A central problem in solar physics is why the sun's atmosphere gets hotter as it rises, from about 8,500 degrees Fahrenheit (5,000 kelvins) at the photosphere (solar surface) to more than 1.8 million degrees Fahrenheit (1 million Kelvins) in the corona, which extends for millions of miles. Without an outside force heating the corona, its temperature would plummet, says solar physicist Scott McIntosh of the National Center for Atmospheric Research (NCAR) in Boulder, Colo.