Properties of hydromagnetic waves in the polar caps: Ulysses

Abstract

Properties of the large amplitude Alfvén waves which are a dominant feature of the high latitude solar wind have been studied during the pole-to-pole passage of Ulysses. In addition, an effort has begun to identify magnetosonic waves at high latitude in spite of their expected small amplitude and the presence of the large amplitude Alfvén waves. The power in the latter is essentially the same in both solar hemispheres without evidence of any significant north-south asymmetry. The increased power as Ulysses approached the poles is not, in fact, dependent on latitude but is caused by a power law dependence on distance. The Alfvénicity of the fluctuations, as indicated by the covariance of the field and velocity, increases to high levels (0.8) at high latitudes. Principal axis analyses indicate that the field vector appears to wander randomly over a hemisphere whose radius is the nearly constant field magnitude. This behavior is consistent with a stochastic model introduced by Barnes as demonstrated by a statistical analysis of the field variations observed by Ulysses. The principal axis analysis also shows that the direction associated with the minimum eigenvalue is nearly radial. The strategy for identifying magnetosonic waves involves searching for them in the weak interaction regions accompanying microstreams. Simultaneous variations in the magnetic, kinetic, and total pressures are qualitatively consistent with the presence of magnetosonic waves.