Brown dwarf, dazzling aurora

"The sky above a failed star in a distant constellation shimmers with a
beautiful green and yellow aurora one million times brighter than the
northern lights. The spectacular light show is the first confirmed
aurora on a body outside the solar system, and the most powerful ever
recorded."

http://www.theguardian.com/science/2...orthern-lights

But this otherwise fine article doesn't describe how the auroras are formed!