arXiv subjects

Dae-Young Lee

Publications and source records attributed to Dae-Young Lee.

5 recordsLinked to original sources

arXiv2026

Ion-Scale Current Sheets Embedded in Reconnection Jet Shear Layer of the Near-Sun Heliospheric Current Sheet

Context. Magnetic reconnection in the heliospheric current sheet (HCS) plays an important role in restructuring the solar wind magnetic topology and generating plasma jets and magnetic islands. While large-scale signatures of HCS reconnection have been reported in many observational studies, the kinetic-scale structure embedded within reconnection regions remains less well understood. Aims. We investigate the ion-scale currents sheets (CSs) embedded within an HCS reconnection region and their relationship to the flow-shear layer at the edge of a reconnection jet. Methods. We analyzed an HCS crossing observed by the Parker Solar Probe on March 29, 2024, using high-time-resolution magnetic field measurements. We focused on ion-scale magnetic transitions within two brief intervals of flow-shear layer at the edges of the reconnection jet and examined them in a local LMN coordinate system. Results. Twelve representative CSs are identified, whose duration is on average $\sim$0.06 sec, corresponding to spatial scales of only a few ion inertial lengths. They are classified into three types based on the behavior of the out-of-plane magnetic component $B_{M}$: (1) CSs showing clear bipolar $B_{M}$ variations without bifurcation in reconnecting-field ($B_{L}$), (2) CSs with both bipolar $B_{M}$ variations and bifurcated $B_{L}$ profiles characterized by a plateau structure, and (3) CSs where strong fluctuations obscure an otherwise expected bipolar signature. Conclusions. The reconnection jet shear layer in the HCS may serve as an active site that hosts a chain of ion-scale CSs. This provides new insight into the multiscale structure of HCS reconnection and suggests that flow shear layers may play an important role in generating secondary kinetic-scale structures.

astro-ph.SR↗

arXiv2026

Wave Activity at MHD-ion Scales Associated with Switchbacks

Magnetic switchbacks (SB) -- the localized magnetic structures with magnetic field direction inclined at an angle $\theta$ relative to the background $B_0$ -- in the young solar wind have been associated with enhanced ion-scale wave activity and local plasma heating. It remains debated whether the apparent wave-power increase is intrinsic or mainly caused by sampling geometry. In this work, we analyze magnetic and electric field fluctuations measured by Parker Solar Probe, focusing on the 0.1--3~$f_{cp}$ frequency band that spans the transition from the MHD inertial range to ion-kinetic scales. By decomposing magnetic fluctuations into field-aligned and transverse components and comparing SB and non-SB intervals at the same local magnetic field angle, we test whether SBs sample an anisotropic cascade from different viewing angles or host intrinsically amplified wave activity. We find that the transverse magnetic power $\delta B_{\perp}$ is systematically enhanced inside switchbacks across a wide range of magnetic field rotation angles $\theta$. The enhancement persists even at small and intermediate deflections, where geometric projection alone predicts weak power, indicating an intrinsic origin beyond sampling geometry. The inertial-range spectral indices also remain similar between SB and non-SB intervals despite the enhanced wave power inside SBs, suggesting that the underlying turbulence cascade is largely preserved. This excess $\delta B_{\perp}$ coincides with elevated proton temperatures and enhanced electric-field fluctuations, supporting the interpretation that SBs act as localized sites of cross-scale energy transfer and ion-scale dissipation in the near-Sun solar wind.

physics.space-ph↗

arXiv2026

MORPH Wheel: A Passive Variable-Radius Wheel Embedding Mechanical Behavior Logic for Input-Responsive Transformation

This paper introduces the Mechacnially prOgrammed Radius-adjustable PHysical (MORPH) wheel, a fully passive variable-radius wheel that embeds mechanical behavior logic for torque-responsive transformation. Unlike conventional variable transmission systems relying on actuators, sensors, and active control, the MORPH wheel achieves passive adaptation solely through its geometry and compliant structure. The design integrates a torque-response coupler and spring-loaded connecting struts to mechanically adjust the wheel radius between 80 mm and 45 mm in response to input torque, without any electrical components. The MORPH wheel provides three unique capabilities rarely achieved simultaneously in previous passive designs: (1) bidirectional operation with unlimited rotation through a symmetric coupler; (2) high torque capacity exceeding 10 N with rigid power transmission in drive mode; and (3) precise and repeatable transmission ratio control governed by deterministic kinematics. A comprehensive analytical model was developed to describe the wheel's mechanical behavior logic, establishing threshold conditions for mode switching between direct drive and radius transformation. Experimental validation confirmed that the measured torque-radius and force-displacement characteristics closely follow theoretical predictions across wheel weights of 1.8-2.8kg. Robot-level demonstrations on varying loads (0-25kg), slopes, and unstructured terrains further verified that the MORPH wheel passively adjusts its radius to provide optimal transmission ratio. The MORPH wheel exemplifies a mechanically programmed structure, embedding intelligent, context-dependent behavior directly into its physical design. This approach offers a new paradigm for passive variable transmission and mechanical intelligence in robotic mobility systems operating in unpredictable or control-limited environments.

cs.RO↗

arXiv2025

Magnetic flux ropes within reconnection exhausts close to the centers of heliospheric current sheets near the Sun

Understanding the relationship between magnetic flux ropes and magnetic reconnection is fundamental to both space and astrophysical plasma studies. In this study, we report on two consecutive heliospheric current sheet (HCS) crossings by Parker Solar Probe (PSP), separated by ~10.5 hours, at a heliocentric distance of ~12 solar radii. For each crossing, we identified a series of flux ropes embedded within reconnection exhausts on the sunward side of X-line. Their passage durations are <20sec, corresponding to spatial scales of a few thousands kilometers, still larger by three orders of magnitude than ion inertial length. This identification was possible particularly during intervals when PSP was closest to the HCS center. These flux ropes are distinguishable from the background exhausts by enhancements in magnetic field strength, significantly in the guide field component, travel speed slightly faster (typically by <10km/s) than surrounding outflows, and often accompanied by, though not always, increased density and reduced temperature. We attribute their origin to secondary reconnection within the exhausts and subsequent merging of smaller flux ropes into larger structures, consistent with predictions by various simulations. We suggest that such flux ropes are most readily identifiable at the HCS center where the background magnetic field is weakest so that the relative enhancement in flux rope field becomes most prominent. This observational advantage is particularly notable closer to the Sun where the high ambient magnetic field strength can otherwise obscure such structures unless the spacecraft trajectory remains within the HCS central region for a sufficient duration.

astro-ph.SR↗

arXiv2025

Switchbacks near Boundaries of Small-scale Magnetic Flux Ropes in the Young Solar Wind from Parker Solar Probe Observations

The Parker Solar Probe (PSP) mission has revealed frequent occurrences of switchbacks (SBs) and small-scale magnetic flux ropes (SMFRs) as prominent structures within the solar wind. These mesoscale features are observed across all heliocentric distances, with heightened activity in the young solar wind, such as successive SMFRs, blobs, and SBs using PSP in situ observations. One study, in particular, focuses on SMFRs observed during the intervals of PSP co-rotating with the Sun, which suggests a similar source of the observed solar wind. In this letter, we identified SBs at the boundaries of SMFRs as a regularly observed phenomenon and found instances where SBs and SMFRs co-occur, with the significance level $α<0.05$. The SMFR-related SBs - observed at the leading and trailing edges of an SMFR - exhibit well-organized axial co-orientations, with their polarity flipping, meaning the radial direction remains constrained while the transversal field reverses. Furthermore, the axial field directions of SMFRs-related SBs appear to be more closely connected than to another SB that is spatially closer and are linked to the SMFR orientation. Our analysis of their relative geometry, which examines the alignment between SBs and the SMFR axis, reveals a distinct tendency emphasizing their correlation, further supporting the idea that the axes of SMFR-related SBs are presumably determined by the SMFR orientation. Observations suggest that a fraction of SBs is spatially and temporally associated with SMFRs, implying that processes related to SMFR boundaries may contribute to SB formation, or that SBs tend to develop in magnetic environments shaped by SMFRs.

astro-ph.SR↗