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

Abstract

Data taken shortly after the launch of the Pioneer Venus spacecraft (PVO) has allowed us to compare an interplanetary coronal mass ejection (ICME) event observed nearly simultaneously on August 27, 1978, by the PVO and ISEE 3 spacecraft. During this period the spacecraft are in close proximity, separated by 0.02 AU radially and 12° (0.21 AU) in azimuth. Both PVO and ISEE 3 observe similar magnetic flux rope signatures within the ICME. Cylindrically symmetric flux rope fits to the observations indicate the handedness 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 observations returns flux rope diameters that are less than the azimuthal separation distance between the spacecraft. Thus, if the cylindrically symmetric model were correct, the two observations could not occur within the same rope, meaning that at least two separate flux ropes would be associated with the same ICME event. The axial orientations of these two ropes differ slightly, with the PVO rope having a clock angle of 238° and a cone angle of 82° and the ISEE 3 rope having a clock angle of 196° and a cone angle of 56° in solar equatorial coordinates. Applying instead a noncylindrically symmetric flux rope model and simultaneously inverting both time series results in a single stretched rope having a cross‐flow diameter of 0.86 AU, ∼4 times its radial thickness, a clock angle of 220°, and a cone angle of 74°. The stretched rope contains ∼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 symmetric fit at ISEE 3. These values are consistent with the amount of magnetic flux observed in active regions on the Sun. While the individual cylindrically symmetric models adequately fit the single spacecraft data, the stretched rope model provides the simplest explanation of the multipoint observations and is consistent with a CME expanding in azimuthal diameter as it attempts to subtend a constant angle at the Sun of 45° as it convects outward. 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.