Investigating the Asymmetry of Magnetic Field Profiles of “Simple” Magnetic Ejecta through an Expansion-modified Flux Rope Model

Abstract

Magnetic clouds (MCs) are most often fitted with flux rope models that are static and have symmetric magnetic field profiles. However, spacecraft measurements near 1 au show that MCs usually expand when propagating away from the Sun and that their magnetic field profiles are asymmetric. Both effects are expected to be related, since expansion has been shown to result in a shift of the peak of the magnetic field toward the front of the MC. In this study, we investigate the effects of expansion on the asymmetry of the total magnetic field strength profile of MCs. We restrict our study to the simplest events, i.e., those that are crossed close to the nose of the MC. From a list of 25 such “simple” events, we compare the fitting results of a specific expanding Lundquist model with those of a classical force-free circular cross-sectional static Lundquist model. We quantify the goodness of the fits by the χ 2 of the total magnetic field and identify three types of MCs: (i) those with little expansion, which are well fitted by both models; (ii) those with moderate expansion, which are well fitted by the expanding model, but not by the static model; and (iii) those with expansion, whose asymmetry of the magnetic field cannot be explained. We find that the assumption of self-similar expansion cannot explain the measured asymmetry in the magnetic field profiles of some of these magnetic ejecta (MEs). We discuss our results in terms of our understanding of the magnetic fields of the MEs and their evolution from the Sun to Earth.