Learning about coronal heating from solar wind observations

Vlasov theory describing the interaction of Alfvén-cyclotron fluctuations and ions in the collisionless solar wind predicts that alpha particles should be strongly scattered perpendicular to the background magnetic field when the alpha/proton relative velocity vαp is negative or has a sufficiently small positive value relative to the Alfvén speed vA. This theory also predicts that, if vαp∕vA is positive and sufficiently large, it is the protons which are scattered in the perpendicular direction, although less strongly. Here proton and alpha particle anisotropies measured in the solar wind near 1AU (AU, astronomical unit) by the plasma and magnetic field instruments on the Advanced Composition Explorer spacecraft are reported which are consistent with these predicted signatures. This indicates that Alfvén-cyclotron heating of ions is active in the solar wind; by using this medium to study this fundamental process, a greater understanding may be obtained of how Alfvén-cyclotron scattering contributes to heating of ions in the solar corona.