Magnetic clouds:: A statistical study of magnetic helicity

We select a set of 20 magnetic clouds (MCs) observed by the spacecraft Wind and reconstruct their local magnetic structure from in situ observations under different models. In particular, we quantify their relative magnetic helicity per unit length (H-r/L) under the assumption of a cylindrical geometry. We investigate how model-dependent are the results using four models (two force-free and two non-force-free) with a significantly different twist distribution in their magnetic field: (a) a linear force-free field, (b) a uniformly twisted field, (c) a non-force-free field with constant current (J) and (d) a non-force-free field with an azimuthal component of J depending linearly on the radius and with a constant axial component of J. We find that the dispersion of the mean H-r/L for the 20 MCs is one order of magnitude larger than the dispersion of the H-r/L value using different models for a given event. In this sense, magnetic helicity per unit length is a well-determined magnitude considering these four models. (c) 2005 Elsevier Ltd. All rights reserved.