Intense night-time equatorial radio scintillations by sporadicE layers

Almost saturated scintillations of radio beacons from geostationary satellites received at an equatorial station during night-time have been shown to occur even during complete absence of spreadF on the vertical incidence ionograms at the same location. These scintillation events were observed when the ionograms showed blanketing type of sporadicE layers simultaneously at different heights. It is suggested that strong equatorial radio wave scintillations during night-time are caused by multiple scattering between different levels of large plasma density gradients in theF or sometimes in theE regions of the ionosphere.     

HF doppler observations on the occurrence of equatorial spread-F

HF doppler observations of the vertical drift velocity and group height of the 5.5 MHz plasma frequency level of the post-sunset bottomside F-region obtained on a fewESF (equatorial spread-F) and non-ESF days at Trivandrum are presented. The results show that on the non-ESF days, the maximum group height attained is about 400 km and the maximum velocity is less than 30 m/sec. On theESF days, however, the corresponding values are found to be in the range of 400–650 km and 30–50 m/sec. TheESF onset is found to be significantly delayed relative to the velocity peak indicating that it is more closely linked to the layer height than to the drift velocity.     

Nonlinear Rayleigh-Taylor instability in partially ionized plasma and the equatorial spread-F

The nonlinear evolution of the collisional gravitation induced RayleighTaylor (R-T) instability in the equatorialF region is investigated taking into account the finite larmor radius (FLR) effects and the complete ion inertial term in ion equation of motion. A special class of coherent weakly nonlinear modes as solutions to the wave equation describing R-T instability driven modes is obtained. The leading nonlinear effects in the wave equation are found to appear throughV _l, the ion diamagnetic drift which essentially gives the FLR corrections. It is shown that the R-T modes in the equatorialF region can evolve into coherent, nonlinear, almost sinusoidal, stationary wave structures. These structures are found to travel with a constant phase velocity and to have slightly distorted sinusoidal shapes. These results seem to have a good agreement with many of the recent rocket and satellite observations of the equatorial spreadF irregularities.     

On the growth and decay of irregularities in the ionosphere

It has been demonstrated that VHF/UHF scintillation data can be used to evaluate important physical parameters such as scale sizes, strength, growth and decay of irregularities in the equatorial ionosphere. These parameters are important in constructing electroject models. It is shown that large scale irregularities are generated first which later break into smaller scale sizes. During the decay phase, the small scale irregularities disappear first after followed by large scale irregularities. The generation and destruction time of these irregularities has been estimated to be around 20 min. In addition these irregularities affect propagation of radio waves from HF to UHF range which suggests the existence of a wide spectrum of irregularities in the ionosphere. It has been found that the scale sizes of daytimeE-region irregularities are smaller than those in theF-region during night-time. The growth rate of the irregularities seems to be larger in theE-region than in theF-region of the ionosphere.     

Daily variations ofE andF region drifts over Tiruchirapalli

Daily variations of the E-W and N-S components of ionospheric drifts, measured using closely spaced receiver method over Tiruchirapalli near magnetic equator, are described for the periods 1973–75 forE region and 1974–75 forF region. N-S component is not observed except for a few occasions during summer months for theE region. E-W component is generally eastward during night hours and westward during day hours. There is a large day-to-day variability in the magnitude as well as in the evening reversal time. Daily variations primarily consist of the diurnal component with average amplitude of about 90 m/sec forF region and of about 80 m/sec forE region. The drift speeds are found to decrease with magnetic activity. It is concluded that the abnormal electrojet region extends upto north of Tiruchirapalli in the Indian zone and drifts here can be used as a monitor of electric field.     

On the formation of a sharp layer of metallic constituents at sunrise in the lower thermosphere

Results are presented of two instrumented rocket experiments performed from an equatorial station, one at night and the other shortly after sunrise. The ion neutral composition as well as electron density and the amplitude of plasma irregularities were monitored. During the latter flight, a sharp layer of ionisation with its lower boundary at 100 km was observed. The layer had a half width close to 1 km and a peak electron density of 5·6× 10⁴cm^?3. Large amplitude of plasma irregularities, noticed on the negative gradient portion of the layer indicates a downward direction of the polarisation electric field during the observations. The resulting downward drift of photoions as they are produced at sunrise followed by the local decrease of the drift is suggested to be the cause of the layer formation at that altitude. The long lasting nature of such layers once identified on ionograms indicates that they are constituted of metallic ions possibly of micrometeoritic origin deposited overnight in the lower thermosphere. The required photoionisation rate of production of the metallic ions at sunris eis about 2 cm^?3 sec^?1.     

Geomagnetic disturbance effects on equatorial spreadF

Geomagnetic disturbance effects on the occurrence of range and frequency spread at Huancayo are studied for the period 1957–74. The occurrence of frequency spread is decreased on disturbed days forD-months andE-months while duringJ-months an increase is noted in the post-midnight period. The occurrence of range spread is decreased on disturbed days in the pre-midnight hours duringD andE-months. Post-midnight hours ofD andE-months and all hours of night duringJ-months show an increase of range spread on disturbed days, which is most prominent during low sunspot years. The mean occurrence of frequency spread (2300–0100 LT) and range spread (2000–2300 LT) decrease with increasing ΣKp. On the other hand, occurrence of range spread (0300–0500 LT) increases with increasing ΣKp, and is suggested as the consequence of the changes of the horizontal electric field in theF-region associated with the geomagnetic storms.     

Meridional drift of large-scale solar magnetic fields

It is shown that the meridional drift of large-scale fields starts in the equatorial zone and continues over 15–16 yrs (16–17 according to another estimate), i.e., during three fourths of the 22-year cycle. There is an abrupt retardation of the drift at latitudes of 30°–50°, and a stagnation region where the drift rate does not exceed several meters per second arises. The drift becomes rapid again at higher latitudes. The stagnation region coincides with the area in which the radial gradient of the rotational velocity is close to zero in the convective zone. This drift is compared with helio-seismological data on the rotation in the convective zone. A model taking into account some elements of dynamo theory is proposed.     

Remote probing ofD-region irregularities

The nature of theD-region irregularities responsible for partial reflections is studied using amplitude, phase and range information taken at Buckland Park, South Australia, utilising a 178-dipole receiving array. Stratifications with distinct layer type structures below 80 km are noticed. A major part of the echo amplitude consists of specularly reflected signals.     

Magnetic properties of sheet silicates; 2:1:1 layer minerals

Magnetization, susceptibility and Mössbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of theT _2g triplet of 1,120K. Predominant exchange interactions in the iron-rich samples are ferromagnetic withJ=1.2 K, as for other trioctahedral ferrous minerals. A peak in the susceptibility of thuringite occurs atT _m=5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the Mössbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4 Å^?1). A factor favouring this random ferromagnetic ground state over the planar antiferromagnetic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illustrate how long range ferromagnetic order may be destroyed by vortices forming around the Fe³⁺-Fe³⁺ pairs.     

Lower Cretaceous ammonites from the Neuquén Basin, Argentina: The Hauterivian genus Spitidiscus

Spitidiscus is a widely-distributed Hauterivian genus that briefly invaded the Neuquén Basin, Argentina, in mid Hauterivian times, well after it first evolved in the west Tethyan area. Its appearance in Argentina is linked with a globally-significant mid Hauterivian sea-level rise. This is marked in the basin by a sharp facies change in the Agrio Formation, from non-marine sandstones of the Avilé Member to marine sediments, often laminated black shales, with Spitidiscus at the base of the overlying Agua de la Mula Member. Our extensive field work has shown that Spitidiscus occurs across the whole basin, where it is represented by two species, Spitidiscus riccardii Leanza and Wiedmann and Spitidiscus kilapiae sp. nov. For most of its vertical range Spitidiscus occurs alone, characterising the S. riccardii Zone. But at the top of its range it is joined by the first crioceratitid ammonites, their appearance marking the base of the Crioceratites schlagintweiti Zone.     

Superposed folding in the Honakere arm of the Chitradurga-Karighatta schist belt in the Dharwar tectonic province,southern India,and its bearing on the Sargur-Dharwar relation

The supracrustal enclave within the Peninsular Gneiss in the Honakere arm of the Chitradurga-Karighatta belt comprises tremolite-chlorite schists within which occur two bands of quartzite coalescing east of Jakkanahalli(12°39′N; 76°41′E), with an amphibolite band in the core. Very tight to isoclinal mesoscopic folds on compositional bands cut across in the hinge zones by an axial planar schistosity, and the nearly orthogonal relation between compositional bands and this schistosity at the termination of the tremolite-chlorite schist band near Javanahalli, points to the presence of a hinge of a large-scale, isoclinal early fold (F₁). That the map pattern, with an NNE-plunging upright antiform and a complementary synform of macroscopic scale, traces folds 'er generation (F ₂),is proved by the varying attitude of both compositional bands (S₀) and axial pranar schistosity (S ₁), which are effectively parallel in a major part of the area. A crenulation cleavage (S ₂) has developed parallel to the axial planes of theF ₂ folds at places. TheF ₂ folds range usually from open to rarely isoclinal style, with theF ₁ andF ₂ axes nearly parallel. Evidence of type 3 fold interference is also provided by the map pattern of a quartzite band in the Borikoppalu area to the north, coupled with younging directions from current bedding andS ₀ -S ₁ inter-relation. Although statistically theF ₁ andF ₂ linear structures have the same orientation, detailed studies of outcrops and hand specimens indicate that the two may make as high an angle as 90°. Usually, in these instances, theF ₁ lineations are unreliable around theF ₂ axes, implying that theF ₂ folding was by flexural slip. In zones with very tight to almost isoclinalF ₂ folding, however, buckling attendant with flattening has caused a spread of theF ₁ lineations almost in a plane. Initial divergence in orientation of theF ₁ lineations due to extreme flattening duringF ₁ folding has also resulted in a variation in the angle between theF ₁ andF ₂lineations in some instances. Upright later folding (F₃) with nearly E-W strike of axial planes has led to warps on schistosity, plunge reversals of theF ₁ andF ₂ axes, and increase in the angle between theF ₁ andF ₂ lineations at some places. Large-scale mapping in the Borikoppalu sector, where the supposed Sargur rocks with ENE ‘trend’ abut against the N-‘trending’ rocks of the Dharwar Supergroup, shows a continuity of rock formations and structures across the hinge of a large-scaleF ₂ fold. This observation renders the notion, that there is an angular unconformity here between the rocks of the Sargur Group and the Dharwar Supergroup, untenable.     

Excitation of electromagnetic lower hybrid instability in the region of bouncing electron beams

The region of bouncing electron beams in the earth’s magnetosphere can be unstable against a non-resonant electromagnetic lower hybrid instability. The instability is purely growing in the rest frame of the plasma, and can be excited either by the temperature anisotropy or the drift velocity of the bouncing electron beams. The growth rates of the instability decrease with the increase of cold electron density. Consequently the growth rate is maximum at the equator where the cold electron density is minimum. The intense turbulence generated by this instability could broaden the bouncing electron beams thereby explaining the observed wider cone width of the beams at the equator. The instability could generate magnetic pulsations in the frequency range of orderPc ₁?Pc₃ with typical wavelength ≈ (3–10) km in the, magnetosphere during magnetic storms or substorms.     

带有复合摩擦阻尼系统的基础隔震结构分析

针对常规摩擦阻尼器的起滑力难以确定的问题,提出了复合摩擦阻尼系统（CFDS）,它由非线性硬弹簧和摩擦阻尼器串联而成。当地震较小时,摩擦阻尼器处于附着状态,非线性硬弹簧发挥作用,防止隔震层位移过大;当地震较大时,非线性硬弹簧的变形则保持不变,摩擦阻尼器处于滑动状态,通过摩擦阻尼耗散能量。建立了带有复合摩擦阻尼系统的基础隔震结构的运动方程,指出摩擦阻尼器处于附着和滑移2种状态下边界条件。据此研究了常规强震和近断层脉冲型地震动作用下复合摩擦阻尼装置对橡胶隔震结构的减震效果。仿真分析表明：安装复合摩擦阻尼系统,在显著降低隔震层位移的同时,对上部结构的层间位移和加速度均有一定的减震效果;并且,在整个运动过程中,摩擦阻尼器会在附着和滑移2种状态之间来回切换。详细研究了复合摩擦阻尼系统的３个参数对隔震结构的减震效果。结果表明：在不同地震波作用下,由于摩擦阻尼系统参数的变化导致隔震结构地震响应的变化趋势是不同的,因此最优参数的取值与输入地震波类型和地面加速度峰值有关,并且不存在使隔震层位移和上部结构加速度同时达到最小的最优参数,因此需要设定合适的控制目标以确定适当的阻尼系统参数。     

Partial mixing in early-type main-sequence B stars

Partial mixing of material in the radiative envelopes and convective cores of rotating main sequence stars with masses of 8 and 16 M _⊙ is considered as a function of the inital angular momentum of the stars. Losses of rotational kinetic energy to the generation of shear turbulence in the radiative envelope and the subsequent mixing of material in the envelope are taken into account. With an initial equatorial rotational velocity of 100 km/s, partial mixing develops in the upper part of the layer with variable chemical composition and the lower part of the chemically homogeneous radiative envelope. When the initial equatorial rotational velocity is 150–250 km/s, the joint action of shear turbulence and semi-convection leads to partial mixing in the radiative envelope and central parts of the star. The surface abundance of helium is enhanced, with this effect increasing with the angular momentum of the star. With an initial equatorial rotational velocity of 250 km/s, the ratio of the surface abundances of helium and hydrogen grows by ~30% and ~70% toward the end of the main-sequence evolution of an 8 M _⊙ and 16 M _⊙ star, respectively. The transformation of rotational kinetic energy into the energy of partial mixing increases with the angular momentum of the star, but does not exceed ~2%?3% in the cases considered.     

Sand movement patterns in the Western Desert of Egypt: an environmental concern

Wind action is the most dominant agent for erosion and deposition in the vast Western Desert of Egypt. Analysis of wind data from seven meteorological stations distributed along the Western Desert reveals that this desert is characterized by high-energy wind environments along the northern and southern edges and low-energy wind environments throughout the rest of the desert. Accordingly, sand drift potential follows the pattern of wind energy. Maximum sand drift potential was observed at the southern edge (571 vector units, which equals 40 m³/m width/year). Sand drift direction was observed towards the southeast except at the southern part of the desert where the trend of sand movement was towards southwest. The major dune type recognized on satellite images was the simple linear type. Linear dunes are generally associated with bimodal wind regime. Rates of sand drift potential and sand dune migration were greatest at East of Owinate region at the extreme southern part of the desert. Measurements of crescentic sand dune advance from two satellite images reveal a maximum advance rate of about 9 m/year at the southern part of the desert. Dune movement creates potential hazard to the infrastructures in this open desert.     

Theory for preliminary negative impulse in storm sudden commencement inH at equatorial stations

It is shown that the storm sudden commencement (SSC) inH field at low latitude station consists of only a positive excursion when the interplanetary shock due to the solar plasma impinging on the magnetosphere is associated with a southward excursion of the interplanetary magnetic field (IMF). When the signature of SSC at low latitude station consists of a preliminary negative excursion preceding the main positive excursion of theH field, the solar plasma causing the compression is associated with a northward excursion of the IMF. It is suggested that the signature of SSC(H) at equatorial stations is the result of combined effect of the compression of magnetosphere by the solar plasma as well as due to the electric field effects associated with the velocity of the solar plasma (v) interacting with the northward component (Bz) of the interplanetary field (i.e.,E =−v _x Bz).     

The life cycle and recruitment of the sand shrimp,Crangon septemspinosa,in the Mystic River estuary,Connecticut

Sampling theC. septemspinosa population of the Mystic River Estuary simultaneously in deep water and along the shoreline indicated that this population has two major reproductive periods. Berried females move into the estuary in early spring and late autumn to hatch their eggs. Two-and three-year-old females produce the larvae in the spring and 1.0- to 1.5-year-old females give rise to the larvae in winter. All larvae are carried seaward by the tidal currents and eventually settle as juveniles in the deep water near the mouth of the estuary or on the continental shelf. Juveniles from the spring hatch migrate to the shallow shoreline where they grow, rapidly, 0.15 mg/day. Those that hatch during the late autumn do not migrate shoreward. A model of this life cycle and evidence to support two recruitment mechanisms that involve inshore migration of adults and the offshore drift of the larvae are presented.     

The role of physical-biological coupling in the benthic boundary layer under megatidal conditions: The case of the dominant species of theAbra alba community in the Eastern Baie de Seine (English Channel)

Recent evidence has shown that postlarvae and juveniles of the most dominant species of theAbra alba community of the eastern Baie de Seine are able to re-enter the water column after larval settlement and undergo active and/or passive redistribution. However, few subtidal studies have been conducted to investigate and quantify the processes controlling the abundance of drifting postlarvae and juveniles. The purpose of this study was to verify the reproducibility or the variability of this phenomenon and to assess coupling between the intensity of drifting and that of bed shear stress, which is dependent on tidal currents, wave characteristics, and sediment features. This study was conducted in June and July 1995 at two sites in the bay, one located at the center of the muddy fine-sediment community and the other at its external border with the fine clean sands of theOphelia borealis community. At each site, suprabenthic sledge samples were taken during specific tidal conditions (low tide, slack water, and flood and ebb peaks) during a neap and a spring tide. Physical data (current velocity, turbidity, and suspended matter content) and meiobenthic samples were taken at the same time. Under megatidal conditions, drifting was a quantitatively important dispersal mechanism for at least 15 of the fine-sediment dominant species belonging to various groups such as polychaetes, molluscs (gastropods and bivalves), and echinoderms. The fluctuation in abundance of drifters was related to tidal range, tidal period, location of the site, and the recruitment dynamics. However, species differed strongly in theri response to hydrodynamical parameters. Three types of drifting species were observed: species that were only present in the water column (‘undet. cardiids’ and Mytilidae); species that were both drifting and present at the seaded (Abra alba, Phaxas pellucidus, Eteone picta, Mysella bidentata, Ophiura texturata, Owenia fusiformis, Pholoe minuta, Phyllodoce lineata, Pectinaria koreni, andSpisula subtruncata); species that were present at the seabed but did not drift during the sampling period (Acrocnida brachiata, Eteone longa, Eumida sanguinea, Glycinde normanni, Magelona mirabilis, Nemertean sp1,Nephtys sp.,Spiophanes bombyx, andTubulanus polymorphus). Each drifting pattern is related to eco-ethological species characteristics and may have very different consequences on population dynamics.     

Differential rotation of a star induced by meridional circulation

Global meridional flows in stars transport angular momentum, thus giving rise to nonuniform rotation. The pattern of differential rotation associated with slow meridional circulation depends on the direction of this circulation. A flow directed from the poles to the equator at the surface and from the equator to the poles in deep layers results in relatively fast rotation of the equatorial zone. If the circulation is directed oppositely, the angular velocity increases from the equator to the poles. Relatively fast rotation at the poles may also result from fast circulation, irrespective of its direction. A simple illustrative explanation is given here to these results. Analytical estimates are supported by numerical calculations. The time variations in the meridional flow observed on the Sun should contribute to torsional oscillations.