Magnetic Helicity Spectrum of Solar Wind Fluctuations as a Function of the Angle With Respect to the Local Mean Magnetic Field

Magnetic field data acquired by the Ulysses spacecraft in high-speed streams over the poles of the Sun are used to investigate the normalized magnetic helicity spectrum σm as a function of the angle θ between the local mean magnetic field and the flow direction of the solar wind. This spectrum provides important information about the constituent modes at the transition to kinetic scales that occurs near the spectral break separating the inertial range from the dissipation range. The energetically dominant signal at scales near the thermal proton gyroradius k⊥ρi ∼ 1 often covers a wide band of propagation angles centered about the perpendicular direction, θ ≃ 90° ± 30°. This signal is consistent with a spectrum of obliquely propagating kinetic Alfvén waves with k⊥ ≫ k∥ in which there is more energy in waves propagating away from the Sun and along the direction of the local mean magnetic field than toward the Sun. Moreover, this signal is principally responsible for the reduced magnetic helicity spectrum measured using Fourier transform techniques. The observations also reveal a subdominant population of nearly parallel propagating electromagnetic waves near the proton inertial scale k∥c/ωpi ∼ 1 that often exhibit high magnetic helicity |σm| ≃ 1. These waves are believed to be caused by proton pressure anisotropy instabilities that regulate distribution functions in the collisionless solar wind. Because of the existence of a drift of alpha particles with respect to the protons, the proton temperature anisotropy instability that operates when Tp⊥/Tp∥ > 1 preferentially generates outward propagating ion-cyclotron waves and the fire-hose instability that operates when Tp⊥/Tp∥ < 1 preferentially generates inward propagating whistler waves. These kinetic processes provide a natural explanation for the magnetic field observations.