For decades, sudden brightenings of auroras—also called the northern and southern lights—have puzzled scientists. Such events have been linked to so-called geomagnetic substorms, disturbances in the outer atmosphere that create "power surges" in the polar lights. But what triggers these substorms has remained a mystery. The research not only could help scientists predict dramatic sky shows, but also should prove a boon for the many technologies—from spacecraft to power grids—that are disrupted by substorms. Now new data show that powerful explosions in the "tail" of magnetic field lines streaming away from Earth release energy that dramatically brightens auroras… more »