Compression Front Research Articles

Observations of compression and fracture in polymer networks subjected to impact loading

Compression and fracture during high rate impacts were quantified in a series of model transparent polymer networks using high speed videography with sub-microsecond temporal resolution. Sample transparency enabled direct quantification of the compression front velocities, as well as time-resolved measurements of crack initiation and growth. Compression front propagation reveals multiple fronts and reflections, indicating a complex loading profile under these conditions. The front velocity is found to correlate linearly with the longitudinal sound velocity. Radial crack growth was measured both in the radial direction as well inward growth towards the sample impact surface. Measurements of the crack length reveal a deceleration period at short times, transitioning to a steady-state velocity at longer times, and crack arrest at still longer times. Crack lengths were found to correlate with quasi-static fracture toughness, and crack velocities were compared to longitudinal sound velocities.

Early-orogenic deformation in the Ionian zone of the Hellenides: Effects of slab retreat and arching on syn-orogenic stress evolution

In this work we report on early-orogenic fracture patterns affecting a Cretaceous to Eocene sedimentary succession exposed in western Greece. These rocks belong to the Cenozoic External Hellenides, which form the western portion of the Aegean orocline. The analysis of fracture type, orientation, and crosscutting relationships provides constraints on the stress and strain patterns during the early stages of orocline formation. The fracture patterns in the study area includes early-orogenic extensional fractures arranged into two mutually orthogonal sets. These developed during progressive burial in the forebulge-foredeep system, ahead of the advancing compressive front. Tectonic solution seams at a high angle to bedding postdate these extensional structures. Solution seams are arranged in different sets, oblique to each other, and developed in the early stages of thrusting and folding. Extensional structures and solution seams are oblique to each other. Their orientation and crosscutting relationships provide evidence for clockwise vertical axis rotation of stress directions with time. This is consistent with the progressive arching of the forebulge-thrust belt system during progressive slab retreat.

Magnetically Induced Current Piston for Generating Extreme-ultraviolet Fronts in the Solar Corona

Abstract Single-pulse, globally propagating coronal fronts, called Extreme-ultraviolet (EUV) waves, were first observed in 1995 by the Extreme-ultraviolet Imaging Telescope and every observed EUV wave since has been associated with a coronal mass ejection (CME). The physical mechanism underlying these waves has been debated for two decades with wave or pseudo-wave theories being advocated. We propose a hybrid model where EUV waves are compressional fronts driven by a reverse electric current layer induced by the time-dependent CME core current. The reverse current layer flows in a direction opposite to the CME core current and is an eddy current layer necessary to maintain magnetic flux conservation above the layer. Repelled by the core current, the reverse current layer accelerates upward so it acts as a piston that drives a compressional perturbation in the coronal regions above. Given a sufficiently fast piston speed, the compressional perturbation becomes a shock that separates from the piston when the piston slows down. Since the model relates the motion of the EUV front to CME properties, the model provides a bound for the core current of an erupting CME. The model is supported and motivated by detailed results from both laboratory experiments and ideal 3D magnetohydrodynamic simulations. Overlaps and differences with other models and spacecraft observations are discussed.

Comprehensive Analysis of the Formation of a Shock Wave Associated with a Coronal Mass Ejection

Abstract On 2014 November 1, a solar prominence eruption associated with a C2.7 class flare and a type II radio burst resulted in a fast partial halo coronal mass ejection (CME). Images acquired in the extreme ultraviolet (EUV) by the Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) and PROBA2/SWAP and in white light (WL) by Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph show the expansion of a bright compression front ahead of the CME. In this work, we present a detailed investigation of the CME-driven shock associated with this event following the early evolution of the compression front observed near the Sun up to the extended corona. Our aim is to shed light on the long-debated issue concerning the location and timing of shock formation in the corona. Through differential emission measure analysis, we derived, for the first time, the compression ratio across the expanding EUV front observed by AIA at different temperature ranges: higher compression ratios corresponded to higher plasma temperature ranges, as expected. Moreover, comparison between up- and downstream temperatures and those expected via adiabatic compression shows that no additional heating mechanisms occurred in the early front expansion phase, implying that the shock formed beyond the AIA field of view. Finally, the analysis of the associated type II radio burst, in combination with the inferred coronal density distribution, allowed us to identify a well-defined region located northward of the CME source region as the site for shock formation and to outline its kinematics in accordance with the evolution of the expanding front as obtained from the EUV and WL data.

Intense Pulses in Relaxing Media with Limited “Memory Time,” Power-Law and Nonanalytic Nonlinearities

Processes that accompany propagation of time-limited pulsed signals in a relaxing medium are investigated for the case of a nonlinear medium with power-law (quadratic or cubic) nonlinearity or nonanalytic nonlinearity (modular or quadratically cubic one). Instead of ordinary integro-differential equations with exponential or fractional-power kernels, a simplified model of a medium with finite “memory time” is used. Such a medium “remembers” its prehistory within a limited time interval, and the corresponding kernel of the integral term is nonzero only within a finite interval. For this model, the problem is reduced to solving a difference-differential equation, which considerably reduces the amount of calculations, as compared to the initial integral equation. The processes that accompany evolution of pulses, namely, the formation of compression and rarefaction shock fronts and the appearance of triangular and trapezoidal nonlinear structures, are described. Effect of relaxation time on these processes is revealed.

Possibility of useful mechanical energy from noise: the solitary wave train problem in the granular chain revisited

A momentary velocity perturbation at an edge of a granular chain with the grains barely touching one another and held between fixed walls propagates as a solitary wave whereas a long lived perturbation, even if it is noisy, ends up as a solitary wave train. Here, we extend our earlier work but with a force instead of a velocity perturbation. Such a perturbation can propagate an extended compression front into the system. We find that a snapshot of the distribution of grain compressions in the solitary wave train shows parabolic as opposed to an approximate exponential decay with the leading edge at the front of the traveling pulse and the trailing edge following it. The system’s time evolution depends on three independent parameters-the material properties, duration of perturbation and the characteristic amplitude of the perturbation. Hence, the coefficients used to describe the parabolic decay of the grain compressions in the solitary wave train depend on these three parameters. When a random finite duration force perturbation is applied we find that the randomness is smoothed out by the system, which in turn suggests that long granular chains (or equivalent systems, such as circuits) can be potentially useful in converting random noisy signals to organized solitary wave trains and hence to potentially usable energy.

MAVEN Observations of Solar Wind‐Driven Magnetosonic Waves Heating the Martian Dayside Ionosphere

AbstractWe present Mars Atmosphere and Volatile EvolutioN observations of large‐amplitude magnetosonic waves propagating through the magnetosheath into the Martian ionosphere near the subsolar point on the dayside of the planet. The observed waves grow in amplitude as predicted for a wave propagating into a denser, charged medium, with wave amplitudes reaching 25 nT, equivalent to ∼40% of the background field strength. These waves drive significant density and temperature variations (∼20% to 100% in amplitude) in the suprathermal electrons and light ion species (H+) that correlate with compressional fronts of the magnetosonic waves. Density and temperature variations are also observed for the ionospheric electrons, and heavy ion species (O+ and O ); however, these variations are not in phase with the magnetic field variations. Whistler waves are observed at compressional wave fronts and are thought to be produced by unstable, anistropic suprathermal electrons. The magnetosonic waves drive significant ion and electron heating down to just above the exobase region. Ion heating rates are estimated to be between 0.03 and 0.2 eVs−1 per ion, and heavier ions could thus gain escape energy if located in this heating region for ∼10–70 s. The measured ionospheric density profile indicates severe ionospheric erosion above the exobase region, and this is likely caused by substantial ion outflow that is driven by the observed heating. The effectiveness of these magnetosonic waves to energize the plasma close to the exobase could have important implications for the long‐term climate evolution for unmagnetized bodies that are exposed to the solar wind.

Propagation and dispersion of shock waves in magnetoelastic materials

Previous studies examining the response of magnetoelastic materials to shock waves have predominantly focused on applications involving pulsed power generation, with limited attention given to the actual wave propagation characteristics. This study provides detailed magnetic and mechanical measurements of magnetoelastic shock wave propagation and dispersion. Laser generated rarefacted shock waves exceeding 3 GPa with rise times of 10 ns were introduced to samples of the magnetoelastic material Galfenol. The resulting mechanical measurements reveal the evolution of the shock into a compressive acoustic front with lateral release waves. Importantly, the wave continues to disperse even after it has decayed into an acoustic wave, due in large part to magnetoelastic coupling. The magnetic data reveal predominantly shear wave mediated magnetoelastic coupling, and were also used to noninvasively measure the wave speed. The external magnetic field controlled a 30% increase in wave propagation speed, attributed to a 70% increase in average stiffness. Finally, magnetic signals propagating along the sample over 20× faster than the mechanical wave were measured, indicating these materials can act as passive antennas that transmit information in response to mechanical stimuli.

Metastable states, relaxation mechanisms, and fracture of liquids under severe loading conditions

The relaxation properties and fracture of glycerol, silicone oil, transformer oil, and water have been studied experimentally under shock wave loading. The power-law strain rate dependences of the stress amplitude and spall strength were found for the compression and rarefaction fronts, respectively. It was shown that temperature has a strong influence on the spall strength of glycerol near the phase transformation temperature. The power laws reflect a self-similar nature of the momentum transfer and fracture mechanisms of liquids that are conventionally observed in solids and governed by the mechanisms of defect-induced structural relaxation. The mechanisms of viscoelasticity are related to the metastable states that may give rise to a collective behavior of displacement field fluctuations (microshearing) in liquids and thus provide a viscoplastic response of liquids under high strain rate loading.

Experimental arguments in favour of heat transfer in compressible fluids by Pressure Gradient Elastic Waves

The results of extensive scientific research show that sound has an effect on thermal processes in gases. The Ranque and Hartmann-Sprenger effects belong to this class of phenomena. Existing conventional theories cannot explain the heat transfer processes in devices based on these effects.The concept of Pressure Gradient Elastic Waves provides a physical description of heat transfer in these processes. Pressure Gradient Elastic Waves are waves of sound type. These waves arise in compressible fluids (in gases) as a result of the existence of a pressure gradient within the volume of the gas, and in the presence of initial density fluctuations (under the influence of sound). Under these conditions the pressure forces act on micro volumes having density fluctuations along the pressure gradient vector. However the resultant forces act on fluctuations of rarefaction and on fluctuations of compression in opposite directions. The compression front of Pressure Gradient Elastic Wave propagates in the direction of increasing pressure, while the rarefaction front propagates in the opposite direction i.e. in the direction of reducing pressure. Thus these waves carry energy from the low pressure zone to the high pressure zone. This heat transfer manifests itself in the heating of the wall bounding the high pressure zone and in the cooling of the low pressure region.The article presents the results of experiments performed by the author on a short vortex chamber and on Sprenger heat tubes. The maximum possible extent of heating and cooling are estimated. It is shown that conventional theories, previously used to explain the Ranque and Hartmann-Sprenger effects, fundamentally cannot explain the results obtained.

A DATA-DRIVEN ANALYTIC MODEL FOR PROTON ACCELERATION BY LARGE-SCALE SOLAR CORONAL SHOCKS

ABSTRACT We have recently studied the development of an eruptive filament-driven, large-scale off-limb coronal bright front (OCBF) in the low solar corona, using remote observations from the Solar Dynamics Observatory’s Advanced Imaging Assembly EUV telescopes. In that study, we obtained high-temporal resolution estimates of the OCBF parameters regulating the efficiency of charged particle acceleration within the theoretical framework of diffusive shock acceleration (DSA). These parameters include the time-dependent front size, speed, and strength, as well as the upstream coronal magnetic field orientations with respect to the front’s surface normal direction. Here we present an analytical particle acceleration model, specifically developed to incorporate the coronal shock/compressive front properties described above, derived from remote observations. We verify the model’s performance through a grid of idealized case runs using input parameters typical for large-scale coronal shocks, and demonstrate that the results approach the expected DSA steady-state behavior. We then apply the model to the event of 2011 May 11 using the OCBF time-dependent parameters derived by Kozarev et al. We find that the compressive front likely produced energetic particles as low as 1.3 solar radii in the corona. Comparing the modeled and observed fluences near Earth, we also find that the bulk of the acceleration during this event must have occurred above 1.5 solar radii. With this study we have taken a first step in using direct observations of shocks and compressions in the innermost corona to predict the onsets and intensities of solar energetic particle events.

Non-equilibrium Thermodynamics of Rayleigh-Taylor instability

Rayleigh-Taylor instability (RTI) has been studied here as a non-equilibrium thermodynamics problem. Air masses with temperature difference of 70K, initially with heavier air resting on lighter air isolated by a partition, are allowed to mix by impulsively removing the partition. This results in interface instabilities, which are traced here by solving two dimensional (2D) compressible Navier-Stokes equation (NSE), without using Boussinesq approximation (BA henceforth). The non-periodic isolated system is studied by solving NSE by high accuracy, dispersion relation preserving (DRP) numerical methods described in Sengupta T.K.: High Accuracy Computing Method (Camb. Univ. Press, USA, 2013). The instability onset is due to misaligned pressure and density gradients and is evident via creation and evolution of spikes and bubbles (when lighter fluid penetrates heavier fluid and vice versa, associated with pressure waves). Assumptions inherent in compressible formulation are: (i) Stokes' hypothesis that uses zero bulk viscosity assumption and (ii) the equation of state for perfect gas which is a consequence of equilibrium thermodynamics. Present computations for a non-equilibrium thermodynamic process do not show monotonic rise of entropy with time, as one expects from equilibrium thermodynamics. This is investigated with respect to the thought-experiment. First, we replace Stokes' hypothesis, with another approach where non-zero bulk viscosity of air is taken from an experiment. Entropy of the isolated system is traced, with and without the use of Stokes' hypothesis. Without Stokes' hypothesis, one notes the rate of increase in entropy to be higher as compared to results with Stokes' hypothesis. We show this using the total entropy production for the thermodynamically isolated system. The entropy increase from the zero datum is due to mixing in general; punctuated by fluctuating entropy due to creation of compression and rarefaction fronts originating at the interface and reflecting from the walls.

Experimental observations of turbulent flame propagation effected by flame acceleration in the end gas of closed combustion chamber

In present study, a new designed constant volume combustion bomb (CVCB) equipped with an orifice plate and visualized by high speed schlieren photography has been employed to study the turbulent flame propagation effected by flame acceleration in the end gas region. The orifice plate is employed to achieve flame acceleration and obtain different turbulent flames at different equivalence ratios. We investigate the flame propagation speed, the formation mechanism of compression front, the influence of flame acceleration on the end-gas as well as the flame structure changes. The results show that the laminar flame can be accelerated and transferred to a wrinkled turbulent flame through the orifice plate significantly and there forms a clear compression front ahead of the turbulent flame. The flame propagation speed without orifice plate shows the trend of initial increase and consequence decrease in the confined space. Moreover, the turbulent flame propagation speed shows the significantly different trend and demonstrates the “M” shape evolution including the self-acceleration, unstable and deceleration process. Finally, the evolution of turbulent flame in the end gas of the confined chamber was described in detail with three stages: compression front formation, flame front distortion and reverse propagation.

Nonideal detonation regimes in low density explosives

Measurements using Velocity Interferometer System for Any Reflector (VISAR) were performed for three high explosives at densities slightly above the natural loose-packed densities. The velocity histories at the explosive/window interface demonstrate that the grain size of the explosives plays an important role. Fine-grained materials produced rather smooth records with reduced von Neumann spike amplitudes. For commercial coarse-grained specimens, the chemical spike (if detectable) was more pronounced. This difference can be explained as a manifestation of partial burn up. In fine-grained explosives, which are more sensitive, the reaction can proceed partly within the compression front, which leads to a lower initial shock amplitude. The reaction zone was shorter in fine-grained materials because of higher density of hot spots. The noise level was generally higher for the coarse-grained explosives, which is a natural stochastic effect of the highly non-uniform flow of the heterogeneous medium. These results correlate with our previous data of electrical conductivity diagnostics. Instead of the classical Zel'dovich–von Neumann–Döring profiles, violent oscillations around the Chapman–Jouguet level were observed in about half of the shots using coarse-grained materials. We suggest that these unusual records may point to a different detonation wave propagation mechanism.

Foam-like compression behavior of fibrin networks.

The rheological properties of fibrin networks have been of long-standing interest. As such there is a wealth of studies of their shear and tensile responses, but their compressive behavior remains unexplored. Here, by characterization of the network structure with synchronous measurement of the fibrin storage and loss moduli at increasing degrees of compression, we show that the compressive behavior of fibrin networks is similar to that of cellular solids. A nonlinear stress-strain response of fibrin consists of three regimes: (1) an initial linear regime, in which most fibers are straight, (2) a plateau regime, in which more and more fibers buckle and collapse, and (3) a markedly nonlinear regime, in which network densification occurs by bending of buckled fibers and inter-fiber contacts. Importantly, the spatially non-uniform network deformation included formation of a moving "compression front" along the axis of strain, which segregated the fibrin network into compartments with different fiber densities and structure. The Young's modulus of the linear phase depends quadratically on the fibrin volume fraction while that in the densified phase depends cubically on it. The viscoelastic plateau regime corresponds to a mixture of these two phases in which the fractions of the two phases change during compression. We model this regime using a continuum theory of phase transitions and analytically predict the storage and loss moduli which are in good agreement with the experimental data. Our work shows that fibrin networks are a member of a broad class of natural cellular materials which includes cancellous bone, wood and cork.

Rarefaction and compressional standing slow mode structures in Mercury's magnetosheath: 3D MHD simulations

We show that slow mode compressional fronts form upstream of the day side magnetopause in MHD simulations of Mercury's magnetosphere. The strongest compressional fronts are located upstream of the magnetopause with strong magnetic shear. Compressional fronts are crossed by magnetic field lines connecting the interplanetary magnetic field and the planet's intrinsic field, their role is to bend the magnetic field in the magnetosheath towards the magnetopause. Besides these compressional fronts, already observed in space and theoretically discussed by various authors for the case of the Earth, we observe the formation of a slow mode standing rarefaction wave spatially growing over a substantial fraction of the distance between the bow shock and the magnetopause. The slow mode source region for the rarefaction waves is located in the magnetosheath, near the bow shock's nose. The generated standing rarefaction waves, however, form even at large distances from the source region along the magnetospheric flanks. They fine-tune the magnetic field line draping and plasma flow around the magnetopause. In ideal MHD the magnetospheres of Mercury, the Earth and the giant planets do closely resemble each other, we therefore expect the mentioned slow mode structures not to be specific to Mercury.

Multipoint observation of the response of the magnetosphere and ionosphere related to the sudden impulse event on 19 November 2007

The aim of this study is to provide a complete scope of a magnetic sudden impulse (SI) event along its way through interplanetary space and the magnetosphere until its arrival to the ground. In our case, we chose the event of 19th November 2007 because of the availability of enough well-located spacecraft at that moment for our purpose. We have used a 16 spacecraft data set. We calculated the mass flux variation and the change in magnetic field components across the discontinuity. Thus, we identified the solar wind discontinuity as a shock. We also calculated the orientation of the solar wind shock front. Then, we examined the effects of the shock front propagation in detail. With this large data set, we obtained a global view of the travelling wave front and identified the effects of the compressional wave front. Thus, we determined in detail the shock front passing through the different parts of the magnetosphere. We described the compressional effects in the bow shock, the magnetosheath, and the magnetopause and we depicted the propagation inside the inner magnetosphere. Moreover, we used an extensive data set from magnetic observatories on the ground and so we studied the global distribution of the SI waveform. Finally, the comparison of the observational facts with those derived from the theoretical model showed a good consistency. On the basis of the waveforms and polarizations of this SI, we determined the location in latitude where ionospheric currents (ICs) changed their sense. And also, we related polarization at ground to polarization measured by GOES spacecraft.

Multiwavelength observation of a large-scale flux rope eruption above a kinked small filament

We analyse multiwavelength observations of a western limb flare (C3.9) occurred in AR NOAA 111465 on 30 April 2012. The high resolution images recorded by SDO/AIA 304, 1600 \AA\ and Hinode/SOT H$\alpha$ show the activation of a mini-filament (rising speed$\sim$40 km s$^{-1}$) associated with kink instability and the onset of a C-class flare near the southern leg of the filament. The first magnetic reconnection occurred at one of the footpoints of the filament causing the breaking of its southern leg. The filament shows unwinding motion of the northern leg and apex in the counterclockwise direction and failed to erupt. A flux-rope (visible only in hot channels, i.e., AIA 131 and 94 \AA\ channels and Hinode/SXT) structure was appeared along the neutral line during the second magnetic reconnection taking place above the kinked filament. Formation of the RHESSI hard X-ray source (12-25 keV) above the kinked filament and simultaneous appearance of the hot 131 \AA\ loops associated with photospheric brightenings (AIA 1700 \AA) suggest the particle acceleration along these loops from the top of the filament. In addition, EUV disturbances/waves observed above the filament in 171 \AA\ also show a close association with magnetic reconnection. The flux rope rises slowly ($\sim$100 km s$^{-1}$) producing a rather big twisted structure possibly by reconnection with the surrounding sheared magnetic fields within $\sim$15-20 minutes, and showed an impulsive acceleration reaching a height of about 80--100 Mm. AIA 171 and SWAP 174 \AA\ images reveal a cool compression front (or CME frontal loop) surrounding the hot flux rope structure.

Frontal compression along the Apennines thrust system: The Emilia 2012 example from seismicity to crustal structure

The evolution of the Apennines thrust-and-fold belt is related to heterogeneous process of subduction and continental delamination that generates extension within the mountain range and compression on the outer front of the Adria lithosphere. While normal faulting earthquakes diffusely occur along the mountain chain, the sparse and poor seismicity in the compressional front does not permit to resolve the ambiguity that still exists about which structure accommodates the few mm/yr of convergence observed by geodetic data. In this study, we illustrate the 2012 Emilia seismic sequence that is the most significant series of moderate-to-large earthquakes developed during the past decades on the compressional front of the Apennines. Accurately located aftershocks, along with P-wave and Vp/Vs tomographic models, clearly reveal the geometry of the thrust system, buried beneath the Quaternary sediments of the Po Valley. The seismic sequence ruptured two distinct adjacent thrust faults, whose different dip, steep or flat, accounts for the development of the arc-like shape of the compressional front. The first shock of May 20 (Mw 6.0) developed on the middle Ferrara thrust that has a southward dip of about 30°. The second shock of May 29 (Mw 5.8) ruptured the Mirandola thrust that we define as a steep dipping (50–60°) pre-existing (Permo-Triassic) basement normal fault inverted during compression. The overall geometry of the fault system is controlled by heterogeneity of the basement inherited from the older extension. We also observe that the rupture directivity during the two main-shocks and the aftershocks concentration correlate with low Poisson ratio volumes, probably indicating that portions of the fault have experienced intense micro-damage.

Crustal structure in the area of the 2002 Molise earthquake: Clues for the evolution of the southern Apennines

AbstractThe Apennines is a mountain belt with paired extension and compression that are explained by the subduction and delamination of the Ionian/Adria lithosphere. During the belt formation, the Adria crust is flexed downward and then incorporated into the wedge after peeling off along some weak level whose origin is still discussed. In southern Apennines, this process seems to have stopped at about 0.7 Ma by the development of a tear in the subducting lithosphere and consequent extension that spread along the mountain range. Seismological data acquired after the 2002 Mw 6.0 Molise strike‐slip earthquake yield modeling the structure of the Adria crust as congealed close to the leading edge of the belt. Tomography images show inverted normal faults within the sedimentary cover (Apulian units). Fault inversion, incomplete close to the compressional front, testifies for a final squeezing of the entire, previously flexed, Apulian continental margin. Pronounced high Vp and low Vp/Vs anomalies are observed at depth greater than 12 km and undoubtfully attributed to heterogeneities of the basement, solving the long‐lasting contention on the nature of similar high‐speed bodies observed beneath the Apennines. Beneath these bodies, we observe a strong reflectivity, suggesting the presence of a fluid‐filled layer in the middle lower crust that may act as a decoupling level in the Apennines tectonics. The deep fluids are also favoring the rupture of strike‐slip earthquakes, like those that occurred in 2002, and the intermediate‐depth seismicity diffused in the Apulian region.