This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Earth's Aurora

by Dr Jamie Love Creative Commons Licence 1997 - 2011

As an amateur astronomer learns about the night sky s/he eventually learns about the Earth's aurora. Other planets may have an aurora, but we will limit this lesson to the Earth's aurora because it's the only one you are likely to see and, if you see it, it will astound you!

The aurora is a luminous phenomenon that occasionally occurs in the sky near the high latitudes. (That is, near the poles.)

More specifically, auroras occur around the Earth's magnetic poles - those around the northern magnetic pole are called the northern lights or aurora borealis while those around the southern magnetic pole are called southern lights or aurora australis. The regions of the Earth in which aurora activity is very high are called auroral ovals. As the name implies, these regions are not perfect circles and, because the magnetic poles are not situated directly over the geographical poles, these regions are not shared equally around the Earth's high latitudes. The Earth's magnetic poles are tilted 11 degrees from its spin axis. The magnetic northern pole is in Canada so folks in North America are more likely to see the aurora borealis than folks in Europe at the same latitude. On the opposite side of the world, the south magnetic pole is also biased, but in the opposite direction, so folks in Australia are more likely to see the aurora australis than people in South America sharing the same latitude.

OK, so what is the aurora?

In a word - "beautiful". No, one word isn't enough - "amazing", "dazzling" and "eerie" also describe the aurora. These displays appear as colored arcs, bands, rays and curtains of light in a variety of colors but mostly greens and reds.

What causes them?

Magic! Well, that's the short answer but I think, as an astronomy student, you should know the long answer because it's more scientific (and correct).

Auroras are caused by charged particles, blown away from the Sun, interacting with atoms, ions and molecules in the Earth's upper atmosphere. These particles are attracted to the Earth's auroral ovals by the Earth's magnetic field. To understand these phenomena I must first teach you about the Sun's solar wind and the Earth's magnetic field.

You learned a great deal about the Sun last month but there's still plenty more that could be learned. Here I want to tell you about what happens on and near the surface of the Sun. What I am going to describe probably also occurs on the surface of all stars. Remember, the Sun is just an average star but it's so close to us that we can make very detailed studies of it.

You will recall that the surface of the Sun is a mere 6000oC but the Sun's corona, its "atmosphere", is about two million degrees (due to complicated and poorly understood phenomena). At those temperatures atoms are ionized - electrons are stripped from their nuclei - and this produces an "ionized gas" that we call a plasma. Plasmas, like all things that carry a charge, are strongly influenced by magnetic fields and the Sun's magnetic field is extremely powerful. Indeed, the entire Solar System itself has a huge magnetic field, just like the Earth's, extending past Neptune! This "Solar System magnetic field" is created almost entirely by the Sun - although only 0.00001 of the Sun's energy is in this magnetic field. Close to the Sun this field holds the plasma in its grip, imparting a great deal of energy to the ions in the process. Eventually the plasma escapes the corona through a hole or weakness in the Sun's magnetic field. The rotation of the Sun's magnetic field adds a bit of spin to the stream of escaping ions so they tend to fly out from the Sun in an arc which produces a spiral as it progresses. This pattern is similar to one that you would produce using water from a garden hose if you swung the hose over your head in a circle. The Sun's spirals of plasma are called the solar winds.

Solar winds are produced by the Sun all the time and each year the Sun losses about a trillion (1,000,000,000,000) tons as the solar winds. But don't worry. That's less than a trillionth of the Sun's total mass anyway. These winds are responsible for repelling the tails of comets (which you will learn about in August) and creating the aurora.

As the solar wind passes the Earth it has a speed between 300 and 400 kilometers per second. Of course, this is very fast but it's not a very dense wind. In our neighborhood there are on average about 10 solar wind ions in each cubic centimeter. That means every square centimeter in our neighborhood has about a million ions each second streaking through it at hundreds of kilometers per second! Some people have suggested that, by using huge "solar sails", we might one day sail spaceships around our Solar System using these solar winds. The wind slows down a bit as it encounters gas and dust in interplanetary space but the solar wind continues to the "edge" of the Solar System which is defined as the heliopause - the boundary where the solar wind slams up against the interstellar medium (gas and dust between the stars).

Overlaying this constant wind from the Sun is a more energetic "weather pattern". There are gusts in the solar wind influenced by sunspot activity. Sunspots are dark patches on the photosphere caused by localized and intense magnetic fields on the Sun. These fields can suppress the convection currents that bring hot materials up to the photosphere. The spots are dark because the convection cells under them are not working well. That part of the Sun becomes cooler (by about a thousand degrees) and therefore darker, creating a spot on the photosphere. (NEVER STARE AT THE SUN! ASTRONOMERS USE SPECIAL FILTERS AND "TRICKS" TO SEE THESE SPOTS!) When these spots finally release their magnetic energy, a gust of materials escapes with it. These produce solar flares (containing large amounts of material) and coronal mass ejections (containing larger amounts of material) which greatly increase the density of particles in the solar wind.

These gusts, unlike the general solar wind, are localized - the materials they release move away from the Sun in a specific direction. Sometimes that direction happens to be in the direction of Earth! When that happens we experience solar storm activity. That increases the intensity and frequency of the auroras and sometimes causes other electrical disturbances in our area that can knock out communications and even powerlines!

This drawing shows the Sun giving off two coronal masses but, because we see them at the edge of the Sun (in this drawing), we know they are not headed towards us - these materials are destined to go flying off at a right angle to our line of sight. It's the ejections that are headed towards us that are the most important (for us) but they are also the ones that are the most difficult to observe.

A solar storm can occur at anytime but there is a certain regularity to most sunspot activity. The Sun experiences cycles of sunspot activity that last eleven years. That means about every eleven years there is an increase in the number, frequency and intensity of sunspot activity and, along with it, an increase in the phenomena that activity produces - such as aurora. We call this cycle the solar cycle.

The solar cycle reached its maximum in 1968-69, 1979-80 and 1990-91 so we expect a lot of sunspot activity in the years 2001 and 2002 as the Sun reaches its maximum activity through its current solar cycle. These should be good years for auroras but there's no telling. The cycle is not perfectly regular and each maxima (maximum event) is not equally energetic. Some maximums are better than others. Sometimes it's as if there was no maxima at all!
Between 1645 and 1715 there was very little sunspot activity. This period of time is called the Maunder Minimum after E. W. Maunder who first noticed this minimum while studying historic records of sunspot activity. [Recall that Galileo Galilei discovered sunspots in the early 17th century. Once they were discovered sunspots became a new and exciting area of astronomical research and many observers kept meticulous notes of what they saw and when they saw it.] There is some speculation that sunspot activity affects our climate. It is certainly true that during the Maunder Minimum the Earth experienced quite a cold spell.

Yeah, but what causes the aurora?

Now we move on to the second part of this lesson - the Earth's magnetosphere.

Any planet with a magnetic field can have a magnetosphere. Jupiter has a very powerful magnetic field so it has a very strong magnetosphere, while Mars has a very weak (almost non-existent) magnetic field so it barely has a magnetosphere at all. A magnetosphere is the region around a "magnetic planet" in which the charged particles of the solar wind are controlled by the planet's magnetic field rather than the Sun's magnetic field.

Remember, the magnetic field created by the Sun extends throughout the Solar System. The shape of the Earth's magnetosphere is a compromise between our planet's magnetic field and the Sun's magnetic field along with the solar wind. On their own, the magnetic fields of either the Sun or the Earth should be a kindof "spherical torus". What I mean is that the field lines, the lines that define the intensity and shape of the magnetic field, should produce a shape that is spherical but that dips down at the magnetic poles. This is what you would expect of a simple bar magnet as shown here in cross section.

Of course, the Sun also has a magnetic field and it's much more powerful than the Earth's. The two magnetic fields interact with each other as shown here. However, this diagram is very overly simplified. Of course, neither the sizes nor the distances between the Sun and the Earth are to the correct scale. (If I drew them to scale all you would see is a lot of empty space and the Sun.)

Not so obvious is the magnetic field density. The Sun's magnetic field is much larger and stronger than the Earth's. However, the strength of a magnetic field decreases with its distance from its source. That is, when the Sun's magnetic field gets to within the vicinity of the Earth, it's much weaker. Indeed, in the vicinity of the Earth it is the Earth's magnetic field that dominates!
(This effect, that a field's strength decreases as you move away from the source of the field, is a common effect in physics and applies to many forces including gravity.)

That the Earth's magnetic field dominates the magnetism near the Earth should not surprise you. After all, a compass points to the Earth's (magnetic) pole, not the Sun's. At least it does on Earth. However, if you were to take your compass a few million kilometers away from here you would discover that it points towards the Sun's pole.

Also, the above diagram is not the complete story because it does not include the effect of the solar winds. (Remember them?) The wind is constantly blowing against the Earth and it forces itself against the Earth's magnetic field. In doing so it radically changes the shape of the Earth's magnetosphere. The Earth's magnetic field gets compressed on the sunward side and swept back on the other side by the powerful solar wind. The final shape of our magneto"sphere" is like a teardrop with its tail pointing away from the Sun (like a comet - and for the same reason). The boundary of the magnetosphere is called the magnetopause. If it were not for the Earth's magnetic field and the magnetosphere it produces, the solar wind would slam into the Earth at full force. Fortunately, that doesn't happen. (Mars and the Moon, however, are constantly pelted with the plasma of the solar wind. That's because they don't have a magnetic field - to speak of.)

But even this image is not really accurate because I have made the Earth very large compared to the fields.

On the sunward side the solar wind compresses the Earth's magnetosphere to within 60,000 kilometers of the Earth. (That's about 4 or 5 times the diameter of the Earth.) On the opposite side, the magnetosphere stretches in a long magnetotail extending about ten times longer - beyond the orbit of the Moon! This image on the right gives a more accurate representation of the size of the magnetosphere with respect to the Earth.

But understand that the size of the magnetopause, and even some of its over all shape, can change if there is a lot of solar activity. During the solar maxima the amount of particles blowing out from the Sun is much greater so the solar wind will push the magnetosphere closer to the Earth (on the sunward side) which brings the sunward side magnetopause closer to the Earth.

Does the solar wind try to follow these magnetic lines?

Sort of.

As the solar wind slams into the sunward side of the magnetopause another boundary, called the bow shock, is formed. The region between the magnetopause and the bow shock is called the magnetosheath and it's made of particles from the solar wind that have been slowed down from collision with the bow shock. It is these particles- that get trapped in the magnetosheath and spiral down along the Earth's magnetic field lines - which create the aurora. The magnetic poles of the Earth act as a channel, or funnel, for these ions to enter our world. Notice that the magnetosphere has a dip at each pole. Each dip, called a cusp, funnels particles from the magnetosheath down to Earth.

Not all of this plasma makes it down the funnel. Some of it gets trapped around the Earth. Inside the magnetosphere (and not shown in my drawings) are two zones, or "belts" of very high radiation. They were discovered in 1958 by James Van Allen and are known as Van Allen zones or belts. [Actually they were discovered in that year by Explorer 1, the first successful American satellite, but Van Allen explained them. ] These zones consist of high-energy charged particles trapped in the Earth's magnetic field. They zip from pole to pole in complicated helical paths. The lower zone extends from 1000 to 5000 kilometers (above the equator) and contains a mix of electrons and protons. The upper zone ranges between 15,000 and 25,00 kilometers (above the equator) and is made mostly of electrons.

The aurora is different from the Van Allen zones because it is made from plasma that has successfully navigated through the trapping effect of those zones, helped by the cusps. These particles, therefore, penetrated much deeper into our atmosphere where they cause the upper air to glow.

But, HOW do they make the air GLOW?

The light is produced in a way very similar to the way your TV or computer monitor works. That is, assuming yours is NOT a flat screen. Ordinary TVs and monitors use a cathode ray tube to accelerate electrons to incredibly high speeds and then slam those electrons into the back of the tube (screen). When they hit the chemical on that side of the tube the energy from those high-speed electrons is converted to light. By controlling the position at which the electrons strike the screen, a glowing image is formed on the screen.

The aurora is produced when charged particles of the now captured solar wind, are dragged into the Earth's upper atmosphere. The color produced depends upon the atom or molecule struck and the amount of energy carried by the plasma. High altitude oxygen (about 300 kilometers up) produces a red glow. This is rare. Normally, the plasma makes it down to a lower altitude (about 100 kilometers from the surface) before colliding with oxygen and, under those conditions, the oxygen atoms give off a yellow-green glow. These are the brightest aurora and the most common. Blue light is created when ionized nitrogen molecules are excited by the plasma while neutral nitrogen produces a red glow. The greater density of nitrogen in the lower part of the atmosphere causes the purple-red borders and edges along the bottom of many auroras. Other "colors" are produced but they are not in the visible part of the spectrum (where we have the ability to see). Details of how this works are in the realm of quantum mechanics.

When's the best time to see the aurora?

  1. at times of maximum solar activity
  2. when the sunspots increase
  3. when there is a large coronal mass ejection.
When these occur the magnetosphere is pushed back, the bow wave moves closer to the Earth, and more of the plasma is funneled down to the Earth. So, the best time to observe the aurora is during the solar cycle maxima. The years 2001 and 2002 should be good but after that don't expect big displays for another decade.

As far back as 1733 someone (Mairan) observed that September and March have the most auroras while January and July have the least. I'm not entirely sure why that is so but it seems to be a fairly accurate statistic. (I think it has something to do with the way the Earth's orbit carries it in and out or the Sun's magnetic equatorial plane.)

However, you should keep in mind that the aurora can occur any night. Or day! These same events occur during the day but the daylight mutes the effect and the plasma is more likely to work its way towards the Earth on the night side anyway.

Of course it helps to be near one of the magnetic poles but not necessarily on top of one. Remember the auroral ovals. The greatest activity occurs along the edges of the spiraling magnetic lines. When solar activity is high, lots of plasma will be hurled down the walls of this magnetic funnel. This often causes the ovals to grow larger and that causes the aurora line of activity to move further away from the magnetic poles.

Have you seen aurora?

Yes, several times. The first time was on a cold winter night while I was living in northern Wisconsin. We came out of the Student Union building and there were the lights! Mostly eerie yellows and greens with touches of blue. They made curtains in the sky that seemed to be flapping as they moved, sometimes very quickly. There were also some rays. It looked like the aurora was reaching down to the cloud tops but that was just an illusion. In fact, the display was occurring much further up. We watched the show for about an hour. By then we were getting pretty cold! It was a great event. Since then I have seen the aurora a few other times. The display I saw one August night was less spectacular but a better view because I was far away from any streetlights. I was on an island in Lake Superior and I feel asleep (in my sleeping bag) watching the aurora's show. But you don't have to be at high latitudes. It just helps. I have seen aurora as far south as southern California.

If you've never seen the aurora I hope that one day you will. They are truly a magical event - made no less magical by understanding them.

See you next month.
Wishing you "Clear Skies" (for your aurora hunting and other activities).
Jamie (Dr Love)




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.