Model of a rotating magnetic cloud.

The authors investigate the possibility that magnetic clouds rotate while they propagate antisunwards. They model magnetic clouds as magnetic flux ropes which rotate rigidly about the axis of symmetry. The authors develop an ideal MHD model in which they relate the evolution of the magnetic structure to the time evolution of the angular frequency. The corresponding effective potential gives rise to two modes of evolution, expansion and oscillation, depending on the energy and on the value of a dimensionless parameter, K. Parameter K depends on the gas pressure, the ratio of the magnetic field components and the frequency of rotation. There is a critical value of K, K_C, above which the oscillatory regime disappears and the flux rope invariably expands, regardless of the energy. Below K_C the energy determines whether the configuration is confined or unbounded. The authors study a data example and present features which are interpret as signatures of rotation. The angular speed is comparable to the Alfven speed, and the core of the rotating cloud completes on average one full revolution every three days at 1 AU.