Deuk Yong Lee

Context . Magnetic reconnection in the heliospheric current sheet (HCS) plays an important role in restructuring 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 aim to investigate the ion-scale current 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 layers 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 ∼0.06 s, corresponding to spatial scales of only a few ion inertial lengths. Eight of the twelve CSs exhibit localized bipolar variations in the transverse B _M component, with several also showing bifurcated B _L profiles characterized by plateau-like structure. In the remaining events, strong transverse fluctuations prevented the identification of a clear bipolar B _M 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.

Magnetic switchbacks (SBs)—the localized magnetic structures with magnetic field direction inclined at an angle θ 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 comparable magnetic field orientations, we test whether the enhanced wave power can be explained solely by geometric sampling of an anisotropic turbulent cascade. We find that the transverse magnetic power δB _⊥ is systematically enhanced inside SBs across a wide range of magnetic field rotation angles θ . 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 δB _⊥ 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.

The 20.5 km terrestrial free-space optical communication (FSOC) link between Mt. Gamak and Mt. Daebong operates in a propagation regime where the transmission distance exceeds the Rayleigh range by approximately four times, rendering diffractive beam spreading dominant. Under this far-field condition, the transmitter can be physically approximated as a point source, and a spherical wave model becomes more appropriate than the conventional plane wave assumption. In this study, a turbulence analysis framework based on Rytov theory is developed to accurately quantify scintillation and angle-of-arrival fluctuations along the long-range horizontal path. Specifically, by integrating a Zernike tip-tilt removal filter into the spherical wave model, we numerically separate the large-scale eddy components suppressed by the control bandwidth from the residual log-irradiance variance. Numerical simulations show that the spherical wave model predicts approximately 39% lower angle-of-arrival variance compared with the plane wave assumption, thereby preventing excessive system design margins. Furthermore, the analysis verifies that a 200 Hz tip-tilt compensation effectively suppresses beam wander from 2.56 μm to 0.33 μm within the diffraction limit, stabilizing the received irradiance. This study provides a rigorous and practically applicable design framework for long-range terrestrial FSOC systems and future satellite–ground laser communication terminals.

Context. Magnetic flux ropes are frequently observed in association with magnetic reconnection in space plasmas, yet their formation, evolution, and observational visibility are not fully understood, particularly with respect to the near-Sun heliosphere. Aims. We investigate the properties and origin of magnetic flux ropes embedded within reconnection exhausts during heliospheric current sheet (HCS) crossings close to the Sun. We assess the conditions under which such structures are most readily identifiable in in situ measurements. Methods. We analyzed high-resolution magnetic field and plasma observations from Parker Solar Probe (PSP) during two consecutive HCS crossings, separated by ∼10.5 hours, at a heliocentric distance of ∼12 R _⊙ . Flux ropes were identified, with particular attention paid to intervals when PSP traversed the HCS central regions. Results. For each crossing, we identified a series of flux ropes embedded within reconnection exhausts. Their passage durations are below 20 seconds, corresponding to spatial scales of a few thousands kilometers. This result is still greater by three orders of magnitude than the ion inertial length. This identification was possible particularly during intervals when PSP was closest to the central region of HCS. These flux ropes can be distinguished from the background exhausts by enhancements in magnetic field strength, which are shown to be particularly significant in the guide field component. In addition, they travel at speeds that diverge slightly (typically by < 10 km/s) from the speeds of the surrounding outflows, along with the possibility of increased density and reduced temperatures. Conclusions. We attributed the origin of magnetic flux ropes to a secondary reconnection within the exhausts and subsequent merging of smaller flux ropes into larger structures, consistent with predictions by various simulations. We stress 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.

Loss of relativistic electrons with energies above 0.5 MeV through the magnetopause, commonly referred to as magnetopause shadowing, has been extensively investigated using both simulations and observations. However, direct and systematic observational evidence remains limited. In this study, we extend a previously developed magnetopause model by incorporating inward magnetopause motion, which generates induced electric fields within the boundary layer. These fields are amplified during compression and further enhanced by magnetic field strengthening on the magnetospheric side. Relativistic electrons (0.5–a few MeV) are influenced by the induced fields primarily through gyroradius reduction caused by energy loss and through drift toward the magnetosphere, both of which inhibit escape. Stronger electric fields extend the energy range of confined electrons toward higher energies. Depending on the local field geometry, the induced electric field can either accelerate or decelerate electrons. In contrast, ultra‐relativistic electrons (several MeV), with gyroradii comparable to the magnetopause thickness, are less affected and more likely to escape directly. When a finite magnetic field component normal to the magnetopause is present, electrons across a broad energy range (0.5–10 MeV) can escape efficiently, as this component guides their motion across the boundary and reduces sensitivity to the induced field. In such cases, energy variations still occur but become less significant with increasing normal component strength, owing to shorter residence times within the magnetopause.

Surface treatments play a crucial role in modifying the surface properties and biological performance of dental ceramics. This study investigated the effects of surface conditions on the wettability, cytocompatibility, and bacterial resistance of 4 mol% Y₂O₃-stabilized tetragonal zirconia polycrystal (4Y–TZP) and two lithium disilicate (Li₂Si₂O₅) glass ceramics (Amber^® Mill (AM) and Amber^® Mill Abut-Crown (AC)). Human gingival fibroblast (HGF-1) responses and biofilm formation on the machined, polished, and glazed samples were evaluated. The polished 4Y–TZP sample exhibited the highest water contact angle (WCA; 71.3°), while that of the AC samples decreased as the sample was machined (58.4°), polished (46.8°), and glazed (14.0°). The wettability, cytocompatibility, and bacterial resistance of the dental ceramics were significantly influenced by material type and surface condition. Among the surface-treated samples, the glazed specimens exhibited the lowest WCA and bulk density; thus, wettability is an important factor for cell proliferation and bacterial resistance. Among all samples, HGF-1 cells adhered well to the glazed ceramics and significantly proliferated over time. Particularly, the 4Y–TZP and AC glazed samples exhibited the lowest biomass and strong resistance to biofilm formation and bacterial adhesion. Thus, the glaze dramatically affected HGF-1 cell growth and antibiofilm formation.

We present a comprehensive statistical analysis of ion-scale waves including dual-band ion-scale waves (DBIWs) observed by the Parker Solar Probe in the solar wind. DBIWs are characterized by the simultaneous occurrence of distinct left- and right-handed polarized wave packets at higher and lower frequencies, respectively, in the spacecraft frame, implying the presence of bidirectionally propagating L-mode waves. We identify and compare DBIWs to single-band ion-scale waves (SBIWs) and investigate their spatial distributions, background magnetic field conditions, and associated inertial range turbulence characteristics. We find that DBIWs exhibit a strong preferential occurrence at heliocentric distances between 0.2 and 0.25 au (equivalently 43 and 54 R _S ) in contrast to SBIWs, the occurrence rate of which peaks closer to the Sun and decreases with the heliocentric distance. DBIWs are also preferentially observed under conditions of strong Alfvénic turbulence, characterized by low magnetic compressibility and perpendicular-dominant fluctuations. These findings support a scenario in which stochastic ion heating via sufficiently large perpendicular Alfvénic fluctuation generates the ion temperature anisotropies required to excite bidirectional ion cyclotron waves. This work advances our understanding of kinetic wave generation in the solar wind and its connection to large-scale turbulence.

We investigate the formation of magnetic islands and the onset of tearing instability within kinetic Alfvén solitary waves in a two-fluid space plasma. These localized structures, featuring internal magnetic shear, naturally support magnetic reconnection processes analogous to tearing modes in magnetically confined fusion plasmas. Treating the solitary wave as a quasi-static background equilibrium, we analyze the resulting topological modifications by making use of the magnetic flux function, in analogy with tokamak plasmas. We find that the spatial size of the magnetic island scales with the square root of the magnetic flux perturbation. Introducing the tearing stability index Δ ′ , we derive the growth rate of the tearing mode using a resistive magnetohydrodynamic framework. Our analysis reveals that tearing modes grow only when the mode’s wavenumber exceeds the inhomogeneity scale, becoming marginally stable when the two are comparable. The emergence of magnetic islands has significant implications for cross-field particle transport, turbulence, and acceleration. In particular, the electron diffusion across magnetic fields may be governed by the island size rather than the Larmor radius, potentially accounting for anomalous transport. Furthermore, in certain regimes, unbounded island growth may signal the collapse of the solitary wave structure, enabling a large-scale transfer of magnetic energy to particle energy. These results suggest a unifying mechanism connecting magnetic reconnection in space plasmas with tearing instabilities in laboratory fusion plasmas.

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 corotating with the Sun, which suggests a similar source of the observed solar wind. In this paper, 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 SMFR-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 are 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.

In this study, we identified 188 heliospheric current sheet (HCS) crossing events observed by Voyager 2 between 1977 and 1990, spanning heliocentric distances from 1 to 33.6 au. The identified HCS crossings revealed well-defined trends including a >30% decrease in magnetic field intensity and >20% increases in proton density and temperature. The Harris sheet model fitting to all 188 HCS crossing events indicates that the estimated HCS thickness spans over 2 orders of magnitude, ranging from 7 × 10 3 to 2.5 × 10 6 km, with a median value of 3.3 × 10 5 km. For 84.6% of the events, the HCS planes lie within − 30 ° to 30 ° in latitude from the equatorial plane, whereas the others indicate highly tilted HCS planes. Similarly, for 85.6% of the events, the HCS planes align with the Parker spiral field from the Parker field direction (within 30°), whereas the others exhibit significant longitudinal tilt. Additionally, a substantial fraction of HCS crossings indicate a significant normal magnetic field component (e.g., B normal / B > 0.3 for 38.8% of the total events), implying rotational discontinuities. Comparing our results with previous studies on HCS thickness from near the Sun to the heliosheath, we find a general increase in thickness with distance. Notably, at 1–33.6 au, HCS thickness remains significantly larger than the ion inertial length, implying conditions generally unfavorable for magnetic reconnection. This contrasts with recent reports of frequent reconnection at HCSs closer to the Sun. Our findings provide a comprehensive insight into the HCS structures across a wide range of heliospheric distances.