Small-scale Magnetic Flux Ropes with Field-aligned Flows via the PSP In Situ Observations

Abstract

Magnetic flux rope, formed by the helical magnetic field lines, can sometimes maintain its shape while carrying significant plasma flow that is aligned with the local magnetic field. We report the existence of such structures and static flux ropes by applying the Grad-Shafranov-based algorithm to the Parker Solar Probe in situ measurements in the first five encounters. These structures are detected at heliocentric distances, ranging from 0.13 to 0.66 au, in a 4-month time period. We find that flux ropes with field-aligned flows, although they occur more frequently, have certain properties similar to those of static flux ropes, such as the decaying relations of the magnetic fields within structures with respect to heliocentric distances. Moreover, these events are more likely with magnetic pressure dominating over the thermal pressure. About one-third of events are detected in the relatively fast solar wind. Taking into account the high Alfvénicity, we also compare with switchback spikes identified during three encounters and interpret their interrelations. We find that some switchbacks can be detected when the spacecraft traverses flux-rope-like structures. The cross-section maps for selected events are presented via the new Grad-Shafranov-type reconstruction. Finally, the possible evolution of the magnetic flux rope structures in the inner heliosphere is discussed.

Effective Polytropic Index of Small‐Scale and Force‐Free Magnetic Flux Ropes in the Solar Wind

“…Finally, it is noteworthy that a large number of nonforce‐free flux ropes in the solar wind have recently been identified (Chen et al., 2019, 2021). Particle‐in‐cell simulations by Lu et al.…”

“…Finally, it is noteworthy that a large number of nonforce-free flux ropes in the solar wind have recently been identified (Chen et al, 2019(Chen et al, , 2021. Particle-in-cell simulations by Lu et al (2021) indicated that force-free and nonforce-free flux ropes have different particle energization processes.…”

“…In the past studies, the magnetic flux ropes were satisfied the constant‐alpha, force‐free flux rope model. Recently, a large data set of small‐scale flux ropes is identified in the solar wind by the automated detection based on the Grad‐Shafranov reconstruction algorithm (Chen et al., 2019, 2021), which is considered as a nonforce‐free flux rope model. The origin of small‐scale flux ropes can be from the Sun (e.g., Feng et al., 2015; Rouillard et al., 2011) or from the heliospheric current sheet (e.g., Feng et al., 2015; Moldwin et al., 2000).…”

Effects of Pressure Anisotropy on the Geometry of Magnetic Flux Rope

“…Magnetic flux ropes/islands are of a helical magnetic field structure for which the twist of magnetic field lines increases with increasing distance from the flux rope center. These twisted field structures are a by-product of magnetic reconnection, ranging in durations from a few minutes up to a few days in the solar wind (e.g., Burlaga 1988;Lepping et al 1990;Cartwright & Moldwin 2008;Gosling et al 2010;Feng et al 2015;Chen & Hu 2020, 2022Chen et al 2021), while in the Earth's magnetosphere from a few seconds to a few minutes (e.g., Slavin et al 2003;Teh et al 2010Teh et al , 2014aTeh et al , 2014bTeh et al , 2017Eastwood et al 2016;Wang et al 2016). A typical magnetic flux rope has a significant axial/core field that is increased largely at the center of the flux rope where the magnetic field strength is therefore significantly enhanced.…”

“…It is noteworthy that the extended GS equations by Sonnerup et al (2006) are different from those by Teh (2018Teh ( , 2019 under the same assumptions because the entropy equation and the double-polytropic energy laws (Hau & Sonnerup 1993) are used in the derivation of Sonnerup et al (2006). Recently, Chen et al (2021) and Chen & Hu (2022) have implemented an automated detection based on the extended GS equation by Teh (2018) to identify a large data set of small-scale magnetic flux ropes observed by the Parker Solar Probe in the solar wind. Moreover, the extended GS reconstruction with pressure anisotropy (Teh 2019) was applied for examining the geometry of mirror structures observed in the Earth's magnetosheath (Teh 2019) and the time-dependent mirror structure from double-polytropic MHD simulation (Teh & Zenitani 2021).…”