Mystery Source of Solar Wind Heating Identified The solar wind, shown here in a plot of data from the Ulysses spacecraft, flows away from the sun at a million miles per hour and is heated by a "turbulent cascade." Credit: NASALarger ImageThe solar wind is hotter than it should be, and for decades researchers have puzzled over the unknown source of energy that heats it. In a paper published in the June 12 issue of Physical Review Letters, NASA scientists say they may have found the answer."The energy source is turbulence," says co-author Melvyn Goldstein, chief of the Geospace Physics Laboratory at NASA's Goddard Space Flight Center, Greenbelt, Md. "The sun heats the solar wind by stirring it up."