Context. Magnetic clouds (MCs) are large-scale interplanetary transient structures in the heliosphere that travel from the Sun into the interplanetary medium. The internal magnetic field lines inside the MCs are twisted, forming a flux rope (FR). This magnetic field structuring is determined by its initial solar configuration, by the processes involved during its eruption from the Sun, and by the dynamical evolution during its interaction with the ambient solar wind. Aims. One of the most important properties of the magnetic structure inside MCs is the twist of the field lines forming the FR (the number of turns per unit length). The detailed internal distribution of twist is under debate mainly because the magnetic field (B) in MCs is observed only along the spacecraft trajectory, and thus it is necessary to complete observations with theoretical assumptions. Estimating the twist from the study of a single event is difficult because the field fluctuations significantly increase the noise of the observed B time series and thus the bias of the deduced twist. Methods. The superposed epoch applied to MCs has proven to be a powerful technique, permitting the extraction of their common features, and removing the peculiarity of individual cases. We apply a superposed epoch technique to analyse the magnetic components in the local FR frame of a significant sample of moderately asymmetric MCs observed at 1 au. Results. From the superposed profile of B components in the FR frame, we determine the typical twist distribution in MCs. The twist is nearly uniform in the FR core (central half part), and it increases moderately, up to a factor two, towards the MC boundaries. This profile is close to the Lundquist field model limited to the FR core where the axial field component is above about one-third of its central value.
Magnetic clouds (MCs) are a subset of ejecta, launched from the Sun as coronal mass ejections. The coherent rotation of the magnetic field vector observed in MCs leads to envision MCs as formed by flux ropes (FRs). Among all the methods used to analyze MCs, Lepping's method (Lepping, Jones, and Burlaga, 1990, J. Geophys. Res. 95, 11957) is the broadest used. While this fitting method does not require the axial field component to vanish at the MC boundaries, this idea is largely spread in publications. Then, we revisit Lepping's method to emphasize its hypothesis and the meaning of its output parameters. As originally defined, these parameters imply a fitted FR which could be smaller or larger than the studied MC. We rather provide a re-interpretation of Lepping's results with a fitted model limited to the observed MC interval. We find that, typically the crossed FRs are asymmetric with a larger side both in size and magnetic flux before or after the FR axis. At the boundary of the largest side we find an axial magnetic field component distributed around zero which we justify by the physics of solar eruptions. In contrast, at the boundary of the smaller side P. Démoulin
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