Characteristics of Suprathermal Electrons in Small-Scale Magnetic Flux Ropes and Their Implications on the Magnetic Connection to the Sun

Abstract

Small-scale magnetic flux ropes (SMFRs) are observed more frequently than larger-scale magnetic flux ropes (e.g., magnetic clouds) in interplanetary space. We selected 235 SMFRs by applying cylindrical linear force-free fitting to 20-year observations of the Wind satellite, which meets the criteria of low beta, low temperature, an enhanced magnetic field, and a rotation feature. By examining the pitch angle distribution of suprathermal electrons for these events, we found that approximately 45.1% of the SMFRs were accompanied by unidirectional beams (strahl). A much smaller percentage of SMFRs (∼10.7%) were associated with bidirectional beams. We also found a small percentage (∼7.2%) of (sunward) conic distributions during SMFR events. Last, the remaining ∼37.0% of SMFRs were associated with complex electron distributions. The unidirectional beams and most of the conics (together corresponding to ∼50% of the total 235 SMFRs) imply open-field SMFRs with only one end connected to the Sun. For ∼37.7% of the unidirectional beam SMFRs, the local IMF field polarity was orthogonal or inverted (possibly due to interchange reconnection). Based on the solar wind conditions around the bidirectional beams, we suggest that more than half of the bidirectional beams were not necessarily closed-field-line SMFRs.

The S-Web Origin of Composition Enhancement in the Slow-to-moderate Speed Solar Wind

“…One may expect different types of reconnection-generated magnetic structures at the boundaries of HS and PS regions due to the topological differences, for example as discussed by Edmondson & Lynch (2017) and Higginson & Lynch (2018), but further numerical modeling of their origin and heliospheric evolution will be needed. This work complements previous statistical studies characterizing magnetic field and plasma properties within coherent intervals or by solar wind type (e.g., Ko et al 2018;Borovsky et al 2019;D'Amicis et al 2019;Borovsky et al 2021), as well as those studies of specific, small-scale structures (Khabarova et al 2021;Gershkovich et al 2022) such as small magnetic flux ropes (e.g., Feng et al 2008;Yu et al 2016;Murphy et al 2020;Choi et al 2021). Importantly, our attempt to relate various in situ properties of the structured variability in slow-to-moderate speed solar wind through an application of the H m -PVI methodology represents a significant extension of previous work where coherent magnetic structures identified "by eye" were shown to be coincident with structure in the proton density and A He observations (e.g., Kepko et al 2016;Di Matteo et al 2019).…”

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

“…In this case, we can imagine that the flux rope axis makes a significant azimuthal angle to the background IMF direction, consistent with the main finding in the present work. On the other hand, Choi et al (2021) have shown that the majority of low-β SMFRs at 1 au (practically the same event group as examined in the present paper) are connected to the Sun at one end (thus open field lines), and there is a very small fraction of SMFRs with closed field lines. Therefore, it needs to be verified whether flux ropes generated by interplanetary HCS reconnection meet this observational constraint for magnetic connectivity to the Sun.…”

“…On the other hand, it has been suggested (Yurchyshyn 2008;Kay et al 2015;Szabo et al 2020) that significant evolution of solar ejecta (for example, a tendency to align with the HCS) occurs during propagation through interplanetary space, possibly changing their axial orientations (say, at 1 au) from those at generation points near the Sun. However, although many SMFRs are found near the HCS, some are not (Choi et al 2021). Therefore, if most SMFRs were indeed launched near the Sun, a possible propagation effect implies that some other mechanism in interplanetary space, whether close to the HCS or not, must play a significant role in the robust statistical trend of the axial orientations found at 1 au in the present paper.…”

“…The small-scale magnetic flux ropes (SMFRs) in the solar wind are known to have properties similar to those of the wellknown magnetic clouds (MCs), but they have been observed more frequently than MCs (e.g., Yu et al 2016;Hu et al 2018;Choi et al 2021) and in a large range of heliospheric distances from <0.2 au to several au from the Sun (Cartwright & Moldwin 2010;. They have been identified and modeled by a few different methods, including fitting with the force-free model (e.g., Moldwin et al 2000;Feng et al 2008), Grad-Shafranov reconstruction (e.g., Zheng et al 2017), and magnetic helicity (Zhao et al 2020a(Zhao et al , 2020b.…”

“…Our recent work based on suprathermal electron data (Choi et al 2021) has shown that, for a majority of low-β SMFRs, the flux rope field lines are open with only one end connected to the Sun, and only a limited number of SMFRs have real closed field lines. In the present work, we proceed to further understand the geometrical structure of SMFRs.…”