The aurora borealis is a thing of great beauty to see. But the difficulty’s always in knowing when to look at the night sky to find it. Mobile applications now make it possible to predict the aurora – and not miss out on that light-drenched spectacle!
What makes the auroras happen?
The auroras are created by particles (essentially electrons and protons) emitted by the Sun in the form of solar wind. As they approach the Earth, these particles strike the Earth's magnetosphere, in other words the magnetic field of our planet. Under certain conditions, a percentage of the particles enters the atmosphere in high-latitude regions and strikes the air molecules. By reaction, the air molecules will emit the light characteristic of auroras.
How do we know how good the chances of observing auroras are? Here are some parameters that might help.
- First of all there’s the Kp-index (Planetary K), which allows us to measure geomagnetic activity thanks to magnetometers situated around the world. The Kp-index is graduated 0 to 9 and measured during a three-hour interval. For us to observe the aurora at the latitude of Montréal, the Kp-index has to exceed 5 (Kp > 5).
- The interplanetary magnetic field is an important component in triggering auroras.
- The total value of this field, represented by the Bt-index, must amount to a minimum of 10 nanoteslas (Bt > 10nT).
- The Bz component must be orientated southward to line up with the Earth’s magnetic field, which is orientated towards the north (a bit like how two oppositely charged magnets attract each other). The Bz index must be negative and lower than -10 nanoteslas (Bz < -10 nT).
- Finally, ideally the speed of the solar wind should be greater than 700 km/s.
Applications for predicting auroras
Fortunately, for those who might need a helping hand there are now a host of free applications out there. Here are some suggestions:
In my view one of the best sites on the auroras. The site’s available on the Web, for tablet or mobile device. In addition, the app is available in 11 languages, including English and French. The homepage quickly provides all the relevant indexes for predicting auroras. There are also short explanations of a number of elements related to auroras.
For tablet and mobile device. This site is well done and easy to consult. Some of the graphics are small on a mobile device, but the relevant indexes are very clear and easy to read. The tablet version doesn’t suffer from the same shortcoming.
Also accessible on tablet and mobile device. The forecast page displays the information relevant to auroras in a user-friendly way. As a bonus: a photo gallery of auroras taken by fans around the planet.
In closing, let me remind you that the presence of favorable indexes is no guarantee that you’ll be able to observe auroras. And of course, you also need a clear sky!