Heating the Sun’s corona

It’s one of the great natural mysteries: How do the Sun’s corona and wind become thousands of times hotter than the Sun’s surface? Somehow, energy makes its way up into the corona against a temperature gradient and is converted to heat. A new analysis of data collected by NASA’s Wind spacecraft doesn’t solve the mystery, but it is consistent with a popular explanation. The analysis was done by Justin Kasper of the Harvard-Smithsonian Center for Astrophysics, Alan Lazarus of MIT, and Peter Gary of Los Alamos National Laboratory. They looked at 14 years of in situ observations of particles and fields made as Wind flew in and out of the solar wind. The team focused on the two most abundant ion species in the solar wind, H⁺ and He²⁺. Because He²⁺ is four times heavier than H⁺ and carries twice the charge, the two species’ kinematics can discriminate among various models for transport and heating. Kasper, Lazarus, and Gary found strong evidence for one picture of coronal heating: Ions are carried upward by magnetohydrodynamic disturbances known as Alfvén waves; heating occurs when ions entrained in the waves fall into resonance at their respective cyclotron frequencies. Confirming or refuting the resonance model will require a spacecraft to enter and explore the corona. That’s the aim of two missions planned for the next decade: NASA’s Solar Probe and ESA’s Solar Orbiter. (J. C. Kasper, A. J. Lazarus, S. P. Gary, Phys. Rev. Lett., in press.) — Charles Day