Magnetic clouds and the pinch effect

Magnetic clouds are characterized by a typical diameter of 0.25 AU, high magnetic field strength, and a magnetic field change predominantly by rotation of one component nearly parallel to a plane; i.e., the field in the cloud is nearly toroidal. They are a narrowly defined subclass of all interplanetary signatures of coronal mass ejections, whose geometry is thought to be quasi‐cylindrical or quasi‐spherical because of their magnetic field properties. The magnetic pressure is higher inside than the sum of the ion pressure and magnetic pressure outside clouds, leading to the suggestion that clouds are expanding at 1 AU. However, the suspected geometry of clouds is such that expansion need not occur because the imbalance can be counteracted by magnetic tension: the pinch effect. Pinch models that reproduce the observed properties of clouds are described here. The evidence for expansion of clouds at 1 AU is nevertheless quite strong, so another reason for its existence must be found. It is shown here that the expansion observations can be reproduced by taking into account the effects of geometrical distortion on the low plasma β clouds as they are advected away from the Sun. This hypothesis for the reason there is an increasing separation of the front and back edges of clouds with distance from the Sun has consequences that can be verified by coordinated spacecraft observations.