Question: Why does the Coronal Mass Ejections cause the northern lights when they clash with the Earth's magnetic field?

Emma Davies answered on 18 Jun 2019:

This is a really intelligent question! It’s all due to a process called Magnetic Reconnection. Coronal Mass Ejections are made up of hot energetic particles (plasma) as well as a twisted magnetic field. This means that when the magnetic field of the CME meets the magnetic field of the Earth, if they are oppositely directed, it’s just like the South end of a magnet meeting the North end of another magnet. In this process, a lot of energy is released and the energetic particles burst through the gap that’s briefly formed in Earth’s magnetic field and they get into our atmosphere (but really high up). These particles then bump into the different molecules in the atmosphere again releasing energy. But different molecules release different energies, or wavelengths of light – this is the Northern Lights! If I remember correctly, oxygen creates a red colour and nitrogen creates the more common green colour we see!