A phenomenon first detected in the solar wind may help solve a long-standing mystery about the sun: why the solar atmosphere is millions of degrees hotter than the surface.

Images from the Earth-orbiting Interface Region Imaging Spectrograph, aka IRIS, and the Atmospheric Imaging Assembly, aka AIA, show evidence that low-lying magnetic loops are heated to millions of degrees Kelvin.

Researchers at Rice University, the University of Colorado Boulder and NASA ’s Marshall Space Flight Center make the case that heavier ions, such as silicon, are preferentially heated in both the solar wind and in the transition region between the sun’s chromosphere and corona.