Banana-polarized solitons in anisotropic plasmas related to Ulysses observations

Abstract

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 _$\parallel$ ) and transverse (T _$\perp$ ) 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 _$\parallel$ /T _$\perp$ >1 and β _$\perp$ ~ 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.