constant flux, it breathes like a living organism.

Today we have learned a remarkable lot through direct measurements by means of orbital satellites. But all these measurements have been carried at one, or at best, at two or three points of a vast complex system. In actual fact, polar auroras alone furnish an insight into the spatial structure of a rapidly changing three-dimensional picture. A series of substorms following one after the other in quick succession may produce a qualitatively new phenomenon - a magnetic storm. In that case the trap of energized particles, i.e. the radiation belt around the earth - swells and shifts toward its atmosphere. An excess of charged particles is dumped then well beyond the polar latitudes.

Now we can obtain a global image of auroras every minute. Yet it took Olga Khorosheva, a young researcher at our Institute of Nuclear Physics, a few months in 1960 to plot from ground photos the picture of circular auroras hovering above the globe.

GLOW OF THE SKY OVER MOSCOW

Do we have polar auroras here in Moscow? Yes, we do, especially during strong magnetic storms. Such phenomena are also observed far southwards, e.g. in Kiev.

The first ever description of northern lights, as far as I remember, is given in the old Russian chronicle The Tale of Bygone Years. As the chronicler tells us, his contemporaries could see the glowing sky on August 15,1091, as

*See: M. Panasyuk, "Breakthrough into Outer Space", Science in Russia, No. 4, 2000. - Ed.

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the mortal remains of the Reverend Feodosiy (Theodosius), Father Superior of the Pechera Monastery, were translated and reinterred... This phenomenon was taken as a heavenly sign and blessing. Here's what the chronicle says:

"...They saw three poles of a glowing arch above the church where Feodosiy was to be put to rest... As they were on their way thither, they saw a great glow over the cave, they saw plenty of candles above the cave, but they could see nothing on approaching the cave..."

Now we know: that year saw a maximum of solar activity and, probably, a strong magnetic storm that could have shifted the auroral zone as far south as Kiev.

Camille Flammarion, the French astronomer, watched an auroral display over Paris on the 24th October of 1870 when the French capital was besieged by Prussian troops. He, armed with his spyglasses and all, was mobilized to keep tabs on enemy movements. Yet Flammarion, in violation of the military discipline, kept his spyglass on the heavenly glow instead of the enemy and, to add insult to injury, even read a lecture to National Guards and a crowd of idlers attracted by the show. He says he saw a glow in the shape of a red moire curtain and, besides, green beams edged by crimson hues. This description, by the way, tallies with the picture in our perception.

But more often than not, the people of temperate latitudes would regard such auroral shows as bad omens presaging such misfortunes as the plague, war or floods. On some occasions the populace, frightened out of their wits, would descry real battles in the heavens, with warriors, monstrous dragons and other specters clashing with one another. A Nuremberg engraver, W. Drechsler, was one of those who watched the heavenly glow on the fifth of October 1591, which he depicted in an engraving now in the custody of the Nuremberg Picture Gallery The artist did not give his imagination free rein and pictured what he really saw.

But at this point I must disappoint Moscow residents: they have a slim chance of watching celestial illuminations in the inner city. Why? Because urbanites are not in the habit of star gazing in the literal sense. Besides, the lights of a big city are a major interference.

ALWAYS ON THE WATCH

In our country regular observations of auroras began during the Second International Polar Year of 1932-33. In 1957 a research team under Professor Alexander Lebedinsky at our Institute of Nuclear Physics designed a setup equipped with a system of mirrors and a camera making it possible to take pictures of the entire sky. Professor Lebedinsky also coached a class of young watchers for work at polar stations.

Subsequently the Polar Institute of Geophysics (RAS Kola Research Center) joined in the effort. The founder and first director of this Institute was Sergei Isayev, an eminent expert in polar auroras. Then

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came other research institutes and observatories, such as the IZMIRAN Institute based in Archangel and involved with terrestrial magnetism and propagation of radiowaves; the observatories of Norilsk and Tixi as well as field stations of the Institute of Cosmic Physics and Aeronomy.

All these research teams of auroral watchers keep alive and kicking in the teeth of the negative effects of a crisis situation in this country.

BALLOONS, ROCKETS, SPUTNIKS

As found in the course of observations and research experiments, the electrons responsible for polar lights expend part of their energy for the generation of braking X- radiation (bremstrahlung). Owing to its high penetrating power, this kind of radiation can be registered at altitudes of 25 to 30 km.

Joint experiments conducted in the 1970s, 1980s, 1990s by Russian and European scientists in the Arctic yielded a wealth of data on spatial-temporal and energy characteristics of auroral particles, electric fields and waves.

Today a new generation of balloons enables us to carry out long overflights of arctic regions not only in summer when northern lights are not visible due to the solid polar day, but also during the continuous polar night of November and December. Television apparatuses lifted by balloons and safeguarded against the vagaries of arctic weather and exposure to moonlight ensure top quality of recordings and snapshots.

Rockets have ushered in a new important stage in the study of auroral displays. It takes a lot of patience, accuracy and good luck to hit the mark. I recall the famous OMEGA rocket experiment conducted thirty years ago by a research team under Academician Roald Sagdeyev as a rocket equipped with an electron gun was launched at one end of a magnetic line of force on the French Island of Kerguelen in the southern Indian Ocean, while an artificial aurora borealis was registered at a conjugate point in the Archangel region up in the North.

It would be in place to recall here that the first measurements of energized particles outside the terrestrial atmosphere were carried out by our Institute's research team under Academician Sergei Vernov, while Konstantin Gringaus and his group at the RAS Institute of Space Studies pioneered in plasma measurements. And here we should likewise mention the INTERBALL experiment, an international project that provided for the launching of a series of orbital satellites. As far as the piloted space flights are concerned, they cover only middle latitudes, and so the crews can take only a side view of auroral shows on the horizon. But satellites with an elliptical orbit enable a full overview of the polar zone and its glows: this is a luminescent ring at 65- 75 ⁰ N in the nighttime and 70-85 ⁰ N on the daylight side above the rotation axis of the earth.

UNDER THE ACTION OF SOLAR PLASMA

Polar illuminations are an important source of information for the understanding of complex fundamental processes known as magnetic storms and magnetospheric substorms. These phenomena, what are they?

We can see the configuration of the terrestrial magnetic dipole stretch under the effect of the solar wind flowing about. The solar wind is a rarefied flux of solar plasma. Suppose you break a substance into minute parts, sever all bonds and get a mixture of negative electrons and positive nuclei. That's the plasma. In a heterogeneous and variable magnetic field it can build complex configurations with sharp boundaries and powerful electrical layers of current. These configurations are unstable, they are readily destroyed by processes which we call plasma instabilities; these

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have a multitude of species and subspecies. Magnetic storms and sub-storms are a component part of the dramatic restructuring process that results in a release and redistribution of significant fluxes of wave and kinetic energy. These fluxes differ in their power and scope: while substorms induce polar illuminations only in the high latitudes, magnetic storms may cause auroras in Moscow, Kiev or Paris, that is much farther south.

A typical substorm lasts for about an hour. At first it shows itself as a blurred dull band, half a rainbow wide, stretching from east to west - we call it a diffuse arch. Then it slowly moves southwards, slows down, comes to a halt, flares up, dies down again and-all of a sudden, produces a bright flash; iridescent in flitting beams, the aurora twists and, in a minute or so, breaks into a dazzling show of colorful fireworks. This means that the breakup is on, a visible manifestation of the plasma instability we have mentioned, a process taking place within the arch-like tubes of magnetic force and resting on the polar atmosphere some 30 to 70 thousand kilometers away from the earth in the equatorial plane.

During the breakup the charged particles, accelerated, fill in our planet's magnetic trap, the outer radiation belt. This trap may be compared to a leaking pail full of holes which is rocked to and fro, splashing out the particles from it. That makes polar illuminations endure in their vivid, dynamic gamut of color. On active days a substorm may have two or three reruns; depending on the eleven-year cycle of solar activity, a month could have two to fifteen days like that.

So auroras are by no means a rare phenomenon. But here we should make allowances for weather conditions - cloud cover, snowfall or just frost-and-wind - all that may interfere with the vivid picture. That's why even denizens of the high altitudes cannot admire it all too often.

The rarest kind of aurora borealis is induced by energized protons accelerated on the sun. This red luminescence, well-nigh uniform from the northern horizon all the way to the zenith, is a spectacular show indeed.

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