Aurora

The Aurora (also known as the Northern Lights, aurora borealis or auror australis) is a luminous glow of the upper atmosphere which is caused by energetic particles that enter the atmosphere from above, primarily from the solar wind.

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 guided to the high latitude atmosphere.

As they 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 with 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 a photon, i.e. by making light.

The composition and density of the atmosphere and the altitude of the aurora determine the possible light emissions and therefor the colour of an aurora.

When an excited atom or molecule returns to the ground state, it sends out a photon with a specific energy. This energy depends on the type of atom and on the level of excitement, and we perceive the energy of a photon as colour. The upper atmosphere consists of air just like the air we breathe. At very high altitudes there is atomic oxygen in addition to normal air, which is made up of molecular nitrogen and molecular oxygen.

The energetic electrons in an aurora are strong enough to occasionally split the molecules of the air into nitrogen and oxygen atoms. The photons that come out of an aurora have therefore the signature colours of nitrogen and oxygen molecules and atoms. Oxygen atoms, for example, strongly emit photons in two typical colors: green and red. The red is a brownish red that is at the limit of what the human eye can see, and although the red auroral emission is often very bright, we can barely see it.

The green emission from oxygen atoms has a peculiar thing about it: usually an excited atom or molecule returns to the ground state right away, and the emission of a photon is a matter of microseconds or less. The oxygen atom, however, takes its time. Only after about a 3/4 second does the excited atom return to the ground state to emit the green photon. For the red photon it takes almost 2 minutes! If the atom happens to collide with another air particle during this time, it might just turn its excitation energy over to the collision partner, and thus never radiate the photon.

Collisions are more likely when the atmospheric gas is dense, so they happen more often the lower down we go. This is why the red color of oxygen only appears at the very top of an aurora, where collisions between air molecules and atoms are rare. Below about 100 km (60 miles) altitude even the green color doesn't get a chance. This happens when we see a purple lower border: the green emission gets quenched by collisions, and all that is left is the blue/red mixture of the molecular nitrogen emission.

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Aurora Links & Resources

The current extent and position of the auroral oval at each pole, extrapolated from measurements taken during the most recent polar pass of the NOAA POES satellite. Click images for enlarged view. The Aurora Page A source for Information, links and images about the "Northern Lights". Auroral Activity Extrapolated from NOAA POES The Auroral ovals at the north and south poles along with information on how to view an aurora and some movies. Real Time Auroral Oval This picture updates every 5 minutes when the activity level is significant. Aurora Forecast Aurora forecast information for Alaska, North America, Europe and the North and South Polar Regions. Hourly STD DMSP/POLAR Auroral Activity Report Ground-based sightings of auroral activity are provided on the following map. Aurora Forecast for Your Region Get an aurora forecast map for where you live. Aurora Forecast for Your Region Get an aurora forecast map for where you live. Aurora Pictures and Movies Take a look at the auroras seen by people around the world. The Aurora Alarm The Aurora Alarm provides this service free of charge to nearly 1, 000 users. The only system of its kind in the world, it uses data from a detector site in eastern Washington to alert users in real time when the aurora appears in the sky. Nordlys Northern Lights Photos, mythology and other auroral manifestations appear on this Norwegian site. AuroraWatch This web site allows you to monitor geomagnetic activity in real time, and will let you know when aurora may be visible from the UK. Aurora WebCam AuroraWebCam.com is an international project. This free site provides the 1st private worldwide live webcast of auroras in history. Auroras - Paintings in the Sky This site will show you what auroras look like on Earth and from space, explain how they are created, and show you where they can be found. Teach yourself using the "Self-Guided Lesson" button. Nordlys Northern Lights Discover the northern lights phenomena through text, pictures, audio and video. Welcome to the worlds largest site dedicated to the northern lights.

Aurora eBooks & Articles

Shooting the Aurora Borealis Here are a few hints to get you started taking your own aurora photos. Don't be afraid to experiment with different exposures because night photography really is an experiment. When the aurora is extremely bright (casting a shadow) it is usually moving too fast for 100 speed film...