Wave-induced Transport Research Articles

Solitary wave-induced boulder transport on a beach

The influence of sediment on a slope on boulder transport is studied experimentally at the Hydrodynamics Laboratory of the University of Oslo. Experiments are conducted in a small 3 m long and 10 cm wide wave tank, filled with 5 cm of water. The transport of boulders is induced by breaking solitary waves with an amplitude normalized by water depth a/hw= 0.5. Generated solitary waves propagate towards a 1:10 beach. The experiments are conducted in two set-ups: (i) empty Polymethyl methacrylate (PMMA) bottom of the flume, (ii) the slope covered by a thin layer of 65μm sand. The boulders are represented by concrete blocks of different shape and size. They are placed alternately (one at a time) at different locations on the beach slope with respect to wave breaking point. Transport of boulders and their dynamics is studied with respect to boulder characteristics (size, orientation), their initial position and inclusion of sediment. It is shown that presence of sediment enhances boulder transport. In particular, in this set-up, the presence of sediment increased the boulder transport in 2–5 times. The maximum displacement increase was observed for boulders with the smallest length and height and the largest width initially located at the breaking position. Another interesting result regards the type of boulder motion. The boulders experienced either sliding or turning over. Boulders whose height was at least twice as large as their length exhibited turning-over. This held for boulders placed both on an empty PMMA slope and on a sedimentary slope. However, the largest boulder displacement on an empty PMMA slope occurred due to turning over, while on a sedimentary slope it occurred due to sliding.

Laboratory investigations of wave-induced transport of plastic debris over a rippled bottom

Laboratory experiments are conducted in a wave flume to investigate the effect of water waves on the transport of plastic pellets over a rippled bottom. The horizontal velocities of plastic debris are analyzed over the rippled bottom for different wave conditions and plastic elements with different properties. Laboratory investigations determined the characteristic transport patterns of wave-induced plastic debris with a density of ∼2.0g/cm3 moving along the rippled bottom. In the first, swing-type motion, the grains move only in the ripple trough with velocities lower than 0.10 m/s. For sliding-type movement, the grains move along the entire rippled surface with velocities in the range of 0.10–0.13 m/s. For higher velocities in the range of 0.15–0.20 m/s, a saltation-type motion becomes dominant. The results show that plastic grains may move up to 2–3 cm above the ripple crest depending on hydrodynamic conditions. The analysis shows that for velocity-skewed flows, sliding-type motion and onshore transport dominate. For acceleration-skewed flows, saltation-type motion and offshore transport dominate, which is attributed to higher boundary layer thickness and phase lag effects. The analysis of the relationship between the particle Reynolds number and the thickness of the turbulent boundary layer reveals that for values of Rep≥1000 and a boundary layer thicknessδ≥50 mm saltation-type motion becomes dominant. The direction of transport is affected not only by the density of the sediment and the wave skewness coefficients but also by the dimensions of the bottom ripples. The laboratory investigations also provide insight into the hydrodynamic conditions affecting the transport of plastic debris along the bottom covered with ripples in oscillating nonlinear water flows.

Microplastic retention in marine vegetation canopies under breaking irregular waves

The present study provides indications and underlying drivers of wave-induced transport and retention potential of microplastic particles (MP) in marine vegetation canopies having different densities. The anthropogenic occurrence of MP in coastal waters is well documented in the recent literature. It is acknowledged that coastal vegetation can serve as a sink for MP due to its energy dissipating features, which can mimic a novel ecosystem service. While the transport behavior of MP in vegetation has previously been investigated to some extent for stationary flow conditions, fundamental investigations for unsteady surf zone flow conditions under irregular waves are still lacking. Herein, we demonstrate by means of hydraulic model tests that a vegetation's retention potential of MP in waves increases with the vegetation shoot density, the MP settling velocity and decreasing wave energy. It is found that particles migrating by traction (predominantly in contact with the bed) are trapped in the wake regions around a canopy, whereas suspended particles are able to pass vegetated areas more easily. Very dense canopies can also promote the passage of MP with diameters larger than the plant spacing, as the canopies then show characteristics of a solid sill and avoid particle penetration. The particle migration ability through a marine vegetation canopy is quantified, and the key drivers are described by an empirical expression based on the particle settling velocity, the canopy length and density. The findings of this study may contribute to improved prediction and assessment of MP accumulation hotspots in vegetated coastal areas and, thus, may help in tracing MP sinks. Such knowledge can be considered a prerequisite to develope methods or new technologies to recover plastic pollutants and rehabilitate valuable coastal environments.

Assessing shoreface sediment transport at Costinha beach, Aveiro, Portuguese Northwest coast

Coastal zones are dynamic environments where sediment transport significantly influences erosion, deposition, and ecosystem stability. Accurate modelling of the complex interactions between hydrodynamics, sediment transport, and morphological changes in high-energy shoreface areas is crucial for sustainable coastal management. This study aims to assess sediment dynamics and understand the dominant physical drivers of sediment transport in the shoreface by coupling high-resolution numerical modelling with a shoreface sand tracer experiment. Results from the field experiment showed high diffusion and a net onshore and downdrift movement of the tracer in agreement with the observed hydrodynamic data. The numerical modelling strategy, validated with in situ hydrodynamic data and tracer results, demonstrated good performance and highlighted the dominant modes of shoreface sediment transport. While during low energetic wave conditions, the dominant transport mechanism involves the slow-rate bed load sediment movement toward the shore, under moderate energetic wave conditions, longshore wave-induced suspended transport becomes the prevailing mode of transport. The study provides valuable insights to increase the understanding of sediment transport in the shoreface and highlights the importance of coupling numerical modelling with sediment tracer experiments.

Relationship between Large-Scale Variability of North Pacific Waves and El Niño-Southern Oscillation

Ocean waves are crucial for driving various oceanic processes. In this study, the spatial and temporal distribution of significant wave height (SWH) in the North Pacific (NP) is analyzed using the 42-year ERA5 reanalysis dataset from European Centre for Medium-Range Weather Forecasts (ECMWF). The presence of an ENSO signal is confirmed in wave fields of the North Pacific. Furthermore, the spatial distributions of swells and wind waves are analyzed using the Empirical Orthogonal Function (EOF) method, revealing that waves can transport large-scale signals from the NP to lower latitudes through swells. In addition, our research reveals a relationship between ENSO and Stokes drift in the NP. Stokes drift contributes positively to the maintenance of stable sea surface temperatures (SSTs) by transporting more (less) water towards the equatorial Pacific during El Niño (La Niña year) years. It is further noted that during strong ENSO events, the strength of the Stokes drift anomaly intensifies accordingly, which implies a strong link between wave-induced transport and ENSO.

Wave-induced Mixing and Transport in the Middle Atmosphere-Lower Thermosphere (MLT)

Wave-induced Mixing and Transport in the Middle Atmosphere-Lower Thermosphere (MLT)

Wave-driven electron inward transport in a magnetic nozzle

Plasma flows in divergent magnetic fields resembling a magnetic nozzle can be found over wide scales ranging from astrophysical objects to terrestrial plasma devices. Plasma detachment from a magnetic nozzle is a frequent occurrence in natural plasmas, e.g., plasma ejection from the Sun and release from the Sun’s magnetic field, forming the solar wind. Plasma detachment has also been a challenging problem relating to space propulsion devices utilizing a magnetic nozzle, especially the detachment of the magnetized electrons having a gyro-radius smaller than the system’s scale is required to maintain zero net current exhausted from the system. Here we experimentally demonstrate that a cross-field transport of the electrons toward the main nozzle axis, which contributes to neutralizing the ions detached from the nozzle, is induced by the spontaneously excited magnetosonic wave having the frequency considerably higher than the ion cyclotron frequency and close to the lower hybrid frequency, driving an E × B drift that only effects the electrons. Wave-induced transport and loss have been one of many important issues in plasma physics over the past several decades. Conversely, the presently observed electron inward transport has a beneficial effect on the detachment by reducing the divergence of the expanding plasma beam; this finding will open a new perspective for the role of waves and instabilities in plasmas.

Large deltas, small deltas: Toward a more rigorous understanding of coastal marine deltas

Deltas are subaerial landforms that cap underlying deposits with subaqueous extensions that result from a river feeding sediment directly into a standing body of water at a rate that overwhelms any effective dispersal processes derived from the ambient basin. This definition encapsulates both the terrestrial surface expression and the geological focus on the entire sediment mass. Environmental studies also focus on the ecology of deltaic wetlands, their drowning history, and related sustainability issues including societal considerations, history, and culture. A mean 76 ± 16% drop in hydraulic energy occurs in all subaerial deltas regardless of size, given the break in gradients separating fluvial and deltaic surfaces, driving an ever-decreasing bed-material transport, shallowing of distributary channels and concomitant overbank flooding. A delta's sediment mass grows from the addition of new river loads but can also include aeolian and marine sediment derived from outside the delta domain, growth of peat and other biomass, and inputs from human action. Removal of sediment is via river plumes interacting with marine currents, wave-induced transport, sediment failures and gravity flows, high-tide inundation onto the delta plain, tidal channel widening and deepening, and human action (peat, clay, sand and gravel mining). A delta's trapping efficiency ranges from 0 for small-load rivers that discharge directly into an energetic ocean, to 80% for large deltas, and up to 100% for some semi-enclosed bayhead deltas, including fjords. The global (ensemble) subaerial delta aggradation rate is ∼1.6 mm/y if 70% of the global sediment load exits the river mouth(s), a reminder of how much sediment can be expected to be delivered to the surfaces of global deltas at a time when the 2022 CE sea level rise is ∼4 mm/y. At the planetary scale, deltas are environmentally complex given Earth's range in climate, hydrodynamics, tectonic settings, relative sea-level provinces, sediment input, redistribution processes, and human actions. Under natural conditions, the subaerial portion of deltas adapt to change by advancing, retreating, switching, aggrading, and/or drowning, whereas many modern deltas are structurally constrained by societal needs. The 89 large and mud-rich coastal marine deltas (i.e. subaerial area > 1000 km2) account for 84.3% of Earth's total deltaic area that hosts >89% of all humans occupying deltas, many living within megacities. The 885 medium-size deltas (i.e. subaerial areas 10–1000 km2) account for 15.5% of the global delta area and 10.5% of humans living on deltas, with characteristics that fall between the small and large delta categories. The 1460 small and essentially sandy deltas (1–10 km2), including all fjord deltas, are impacted less from human action (with exceptions) and most are better able to withstand climate change. Recognizing the limits of big data in capturing delta complexity, field data remains a necessary gold standard for site investigators.

Transport of Microplastics From the Daugava Estuary to the Open Sea

This study considers the transport of microplastics (MPs) from inland waters (rivers and lakes) to coastal waters and then to the open sea. A three-dimensional MP Eulerian tracer model based on the HIROMB-BOOS model (HBM) with wave-induced transport and biofouling process is used. Multilayer two-way nested model grids with 3–0.5–0.25–0.05 nautical mile resolutions are applied to resolve relevant riverine–estuarial–coastal hydrodynamics with a focus on the southern waters in the Gulf of Riga. The major river of the Gulf of Riga, Daugava, is governed by the Riga Hydro Power Station (RHPS) with high daily and weekly variability of the runoff creating more intense outflows during its working hours. This gives additional complexity when calibrating this model. The model results are validated against MP observations that are collected on various cruises in the summer of 2018 in the Gulf of Riga. There exists a strong synoptic variability in the observations, which are also reproduced. As the rivers are the primary source of MPs, a special backtracking algorithm was developed to find the most possible source of pollutants at a given location and time. The backtracking algorithm includes optimization with respect to salinity in order to prefer trajectories coming from freshwater and, consequently, MP sources. Lagrangian drift studies are performed for events with high precipitation in the estuary domain when sewer overflow at wastewater treatment plants (WWTPs) can occur, and the results are compared with different MP components in observations.

Enhancing one-dimensional particle-in-cell simulations to self-consistently resolve instability-induced electron transport in Hall thrusters

The advent of high-power Hall thrusters and the increasing interest toward their use as a primary propulsion system for various missions have given a new boost to the efforts aiming at self-consistent predictive modeling of this thruster technology. In this article, we present a novel approach, which allows enhancing the predictive capability of one-dimensional particle-in-cell (PIC) simulations to self-consistently capture the wave-induced electron transport due to the azimuthal instabilities in Hall thrusters. The so-called “pseudo-2D” PIC scheme resulting from this approach is extensively tested in several operating conditions. The results are compared against a well-established 2D3V axial–azimuthal reference case in terms of the axial profiles of the time-averaged plasma properties, the azimuthal electric field fluctuations and their dispersion features, and the contributions of the force terms in the electron azimuthal momentum equation to the cross-field mobility. We have demonstrated that the pseudo-2D PIC provides a prediction of the above aspects that compares very closely in almost all conditions with those from the full-2D simulation. In addition, the sensitivity of the pseudo-2D simulation results to the numerical parameters associated with our approach is assessed in detail. The outcomes of these analyses have casted light on the next steps to further improve the approach.

Net sheet-flow sediment transport rate: Additivity of wave propagation and nonlinear waveshape effects

Net wave-induced sheet-flow sediment transport rate, Qnet, can be produced by two co-existing effects: nonlinear waveshape and wave propagation, which alter the intra-wave boundary layer flow and sediment concentration. A intriguing question is whether the two effects can be simply added (the additivity hypothesis). This paper addresses this question based on simulations of a one-phase numerical model for sheet-flow sediment transport and examinations of the governing equations for boundary layer flow and sediment concentration. The additivity for the mean velocity (or streaming) is first demonstrated under typical field conditions. The main reason is that the two driving forces, i.e., momentum transfer produced by wave propagation and intra-wave asymmetry of flow turbulence produced by nonlinear waveshape, do not affect each other. The net sediment flux is separated into wave- and current-related components. Since the mean concentration profile is determined by the first-order process, the ‘current’-related flux produced by the two effects, following the mean velocity, can be superimposed. After examining the leading Fourier components, the wave-related flux is also found to follow the additivity hypothesis. The findings are confirmed by numerical experiments. The implication of this work is that a practical formula for Qnet can be designed with two separate components for nonlinear-waveshape and wave-propagation effects.

The radial phase variation of reversed-shear and toroidicity-induced Alfvén eigenmodes in DIII-D

The eigenfunction of an instability contains information about energy flow in the wave. In this study, the amplitude and phase of electron cyclotron emission radiometer data from hundreds of DIII-D reversed shear Alfvén eigenmodes (RSAE) and toroidicity-induced Alfvén eigenmodes (TAE) are analyzed along the outboard horizontal midplane. The radial phase profile can be flat, linearly rising or falling, convex or concave; in other words, a wide variety of shapes is observed. For a particular mode, often the radial phase profile remains approximately constant as the mode evolves in time but sometimes it changes rapidly. Many TAEs and some RSAEs have phase profiles that are rather flat where the mode amplitude is largest but rise steadily by at large major radius. Rapid phase changes are observed when the frequencies of an RSAE and TAE overlap and the modes couple. The phase profile depends weakly on the fast-ion gradient that would appear in the absence of wave-induced transport. Linear and quadratic fits to the phase profiles, together with many plasma parameters, are assembled into RSAE and TAE databases. In both cases, large variability is observed. For RSAEs, the strongest phase dependencies are on electron temperature T e, RSAE mode frequency, and the density of carbon impurities. For TAEs, the strongest dependencies are on beam power and major radius of the mode. In general, the average RSAE radial phase profile is essentially flat but the TAE profile has non-zero slope and curvature.

Wave-Induced Transport of a Particle on a Beam Surface

Wave-Induced Transport of a Particle on a Beam Surface

Impacts of Hurricane Irma (2017) on wave-induced ocean transport processes

The intensity of major tropical cyclones has increased during the past decade. Their effect is particularly acute in coastal areas where they cause extensive damage leading to an influx of debris, sediments and waste to the sea. However, most operational coastal ocean models do not represent heavy-wind transport processes correctly if the hydrodynamics is not coupled with the wind-generated waves. This may lead to significant errors in ocean simulations under tropical cyclone conditions. Here, we investigate current–wave interactions during a major hurricane and assess their impact on transport processes. We do that by coupling the unstructured-mesh coastal ocean model SLIM with the spectral wave model SWAN, and applying it to the Florida Reef Tract during Hurricane Irma (September 2017). We show that the coupled model successfully reproduces the wave behavior, the storm surge and the ocean currents during the passage of the hurricane. We then use the coupled and uncoupled wave–current model to simulate the transport of passive drifters. We show that the wave radiation stress gradient alone can lead to changes of up to 1 m/s in the modeled currents, which in turn leads to differences of up to 5 km in the position of drifting material over the duration of the hurricane. The Stokes drift however appears to cause deflections up to 4 times larger and hence dominates wave-induced transport. Wave–current interactions therefore strongly impact the transport of drifting material such as sediments and debris in the aftermath of a hurricane. They should thus be taken into account in order to correctly assess its overall impact.

Effects of different forest cover configurations on reducing the solitary wave-induced total sediment transport in coastal areas: An experimental study

The sediment transport phenomenon is considered to be an important factor in improving coastline hydraulic performance. This research studied the effects of long waves as simulated solitary waves on the sediment transport to estimate the sediment transport rate in different coastal forest cover (CFC) densities. To examine how trees altered the wave characteristics, artificial trees were used experimentally to simulate the aligned and staggered configurations in totally 32 cover densities and 3 different wave heights. Results revealed that the CFC could averagely reduce the total sediment transport rate by up to 41.18% (in the densest case by 54% and 45% in staggered and aligned configurations, respectively, compared with the no-CFC case). The optimum density limit with maximum reduction effect was determined as 40–50% for the staggered and 70–80% for the aligned pattern. The configuration effects on the CFC efficiency were also investigated and showed the superiority of the zigzag configuration over the aligned arrangement. This study also used the laboratory data to develop two equations for calculating the sediment transport rate in the presence of coastal cover.

Late Holocene evolution of São Tomé cape (Rio de Janeiro, Brazil): Insights from geomorphological, geophysical and geochronological data

At wave-dominated coasts it is common to find coastal barriers that might be in a progradational pattern and can compose sedimentary capes. Those prograding coastal barriers can be characterized by beach ridges or barrier spits, or even by a more complex pattern depending on the processes controlling their formation. In this study geomorphological mapping, topographic surveys, ground-penetrating radar profiles and optically stimulated luminescence (OSL) dating were performed on the coastal barrier systems of São Tomé cape (north of Rio de Janeiro state, Brazil). The aim was to reconstruct its spatial and temporal evolution and, by this, better understand the interplay between sedimentary cape formation and its driving mechanisms as well as their function as potential archives for long-term environmental dynamics. The study site is characterized by lagoonal and fluvio-lagoonal depressions, and a lagoon surrounded by beach ridges that are interleaved with barrier spits and erosional truncations. Both types of coastal barriers reach up to 6 m above sea-level. Shallow geophysical analysis identified seven radar facies that characterize the stratigraphy of the targeted morphologies. Beach ridges are mainly composed by planar, seaward dipping, subparallel, continuous reflectors which represents their progradation, while barrier spits have planar to convex up, parallel, landward migrating reflectors. Geochronological results demonstrate that the coastal barriers were formed during the Late Holocene. Beach ridge formation until ~2700 years ago was interrupted by the formation of a barrier spit system between ~2300 and ~2000 years ago. This barrier spit system was responsible for the formation of an enclosed lagoon, the Salgada lagoon, and was truncated by a set of beach ridges formed since ~2000 years ago and continuing to the present shoreline. The different types of coastal barriers indicate a change in hydrodynamic processes during the Late Holocene, with an alternation between the predominance of wave-induced cross-shore transport, responsible for the formation of beach ridges, and the predominance of longshore transport, responsible for the formation of barrier spits. Besides that, erosional truncations and unconformities indicate the interruption of shoreline progradation and the impact of erosion due to the storms. This illustrates the complex and highly dynamic development of São Tomé Cape with wave climate and storms as its most important drivers. More generally, this study shows that sedimentary capes may be regarded as geoarchives with potential for the reconstruction of coastal processes over Holocene timescales.

Modeling the hydrodynamics and morphodynamics of sandbar migration events

A number of models are available for science and engineering purposes that numerically simulate nearshore hydrodynamics and the corresponding morphological evolution. However, the models include adjustable coefficients in parameterizations for physical processes that need to be calibrated, and thus there remains room for improvement by including additional physics. One such model is XBeach, which can simulate erosion during storms with proper calibration based on observations. The modeled sediment transport, especially in the cross-shore direction, is sensitive to the adjustable coefficients, with preferred values that are site and event specific. Here, the skill of XBeach is investigated by comparing 1-dimensional (cross-shore) depth-averaged simulations with observations of waves, currents, and sandbar migration across an Atlantic Ocean beach. Calibration of coefficients improved the agreement of the computed results with observed wave heights, offshore-directed mean currents (undertow), the wave-orbital-velocity third moments (skewness and asymmetry), and onshore/offshore sandbar migration although the proposed coefficient values depend on the parameterizations used. For example, including a variable breaking-wave roller energy model resulted in more skillful predictions of undertow than using the default constant coefficient value. Using the calibrated roller coefficients and corresponding undertow, XBeach simulated the observed offshore migration of the sandbar. Onshore transport in XBeach is driven by non-sinusoidal wave-orbital velocities, and proposed values for coefficients depend on the parameterization used to estimate skewness and asymmetry and the associated transport, as well as on incident wave conditions. XBeach calculations of cross-shore sediment transport rates were compared with those estimated by a commonly used sediment transport formula based on laboratory experiments. The inter-comparison suggests that using a wave-induced onshore transport parameter 3 or 4 times larger than the default value may at least in part compensate for the lack of bottom-boundary-layer-streaming-driven-onshore transport in XBeach.

Longshore drift produced by climate-modulated monsoons and typhoons in the South China Sea

Abstract Monsoons and typhoons impact the tropical coastal zones through their signature on winds and waves, leading to increased vulnerability – through erosion, marine submersion and flooding – of an ever growing coastal population. This study addresses wave-driven coastal impacts in the South China Sea (SCS), particularly along the Vietnam coast. Using 38 years of offshore wave fields from ERA-Interim reanalysis, we assess the seasonal and interannual variability of wave patterns, storminess, and wave-induced alongshore sediment transport (longshore drift). Results suggest a large seasonal coastal impact due to high-energy, northeasterly waves generated by winter monsoon winds. The net annual sediment transport is southward with significant interannual and decadal variations (20% standard deviation with particular years at 40% of the mean), presenting strong correlations with the El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) regime changes. The regime shift of 1998, from a warm to a cold PDO phase characterized by more La Nina events, is associated with an increase in winter wave energy and thus higher sediment transport (about 10%). The typhoon activity in the SCS is partly associated with that of the Pacific Northwest basin, but as a large semi-enclosed sea, presents local differences. It is positively correlated with La Nina in summer and with El Nino Modoki in winter. The summer correlation to ENSO phases is opposite to that of the whole Northwest Pacific due to competing local and remote mechanisms driving cyclone formation and trajectory. For the same reason, the effects of ENSO and PDO phases are opposite in the SCS: during the last cold PDO phase, typhoon frequency was reduced by 20%, with significant net impact on estimated sediment transport. The typhoon contribution to sediment transport is a 15% reinforcement of southward transport due essentially to winter activity. If winter monsoon and typhoons appear to work together on average, their low-frequency variability are out-of-phase. This is particularly clear at decadal scale, as cold PDO phases seem favorable to strong winter monsoon waves but detrimental to SCS typhoon numbers. These results confirm that regional climate variability (together with human factors affecting river sediment supply, coastal management of beach-dune systems, land subsidence, mangrove deforestation) is an essential part of coastal vulnerability that needs to be better assessed.

Importance of Wave Non-linearity for 3D Morphodynamic Modelling

Mengual, B.; Bertin, X., and Martins, K., 2020. Importance of wave non-linearity for 3D morphodynamic modelling. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1201-1205. Coconut Creek (Florida), ISSN 0749-0208.The effect of wave non-linearity on morphological changes of a sandbar is investigated through a realistic application at Duck Beach (North Carolina, USA) of a 3D state-of-the-art process-based morphodynamic model, which couples sediment transport, currents and waves (vortex force formalism). From simplified 1D/2DH models, previous studies highlighted that acceleration skewness of non-breaking waves over a sandbar could promote its progressive onshore migration. This process can counterbalance the offshore migration occurring under breaking waves through the development of strong offshore-directed “undertow” currents near the seabed. Based on the existing literature, an additional bedload flux associated to acceleration skewness of waves is implemented in the 3D model. Numerical experiments with and without this additional term clearly demonstrate the need to account for this supplementary wave-induced transport to reproduce onshore migration phases of the sandbar. Effectively, even a model integrating wave asymmetry effects on bedload flux estimates and 3D wave-current interactions fails to reproduce the observed onshore migration of the sandbar.

Two-step Dropouts of Radiation Belt Electron Phase Space Density Induced by a Magnetic Cloud Event

Abstract We report a two-step dropout event of radiation belt electron phase space density (PSD) induced by a typical magnetic cloud (MC) that drove an intense geomagnetic storm. The first and second steps of PSD dropout occurred, respectively, in the initial and main phases of the storm with a short-time partial recovery between the two dropouts. In this event, the initial phase after the sudden commencement lasted for near 21 hr, which gives an ideal opportunity to investigate the nature of the radiation belt electron dropout by isolating the main phase from any losses occurring during the initial phase. Detailed analysis shows that the first step of the dropout in the initial phase is likely associated with the magnetopause shadowing effect in combination with ultra-low frequency wave-induced outward transport caused by sustaining enhanced dynamic pressure activity before the MC. Comparably, the prolonged strong southward interplanetary magnetic field inside the MC that resulted in the storm main phase is supposed to play an important role in the second step of significant electron losses to the interplanetary space. Additionally, the partial recovery of electron PSD between the two steps of the dropout is possibly due to the acceleration processes via wave-particle interactions with whistler-mode chorus waves. Our study demonstrated that persistently enhanced solar wind dynamic pressure, which is frequently observed inside interplanetary coronal mass ejections and corotating interaction regions, can play an important role in modulating the radiation belt electron dynamics before the storm main phase driven by these solar wind disturbances.