Auroras: What are they? When to watch them?

Some days ago, National Geographic published amazing photos of a ghostly green aurora borealis over Iceland (Image 1 and Image 2). Not only I wanted to share those images, but also to provide some information about auroras and how to plan a trip to watch them. A previous article about solar wind could be an introduction about such phenomena.
Image 1. Aurora Borealis in the late night skies above the frozen lake Kleifarvatn in Iceland on March 17, 2013.

What is the aurora?
The aurora is a display of milky greenish light in the night sky. Aurora is also a sign of the electrical connection between our planet and the Sun. As explained in the solar wind article, solar wind carries the Sun's energy towards the Earth. Such energy is a stream of electrically charged particles, that are deflected by the magnetic field. Depending on the solar wind strength, some energy may leak into the magnetosphere, where it is converted into electromagnetic energy. Once this energy is large enough, it is released.
Image 2. Aurora Borealis in the late night skies above the frozen lake Kleifarvatn in Iceland on March 17, 2013.

Where do auroras occur?
Auroras occur in ring shaped areas around the magnetic poles, a.k.a auroral ovals (Image 3). The northern auroral oval traces a path across central Alaska and Canada, Greenland, northern Scandinavia and Russia. The southern auroral oval traces a path mostly over the oceans circling Antarctica, but it can occasionally reach the far edges of New Zealand, Chile, and Australia. Although auroras typically occur near the poles, depending on the solar activity they may also occur at lower latitudes. During periods of high sunspot, aurora may occur in regions as far south as Texas or Florida. Moreover, the 1909 geomagnetic storm (the most potent geomagnetic storm on record) brought the aurora to the tropical Singapore.
Image 3. Auroral ovals.
Source: NASA

Some tips if you want to plan a trip for watching the aurora
  • The best months are Semptember and March, as the Earth's orbit reaches its maximum of solar activity at these times of year.
  • Auroras are expected when solar activity reaches the peak of its 11 year solar cycle. This year (2013) was peak year. Anyway, you are advised to check the solar activity predictions.

References & Additional Reading
Aurora... fabled glowing lights of the Sun - Earth connection
Ghostly Green Aurora
New Solar Cycle Prediction
Solar Wind

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