Anisotropy and Alfvénicity of hourly fluctuations in the fast polar solar wind

Abstract

Ulysses magnetic and plasma data obtained in the fast polar solar wind are used to study the nature of fluctuations in the frequency range between the turbulent inertial range and the lower-frequency range dominated by quasi-static structures originating at the Sun. For daily variations of hourly averages of the magnetic field components the anisotropy of the fluctuations is less than in the higher-frequency range, and the minimum variance direction loses its alignment with the magnetic field in favor of a more radial direction as the solar distance increases. The anisotropy of the magnetic fluctuations also increases with solar distance, while the Alfvénicity, as measured by the correlation of the field and velocity components, decreases. Most of the observed frequency and distance trends are similar to those observed in fast solar wind streams in the ecliptic, despite the difference in stream structure between high and low latitudes. One major difference from higher-frequency (inertial range) variations at high latitudes is the continuing decline of the correlation between velocity and field vectors between 2 and 4.5 AU. It is suggested that the relative contribution of discontinuities and planar magnetic structures grows with solar distance relative to the contribution of Alfvén waves.