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21.
The coupling of thermal and ideal MHD effects in a sheared magnetic field is investigated. A slab geometry is considered so that the Alfvén mode can be decoupled from the system. With the total perturbed pressure approximately zero, the fast mode is eliminated and a system of linearized equations describing magnetic effects on the slow mode and thermal mode is derived. These modes evolve independently on individual fieldlines. One of the main features of this approach is that the influence of the dense photosphere can be included. A variety of different conditions that simulate the photospheric boundary are presented and the different results are discussed. A choice of field geometry and boundary conditions is made which removes mode rational surfaces so that there are no regions in which parallel thermal conduction can be neglected. This provides a stabilizing mechanism for the thermal mode. Growth rates are reduced by 30–40% and there is complete stabilization for sufficiently short fieldlines. The influence of dynamic and thermal boundary conditions on the formation of prominences is discussed. 相似文献
22.
The influence of wind and river pulses on an estuarine turbidity maximum: Numerical studies and field observations in Chesapeake Bay 总被引:1,自引:0,他引:1
E.?W.?NorthEmail author S.?Y.?Chao L.?P.?Sanford R.?R.?Hood 《Estuaries and Coasts》2004,27(1):132-146
The effect of pulsed events on estuarine turbidity maxima (ETM) was investigated with the Princeton Ocean Model, a three-dimensional
hydrodynamic model. The theoretical model was adapted to a straight-channel estuary and enhanced with sediment transport,
erosion, deposition, and burial components. Wind and river pulse scenarios from the numerical model were compared to field
observations before and after river pulse and wind events in upper Chesapeake Bay. Numerical studies and field observations
demonstrated that the salt front and ETM had rapid and nonlinear responses to short-term pulses in river flow and wind. Although
increases and decreases in river flow caused down-estuary and up-estuary (respectively) movements of the salt front, the effect
of increased river flow was more pronounced than that of decreased river flow. Along-channel wind events also elicited non-linear
responses. The salt front moved in the opposite direction of wind stress, shifting up-estuary in response to down-estuary
winds and vice-versa.
Modeled pulsed events affected suspended sediment distributions by modifying the location of the salt front, near-bottom shear
stress, and the location of bottom sediment in relation to stratification within the salt front. Bottom sediment accumulated
near the convergent zone at the tip of the salt front, but lagged behind the rapid response of the salt front during wind
events. While increases in river flow and along-channel winds resulted in sediment transport down-estuary, only reductions
in river flow resulted in consistent up-estuary movement of bottom sediment. Model predictions suggest that wind and river
pulse events significantly influence salt front structure and circulation patterns, and have an important role in the transport
of sediment in upper estuaries. 相似文献
23.
3D simulations of basin-scale lunar impacts are carried out to investigate: (a) the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins; and (b) the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the South Pole-Aitken (SPA) basin. The simulations demonstrate that a basin-forming impact produces a massive, hot, partially ionized cloud of vapor and melt that expands thermally around the Moon, converging near the basin antipode approximately 1 h after the impact for typical impact parameters. In agreement with previous work, analytic calculations of the interaction of this vapor-melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region. The time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode. The latter produce antipodal shock stresses within the range of 5-25 GPa where stable shock remanent magnetization (SRM) of lunar soils has been found experimentally to occur. Calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources, a possibility that is consistent with enhanced magnetic anomalies observed peripheral to SPA. The latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle occurring near the antipodes of the Imbrium and Serenitatis impacts. Together with converging seismic compressional waves, these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of SPA near the Imbrium antipode, allowing the ascent of magma with enhanced KREEP concentrations. 相似文献
24.
25.
We present two-dimensional numerical magnetohydrodynamics simulations of a coronal X-ray bright point (XBP) caused by a cancelling magnetic feature (CMF). Cancellation is driven by converging motions of two magnetic bipolar sources. These sources are initially disconnected from each other so that both, the CMF and the associated reconnection/heating event (i.e. the XBP), are modelled in a self-consistent way. In the initial state, there is no X-point but two separatrices are present. Hence, the reconnection/heating and the cancellation phases have not yet started. Our numerical experiments end shortly after the converging magnetic bipole has fully cancelled. By this time, reconnection in the inner domain has ceased and occurs only at the base. Solving the energy equation with various heating and cooling terms included, and considering different bottom boundary conditions, reveals that the unrealistically high temperatures produced by Ohmic heating are reduced to more moderate temperatures of 1.5–2 MK consistent with observations of XBPs, if thermal conduction is included and density and temperature are fixed at the base. 相似文献
26.
This paper is a demonstration of how the WKB approximation can be used to help solve the linearised 3D MHD equations. Using Charpit’s method and a Runge?–?Kutta numerical scheme, we have demonstrated this technique for a potential 3D magnetic null point, B=[x,ε y,?(ε+1)z]. Under our cold-plasma assumption, we have considered two types of wave propagation: fast magnetoacoustic and Alfvén waves. We find that the fast magnetoacoustic wave experiences refraction towards the magnetic null point and that the effect of this refraction depends upon the Alfvén speed profile. The wave and thus the wave energy accumulate at the null point. We have found that current buildup is exponential and the exponent is dependent upon ε. Thus, for the fast wave there is preferential heating at the null point. For the Alfvén wave, we find that the wave propagates along the field lines. For an Alfvén wave generated along the fan plane, the wave accumulates along the spine. For an Alfvén wave generated across the spine, the value of ε determines where the wave accumulation will occur: fan plane (ε=1), along the x-axis (0<ε<1) or along the y-axis (ε>1). We have shown analytically that currents build up exponentially, leading to preferential heating in these areas. The work described here highlights the importance of understanding the magnetic topology of the coronal magnetic field for the location of wave heating. 相似文献
27.
Katharine A. Smith Elizabeth W. North Fengyan Shi Shih-Nan Chen Raleigh R. Hood Evamaria W. Koch Roger I. E. Newell 《Estuaries and Coasts》2009,32(4):748-757
Seagrass beds have declined in Chesapeake Bay, USA as well as worldwide over the past century. Increased seston concentrations,
which decrease light penetration, are likely one of the main causes of the decline in Chesapeake Bay. It has been hypothesized
that dense populations of suspension-feeding bivalves, such as eastern oysters (Crassostrea virginica), may filter sufficient seston from the water to reduce light attenuation and enhance seagrass growth. Furthermore, eastern
oyster populations can form large three-dimensional reef-like structures that may act like breakwaters by attenuating waves,
thus decreasing sediment resuspension. We developed a quasi-three-dimensional Seagrass-Waves-Oysters-Light-Seston (SWOLS)
model to investigate whether oyster reefs and breakwaters could improve seagrass growth by reducing seston concentrations.
Seagrass growth potential (SGP), a parameter controlled by resuspension-induced turbidity, was calculated in simulations in
which wave height, oyster abundance, and reef/breakwater configuration were varied. Wave height was the dominant factor influencing
SGP, with higher waves increasing sediment resuspension and decreasing SGP. Submerged breakwaters parallel with the shoreline
improved SGP in the presence of 0.2 and 0.4 m waves when sediment resuspension was dominated by wave action, while submerged
groins perpendicular to the shoreline improved SGP under lower wave heights (0.05 and 0.1 m) when resuspension was dominated
by along-shore tidal currents. Oyster-feeding activity did not affect SGP, due to the oysters’ distance from the seagrass
bed and reduced oyster filtration rates under either low or high sediment concentrations. Although the current implementation
of the SWOLS model has simplified geometry, the model does demonstrate that the interaction between oyster filtration and
along-shore circulation, and between man-made structures and wave heights, should be considered when managing seagrass habitats,
planning seagrass restoration projects, and choosing the most suitable methods to protect shorelines from erosion. 相似文献
28.
Zipper reconnection has been proposed as a mechanism for creating most of the twist in the flux tubes that are present prior to eruptive flares and coronal mass ejections. We have conducted a first numerical experiment on this new regime of reconnection, where two initially untwisted parallel flux tubes are sheared and reconnected to form a large flux rope. We describe the properties of this experiment, including the linkage of magnetic flux between concentrated flux sources at the base of the simulation, the twist of the newly formed flux rope, and the conversion of mutual magnetic helicity in the sheared pre-reconnection state into the self-helicity of the newly formed flux rope. 相似文献
29.
30.