The aurora last night was a prime example of what I want to illustrate – why does the aurora flare up? In the timelapse below the aurora dances on the horizon before exploding into one of the best shows I have seen overhead. During my time in Alaska I have tried to glean scientific information on the aurora. Last week I attended a talk by Dr. Akasofu who has been studying the aurora for 50 years, and his talk was focused on the very question I pose here.
So first, the setting. You are on top of a large hill in Alaska and it’s 11:00 PM. As you stare into the inky darkness of the moonless night a green band of light plays in front of your eyes, and it is OK, but it’s not a jaw-dropper. Often time that is the form of the aurora. But suddenly as you watch the green smudge it goes super-nova expanding rapidly in size, color, and intensity. In fact, it’s so intense that the snow is lit up green and even your coat might be. Over your head and on all sides, the aurora builds in greens and reds. Pulses of light can be seen on the far horizon which flow towards you like a wave over your head breaking in unpredictable patterns. Green light shoots in all directions.
Why did that happen? I always assumed the high intensity auroral moments were created by extra energy (solar wind) entering the system. In contrary to that, the research conducted by Dr. Akasofu and other suggests the aurora is a circuit. Incoming solar wind is pushed against the earths magnetosphere where it reacts in an auroral sub-storm. If more energy is input into the system than can be output it starts to build up in a ‘secondary circuit’. The extra energy is stored and builds up within the atmosphere. When the conditions are right the energy is released in ONE pulse of energy causing the aurora to erupt suddenly. It also explains why eruptions last roughly the same amount of time (1 hour) since a finite amount of energy can be built up.
Based on this model, the aurora goes through three phases. Growth which is aurora formed directly by solar winds and is often manifested by low-grade auroras. Expansion which is the unloading of the secondary circuit and direct solar wind. And finally recovery, which is just driven by solar winds.
I hope you’ve enjoyed some of the science of the aurora! I’ll put my disclaimer on the end that I disseminated the information of the talk to you the best I could, and I hope I got it right!