Magnetic clouds of oblate shapes

There is a growing evidence that cross-sections of magnetic clouds (interplanetary magnetic flux ropes) have oblate shapes. This follows from theoretical results as well as from interpretation of in situ measurements. MHD simulations show that a leading part of a cloud, an apex, has an oblate cross-section, and this may be very extreme in some cases. Interpretations of magnetic cloud observations by non-force-free models, multispacecraft observations, and analyses of the cloud bow shock stand-off distance also indicate that a cross-section of some magnetic clouds is oblate. Recently we have found a force-free solution with constant alpha in an elliptic cylinder. It is a direct generalization of the widely used Lundquist constant-alpha force-free solution inside a circular cylinder. Comparisons of this solution and the Lundquist solution with observations are shown. (C) 2004 Elsevier Ltd. All rights reserved.