Compression Front Research Articles

Radio Signature of the Strong Compression between a Streamer and a Coronal Hole Boundary

We present evidence of the first detection of the radio signature at metric wavelengths of the strong compression between a helmet streamer (HS) and the boundary of a coronal hole (CH) using radio observations from the Callisto MEXICO-LANCE and ALASKA-HAARP systems and white-light observations obtained by the STEREO-A/COR1-COR2 coronagraphs. The event occurred very close to the Sun (∼3.4 solar radii) and produced an intense and unusually broad drifting radio feature at metric wavelengths after a downward-drifting band of emission related to a metric Type II radio burst. The compression is caused by the interaction between an expanding structure (coronal mass ejection/shock) and the HS against the CH boundary. Observations in white light show a sharp compressive feature that propagates radially outward, while STEREO-A/EUVI images show loop oscillations at the same position angle, indicating that the interaction occurs across a range of heights. The loop oscillations cease when the compressive front loses its sharp boundary. This transition indicates a reduction of the density compression at the front and the cessation of the radio emission.

River linear inversion to assess drainage base-level fall history in North-western Apennines and implications on the Alessandria Basin tectonic activity

Drainage network systems are responsive elements to recent active tectonics from among all the topographic features. In geodynamically active areas, fluvial landscapes can record different processes through the formation and current presence of features related to spatial-temporal variation in base-level fall and vertical incision of stream channels. This study focuses on the tectonic evolution of the Alessandria Basin, a synorogenic tectonic basin located at the junction between the Alps and the Apennines, that experienced progressive subsidence during the overthrusting of the Monferrato Thrust Front (the westernmost outer arc of the Apennine belt) onto the Po Foreland Basin. Even though several studies have assessed the Neogene tectonic evolution at a regional scale, rates and timing of the Quaternary activity are still poorly understood in terms of both Alps/Apennines uplift and activity of the compressive front of the Monferrato Arc. In this paper, we applied the method of the river profile linear inversions to reconstruct the base-level fall history of 6 catchments that drain into the Alessandria Basin. We used nine 10Be-derived basin-average denudation rates to constrain the erodibility parameter needed to infer base-level fall rates from χ-transformed river profiles. The results describe the tectonic history of the area in the last ∼5 Ma, documenting increases in base-level fall rate with an initial peak between 3 and 2.5 Ma, and a second between 2 and 1.5 Ma. While the first peak is coeval with the uplift phase that involved most of the northern-central Apennine, the second one suggests an acceleration in subsidence of the Alessandria Basin concurrently with the uplift of the Monferrato Thrust Front.

Active Shortening Simultaneous to Normal Faulting Based on GNSS, Geophysical, and Geological Data: The Seismogenic Ventas de Zafarraya Fault (Betic Cordillera, Southern Spain)

AbstractThe central Betic Cordillera, southern Spain, is affected by an uplift related to the NNW–SSE Eurasia‐Nubia convergence and shallow ENE–WSW orthogonal extension accommodated by the extensional system of the Granada Basin. The combination of geophysical, geodetic, and geological data reveals that the southwestern boundary of this extensional system is a seismically active compressional front extending from the W to the SW of the Granada Basin. The near‐field Global Navigation Satellite System data determine NNE–SSW shortening of up to 2 mm/yr of the compressional front in the Zafarraya Polje. In this setting, the normal Ventas de Zafarraya Fault developed as a result of the bending‐moment extension of the Sierra de Alhama antiform and was last reactivated during the 1884 Andalusian earthquake (Mw 6.5). The uplift in the central Betic Cordillera together with the subsidence in the Western Alborán Basin may facilitate a westward to southwestward gravitational collapse through the extensional detachment of the Granada Basin. The heterogeneous crust of the Betic Cordillera would generate the compressional front, which is divided into two sectors: thrusting to the west, and folding associated with buttressing to the south. Our results evidence that basal detachments, linking extensional fault activity with compressional fronts, may determine the activity of local surface structures and the geological hazard in densely populated regions.

Active Collapse in the Central Betic Cordillera: Development of the Extensional System of the Granada Basin

The Betic Cordillera was formed by the collision between the Alboran Domain and the South Iberian paleomargin in the frame of the NW–SE convergent Eurasia–Nubia plate boundary. The central region is undergoing a heterogeneous extension that has not been adequately analysed. This comprehensive study addressed it by collecting structural geologic, seismologic, and geodetic data. The region west of the Sierra Nevada is deformed by the extensional system of the Granada Basin, which facilitates E–W to NE–SW extension. Moreover, the southern boundary of Sierra Nevada is affected by a remarkable N–S extension related to E–W normal to normal–dextral faults affecting the shallow crust. However, geologic and geodetic data suggest that the western and southwestern Granada Basin boundary constitutes a compressional front. These data lead to the proposal of an active extensional collapse from the uplifted Sierra Nevada region to the W–SW–S, over an extensional detachment. The collapse is determined by the uplift of the central Betics and the subsidence in the Alboran Basin due to an active subduction with rollback. Our results indicate that the central Betic Cordillera is a good example of ongoing extensional collapse in the general context of plate convergence, where crustal thickening and thinning simultaneously occur.

Seismic source identification of the 9 November 2022 Mw 5.5 offshore Adriatic sea (Italy) earthquake from GNSS data and aftershock relocation

The fast individuation and modeling of faults responsible for large earthquakes are fundamental for understanding the evolution of potentially destructive seismic sequences. This is even more challenging in case of buried thrusts located in offshore areas, like those hosting the 9 November 2022 Ml 5.7 (Mw 5.5) and ML 5.2 earthquakes that nucleated along the Apennines compressional front, offshore the northern Adriatic Sea. Available on- and offshore (from hydrocarbon platforms) geodetic observations and seismological data provide robust constraints on the rupture of a 15 km long, ca. 24° SSW-dipping fault patch, consistent with seismic reflection data. Stress increase along unruptured portion of the activated thrust front suggests the potential activation of longer portions of the thrust with higher magnitude earthquake and larger surface faulting. This unpleasant scenario needs to be further investigated, also considering their tsunamigenic potential and possible impact on onshore and offshore human communities and infrastructures.

Slow-mode rarefaction and compression fronts in the Hermean magnetosphere: From MESSENGER insights to future BepiColombo observations

Context. Standing slow-mode rarefaction and compression front structures may appear in the Mercury magnetosheath under particular solar wind conditions. Aims. The aim of the study is to identify the wind conditions required for the formation of slow-mode structures (SMS) in the Mercury magnetosphere by comparing MESSENGER magnetometer data and magnetohydrodynamics simulations. Methods. We used the magnetohydrodynamics code PLUTO in spherical coordinates to reproduce the interaction of the solar wind with the Mercury magnetosphere. First, the optimal wind conditions for the SMS formation were identified with respect to the orientation of the interplanetary magnetic field (IMF) and dynamic pressure. Next, the magnetic field calculated in the simulations along the MESSENGER trajectory was compared to MESSENGER magnetometer data to identify tracers of the satellite encounter with the SMS. Results. Optimal wind conditions for the formation of SMS require that the IMF is oriented in the northward or radial directions. The MESSENGER orbit on 8th September 2011 takes place during wind conditions that are close to the optimal configuration for SMS formation near the north pole, leading to the possible intersection of the satellite trajectory with the SMS. MESSENGER magnetometer data show a rather strong decrease in the magnetic field module after the satellite crossed nearby the compression front that is observed in the simulation, providing indirect evidence of the SMS.

The role of pore fluids in supershear earthquake ruptures

The intensity and damage potential of earthquakes are linked to the speed at which rupture propagates along sliding crustal faults. Most earthquakes are sub-Rayleigh, with ruptures that are slower than the surface Rayleigh waves. In supershear earthquakes, ruptures are faster than the shear waves, leading to sharp pressure concentrations and larger intensities compared with the more common sub-Rayleigh ones. Despite significant theoretical and experimental advances over the past two decades, the geological and geomechanical controls on rupture speed transitions remain poorly understood. Here we propose that pore fluids play an important role in explaining earthquake rupture speed: the pore pressure may increase sharply at the compressional front during rupture propagation, promoting shear failure ahead of the rupture front and accelerating its propagation into the supershear range. We characterize the transition from sub-Rayleigh to supershear rupture in fluid-saturated rock, and show that the proposed poroelastic weakening mechanism may be a controlling factor for intersonic earthquake ruptures.

Shock compression of semiflexible polymers.

We employ molecular dynamics simulations to investigate the shock compression of linear semiflexible polymers. While the propagation velocity of a shock primarily depends on density, both chain rigidity and chain orientation significantly influence the shock width and the final temperature of the system. In general, the shock wave triggers molecular buckling in chains oriented perpendicular to the compression front. Following the passage of the front, the semiflexible chains buckle with a wavelength that decreases with the compression speed as λm ∼ up-0.2, and subsequent relaxation leads to a banana-like liquid crystal phase. In ordered systems with molecules oriented perpendicular to the compression front, the shock width increases by a factor of up to 10 compared to a similar isotropic system, resulting in enhanced shock energy absorption. These findings indicate that chain stiffness plays a critical role in the impact absorption properties of polymeric materials.

Failed Solar Eruption of a Multithermal Flux Rope

Abstract A magnetic flux rope (FR), hosting hot plasma, is thought to be central to the physics of coronal mass ejections. Such FRs are widely observed with passbands of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory that are sensitive to emission from the hot plasma around 10 MK. In contrast, observations of warmer (around 1 MK) counterparts of FRs are sparse. In this study, we report the failed eruption of a multithermal FR, hosting both hot and warm plasma. On 2015 May 1, a hot channel appeared in the AIA high-temperature passbands out of the southeastern solar limb to the south of a nearby flare, and then erupted outward. During the eruption, it rotated perpendicular to the erupting direction. The hot channel stopped erupting, and disappeared gradually, showing a failed eruption. During the hot channel eruption, a warm channel appeared sequentially in the AIA low-temperature passbands. It underwent a similar evolution, including the failed eruption, rotation, and disappearance, to the hot channel. A bright compression front is formed in front of the warm channel eruption in AIA low-temperature images. Under the hot and warm channel eruptions, a small flare occurred, upon which several current sheets, connecting the erupting channels and the underneath flare, formed in the AIA high-temperature passbands. Investigating the spatial and temporal relation between the hot and warm channels, we suggest that both channels twist together, constituting the same multithermal FR that has plasma with the high and low temperatures.

On the critical threshold for detonation

This paper considers the stress threshold required to shock an explosive so that it runs to full detonation. At present it is assumed that a reaction zone composes localised burning regions that increase in density as pressure is increased. However, a series of burn rate experiments and investigations into the shock response of explosives indicate a critical stress level at which response changes. The stress at which the theoretical shear strength is exceeded under shock loading is called the Weak Shock Limit. This is the gateway to the strong shock region in which a compression front travels faster than an elastic wave. In this region experiments suggest that the response becomes independent of microstructure and that there is a transition in an energetic crystal's electronic state. This results in pre-explosion conductivity and luminescence, consistent with a reduced bandgap in the homogeneous flow. The rate-limiting steps in explosive decomposition are generally accepted to follow Arrhenius kinetics, and it is hypothesised here that the activation energy then decreases above the weak shock limit. This allows reaction to move from temperature-dependant burn at mesoscale hot spots, to temperature-dependant, homogeneous reaction behind the shock wave. The shock can then quickly build to detonation even through the stress for this transition is an order of magnitude less than the Von Neumann spike pressure. This hypothesis has a range of implications that may be tested in further experiments. If explicitly proven, the weak shock limit will allow the description of the transition to, and propagation of, detonation within continuum codes. It will thus represent a vital quantity to consider for the safe handling of energetic materials.

Tracing the ICME plasma with a MHD simulation

The determination of the chemical composition of interplanetary coronal mass ejection (ICME) plasma is an open issue. More specifically, it is not yet fully understood how remote sensing observations of the solar corona plasma during solar disturbances evolve into plasma properties measured in situ away from the Sun. The ambient conditions of the background interplanetary plasma are important for space weather because they influence the evolutions, arrival times, and geo-effectiveness of the disturbances. The Reverse In situ and MHD APproach (RIMAP) is a technique to reconstruct the heliosphere on the ecliptic plane (including the magnetic Parker spiral) directly from in situ measurements acquired at 1 AU. It combines analytical and numerical approaches, preserving the small-scale longitudinal variability of the wind flow lines. In this work, we use RIMAP to test the interaction of an ICME with the interplanetary medium. We model the propagation of a homogeneous non-magnetised (i.e. with no internal flux rope) cloud starting at 800 km s−1at 0.1 AU out to 1.1 AU. Our 3D magnetohydrodynamics (MHD) simulation made with the PLUTO MHD code shows the formation of a compression front ahead of the ICME, continuously driven by the cloud expansion. Using a passive tracer, we find that the initial ICME material does not fragment behind the front during its propagation, and we quantify the mixing of the propagating plasma cloud with the ambient solar wind plasma, which can be detected at 1 AU.

Traveling fronts in dissipative granular chains and nonlinear lattices

We consider an infinite chain of particles with nonlinear elastic and dissipative nearest neighbors interactions. Assuming the existence of a traveling front between uniformly compressed (or stretched) states, we obtain jump conditions relating the wave speed and limiting particle velocities to the relative displacements at infinity. Using this result, we characterise compression fronts in chains of touching beads, for viscoelastic contact laws that include a nonlinear elastic force (generalised Hertz contact) and viscous dissipation. We compute compression fronts numerically for the generalised Kuwabara–Kono model in which the viscous contact force is proportional to the derivative of the elastic force, without precompression of the chain. Steady fronts are obtained both as the end result of the compression of one end of the chain and using a shooting method which provides numerically exact traveling waves. Depending on the magnitudes of contact damping and strain applied downstream, we obtain either overdamped (monotonic) or underdamped (oscillatory) compression fronts. To explain this transition and approximate the front profiles, we consider a continuum limit valid when the exponent of the contact nonlinearity is close to (and above) unity. Using multiscale expansions, we formally derive two different amplitude equations for long waves, a Burgers equation with logarithmic nonlinearity, and a logarithmic Korteweg–de Vries (KdV)–Burgers equation for small contact damping. Both models possess traveling front solutions that are in good agreement with the front profiles computed numerically in the granular chain. The analysis of the logarithmic KdV–Burgers equation allows one to approximate the critical damping corresponding to the transition from underdamped to overdamped fronts.

Fast electron transport dynamics and energy deposition in magnetized, imploded cylindrical plasma.

Inertial confinement fusion approaches involve the creation of high-energy-density states through compression. High gain scenarios may be enabled by the beneficial heating from fast electrons produced with an intense laser and by energy containment with a high-strength magnetic field. Here, we report experimental measurements from a configuration integrating a magnetized, imploded cylindrical plasma and intense laser-driven electrons as well as multi-stage simulations that show fast electrons transport pathways at different times during the implosion and quantify their energy deposition contribution. The experiment consisted of a CH foam cylinder, inside an external coaxial magnetic field of 5 T, that was imploded using 36 OMEGA laser beams. Two-dimensional (2D) hydrodynamic modelling predicts the CH density reaches , the temperature reaches 920 eV and the external B-field is amplified at maximum compression to 580 T. At pre-determined times during the compression, the intense OMEGA EP laser irradiated one end of the cylinder to accelerate relativistic electrons into the dense imploded plasma providing additional heating. The relativistic electron beam generation was simulated using a 2D particle-in-cell (PIC) code. Finally, three-dimensional hybrid-PIC simulations calculated the electron propagation and energy deposition inside the target and revealed the roles the compressed and self-generated B-fields play in transport. During a time window before the maximum compression time, the self-generated B-field on the compression front confines the injected electrons inside the target, increasing the temperature through Joule heating. For a stronger B-field seed of 20 T, the electrons are predicted to be guided into the compressed target and provide additional collisional heating.This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 2)’.

Temporal evolution of short-lived penumbral microjets

Context. Penumbral microjets (PMJs) is the name given to elongated jet-like brightenings observed in the chromosphere above sunspot penumbrae. They are transient events that last from a few seconds to several minutes, and their origin is presumed to be related to magnetic reconnection processes. Previous studies have mainly focused on their morphological and spectral characteristics, and more recently on their spectropolarimetric signals during the maximum brightness stage. Studies addressing the temporal evolution of PMJs have also been carried out, but they are based on spatial and spectral time variations only. Aims. Here we investigate, for the first time, the temporal evolution of the polarization signals produced by short-lived PMJs (lifetimes < 2 min) to infer how the magnetic field vector evolves in the upper photosphere and mid-chromosphere. Methods. We use fast-cadence spectropolarimetric observations of the Ca II 854.2 nm line taken with the CRisp Imaging Spectropolarimeter at the Swedish 1 m Solar Telescope. The weak-field approximation (WFA) is used to estimate the strength and inclination of the magnetic field vector. By separating the Ca II 854.2 nm line into two different wavelength domains to account for the chromospheric origin of the line core and the photospheric contribution to the wings, we infer the height variation of the magnetic field vector. Results. The WFA reveals larger magnetic field changes in the upper photosphere than in the chromosphere during the PMJ maximum brightness stage. In the photosphere, the magnetic field inclination and strength undergo a transient increase for most PMJs, but in 25% of the cases the field strength decreases during the brightening. In the chromosphere, the magnetic field tends to be slightly stronger during the PMJs. Conclusions. The propagation of compressive perturbation fronts followed by a rarefaction phase in the aftershock region may explain the observed behavior of the magnetic field vector. The fact that such behavior varies among the analyzed PMJs could be a consequence of the limited temporal resolution of the observations and the fast-evolving nature of the PMJs.

Umbral chromospheric fine structure and umbral flashes modelled as one: The corrugated umbra

Context.The chromosphere of the umbra of sunspots features an assortment of dynamic fine structures that are poorly understood and often studied separately. Small-scale umbral brightenings (SSUBs), umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral dark fibrils are found in any observation of the chromosphere with sufficient spatial resolution performed at the correct umbral flash stage and passband. Understanding these features means understanding the dynamics of the umbral chromosphere.Aims.We aim to fully understand the dynamics of umbral chromosphere through analysis of the relationships between distinct observed fine features and to produce complete models that explain both spectral profiles and the temporal evolution of the features. We seek to relate such understanding to umbral flashes.Methods.We studied the spatial and spectral co-evolution of SDFs, SSUBs, and umbral flashes in Ca II8542 Å spectral profiles. We produced models that generate the spectral profiles for all classes of features using non-local thermodynamic equilibrium radiative transfer with a recent version of the NICOLE inversion code.Results.We find that both bright SSUBs and dark SDF structures are described with a continuous feature in the parameter space that is distinct from the surroundings even in pixel-by-pixel inversions. We find a phase difference between such features and umbral flashes in both inverted line-of-sight velocities and timing of the brightenings. For umbral flashes themselves we resolve, for the first time in inversion-based semi-empirical modelling, the pre-flash downflows, post-flash upflows, and the counter-flows present during the umbral flash phase. We further present a simple time-dependent cartoon model that explains the dynamics and spectral profiles of both fine structure, dark and bright, and umbral flashes in umbral chromospheres.Conclusions.The similarity of the profiles between the brightenings and umbral flashes, the pattern of velocities obtained from the inversions, and the phase relationships between the structures all lead us to put forward that all dynamic umbral chromospheric structures observed to this date are a locally delayed or locally early portion of the oscillatory flow pattern that generates flashes, secondary to the steepening large-scale acoustic waves at its source. Essentially, SSUBs are part of the same shock or merely compression front responsible for the spatially larger umbral flash phenomenon, but out of phase with the broader oscillation.

Paleomagnetism of the Middle Cenozoic Mula Basin (East Tibet): Evidence for km‐Scale Crustal Blocks Rotated by Midlower Crust Drag

AbstractExisting models describing continental crust deformation require the coexistence of strike‐slip faults and crustal blocks rotating between them, although the dimension and shape of the blocks and the location and offset of the faults are mostly unconstrained. Here we report on the paleomagnetism of middle Cenozoic (<45 Ma) continental red beds exposed along the 40 km long and 2–8 km wide NW‐trending Mula basin (East Tibet), unconformably lying above Triassic marine strata and plutons and mildly deformed by two subparallel thrust faults. A tectonic magnetic fabric and magnetic lineations subhorizontal and parallel to the compressive fronts show that thrust tectonics guided basin formation and continued soon after sediment deposition. Characteristic and high‐temperature components isolated at 17 sites support a positive fold test and suggest primary detrital magnetization acquisition. The comparison with East Asia paleopoles defines several 2–5 km wide crust fragments yielding variable rotations from ~30° counterclockwise to ~90° clockwise without clear rotation trend. No strike‐slip fault with offset exceeding 1 km occurs among blocks, and no regional‐scale strike‐slip fault is documented at basin vicinity, implying that the East Tibet rotation pattern is different from all existing block rotation models. A regional high thermal flow and vigorous geothermal activity are consistent with the occurrence of a ductile crust layer identified by seismological data at 13–30 km depths. We suggest that midlower crust, flowing SE‐ward toward Indochina, drag upper crust fragments that were randomly rotated depending on the local torque exerted on lower block boundaries by a ductile crust flow.

The Herschel view of the dense core population in the Ophiuchus molecular cloud

Context. Herschel observations of nearby clouds in the Gould Belt support a paradigm for low-mass star formation, starting with the generation of molecular filaments, followed by filament fragmentation, and the concentration of mass into self-gravitating prestellar cores. In the case of the Ophiuchus molecular complex, a rich star formation activity has been documented for many years inside the clumps of L1688, the main and densest cloud of the complex, and in the more quiescent twin cloud L1689 thanks to extensive surveys at infrared and other wavelengths. Aims. With the unique far-infrared and submillimeter continuum imaging capabilities of the Herschel Space observatory, the closeby (d = 139 pc) Ophiuchus cloud was extensively mapped at five wavelengths from 70 to 500 μm with the aim of providing a complete census of dense cores in this region, including unbound starless cores, bound prestellar cores, and protostellar cores. Methods. Taking full advantage of the high dynamic range and multi-wavelength nature of the Herschel data, we used the multi-scale decomposition algorithms getsources and getfilaments to identify an essentially complete sample of dense cores and filaments in the cloud and study their properties. Results. The densest clouds of the Ophiuchus complex, L1688 and L1689, which thus far are only indirectly described as filamentary regions owing to the spatial distribution of their young stellar objects, are now confirmed to be dominated by filamentary structures. The tight correlation observed between prestellar cores and filamentary structures in L1688 and L1689 supports the view that solar-type star formation occurs primarily in dense filaments. While the sub clouds of the complex show some disparities, L1689 being apparently less efficient than L1688 at forming stars when considering their total mass budgets, both sub clouds share almost the same prestellar core formation efficiency in dense molecular gas. We also find evidence in the Herschel data for a remarkable concentric geometrical configuration in L1688 which is dominated by up to three arc-like compression fronts and has presumably been created by shockwave events emanating from the Sco OB2 association, including the neighboring massive (O9V) star σ Sco. Conclusions. Our Herschel study of the well-documented Ophiuchus region has allowed us to further analyze the influence of several early-type (OB) stars surrounding the complex, thus providing positive feedback and enhancing star formation activity in the dense central part of the region, L1688.

Present-day structural frame of the Santander Massif and Pamplona Wedge: The interaction of the Northern Andes

Based on regional geology and stress tensor analysis from fault slickensides, we propose a structural model to explain the present-day configuration of a crystalline core in the northern Andes of Colombia (the Santander Massif - SM). The SM has undergone transpressional tectonics in a domino-style, controlled by longitudinal sinistral strike-slip faults as the Bucaramanga Fault. The tectonic style also exhibits NE-SW trending transverse inner faults with dextral strike-slip kinematics. The transpressional regime of the SM differs from the compressive regime that characterizes nearby blocks (northern Perijá Range, southern Floresta Massif, and eastern Pamplona Wedge). Transpressional structures result from a regional W-E horizontal compression corresponding to the current stress field as determined here by cross-cutting relations, mainly observed in the Pamplona Wedge. Stress tensors also show maximum horizontal stress (SHmax) in radial pattern towards the wedge deformation front, whose influence spreads over the SM western border. The W-E regional compression also explains the tectonic syntaxes formed by the Pamplona Wedge to the north and the Sierra Nevada de Güicán or El Cocuy to the south. These two areas are characterized by the opposite outward vergence of their compressive deformation fronts.

Localized Heating of the Martian Topside Ionosphere Through the Combined Effects of Magnetic Pumping by Large‐Scale Magnetosonic Waves and Pitch Angle Diffusion by Whistler Waves

AbstractWe present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of periodic ( 25 s) large‐scale (hundreds of km) magnetosonic waves propagating into the Martian dayside upper ionosphere. These waves adiabatically modulate the superthermal electron distribution function, and the induced electron temperature anisotropies drive the generation of observed electromagnetic whistler waves. The localized (in altitude) minimum in the ratio pe/ ce provides conditions favorable for the local enhancement of efficient wave‐particle interactions, so that the induced whistlers act back on the superthermal electron population to isotropize the plasma through pitch angle scattering. These wave‐particle interactions break the adiabaticity of the large‐scale magnetosonic wave compressions, leading to local heating of the superthermal electrons during compressive wave “troughs.” Further evidence of this heating is observed as the subsequent phase shift between the observed perpendicular‐to‐parallel superthermal electron temperatures and compressive wave fronts. This heating mechanism may be important at other unmagnetized bodies.

Design of carbonated r‐MgO‐PC binder system: Effect of r‐MgO replacement level and water‐to‐binder ratio

AbstractThis study investigates the influence of reactive MgO (r‐MgO) replacement levels and water‐to‐binder (w/b) ratios on the compressive strength, carbonation front, and pore structure of mortars with r‐MgO and Portland cement (r‐MgO‐PC) as binder. The experimental results reveal that an increase in the w/b ratio decreases the carbonation degree for mortars with 20% r‐MgO, but increases the carbonation degree for mortars with 60% r‐MgO. The r‐MgO replacement level together with the w/b ratio and carbonation degree impacts the type and quantity of the carbonation products, namely Mg‐calcite, nesquehonite and the hydrated amorphous Mg carbonate, which affects the pore structure and compressive strength of the matrix. The maximum strength for mortars with 20% r‐MgO occurs at the lowest w/b ratio (0.45), while the maximum strength for mortars with 60% r‐MgO occurs at the intermediate w/b ratio (0.65). A design of r‐MgO replacement level, w/b ratio in conjunction with the drying and carbonation curing regimes is needed for further advancement and application r‐MgO‐PC system.