These days I am spending a few months in northern Sweden, to start a collaboration with computer scientists and physicists from Lulea University of Technology on neuromorphic computing (I'll soon write about that, stay tuned). The rather cold weather of March (sub-zero temperatures throughout the day) is compensated by having access to the night show of northern lights, which are often visible from these latitudes (66 degrees north).
At the not-so-young age of 57, I came here without having seen northern lights, ever. So, having received an alert from a colleague, it was with some trepidation that last Tuesday evening I dressed up very warmly and walked half mile east, toward the lake near the University Campus. I had previously studied the map and figured out that must be the place with the least light pollution...
The show was awesome. A geomagnetic disturbance had been forecast to hit Earth late on March 14, and indeed, there it was - green dancing structures in the sky! I tried to take a few pictures with my cellphone, but due to the cold weather and the crappy hardware I was not able to pull off anything worth showing here. Fortunately, the same spot had been chosen by a few chinese PhD students, who later shared their pictures with me. So below you can see more or less what I could see on Tuesday night.
Aurorae are definitely one of the most beautiful natural phenomena you can see in the sky. During the strongest geomagnetic storms the sky can lit up with pulsating structures of all colors, from red to blue - although these are rare events, some pink-reddish tones are not uncommon; but blue is indeed rare. The kind of shapes you see in the sky are also awe-inspiring, and they exhibit fast (order one second) as well as slower variations, giving rise to a real show. These shapes have only recently been closely connected to the different phenomena that take part in the upper layers of our atmosphere.
While the mix of observed colours is not difficult to figure out - it depends on the different atoms that emit light when struck by solar wind particles (electrons and ions, that flow to Earth ejected by solar flares, and get to move and dance in the geomagnetic field at altitudes of 100 to several hundred kilometers), understanding precisely how the different shapes of emission regions and their moving patterns are generated is much less straightforward.
The most common structures are called "arcs", or "curtains". They appear as 10-20 degree wide bands in the sky, from east to west, often with a sharper , more defined side and a blurrier one. They are typically green, with a pinkish side hardly detectable in all but the strongest events. They move about slowly, but can occasionally "lit up" within seconds when a stream of electrons reinforces the emission.
Arcs may pulsate and exhibit longitudinal bands that move about and appear and disappear at very fast rate. They may also evolve into spikes - bright vertical columns caused by the precipitation of electrons. Then, sometimes the structures evolve to "coronas", which lit up the whole sky with irregular structures comprising spikes, cloud-looking patches, and other formations.
The physics that describes the dynamics of electrons and ions in the small magnetic field of the upper atmosphere allows us to understand how these structures are generated and evolve in time, but a forecast of the precise forms is not presently possible. What can be forecast with reasonable accuracy is instead the overall level of auroral activity in different locations around the globe. This is possible by correlating information from Sun observations with measurements on Earth. A good site for short-term forecasts is the Space Weather Prediction Center.
