Observations of heliospheric planar and offset-planar magnetic structures

Abstract

Planar magnetic structures are regions of solar wind plasma where the magnetic field orientation is constrained within a fixed plane for durations of several hours. Within these structures, magnetic field discontinuities are seen, with the discontinuity surfaces being almost co-planar. The results of a survey of planar magnetic strutures detected by the Ulysses spacecraft are presented. When searching for these events using minimum variance analysis, a large population was detected of magnetic field structures whose directions described small circles in the spacecraft's field of view (as opposed to great circles described by 'classic' planar magnetic structures). It is likely that these 'small-circle' events represents Alfvenic structures. Variations in the nature of the structures at various heliographic latitudes and differing solar wind conditions are described. Implications of the results of the survey are discussed.