For much of human existence, we have looked into the night sky with wonder at thousands of points of light that seem to form endless patterns.
Ancient astronomers spent a lot of time trying to understanding all that the stars could tell us. But in their observations, they often missed the most important star of all - our sun.
The sun is the closest star to the planet Earth at only 500 light-seconds away.That is, it only takes 500 seconds or eight minutes and 20 seconds for light that has left the surface of the sun to reach Earth. (It takes millions of years for the energy released in the sun's core to get to the surface.)
The sun is also very bright so it really isn't any surprise that early astronomers did not recognize it as a star nor study its surface. Viewing the sun with the naked eye can destroy your retina.
As far as life on Earth is concerned, the sun is the main source of all of the energy that we consume. We receive this energy in the form of electromagnetic radiation across a broad range of frequencies or wavelengths but the most important component is the region that we call visible light.
However, electromagnetic radiation or light is not the only thing we get from the sun. As the various nuclear reactions occur within the interior, fragments of atoms collide and atomic nuclei are fused. The sun is a giant explosion and, as such, it is constantly blowing off atoms, ions, and molecular species.
This collection of particles is called "the solar wind". It is responsible for the auroras seen throughout the solar system.
To explain the origin of Earth's auroras - the Aurora Borealis and the Aurora Australis - we need to start with the fact that the Earth is shielded by a magnetic bubble and, for the most part, the magnetic field lines within the bubble shuttle the particles in the solar wind around the planet.
An analogy is to consider a post or tree in a river. The water flows around the object with a distinctive teardrop shaped pattern.
Every now and then, the Solar wind can overcome our magnetic shielding. When this happens the particles within the wind dip into the upper atmosphere where some truly amazing atomic chemistry occurs.
Atoms are composed of electrons in orbitals around a nucleus. The energy of each orbital is quite precise. To jump from one orbital to the next requires either the absorption or emission of energy in the form of electromagnetic radiation.
When energy is emitted in the visible region of the electromagnetic spectrum, we see fluorescence or the emission of a single wavelength of light. The light is particular intense because all of the atoms involved release light at the same wavelength.
When the solar wind overcomes out magnetic shield and intrudes into the upper atmosphere, the particles in the wind strike atoms and molecules of oxygen, nitrogen, argon, and other gases found in the upper reaches. These molecules become excited. They absorb the energy from the incoming particle by either shifting electrons to higher orbitals or by losing the electrons entirely.
The electrons in the excited molecules then collapse back to lower levels or to the ground state and emit photons of light all of the same wavelength or colour.
The result is an aurora - large bands of colour in the sky that result from the interaction of the particles in the solar wind sneaking through our defensive magnetic shields and exciting molecules in the upper atmosphere resulting in the emission of specific wavelengths of light.
That is probably not the most poetic way of describing an aurora but it is essentially what happens. The different colours are generated by different molecules. For example, the dominant blue colour results from the excitation of molecular nitrogen resulting in the emission of light at 425 and 390 nanometres. The brilliant greens (557.7 nm) and reds (630.0 nm) are produced by atomic oxygen.
Modern astronomers are now able to look at the aun. We use instruments designed to shield us from the damage caused by the more intense portions of the electromagnetic spectrum. Several satellites and telescopes are specifically designed to monitor the sun's activity.
Solar storms can result in coronal mass ejections (CME) and it is these belches of solar particles that can result in a particularly strong solar wind, which will overcome Earth's defenses. Because the light from the sun gets to us in just over eight minutes, we can see the release of the particles a few days in advance of their arrival at Earth.
Modern astronomers can not only observe the sun but they can now predict when an aurora is likely. We should be seeing some good ones over the next few days.
