Stability of the system of stars,gas and magnetic fields

Linear stability of a system of stars, gas and magnetic fields under the existence of a relative motion between the stars and the gas is investigated by the use of the magnetohydrodynamic and the polytropic equations for the gas and the collisionless Boltzmann equation for the stars together with the Poisson equation. The star system is supposed to have the anisotropic Schwarzschild distribution. The critical wavenumber is calculated and it is found that the system becomes universally unstable under some conditions.     

Corrected sidereal anisotropy for underground muons

Data from underground muon telescopes in New Mexico and Bolivia are analyzed in sidereal time and anti-sidereal time to study anisotropies in the rigidity range 20 GV to a few 100s of GV. Using both vertical and North- and South-pointing telescopes in both hemispheres, a latitude range of 70°N–50°S is covered. The distribution of cosmic rays in the inner heliosphere gives rise to a diurnal variation in anti-sidereal time, and also produces a spurious contribution to the anisotropy in sidereal time. It is shown that the anti-sidereal variation is of the P₂¹ type, having opposite phase in the Northern and Southern Hemispheres, and maximum amplitude at mid latitudes. The anti-sidereal data are used to correct the sidereal data, using the Nagashima method (Nagashima, Sakakibara, Fenton and Humble, 1985); the resulting corrected sidereal vectors for Northern Hemisphere telescopes have their sidereal maxima close to 3 h sidereal time, in reasonable agreement with sidereal data at higher energies from small air showers. The Nagashima correction also appears to eliminate effects due to the reversal of the Sun's polar magnetic field which show up in the unconnected sidereal data, and which also remain in corrected data using an alternative correction.     

The Kuroshio Current System as a jet and twin “relative” recirculation gyres embedded in the Sverdrup circulation

By analyzing the results of a realistic ocean general circulation model (OGCM) and conducting a series of idealized OGCM experiments, the dynamics of the Kuroshio Current System is examined. In the realistic configuration, the Kuroshio Current System is successfully simulated when the horizontal resolution of OGCMs is increased from 1/2° to 1/10°. The difference between the two experiments shows a jet, the model’s Kuroshio Extension, and a pair of cyclonic and anticyclonic, “relative,” recirculation gyres (RRGs) on the northern and southern flanks of the jet. We call them recirculation gyres because they share some features with ordinary recirculation gyres in previous studies, and we add the adjective “relative” to emphasize that they may not be apparent in the total field. Similar zonal jet and RRGs are obtained also in the idealized model with a rectangular basin and a flat bottom with a horizontal resolution of 1/6°. The northern RRG is generated by the injection of high potential vorticity (PV) created in the viscous sublayer of the western boundary current, indicating the importance of a no-slip boundary condition. Since there is no streamline with such high PV in the Sverdrup interior, the eastward current in the northern RRG region has to lose its PV anomaly by viscosity before connecting to the interior. In the setup stage this injection of high PV is carried out by many eddies generated from the instability of the western boundary current. This high PV generates the northern RRG, which induces the separation of the western boundary current and the formation of the zonal jet. In the equilibrium state, the anomalous high PV values created in the viscous sublayer are carried eastward in the northern flank of the zonal jet. The southern RRG is due to the classical Rhines–Young mechanism, where low PV values are advected northward within the western boundary inertial sublayer, and closed, PV-conserving streamlines form to the south of the Kuroshio Extension, allowing slow homogenization of the low PV anomalies. The westward-flowing southern branch of this southern RRG stabilizes the inertial western boundary current and prevents its separation in the northern half of the Sverdrup subtropical gyre, where the western boundary current is unstable without the stabilizing effect of the southern RRG. Therefore, in the equilibrium state, the southern RRG should be located just to the north of the center of the Sverdrup subtropical gyre, which is defined as the latitude of the Sverdrup streamfunction maximum. The zonal jet (the Kuroshio Extension) and the northern RRG gyre are formed to the north of the southern RRG. This is our central result. This hypothesis is confirmed by a series of sensitivity experiments where the location of the center of the Sverdrup subtropical gyre is changed without changing the boundaries of the subtropical gyre. The locations of the zonal jets in the observed Kuroshio Current System and Gulf Stream are consistent as well. Sensitivities of the model Kuroshio Current System are also discussed with regard to the horizontal viscosity, strength of the wind stress, and coastline.     

http://dx.doi.org/10.1016/j.gsf.2016.10.001

The Francevillian Group in Gabonese Republic was recently established as a typical sedimentary sequence for the Paleoproterozoic.However,its age is rather poorly constrained,mainly based on Rb-Sr and Nd-Sm datings.This study reports new zircon data obtained from Chaillu massif and N'goutou complex,which constrain the protolith age of the basement orthogneisses and the igneous age of an intrusive granite,respectively.Most zircons from the orthogneisses are blue and exhibit oscillatory zoning in cathode-luminescence images.Zircons with lower common lead abundances tend to be distributed close to the concordia curve.Two age clusters around 2860 Ma and 2910 Ma are found in zircons plotted on the concordia curve.Based on the Th/U ratios of zircons,these ages correspond to the protolith ages of the orthogneisses,and the zircons are not metamorphic in origin.Syenites and granites were collected from the N'goutou complex that intrudes into the FA and FB units of the Francevillian Group.The granitoids exhibit chemical composition of A-type granite affinity.Half of zircons separated from the granite are non-luminous,and the remaining half exhibit obscure internal textures under cathode-luminescence observation.All zircon grains contain significant amounts of common lead;the lead isotopic variability is probably attributed to the mixing of two components in the zircons.The zircon radiogenic ~(207)Pb/~(206)Pb ratio is 0.13707 ± 0.0010.corresponding to a ~(207)Pb/~(206)Pb age of 2191 ± 13 Ma.This constrains the minimum depositional age of the FA and FB units.Furthermore,the FB unit consists of manganese-rich carbonate rocks and organic carbon-rich black shales with macroscopic fossils.Based on our age constraints,these organisms appeared in the study area just after the last Paleoproterozoic Snowball Earth event,in concert with global scale oxidation event encompassing the Snowball Earth.     

Long-term climate response to stabilized and overshoot anthropogenic forcings beyond the twenty-first century

From multi-ensembles of climate simulations using the Community Climate System Model version 3, global climate changes have been investigated focusing on long-term responses to stabilized anthropogenic forcings. In addition to the standard forcing scenarios for the current international assessment, an overshoot scenario, where radiative forcings are decreased from one stabilized level to another, is also considered. The globally-averaged annual surface air temperature increases during the twenty-first century by 2.58 and 1.56°C for increased forcings under two future scenarios denoted by A1B and B1, respectively. These changes continue but at much slower rates in later centuries under forcings stabilized at year 2100. The overshoot scenario provides a different pathway to the lower B1 level by way of the greater A1B level. This scenario results in a surface climate similar to that in the B1 scenario within 100 years after the forcing reaches the B1 level. Contrasting to the surface changes, responses in the ocean are significantly delayed. It is estimated from the linear response theory that temperature changes under stabilized forcings to a final equilibrium state in the A1B (B1) scenario are factors of 0.3–0.4, 0.9, and 17 (0.3, 0.6, and 11) to changes during the twenty-first century, respectively, for three ocean layers of the surface to 100, 100–500, and 500 m to the bottom. Although responses in the lower ocean layers imply a nonlinear behavior, the ocean temperatures in the overshoot and B1 scenarios are likely to converge in their final equilibrium states.     

The insensitivity of the cosmic ray galactic anisotropy to heliomagnetic polarity reversals

Using the cosmic ray sidereal and anti-sidereal diurnal variations observed underground in London and Hobart during the period 1958–1983, it is demonstrated that: (1) the phase changes of the apparent sidereal diurnal variation observed only in the Northern Hemisphere cannot be attributed to the change of the heliomagnetospheric modulation of galactic cosmic ray anisotropy caused by the polarity reversal of the solar magnetic field, but that they are due to the fluctuation of the spurious sidereal variation produced from the anisotropy responsible for the solar semi-diurnal variation; (2) the spurious sidereal variation can be eliminated from the apparent variation by using the observed anti-sidereal diurnal variation; and (3) after the elimination, the sidereal diurnal variations in the Northern and Southern Hemispheres almost coincide with each other and are stationary throughout the period, regardless of the polarity reversal of the heliomagnetosphere. The origin of the corrected sidereal variation is discussed.     

Optimal displacement feedback control law for active tuned mass damper

Optimal displacement feedback control law is derived for a vibration control of a single‐degree‐of‐freedom structure with an active tuned mass damper (ATMD). Analytical expressions of the linear quadratic regulator (LQR) feedback gains for the ATMD are derived by solving the Ricatti equation straightforwardly. Based on these solutions, it is found that if the stiffness of the tuned mass damper (TMD) is calibrated to satisfy a certain condition, the control law is simplified to be composed of the feedback gains only for the displacement of the structure and the velocity of the auxiliary mass stroke, which is referred to as ‘optimal displacement feedback control’. The mean‐square responses of the structure as well as the auxiliary mass against Gaussian white noise excitations are evaluated by solving the Lyapunov equation analytically based on the stochastic optimal control theory. Using these analytical solutions, the optimal damping parameter for the auxiliary mass is also derived. Finally, the optimal displacement feedback control law is presented. Copyright © 2001 John Wiley & Sons, Ltd.     

Crustal structure and tectonics of the Hidaka Collision Zone, Hokkaido (Japan), revealed by vibroseis seismic reflection and gravity surveys

This study is the first integrated geological and geophysical investigation of the Hidaka Collision Zone in southern Central Hokkaido, Japan, which shows complex collision tectonics with a westward vergence. The Hidaka Collision Zone consists of the Idon'nappu Belt (IB), the Poroshiri Ophiolite Belt (POB) and the Hidaka Metamorphic Belt (HMB) with the Hidaka Belt from west to east. The POB (metamorphosed ophiolites) is overthrust by the HMB (steeply eastward-dipping palaeo-arc crust) along the Hidaka Main Thrust (HMT), and in turn, thrusts over the Idon'nappu Belt (melanges) along the Hidaka Western Thrust (HWT). Seismic reflection and gravity surveys along a 20-km-long traverse across the southern Hidaka Mountains revealed hitherto unknown crustal structures of the collision zone such as listric thrusts, back thrusts, frontal thrust-and-fold structures, and duplex structures. The main findings are as follows. (1) The HMT, which dips steeply at the surface, is a listric fault dipping gently at a depth of 7 km beneath the eastern end of the HMB, and cutting across the lithological boundaries and schistosity of the Hidaka metamorphic rocks. (2) A second reflector is detected 1 km below the HMT reflector. The intervening part between these two reflectors is inferred to be the POB, which is only little exposed at the surface. This inference is supported by the high positive Bouguer anomalies along the Hidaka Mountains. (3) The shallow portion of the IB at the front of the collision zone has a number of NNE-dipping reflectors, indicative of imbricated fold-and-thrust structures. (4) Subhorizontal reflectors at a depth of 14 km are recognized intermittently at both sides of the seismic profile. These reflectors may correspond to the velocity boundary (5.9–6.6 km/s) previously obtained from seismic refraction profiling in the northern Hidaka Mountains. (5) These crustal structures as well as the back thrust found in the eastern end of the traverse represent characteristics of collisional tectonics resulting from the two collisional events since the Early Tertiary.     

Direct measurements of deep current at 162°E south of the equator in the Melanesian Basin: a trial to detect a cross-equatorial deep western boundary current

We conducted 1-year-long mooring observations four times below 2000?m, slightly south of the equator (2°39?? to 4°35??S) at 162°E in the Melanesian Basin in order to detect the southward deep western boundary return current crossing the equator. Contrary to our initial expectation of the deep flow scheme in the equatorial western boundary region, the observed results indicated a fairly complicated flow configuration. We analyzed the results with the help of a high-resolution model simulation. The ensemble average of the horizontal flow at each level near the deep western boundary indicates a significant westward flow at 2000 and 2250?m, with an insignificant southward component at 2500 and 2750?m. The annual mean meridional transports are very small (>1?Sv) and insignificant, with an ensemble-averaged value of 0.3?Sv (southward) ±0.4?Sv at most. Combining this with high-resolution model results, it is deduced that the southward transport of the deep western boundary current (DWBC) leaving the equator may be smaller than those obtained by low-resolution models, because of trapping of its fairly large fraction in the equatorial zone. Annual-scale flow patterns are classified into several categories, mainly based on the meridional-flow dominating or the zonal-flow dominating pattern. A case of the meridional-flow dominating patterns may possibly capture an annual-scale variability of DWBC, because its meridional transport variation, though somewhat weak, is consistent with that simulated. The zonal-flow dominating regime includes two types: long-lasting, almost steady westward flows and long-term zonal flow oscillations. The former seems to comprise well-known zonally elongated and meridionally narrow structures of the zonal flow beneath the thermocline in the equatorial region. The ensemble-averaged flow mentioned above is dominated by this type at the upper two levels 2000 and 2250?m, with total westward transport of 1.6?±?0.7?Sv. The latter type seems to be a manifestation of the vertically propagating equatorial annual Rossby waves.     

Non-adiabatic linear pulsation models for low-mass helium stars

Non-adiabatic linear pulsation models have been calculated for low-mass stars with effective temperatures between 16 000 and 35 000 K, and with surface gravities in the range 3⊙, X =0.00, Z =0.02. It is shown that the Z -bump instability persists to low masses ( M ∼0.4 M_⊙) but is suppressed either by a reduction in metallicity Z or by a selective enhancement of the carbon abundance. An unexpected result is the discovery that Z -bump instability persists at hydrogen abundances X >0.3, although the position of the red edge is sensitive to X . We have found that non-radial pulsations are also excited in the same instability region as radial pulsations.
The implications of these results for individual low-mass helium stars are discussed. It is concluded that Z -bump driven pulsations (radial and/or non-radial) may be excited in some helium-rich subdwarf B stars, representing a possible major extension to the class of variable stars represented by the prototype V652 Her.