Solar WindstormAn aurora borealis, or northern lights, twists across the heavens above Ersfjord (map), Norway, in the first hour of September 15. Three days earlier NASA's Solar Dynamics Observatory, a sun-monitoring satellite, witnessed a magnetic eruption on the sun, which unleashed a gigantic cloud of charged particles into space. The southern part of the particle cloud, or solar wind, grazed Earth's magnetic field on the 14th and 15th, resulting in a particularly good night for aurorae.During the Northern Hemisphere's autumn and spring, solar magnetic fields are oriented in just the right way to cause "rips" in Earth's magnetic field. The resulting increase in solar wind on Earth encourages auroral sky shows but can also damage satellite technology and electrical grids on the ground."Changes in the magnetic field for both the sun and Earth really determine what [particles] get launched into space and hit the Earth," said John Manuel, a research scientist with the Canadian Space Agency."The orientation and variability in the magnetic fields are really what end up making it more or less favorable for auroras here on Earth."(Also see pictures of unusual aurorae triggered by a solar storm in August.)—Andrew Fazekas

Photograph by Thilo Bubek