Local Density Enhancement Research Articles

High-resolution observations of 12CO and 13CO J = 3 2 towards the NGC 6334 extended filament. I. Emission morphology and velocity structure

NGC 6334 is a giant molecular cloud (GMC) complex that exhibits elongated filamentary structure and harbours numerous OB-stars, H II regions, and star-forming clumps. To study the emission morphology and velocity structure of the gas in the extended NGC 6334 region using high-resolution molecular line data, we made observations of the 12CO and 13CO $J=3 2$ lines with the LAsMA instrument at the APEX telescope . The LAsMA data provided a spatial resolution of 20" (sim 0.16 pc) and sensitivity of 0.4 K at a spectral resolution of 0.25 km $. Our observations revealed that gas in the extended NGC 6334 region exhibits connected velocity coherent structure over sim 80 pc parallel to the Galactic plane. The NGC 6334 complex has its main velocity component at $ -3.9$ km $ with two connected velocity structures at velocities $ -9.2$ km $ (the `bridge' features) and $ km $ (the northern filament, NGC 6334-NF). We observed local velocity fluctuations at smaller spatial scales along the filament that are likely tracing local density enhancement and infall, while the broader V-shaped velocity fluctuations observed towards the NGC 6334 central ridge and G352.1 region located in the eastern filament EF1 indicate globally collapsing gas onto the filament. We investigated the 13CO emission and velocity structure around 42 WISE H II regions located in the extended NGC 6334 region and found that most H II regions show signs of molecular gas dispersal from the centre (36 of 42) and intensity enhancement at their outer radii (34 of 42). Furthermore most H II regions (26 of 42) are associated with least one ATLASGAL clump within or just outside of their radii, the formation of which may have been triggered by H II bubble expansion. Typically towards larger H II regions we found visually clear signatures of bubble shells emanating from the filamentary structure. Overall the NGC 6334 filamentary complex exhibits sequential star formation from west to east. Located in the west, the GM-24 region exhibits bubbles within bubbles and is at a relatively evolved stage of star formation. The NGC 6334 central ridge is undergoing global gas infall and exhibits two gas bridge features possibly connected to the cloud-cloud collision scenario of the NGC 6334-NF and the NGC 6334 main gas component. The relatively quiescent eastern filament (EF1 - G352.1) is a hub-filament in formation, which shows the kinematic signature of global gas infall onto the filament. Our observations highlight the important role of H II regions in shaping the molecular gas emission and velocity structure as well as the overall evolution of the molecular filaments in the NGC 6334 complex.

Internal magnetic field structures observed by PSP/WISPR in a filament-related coronal mass ejection

Context. We investigated the coronal mass ejection (CME) related to an eruptive filament over the southwestern solar limb on December 8, 2022, at around 8 UT. We tracked localized density enhancements reflecting the magnetic structures using white-light data taken with the Wide-field Instrument for Solar PRobe (WISPR) aboard the Parker Solar Probe (PSP). Aims. We aim to investigate the 3D location, morphology and evolution of the internal magnetic fine structures of CMEs. Specifically, we focused on the physical origin of the features in the WISPR images, how the white-light structures evolve over time, and their relationship with the source region, filament, and the flux rope. Methods. The fast tangential motion of the PSP spacecraft during its perihelion permits a single event to be viewed from multiple angles in short times relative to the event’s evolution. Hence, three-dimensional information of selected CME features can be derived from this single spacecraft using triangulation techniques. Results. We grouped small-scale structures with roughly similar speeds, longitude, and latitude into three distinct morphological groups. We found twisted magnetic field patterns close to the eastern leg of the CME that may be related to “horns” outlining the edges of the flux-rope cavity. We identified aligned thread-like bundles close to the western leg, and they may be related to confined density enhancements evolving during the filament eruption. High density blob-like features (magnetic islands) are widely spread in longitude (∼40°) close to the flanks and the rear part of the CME. We also note that the large-scale outer envelope of the CME, seen clearly from 1 AU, was not well observed by PSP. Conclusions. We demonstrate that CME flux ropes, apart from the blobs, may comprise different morphological groups with a cluster behavior; the blobs instead span a wide range of longitudes. This finding may hint at either the three-dimensionality of the post-CME current sheet (CS) or the influence of the ambient corona in the evolutionary behavior of the CS. Importantly, we show that the global appearance of the CME can be very different in WISPR (0.11–0.16 AU) and the instruments near 1 AU because of the shorter line-of-sight integration of WISPR.

Differential proteomics profile of microcapillary networks in response to sound pattern-driven local cell density enhancement

Spatial cell organization and biofabrication of microcapillary networks in vitro has a great potential in tissue engineering and regenerative medicine. This study explores the impact of local cell density enhancement achieved through an innovative sound-based patterning on microcapillary networks formation and their proteomic profile. Human umbilical vein endothelial cells (HUVEC) and human pericytes from placenta (hPC-PL) were mixed in a fibrin suspension. The mild effect of sound-induced hydrodynamic forces condensed cells into architected geometries showing good fidelity to the numerical simulation of the physical process. Local cell density increased significantly within the patterned areas and the capillary-like structures formed following the cell density gradient. Over five days, these patterns were well-maintained, resulting in concentric circles and honeycomb-like structures.Proteomic analysis of the pre-condensed cells cultured for 5 days, revealed over 900 differentially expressed proteins when cells were preassembled through mild-hydrodynamic forces. Gene ontology (GO) enrichment analysis identified cellular components, molecular functions, and biological processes that were up- and down-regulated, providing insights regarding molecular processes influenced by the local density enhancement. Furthermore, we employed Ingenuity Pathway Analysis (IPA) to identify altered pathways and predict upstream regulators. Notably, VEGF-A emerged as one of the most prominent upstream regulators.Accordingly, this study initiates the unraveling of the changes in microcapillary networks at both molecular and proteins level induced by cell condensation obtained through sound patterning. The findings provide valuable insights for further investigation into sound patterning as a biofabrication technique for creating more complex microcapillary networks and advancing in vitro models.

Submillimetre galaxies as laboratories for dust grain coagulation

ABSTRACT Coagulation in the dense interstellar medium (ISM) is an important process that determines the size of the largest grains. We use submillimetre galaxies (SMGs) as laboratories of grain coagulation, since some of them host the densest ISM on a galactic scale among various populations of galaxies known. We examine how large the grains can be in such dense environments based on the mean ISM density estimated from the observed typical dust mass density in SMGs. We also consider local density enhancement based on a model of supersonic turbulence, which is expected from strong stellar feedback. In the unlimited coagulation model, in which we do not impose any coagulation threshold velocity, grains as large as $\sim 30~\rm{\mu m}$ can form under the observationally estimated mean gas density if the Mach number of turbulence is $\mathcal {M}\gtrsim 3$. We exclude this possibility since the observed emissivity index β ≃ 2 in the far-infrared (FIR) indicates that such large grains cannot actively form in SMGs. This means that coagulation does not proceed in an unlimited way: 30-$\rm{\mu m}$ grains should have velocities larger than the coagulation threshold. If we use a coagulation threshold (upper limit) grain velocity (∼0.08 km s−1) taken from a theoretical study, grains likely grow only up to $\rm{\mu m}$ size, which is small enough not to affect the FIR emissivity index. The above results indicate that SMGs can be used to constrain the physical processes relevant to coagulation.

Simulation of localized electron density enhancement caused by high-pressure H2O gas release in ionosphere

Based on the continuity and momentum equations, a self-consistent simulation model is developed for describing the localized electron density enhancement caused by high-pressure H2O gas release in the ionosphere. The chemical reaction and momentum exchange process between the neutral gas and ionospheric plasma species are considered in the theoretical simulation model. The finite element method is used to solve the simulation model for H2O gas release, and the expansion and ionospheric disturbance process at the early stage of high-pressure gas release are studied. It is shown that the space expansion of the released gas is mainly dominated by the pressure difference between the H2O gas and the ionospheric plasma at the early stage of release. Then the diffusion process becomes the dominant process of the space transport of H2O molecules. An electron depletion region forms near the center of the release region due to the chemical reaction and collision process with H2O molecules. Meanwhile, an electron density enhancement region forms on both sides along the magnetic field direction due to the electron snowplow effect. With the increase in the released mass of H2O gas, the intensity and duration of the electron density enhancement increase gradually. With the release position rising, it is found that the intensity of the electron density enhancement has a peak near 380 km and the duration increases slightly with the increasing release height.

Modelling Local Bubble analogs: synthetic dust polarization maps

ABSTRACT We present a study of synthetic observations of polarized dust emission at 353 GHz as seen by an observer within a cavity in the interstellar medium (ISM). The cavity is selected from a magnetohydrodynamic simulation of the local ISM with time-dependent chemistry, star formation, and stellar feedback in form of supernova explosions with physical properties comparable to the Local Bubble ones. We find that the local density enhancement together with the coherent magnetic field in the cavity walls makes the selected candidate a translucent polarization filter to the emission coming from beyond its domains. This underlines the importance of studying the Local Bubble in further detail. The magnetic field lines inferred from synthetic dust polarization data are qualitatively in agreement with the all-sky maps of polarized emission at 353 GHz from the Planck satellite in the latitudes interval 15° ≲ |b| ≲ 65°. As our numerical simulation allows us to track the galactic mid-plane only out to distances of $250\,$ pc, we exclude the region |b| ≲ 15° from our analysis. At large galactic latitudes, our model exhibits a high degree of small-scale structures. On the contrary, the observed polarization pattern around the Galactic Poles is relatively coherent and regular, and we argue that the global toroidal magnetic field of the Milky Way is important for explaining the data at |b| ≳ 65°. We show that from our synthetic polarization maps, it is difficult to distinguish between an open and a closed galactic cap using the inferred magnetic field morphology alone.

INCREASING LOCAL CELL DENSITY BY SOUND TO ENGINEER VASCULARIZED BONE CONSTRUCTS

Successful application of patient derived cells to engineer vascularized bone grafts is often hampered by low cell numbers and lengthy in vitro expansion. With sound induced morphogenesis (SIM), local cell density enhancement was shown to improve microvasculature formation at lower cell concentration than conventional methods [1]. SIM takes advantage of hydrodynamic forces that act on cells to arrange them within a hydrogel. Following, we are evaluating the potential of cell-hydrogel biografts with high local cell density to improve the therapeutic efficacy in clinical scenarios such as anastomosis or bone formation within non-union fractures.To assess anastomosis, human umbilical vein endothelial cells (HUVEC) and human mesenchymal stromal cells (MSC) were mixed at a 1:1 ratio in PEG-based or Dextran-based hydrogels at a final concentration of 2×106 cells×mL-1. For ectopic bone formation, MSC were resuspended in PEG-based hydrogels at 2×106 or 5×106 cells×mL-1, with or without BMP-2. Cells were assembled into distinct patterns at a frequency of 60 Hz. Four biografts of 4 × 9 mm2 were implanted at the back of nude mice (total of 7 animals) and harvested after 2 or 8 weeks. Explants were fixed and imaged as whole constructs or embedded in paraffin for histological analysis.Upon explantation, microscopic evaluation indicated that HUVEC were retained within the PEG-hydrogel after 2 weeks and formed a pre-vascular network. In the second study, ectopic bone formation was more pronounced in areas of higher local cell density based on visual inspection. Ongoing experiments are further characterizing bone formation by micro-CT and histological evaluation.Our results indicate that local cell density enhancement by sound requires a lower initial cell concentration than conventional, static seeding methods to achieve comparable microvasculature structures or local osteogenesis.

Multiline observations of CH3OH, c-C3H2, and HNCO toward L1544

Context. Pre-stellar cores are the basic unit for the formation of stars and stellar systems. The anatomy of the physical and chemical structures of pre-stellar cores is critical for understanding the star formation process. Aims. L1544 is a prototypical pre-stellar core that shows significant chemical differentiation surrounding the dust peak. We aim to constrain the physical conditions at the different molecular emission peaks. This study allows us to compare the abundance profiles predicted from chemical models with the classical density structure of the Bonnor-Ebert (BE) sphere. Methods. We conducted multi-transition pointed observations of CH3OH, c-C3H2, and HNCO with the IRAM 30m telescope toward the dust peak and the respective molecular peaks of L1544. Using this data set, with nonlocal-thermodynamic-equilibrium radiative transfer calculations and a one-dimensional model, we revisit the physical structure of L1544 and benchmark the observations with the abundance profiles from current chemical models. Results. We find that the HNCO, c-C3H2, and CH3OH lines in L1544 trace progressively higher-density gas, from ~104 to several times 105 cm−3. Particularly, we find that to produce the observed intensities and ratios of the CH3OH lines, a local gas density enhancement above that of the BE sphere is required. This suggests that the physical structure of an early-stage core may not necessarily follow a smooth decrease in gas density profile locally, but can be intercepted by clumpy substructures that surround the gravitational center. Conclusions. Multiple transitions of molecular lines from different molecular species can provide a tomographic view of the density structure of pre-stellar cores. The local gas density enhancement deviating from the BE sphere may reflect the impact of accretion flows that appear asymmetric and are enhanced at the meeting point of large-scale cloud structures.

Molecular Gas Structures Traced by 13CO Emission in the 18,190 12CO Molecular Clouds from the MWISP Survey

After morphological classification of 18,190 12CO molecular clouds, we further investigate the properties of their internal molecular gas structures traced by the 13CO (J = 1−0) line emissions. Using three different methods to extract the 13CO gas structures within each 12CO cloud, we find that ∼15% of the 12CO clouds (2851) have 13CO gas structures and these 12CO clouds contribute about 93% of the total integrated flux of 12CO emission. In each of the 2851 12CO clouds with 13CO gas structures, the 13CO emission area generally does not exceed 70% of the 12CO emission area, and the 13CO integrated flux does not exceed 20% of the 12CO integrated flux. We reveal a strong correlation between the velocity-integrated intensities of 12CO lines and those of 13CO lines in both 12CO and 13CO emission regions. This indicates the H2 column densities of molecular clouds are crucial for the 13CO line emission. After linking the 13CO structure detection rates of the 18,190 12CO molecular clouds to their morphologies, i.e., nonfilaments and filaments, we find that the 13CO gas structures are primarily detected in 12CO clouds with filamentary morphologies. Moreover, these filaments tend to harbor more than one 13CO structure. That demonstrates filaments not only have larger spatial scales, but also have more molecular gas structures traced by 13CO lines, i.e., local gas density enhancements. Our results favor the turbulent compression scenario for filament formation, in which dynamical compression of turbulent flows induces local density enhancements. The nonfilaments tend to be in the low-pressure and quiescent turbulent environments of the diffuse interstellar medium.

Tuning and switching effects of quasi-BIC states combining phase change materials with all-dielectric metasurfaces

Emission enhancement of quantum emitters is particularly relevant in the development of single-photon sources, which are key elements in quantum information and quantum communications. All-dielectric metasurfaces offer a route towards strong enhancement of local density of optical states via engineering of high quality factor optical modes. In particular, the recently proposed concept of quasi-bound states in the continuum (quasi-BIC) allows for precise control of such resonances in lattices with an asymmetric unit cell. Still, the spectral band of emission enhancement is usually fixed by the geometric parameters of the metasurface. Here, we propose to utilize phase change materials to tune the properties of light-emitting metasurfaces designed to support quasi-BIC states in the telecom wavelength range. In our design, a thin layer of a phase change material, which provides strong contrast of refractive index when switched from the amorphous to the crystalline state, is located on top of the resonators made of amorphous silicon (a-Si). Depending on the selected phase change material, we numerically demonstrate different functionalities of the metasurface, In particular, for low-loss Sb2Se3 we evidence spectral tuning effects, whereas for Ge2Sb2Te5, we report an “on”/“off” switching effect of the quasi-BIC resonance. Furthermore, we investigate the influence of the crystallization fraction and the asymmetry parameter of the metasurface on the results. This work provides concrete design blueprints for switchable metasurfaces, offering new opportunities for nanophotonics devices or integrated photonic circuits.

Categorization model of moving small-scale intensity enhancements in solar active regions

Context. The small-scale moving intensity enhancements remotely observed in the extreme ultraviolet images of the solar active regions, which we refer to as active region moving campfires (ARMCs), are related to local plasma temperature and/or density enhancements. Their dynamics is driven by the physical processes in the entire coronal plasma. Our previous study of ARMCs indicates that they have characteristic velocities at around the background sound speed. In the present paper, we further investigate the dynamical and statistical properties of ARMCs. Aims. The main goal of our work is to carry out a simultaneous analysis of EUV images from two observational missions, SDO/AIA and Hi-C 2.1. The aims of the performed cross-validating analysis of both SDO/AIA and Hi-C 2.1 data were to reveal how the observed moving features are distributed over the studied active region, AR12712, and to perform a statistical hypothesis test of the existence of different groups of ARMCs with distinct physical characteristics. Methods. We use the statistical model of intensity centroid convergence and tracking that was developed in our previous paper. Furthermore, a Gaussian mixture model fit of the observed complex of moving ARMCs is elaborated to reveal the existence of distinct ARMC groups and to study the physical characteristics of these different groups. Results. In data from the 171 Å, 193 Å and 211 Å channels of SDO/AIA, we identified several groups of ARMCs with respect to both blob intensity and velocity profiles. The existence of such groups is confirmed by the cross-validation of the 172 Å data sets from Hi-C 2.1. Conclusions. The ARMCs studied in this paper have characteristic velocities in the range of the typical sound speeds in coronal loops. Hence, these moving objects differ from the well-known rapid Alfvénic velocity jets from magnetic reconnection sites. This is also proven by the fact that ARMCs propagate along the active region magnetic structure (strands). The nature of the discovered statistical grouping of the ARMC events is not known. Further theoretical studies and modeling is required to reveal this nature.

Low-latitude plasma blobs above Africa: Exploiting GOLD and multi-satellite in situ measurements

Low-latitude plasma blobs are localized density enhancements of electron density that are occasionally observed in the night-time tropical ionosphere. Two-dimensional (2D) imaging of this phenomenon has been rare and frequently restricted to Central/South America, which is densely covered with ground-based airglow imagers and Global Navigation Satellite System (GNSS) receivers. In Africa, on the contrary, no 2D image of a blob has been reported. Here we present two low-latitude blob events above Africa, one in the Northern summer and the other in winter, in the 2-dimensional Far-UltraViolet (FUV) images from the Global-scale Observations of the Limb and Disk (GOLD) mission. Additionally, multiple satellites (four spacecraft per event) on the Low-Earth-Orbit (LEO) encountered the blob events, some within the GOLD images and some outside. The LEO data support the robustness of GOLD observations and bridge time gaps between the consecutive images. Properties of the two blob events above Africa generally support the conclusions in a previous case study for Central/South America. Plasma therein exhibited higher O+ fraction and faster ion flow toward outer L-shells than the ambient. The blobs were conjugate to locally intensified Equatorial Ionization Anomaly crests without conspicuous equatorward-westward propagation. Our results demonstrate the usefulness of GOLD and multiple LEO satellites in monitoring the ionosphere above Africa, which is a fascinating laboratory of low-latitude electrodynamics but still waiting for more observatories to be deployed.

Statistical Study of Foreshock Bubbles, Hot Flow Anomalies, and Spontaneous Hot Flow Anomalies and Their Substructures Observed by MMS

AbstractForeshock transients such as foreshock bubbles (FBs), hot flow anomalies (HFAs), and spontaneous hot flow anomalies (SHFAs) display heated, tenuous cores and large flow deflections bounded by compressional boundaries. THEMIS and Cluster observations show that some cores contain local density enhancements which can be studied to better understand the evolution processes of foreshock transients. However, closer examinations of these substructures were not feasible until the availability of the higher resolution data from the Magnetospheric Multiscale mission (MMS). We identify 164 FB‐like, HFA‐like, and SHFA events from two MMS dayside phases for a statistical study to investigate their solar wind conditions, properties, and substructure properties. Occurrence rates of the three event types are higher for lower magnetic field strengths, higher solar wind speeds and Mach numbers, and quasi‐parallel bow shocks. Events usually span up to 3 RE along the bow shock surface and extend up to 6 RE upstream from the bow shock. Though events with and without substructures exhibit similar solar wind conditions, events with substructures are more likely to have longer core durations and larger sizes. Substructure densities display a positive correlation with bulk flows and a negative correlation with temperatures. Substructure sizes vary between 4 and 24 ion inertial lengths, indicating multiple formation mechanisms. Substructures could be the boundary between two foreshock transient events that have merged into a single event, fast‐mode variations, generated by slow or mirror mode instabilities, or produced from instabilities due to parameter gradients at the compressional boundaries or shocks.

Isolated Proton Aurora Driven by EMIC Pc1 Wave: PWING, Swarm, and NOAA POES Multi‐Instrument Observations

AbstractWe report the concurrent observations of F‐region plasma changes and field‐aligned currents (FACs) above isolated proton auroras (IPAs) associated with electromagnetic ion cyclotron Pc1 waves. Key events on March 19, 2020 and September 12, 2018 show that ground magnetometers and all‐sky imagers detected concurrent Pc1 wave and IPA, during which NOAA POES observed precipitating energetic protons. In the ionospheric F‐layer above the IPA zone, the Swarm satellites observed transverse Pc1 waves, which span wider latitudes than IPA. Around IPA, Swarm also detected the bipolar FAC and localized plasma density enhancement, which is occasionally surrounded by wide/shallow depletion. This indicates that wave‐induced proton precipitation contributes to the energy transfer from the magnetosphere to the ionosphere.

HaloSat Observations of Heliospheric Solar Wind Charge Exchange

Abstract X-ray emission from solar wind charge exchange (SWCX) produced in interplanetary space contaminates every astrophysical observation, regardless of the line of sight. Unfortunately, the primary SWCX emission lines also happen to be important diagnostics of astrophysical plasmas. Models of SWCX emission are limited by two main uncertainties: the local solar wind fluxes along the line of sight and the charge exchange cross sections. The He cone, a localized density enhancement of helium neutrals, is the only heliospheric SWCX emission feature that is small enough and bright enough to be observationally isolated from the X-ray background and the broader SWCX emission. HaloSat, an X-ray CubeSat mission, has recently completed two series of specialized observations, near and far from the ecliptic plane, during two Earth transits of the He cone. These observations were used to test the predictions of an SWCX emission model against the emission observed at low ecliptic latitude, where the solar wind data are monitored, and at high ecliptic latitude, where the solar wind data are extrapolated. The measured SWCX emission for the set of observations near the ecliptic plane was consistent with the line intensities predicted by the model but underpredicted for the set of observations at high ecliptic latitude near the south ecliptic pole. Additionally, high-temperature Galactic halo emission components are reported for both spectral sets.

Plasmon enhanced upconversion emission in Tm3+/Yb3+/lithium niobate single crystal

Light conversion efficiency, as the core issue of luminescence applications, has become the focus of upconversion (UC) study. Here, we have coated Au nano-films in different thicknesses on c-cut Tm3+/Yb3+/lithium niobate (LN) single crystal wafers, and discovered the luminescence intensity could be drastically enhanced by 15.1 and 26.5 folds for blue and red emission, respectively, in a proper Au thickness. Compared to the Au-free sample, optical measurements demonstrate a moderate enhancement of local optical power density. However, considering the non-linear relationship between the coating thickness and the enhancement factor, we attribute the drastic UC enhancement to the size-dependent localized surface plasmon resonance (LSPR); and accordingly, adopt an analytical optical absorption model to calculate the electron density of Au coating, which manipulates the resonance intensity and enhancement factor. The above explanation is further verified by the variation of LSPR frequency. These findings interpret the interaction between nano-scale Au coating and crystal surface, and demonstrate the feasibility and superiority of LSPR-enhanced UC luminescence in crystalline materials.

Energy Dissipation via Magnetic Reconnection Within the Coherent Structures of the Magnetosheath Turbulence

AbstractA series of intermittent coherent structures was observed in magnetosheath turbulence in the form of magnetic peaks. These magnetic peaks are always accompanied with enhancement of local current density, and three of them are studied in detail because of their intense current density. Based on the magnetic field signals, magnetic curvatures, and the toroidal magnetic field lines, three peaks are identified as magnetic flux ropes. In each trailing part of these three peaks, an extremely thin electron current layer was embedded within a much broader ion‐scale current layer. The energy dissipation is evident within the peaks and direct evidence of magnetic reconnection was found within the thinnest electron current layer. The electrons were heated mainly in two regions of magnetic peaks, that is, the reconnecting current layer by parallel electric field and the trailing edges by Fermi and betatron mechanisms. These results suggest that the ion‐scale magnetic peaks are coherent structures associated with energy dissipation and electron heating in the magnetosheath. Thin current layers can be formed in magnetic peaks, and magnetic reconnection can play a significant role for the energy dissipation in magnetic peaks.

Observing exceptional point degeneracy of radiation with electrically pumped photonic crystal coupled-nanocavity lasers

Controlling gain and loss of coupled optical cavities can induce non-Hermitian degeneracies of eigenstates, called exceptional points (EPs). Various unconventional phenomena around EPs have been reported, and are expected to incorporate extra functionalities into photonic devices. The eigenmode exactly under EP degeneracy is also predicted to exhibit enhanced radiation. However, such responses have yet to be observed in on-chip lasers because of both the limited controllability of their gain and loss and the lifting of degeneracy by pump-induced cavity detuning. Here, we report, to the best of our knowledge, the first non-Hermitian nanophotonic platform based on two electrically pumped photonic crystal lasers and its spontaneous emission at EP degeneracy. Systematically tuned and independent current injection to our wavelength-scale active heterostructure cavities enables us to demonstrate the clear EP phase transition of their spontaneous emission, accompanied with the spectral coalescence of coupled modes and reversed pump dependence of the intensity. Furthermore, we find experimentally and confirm theoretically the peculiar squared Lorentzian emission spectrum very near the exact EP, which indicates a four-fold enhancement of the photonic local density of states induced purely by the degeneracy. Our results open a new pathway to engineer the light–matter interaction by non-Hermiticity and explore larger reconfigurable laser arrays for further non-Hermitian features and physics.

Extreme Positive Ionosphere Storm Triggered by a Minor Magnetic Storm in Deep Solar Minimum Revealed by FORMOSAT‐7/COSMIC‐2 and GNSS Observations

AbstractThis study examines an unexpected and extreme positive ionospheric response to a minor magnetic storm on August 5, 2019 by using global ionosphere specification (GIS) 3D electron density profiles obtained by assimilating radio occultation total electron content (TEC) measurements of the recently launched FORMOSAT‐7/COSMIC‐2 satellites, and ground‐based global navigation satellite system (GNSS) TEC. The results reveal ∼300% enhancement of equatorial ionization anomaly (EIA) crests, appearing over 200–300 km altitudes, and a much intense localized density enhancement over the European sector. These are the most intense ionospheric response that has ever been detected for a small magnetic storm with Dst ∼ −53 nT (SYM‐H ∼ −64 nT). The enhancements are validated by using global ionosphere map (GIM) TEC and ground‐based GNSS TEC. The GIS vertical electron density structures during the storm are examined to understand the physical processes giving rise to such an intense ionosphere response during deep solar minimum conditions when the background electron density is very low. Altitude variations and poleward shifts of the locations of the EIA crests indicate that prompt penetration electric fields (PPEF) play an important role in producing the observed positive storm responses, with the storm‐induced equatorward circulation supporting the plasma accumulation against recombination losses. Additional physical mechanisms are required to fully explain the unexpected electron density enhancements for this minor storm event.

Sub-to-supercritical properties and inhomogeneity of JP-10 using molecular dynamics simulation

JP-10, as one single-component aviation fuel, usually undergoes a sub-to-supercritical phase transition through channels due to the extreme temperature and pressure when applied in the regeneration cooling process. Predicting JP-10 properties during this transition has always been one task which is far from trivial. In this study, equilibrium molecular dynamics simulations with three forcefields (OPLS-AA, COMPASS (II) and TraPPE-UAf) are employed to model JP-10 under the pressure from 1.48 MPa to 6.00 MPa and temperature from 250 K to 900 K. The OPLS forcefield achieves the most satisfactory result in predictions of density, viscosity, thermal conductivity and critical parameters, with few mispredictions in the transcritical area. To further explore the divergences, inhomogeneity, as an inherent property of supercritical fluids which disturbs EMD simulations near the critical point, is described by Voronoi tessellation and quantitatively analyzed by evaluating the local density augmentation and enhancement factor under various conditions. Simulation results show an increasing tendency in local density augmentation of supercritical JP-10 with the decreasing of the bulk density in the range of this work.