Banana-polarized solitons in anisotropic plasmas related to Ulysses observations

Localized magnetic compressions associated with a rotation in the magnetic field direction have recently been reported after surveying Ulysses high-resolution data. Although the observed structures have similarities to usual Hall-MHD solitons, the banana-type polarization of the described rare events is a decisive signature which counts against such an interpretation. The present paper considers stationary wave forms in an anisotropic plasma with different temperatures parallel (T parallel to) and transverse (T perpendicular to) to the ambient magnetic field in the phase velocity gap between the slow and the intermediate mode. It is shown that for slow solar wind conditions (A = T parallel to/T perpendicular to > 1 and beta perpendicular to approximate to 1) bright solitons with banana-type polarization (bright bananas'') exist which in many respects resemble the observed Ulysses magnetic events. The necessary condition for the appearance of bright bananas'' is that the slow mode becomes faster than the intermediate mode which is achieved for sufficiently strong temperature anisotropy A.