Flux Ropes and Spheromaks: A Numerical Study

Abstract

Propagation of magnetic clouds as flux ropes or spheromaks is studied with use of a self-consistent MHD numerical approach. Flux ropes are approximated by cylinders in a 2.5-D treatment, while spheromaks (plasmoids) described initially as spherical clouds are treated in 3-D. We deal with the reliability of 2.5-D solutions, comparing results of different codes, different mesh resolutions and different conditions for the cloud's injection. Despite this variety, we find that the results of the cloud's propagation are very similar and consistent. Indeed, we find that the results are independent of code mesh size and the numerical technique used. Cylindrical clouds are deflected from straight, radial propagation to the side where the inner and outer magnetic fields have the opposite sense. The magnitude of the deflection depends on the magnetic field strength and the gradient of the ambient magnetic field near the cloud's boundary. Spherical clouds with a poloidal field and axes aligned along the background magnetic field are not deflected. But they evolve into toroids, closed flux ropes. These results were obtained using a unipolar background magnetic field, that is, we do not consider interactions with the heliospheric current sheet.