Constraints on the Global Structure of Magnetic Clouds: Transverse Size and Curvature
We present direct evidence that magnetic clouds (MCs) have highly flattened and curved cross section resulting from their interaction with the ambient solar wind. Lower limits on the transverse size are obtained for three MCs observed by ACE and Ulysses from the latitudinal separation between the two spacecraft, ranging from 40$^{\circ}$ to 70$^{\circ}$. The cross-section aspect ratio of the MCs is estimated to be no smaller than $6:1$. We offer a simple model to extract the radius of curvature of the cross section, based on the elevation angle of the MC normal distributed over latitude. Application of the model to Wind observations from 1995 - 1997 (close to solar minimum) shows that the cross section is bent concavely outward by a structured solar wind with a radius of curvature of $\sim$ 0.3 AU. Near solar maximum, MCs tend to be convex outward in the solar wind with a uniform speed; the radius of curvature is proportional to the heliographic distance of MCs, as demonstrated by Ulysses observations between 1999 and 2003. These results improve our knowledge of the global morphology of MCs in the pre-Stereo era, which is crucial for space weather prediction and heliosphere studies.