Three-dimensional numerical cosmological radiative transfer in an inhomogeneous medium     

Alexei O. Razoumov  Douglas Scott 《Monthly notices of the Royal Astronomical Society》1999,309(2):287-298

A numerical scheme is proposed for the solution of the three-dimensional (3D) radiative transfer equation with variable optical depth. We show that time-dependent ray tracing is an attractive choice for simulations of astrophysical ionization fronts, particularly when one is interested in covering a wide range of optical depths within a three-dimensional clumpy medium. Our approach combines the explicit advection of radiation variables with the implicit solution of local rate equations given the radiation field at each point. Our scheme is well suited to the solution of problems for which line transfer is not important, and could, in principle, be extended to those situations also. This scheme allows us to calculate the propagation of supersonic ionization fronts into an inhomogeneous medium. The approach can be easily implemented on a single workstation and should also be fully parallelizable.  相似文献   

Residual and tidal circulation revealed by VHF radar surface current measurements in the southern Channel Isles region (English Channel)     

Alexei Sentchev  Philippe Forget  Yves Barbin 《Estuarine, Coastal and Shelf Science》2009

Two very high-frequency radars (VHFRs), operating in the southern Channel Isles region (English Channel) in February–March 2003, provided a continuous 27-day long dataset of surface currents at 2 km resolution over an area extending approximately 20 km offshore. The tidal range in the region of study is one of the highest in the world and the coastal circulation is completely dominated by tides. The radar data resolve two modes which account for 97% of the variability of the surface current velocities, with the major contribution of the first mode. This mode accounts for oscillating tidal currents whereas the second mode represents motions emerging from the interaction of tidal currents with capes and islands (eddy in the vicinity of the Point of Grouin and jet south of Chausey). A fortnightly modulation of the modal amplitudes causes the exceptional (more than 600%) variability of currents which is well captured by the VHFR observations. The radar data revealed that tidal circulation in the region is flood-dominated with a strong asymmetry of current velocity curve. Wind events and fortnightly variability affect the course of tidal cycle by modifying the magnitude and duration of ebb and flood. In addition to expected features of coastal circulation (tidally dominated flow, eddies) and high wind-current coupling, the residual currents revealed a strong cross-shore structure in the mean and a significant variability which has the same order of magnitude.  相似文献   

Particle Acceleration in Collapsing Magnetic Traps with a Braking Plasma Jet     

Alexei Borissov  Thomas Neukirch  James Threlfall 《Solar physics》2016,291(5):1385-1404

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What helicity can tell us about solar magnetic fields     

Alexei A. Pevtsov 《Journal of Astrophysics and Astronomy》2008,29(1-2):49-56

Concept of magnetic/current helicity was introduced to solar physics about 15 years ago. Earlier studies led to discovery of such fundamental properties as hemispheric helicity rule, and role of helicity in magnetic reconnection and solar eruptions. Later, the concept was successfully applied in studies of different solar processes from solar dynamo to flare and CME phenomena. Although no silver bullet, helicity has proven to be a very useful “tool” in answering many still-puzzling questions about origin and evolution of solar magnetic fields. I present an overview of some helicity studies and briefly analyze their findings.  相似文献   

A Comparison Between Nonlinear Force-Free Field and Potential Field Models Using Full-Disk SDO/HMI Magnetogram     

Tilaye Tadesse  T. Wiegelmann  P. J. MacNeice  B. Inhester  K. Olson  A. Pevtsov 《Solar physics》2014,289(3):831-845

Measurements of magnetic fields and electric currents in the pre-eruptive corona are crucial to the study of solar eruptive phenomena, like flares and coronal mass ejections (CMEs). However, spectro-polarimetric measurements of certain photospheric lines permit a determination of the vector magnetic field only at the photosphere. Therefore, there is considerable interest in accurate modeling of the solar coronal magnetic field using photospheric vector magnetograms as boundary data. In this work, we model the coronal magnetic field above multiple active regions with the help of a potential field and a nonlinear force-free field (NLFFF) extrapolation code over the full solar disk using Helioseismic and Magnetic Imager (SDO/HMI) data as boundary conditions. We compare projections of the resulting magnetic field lines with full-disk coronal images from the Atmospheric Imaging Assembly (SDO/AIA) for both models. This study has found that the NLFFF model reconstructs the magnetic configuration closer to observation than the potential field model for full-disk magnetic field extrapolation. We conclude that many of the trans-equatorial loops connecting the two solar hemispheres are current-free.  相似文献   

Lake stratigraphy implies an 80 000 yr delayed melting of buried dead ice in northern Russia     

Mona Henriksen  Jan Mangerud  Alexei Matiouchkov  Aage Paus  John Inge Svendsen 《第四纪科学杂志》2003,18(7):663-679

Sediment cores from lakes Kormovoye and Oshkoty in the glaciated region of the Pechora Lowland, northern Russia, reveal sediment gravity flow deposits overlain by lacustrine mud and gyttja. The sediments were deposited mainly during melting of buried glacier ice beneath the lakes. In Lake Kormovoye, differential melting of dead ice caused the lake bottom to subside at different places at different times, resulting in sedimentation and erosion occurring only some few metres apart and at shifting locations, as further melting caused inversion of the lake bottom. Basal radiocarbon dates from the two lakes, ranging between 13 and 9 ka, match with basal dates from other lakes in the Pechora Lowland as well as melting of ice‐wedges. This indicates that buried glacier ice has survived for ca. 80 000 years from the last glaciation of this area at 90 ka until about 13 ka when a warmer climate caused melting of permafrost and buried glacier ice, forming numerous lakes and a fresh‐looking glacial landscape. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Keck infrared observations of GRO J0422+32 in quiescence     

Mark T. Reynolds  Paul J. Callanan  Alexei V. Filippenko 《Monthly notices of the Royal Astronomical Society》2007,374(2):657-663

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What controls the C iv line profile in active galactic nuclei?     

Alexei Baskin  Ari Laor 《Monthly notices of the Royal Astronomical Society》2005,356(3):1029-1044

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On the origin of the C iv Baldwin effect in active galactic nuclei     

Alexei Baskin  Ari Laor 《Monthly notices of the Royal Astronomical Society》2004,350(2):L31-L35

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Sea Ice Leads Detection Using SARAL/AltiKa Altimeter     

Elena A. Zakharova  Sara Fleury  Kévin Guerreiro  Sascha Willmes  Frédérique Rémy  Alexei V. Kouraev 《Marine Geodesy》2015,38(3):522-533

Sea ice leads play an essential role in ocean-ice-atmosphere exchange, in ocean circulation, geochemistry, and in ice dynamics. Their precise detection is crucial for altimetric estimations of sea ice thickness and volume. This study evaluates the performance of the SARAL/AltiKa (Satellite with ARgos and ALtiKa) altimeter to detect leads and to monitor their spatio-temporal dynamics. We show that a pulse peakiness parameter (PP) used to detect leads by Envisat RA-2 and ERS-1,-2 altimeters is not suitable because of saturation of AltiKa return echoes over the leads. The signal saturation results in loss of 6–10% of PP data over sea ice. We propose a different parameter—maximal power of waveform—and define the threshold to discriminate the leads. Our algorithm can be applied from December until May. It detects well the leads of small and medium size from 200 m to 3–4 km. So the combination of the high-resolution altimetric estimates with low-resolution thermal infra-red or radiometric lead fraction products could enhance the capability of remote sensing to monitor sea ice fracturing.  相似文献