Science Of The Aurora
The Aurora Borealis (and Aurora Australis in the southern hemisphere) is a result of charged particles from the Sun interacting with the Earth’s atmosphere. These particles, primarily electrons and protons, are ejected from the Sun in a stream called the solar wind.
As the solar wind reaches Earth, it’s deflected by our planet’s magnetic field. This magnetic field acts like a shield, protecting us from the harmful radiation of the Sun. However, some of these charged particles get trapped in the Earth’s magnetic field lines and are funneled towards the poles. When these charged particles collide with atoms and molecules in the Earth’s atmosphere, they transfer energy to them. This causes the atoms and molecules to release energy in the form of light - photons - as they return to their natural state. These photons are what see as the aurora. As the solar wind increases in speed and the interplanetary magnetic field embedded in the solar wind turns southward, the geomagnetic activity will increase and the aurora will become brighter.
So the combination of solar wind speed and direction (Bz) are significant when trying to predict good conditions for the aurora. In general solar wind speeds above 400 km/s and a strong southerly direction -10 Bz or lower are needed for the best viewing.
Why can the aurora be different colours?
The color of the aurora depends on the type of atom or molecule that is excited and the energy level of the collision. For example, green auroras are caused by collisions with oxygen atoms, red from high altitude oxygen, while hints of purple and blue are caused by collisions with nitrogen.
Due to different concentrations of gases at different levels of the atmosphere this means aurora colour is strongly linked with altitude.
- Red can occur rarely at around 200-500 km. At this altitude the oxygen is less concentrated and releases photons at a higher frequency than oxygen lower in the atmosphere.
- Green occurs at around 100-300 km above Earth. Here there are high concentrations of oxygen. This is the altitude where most collisions occur which coupled with the eyes ability to detect the green part of the spectrum makes this the predominant colour.
- Blue and Purple occur below 100 km. Here the aurora is caused by interactions with nitrogen molecules. If they occur you will usually see these colours towards the bottom of the aurora display.
The Aurora Oval
The aurora oval typically appears as a ring-shaped area centered around the Earth’s magnetic poles.
The brightness of the aurora within this oval can vary significantly. Areas within the oval may experience vibrant displays, while regions outside may see little to no activity.
The oval can change shape and size based on solar activity and geomagnetic conditions. It is not static; it can expand and contract depending on the intensity of solar wind and geomagnetic storms (measured by Kp-index). During strong solar events, the oval can extend further south.
The NOAA provide many incredibly useful tools for aurora forecasting. The Ovation map (shown above) might be the best. While the Kp-index gives you a general idea of how strong the geomagnetic activity is, the Ovation map pinpoints exactly where you might see the Aurora. Think of it as a real-time map powered by data about solar winds and the magnetic field. Ovation predicts where the Aurora will be visible 30 to 40 minutes in advance. It even tells you how strong the Aurora will be. Please remember though, that even if the ovation map suggests you should see the aurora it may not matter if you’re in an area with too much light pollution or under cloudy skies.
See also
Northern Lights PhotographyNorthern Lights PlaylistPlanning Your Northern Lights Adventure