In 2012, a research group proved that the source of sounds associated with the Northern Lights is located close to the ground at an altitude of approximately 70 meters. Now, by combining his measurements with the temperature profiles measured by the Finnish Meteorological Institute, a researcher has found an explanation for the mechanism that creates the sound.  According to the new inversion layer hypothesis, the popping and crackling sounds associated with the Northern Lights are born when the related geomagnetic storm activates the charges that have accumulated in the atmosphere's inversion layer causing them to discharge. 

Source: Acoustics researcher finds explanation for auroral sounds