A class in Maine submitted the following questionnaire to the Space Sciences Laboratory at UCB.

Intro: What is an aurora?

An aurora is a special interaction of three important features of a planet: the atmosphere, the magnetic field, and the surrounding space plasma. Let's discuss each one of these individually and then see how they work together to form the aurora.

The Atmosphere:

The atmosphere consists of the mixture of gases bound to the Earth by gravity. Nitrogen and oxygen are the most abundant gases in our atmosphere.

Plasma:

Our solar system is bathed in plasma. The term "plasma" means a collection of atoms in which the electrons are dissociated from the nuclei. A plasma is therefore a collection of negatively charged electrons and positively charged ions. Some sources of plasma are: the solar wind, lightning bolts, neon lights, and nuclear fusion reactors.

Our Earth is enveloped in plasma. Most of it comes straight from the sun in the form of solar wind. Some originates at Earth itself; when sunlight strikes our atmosphere, it energizes some of the atoms into the plasma state.

The Magnetic Field:

The magnetic field of most planets is similar to that of a simple bar magnet. In the case of Earth, the magnetic field lines exit near the South Pole and loop around to reenter near the North Pole. (There is nothing physical about the "lines"; they are simply a useful way to convey the strength and orientation of the magnetic field.) Magnetic fields affect charged particles. If you've ever held a magnet near a television, you've seen how it distorts the picture. This happens because the picture is formed by streaming electrons, charged particles. Charged particles have difficulty moving across magnetic field lines, but move more easily parallel to them.

The Earth's magnetic field affects the movement of the surrounding plasma the same way a bar magnet affects the electrons in your tv. Since the Earth's magnetic field lines are all oriented in the north/south direction, the plasma particles mostly travel north or south, and movement east or west is difficult. When the particles approach the poles, one of two things can happen: (1) if they reach the atmosphere, they will collide with molecules and stay there, or (2) instead of entering the atmosphere, they may reflect back from the pole to the other hemisphere. Thus, plasma particles bounce back and forth between the northern and southern hemispheres until they are lost in the atmosphere.

The Interaction of Field, Plasma, and Atmosphere:

When plasma is directed by magnetic field lines down toward the Earth's surface, the charged plasma particles collide with atmospheric gas molecules like oxygen and nitrogen. If the plasma particles have sufficient energy, they can excite the gas atoms (or molecules) into a higher energy state. These excited atoms release their energy in the form of light. We see this light as an aurora.

Your television makes a good model for the aurora. By means of a field, electrons in the tube are directed to the surface of your screen where they collide with a coating on the glass. When they hit, the energy of the electrons is released in the form of light, thereby creating a picture. Comparing this to an aurora: the electrons symbolize the plasma, the coated glass is the atmosphere, and the picture is the aurora.



1. Why are auroras different colors?

Colors are our perception of different wavelengths of light, so we may as well ask: Why do auroras emit different wavelengths of light?

When plasma particles strike our atmosphere, they excite atoms into higher energy states. When these atoms relax back to the lower energy state they emit energy in the form of light waves. Different atoms are capable of emitting different wavelengths of light. For example, red and yellow-green auroras are due to the relaxation of oxygen, while nitrogen is responsible for blue auroras. Nitrogen may also emit red light, which is sometimes seen as the lower border of auroral arcs.

Auroral spectra (color compositions) depend highly on the altitude at which they occur. Certain altitude ranges are more conducive to the relaxation of specific atmospheric atoms than others. (Auroras occur above about 90 kilometers.) For example, excited oxygen has more of a chance to relax above 200 kilometers (where collisions are rarer due to low atmospheric density) so you see red auroras higher in the sky.



2. Do auroras give off heat?

Infrared radiation (IR) is a form of heat. In addition to optical radiation (the reds, greens and blues we see) IR is also emitted by auroras. Oxygen and nitrogen are mostly responsible for the visible emissions. Far less prevalent gases like nitrogen monoxide, carbon dioxide, and helium are responsible for IR emissions.

Auroras also emit light beyond the other end of the visible spectrum. IR is beyond the color red, while ultraviolet light (UV) is just beyond blue. Strong auroral UV emissions are detected by the present POLAR and IMAGE spacecraft.



3. If auroras didn't exist how would the world be affected?

One cannot answer this question without speculating the reason for the aurora's lack of existence.

Auroras would not exist without the sun because there would be no plasma to excite the atoms in the atmosphere. Without the sun, there could be no solar system, and we would not be here to ask about auroras.

We would see no auroras if the Earth had no magnetic field. The magnetic field is what directs the plasma into the atmosphere to cause the brilliant light display. The Earth's magnetic field also protects us from energetic particles streaming from the sun. It acts like a shield which directs the solar wind around the Earth, sheltering life on the surface. We would suffer much higher daily radiation doses without the magnetic field.

Finally, auroras do not exist where there is no atmosphere. Atmospheric gas atoms emit the light we see when plasma particles excite them. Without an atmosphere, Earth would be a forbidding place.



4. Is plasma present during an aurora?

Yes, plasma particles directed by the Earth's magnetic field collide with atoms in the atmosphere. These collisions give rise to light emissions we call the aurora.



5. What are some of the early theories about auroras?

Galileo Galilei thought auroras might be caused by air passing out of Earth's shadow, so that it was illuminated by the sun. Rene Descartes guessed that auroras were caused by reflections from ice crystals in the air. Primitive tribes thought auroras were manifestations of their ancestors fighting the enemy.



6. Are auroras seen on other planets?

The planets must have magnetic fields and an atmosphere to produce auroras. The interaction of plasma and magnetic field at planets like Jupiter, Saturn, and Uranus is analagous to that of Earth. Venus and Mars do not have magnetospheres so you probably wouldn't see auroras there.



7. What would happen if you traveled through an aurora?

Plasma presents a hazard. Since electrons in the plasma generally travel faster than ions, the body collects them faster. This causes charging to a high (negative) potential. That's okay as long as all parts charge equally. Problems arise if certain parts collect more electrons than others. For example, imagine a craft flying headlong through plasma. The nose is exposed to more electrons than the tail so a potential difference results. If the potential grows too great, electrons may discharge. Over time, such electric shocks cause degradation of the craft. Satellite designers must take care to avoid this sort of problem.

Radiation is another significant problem. High energy particles can damage sensitive electronic components. In the auroral zone, particles have low enough energies to be sheilded by metal. In contrast, particles in the radiation belts typically have energies 1000 times greater, and shielding is not feasible.



8. Why don't auroras happen near the equator?

Auroras are usually seen about 20 degrees from either magnetic pole. The magnetic field lines that contact the Earth there are connected to the processes that accellerate plasma particles during the interaction of the solar wind with the Earth's magnetic field. Since plasma must follow magnetic field lines, most of this accellerated plasma reaches the atmosphere at high latitudes.

The "shape" of the magnetic field about the Earth can vary wildly depending on the activity of the sun. When the magnetic field changes violently, this is called a "magnetic storm". During these storms, auroras may be seen much closer to the equator than usual. Auroras have been observed in Hawaii.



9. Additional information:

Auroras near the North Pole are called "aurora borealis", meaning "dawn of the north". Those in the South are called "aurora australis", meaning "dawn of the south".

Good places to view the aurora are Alaska (Fairbanks), central Canada (Ft. Churchill), southern Greenland, northern Scandinavia (Tromso), and the northern part of Russia. The sky has to be clear and dark, so there is no chance to see the aurora in June/July in the northern hemisphere.


Response by: Joseph M. Rauch-Leiba, Jr. Dev. Eng., FAST Science Operations Center
Qualifications: Physics B.A., UC Berkeley, 1996
Email Address: rauch@ssl.berkeley.edu