Meridional circulation is a crucial component of the Sun's internal dynamics, but its inference in the deep interior is complicated by a systematic center-to-limb effect in helioseismic measurement ...techniques. Previously, an empirical method, removing travel-time shifts measured for the east-west traveling waves in the equatorial area from those measured for the north-south traveling waves in the central meridian area, was used, but its validity and accuracy need to be assessed. Here, we develop a new method to separate the center-to-limb effect, , and the meridional-flow-induced travel-time shifts, , in a more robust way. Using 7-year observations of the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, we exhaustively measure the travel-time shifts between two surface locations along the solar disk's radial direction for all azimuthal angles and all skip distances. The measured travel-time shifts are a linear combination of and , which can be disentangled by solving the linear equation set. The is found to be isotropic relative to the azimuthal angle, and the is then inverted for the meridional circulation. Our inversion results show a three-layer flow structure, with equatorward flow found between about 0.82 and 0.91 R☉ for low latitude areas and between about 0.85 and 0.91 R☉ for higher latitude areas. Poleward flows are found below and above the equatorward flow zones, indicating a double-cell circulation in each hemisphere.
Abstract
Several recent studies utilizing different helioseismic methods have confirmed the presence of large-scale vorticity waves known as solar Rossby waves within the Sun. Rossby waves are ...distinct from acoustic waves, typically with longer periods and lifetimes, and their general properties, even if only measured at the surface, may be used to infer properties of the deeper convection zone, such as the turbulent viscosity and entropy gradients that are otherwise difficult to observe. In this study, we utilize 12 yr of inverted subsurface velocity fields derived from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager’s time–distance and ring-diagram pipelines to investigate the properties of the solar equatorial Rossby waves. By covering the maximum and the decline phases of Solar Cycle 24, these data sets enable a systematic analysis of any potential cycle dependence of these waves. Our analysis provides evidence of a correlation between the average power of equatorial Rossby waves and the solar cycle, with stronger Rossby waves during the solar maximum and weaker waves during the minimum. Our result also shows that the frequency of the Rossby waves is lower during the magnetic active years, implying a larger retrograde drift relative to the solar rotation. Although the underlying mechanism that enhances the Rossby wave power and lowers its frequency during the cycle maximum is not immediately known, this observation has the potential to provide new insights into the interaction of large-scale flows with the solar cycle.
Abstract
Using time–distance local helioseismology flow maps within 1 Mm of the solar photosphere, we detect inflows toward activity belts that contribute to solar-cycle scale variations in the ...near-surface meridional flow. These inflows stretch out as far as 30° away from the active region centroids. If active region neighborhoods are excluded, the solar-cycle-scale variation in the background meridional flow diminishes to below 2 m s
−1
, but still shows systematic variations in the absence of active regions between sunspot cycles 24 and 25. We therefore propose that the near-surface meridional flow is a three-component flow made up of a constant baseline flow profile that can be derived from quiet-Sun regions, variations due to inflows around active regions, and solar-cycle-scale variation of about 2 m s
−1
. Torsional oscillation, on the other hand, is found to be a global phenomenon, i.e., exclusion of active region neighborhoods does not significantly affect its magnitude or phase. This nonvariation in torsional oscillation with distance away from active regions and the three-component breakdown of the near-surface meridional flow serve as vital constraints for solar dynamo models and surface flux-transport simulations.
As the first example of a photocatalytic system for splitting water without additional cocatalysts and photosensitizers, the comparatively cost‐effective Cu2I2‐based MOF, Cu‐I‐bpy ...(bpy=4,4′‐bipyridine) exhibited highly efficient photocatalytic hydrogen production (7.09 mmol g−1 h−1). Density functional theory (DFT) calculations established the electronic structures of Cu‐I‐bpy with a narrow band gap of 2.05 eV, indicating its semiconductive behavior, which is consistent with the experimental value of 2.00 eV. The proposed mechanism demonstrates that Cu2I2 clusters of Cu‐I‐bpy serve as photoelectron generators to accelerate the copper(I) hydride interaction, providing redox reaction sites for hydrogen evolution. The highly stable cocatalyst‐free and self‐sensitized Cu‐I‐bpy provides new insights into the future design of cost‐effective d10‐based MOFs for highly efficient and long‐term solar fuels production.
No additives required: A low‐cost Cu2I2‐based MOF exhibits efficient photocatalytic H2 production without additional photosensitizers and cocatalysts. DFT calculations reveal a good band alignment with the water redox energy levels. The proposed mechanism demonstrates that Cu2I2 clusters in Cu‐I‐bpy (bpy=4,4′‐bipyridine) serve as photoelectron generators to accelerate copper(I) hydride interaction for hydrogen evolution.
Abstract
Previous studies analyzing the evanescent nature of acoustic waves in the lower solar atmosphere, up to 300 km above the photosphere, have shown an unexpected phase shift of an order of 1 s ...between different heights. Those studies investigated the spectral line Fe
i
λ
6173.3, commonly used for helioseismic measurements. Such phase shifts can contribute to a misinterpretation of the measured travel times in local helioseismology, complicating inferences of, e.g., the deep meridional flow. In this study, we carry out phase shift computations using a simulated, fully radiative, and convective atmosphere from which the Fe
i
λ
6173.3 line is synthesized. The resulting phase shifts as functions of frequency across multiple heights show nonzero values in evanescent waves, similar to what was found in observational data. Comparing the Doppler velocities estimated from the synthesized absorption line with the true velocities directly obtained from the simulated plasma motions, we find substantial differences in phase shifts between the two. This leads us to hypothesize that the nonadiabaticity of the solar atmosphere yields extra phase shift contributions to Doppler velocities. Finally, computing phase differences for different viewing angles reveals a systematic center-to-limb variation, similar to what is present in observations. Overall, this study helps to improve our understanding of the physical cause of the helioseismic center-to-limb effect.
Urban expressway is the main artery of traffic network, and an in-depth analysis of the crashes is crucial for improving the traffic safety level of expressways. This study intended to address the ...injury severity of expressways in Beijing by proposing Bayesian ordered logistic regression model. Crash data were collected from urban express rings and expressways in 2015 and 2016. The results showed that crash location, time and crash season are significant variables influencing injury severity. The findings revealed that the proposed model can address the ordinal feature of injury severity, while accommodating the data with small sample sizes that may not adequately represent population characteristics. The conclusions can provide the management departments with valuable suggestions for the injury prevention and safety improvement on the urban expressways.
ABSTRACT Atmosphere above sunspots is abundant with different types of waves. Among these waves are running penumbral waves in the chromosphere, quasi-periodic oscillations in the lower coronal ...loops, and recently reported running waves in sunspots' photosphere, all of which were interpreted as magnetoacoustic waves by some authors. Are these waves in different atmospheric layers related to each other, what is the nature of these waves, and where are the ultimate sources of these waves? Applying a time-distance helioseismic analysis over a suite of multi-wavelength observations above a sunspot, we demonstrate that the helioseismic p-mode waves are able to channel up from the photosphere through the chromosphere and transition region into the corona, and that the magnetoacoustic waves observed in different atmospheric layers are a same wave originating from the photosphere but exhibiting differently under different physical conditions. We also show waves of different frequencies travel along different paths, which can be used to derive the physical properties of the atmosphere above sunspots. Our numerical simulation of traveling of waves from a subphotospheric source qualitatively resembles the observed properties of the waves and offers an interpretation of the shapes of the wavefronts above the photosphere.
Integrins are a family of cell surface receptors crucial to fundamental cellular functions such as adhesion, signaling, and viability, deeply involved in a variety of diseases, including the ...initiation and progression of cancer, of coronary, inflammatory, or autoimmune diseases. The natural ligands of integrins are glycoproteins expressed on the cell surface or proteins of the extracellular matrix. For this reason, short peptides or peptidomimetic sequences that reproduce the integrin-binding motives have attracted much attention as potential drugs. When challenged in clinical trials, these peptides/peptidomimetics let to contrasting and disappointing results. In the search for alternative utilizations, the integrin peptide ligands have been conjugated onto nanoparticles, materials, or drugs and drug carrier systems, for specific recognition or delivery of drugs to cells overexpressing the targeted integrins. Recent research in peptidic integrin ligands is exploring new opportunities, in particular for the design of nanostructured, micro-fabricated, cell-responsive, stimuli-responsive, smart materials.
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