Slow Mode Research Articles

Bifurcations at the Stability Transition of Earthquake Faulting

AbstractTectonic faults typically break in a single rupture mode within the range of styles from slow slip to dynamic earthquake failure. However, in increasingly well‐documented instances, the same fault segment fails in both slow and fast modes within a short period, as in the sequences that culminated in the 2011 Mw = 9.0 Tohoku‐Oki, Japan, and 2014 Mw = 8.2 Iquique, Chile, earthquakes. Why slow slip alternates with dynamic rupture in certain regions but not in others is not well understood. Here, we integrate laboratory experiments and numerical simulations to investigate the physical conditions leading to cycles where the two rupture styles alternate. We show that a bifurcation takes place near the stability transition with sequences encompassing various rupture modes under constant loading rate. The range of frictional instabilities and slip cycles identified in this study represents important end‐members to understand the interaction of slow and fast slip on the same fault segment.

Diffraction at the Open-Ended Dielectric-Loaded Circular Waveguide: Rigorous Approach

An elegant and convenient rigorous approach for solving circular open-ended dielectric-loaded waveguide diffraction problems is presented. It uses the solution of corresponding Wiener-Hopf-Fock equation and leads to an infinite linear system for reflection coefficients (S-parameters) of the waveguide, the latter can be efficiently solved numerically using the reducing technique. As a specific example directly applicable to beam-driven radiation sources based on dielectric-lined capillaries, diffraction of a slow TM symmetrical mode at the open end of a circular waveguide with uniform dielectric filling is considered. A series of such modes forms the wakefield (Cherenkov radiation field) generated by a charged particle bunch during its passage along the waveguide axis. Calculated S-parameters were compared with those obtained from COMSOL simulation and an excellent agreement is shown. This method is expected to be very convenient for analytical investigation of various electromagnetic interactions of Terahertz (THz) waves (both free and guided) and charged particle bunches with slow-wave structures prospective in context of modern beam-driven THz emitters, THz accererators and THz-based bunch manipulation and bunch diagnostic systems.

When local access matters: A detailed analysis of place, neighbourhood amenities and travel choice

How cities can become more conducive to sustainable travel patterns is a pressing matter in urban research. In the built environment–travel literature, several features potentially promoting sustainable mobility remain under-researched. One concerns the precise role of local accessibility – in terms of the number, variety and specific types of amenities – in promoting the slow modes of walking and cycling and in reducing car use. Another is the possibility that the standard assumption of a linear relationship between density and (un)sustainable travel is flawed. To fill these gaps, we explore specific local amenity supply types that stimulate a large share of residents to walk or cycle, avoiding long drives, to accomplish their daily activities. In a case study of the Västra Götaland region in Sweden, we analyse a combination of high-quality micro data from registers and the national travel survey using a stepwise statistical approach. We find evidence of non-linear discontinuities and significant geographical variation in how local accessibility affects travel behaviour. While more neighbourhood amenities contribute to more walking/cycling, the effect of establishing additional amenities varies amongst neighbourhood types and travel variables. Local everyday services, for example, grocery stores and preschools, are critical in triggering change. The findings inform urban planning by showing how a basic supply of essential amenities could contribute to better prerequisites for walking or cycling and reduced car dependence by inducing residents to choose local options.

Computational insights on intensification of hydrodenitrogenation in a trickle bed reactor using periodic flow modulation

Trickle bed reactors (TBRs) are widely used in petroleum industries, which are subjected to stringent environmental laws. Hydrodenitrogenation (HDN), a crucial process for nitrogen removal employed in crude oil processing, is typically carried out in steady state TBRs. Acquainted with this operation, researchers are focused on exploring the efficacy of flow modulation in TBRs for improved reaction conversion. A CFD model of a hydrotreating reactor is used in this work to examine potential benefits of periodic operation for a HDN reaction. Results of continuous flow were compared with time averaged conversions for slow and fast modes of on-off and min–max operations having different split ratios. All flow modulated cases proved to be beneficial than steady state operation with respect to enhanced time averaged conversion. However, this enhancement deteriorated with increase in cycle time for slow mode of operation. Additionally, this was associated with substantial undulations in overall pressure drop, which is detrimental to process safety. Nonetheless, fast mode of on–off and min–max operations at low split ratio resulted in maximum improvement of about 47% and 35%, respectively, which revealed the efficacy of fast mode flow modulation in achieving maximum conversion with lower pressure drop undulations.

A new class of Ion-acoustic solitons that can exist below critical Mach number

It is commonly believed that ion-acoustic solitons can only exist above the critical Mach number in a plasma system. A new class of ion-acoustic solitons that can exist below the critical Mach number is reported for the first time in a three-component plasma consisting of hot Maxwellian electrons, and two counterstreaming ion beams. The analysis is based on the Sagdeev pseudopotential technique, and considers a simple case of two counterstreaming proton beams with equal density and streaming velocity. Linear stability analysis shows that the slow ion-acoustic modes become unstable due to ion beam instability when the beam velocity normalized with the ion acoustic speed, U0, is in the range of 0.55 ≤ U0 ≤ 1.14. It is shown that when the normalized streaming velocity is below or at a threshold value, Uth = 1.14, only the regular solitons having Mach numbers greater than critical Mach number can exist. However, when the streaming velocity exceeds the threshold value (all modes are stable), both regular and the new class of ion-acoustic solitons can exist. A special case of unequal ion densities and unequal streaming velocities of the counterstreaming beams is considered in , and similar effects are found. Hence, the new class of slow ion-acoustic solitons can exist in the parametric regime where the system is stable to counterstreaming ion beams instability. The results could be useful in the interpretation of slow electrostatic solitary waves (ESWs) observed in the magnetosphere.

On the receptivity of surface plasma actuation in high-speed boundary layers

Significant amounts of work have been conducted in the area of plasma flow control, while the receptivity of plasma actuation in high-speed boundary layers has not had much attention over the last two decades. In the present study, the receptivity of a Mach 4.5 flat-plate boundary layer to plasma heating actuation produced by pulsed-DC surface dielectric barrier discharge (SDBD) has been studied by direct numerical simulation (DNS) and stability analysis. With the help of multimode decomposition technology, the amplitude of normal modes can be obtained. The results show that both fast and slow modes can be excited by plasma actuation, and the receptivity maximum is observed near the lower neutral branch. Because the pulsed-DC SDBD actuation is typical periodic pulse signals, when the total power remains constant, the Fourier components with multiples of actuation frequency have the same energy, regardless of the waveform, period, and width of the actuation signal. Such characteristics benefit the robustness of the pulsed-DC SDBD actuator. A theoretical prediction method by combining the receptivity model and linear parabolized stability equations is considered in the present study, and good agreement with the DNS results is achieved.

Anisotropy of Velocity Centroids and the Signature of Different MHD Modes in the Turbulent ISM

Abstract Magnetic turbulence is anisotropic because the directions of motion are constrained by the magnetic field. Such anisotropy can be observed in velocity centroids obtained from spectroscopic observations. We use magnetohydrodynamics (MHD) simulations to produce synthetic spectroscopic observations (position–position–velocity data) and study the anisotropy in the structure function of velocity centroid maps. We decomposed the velocity in the simulations into Alfvén, slow, and fast modes and studied how each of them contributes to the observed anisotropy. We found that when the angle between the line of sight and the mean magnetic field is large the Alfvén mode dominates the observed anisotropy, while for smaller angles the anisotropy is not large enough to be used to probe the magnetization of the medium, and it is dominated by the slow mode. Our results are in fair agreement with the theoretical predictions in Kandel et al.

Analysis of linear viscoelasticity of aging soft glasses

Aging soft glassy materials do not follow time-translational invariance and violate the principles of linear viscoelasticity, such as the relation between the dynamic moduli in the frequency domain and the stress relaxation modulus in the time domain. Using an aqueous suspension of hectorite clay, a model aging soft glassy material, we account for time-dependent behavior by transforming the experimentally obtained stress relaxation and dynamic moduli from the real-time domain to the effective-time domain by normalizing the real-time by a time-dependent relaxation time. We find that the two sets of experiments probe different aging and deformation timescales, and simultaneous analysis provides insights into material behavior over a wide range of timescales. Careful analysis of the dynamic moduli reveals contamination of (slow) aging α modes by (fast) nonaging β modes, which is not clearly evident in the stress relaxation modulus. On the other hand, the stress relaxation measurements probe timescales long enough to characterize the terminal relaxation of the suspension. A toy Maxwell model is used to elucidate the interaction of the slow aging and fast nonaging modes, and their effect of measurements. The synergistic relationship between the two sets of experiments offers guidelines for characterizing time-dependent materials by generalizing the principles of linear viscoelasticity.

Wave Composition, Propagation, and Polarization of Magnetohydrodynamic Turbulence within 0.3 au as Observed by Parker Solar Probe

Abstract Turbulence, a ubiquitous phenomenon in interplanetary space, is crucial for the energy conversion of space plasma at multiple scales. This work focuses on the propagation, polarization, and wave composition properties of the solar wind turbulence within 0.3 au, and its variation with heliocentric distance at magnetohydrodynamic scales (from 10 s to 1000 s in the spacecraft frame). We present the probability density function of propagation wavevectors (PDF (k ∥, k ⊥)) for solar wind turbulence within 0.3 au for the first time: (1) wavevectors cluster quasi-(anti-)parallel to the local background magnetic field for kd i < 0.02, where d i is the ion inertial length; (2) wavevectors shift to quasi-perpendicular directions for kd i > 0.02. Based on our wave composition diagnosis, we find that: the outward/anti-sunward Alfvén mode dominates over the whole range of scales and distances, the spectral energy density fraction of the inward/sunward fast mode decreases with distance, and the fractional energy densities of the inward and outward slow mode increase with distance. The outward fast mode and inward Alfvén mode represent minority populations throughout the explored range of distances and scales. On average, the degree of anisotropy of the magnetic fluctuations defined with respect to the minimum variation direction decreases with increasing scale, with no trend in distance at any scale. Our results provide comprehensive insight into the scenario of transport and transfer of the solar wind fluctuations/turbulence in the inner heliosphere.

Target search and recognition mechanisms of glycosylase AlkD revealed by scanning FRET-FCS and Markov state models

DNA glycosylase is responsible for repairing DNA damage to maintain the genome stability and integrity. However, how glycosylase can efficiently and accurately recognize DNA lesions across the enormous DNA genome remains elusive. It has been hypothesized that glycosylase translocates along the DNA by alternating between a fast but low-accuracy diffusion mode and a slow but high-accuracy mode when searching for DNA lesions. However, the slow mode has not been successfully characterized due to the limitation in the spatial and temporal resolutions of current experimental techniques. Using a newly developed scanning fluorescence resonance energy transfer (FRET)-fluorescence correlation spectroscopy (FCS) platform, we were able to observe both slow and fast modes of glycosylase AlkD translocating on double-stranded DNA (dsDNA), reaching the temporal resolution of microsecond and spatial resolution of subnanometer. The underlying molecular mechanism of the slow mode was further elucidated by Markov state model built from extensive all-atom molecular dynamics simulations. We found that in the slow mode, AlkD follows an asymmetric diffusion pathway, i.e., rotation followed by translation. Furthermore, the essential role of Y27 in AlkD diffusion dynamics was identified both experimentally and computationally. Our results provided mechanistic insights on how conformational dynamics of AlkD-dsDNA complex coordinate different diffusion modes to accomplish the search for DNA lesions with high efficiency and accuracy. We anticipate that the mechanism adopted by AlkD to search for DNA lesions could be a general one utilized by other glycosylases and DNA binding proteins.

Magnetohydrodynamic Waves in the Solar Corona

The corona of the Sun is a unique environment in which magnetohydrodynamic (MHD) waves, one of the fundamental processes of plasma astrophysics, are open to a direct study. There is striking progress in both observational and theoretical research of MHD wave processes in the corona, with the main recent achievements summarized as follows: ▪ Both periods and wavelengths of the principal MHD modes of coronal plasma structures, such as kink, slow and sausage modes, are confidently resolved. ▪ Scalings of various parameters of detected waves and waveguiding plasma structures allow for the validation of theoretical models. In particular, kink oscillation period scales linearly with the length of the oscillating coronal loop, clearly indicating that they are eigenmodes of the loop. Damping of decaying kink and standing slow oscillations depends on the oscillation amplitudes, demonstrating the importance of nonlinear damping. ▪ The dominant excitation mechanism for decaying kink oscillations is associated with magnetized plasma eruptions. Propagating slow waves are caused by the leakage of chromospheric oscillations. Fast wave trains could be formed by waveguide dispersion. ▪ The knowledge gained in the study of coronal MHD waves provides ground for seismological probing of coronal plasma parameters, such as the Alfvén speed, the magnetic field and its topology, stratification, temperature, fine structuring, polytropic index, and transport coefficients.

Amoeboid Swimming Is Propelled by Molecular Paddling in Lymphocytes

Mammalian cells developed two main migration modes. The slow mesenchymatous mode, like crawling of fibroblasts, relies on maturation of adhesion complexes and actin fiber traction, whereas the fast amoeboid mode, observed exclusively for leukocytes and cancer cells, is characterized by weak adhesion, highly dynamic cell shapes, and ubiquitous motility on two-dimensional and in three-dimensional solid matrix. In both cases, interactions with the substrate by adhesion or friction are widely accepted as a prerequisite for mammalian cell motility, which precludes swimming. We show here experimental and computational evidence that leukocytes do swim, and that efficient propulsion is not fueled by waves of cell deformation but by a rearward and inhomogeneous treadmilling of the cell external membrane. Our model consists of a molecular paddling by transmembrane proteins linked to and advected by the actin cortex, whereas freely diffusing transmembrane proteins hinder swimming. Furthermore, continuous paddling is enabled by a combination of external treadmilling and selective recycling by internal vesicular transport of cortex-bound transmembrane proteins. This mechanism explains observations that swimming is five times slower than the retrograde flow of cortex and also that lymphocytes are motile in nonadherent confined environments. Resultantly, the ubiquitous ability of mammalian amoeboid cells to migrate in two dimensions or three dimensions and with or without adhesion can be explained for lymphocytes by a single machinery of heterogeneous membrane treadmilling.

Powering galactic superwinds with small-scale AGN winds

ABSTRACT We present a new implementation for active galactic nucleus (AGN) feedback through small-scale, ultrafast winds in the moving-mesh hydrodynamic code arepo. The wind is injected by prescribing mass, momentum, and energy fluxes across a spherical boundary centred on a supermassive black hole according to available constraints for accretion disc winds. After sweeping-up a mass equal to their own, small-scale winds thermalize, powering energy-driven outflows with dynamics, structure, and cooling properties in excellent agreement with those of analytic wind solutions. Momentum-driven solutions do not easily occur, because the Compton cooling radius is usually much smaller than the free-expansion radius of the small-scale winds. Through various convergence tests, we demonstrate that our implementation yields wind solutions, which are well converged down to the typical resolution achieved in cosmological simulations. We test our model in hydrodynamic simulations of isolated Milky Way – mass galaxies. Above a critical AGN luminosity, initially spherical, small-scale winds power bipolar, energy-driven superwinds that break out of the galactic nucleus, flowing at speeds $\gt 1000 \rm \, km \, s^{-1}$ out to $\sim 10 \, \rm kpc$. These energy-driven outflows result in moderate, but long-term, reduction in star formation, which becomes more pronounced for higher AGN luminosities and faster small-scale winds. Suppression of star formation proceeds through a rapid mode that involves the removal of the highest density, nuclear gas, and through a slower mode that effectively halts halo gas accretion. Our new implementation makes it possible to model AGN-driven winds in a physically meaningful and validated way in simulations of galaxy evolution, the interstellar medium and black hole accretion flows.

Properties of Magnetohydrodynamic Modes in Compressively Driven Plasma Turbulence

We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing the data of MHD simulations into linear MHD modes - namely the Alfven, slow magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture of solenoidal and compressive driving, while varying the Alfven Mach number (MA), plasma beta, and the sonic Mach number from sub-sonic to trans-sonic. We find that the proportion of fast and slow modes in the mode mixture increases with increasing compressive forcing. This proportion of the magnetosonic modes can also become the dominant fraction in the mode mixture. The anisotropy of the modes is analyzed by means of their structure functions. The Alfven mode anisotropy is consistent with the Goldreich-Sridhar theory. We find a transition from weak to strong Alfvenic turbulence as we go from low to high MA. The slow mode properties are similar to the Alfven mode. On the other hand the isotropic nature of fast modes is verified in the cases where the fast mode is a significant fraction of the mode mixture. The fast mode behavior does not show any transition in going from low to high MA. We find indications that there is some interaction between the different modes and the properties of the dominant mode can affect the properties of the weaker modes. This work identifies the conditions under which magnetosonic modes can be a major fraction of turbulent astrophysical plasmas, including the regime of weak turbulence. Important astrophysical implications for cosmic ray transport and magnetic reconnection are discussed.

Slow Coherency Identification and Power System Dynamic Model Reduction by Using Orthogonal Structure of Electromechanical Eigenvectors

Identifying generator coherency with respect to slow oscillatory modes has numerous power system use cases including dynamic model reduction, dynamic security analysis, or system integrity protection schemes (e.g., power system islanding). Despite their popularity in both research and industry, classic eigenvector-based slow coherency techniques may not always return accurate results. The multiple past endeavors to improve their accuracy often lack a solid mathematical foundation. Motivated by these deficiencies, we propose an alternative consistent approach to generator slow coherency. Firstly, a new approach is introduced to accurately detect slow coherent generators by effectively minimizing generic normalized cuts. As a by-product, the new approach can also guide the choice of the number of slow coherent groups. Secondly, it is shown that the combination of the the proposed slow coherency approach and an enhanced version of the inertial generator aggregation method allows to produce accurate dynamic equivalents even if the selected number of generator groups is relatively low.

A unified quantum model susceptible to elucidate the dissimilarity of IR spectral density of dicarboxylic acid crystals: Phthalic and terephthalic acid crystals cases

Over the last decades, several approaches have been developed for elucidating the infrared spectral density of dicarboxylic acid crystals, which has been served as prototype for determining hydrogen bonds dynamics. These approaches differ in how accurately the simulated spectra can superimpose the experimental ones. In this study, we present a superdimer quantum approach susceptible to elucidate the infrared spectral properties of some particular dicarboxylic acid crystals using a newly proposed algorithm, which favors the rule of Davydov coupling in the generation of the spectra. The approach, which is herein effectively applied to terephthalic and phthalic acid dimer crystals, ascribes the non-conventional IR spectral properties of these particular acid crystals to the existence of superdimer structure in their lattices. In this superdimer structure, a strong vibronic coupling mechanism, namely Davydov coupling, takes place between the proton stretching vibrations in the (COOH)₂ cycles. This strong coupling exciton, generated by the resonance arising in the two coupled (COOH)₂ cycles of the aromatic rings of the superdimer, in conjunction with the strong anharmonic coupling between the fast and slow modes of each hydrogen bonds provide a strong support basis for a common explanation of the physical properties of these two different crystalline systems. The numerical simulations, involving the implications of the superdimer model, are systematically correlated with the experimental spectra. A decent agreement between the evaluated spectra and the experimental bandshapes of terephthalic and phthalic dicarboxylic acid crystals was obtained using a set of physically sound parameters as inputs in the theoretical formulation. The superdimer quantum approach thereby underscore the potential of the dynamical cooperative interactions between "Davydov coupling" and "strong anharmonic coupling" mechanisms in the generation of the spectral features of terephthalic and phthalic dicarboxylic acid crystals, suggesting that the congregated effects of these two mechanisms can be considered as the most reliable source of the non-conventional IR spectral properties observed. It is therefore expected that this novel algorithm reduces the discrepancies between the simulated spectra compared to the experimental one and simplify the computation of spectra in more complex hydrogen bonded systems.

Efficient coupling to slow light in a silicon slot waveguide with internal corrugations

We modeled, simulated and experimentally observed efficient slow light coupling to internally corrugated slot Bragg grating (ICSBG) waveguide on a silicon-on-insulator platform. A short step taper is utilized to evanescently couple to the slow light mode near the stopband edge, resulting in minimization of Fabry–Perot reflections due to group index mismatch and a minimum slow light coupling enhancement of ∼1 dB. This is important as it results in a lower insertion loss ICSBG waveguide, which is capable of low loss slow light propagation and therefore is an attractive option as a phase shifter structure for an on-chip slow light enhanced electro-optic modulator or integrated optic sensor applications.

Effect of Electrical Resistivity on the Damping of Slow Sausage Modes

Abstract Recent observations of slow surface sausage modes in a photospheric magnetic pore have been shown to be heavily damped. Numerical calculations have shown that electrical resistivity plays a significant role in this damping process. The aim of the present paper is to make an independent analytical derivation that would confirm the importance of electrical resistivity in the damping of these modes. An analytical dispersion relation in the framework of resistive magnetohydrodynamics is derived for sausage modes in a straight cylinder with a circular cross section and a discontinuous boundary. The effect of electrical resistivity on the damping of slow sausage modes in photospheric pore conditions is then studied, by solving the obtained dispersion relation numerically. The obtained results agree with those from the numerical calculations.

Determinants of transport mode choice in metropolitan areas the case of the metropolitan area of the Valley of Mexico

The following paper examines the determinants of transport mode choice for trips in the Metropolitan Area of the Mexican Valley. Using a data set from the Mexican household travel survey (Encuesta Origen - Destino) conducted in 2017 and multinomial logistic regression, the influence of transportation supply, spatial characteristics and socio-economic factors on individual mode choice is estimated. To our knowledge, this is the first analysis of mobility behaviour in the Metropolitan Area of the Mexican Valley with this newly developed data set.The main findings show that a better Mass Rapid Transit infrastructure is not consistently positively associated with the likelihood of using this service. While a better road infrastructure is promoting the use of Mass Rapid Transit services over driving, it is negatively linked with the likelihood of walking and bicycling. These slow modes are mainly interfered with the distance to the city centre: the further away from the centre, the higher the likelihood of walking and bicycling instead of using the car.With respect to socio-economic characteristics of individuals, women and the young found to have a higher likelihood of using public transport than the car. In contrast, having children is negatively associated with the likelihood of using public transport. Moreover, a higher education of individuals is found to be positively associated with car use, while car availability is negatively associated with all other transport mode options.

Understanding the viscosity decline of poly(amic acid) solution from the correlation point of view

The effects of storage time and temperature on the properties of poly(amic acid) (PAA) solution were re-examined by dynamic and static light scattering. One fast and one slow relaxation modes were observed in fresh prepared semidilute solution. The fast mode corresponds to cooperative diffusion of chain segments inside each “blob” while the slow mode comes from the long-range correlated concentration fluctuation of transient network. Chains relaxation during storage at room temperature (20 °C) leads to the disentanglement of transient network and viscosity drop, thereby only one fast relaxation mode was observed which was related to the mutual diffusion of separated polymer chains. Cooling (to 2 °C) rejuvenates the solution to a certain extent, by which the slow relaxation mode can be regenerated and viscosity re-rises. These results indicate that PAA solutions experience a slow thermodynamic and kinetic transition during storage. Hydrolysis may not be the only reason responsible for the viscosity decrease of the PAA solution, the arrangement of chain conformation during storage should be accounted for.