Loop-like Structures Research Articles

A type II radio burst associated with solar filament–filament interaction

Aims. Solar radio type II bursts are often associated with coronal shocks driven by solar eruptions. In this study, we report a type II burst associated with filament–filament interaction. Methods. Combining the high-quality multiwavelength observations from CHASE, SDO, STEREO, and CALLISTO, we conducted a detailed study of the type II burst associated with filament–filament interaction. Results. On 2023 September 11, an erupting filament (F1) likely disturbed a nearby long filament (F2), causing F2 to subsequently erupt. As a result of possible magnetic reconnection between ejective materials from the two filaments, loop-like structures formed perpendicular to them. Subsequently, the expansion of these loop-like structures triggered a strong coronal mass ejection (CME). Interestingly, a type II burst appeared on the solar spectrum around the time when the loop-like structures formed and the CME appeared above the occulting disk of STEREO/COR1. By converting the frequency of the type II burst to the coronal height using polarization brightness data recorded by the COR1 coronagraph and the spherically symmetric polynomial approximation technique, we determined the formation height of the type II burst to be around 1.45 R⊙, with a speed of approximately 440 km s−1. This is comparable to the observed height of the CME (∼1.43 R⊙), although slightly lower in speed (540 km s−1). Conclusions. All these results indicate that the type II burst was closely associated with filament–filament interactions and was possibly excited by the accompanying CME at the flank. We suggest that the filament–filament interactions played an important role in producing the type II burst by acting as a piston to trigger a strong CME.

Epidermal growth factor-like domain 7 drives brain lymphatic endothelial cell development through integrin αvβ3

In zebrafish, brain lymphatic endothelial cells (BLECs) are essential for meningeal angiogenesis and cerebrovascular regeneration. Although epidermal growth factor-like domain 7 (Egfl7) has been reported to act as a pro-angiogenic factor, its roles in lymphangiogenesis remain unclear. Here, we show that Egfl7 is expressed in both blood and lymphatic endothelial cells. We generate an egfl7cq180 mutant with a 13-bp-deletion in exon 3 leading to reduced expression of Egfl7. The egfl7cq180 mutant zebrafish exhibit defective formation of BLEC bilateral loop-like structures, although trunk and facial lymphatic development remains unaffected. Moreover, while the egfl7cq180 mutant displays normal BLEC lineage specification, the migration and proliferation of these cells are impaired. Additionally, we identify integrin αvβ3 as the receptor for Egfl7. αvβ3 is expressed in the CVP and sprouting BLECs, and blocking this integrin inhibits the formation of BLEC bilateral loop-like structures. Thus, this study identifies a role for Egfl7 in BLEC development that is mediated through the integrin αvβ3.

Comparative ultrastructure of the olfactory system in the East African root rat (Tachyoryctes splendens) and the naked mole rat (Heterocephalus glaber).

The ultrastructure of the olfactory system of most fossorial rodents remains largely unexplored. This study sought to investigate the functional structure of the olfactory mucosa and olfactory bulb of two species of fossorial rodents that have distinct behaviour and ecology, the East African root rat (RR) and the naked mole rat (NMR). Transmission electron microscopy and scanning electron microscopy were employed. The basic ultrastructural design of the olfactory system of the two species was largely comparable. In both species, the olfactory mucosa comprised an olfactory epithelium and an underlying lamina propria. The olfactory epithelium revealed olfactory knobs, cilia and microvilli apically and sustentancular cells, olfactory receptor neurons and basal cells in the upper, middle and basal zones, respectively. The lamina propria was constituted by Bowman's glands, olfactory nerve bundles and vasculature supported by loose connective tissue. Within the olfactory bulb, intracellular and extracellular structures including cell organelles, axons and dendrites were elucidated. Notable species differences were observed in the basal zone of the olfactory epithelium and on the luminal surface of the olfactory mucosa. The basal zone of the olfactory epithelium of the RR consisted of a single layer of flattened electron-dense horizontal basal cells while the NMR had juxtaposed electron-dense and electron-lucent heterogenous cells, an occurrence seen as being indicative of quiescent and highly proliferative states of the olfactory epithelia in the two species, respectively. The olfactory epithelial surface of the NMR comprised an elaborate cilia network that intertwined extensively forming loop-like structures whereas in the RR, the surface was rugged and consisted of finger-like processes and irregular masses. With gross and histological studies showing significant differences in the olfactory structures of the two species, these findings are a further manifestation that the olfactory system of the RR and the NMR have evolved differently to reflect their varied olfactory functional needs.

Dynamics and Structures of Amyloid Aggregates under Fluid Flows.

In this work, we investigate how fluid flows impact the aggregation mechanisms of Aβ40 proteins and Aβ16-22 peptides and mechanically perturb their (pre)fibrillar aggregates. We exploit the OPEP coarse-grained model for proteins and the Lattice Boltzmann Molecular Dynamics technique. We show that beyond a critical shear rate, amyloid aggregation speeds up in Couette flow because of the shorter collisions times between aggregates, following a transition from diffusion limited to advection dominated dynamics. We also characterize the mechanical deformation of (pre)fibrillar states due to the fluid flows (Couette and Poiseuille), confirming the capability of (pre)fibrils to form pathological loop-like structures as detected in experiments. Our findings can be of relevance for microfluidic applications and for understanding aggregation in the interstitial brain space.

Are Coronal Loops Projection Effects?

We report results of an in-depth numerical investigation of three-dimensional projection effects that could influence the observed loop-like structures in an optically thin solar corona. Several archetypal emitting geometries are tested, including collections of luminous structures with circular cross sections of fixed and random size, and light-emitting structures with highly anisotropic cross sections, as well as two-dimensional stochastic current density structures generated by fully developed magnetohydrodynamic turbulence. A comprehensive set of statistical signatures is used to compare the line-of-sight (LOS) integrated emission signals predicted by the constructed numerical models with the loop profiles observed by the extreme ultraviolet telescope on board the flight 2.1 of the High-Resolution Coronal Imager (Hi-C). The results suggest that typical cross-sectional emission envelopes of the Hi-C loops are unlikely to have high eccentricity, and that the observed loops cannot be attributed to randomly oriented quasi-two-dimensional emitting structures, some of which would produce anomalously strong optical signatures due to an accidental LOS alignment, as expected in the ''coronal veil“ scenario proposed recently by Malanushenko et al. The possibility of apparent loop-like projections of very small (close to the resolution limit) or very large (comparable with the size of an active region) light-emitting sheets remains open, but the intermediate range of scales commonly associated with observed loop systems is most likely filled with true quasi-one-dimensional (roughly axisymmetric) structures embedded into the three-dimensional coronal volume.

Exploring the correlation between dark matter, intracluster light, and globular cluster distribution in SMACS0723

We present a free-form model of SMACS0723, the first cluster observed with JWST. This model does not make any strong assumptions on the distribution of mass (mostly made up of dark matter) in the cluster and we use it to study the possible correlation between dark matter with the intracluster light and distribution of globular clusters (GCs). To explore the uncertainty in mass modeling, we derived three lens models based on spectroscopically confirmed systems and new candidate systems with redshifts predicted by the lens model derived from the spectroscopic systems. We find that beyond the radius of influence for the brightest cluster galaxy (BCG), the total mass does not trace the intracluster light (ICL), implying the need for a dark component (dark matter). Two loop-like structures observed in the intracluster light do not have any obvious correspondence with the total mass (of mostly dark matter) distribution. The radial profiles of the ICL and the distribution of GCs are similar to each other, but they are steeper than the profile of the lens model. More specifically, we find that the total mass is shallower by 1 dex in log scale than both ICL and GC profiles. This is in excellent agreement with current N-body simulations of cold dark matter.

Formation and Dynamics in an Observed Preeruptive Filament

The formation of filaments/prominences is still a debated topic. Many different processes have been proposed: levitation, injection of cool plasma, merging filaments, and cooling plasma in hot loops. We take the opportunity to make a multiwavelength analysis of the formation of an active-region filament, combining several UV and EUV observations including the new Ne vii 465 Å filtergrams provided by the Solar Upper Transition Region Imager on board the Space Advanced Technology satellite. The filament is mainly observed at the limb for 3 hr. It is progressively formed through a series of stages, including emergence and cooling of hot loops, reconnection between small filaments, material transfer in a large filament channel, and reconnection between filaments and emerged hot loops. From the observations at 465 Å, we find that the new-formed filaments show bright structures as in 304 Å, while the long-lived stable filaments display dark morphology as in 211 Å. This suggests that the plasma around 0.5 MK would be an essential component of new-formed filaments and the material temperature in filaments would be variable during their evolution. The filament formed by the recombination of two filaments and an emerged hot loop finally erupts. After reconnection, the final filament shows a highly twisted structure of both bright and dark strands, which is surrounded by several weak and dispersive looplike structures. This eruptive filament has a complex multichannel topology and covers a wide range of temperatures.

Small-scale loops heated to transition region temperatures and their chromospheric signatures in the simulated solar atmosphere

Context. Recent observations have revealed loop-like structures at very small scales visible in observables that sample the transition region (TR) and even coronal temperatures. These structures are referred to as either ‘unresolved fine structures’, ‘dynamic cool loops’, ‘miniature hot loops’ or ‘campfires’ depending on the observables in which they are detected. Their formation remains unclear. Aims. Realistic magnetohydrodynamic simulations and forward synthesis of spectral lines are used to investigate how these features occur. Methods. Computations were carried out using the MURaM code to generate model atmospheres. The synthetic Hα and Si IV spectra are calculated at two angles (μ = 1, μ = 0.66) using the Multi3D code. We traced magnetic field lines in the model and examined the evolution of the underlying field topology. Results. The synthetic Hα Dopplergrams reveal loops that evolve dramatically within a few minutes. The synthetic Hα line profiles show observed asymmetries and Doppler shifts in the line core. However, they also show strong emission peaks in the line wings, even at the slanted view. The synthetic Si IV emission features partly coincide with structures visible in Hα Dopplergrams and partly follow separate magnetic field threads. Some are even visible in the emission measure maps for the lg(T/K) = [5.8, 6.2] temperature interval. The emission areas trace out the magnetic field lines rooted in opposite polarities in a bipolar region. Conclusions. The model shows that a loop-like structure in a bipolar system with footpoints undergoing rapid movement and shuffling can produce many small-scale recurrent events heated to high temperatures. It demonstrates that heating to different temperatures occurs and can be confined to a small part of the loop, at the location where resistive and viscous heating increases. The model largely reproduces the observed features in terms of size, lifetime and morphology in chromospheric, TR and coronal observables. The morphology and evolution of the resulting observable features can vary depending on the viewing angle.

Statistical properties of the population of the Galactic centre filaments – II. The spacing between filaments

ABSTRACT We carry out a population study of magnetized radio filaments in the Galactic centre using MeerKAT data by focusing on the spacing between the filaments that are grouped. The morphology of a sample of 43 groupings containing 174 magnetized radio filaments are presented. Many grouped filaments show harp-like, fragmented cometary tail-like, or loop-like structures in contrast to many straight filaments running mainly perpendicular to the Galactic plane. There are many striking examples of a single filament splitting into two prongs at a junction, suggestive of a flow of plasma along the filaments. Spatial variations in spectral index, brightness, bending, and sharpening along the filaments indicate that they are evolving on a 105−6-yr time-scale. The mean spacings between parallel filaments in a given grouping peaks at ∼16 arcsec. We argue by modeling that the filaments in a grouping all lie on the same plane and that the groupings are isotropically oriented in 3D space. One candidate for the origin of filamentation is interaction with an obstacle, which could be a compact radio source, before a filament splits and bends into multiple filaments. In this picture, the obstacle or sets the length scale of the separation between the filaments. Another possibility is synchrotron cooling instability occurring in cometary tails formed as a result of the interaction of cosmic ray driven Galactic centre outflow with obstacles such as stellar winds. In this picture, the mean spacing and the mean width of the filaments are expected to be a fraction of a parsec, consistent with observed spacing.

Kraenkel-Manna-Merle saturated ferromagnetic system: Darboux transformation and loop-like soliton excitations

In this article, we reinvestigate the Kraenkel-Manna-Merle (KMM) system, which can describe the nonlinear ultra-short wave pulse motions in certain saturated ferromagnetic materials. Starting from the Lax pair of the system, we construct the Darboux transformation of this system. Then, new one-, two- and three-soliton solutions are obtained where free functions with respect to the time variable is involved. A series of novel loop-like soliton structures are excited by choosing the free functions as certain particular functions. The soliton excitation dynamics is also discussed in detailed.

Adult mouse kidney stem cells orchestrate the de novo assembly of a nephron via Sirt2‐ modulated canonical Wnt/β‐catenin signaling

End‐stage renal disease affects an estimated 2.4 million people annually worldwide. With cases increasing 8% every year, regenerative strategies are needed to help alleviate, and potentially replace, the increasing demand for kidney transplants. An important question remains whether stem cells reside in the adult mammalian kidney. Here we developed a methodology for isolation of kidney stem cells from adult mouse kidney by using Sca1 and Oct4 stem cell markers. Using these cells, we generated three types of kidney‐like structures. First, we observed the assembly of self‐organizing three‐dimensional kidney organoids. These kidney organoids show characteristic of kidney structures and contain podocytes and endothelial cells that form networks of capillary loop‐like structures. We also observed the de novogeneration of a self‐organizing nephron (and its associated collecting duct) from monolayer cultures. Comma‐ and S‐shaped bodies that expressed activated Notch1 were present during this formation, suggesting that Notch signaling played a key role in driving nephrogenesis. This is consistent with our current knowledge of nephron formation in the mouse embryo. Furthermore, we observed the differentiation of monolayer cells into functionally mature tubules and self‐organizing kidney‐shaped structures that selectively endocytose dextran. Finally, we demonstrated that expression of Sirt2 was upregulated, which modulated the dynamics of canonical Wnt/β‐catenin signaling during kidney organoid development. Altogether, our observations provide the first evidence for the existence of stem cells in the adult mouse kidney. These findings offer powerful tools for future applications, including drug screening, disease modelling, cell therapy, and kidney regeneration.

Propagating brightenings in small loop-like structures in the quiet-Sun corona: Observations from Solar Orbiter/EUI

Brightenings observed in solar extreme-ultraviolet images are generally interpreted as signatures of micro- or nanoflares occurring in the transition region or at coronal temperatures. Recent observations with the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have revealed the smallest of such brightenings (called campfires) in the quiet-Sun corona. Analyzing EUI 174 Å data obtained at a resolution of about 400 km on the Sun with a cadence of 5 s on 30 May 2020, we report here a number of cases in which these campfires exhibit propagating signatures along their apparently small (3–5 Mm) loop-like structures. The measured propagation speeds are generally between 25 km s−1and 60 km s−1. If the loop plasma is assumed to be at a million Kelvin, these apparent motions would be slower than the local sound speed. Furthermore, these brightenings exhibit nontrivial propagation characteristics such as bifurcation, merging, reflection, and repeated plasma ejections. We suggest that these features are manifestations of the internal dynamics of these small-scale magnetic structures and could provide important insights into the dynamic response (∼40 s) of the loop plasma to the heating events and also into the locations of the heating events themselves.

Signatures of Recent Cosmic-Ray Acceleration in the High-latitude Gamma-Ray Sky.

Cosmic-ray (CR) sources temporarily enhance the relativistic particle density in their vicinity over the background distribution accumulated from the Galaxy-wide past injection activity and propagation. If individual sources are close enough to the solar system, their localized enhancements may present as features in the measured spectra of the CRs and in the associated secondary electromagnetic emissions. Large-scale loop-like structures visible in the radio sky are possible signatures of such nearby CR sources. If so, these loops may also have counterparts in the high-latitude γ-ray sky. Using ~10 yr of data from the Fermi Large Area Telescope, applying Bayesian analysis including Gaussian Processes, we search for extended enhanced emission associated with putative nearby CR sources in the energy range from 1 GeV to 1 TeV for the sky region |b| > 30°. We carefully control the systematic uncertainty due to imperfect knowledge of the interstellar gas distribution. Radio Loop IV is identified for the first time as a γ-ray emitter, and we also find significant emission from Loop I. Strong evidence is found for asymmetric features about the Galactic l = 0° meridian that may be associated with parts of the so-called "Fermi Bubbles," and some evidence is also found for γ-ray emission from other radio loops. Implications for the CRs producing the features and possible locations of the sources of the emissions are discussed.

Recognition of single-stranded nucleic acids by small-molecule splicing modulators.

Risdiplam is the first approved small-molecule splicing modulator for the treatment of spinal muscular atrophy (SMA). Previous studies demonstrated that risdiplam analogues have two separate binding sites in exon 7 of the SMN2 pre-mRNA: (i) the 5′-splice site and (ii) an upstream purine (GA)-rich binding site. Importantly, the sequence of this GA-rich binding site significantly enhanced the potency of risdiplam analogues. In this report, we unambiguously determined that a known risdiplam analogue, SMN-C2, binds to single-stranded GA-rich RNA in a sequence-specific manner. The minimum required binding sequence for SMN-C2 was identified as GAAGGAAGG. We performed all-atom simulations using a robust Gaussian accelerated molecular dynamics (GaMD) method, which captured spontaneous binding of a risdiplam analogue to the target nucleic acids. We uncovered, for the first time, a ligand-binding pocket formed by two sequential GAAG loop-like structures. The simulation findings were highly consistent with experimental data obtained from saturation transfer difference (STD) NMR and structure-affinity-relationship studies of the risdiplam analogues. Together, these studies illuminate us to understand the molecular basis of single-stranded purine-rich RNA recognition by small-molecule splicing modulators with an unprecedented binding mode.

Plasma Heating Induced by Tadpole-like Downflows in the Flaring Solar Corona

SummaryAs one of the most spectacular energy release events in the solar system, solar flares are generally powered by magnetic reconnection in the solar corona. As a result of the re-arrangement of magnetic field topology after the reconnection process, a series of new loop-like magnetic structures are often formed and are known as flare loops. A hot diffuse region, consisting of around 5–10 MK plasma, is also observed above the loops and is called a supra-arcade fan. Often, dark, tadpole-like structures are seen to descend through the bright supra-arcade fans. It remains unclear what role these so-called supra-arcade downflows (SADs) play in heating the flaring coronal plasma. Here we show a unique flare observation, where many SADs collide with the flare loops and strongly heat the loops to a temperature of 10–20 MK. Several of these interactions generate clear signatures of quasi-periodic enhancement in the full-Sun-integrated soft X-ray emission, providing an alternative interpretation for quasi-periodic pulsations that are commonly observed during solar and stellar flares.

Source region identification and geophysical effects of stealth coronal mass ejections

We use the example of two Stealth-type coronal mass ejections (stealth CMEs) registered by LASCO C2 on the quiescent Sun on 16 June 2010 and in the active region NOAA 11520 7 July 2012 to demonstrate that these mass ejections emerged in a lower-coronal area where an intense short-term radiation burst was registered in several extreme ultraviolet channels. It is established for the first time that the stealth CME emergence is accompanied by an eruption of small-scale loop-like structures, and the frontal structure (FS) formation is detected. The time dependences of the velocity of these FSs are discussed. The conclusion is made that having reached Earth orbit and impacting on the Earth magnetosphere, the stealth CMEs can lead to magnetic substorm generation.

Structural basis of astrocytic Ca2+ signals at tripartite synapses

Astrocytic Ca2+ signals can be fast and local, supporting the idea that astrocytes have the ability to regulate single synapses. However, the anatomical basis of such specific signaling remains unclear, owing to difficulties in resolving the spongiform domain of astrocytes where most tripartite synapses are located. Using 3D-STED microscopy in living organotypic brain slices, we imaged the spongiform domain of astrocytes and observed a reticular meshwork of nodes and shafts that often formed loop-like structures. These anatomical features were also observed in acute hippocampal slices and in barrel cortex in vivo. The majority of dendritic spines were contacted by nodes and their sizes were correlated. FRAP experiments and Ca2+ imaging showed that nodes were biochemical compartments and Ca2+ microdomains. Mapping astrocytic Ca2+ signals onto STED images of nodes and dendritic spines showed they were associated with individual synapses. Here, we report on the nanoscale organization of astrocytes, identifying nodes as a functional astrocytic component of tripartite synapses that may enable synapse-specific communication between neurons and astrocytes.

New Approaches to the Identification of Sources and the Onset of Stealth Coronal Mass Ejections

New approaches to the identification of sources and the onset of stealth coronal mass ejections (CMEs) are discussed based on the stealth CME of July 7, 2012. It is shown that the generation of this stealth CME is accompanied by various manifestations of short-term and small-scale activity in the region of its formation in the form of bursts of UV radiation (a microflare), as well as the activation and movement of loop-like structures. The features of the behavior of the photospheric magnetic field in the region of observation of the microflare are studied. The formation of a frontal CME structure there can be considered evidence that the stealth CME is generated in a solar atmospheric region with detected manifestations of solar activity.

Electron tomography reveals changes in spatial distribution of UBTF1 and UBTF2 isoforms within nucleolar components during rRNA synthesis inhibition

Upstream binding transcription factor (UBTF) is a co-regulator of RNA polymerase I by constituting an initiation complex on rRNA genes. UBTF plays a role in rDNA bending and its maintenance in “open” state. It exists as two splicing variants, UBTF1 and UBTF2, which cannot be discerned with antibodies raised against UBTF. We investigated the ultrastructural localization of each variant in cells synthesizing GFP-tagged UBTF1 or UBTF2 by using anti-GFP antibodies and pre-embedding nanogold strategy. Detailed 3D distribution of UBTF1 and 2 was also studied by electron tomography. In control cells, the two isoforms are very abundant within fibrillar centers, but their repartition strongly differs. Electron tomography shows that UBTF1 is disposed as fibrils that are folded in coils whereas UBTF2 is localized homogenously, preferentially at their cortical area. As UBTF is a useful marker to trace rDNA genes, we used these data to improve our previous model of 3D organization of active transcribing rDNA gene within fibrillar centers. Finally, when rRNA synthesis is inhibited during actinomycin D treatment or entry in mitosis, UBTF1 and UBTF2 show a similar distribution along extended 3D loop-like structures. Altogether these data suggest new roles for UBTF1 and UBTF2 isoforms in the organization of active and inactive rDNA genes.

Oil mobilization and solubilization in porous media by in situ emulsification

HypothesisFor a wide range of subsurface engineering processes, such as geological carbon sequestration and enhanced oil recovery, it is critical to understand multiphase flow at a fundamental level. To this end, geomaterial microfluidic devices provide visual data that can be quantified to explain the physics of multiphase flow at the length scale of individual pores in realistic rock structures. For surfactant enhanced oil recovery, it is the underlying geometrical states of the capillary trapped oil that dictates the recovery process and the degree to which oil is recovered through either mobilization or solubilization during in situ emulsification. ExperimentsA novel geomaterial microfluidic device is fabricated and its integrity is checked using light microscopy and X-ray micro-computed tomography (μ-CT) imaging. Subsequently, alkaline surfactant (AS) flooding of an oil saturated device is studied for enhanced recovery. The recovery process is analyzed by collecting 2D radiographic projections of the device during water flooding and in situ emulsification. 3D μ-CT images are also collected to quantify the geometrical states of the fluids after each flooding sequence. FindingsOur study reveals the processes of oil cluster mobilization and solubilization in porous media. After water flooding there are numerous oil clusters that are relatively large, extending over multiple pores, forming various loop-like structures. These clusters are mobile under AS flooding accounting for 75% of the recovered oil. The less mobile smaller clusters, isolated to single pores, forming no loop-like structures are immobile. These clusters are solubilized during AS flooding accounting for 25% of the recovered oil. The mobilized clusters coalesce forming an oil bank prior to total solubilization. The remaining oil clusters after AS flooding are highly non-wetting, as indicated by contact angle measurements and would only be recoverable after further solubilization.