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Secrets of the Polar Aurora
For a remarkable two-minute video of aurora, click here
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Alaska is known as a good place for seeing the polar aurora, also known as "Northern Lights." Originally the phenomenon was named "Aurora Borealis," Latin for "northern dawn," since in the lower 48 states or in mid-Europe it may appear (on the rare occasions when it does) as a glow on the northern horizon, as if the sun was rising from the wrong direction. But the southern hemisphere has the same phenomenon, with the glow coming from the south, so scientists prefer to call it simply the "Polar Aurora." Most visitors to Alaska never get to see an aurora, because they come in the summer, when skies are rarely dark enough. Alaskans claim that only around August 16 does the sky get dark enough to see stars, which is when aurora stands out. After that date, your best bet is to go to Fairbanks--and since the brightest auroras occur around midnight (or later, due to Alaska's time zone), you might have to stay up a long time. Perhaps it is better then to ask the night clerk at your hotel to ring you up if a good display becomes visible. Appearance and relation to magnetismWhat does it look like? Most often, you see greenish white ribbons stretching across the sky, roughly from east to west, usually with waves in them. In Fairbanks they could be overhead, in northern Norway or Sweden too, sometimes even in Winnipeg. Further south those ribbons tend to be near the northern horizon. And if you look closely at them, you will note that they contain many parallel rays, running across their width (see picture below). Two things about those rays. One, the bright ones fade while dim ones brighten instead--a bit like flames in a fireplace, and just as mesmerizing. Some auroras are deep red, and these may be just a shapeless glow--or they may have rays, too. And second, the direction of those rays is related to the magnetism of the Earth.
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| The aurora--a woodcut by Fridtjof Nansen |
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Anyone who has ever used a compass knows that the Earth is a giant magnet. The needle of the compass usually points towards one of two points, the magnetic poles of the Earth, located near the geographic poles. But because the compass needle is mounted horizontally, it does not show everything. Actually, the magnetic force points not just northward but also slants down into the Earth. Compass needles carefully balanced on a horizontal axis ("dip needles") point in that slanting direction, when allowed to swing in a north-south vertical plane. In fact, the angle gets steeper the closer one gets to the magnetic pole. At the pole the force is vertical. The rays of the aurora faithfully follow that slanting direction. The Auroral ZoneThat was one clue that the aurora was related to the Earth's magnetism. The other clue was found by keeping tabs on how often aurora was seen in various locations. It turned out that the important factor was distance from the magnetic pole. That pole is separated from the geographic pole, marking the Earth's rotation axis, and currently it is in the Arctic Ocean, just north of Canadian soil. The fact it is displaced towards America means Americans do not have to go as far north to see aurora as do, say, residents of Siberia, on the other side of the globe. Locations about 1500 miles from the magnetic pole are where aurora is seen most frequently: further away or nearer to the magnetic pole, they get more rare (they are quite rare at the magnetic pole itself). Fairbanks, Alaska, at the edge of the "auroral zone, " makes a good observation post. What you usually see there are those quiet curtains and ribbons. But not always. At some times they change shape rapidly, advance, retreat or bulge out in a violent fashion, and they also get quite bright. Scientists call such an active, violent outburst an "auroral substorm, " and satellites still study the release of energy, far in space, which causes it. If you are lucky you may also see a "corona"--a burst of rays radiating in all directions. That is a caused by perspective--like the rays of the sun setting behind a cloud--and it means it the rays of the aurora are arriving right overhead.
Magnetic storms expand the auroral zone to locations more distant from the magnetic pole--such as Washington, London or Beijing--and also create bright auroras. If this happens on a clear night, residents in those cities can see an aurora, but it is a rare treat for them. On the right is a satellite image of an aurora extending to the "lower 48 states" of the US (note Florida, outlined by its city lights). Later that day, in March 1989, the aurora actually spread much further southward, but no satellite was in position to photograph it then. The next picture below is of a 2001 aurora seen in Purcellville, Virginia. Electrons of the AuroraTo early observers, and well into the 20th century, the polar aurora was a great mystery. Not everything is solved even now--but thanks largely to space satellites, we have a fair understanding of the way the aurora is produced. First question-- how high up is it? By comparing photographs taken from separated locations, an altitude of about 60 miles was found for the green aurora, and up to about twice as much for the red one. Clues like that led scientists to conclude that "something out there" was shooting towards us beams of fast electrons, somewhat like the ones painting the picture inside a TV picture tube. In a TV, electrons hit a screen, come to a stop, and their energy is converted to light. Something similar happens with the electrons that cause the aurora: they collide with atoms in the upper fringes of the atmosphere, give up their energy to those atoms and cause them to emit light. And what are electrons? Tiny particles with negative electric charge, contained in all matter. At the center of every atom is a nucleus, containing almost all of its mass and always carrying a positive electric charge. The positive charge attracts electrons and binds them, and jointly the two types yield an ordinary atom, electrically neutral, with no excess charge of either kind. Atoms like this build up you, me, and anything we can see on Earth.
The green and red colors are emitted by atoms of oxygen after they are hit by fast electrons. Each element emits its characteristic colors, and for rarefied oxygen, these appear to us green or red. Typically, a delay of 0.5-1 second exists between collision and the emission (in this case--not in denser surroundings!), and that is why the rays of the aurora brighten and fade so slowly. The beam of electrons which "excites" the oxygen atoms may only last a small fraction of a second, but the afterglow persists 0.5-1 seconds or more. The Aurora and Magnetic Field Lines
And what connects the pattern of the aurora to the region of the Earth's magnetic forces--the "magnetic field" of the Earth, as that region is known? For such a region, extending far into space, a convenient method is needed to describe it there. Such a method is provided by magnetic field lines, or as they were once called, "magnetic lines of force."
Chances are you have seen a drawing of the field lines of a bar magnet. They fan out from one pole, bend around in big curves, and then converge on the other pole. The magnetic pattern near Earth is like that, too--it is as if the Earth had a small (but oh so powerful!) bar magnet in its center: the lines fan out from the region near the south magnetic pole, reach their greatest distances above the equator, then converge again near the north magnetic pole. To define field lines more exactly, imagine you had a compass needle hanging in space, able to tell us the exact direction of the magnetic force, in 3 dimensions. Such a needle will always point in the direction of the magnetic field line at its location. North of the equator such lines converge towards the region near the north magnetic pole, just like those of a bar magnet.
Back to the aurora. Between 1895 and 1907 the Norwegian physicist Kristian Birkeland tried to study its behavior in a lab. Inside a glass vacuum chamber he mounted a sphere with a magnet inside--he called it "terrella," Latin for "little Earth"-- and directed towards it a beam of electrons. To his surprise and gratification, the magnet steered the beam right to a patch around the magnetic poles of his small sphere, producing there, as it hit, a visible glow. He probably thought-- aha, so that is how it is done! It turned out (and I skip a lot) that negative electrons and positive ions alike are guided in space by magnetic field lines. They tend spiral around them, meanwhile sliding along them, like beads on a wire. Because Birkeland's field lines reached the terrella near its magnetic poles, that is where his electrons came down. Similarly magnetic field lines of the Earth guide electrons of the aurora to come down in the auroral zone. No wonder the rays of the aurora pointed along such lines! Each was produced by a ray of electrons, riding its own field line down to the atmosphere. But where did those electrons start from? The Sun's Corona and the Solar WindPhysical processes usually require a source of energy to drive them. Think of energy as a kind of money, paying for every physical process! Any object moving at great speed needs energy to do so--its "kinetic energy"--and if the aurora contains beams of electrons moving at 1/10 the velocity of light, something must have paid the price, must have provided the energy. Not surprisingly, it is the Sun. Indeed, why not? After all, the Sun powers almost every process on Earth: the food we eat, the coal and gasoline we burn, the winds that blow and the rain which waters the land--none would exist without the energy provided by sunlight. With the aurora, however, it is not the light of the Sun, but something more subtle, the so-called solar wind.
The plasma of the corona is far too hot for the Sun's gravity to hold it captive. Instead, it constantly expands away from the Sun and is blown off as the solar wind, filling the solar system and reaching Earth, and far beyond, past Pluto's orbit. The Earth's magnetic field, however, is an obstacle which the solar wind cannot penetrate. Like a river meeting a rock, it splits up and is diverted to flow around it. Around the Earth a cavity is formed, protected from the solar wind and known as the Earth's magnetosphere. And just as a rock in a river leaves a long shielded wake behind it, the Earth's magnetic space has a long tail on the night side--some call it the Earth's "magnetotail. " But even though the solar wind is kept out, it manages to transmit some electric energy to the magnetosphere, by brushing against it--in particular, to the tail region. Let me say here at once, the tail is where most electrons of the aurora seem to come from, which is why in Fairbanks the brightest aurora tends to occur around midnight--even in arctic winter when the sky is dark at most other times, too. At home, energy is carried by electric currents that flow from electric outlets to lamps, appliances and TV. The energy of the solar wind also reaches the magnetosphere (at least in part) by means of electric currents. Satellites have observed those currents near Earth: they flow in and out of the auroral zone, along magnetic field lines--for the main circuit, in on the morning side of midnight, out on the evening side, the two branches connecting (since any electric circuits must be closed!) through the high atmosphere, which (as noted) conducts electricity. One might perhaps say that we are still searching where the plug is.
Author and Curator: Dr. David P. Stern
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Is the polar aurora rare? Depends on where you are! If your home is in Fairbanks, or in Tromso, Norway, or Fort Churchill, Canada--not at all. You won't see it every evening, but it is present frequently enough. In Washington D.C., or London, or Beijing, however, it is a rare event, seen only when the Sun creates "stormy conditions." On such occasions--especially near the peak of the 11-year sunspot cycle--the Sun sends out a dense cloud of hot gas, whose arrival disturbs the Earth's magnetic environment and produces a so-called "magnetic storm" (more details, further below).
During a total eclipse of the Sun one can see the outermost layer of its atmosphere, the corona, a glowing halo around the darkened Sun. It turns out (by examining its light) that the corona is incredibly hot--about a million degrees centigrade, nearly 2 million Fahrenheit. Such extreme heat will tear electrons off any atom, turning the corona into a "soup" of free ions and free electrons, a strange gas known as a "plasma" which (among other things) conducts electricity. If you use fluorescent tubes, or have watched neon lights--it's the plasma inside them (not as hot as the corona) which carries their electric current and produces their light.
Earlier it was claimed that electrons or protons tended to be guided by magnetic field lines, like beads on a wire. The ones which guided radiation belt particles had a typical shape--they came out of Earth's southern polar region, described a wide arc across the equator, and returned back to Earth near the northern pole.
Most people have heard about sunspots--dark markings on the sun (a sunspot group is pictured on the left). Their number rises and falls with an irregular cycle of about 11 years, and some people have speculated (not too convincingly) that it matches the rise and fall of climate, the stock market, warfare and more.
