Authors: Wenyuan Yu (University of New Hampshire), Nada A. Al-Haddad (University of New Hampshire), Charles J. Farrugia (University of New Hampshire), Noé Lugaz (University of New Hampshire), Florian Regnault (University of New Hampshire)

Magnetic ejecta (e.g., magnetic clouds, MCs) are most often fitted with flux rope models which are static and typically have a symmetric magnetic field profile (e.g., Lundquist solution). However, spacecraft measurements near 1 AU show that MCs usually expand when propagating away from the Sun and that their total magnetic field profile is asymmetric. These effects are expected due to expansion. In this study, we investigate the effects of expansion on the asymmetry of total magnetic field strength inside magnetic ejecta by comparing the fitting results of an expanding Lundquist model with the classical force-free circular cross-section static Lundquist model. We select 25  “simple” events (which are crossed close to the nose of the MC, that is, the flux rope axis is approximately along GSE Y or RTN T direction) observed from Wind and STEREO spacecraft. We fit them by using the original static Lundquist and an expanding Lundquist model. We quantify the goodness of the fits by the χ² of the total magnetic field strength and identify three types of MCs: (i) those with little expansion, which are well fitted by both models, (ii) those with moderate expansion, well fitted by the expanding model but not by a static model and (iii) those of which expansion, as measured by the velocity profile at 1~AU, cannot explain the asymmetry of the magnetic field profile. We discuss our results in terms of the relationship between expansion and asymmetry of total magnetic field strength.