An inverse model of the large scale circulation in the South Indian Ocean

An overview of the large-scale circulation of the South Indian Ocean (SIO) (10°S-70°S/20°E-120°E) is proposed based on historical hydrographic data (1903-1996) synthesized with a finite-difference inverse model. The in situ density, potential temperature and salinity fields of selected hydrographic stations are projected on the basis of EOFs. Then the EOF coefficients (the projected values) are interpolated on the model grid (1° in latitude, 2° in longitude) using an objective analysis whose spatial correlation functions are fitted to the data set. The resulting fields are the input of the inverse model. This procedure filters out the small-scale features. Twelve modes are needed to keep the vertical structures of the fields but the first three modes are sufficient to reproduce the large-scale horizontal features of the SIO: the Subtropical Gyre, the Weddell Gyre, the different branches of the Antarctic Circumpolar Current.The dynamics is steady state. The estimated circulation is in geostrophic balance and satisfies mass, heat and potential vorticity conservation. The wind and air-sea heat forcing are annual means from ERS1 and ECMWF, respectively.The main features of the various current systems of the SIO are quantified and reveal topographic control of the deep and bottom circulation. The cyclonic Weddell Gyre, mainly barotropic, transports 45 Sv (1 Sv = 10⁶m³/s), and has an eastern extension limited by the southern part of the Antarctic Circumpolar Current.The bottom circulation north of 50°S is complex. The Deep Western Boundary Currents are identified as well as cyclonic recirculations. South east of the Kerguelen Plateau, the bottom circulation is in good agreement with previous water mass analysis. The comparison between some recent regional analysis and the inverse estimation is limited by the model resolution and lack of deep data.The meridional overturning circulation (MOC) is estimated from the finite difference inverse model. Between 26°S and 32°S the reversal of the current deepens and reaches 1400 m at 32°S. The major part of the deep meridional transport at 32°S is located between the African coast and the Madagascar Ridge, carried by the Agulhas Undercurrent. The mean value for this meridional thermohaline recirculation is 8.8 ± 4.4 Sv between 26°S and 32°S. The Agulhas Undercurrent (11 Sv) is associated with a weak Agulhas Current (55 Sv). The MOC is thus trapped in the western margin of the Southwest Indian Ridge. The corresponding vertical velocity along 32°S between 30°E and 42°E is 7.2 × 10⁻⁵ ± 8.9 × 10⁻⁵ cm s⁻¹. The net meridional heat flux represents −0.53 PW at 18°S and −0.33 PW at 32°S (negative values for southward transports). The intensity of the meridional heat flux is linked to the intensity of the Agulhas Current and to the vertical mixing.     

Neutron scattering and computer simulation studies of ice dynamics

In this article we describe a range of simulations (lattice dynamics and molecular dynamics) of the inelastic incoherent neutron scattering spectra of ices (normal ice, ice II and ice VIII). These simulations use a variety of different intermolecular potentials from simple classic pair-wise (rigid and non-rigid molecule) potentials to sophisticated polarisable potentials. It was found that MCY makes stretching and bending interactions too weak while others do them well. We demonstrate that in order to reproduce the measured neutron spectrum, greater anisotropy (or orientational variation) is required than these potentials presently provide.     

A comparative study on four major cities in Northeastern Thailand using urban land density function

Urban land density is an important factor to understand how cities expand. An “Inverse S-shape Rule” was implemented for the first time to analyze urban land density in Northeastern Thailand using the four cities Khon Kaen, Udon Thani, Nakhon Phanom, and Nong Khai as study sites. Land density function was tested using different data classification techniques from previous studies. Each city was investigated over two different time periods between 2002 and 2015. Declining pattern characteristics of metropolitan area density outward from city centers can be quantified by fitting the parameters to urban land density functions. An inverse S-shape function was identified as the best data fit. The four selected cities showed conventional density variation for decline in urban land area from city centers to outlying areas. Overall trend indicated that cities became more compact over time since the density differences between the urban core and urban fringe were greater with increasing infilling growth within the urban boundary. All four cities increased in size over time; however, the increasing amount of built-up land in the surrounding rural areas did not follow the same trend in each case. Some functional parameters required careful interpretation because of the linear shape of the city as in the case of Nakhon Phanom. Using highly detailed urban data resulted in lower densities of urban areas compared to the conventional pixel-based classification, and this affected the overall shape of the inverse S-shape function. The fitted parameters and their changing trends indicated that the urban land density function was useful for understanding urban form and urban sprawl in Thailand. Results can be used to develop a specific framework for other cities with similar attributes in the future.     

Chaotic Scattering in the Restricted Three‐Body Problem II. Small mass parameters

We study the scattering motion of the planar restricted three‐body problem for small mass parameters μ. We consider the symmetric periodic orbits of this system with μ = 0 that collide with the singularity together with the circular and parabolic solutions of the Kepler problem. These divide the parameter space in a natural way and characterize the main features of the scattering problem for small non‐vanishing μ. Indeed, continuation of these orbits yields the primitive periodic orbits of the system for small μ. For different regions of the parameter space, we present scattering functions and discuss the structure of the chaotic saddle. We show that for μ < μc and any Jacobi integral there exist departures from hyperbolicity due to regions of stable motion in phase space. Numerical bounds for μc are given. This revised version was published online in July 2006 with corrections to the Cover Date.     

Models of the iron Kα fluorescent line and the Compton Shoulder in irradiated accretion disc spectra

We present a full set of model atmosphere equations for the accretion disc around a supermassive black hole irradiated by a hard X-ray lamp of power-law spectral distribution. Model equations allow for multiple Compton scattering of radiation on free electrons, and for large relative photon–electron energy exchange at the time of scattering. We present spectra in specific intensities integrated over the disc surface. Theoretical outgoing intensity spectra show soft X-ray excess below 1 keV, and distinct Kα and Kβ fluorescent lines of iron. We demonstrate the existence of the Compton Shoulder and claim that it can contribute to the asymmetry and equivalent widths of some observed Fe Kα lines in active galactic nuclei. Our models exhibit the effect of limb-brightening in reflected X-rays.     

SELECTION OF BRILLOUIN SHIFT DISCRIMINATOR FOR BRILLOUIN LIDAR

For the measurement of vertical profiles of sound speed in the sea using laser excited Brillouin scattering, a high-resolution measurement of Brillouin frequency shift is required. In this work, a molecular absorption cell was selected as the frequency shift discriminator and several kinds of absorption gases were tried. It was found that the strong line ( # 1095) of ^127 I2 at 18783. 3297 cm^-1 and two absorption lines of ^129 I2 located at the two sides of the # 1095 line of ^127 I2 could be used as frequency shift discriminator to detect the changes of the Brillouin frequency shift. This selection is the best one within the range from 532.0131 run to 532.5154 nm. But it is not perfect and there is a lot of work to do before its practical application.     

Accretion column eclipses in the X-ray pulsars GX 1+4 and RX J0812.4−3114

Sharp dips observed in the pulse profiles of three X-ray pulsars (GX 1+4, RX J0812.4−3114 and A 0535+26) have previously been suggested to arise from partial eclipses of the emission region by the accretion column occurring once each rotation period. We present pulse-phase spectroscopy from Rossi X-ray Timing Explorer satellite observations of GX 1+4 and RX J0812.4−3114, which for the first time confirms this interpretation. The dip phase corresponds to the closest approach of the column axis to the line of sight, and the additional optical depth of photons escaping from the column in this direction gives rise to both the decrease in flux and increase in the fitted optical depth measured at this phase. Analysis of the arrival time of individual dips in GX 1+4 provides the first measurement of azimuthal wandering of a neutron star accretion column. The column longitude varies stochastically with a standard deviation ranging between 2° and 6° depending on the source luminosity. Measurements of the phase width of the dip both from mean pulse profiles and from the individual eclipses demonstrate that the dip width is proportional to the flux. The variation is consistent with that expected if the azimuthal extent of the accretion column depends only upon the Keplerian velocity at the inner disc radius, which varies as a consequence of the accretion rate M˙ .     

Seismic evidence for a sharp lithospheric base persisting to the lowermost mantle beneath the Caribbean

Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method—the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core–mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10–15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection.