’Surfing’ particles: Physicists solve a mystery surrounding aurora borealis

The spectacularly colorful aurora borealis — or northern lights — that fill s the sky in high-latitude regions ha s fascinated people for thousands of years. Now, a team of scientists has resolved one of the final mysteries surrounding its origin. Scientists know that electrons and other energized particles that emanate from the sun as part of the "solar wind" speed down Earth's magnetic field lines and into the upper atmosphere, where they collide with oxygen and nitrogen molecules, kicking them into an excited state. These molecules then relax by emitting light, producing the beautiful green and red hues of the aurora. What has not been well understood is precisely how groups of electrons accelerate through the magnetic field on the last leg of their journey , reaching speeds of up to 45 million mph. In a study published today Communications, that question is answered by physicists from UCLA, Wheaton College, the University of Iowa and the Space Science Institute. A popular theory has been that electrons hitch a ride on Alfvén waves — a type of electromagnetic wave that spacecraft have frequently identified traveling Earthward along magnetic field lines above auroras.