NASA"s fleet of five THEMIS microspacecraft, by providing simultaneous observations of the Earth's magnetic field from multiple points, have explained what starts the Northern and Southern lights dancing. The triggers are magnetic "substorms" about a third of the way from the Earth to the Moon. In this region, the magnetic field lines are stretched until they "snap back" into a new shape in a process called magnetic reconnection. This process releases the energy from the solar wind captured and stored by the magnetic field lines and directs it back toward the planet, creating the curtains of energetic particles which become visible in the polar regions.
COMMENT: THEMIS is a great demonstration of one of the space science benefits of microspacecraft: the ability to deploy an affordable constellation of instruments for multi-point observations.