The Aurora (Part 2: Origin of the Aurora)
Published on Nov 12, 2006 at 2:37 pm.
1 Comment.
Filed under Earth, aurora, physics.
Yesterday, I wrote about observing the aurora. Today, I am going to write a bit about the origin of the aurora. There are two types of aurorae: 
the diffuse aurora, which is shown above, and the disctrete aurora, as is shown to the right. 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 phenominon, has distinct and sharp edges, and is much brighter, so it is visible to ground based observers.
By the early Twentieth Century, geoscientists had realized that the aurora was due to emission of light from excited atoms in the uppermost portions of the Earth’s atmosphere. The colors that you see, mostly red and green, come from the de-excitation of atoms of oxygen and nitrogen. My post yesterday had more information on this. It didn’t take long to realize that the aurora was some sort of electromagnetic phenominon, 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. 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.
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 atospheric atoms with far greater energies, exciting the atoms to higher energies and excitiing 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 is known that more aurorae 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. Many of the things that I had been taught years ago are now believed to be wrong. For example, I was taught that the source of the aurora was likely due to an overloading of the van Allen radiation belts during a geomagnetic storm, and the surplus particles streaming down into the atmosphere. Actually, I am a little miffed that I was taught that, since James van Allen himself disproved that idea in the early 60’s, well before I was taught the wrong thing! Another wrong idea is that the aurora comes from charged particles from the Sun slamming into the atmosphere. We know that the Sun is continually sending particles our way (the Solar wind), and we know that at times the solar wind increaes, and that these increases in the solar wind are associated with aurrorae. In fact, this was Birkeland’s idea. However, the solar wind particles are far too low energy to give rise to the discrete aurora (though they may contribute in part to a diffuse aurora right over the geomagnetic poles). So, if these are not the sources of the aurora, then what is the source?
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.
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 (as ill defined as that is). 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. 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. 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 paticulate radiation (essentially high energy beta radiation and proton radiation). And, I had been taught that the particles populating the van Allen belts were from the Sun. Now, 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.
The magnetosphere deflects the solar wind past Earth. However, the solar wind itself is composed of charged particles, and these charged particles streaming past Earth produce an electric current whose magnetic field interacts with Earth’s magnetic field. An 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. Major fluctuations in the planetary K-Index signify a geomagnetic storm. This data can be monitored 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. I did an extensive post on airline crew radiation exposure some time back.
So, just how do fluctuations in 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, though, 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 aurorae, 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 aurorae.
-Astroprof
(Image credits: NOAA, NASA, NGDC, JAXA)
Donald Mayer on December 22, 2011 at 1:36 pm: 1
Can man travel through this intense radiation within the Van Allen belts; even in a spacecraft; without dying?