Near-orthogonal Orientation of Small-scale Magnetic Flux Ropes Relative to the Background Interplanetary Magnetic Field

Abstract

Small-scale magnetic flux ropes (SMFRs) have been identified at a large range of heliospheric distances from the Sun. Their features are somewhat similar to those of larger-scale flux rope structures such as magnetic clouds (MCs), while their occurrence rate is far higher. In this work, we examined the orientations of a large number of SMFRs that were identified at 1 au by fitting to the force-free model. We find that, while most of the SMFRs lie mostly close to the ecliptic plane, as previously known, their azimuthal orientations relative to the Sun–Earth line are found largely at two specific angles (slightly less than 45° and 225°). This latter feature in turn leads to a strong statistical trend in which the axis of SMFRs lies at a large tilt angle relative to (most often nearly orthogonal to) the corresponding background interplanetary magnetic field directions in the ecliptic plane. This feature is different from previous reports on SMFRs—and in stark contrast to the cases of MCs. This is an important observational constraint that should be considered for understanding SMFR generation and propagation.