Light-induced states and Autler-Townes splitting of laser-coupled states are common features in the photoionization spectra of laser-dressed atoms. The entangled light-matter character of metastable ...Autler-Townes multiplets, which makes them autoionizing polaritons, however, is still largely unexplored. We employ attosecond transient-absorption spectroscopy in argon to study the formation of polariton multiplets between the 3s−1 4p and several light-induced states. We measure a controllable stabilization of the polaritons against ionization, in excellent agreement with ab initio theory. Using an extension of the Jaynes-Cummings model to autoionizing states, we show that this stabilization is due to the destructive interference between the Auger decay and the radiative ionization of the polaritonic components. These results give new insights into the optical control of electronic structure in the continuum and unlock the door to applications of radiative stabilization in metastable polyelectronic systems.
We present a series of controlled fluid injection experiments in the laboratory on a pre‐stressed natural rough fracture with a high initial permeability (∼10−13 m2) in granite using different fluid ...pressurization rates. Our results show that fluid injection on a fracture with a slight velocity‐strengthening frictional behavior exhibits dilatant slow slip in association with a permeability increase up to ∼41 times attained at the maximum slip velocity of 0.085 mm/s for the highest‐rate injection case. Under these conditions, the slip velocity‐dependent change in hydraulic aperture is a dominant process to explain the transient evolution of fracture permeability, which is modulated by fluid pressurization rate and fracture surface asperities. This leads to the conclusion that permeability evolution can be engineered for subsurface geoenergy applications by controlling the fluid pressurization rate on slowly slipping fractures.
Plain Language Summary
Understanding the evolution of fracture permeability during hydraulic stimulation of subsurface reservoirs is the key to characterizing fluid transport and formulating strategies to limit induced seismicity. Accordingly, there is a significant interest in deciphering how the fluid pressurization rate, a constitutive operational parameter during injection, influences the transient permeability change during fracture slip. We conducted a series of experiments in the laboratory using different fluid pressurization rates on a natural rough fracture in granite under a pre‐stressed state. The fracture had a high initial permeability. Our findings show that when fluid is injected into a fracture with a slight velocity‐strengthening frictional behavior, it causes slow slipping with significant permeability enhancement. The change in hydraulic aperture caused by slip velocity is the main reason for the temporary change in permeability, and this effect is modulated by fluid pressurization rate and fracture surface irregularities. Our results suggest that we can modulate the permeability of subsurface geoenergy reservoirs by controlling the fluid pressurization rate on slowly slipping fractures.
Key Points
We conducted fluid injection experiments on a pre‐stressed natural rough fracture in granite at different pressurization rates
The velocity‐strengthening fracture exhibits slow slip accompanied by a significant increase in permeability during fluid injection
Transient fracture permeability is controlled by injection‐induced slip velocity, modulated by pressurization rate and surface asperities
The rich and complex phenomena of the transport of active particles like microorganisms in shear flows are of great significance to various biological and environmental applications. Recent studies ...have shown that the motility and gyrotaxis of algae could greatly influence their transport in waters. However, little attention has been paid to the initial and transient transport regime when the classical Taylor dispersion model is not applicable. To tackle this problem, we resort to Gill's generalized dispersion model for passive particles like solute, which has the potential for accurately describing the entire transport process. For the first time, we extend Gill's model to the active particles, and the effects of swimming, gyrotaxis, and flow shear on the microorganism dispersion in an open‐channel flow have been thoroughly investigated. We first theoretically solve the transient drift and dispersion coefficients, based on which we obtain analytical solutions for concentration distributions of microorganisms, and further validate these results by numerically solving the governing equation. We find that when there is no flow, the longitudinal dispersion of microorganisms can be weakened by the gravitactic accumulation in the vicinity of water surface, while enhanced by a stronger swimming ability of the microorganisms. The effect of the flow shear does not affect the form of the asymptotic concentration distribution, but can greatly enhance the transient drift velocity and the dispersivity. We further analyze the effect of turbulence on microorganisms' dispersion by combing the direct numerical simulation and the random walk simulation. The increase of turbulence is shown to decrease the vertical non‐uniformity of the concentration distribution, as well as the relative contribution of active behavior to both the drift and Taylor dispersivity during transport.
Key Points
For the first time we extend Gill's dispersion model for active particles
The initial transient transport features can be captured by the new model
The turbulence is seen to decrease the relative contribution of active motions
On the solitary wave paradigm for tsunamis Madsen, Per A.; Fuhrman, David R.; Schäffer, Hemming A.
Journal of Geophysical Research - Oceans,
December 2008, Letnik:
113, Številka:
C12
Journal Article
Recenzirano
Odprti dostop
Since the 1970s, solitary waves have commonly been used to model tsunamis especially in experimental and mathematical studies. Unfortunately, the link to geophysical scales is not well established, ...and in this work, we question the geophysical relevance of this paradigm. In part 1, we simulate the evolution of initial rectangular‐shaped humps of water propagating large distances over a constant depth. The objective is to clarify under which circumstances the front of the wave can develop into an undular bore with a leading soliton. In this connection, we discuss and test various measures for the threshold distance necessary for nonlinear and dispersive effects to manifest in a transient wave train. In part 2, we simulate the shoaling of long smooth transient and periodic waves on a mild slope and conclude that these waves are effectively non‐dispersive. In this connection, we discuss the relevance of finite amplitude solitary wave theory in laboratory studies of tsunamis. We conclude that order‐of‐magnitude errors in effective temporal and spatial duration occur when this theory is used as an approximation for long waves on a sloping bottom. In part 3, we investigate the phenomenon of disintegration of long waves into shorter waves, which has been observed, for example, in connection with the Indian Ocean tsunami in 2004. This happens if the front of the tsunami becomes sufficiently steep, and as a result, the front turns into an undular bore. We discuss the importance of these very short waves in connection with breaking and runup and conclude that they do not justify a solitary wave model for the bulk tsunami.
Using the high‐time‐resolution data from the Magnetospheric Multiscale mission, precursor waves upstream of foreshock transient (FT) shocks are statistically investigated using the four‐spacecraft ...timing method. The wave frequencies and wave vectors determined in the plasma rest frame (PRF) are shown to follow the cold plasma dispersion relation for whistler waves. Combining with the feature of the right‐hand polarization in the PRF, the precursors are identified as whistler‐mode waves around the lower hybrid frequency. The occurrence of whistler precursors is independent of the Alfvén Mach number and the FT shock normal angle. More importantly, all the whistler precursors have group velocities pointing upstream in the shock frame, suggesting the dispersive radiation to be a possible generation mechanism. The study improves the understanding of not only the whistler precursors but also the overall FT shock dynamics.
Plain Language Summary
The characteristics of the precursor waves upstream of foreshock transient (FT) shocks are determined in the plasma rest frame using the multi‐point measurements from the Magnetospheric Multiscale mission with appropriate separation scales. The statistical results demonstrate for the first time that the precursors upstream of FT shocks are lower hybrid frequency whistler‐mode waves. The presence or absence of large amplitude whistler precursors does not depend on the FT shock normal angle and the Alfvén Mach number. These results have important implications on the nature of the whistler precursors and the dynamics of the FT shocks.
Key Points
Precursor waves upstream of foreshock transient shocks are found to follow the whistler wave dispersion relation in the plasma rest frame
The occurrence of whistler precursors is independent of the Alfvén Mach numbers and normal angles of the foreshock transient shocks
The observed wave characteristics are consistent with that the precursors are generated through the dispersive radiation mechanism
Abstract
The presence of water condensation in the lunar polar region has been reported. However, the origin and amount of water remain unknown, and information on the migration mechanism of water is ...key for understanding the delivery of water to the Earth–Moon system. We herein report the first identification of the migration process observed as localized and condensed plumes of water ice/gas mixtures ejected from the lunar surface to space in the polar region based on spectral data derived by SELENE/Kaguya. We identified water and other volatile absorption in 10,200 spectra at the nonpermanent shadowed region during the nighttime when no illumination reached the lunar surface, while nearly horizontal solar light at these regions could illuminate the sky. Ray-tracing analyses suggested that the majority of the observed volatile signal did not come from the lunar surface but from the sky (space). The majority of the identified plumes were observed at or near the hydrogen distribution reported previously. Volatile materials were repeatedly identified in some locations and most of the plume events did not coincide with the major meteoroid streams. Rather, more signals were identified during the winter season in each pole. The timing, duration, location, and temperature simulations all suggest that impact events cannot be their main origin, but rather the volatile sublimation from the subsurface (∼10 cm depth). Overall, it was revealed that the lunar polar region is active in terms of volatile migration, which is significant for understanding water delivery to the Earth–Moon system.
•We explore the role of seasonal melting on a “cold and icy” early Mars.•The valley networks may have been formed through this seasonal melting mechanism.•Eccentricity is an important factor for ...producing significant seasonal meltwater.
The nature of the Late Noachian climate of Mars remains one of the outstanding questions in the study of the evolution of martian geology and climate. Despite abundant evidence for flowing water (valley networks and open/closed basin lakes), climate models have had difficulties reproducing mean annual surface temperatures (MAT) > 273 K in order to generate the “warm and wet” climate conditions presumed to be necessary to explain the observed fluvial and lacustrine features. Here, we consider a “cold and icy” climate scenario, characterized by MAT ∼225 K and snow and ice distributed in the southern highlands, and ask: Does the formation of the fluvial and lacustrine features require continuous “warm and wet” conditions, or could seasonal temperature variation in a “cold and icy” climate produce sufficient summertime ice melting and surface runoff to account for the observed features? To address this question, we employ the 3D Laboratoire de Météorologie Dynamique global climate model (LMD GCM) for early Mars and (1) analyze peak annual temperature (PAT) maps to determine where on Mars temperatures exceed freezing in the summer season, (2) produce temperature time series at three valley network systems and compare the duration of the time during which temperatures exceed freezing with seasonal temperature variations in the Antarctic McMurdo Dry Valleys (MDV) where similar fluvial and lacustrine features are observed, and (3) perform a positive-degree-day analysis to determine the annual volume of meltwater produced through this mechanism, estimate the necessary duration that this process must repeat to produce sufficient meltwater for valley network formation, and estimate whether runoff rates predicted by this mechanism are comparable to those required to form the observed geomorphology of the valley networks.
When considering an ambient CO2 atmosphere, characterized by MAT ∼225 K, we find that: (1) PAT can exceed the melting point of water (>273 K) in topographic lows, such as the northern lowlands and basin floors, and small regions near the equator during peak summer season conditions, despite the much lower MAT; (2) Correlation of PAT > 273 K with the predicted distribution of surface snow and ice shows that melting could occur near the edges of the ice sheet in near-equatorial regions where valley networks are abundant; (3) For the case of a circular orbit, the duration of temperatures >273 K at specific valley network locations suggests that yearly meltwater generation is insufficient to carve the observed fluvial and lacustrine features when compared with the percentage of the year required to sustain similar features in the MDV; (4) For the case of a more eccentric orbit (eccentricity of 0.17), the duration of temperatures >273 K at specific valley network locations suggests that annual meltwater generation may be capable of producing sufficient meltwater for valley network formation when repeated for many years; (5) When considering a slightly warmer climate scenario and a circular orbit, characterized by MAT ∼243 K, we find that this small amount of additional greenhouse warming (∼18 K MAT increase) produces time durations of temperatures >273 K that are similar to those observed in the MDV. Thus, we suggest that peak daytime and seasonal temperatures exceeding 273 K could form the valley networks and lakes with either a relatively high eccentricity condition or a small amount of additional atmospheric warming, rather than the need for a sustained MAT at or above 273 K.
The results from our positive-degree-day analysis suggest that: (1) For the conditions of 25° obliquity, 600 mbar atmosphere, and eccentricity of 0.17, this seasonal melting process would be required to continue for ∼(33–1083) × 103 years to produce a sufficient volume of meltwater to form the valley networks and lakes; (2) Similarly, for the conditions of 25° obliquity, 1000 mbar atmosphere, circular orbit, and ∼18 K additional greenhouse warming, the process would be required to continue for ∼(21–550) × 103 years. Therefore, peak seasonal melting of snow and ice could induce the generation of meltwater and fluvial and lacustrine activity in a “cold and icy” Late Noachian climate in a manner similar to that observed in the MDV. A potential shortcoming of this mechanism is that independent estimates of the required runoff rates for valley network formation are much higher than those predicted by this mechanism when considering a circular orbit, even when accounting for additional atmospheric warming. However, we consider that a relatively higher eccentricity condition (0.17) may produce the necessary runoff rates: for the perihelion scenario in which perihelion occurs during southern hemispheric summer, intense melting will occur in the near-equatorial regions and in the southern hemisphere, producing runoff rates comparable to those required for valley network formation (∼mm/day). In the opposite perihelion scenario, the southern hemisphere will experience very little summertime melting. Thus, this seasonal melting mechanism is a strong candidate for formation of the valley networks when considering a relatively high eccentricity (0.17) because this mechanism is capable of (1) producing meltwater in the equatorial region where valley networks are abundant, (2) continuously producing seasonal meltwater for the estimated time duration of valley network formation, (3) yielding the amount of meltwater necessary to incise the valley networks within this time period, and (4) by considering a perihelion scenario in which half of the duration of valley network formation is spent with peak summertime conditions during perihelion in each hemisphere, higher runoff rates are produced than in a circular orbit and the rates may be comparable to those required for valley network formation.
Wavelet analysis (WA) provides superior time-frequency decomposition of complex signals than conventional spectral analysis tools. To illustrate its usefulness in assessing transient phenomena, we ...applied a custom-developed WA algorithm to laser-Doppler (LD) signals of the cutaneous microcirculation measured at glabrous (finger pulp) and nonglabrous (forearm) sites during early recovery after dynamic exercise. This phase, importantly contributing to the establishment of thermal homeostasis after exercise cessation, has not been adequately explored because of its complex, transient form. Using WA, we decomposed the LD signals measured during the baseline and early recovery into power spectra of characteristic frequency intervals corresponding to endothelial nitric oxide (NO)-dependent, neurogenic, myogenic, respiratory, and cardiac physiological influence. Assessment of relative power (RP), defined as the ratio between the median power in the frequency interval and the median power of the total spectrum, revealed that endothelial NO-dependent (5.87 early recovery; 1.53 baseline;
= 0.005; Wilcoxon signed-rank test) and respiratory (0.71 early recovery; 0.40 baseline;
= 0.001) components were significantly increased, and myogenic component (1.35 early recovery; 1.83 baseline;
= 0.02) significantly decreased during early recovery in the finger pulp. In the forearm, only the RP of the endothelial NO-dependent (1.90 early recovery; 0.94 baseline;
= 0.009) component was significantly increased. WA presents an irreplaceable tool for the assessment of transient phenomena. The relative contribution of the physiological mechanisms controlling the microcirculatory response in the early recovery phase appears to differ in glabrous and nonglabrous skin when compared with baseline; moreover, the endothelial NO-dependent influence seems to play an important role.
We address the applicability of wavelet analysis (WA) in evaluating transient phenomena on a model of early recovery to exercise, which is the only exercise-associated phase characterized by a distinct transient shape and as such cannot be assessed using conventional tools. Our WA-based algorithm provided a reliable spectral decomposition of laser-Doppler (LD) signals in early recovery, enabling us to speculate roughly on the mechanisms involved in the regulation of skin microcirculation in this phase.
Earth’s temporarily-captured orbiters (TCOs) are a sub-population of near-Earth objects (NEOs). TCOs can provide constraints for NEO population models in the 1–10-metre-diameter range, and they are ...outstanding targets for in situ exploration of asteroids due to a low requirement on Δv. So far there has only been a single serendipitous discovery of a TCO. Here we assess in detail the possibility of their discovery with the upcoming Large Synoptic Survey Telescope (LSST), previously identified as the primary facility for such discoveries. We simulated observations of TCOs by combining a synthetic TCO population with an LSST survey simulation. We then assessed the detection rates, detection linking and orbit computation, and sources for confusion. Typical velocities of detectable TCOs will range from 1∘/day to 50∘/day, and typical apparent V magnitudes from 21 to 23. Potentially-hazardous asteroids have observational characteristics similar to TCOs, but the two populations can be distinguished based on their orbits with LSST data alone. We predict that a TCO can be discovered once every year with the baseline moving-object processing system (MOPS). The rate can be increased to one TCO discovery every two months if tools complementary to the baseline MOPS are developed for the specific purpose of discovering these objects.
•The discoverability of Earth’s temporary moons with LSST is assessed.•With a tailored approach, a temporary moon is detected with LSST every three months.•Rapid follow-up is needed to constrain the orbit for fast in situ missions.
SUMMARY
Flat slab subduction has been ascribed to a variety of causes, including subduction of buoyant ridges/plateaus and forced trench retreat. The former, however, has irregular spatial ...correlations with flat slabs, while the latter has required external forcing in geodynamic subduction models, which might be insufficient or absent in nature. In this paper, we present buoyancy-driven numerical geodynamic models and aim to investigate flat slab subduction in the absence of external forcing as well as test the influence of overriding plate strength, subducting plate thickness, inclusion/exclusion of an oceanic plateau and lower mantle viscosity on flat slab formation and its evolution. Flat slab subduction is reproduced during normal oceanic subduction in the absence of ridge/plateau subduction and without externally forced plate motion. Subduction of a plateau-like feature, in this buoyancy-driven setting, enhances slab steepening. In models that produce flat slab subduction, it only commences after a prolonged period of slab dip angle reduction during lower mantle slab penetration. The flat slab is supported by mantle wedge suction, vertical compressive stresses at the base of the slab and upper mantle slab buckling stresses. Our models demonstrate three modes of flat slab subduction, namely short-lived (transient) flat slab subduction, long-lived flat slab subduction and periodic flat slab subduction, which occur for different model parameter combinations. Most models demonstrate slab folding at the 660 km discontinuity, which produces periodic changes in the upper mantle slab dip angle. With relatively high overriding plate strength or large subducting plate thickness, such folding results in periodic changes in the dip angle of the flat slab segment, which can lead to periodic flat slab subduction, providing a potential explanation for periodic arc migration. Flat slab subduction ends due to the local overriding plate shortening and thickening it produces, which forces mantle wedge opening and a reduction in mantle wedge suction. As overriding plate strength controls the shortening rate, it has a strong control on the duration of flat slab subduction, which increases with increasing strength. For the weakest overriding plate, flat slab subduction is short-lived and lasts only 6 Myr, while for the strongest overriding plate flat slab subduction is long-lived and exceeds 75 Myr. Progressive overriding plate shortening during flat slab subduction might explain why flat slab subduction terminated in the Eocene in western North America and in the Jurassic in South China.
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