Multispacecraft modeling of the flux rope structure of interplanetary coronal mass ejections: Cylindrically symmetric versus nonsymmetric topologies

Data taken shortly after the launch of the Pioneer Venus spacecraft (PVO) h as allowed us to compare an interplanetary coronal mass ejection (ICME) eve nt observed nearly simultaneously on August 27, 1978, by the PVO and ISEE 3 spacecraft. During this period the spacecraft are in close proximity, sepa rated by 0.02 AU radially and 12 degrees (0.21 AU) in azimuth. Both PVO and ISEE 3 observe similar magnetic flux rope signatures within the ICME. Cyli ndrically symmetric flux rope fits to the observations indicate the handedn ess of the magnetic flux rope seen at PVO is the same as at ISEE 3, but the peak fields are not. Moreover, symmetric modeling of the magnetic rope obs ervations returns flux rope diameters that are less than the azimuthal sepa ration distance between the spacecraft. Thus, if the cylindrically symmetri c model were correct, the two observations could not occur within the same rope, meaning that at least two separate flux ropes would be associated wit h the same ICME event. The axial orientations of these two ropes differ sli ghtly, with the PVO rope having a clock angle of 238 degrees and a cone ang le of 82 degrees and the ISEE 3 rope having a clock angle of 196 degrees an d a cone angle of 56 degrees in solar equatorial coordinates. Applying inst ead a noncylindrically symmetric flux rope model and simultaneously inverti ng both time series results in a single stretched rope having a cross-flow diameter of 0.86 AU, similar to4 times its radial thickness, a clock angle of 220 degrees, and a cone angle of 74 degrees. The stretched rope contains similar to 50 TWb Of magnetic flux, nearly twice the amount given for the cylindrically symmetric fit at PVO and a factor of 5 greater than the symme tric fit at ISEE 3. These values are consistent with the amount of magnetic flux observed in active regions on the Sun. While the individual cylindric ally symmetric models adequately fit the single spacecraft data, the stretc hed rope model provides the Simplest explanation of the multipoint observat ions and is consistent with a CME expanding in azimuthal diameter as it att empts to subtend a constant angle at the Sun of 45 degrees as it convects o utward. Although the inversion technique used does not require the field to be force-free, there are at most only slight deviations from a force-free configuration within the symmetric ropes and significant plasma forces only at the east and west ends of the nonsymmetric flux rope.