By Dr. Mikko Syrjäsuo, Alberta Ingenuity Fellow, University of Calgary
It all begins with our nearest star, the Sun, whose surface is so hot that it gives off a continuous stream of solar wind. The wind from the Sun carries particles with it to the rest of the solar system and beyond. This not-so-gentle breeze is continuous but varying: the average speed is around 400 km per second—by comparison a car's highway speed is about 30 metres per second. However, in gusts or solar storms the wind speed can reach 1000 km per second and it will carry more particles with it.
Although the solar wind howls around and past the Earth, it does not directly reach the surface because Earth's magnetic field acts as a shield. This invisible windshield is called the magnetosphere.
The flow of the solar wind around the Earth's magnetosphere is the ultimate power source for the aurora.
It is like any electrical generator, where a conductor moves in a magnetic field and the motion creates an electric current. The solar wind is ionized gas that can conduct electric currents, and it acts as a moving conductor in the Earth's magnetic field. So together they form a giant generator: this electric dynamo creates currents of millions of amperes in the near-Earth space and the upper atmosphere. Electric currents, or electrons in motion, eventually cause proton and other ion motion as well. A thousand billion watts of power can create quite a lot of action anywhere!
The formation of aurora borealis works a lot like a television. When you plug a TV into the power outlet, the back end of the cathode ray tube starts boiling off electrons. The electrons are accelerated and then guided by electric and magnetic fields so that the electrons hit appropriate locations on the TV screen. The screen is coated with phosphor which gives off light when hit by electrons, and the light forms a picture.
In the cosmic TV, the power is obtained from the solar wind and used to accelerate electrons and protons inside the magnetosphere. The particles are then guided by the Earth's magnetic field until they precipitate into the upper atmosphere and produce the northern lights. Auroral scientists frequently point out that the TV analogy goes even further: the acceleration voltage both in TVs and auroras is about 20,000 volts. After that, the scientists hesitate: the actual mechanism of creating this electric potential in the appropriate place is not really understood. The cosmic show itself is programmed by the variations in the solar wind, of course, but we still do not know enough to explain how the auroral shapes are formed. Nevertheless, we are sure that the show is about the environment of our home planet .
Because the solar wind flows continuously, there is always some aurora. We can even observe auroras during the day. The twist is that we need a dark day, such as in the arctic during mid-winter when the Sun does not rise. The colours in the aurora are due to different collisions. For example, electrons crashing into atmospheric oxygen atoms at different speeds produce either yellowish-green or red. Collisions with nitrogen molecules, on the other hand, produce blue or violet. Brightness of the aurora simply depends on the number of collisions— the more precipitating particles, the brighter the display. All of these collisions occur at altitudes of about 90 kilometres or more.
There still are many unanswered questions that continue to puzzle space physicists. In Canada, we build and use auroral imagers, magnetometers and ionospheric radars, as well as satellite and rocket instruments for studying the aurora. We create mathematical models and simulations to verify whether we're right or wrong. Sometimes, though, it's more fun to put the science aside, lean back, and enjoy the show.