National Space Science Center

Abstract The heliospheric current sheet (HCS) represents the most expansive current sheet within the heliosphere. However, there has been a lack of specific analysis regarding the potential folding and flapping of the HCS. The Parker Solar Probe (PSP) offers a unique opportunity to investigate this phenomenon because it can traverse the HCS multiple times during its perihelion encounters with the Sun. During the perihelion of Encounter 7, PSP successively crossed the HCS five times. Utilizing the minimum variance analysis method and discontinuity type analysis on the in situ measurements, we observed that the HCS exhibits a multifolded structure at small scales, coupled with an upward and downward flapping motion. The flapping velocities are estimated to range from several to hundreds of kilometers per second. By employing a two-step ballistic backmapping of the magnetic field in conjunction with observations from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory, we deduced that the flapping motion may be instigated by a solar flare eruption at the source region. Furthermore, we confirmed that the multiple folds are not merely localized manifestations of the Kelvin–Helmholtz instability. Our research indicates that the HCS is, in reality, a dynamic 3D structure characterized by a complex small-scale morphology and its movements are intricately linked to the activities in the source region, revealing a level of complexity that surpasses previous understandings.

Abstract Bipolar light bridges (BLBs) are bright regions located between sunspot umbrae of opposite magnetic polarity. They are typically characterized by strong magnetic fields and intense flows, which are believed to be closely associated with major solar flares. Despite their importance, their fine structure, formation, and evolution remain poorly understood. In this work, we analyze the observations of a well-defined BLB obtained by the Goode Solar Telescope (GST) at the Big Bear Solar Observatory and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. The high-resolution GST observations reveal that the BLB is composed of fine, penumbral filament-like structures with widths of approximately 100–150 km. The corresponding Doppler velocity maps present a stable pattern of spatially adjacent red- and blueshifted patches within the BLB throughout the 5.5 hr GST observation. HMI observations show that the BLB arises from the converging and shearing motions of sunspots with opposite polarities. Penumbral regions originating from different polarities gradually evolve and interact, eventually forming the BLB. The observed Doppler velocity pattern, characterized by red- and blueshifted patches, can be interpreted as a projection effect of the Evershed flow within the penumbrae. Therefore, we argue that the BLB is formed through the compression and stretching of penumbral structures from oppositely polarized sunspots.

ABSTRACT This paper focuses on the front‐stage driving source of the solid‐state terahertz local oscillator component, which is one of the core components of the system's radio frequency front‐end. To enhance the output power, power handling, and thermal dissipation of the local oscillator link, GaN material with high thermal conductivity, high withstand voltage characteristics, and high electron saturation rate is independently designed as a GaN Schottky diode. By incorporating the thermal resistance matrix composed of drift‐diffusion equations and heat conduction equations into the diode modeling, the discrepancy between the simulated and measured performance of the driving source is significantly reduced. The measured results show that within the frequency range of 109–129 GHz, the driving source achieves a maximum output power of 35.56 mW, thereby validating the proposed design approach.

The spatial distribution of energetic O+ ions in the dayside outer and inner magnetosphere during the early recovery phase of the May 2024 superstorm is observed by MMS satellites over a 10-minute interval. During this short interval, the solar wind dynamic pressure sharply decreases from 40 nPa to 10 nPa, leading to the magnetopause expanding sunward. O+-rich plasma is found in the dayside magnetosheath with high O+ ion number density, Nmax_O+ ~ 4.8 cm-3, and the number density ratio of O+ to H+ is about 0.1. O+ ions in the magnetosheath have energies in the range 3-40 keV. O+ ions, escaping from the ring current, are perpendicularly accelerated by the intense electric field, Ex ~ 50 mV/m, at the dayside magnetopause with high reconnection outflow ~260 km/s into the magnetosheath. The escape of energetic O+ ions, with high number density and temperature, from the ring current into the magnetosheath results in the rapid decay of the ring current energy flux during the early recovery phase of this superstorm. This O+ ion escape can cause the SYMH index to recover by 16 nT. Our study provides evidence for a high-energy O+ ion flux in the magnetosheath, which drives the efficient decay of the ring current and the rapid early recovery phase observed during the May 2024 superstorm.

Atmospheric tides propagating upward from the lower atmosphere undergo nonlinear interactions and modulate ionospheric plasma redistribution, leading to pronounced day-to-day variability in ionospheric parameters. This variability reflects the superposition of multiple tidal components with different periods, zonal wavenumbers, and mode structures, yet the dominant modes remain unclear. A hybrid method that combines space-based observations (ICON/MIGHTI), ground-based measurements (Chinese meteor radar chain), and empirical tidal modes (ETMs) is applied to extract the short-term tidal variability. The method is validated during the 2021 sudden stratospheric warming event, capturing the enhancement of the SW2 tidal amplitude, a strengthened first antisymmetric mode, and the phase advance in E-region neutral winds. Future work will extend this approach to assess the modal contributions of tides to the variability of ionospheric plasma drift.

In Earth’s polar regions, Field-Aligned Electrons (FAEs) have been studied for decades. However, their response to solar wind and IMF conditions still require further investigation. In this study, we used Cluster observation data to examine the influence of solar wind and IMF on polar region FAEs. The FAE event was selected based on an electron flux threshold exceeding 3×108 cm-2s-1 for analysis. Several notable findings were obtained. (1) FAE occurrence rates increase with solar wind dynamic pressure (Psw) increasing for both upward and downward FAEs. In the northern hemisphere, however, the occurrence rates appear to rise more sharply than in the southern hemisphere. (2) The distribution of FAE occurrence shows two peaks in relation to IMF By: a major peak around IMF By = -20 nT and a minor peak around IMF By = +20 nT. (3) FAEs occur most frequently when IMF Bz>0 and IMF By>0, which corresponds to an IMF clock angle between 12:00 and 03:00. (4) Since geomagnetic activity is driven by solar wind–magnetosphere interaction, we also examined FAE occurrence in relation to the geomagnetic activity Kp and AE indices. The results indicate that FAE occurrence depends primarily on increasing AE activity. We discuss potential mechanism underlying these results. Variation in FAE occurrence appears to be largely controlled by magnetospheric configuration and its response to solar wind conditions. Further analysis suggests that FAE are closely associated with FAC in polar space. It is significant to understand the physical process in the polar region.

Snow Water Equivalent (SWE) is a critical parameter in the global and regional water cycle and climate system. However, accurately measuring SWE change using satellite remote sensing remains a challenge. While the Interferometric Synthetic Aperture Radar (InSAR) is a promising technique to retrieve SWE from space, its application has been constrained until recently by the lack of spaceborne observations combining optimal low-frequency (e.g., L-band) radar frequencies with short temporal baselines. Furthermore, interferometric coherence is a key factor that affects the accuracy of the unwrapped phase and the subsequent SWE retrieval. However, the repeat-pass InSAR coherence modelling over snow has not been sufficiently investigated.Our study presents the first demonstration of SWE change retrieval using spaceborne repeat-pass L-band InSAR observations from the Chinese Lutan-1 mission. The study area focused on the Altay region in Xinjiang, China, during the winter of 2023–2024. Continuous interferometric pairs with 4/8-day temporal baselines are processed for phase changes and then estimate SWE variations. The retrieved SWE change shows a good agreement with in-situ SWE observations during the dry snow period (January 12 to February 9, 2024), with a Root Mean Square Error (RMSE) of 9 mm and a correlation coefficient (R) of 0.48 for the 4-day temporal baselines. However, a heavy snowfall event observed from February 9 to 17, 2024, induced severe decorrelation, leading to phase unwrapping errors that pose a challenge to SWE retrieval. To address the decorrelation mechanism of snow, the InSAR coherence model for snow is established based on the assumption of a bivariate Gaussian distribution for the ground and snow surface. The time-series modeled coherence shows a consistent trend with the observed Lutan-1 coherence, capturing effectively the decorrelation process caused by snowfall events and snow compaction processes. Furthermore, validation of the modeled coherence against Lutan-1 observations shows a strong agreement (R=0.87) over the entire study period from January 12 to March 28, 2024.Overall, this study demonstrates the capability of spaceborne L-band InSAR with short revisit intervals to effectively retrieve SWE change under appropriate snow conditions. However, the retrieval accuracy is significantly constrained by severe decorrelation during heavy snowfall events. These results highlight both the potential and challenges of operational SWE monitoring from existing and upcoming L-band SAR missions such as Chinese Lutan-1, NASA’s NISAR, JAXA’s ALOS-4, and ESA’s ROSE-L, which are characterized by short repeat cycles, wide swath coverage, and high spatial resolution.