Text and pictures © 1993-2023 Guillaume Dargaud
Last updated on 2021/11/05
"If my soul could get away from this so-called prison, be granted all the list of attributes generally bestowed on spirits, my first ramble on spirit-wings would not be among the volcanoes of the moon. Nor should I follow the sunbeams to their sources in the sun. I should hover about the beauty of our own good star. I should not go moping among the tombs, not around the artificial desolation of men. I should study Nature's laws in all their crossings and unions; I should follow magnetic streams to their source and follow the shores of our magnetic oceans. I should go among the rays of the aurora, and follow them to their beginnings, and study their dealings and communions with other powers and expressions of matter." — John Muir.
Left: Aurora australis above DdU
Auroras are _the_ thing to see in polar regions in winter. When the sun is very active (activity follows an 11 years cycle), the so-called solar wind emitted during chromospheric eruptions of the sun hits the Earth and gets deviated by its magnetic field and canalized towards the magnetic poles (the south magnetic pole is only 15 km from DdU). The charged particles of the solar wind (protons, electrons...) follow the magnetic lines to the poles and when they hit the ionosphere they ionize the present molecules. The lower limit of auroras is between 60 and 100km, while the higher limit is between 100 and 400 km (seldom 1000km). They are usually as bright as clouds lit by the full moon, but can be much brighter. They happen any time of the year but are visible only at night.
Auroras are often qualified as 'borealis' in the northern hemisphere and 'australis' in the southern one but they are the same phenomena. The term 'northern lights' is also commonly used in Alaska and Canada.
Left: Aurora australis above DdU
Right: Animation showing the evolution of an aurora
The most common kind of aurora is the green one, but they can be red, purple, blue... Different colors in the auroras come from the excitation of different molecules (ionized oxygen 300km up lead to a red color, molecular oxygen 100km up gives a yellow-green color, molecular nitrogen gives up a light red color, ionized nitrogen purple or dark blue...). Unfortunately this picture is only a colorized black & white. It is a truly fantastic show to witness with curtains of light falling from the sky, moving around, changing shape in a matter of seconds. Well worth freezing your butts off outside in the cold winter night !
Auroras are concentrated on a circle around the geomagnetic pole, depending on the solar activity this circle can extend as far as 40° latitude but usually it is localized to polar regions. Since the activity of auroras depends on the solar wind, it has long been believed that auroras happen at the same time and in a symmetric way in the northern and southern hemisphere. The problem is that when it's dark north, it's daylight down south... Recently satellite imagery showing the entire auroral circles seems to have put this idea to rest, probably because the angle of the Earth changes the trajectory of the particles too much for them to maintain similar trajectories in both hemispheres. One last thing: any planet with a strong magnetic field can have auroras, they've been observed by Hubble on Jupiter and Saturn.
I took those pictures in less than ideal conditions. First because I was tired and drunk since it was one of the last days of the midwinter party, it was a windy night that caused the tripod (and the camera) to fall twice on the concrete of the landing pad, and I was cold since I did not want to waste time going back inside to put more clothes on. I took those pictures wearing only blue-jeans by -25°C and all my leg hair fell off afterwards. Exposure time is around 8 seconds, 3200asa film (Kodak Tmax 3200), f=20mm/2.8 at full aperture and hand-held open flash to light up the buildings.
More information and pictures about the northern lights.
Left: A faint aurora above a satellite antenna is revealed by a long exposure.
Right: Another long exposure reveals the faint purple lights above the usual faint green.
Right: Curtains of light waving above the roof telescope.
Left: Some purple shades can be discerned above the green curtains.
Right: The aurora changes quickly and silently position.
Right: 180° fisheye images cover the entire sky and show various auroral curtains crossing path with the much farther Milky Way.
Left: The Milky Way and several auroras.
Above: A fisheye image similar to those above, but unrolled in software to display 360x90°. The Milky Way and the aurora form two great arcs in the night sky. Enlarge your browser window if you have some display room ———>
Above: Several auroral curtains and the Milky Way.
Above: The full moon somewhat dims two auroral curtains.
Earth | Time Lapse View from Space, Fly Over | NASA, ISS from Michael König on Vimeo.