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   

Study of Lake Baikal Ice Cover from Radar Altimetry and In–Situ Observations     

Alexei V. Kouraev  Elena A. Zakharova  Frédérique Rémy  Andrey Ya. Suknev 《Marine Geodesy》2015,38(3):477-486

Comparison of ENVISAT and SARAL missions data shows that AltiKa can be successfully used for ice discrimination methodology and extension of ice conditions time series. Due to shorter wavelength and large bandwidth (480 MHz) which leads to a higher sensitivity to different surface conditions, AltiKa shows more clearly the separation between open water and various ice types. We observe significant decrease of backscatter (25–30 dB) in late spring for both ENVISAT and SARAL and discuss it in the context of ice metamorphism. There is a clear need to continue and expand our dedicated field studies of lake Baikal ice cover to better assess influence of ice structure on altimetric signal.  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   

Neutrinos from SN1987A: flavor conversion and interpretation of results     

Cecilia Lunardini  Alexei Yu. Smirnov   《Astroparticle Physics》2004,21(6):703-720

After recent results from solar neutrino experiments and KamLAND we can definitely say that neutrinos from SN1987A underwent flavor conversion, and the conversion effects must be taken into account in the analysis of the data. Assuming the normal mass hierarchy of neutrinos we calculate the permutation factors p for the Kamiokande-2, IMB and Baksan detectors. The conversion inside the star leads to p=0.28–0.32; the oscillations in the matter of the Earth give partial (and different for different detectors) regeneration of the original signal, reducing this factor down to 0.15–0.20 (at E=40 MeV). We study in details the influence of conversion on the observed signal depending on the parameters of the original neutrino spectra. For a given set of these parameters, the conversion could lead to an increase of the average energy of the observed events up to 50% and of the number of events by a factor of 2 at Kamiokande-2 and by a factor of 3–5 at IMB. Inversely, we find that neglecting the conversion effects can lead up to 50% error in the determination of the average energy of the original spectrum and about 50% error in the original luminosity. Comparing our calculations with experimental data we conclude that the Kamiokande-2 data alone do not favor strong conversion effect, which testifies for small difference of the original and spectra. In contrast, the combined analysis of the Kamiokande and IMB results slightly favors strong conversion effects (that is, large difference of the original spectra). In comparison with the no-oscillation case, the latter requires lower average energy and higher luminosity of the original flux.  相似文献   

Interstellar polarization and the position-angle orientations of type 1 Seyfert galaxies     

Jennifer L. Hoffman †  Ryan Chornock  Douglas C. Leonard †  Alexei V. Filippenko 《Monthly notices of the Royal Astronomical Society》2005,363(4):1241-1246

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Magnetic Connectivity Between Active Regions 10987, 10988, and 10989 by Means of Nonlinear Force-Free Field Extrapolation     

Tilaye Tadesse  T. Wiegelmann  B. Inhester  A. Pevtsov 《Solar physics》2012,277(1):119-130

Extrapolation codes for modelling the magnetic field in the corona in Cartesian geometry do not take the curvature of the Sun’s surface into account and can only be applied to relatively small areas, e.g., a single active region. We apply a method for nonlinear force-free coronal magnetic field modelling of photospheric vector magnetograms in spherical geometry which allows us to study the connectivity between multi-active regions. We use Vector Spectromagnetograph (VSM) data from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) survey to model the coronal magnetic field, where we study three neighbouring magnetically connected active regions (ARs 10987, 10988, 10989) observed on 28, 29, and 30 March 2008, respectively. We compare the magnetic field topologies and the magnetic energy densities and study the connectivities between the active regions. We have studied the time evolution of the magnetic field over the period of three days and found no major changes in topologies, as there was no major eruption event. From this study we have concluded that active regions are much more connected magnetically than the electric current.  相似文献   

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   

Living with uncertainty: from the precautionary principle to the methodology of ongoing normative assessment     

Jean-Pierre Dupuy  Alexei Grinbaum 《Comptes Rendus Geoscience》2005,337(4):457-474

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New age data from the Lesnaya Group: A key to understanding the timing of arc-continent collision, Kamchatka, Russia     

Alexei V. Soloviev  Mikhail N. Shapiro  John I. Garver  Ekaterina A. Shcherbinina  Igor R. Kravchenko-Berezhnoy 《Island Arc》2002,11(1):79-90

Abstract The Lesnaya Group is part of a thick, poorly dated turbidite assemblage that sits in the footwall of a regionally extensive collision zone in which the Cretaceous–Paleocene Olutorsky island arc terrane was obducted onto continental margin basin strata. Nannoplankton from 18 samples from the upper part of the Lesnaya Group yield Paleocene through Middle Eocene assemblages. Detrital zircons from nine sandstone samples have a young population of fission-track ages that range from 43.7 ± 3.4 to 55.5 ± 3.5 Ma (uppermost Paleocene to Middle Eocene). The deformed footwall rocks of the Lesnaya Group and the overlying thrusts of the Olutorsky arc terrane, are unconformably overlain by neoautochthonous deposits which are Lutetian (lower Middle Eocene) and younger. Together, these new data indicate that thrusting, which is inferred to have been driven by collision of the Cretaceous–Paleocene island arc with north-eastern Asia, took place in the mid-Lutetian, at about 45 Ma.  相似文献