波粒相互作用导致环电流质子沉降的卫星共轭观测          下载免费PDF全文

王杰  袁志刚  余雄东  薛祖祥 《地球物理学报》2020,63(6):2131-2140

波粒相互作用是环电流损失的重要机制之一,但波粒相互作用导致的环电流离子沉降而损失迄今为止缺乏直接的观测证据.基于磁层及电离层卫星的协同观测,本文报道了发生在2015年9月7日,由电磁离子回旋波(EMIC波)导致环电流质子沉降的共轭观测事件.在等离子体层的内边界,Van Allen Probe B卫星观测到,存在EMIC波的区域和不存在EMIC波的区域相比,离子通量的投掷角分布的各向异性变弱.我们将Van Allen Probe B卫星沿着磁力线投影到电离层高度,同时在该投影区域内DMSP 16卫星在亚极光区域观测到环电流质子沉降.而且,通过从理论上计算质子弹跳平均扩散系数,我们进一步证实观测的EMIC波确实能将环电流质子散射到损失锥中.本文的研究工作为EMIC波导致环电流质子沉降提供了直接的观测证据,揭示了环电流衰减的重要物理机制:EMIC波将环电流质子散射到损失锥中,从而沉降到低高度大气层中而损失.  相似文献   

Geoheritage at the small scale: tidal-zone bubble-sand structures as diagnostic paleo-environmental indicators,and their geoheritage significance     

V. Semeniuk  M. Brocx 《Australian Journal of Earth Sciences》2019,66(6):913-922

In identifying sites of geoheritage significance, commonly there has been an emphasis on the larger-scale features. However, the story of geology and the significant features that are critical to unravel geological processes and geological history are commonly small in scale. This contribution focuses on bubble sand and bubble-sand structures as features that are small-scale but nonetheless important to geology, and hence are of geoheritage significance. Bubble sand and bubble-sand structures are ubiquitous on modern beaches and tidal flats, occurring in the uppermost tidal zone of sandy beaches, as a distinct layer in a shoaling beach-to-dune stratigraphy, and are a diagnostic indicator of upper-tidal conditions where a rising tide and a concomitantly rising water-table interacts with the upper swash-zone wave processes. On sandy tidal flats, bubble sand and bubble-sand structures may occur in the mid- to upper-tidal zones; here they are also diagnostic indicators of tidal conditions, forming during a rising tide where a rising water-table forces air upwards to be trapped in moist sand. If found in ancient sequences, bubble-sand structures are a powerful environmental indicator of tidal conditions and, for beach sequences, an indicator of the high-tide level and sea level. Bubble-sand structures have been found in a number of ancient sequences throughout the geological record as far back as the Neoproterozoic, e.g. within beach-to-dune stratigraphy in Pleistocene limestones of the Perth Basin and in southeastern USA, and in tidal-flat sands of the Mesozoic Broome Sandstone of the Canning Basin. The bubble-sand structure is a significant geological tool for use in paleo-environmental and paleo-oceanographic reconstructions, and determination of the position of a paleo-water-table. Given the rarity of their preservation, these occurrences of bubble-sand structures are of geoheritage significance in their own right and, depending on age of sequence and how common they are in the region, they may be nationally significant or globally significant.  相似文献   

Numerical Study on Tidal Mixing in the Bohai Sea     

Yifei Zhao  Ting Yu  Hu Wang 《Marine Geodesy》2019,42(1):46-63

The spatial and temporal variability of tidal mixing in Bohai Sea is studied using a numerical approach. In calculating tidal mixing, accurate barotropic tidal current is obtained via a harmonic analysis package utilizing the simulated current output from a high-resolution regional ocean model. And a “small-scale” roughness map is adopted to describe the detailed topographic features of Bohai Sea. It is shown that the tidal mixing estimated in Bohai Sea is much higher than the level of global background, and fluctuates considerably at some regions within a single day. In Liaodong Bay, Bohai Bay and Bohai Strait, the mixing varies greatly, with the peak value of O (10^?2) m² s^?1. The order of magnitude of mixing in Laizhou Bay is about O (10^?5～10^?3) m² s^?1. Mixing with background level of O (10^?5) m² s^?1 only appears in central area. Result also shows that rough topography plays relatively a more important role than tidal current in enhancing diapycnal mixing in Bohai Sea. The distributions of tidal mixing in selected sections reveal that the vertical stratification in Bohai Sea is not obvious, generally renders a barotropic structure.  相似文献   

周期性的或伴有贯穿流的西边界流跨隙流动的迟滞变异过程     

宋新新  袁东亮  王铮 《海洋学报(英文版)》2019,38(4):90-96

使用1.5层准地转约化重力模式研究了周期性的或伴有贯穿流的西边界流跨隙流动的迟滞变异过程。当西边界流变化的周期比罗斯贝波在缺口处调整的时间尺度大得多时，在雷诺数增加和减少过程的霍夫分叉点都发生延迟，从而产生新的雷诺数迟滞区间；并且西边界流流态转变的临界值变化显著；且周期强迫越短，雷诺数迟滞区间越大。当西边界流变化的周期与罗斯贝波在缺口处调整的时间尺度相当时，西边界流在缺口的流态呈无迟滞的周期性变化，且西边界流入侵西海盆的程度随周期减少而变小。此外，当贯穿流的流量大于西边界流的一半时，会显著影响西边界流在缺口处的迟滞变异过程；西边界流向西入侵程度和流态转变发生的临界雷诺值均发生变化，且贯穿流流量越大变化越大。  相似文献   

Storm-induced turbidity currents on a sediment-starved shelf: Insight from direct monitoring and repeat seabed mapping of upslope migrating bedforms     

Alexandre Normandeau  Daniel Bourgault  Urs Neumeier  Patrick Lajeunesse  Guillaume St-Onge  Louis Gostiaux  Cédric Chavanne 《Sedimentology》2020,67(2):1045-1068

The monitoring of turbidity currents enables accurate internal structure and timing of these flows to be understood. Without monitoring, triggers of turbidity currents often remain hypothetical and are inferred from sedimentary structures of deposits and their age. In this study, the bottom currents within 20 m of the seabed in one of the Pointe-des-Monts (Gulf of St. Lawrence, eastern Canada) submarine canyons were monitored for two consecutive years using Acoustic Doppler Current Profilers. In addition, multibeam bathymetric surveys were carried out during deployment of the Acoustic Doppler Current Profilers and recovery operations. These new surveys, along with previous multibeam surveys carried out over the last decade, revealed that crescentic bedforms have migrated upslope by about 20 to 40 m since 2007, despite the limited supply of sediment on the shelf or river inflow in the region. During the winter of 2017, two turbidity currents with velocities reaching 0·5 m sec⁻¹ and 2·0 m sec⁻¹, respectively, were recorded and were responsible for the rapid (<1 min) upstream migration of crescentic bedforms measured between the autumn surveys of 2016 and 2017. The 200 kg (in water) mooring was also displaced 10 m down-canyon, up the stoss side of a bedform, suggesting that a dense basal layer could be driving the flow during the first minute of the event. Two other weaker turbidity currents with speeds <0·5 m sec⁻¹ occurred, but did not lead to any significant change on the seabed. These four turbidity currents coincided with strong and sustained wind speed >60 km h⁻¹ and higher than normal wave heights. Repeat seabed mapping suggests that the turbidity currents cannot be attributed to a canyon-wall slope failure. Rather, sustained windstorms triggered turbidity currents either by remobilizing limited volumes of sediment on the shelf or by resuspending sediment in the canyon head. Turbidity currents can thus be triggered when the sediment volume available is limited, likely by eroding and incorporating canyon thalweg sediment in the flow, thereby igniting the flow. This process appears to be particularly important for the generation of turbidity currents capable of eroding the lee side of upslope migrating bedforms in sediment-starved environments and might have wider implications for the activity of submarine canyons worldwide. In addition, this study suggests that a large external trigger (in this case storms) is required to initiate turbidity currents in sediment-starved environments, which contrasts with supply-dominated environments where turbidity currents are sometimes recorded without a clear triggering mechanism.  相似文献   

Deposition and preservation of fluvio-tidal shallow-marine sandstones: A re-evaluation of the Neoproterozoic Jura Quartzite (western Scotland)     

Bruce K. Levell  Howard D. Johnson  Daniel S. Collins  Marijn van Cappelle 《Sedimentology》2020,67(1):173-206

The 2 to 5 km thick, sandstone-dominated (>90%) Jura Quartzite is an extreme example of a mature Neoproterozoic sandstone, previously interpreted as a tide-influenced shelf deposit and herein re-interpreted within a fluvio-tidal deltaic depositional model. Three issues are addressed: (i) evidence for the re-interpretation from tidal shelf to tidal delta; (ii) reasons for vertical facies uniformity; and (iii) sand supply mechanisms to form thick tidal-shelf sandstones. The predominant facies (compound cross-bedded, coarse-grained sandstones) represents the lower parts of metres to tens of metres high, transverse fluvio-tidal bedforms with superimposed smaller bedforms. Ubiquitous erosional surfaces, some with granule–pebble lags, record erosion of the upper parts of those bedforms. There was selective preservation of the higher energy, topographically-lower, parts of channel-bar systems. Strongly asymmetrical, bimodal, palaeocurrents are interpreted as due to associated selective preservation of fluvially-enhanced ebb tidal currents. Finer-grained facies are scarce, due largely to suspended sediment bypass. They record deposition in lower-energy environments, including channel mouth bars, between and down depositional-dip of higher energy fluvio-ebb tidal bars. The lack of wave-formed sedimentary structures and low continuity of mudstone and sandstone interbeds, support deposition in a non-shelf setting. Hence, a sand-rich, fluvial–tidal, current-dominated, largely sub-tidal, delta setting is proposed. This new interpretation avoids the problem of transporting large amounts of coarse sand to a shelf. Facies uniformity and vertical stacking are likely due to sediment oversupply and bypass rather than balanced sediment supply and subsidence rates. However, facies evidence of relative sea level changes is difficult to recognise, which is attributed to: (i) the areally extensive and polygenetic nature of the preserved facies, and (ii) a large stored sediment buffer that dampened response to relative sea-level and/or sediment supply changes. Consideration of preservation bias towards high-energy deposits may be more generally relevant, especially to thick Neoproterozoic and Lower Palaeozoic marine sandstones.  相似文献   

潮汐和风对马六甲海峡海流的影响     

范锦晓  胡松 《热带海洋学报》2020,39(4):13-24

基于非结构有限体积法海洋模型FVCOM(Finite-Volume Community Ocean Model), 建立了马六甲海峡及其毗邻海域高分辨率水动力数值模型, 研究了风和潮流作用下的余环流结构以及水体输运特征。结果表明, 马六甲海峡航道中央潮流运动以往复流为主, 边缘存在旋转流; 主要研究区域内落潮流速略大于涨潮流速, 东南窄道处流速最大; 因峡道束窄变浅, 在涨落潮过程中潮流发生汇聚与分离; 主要研究区域东南段存在3个显著的潮致余环流; 东北季风驱动时模型响应为海峡海流整体向西北方向流动, 西南季风时反之; 季风期间潮致表层余环流结构被破坏, 但底层余流仍存在水平环流结构, 且随着风速增加, 底层余环流的数目、大小、形状、位置均会产生变化; 季风过渡期余环流结构也会发生部分改变, 尤其是小潮期间风场影响效果显著。  相似文献   

On the morphology of radial sand ridges     

Weina Zhang  Xiaohe Zhang  Huiming Huang  Yigang Wang  Sergio Fagherazzi 《地球表面变化过程与地形》2020,45(11):2613-2630

We use a hydrodynamic model applied to an idealized fan-shaped basin to explore the morphology and dynamics of radial sand ridges in a convergent coastal system. A positive morphological feedback between channel incision and flow redistribution is responsible for the formation of the channel-ridge pattern. The selection mechanism of bottom wavelength is associated with flow concentration in the deeper part of the channels. Our results are compared to sediment and hydraulic dynamics in the radial sand ridges (RSRs) in China. In a convergent, sloping basin the tangentially averaged tidal velocity peaks at 47 km from the apex. This distance is similar to the arc distance, 62 km, where the RSRs are most incised. An offshore shift in tidal phase results in stronger flows near the north coastline, explaining the presence of asymmetric channel patterns. A numerical stability analysis indicates that small radial oscillations with a wavelength of 10° to 15° maximize the velocity in the troughs. This oscillation wavelength also emerges in the RSRs, which display a peak in spectral energy at a radial wavelength between 25° to 37.5°. High-resolution numerical simulations in the RSRs confirm that flow concentration occurs in the deeper part of the channels, keeping them flushed. We therefore conclude that the RSRs display morphometric characteristics similar to other tidal incisions, like tidal inlets and intertidal channels. This result further supports the dominant role of tidal prism and related peak velocities in incising coastal landscapes. © 2020 John Wiley & Sons, Ltd.  相似文献   

Application of computational fluid dynamics simulation for submarine oil spill     

YANG Zhenglong  YU Jianxing  LI Zhigan  CHEN Haicheng  JIANG Meirong  CHEN Xi 《海洋学报(英文版)》2018,37(11):104-115

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基于SODA资料的南海表层风能输入的空间分布与长期趋势研究     

杨倩  崔超然  张宇  刘志宇  管玉平  黄瑞新 《热带海洋学报》2018,37(6):41-48

海面风不仅是驱动上层海洋运动的主要动力, 其能量也是维持海洋表层流动的主要机械能来源。为了分析南海表层流风能输入的变化, 用SODA(Simple Ocean Data Assimilation)(1901—2010)资料估算了风向南海表层流(表层地转流+表层非地转流)的能量输入。结果表明, 风向南海表层流、表层地转流和表层非地转流输入的能量总体均呈减少趋势, 110年间分别减小了约56%、65%和49%。导致风能输入减小的最主要因素是风应力的减弱(减小了35%)。由于南海受季风系统的控制, 风向表层流及其各成分输入的能量呈现出显著的季节性变化。冬季风能输入最强, 高值区位于南海西部及北部区域, 呈一个显著的“回力镖”状结构。这些结果对深入认识南海环流具有理论意义。  相似文献