Swords of Fire: The Northern Lights

To the north was a pale white arc with several rays extending up into the sky. Once in a while a ray would stretch up into Ursa Major. To the northeast and northwest of the auroral arc were remnants of crimson color...The arc had broken up somewhat, and the rays were not as prominent...In a few seconds the arc was full of rays, all about of equal height. They reminded me of pleated curtains...The aurora was pulsing from the top of the arc to a point directly overhead. I've seen this activity before, but it still leaves me in awe. It reminds me of science movies which graphically show how our brain's neurons and synapses communicate information-sort of a pulsing ripple. It was fantastic.

I then noticed that the northwestern end of the arc began to take on a crimson color. It started out as a diffuse glow in the sky behind a large pine tree. Quickly the magnetic field lines shaped the particles into long rays of red daggers. It's quite easy to see why people of the Middle Ages called auroras like this "swords of fire." The pulsing ripples and the crimson daggers continued simultaneously for only 20 minutes or so. Then as quickly as the activity had started, it ceased within a couple of minutes. Clouds were beginning to thicken, and a partial pale white/green arc with minor draperies soon disappeared in the northeast.

The date was March 12, 1989, and the location from where I observed this wonderful display of the northern lights was way up in North Country, Pascoag, Rhode Island!

I don't know why many folks believe one has to travel to Alaska or the North Pole to be successful in observing the Aurora Borealis, the northern lights. Well, it just isn't true. Maybe it's because many of the photographic images of aurora chosen for publication come to us from Fairbanks, where on any dark and clear night the aurora is visible. That doesn't mean it cannot manifest itself at our lower latitude.

The second misconception regarding the aurora is that it is best observed during the wintertime. Not true! Though the aurora doesn't have anything to do with the seasons, its appearance is cyclic. Again I can only surmise that folks have equated the aurora with wintertime because of the preponderance of aurora images from Alaska. It's perpetually dark in Fairbanks during the winter. There's a lot of snow on the ground. The aurora appears every clear night. Voila!

Well, it's time to put the misconceptions to rest. Read on to discover the cause of the northern lights, how they can be observed locally, and when one might present itself.

Our Sun

Ever since man began to gaze at the sky he has been mystified by what he's seen. Always, though, he has tried to comprehend the world around him, and the sky above him. All the naked-eye astronomical bodies of our solar system and all the events that took place in the heavens were scrutinized. Why? It was believed they exerted an influence on the earth and its people. The sun, the most prominent object in the sky, became a great deity to be appeased. The early peoples therefore developed a working relationship with the sky above them. Their fate depended on that knowledge. Hence, astrology was born.

However, as the centuries passed, mankind, particularly the Europeans, did not progress in the understanding of astronomy. During the 16th and 17th centuries astronomical events were not only deep mysteries to the people of that time, but superstitious beliefs were substituted for any scientific inquiry of the phenomenon. Meteor showers were often regarded as a sign of the approaching end of the world. And, comets, Halley's Comet in particular, were thought to be celestial signs foretelling the death of a king or the collapse of a nation. Although the sun was no longer worshipped as a great deity by the masses, it was still thought to be a perfect celestial body (obviously a carryover from primitive times). This notion was held until the early 1600's when the telescope was invented in Europe.

On October 10, 1610, Galileo Galilei trained his newly constructed telescope on the sun and discovered it was not the perfect body it was long thought to be. He viewed a granular surface dotted with dark spots which later came to be known as sunspots. Although dark spots had been seen crossing the solar disk with the unaided eye before Galileo's discovery, they were thought to be celestial bodies intervening between the earth and the sun.

The idea of such imperfections upon the disk of a celestial body was repugnant to the theological philosophy of the Middle Ages. Consequently, Galileo kept silent about his discovery for several months until other astronomers of his time found the courage to publish their findings. Though the idea of an imperfect solar body was only slowly accepted after definite proof was presented, this early inquiry set the stage for the solar research which has continued to up to this present day.

In the almost four centuries since Galileo's discovery, mankind has accumulated a wealth of data about the sun. Astronomers have determined that the sun "burns" by converting tons of hydrogen into helium every second. At the core of the sun is a controlled nuclear explosion, producing a temperature of 15 to 20 million degrees or so. This great temperature and outward radiative pressure, in balance with the intense gravity at the core, maintains the nuclear fusion of hydrogen that causes our sun to shine. The process has been going on for about 5 billion years, and will continue for another 5 billion or so before all the hydrogen is "used up" or converted.

However, the physics at the surface of the sun are quite different. Though the core is extremely hot, the visible surface is only 11,000 degrees F. All the intense heat stays in the core. That which makes it to the surface only does so by convection, the same process that occurs as tomato sauce simmers on a hot stove. Watch it next time...little cells or bubbles come to the surface and pop. Astronomers can watch the same phenomena on the sun. Also they can see the dark spots that Galileo discovered so long ago. Why are they dark? They're cooler, about 8,000 degrees F as compared to the surrounding surface temperature of 11,000 degrees F. That is why they appear dark. If they were isolated from the surface of the sun, they would appear as bright as the surface themselves.

But why are they cooler? An intense magnetic field in the sunspot area prevents hot material from flowing into it, thereby allowing it to cool somewhat. You could consider the sunspot is the shade of a tree on a sunny day here on earth. The ever-changing magnetic field of a sunspot or group of sunspots causes its size and shape to alter constantly.

In addition, astronomers have also observed that the number of sunspots is not constant. They have determined that the occurrence of spots fluctuates during an 11-year cycle, going from a minimum or nonexistent number during the beginning of the cycle, to a maximum number when the solar disk is covered with spots. The rise to peak takes about 5 years, while the decline takes 5 to 6 years on the average. Even this 11-year cycle is not carved in rock. It has been as short as 7 years and as long as 17.

That is why I haven't written about the aurora for several years. The last minimum of solar activity was in November 1996. Since then solar activity has been on the increase. Peak, or solar maximum, is expected to occur in October 2000. (More aurora activity occurs for us at our latitude when solar activity is at peak or just coming down off peak.)

The knowledge that solar activity is cyclic in nature is quite valuable to solar researchers because sunspots are not the only activity on the solar disk. Solar prominences, which are usually seen during total solar eclipses, are jets or clouds of glowing hydrogen gas that shoot out millions of miles into space from the sun's surface. (They are also visible with special instruments and filters).

Also produced above sunspot groups are intense eruptions called flares. These are brief increases in luminosity of a localized region of the solar surface. Since they are associated with sunspots, they too follow the 11-year solar cycle. These very energetic bursts of energy spew ionized (charged) particles that stream into space. If circumstances are just right, the earth gets swept over by these particles.

In addition, very large-scale magnetic storms in the solar corona (outer solar atmosphere) eject voluminous amounts of plasma into space. These CMEs (Coronal Mass Ejections) often intercept the Earth like the flares do. However, CME particles are much more energetic. In fact, some researchers believe most, if not all, of the major geomagnetic storms are CME induced. While flares occur above sunspot groups, CMEs do not have to be associated with sunspot groups. They also ebb and wane with the solar cycle.

Now although the earth averages about 93,000,000 miles from the sun, one might not expect such activity to affect us. On the contrary!! We receive heat and light from the sun, so why can't other solar effects be manifested on the earth in different ways as well. Geomagnetic storms are one of those manifestations, and a fantastic phenomenon does occur on earth as a result of this intense solar activity -- the Aurora Borealis, or northern lights, and its southern counterpart, the Aurora Australis, the southern lights. In Part II this column will focus on the more widely observed northern lights.

The ancient peoples of the world were awed by the northern lights. To their unscientific minds it was thought to be celestial warriors in combat or swords of fire. Some even believed it was the spirits of the dead visiting the earth. In the 1700's and 1800's it was often thought the aurora was caused by a house on fire very far away causing the sky to glow.

We now know differently. The solar flares and CMEs (coronal mass ejections) discussed in a previous column are responsible. Under normal solar activity, aurora are confined to the auroral oval, a region high in our atmosphere above the geomagnetic poles. When highly charged particles ejected from the sun by flares and CMEs intercept the Earth (about two days journey time) they are trapped by the earth's magnetic field. The charged particles collect in the Van Allen Belt, a donut-shaped belt of lethal radiation encircling the earth, and also in the Earth's magnetotail (a tail of the Earth's magnetic field lines pushed downwind by a steady stream of particles called the solar wind).

Now although these particles are always entering the earth's magnetic field, only when a certain concentration of these particles has been attained will the overflow shift and spiral towards the north and south magnetic poles. The amount of the overflow will completely depend upon the intensity of the flare/CME producing the particles. The northern auroral oval expands southward. If enough energy powers this expansion during a major geomagnetic storm, we may eventually see the southern extent of the oval as an arc rising above our northern horizon.

As the earth's magnetic field accelerates the charged particles to very high energies, atoms and molecules of particular elements in our atmosphere are stimulated by the bombardment of these energetic and charged electrons and protons. The bombarded atoms and molecules consequently give off light of their own characteristic color.

The same phenomenon can be seen every night in the city. Neon lights and sodium vapor lamps are light bulbs containing a rarefied gas (similar to our upper atmosphere) that is being stimulated by electricity. The color given off depends entirely on the gas contained within the bulb.

This is exactly what happens in our atmosphere. The colors seen in a display of the northern lights depends on the elements being stimulated. Atomic oxygen produces a yellow-green color, and at extremely high altitudes a red color. Molecular oxygen and nitrogen also produce a red color. Very high displays have a bluish or purplish tint due to ionized molecular nitrogen receiving sunlight. Also, just as the brightness of a light bulb depends on the amount of electricity being supplied to it, so does the brightness of an aurora depend on the energy of the charged particles bombarding the atmospheric elements.

In addition, auroras are not only fascinating to watch because of their varying colors, but also because of the many shapes that can be manifested. The shape completely depends on the fluctuations of the earth's magnetic field. A combination of the auroral arc and drapery or curtain type is one of the finest displays that can be seen. They start out as a green glow, or pale green or white arc on the horizon, slowly moving up or down the sky, sometimes breaking up, becoming thinner or developing rays and often dancing curtains and draperies of colored light. Collections of curtains evolve into soft billowy clouds which can brighten and fade in a matter of seconds. During some exceedingly bright displays, the aurora's shimmering glow has been reported to be as bright as the full moon. These fine displays occur at an altitude of from 65-70 miles, while the lower limit is 40 miles and the upper limit is 600 miles.

If the activity is minor the aurora will most usually be seen to the north. As the energy and number of particles increases the arc will move southward. During the last solar maximum I observed two spectacular displays directly overhead. These are called "coronal" because they consist of a large circular area of activity with streamers moving up and down the sides.

An analogy would be one of those orange cones used by road crews. If you looked up into the cone from the wide base, you would see the circular area of the top with the sides of the cone sloping upwards. That is how the coronal auroral form appears....and it can be an extremely impressive sight. During a very active period the aurora can even appear in a southerly direction, halfway to the southern horizon. All you need are clear skies and a dark location.

With solar maximum predicted in October, 2000, our chances to see an aurora this year increases daily. For us in southern New England to see an aurora display, a flare of great intensity or a CME has to occur on the sun. Besides that, the flare or CME has to be pointing toward the general direction of the earth as well for us to intercept the energetic particles.

How would you know when to expect an auroral display? Well, many of the local meteorologists get teletype messages when scientists believe one will occur at these latitudes. This belief is based on the observance of a major flare or CME and the activity of the earth's geomagnetic field. However, it seems every time they predict an aurora we don't get one. At other times it seems they happen unexpectedly, or at least the information doesn't get to the public. I also alert the local meteorologists when I start receiving reports of auroral activity so they may air that information so everyone can share in Mother Nature's beauty.

Furthermore, with solar activity increasing, you too can help yourself by watching the night sky more regularly. No time is more favorable than another. Aurora can occur just after sunset, or at three in the morning. But if you do hear or read that there is a chance of a display, by all means make an attempt at checking the sky all evening/morning.

If you are lucky and observe an aurora, there are several additional phenomena associated with its presence. A rare event, seldom heard, but always denied by researchers, is crackling and hissing noises during a display. Some scientists believe they are caused by radio waves emitted by the aurora, or from electric currents induced near the observer by the changing magnetic fields. This should not be surprising since the currents can dissipate a trillion watts of power in the lower atmosphere, depending upon the intensity of the display. That amount of energy exceeds the total electrical generating capacity of the United States.

Also because of the ionization of the atmosphere, all modes of communication by radio are rendered useless. Taxi drivers have received orders from distant cities because of this atmospheric interference. Even transformers and transmission lines are affected. If it weren't for protective devices handling these power surges and voltage swings, lines would blow out like fuses in overload. Hydro Quebec had severe damage to its equipment during the March 1989 Great Aurora. Huge strands of cable in generators melted because of electrical overloads.

If you happened to look at a compass during an auroral display or during any geomagnetic storm, you would see it spinning wildly back and forth because of the changing magnetic fields. It has been suggested that our biosphere, especially the weather, could be adversely affected by the chemical changes and huge electromagnetic currents caused by activity on the sun. It has been postulated that the severe winter of 1977-1978 was due to an inactive sun. Though this has not yet been proven, more research is being down on the phenomenon.

In conclusion, now that you know the mechanics of an aurora, I hope you can better appreciate a display when we next experience one. Auroras are simple to observe since they do not require telescopes or binoculars to view them, but just your own unaided (eyeglasses excepted) eyes. Enjoy nature's 'Swords of Fire' - the Aurora Borealis.