Aurora are sporadic, faint visual phenomenon associated with geomagnetic activity that occurs mainly in the high latitude night sky. These can be spectacular and moving. According to Geoffrey Wyatt, Senior Science Educator for the Museum of Applied Arts and Sciences:
"It's hard to convey what it's like standing under the sky as it dances and pulses because of what happened on the Sun several days ago!"
Geoff has recently visited Iceland and his photographs of aurora are impressive, so impressive that we have asked him if we can use them on this blog. The science which explains them is fascinating.
Auroras occur within a band of latitudes known as the auroral oval, the location of which is dependent on geomagnetic activity. They are a result of collisions between atmospheric gasses and precipitating charged particles (mostly electrons) guided by the geomagnetic field from the magnetotail (an extension of the magnetosphere). Each gas, oxygen and nitrogen molecules and atoms, emits a particular colour depending on the energy on the precipitating particles, and atmospheric composition varies with altitude. Since the faster precipitating penetrate deeper certain auroral colors originate preferentially from certain heights in the sky.
The auroral altitude range from 80 to 1000km but typical auroras are 100 to 250km above the ground; the color of typical aurora is yellow-green, from a specific transition of atomic oxygen. Above 250km, auroral light from lower levels in the atmosphere is dominated by blue and red bands from molecular nitrogen and molecular oxygen.
Above 250km auroal light is characterised by a red spectral line of atomic oxygen. To an observer on the ground, the combined light of these three fluctuating primary colours produces an extraordinary visual display.
Auroras in the Northern Hemisphere are called the aurora borealis or “northern lights.”
Auroras in the Southern Hemisphere are called aurora Australis.
The patterns and forms of the aurora include quiescent arcs, rapid moving rays and certain patches and veils as you can see in Geoff's images of the aurora borealis.
Though solar activity on the Sun such as large Sunspot groups, Flares and Surges produce the Aurora activity there are at times during the solar minimum where activity on the Sun is very low and Auroras are still seen. This is caused by streams of solar wind that flow from coronal holes.
REF: Space Environment Service Centre
Monty Leventhal OAM.