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 sigma(m) as a function of the angle theta 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(perpendicular to)rho(i) similar to 1 often covers a wide band of propagation angles centered about the perpendicular direction, theta similar or equal to 90 degrees +/- 30 degrees. This signal is consistent with a spectrum of obliquely propagating kinetic Alfven waves with k(perpendicular to) >> k(parallel to) 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(parallel to)c/omega p(i) similar to 1 that often exhibit high magnetic helicity vertical bar sigma(m)vertical bar similar or equal to 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 T-p perpendicular to/T-p parallel to > 1 preferentially generates outward propagating ion-cyclotron waves and the fire-hose instability that operates when T-p perpendicular to/T-p parallel to < 1 preferentially generates inward propagating whistler waves. These kinetic processes provide a natural explanation for the magnetic field observations.