Aurora is a luminous glow of the upper atmosphere caused by energetic particles entering the atmosphere from above. This definition differentiates aurora from other forms of airglow, and from sky brightness that is due to reflected or scattered sunlight. Airglow features that have "internal" energy sources are more common than aurora. For example, lightening and all associated optical emissions like sprites should not be considered aurora. On Earth, the energetic particles that make aurora come from the geospace environment, the magnetosphere. These energetic particles are mostly electrons, but protons also make aurora. The electrons travel along magnetic field lines. The Earth's magnetic field looks like that of a dipole magnet where the field lines are coming out and going into the Earth near the poles. The auroral electrons are thus guided to the high latitude atmosphere near each pole. As the electrons penetrate into the upper atmosphere, the chance of colliding with an atom or molecule increases the deeper they go. Once a collision takes place, the atom or molecule takes some of the energy of the energetic particle and stores it as internal energy while the electron goes on at a reduced speed. The process of storing energy in a molecule or atom is called "exciting" the atom. An excited atom or molecule can return to the non-excited state (ground state) by sending off or emitting a photon, i.e. by making light.
Anywhere from 10 minutes to all night long, depending on the magnitude of the incoming solar wind. "Coronal holes" consistently produce nice auroras but big solar flares and CMEs-coronal mass ejections are responsible for global-wide aurora displays…the BIG shows!
In northern regions like the Yukon, the best time to view the aurora is from late August to mid-April. During the summer months the night skies are not dark enough to see the aurora borealis even at times when the solar wind activity is strong. In midwinter the temperatures may fall to the minus 40 degree range and it is important to wear appropriate winter clothing (please see “winter clothing packages”) to enjoy the displays.