All About Auroras And its Origin

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The Auroras (The plural is either auroras or aurorae. Both seem to be acceptable) are generally seen near the Earth’s magnetic poles, so they are often very far north or very far south. When in the north, we call them the Aurora Borealis, and when in the south the Aurora Australis. So, let’s discuss, All about Auroras and its origin.

All About Auroras And its Origin

What is Aurora and how they are formed?

The Aurora is glowing in the upper atmosphere caused by excitation of atoms by high energy particles, typically electrons. These high-speed electrons slam into the oxygen and nitrogen atoms high in the atmosphere, exciting them (imparting energy to them). These atoms will then de-excite, emitting energy in the form of light. The particular wavelength (color) of light will depend upon the particular energy level transitions involved. Of course, atoms can be excited at a lower level in the atmosphere, too. But, at the lower levels of the atmosphere, where the air is denser, atoms will bump into one another before they get a chance to de-excite.

These collisions will allow the excited atoms to lose energy to other atoms through collision rather than losing energy through emission. Thus, the transitions leading to these emissions are never seen at low altitudes. Therefore, we call these “forbidden” transitions (They are forbidden transitions because you don’t generally see them in the laboratory, not because they don’t happen!).

Different colors of Auroras

At high altitudes, the gasses of the atmosphere are so rarified that the atoms can go for long enough between collisions that they get a chance to de-excite via photon emissions. Most auroras occur at over 60 miles altitude. The air here is so thin that it is closer to a vacuum than you normally can get in the laboratory with a common vacuum pump. The most common colors of the aurora are reds and greens from atomic oxygen. Normally, the reds are seen at higher altitudes than the greens. Sometimes combinations of colors make the aurora appear orange. There are reports of blue and purple colors. But, by far the typical aurorae are red or green.

Forms of Aurora’s

Auroras can appear in a variety of forms, sometimes as ribbons, and sometimes as vertical spikes, and sometimes simply as a diffuse glow. Though normally seen near the geomagnetic poles, during severe geomagnetic storms (major disturbances in Earth’s magnetic field resulting from interactions between Earth’s magnetosphere and coronal mass ejections from the Sun) the aurora can be much brighter and can sometimes be seen much farther from the geomagnetic poles.

More auroras can be seen when the Sun is most active. The Sun’s activity goes through an approximately 11-year cycle. Interestingly, auroras are somewhat more common near the equinoxes than at other times of the year. However, there is no clear model that explains why this is the case. In fact, there is a lot that we don’t understand about auroras.

But, with recent advances in understanding, we also have gained some ability to predict the likelihood of seeing an aurora. If you are really interested in seeing an Aurora, the University of Alaska at Fairbanks has a website that gives auroral forecasts. NOAA’s Space Environment Center has all sorts of current data and space weather forecasts available. Another wonderful website is Spaceweather.com, which will keep you updated on not just auroral activity, but all sorts of space-related news.

Origin of Aurora

There are two types of auroras: The Diffuse Aurora, and The Discrete Aurora. The diffuse aurora is always present, but is very dim, lacks sharp outlines, and is generally not able to be observed from the ground. The photo at the top of this post is taken from space and is highly enhanced to show the diffuse aurora. The discrete aurora, though, is a transient phenomenon, has distinct and sharp edges and is much brighter, so it is visible to ground-based observers.

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By the early Twentieth Century, geoscientists had realized that the aurora was due to the emission of light from excited atoms in the uppermost portions of the Earth’s atmosphere. It didn’t take long to realize that the aurora was some sort of electromagnetic phenomenon, too. The Earth’s magnetic field was found to be significantly disturbed near the aurora. In 1903, Kristian Birkeland proposed that the magnetic disturbances in the vicinity of the aurora may be due to large electrical currents flowing up and down along the auroral features. However, it took nearly 7 decades for a mathematical model of these currents to be developed.

Diffuse Aurora

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Diffuse Aurora. Image Source: Flickr

In recognition of Birkeland’s groundbreaking research, we call them Birkeland currents. These currents are quite powerful. The current flows upward on the night side of the Earth, and downward on the day side, and from day to night between. Most of the charge carriers of the current, as with most currents, are electrons.

However, electrons are negatively charged, so they move in a direction opposite to the current flow. Thus, electrons are streaming down on the night side of the auroral oval, and upward on the day side. The downward flowing electrons that slam into oxygen and nitrogen atoms exciting them, causing them to emit light. Though electrons are the dominant charge carrier, some protons also are involved, and they move in the direction of the current, so the protons are slamming into atoms on the day side. This is happening all of the time and gives rise to the diffuse aurora.

Discrete Aurora

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Discrete Aurora. Image Source: aurorasaurus.org

However, once in a while something causes the charge carriers (electrons and protons) to be accelerated to extremely high velocities. These very highly energetic particles follow along Earth’s magnetic field lines to slam into the atmospheric atoms with far greater energies, exciting the atoms to higher energies and exciting more atoms. This makes for a brighter aurora.

Also, the charged particles are generally accelerated in bunches and follow along the magnetic field to strike the atmosphere in about the same area. This makes for a sharper, more defined aurora, often showing features of ribbons or spikes. This is believed to be the origin of the discrete aurora. Whatever mechanism is at work with the Aurora, is known that more auroras are visible when the Sun is active and that auroral activity is related to geomagnetic storms.

This much had been worked out by the latter part of the 20th Century. However, it was still a mystery as to where these charged particles came from. Well, we don’t really know for sure. However, in recent years the scientists who study space weather (the study of Earth’s magnetosphere) have made major strides forward in understanding the aurora. To understand the Aurora, you need to understand the magnetosphere and electromagnetism. First of all, let’s look at Earth’s magnetosphere.

Involvement of Planet Earth

Planetary scientists don’t think of the Earth as ending at its surface. They don’t even think of it ending at the top of the atmosphere. Rather, the Earth extends into space through the reach of its magnetic field. The region of space surrounding Earth is dominated by both Earth’s gravity and its magnetic field.

The region of space dominated by Earth’s magnetic field is its magnetosphere. About two centuries ago, Christiaan Huygens showed that electricity and magnetism are related to one another. In fact, we now think of them as different aspects of the same force: the electromagnetic force. Moving charged particles create magnetic fields. But, magnetic fields exert a force on moving charged particles.

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Earth’s Magnetic Field. Credit : Shutterstock

Plasma streaming from the Sun as the solar wind is deflected by Earth’s magnetic field. In fact, the solar wind particles have no direct access into the Earth’s magnetosphere except right along the poles.

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For years, it was assumed that some of these charged particles somehow got into the Earth’s magnetosphere to populate the van Allen radiation belts. These radiation belts are roughly donut-shaped regions surrounding Earth in which charged particles are trapped in Earth’s magnetic field. Anything entering the regions is bombarded by these particles, in a sort of particulate radiation (essentially high energy beta radiation and proton radiation). It appears that the majority of these particles originate with Earth itself. Sunlight, particularly short wavelength light, such as ultraviolet light, seems to be responsible for populating the van Allen belts with ions from the uppermost parts of Earth’s atmosphere.

Earth’s Magnetosphere

The magnetosphere deflects the solar wind past Earth. However, the solar wind itself is composed of charged particles. These charged particles streaming past Earth produce an electric current whose magnetic field interacts with Earth’s magnetic field. Equilibrium is achieved, and the observed planetary magnetic field is in a sense a combination of both the magnetic field generated in the interior of the Earth and the magnetic field resulting from the solar wind.

However, the solar wind is gusty, and so the amount of solar wind keeps changing. This means that the magnetic field in the magnetosphere keeps changing. The farther from Earth, the bigger the effect. Fluctuations in the global magnetic field are monitored and reported as a planetary K-Index.

The K-Index

Major fluctuations in the planetary K-Index signify a geomagnetic storm. You can monitor this data on the internet at SpaceWeather.com or at NOAA’s Space Environment Center webpage. The bigger the planetary K-Index, the more likely the Aurora, and the farther from the geomagnetic poles that an aurora may be seen. It takes a K value of about an 8 or 9 for an aurora to be visible from here in Texas. Also, the bigger the K-Index, the more radiation that airline flight crews are exposed to. And the further south and lower altitudes that intense exposure can be experienced.

How do fluctuations in the magnetic field give rise to Aurora and high-altitude radiation?

Well, it has to do with electromagnetism. Michael Faraday determined that changes in magnetic fields can produce electric voltage. In fact, we use this physical fact in all generators that produce electricity. The more rapid the change or the larger the change, the higher the voltage produced. A voltage differential in space can accelerate electrons and protons to extremely high energies.

This effect is not limited to space. As geomagnetic storms can induce high voltage and damaging currents in phone lines, electric transmission lines, and pipelines on Earth. Now, we are only just coming to grips with understanding auroras, so this may not be the whole story, or we may find out later that it is wrong. But this might be an explanation for at least some of the auroral effects that we see. Certainly, major progress is being made in understanding the auroras.


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