Signatures of wave-ion interactions in the solar wind: Ulysses observations

Abstract

Theory and simulations predict that resonant wave-particle scattering by enhanced magnetic fluctuations yields characteristic signatures in properties of solar wind ions. Here Ulysses/Solar Wind Observations Over the Poles of the Sun (SWOOPS) instrument measurements of protons and alpha particles are examined to seek three types of these signatures: (1) a statistical constraint on the proton temperature anisotropy imposed by the electromagnetic proton cyclotron instability; (2) a correlation between the alpha anisotropy and the alpha/proton relative flow speed driven by electromagnetic alpha/proton instabilities; and (3) relationships among the proton anisotropy, the alpha anisotropy, and the alpha/proton speed induced by the cascade of long-wavelength Alfvén fluctuations to ion cyclotron resonances at shorter wavelengths. No signatures of alpha/proton instabilities are found. However, the first example of the proton anisotropy constraint in the slow solar wind is demonstrated, and a theoretical prediction for Alfvén/cyclotron wave acceleration in the fast wind is similar to a statistical lower bound observed on the alpha/proton speed.