GEOMETRY OF INTERPLANETARY MAGNETIC CLOUDS

Two dimensional magnetohydrodynamic simulations are presented of the d istortion of a magnetic flux rope that is being accelerated through am bient solar wind plasma. The flux rope magnetic field has an axial com ponent parallel to the solar wind field and an azimuthal component, wh ich lies in the simulation plane. As the flux rope moves through the s olar wind plasma, vortices form on its trailing edge and couple strong ly to its interior. If the flux rope azimuthal field is weak, it defor ms into an elongated banana-like shape after a few Alfven transit time s. A strong azimuthal field component tends to inhibit this distortion . If the flux rope is taken to model a magnetic cloud, it is suggested that the shape of the cloud at 1 AU is determined by its distortion i n the inner solar wind. Distortion timescales beyond 1 AU are estimate d as many days. It is estimated that effective drag coefficients somew hat greater than unity are appropriate for modelling flux rope propaga tion.