Learning about coronal heating from solar wind observations

Vlasov theory describing the interaction of Alfven-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 v(alpha p) is negative or has a sufficiently small positive value relative to the Alfven speed v(A). This theory also predicts that, if v(alpha p)/v(A) 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 1 AU (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 Alfven-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 Alfven-cyclotron scattering contributes to heating of ions in the solar corona. (c) 2005 American Institute of Physics.