Horizontal Density Variations Research Articles

Inertial oscillations and frontogenesis driven by a quadratic horizontal density variation

We consider inviscid rotating flow driven by a horizontally quadratic density variation in a horizontally unbounded slab. This configuration permits a similarity solution, removing the dependence on the horizontal coordinate from the vorticity and temperature equations, which are then solved by numerical integration along characteristics. At large values of Rossby number, the flow proceeds to a singularity in a similar manner to the non-rotating flow with the same initial conditions. At small values of Rossby number there are inertial oscillations of growing amplitude, which have been analysed using the method of multiple scales. The oscillations become desynchronised between the upper and lower parts of the domain, and static instability appears for a small fraction of each oscillation period. Eventually the oscillations give way to the rapid formation of a singularity, in contrast to geostrophic adjustment theory which predicts that a singularity will form only if the Rossby number is sufficiently large.

On the Vertical Structure of Radiation-dominated Accretion Disks

The vertical structure of black hole accretion disks in which radiation dominates the total pressure is investigated using a three-dimensional radiation-MHD calculation. The domain is a small patch of disk centered 100 Schwarzschild radii from a black hole of 108 M☉, and the stratified shearing-box approximation is used. Magnetorotational instability converts gravitational energy to turbulent magnetic and kinetic energy. The gas is heated by magnetic dissipation and by radiation damping of the turbulence, and cooled by diffusion and advection of radiation through the vertical boundaries. The resulting structure differs in several fundamental ways from the standard Shakura-Sunyaev picture. The disk consists of three layers. At the midplane, the density is large, and the magnetic pressure and total accretion stress are less than the gas pressure. In lower density surface layers that are optically thick, the magnetic pressure and stress are greater than the gas pressure but less than the radiation pressure. Horizontal density variations in the surface layers exceed an order of magnitude. Magnetic fields in the regions of greatest stress are buoyant, and dissipate as they rise, so the heating rate declines more slowly with height than the stress. Much of the dissipation occurs at low column depth, and the interior is cooler and less radiation-dominated than in the Shakura-Sunyaev model with the same surface mass density and flux. The mean structure is convectively stable.

An Expression for Effective Buoyancy in Surroundings with Horizontal Density Gradients

Abstract An expression is obtained for effective buoyancy per unit volume in inviscid anelastic flow with surrounding horizontal density variations. Effective buoyancy is defined here as the statically forced part of the locally nonhydrostatic, upward pressure-gradient force because this is the part of the net vertical force that is both absolute (i.e., independent of an arbitrary base state) and dependent solely on variations in the specific weight of the air–hydrometeor mixture. For the case where the domain is the half space z > 0 above flat ground, effective buoyancy β(x) at a given field point x is given by β(x) = ∫∞−∞ ∫∞−∞ ∫∞−∞ [g2HρT(x)]/(4π|x − x|) dx dŷ dẑ, where gρT is the total weight of air and hydrometeors per unit volume, the x ≡ (x, ŷ, ẑ) are source points, 2H is the horizontal Laplacian with respect to x, and 2HρT(x, ŷ, −ẑ) ≡ −2HρT(x, ŷ, ẑ).

Deformations of accreting neutron star crusts and gravitational wave emission

Motivated by the remarkably narrow range of measured spin frequencies of ∼20 accreting (and weakly magnetic) neutron stars in the Galaxy, Bildsten conjectured that their spin-up had been halted by the emission of gravitational waves. If so, then the brightest persistent X-ray source on the sky, Scorpius X-1, should be detected by gravitational wave interferometers within 10 years. Bildsten pointed out that small non-axisymmetric temperature variations in the accreted crust will lead to ‘wavy’ electron capture layers, and the resulting horizontal density variations near e− capture layers create a mass quadrupole moment. Neglecting the elastic response of the crust, Bildsten estimated that even e− capture layers in the thin outer crust can develop the quadrupole necessary to balance accretion torque with gravitational waves, for accretion rates We present a full calculation of the crust's elastic adjustment to the density perturbations induced by the temperature-sensitive e− capture reactions. We find that, due to the tendency of the denser material to sink rather than spread sideways, neglecting the elastic response of the crust overestimates, by a factor of 20–50, the Q22 that results from a wavy capture layer in the thin outer crust. However, we find that this basic picture, when applied to capture layers in the deep inner crust, can still generate Q22 in the necessary range, as long as there are ≲5 per cent lateral temperature variations at densities in excess of 1012 g cm−3, and as long as the crustal breaking strain is high enough. By calculating the thermal flow throughout the core and the crust, we find that temperature gradients this large are easily maintained by asymmetric heat sources or lateral composition gradients in the crust. If the composition or heating asymmetries are independent of the accretion rate, then for the induced quadrupole moments have approximately the same scaling, ∝M1/2, as that necessary to balance the accretion torque at the same spin frequency for all M. Temperature gradients in the deep crust lead to a modulation in the thermal emission from the surface of the star that is correlated with Q22. In addition, a ∼0.5 per cent lateral variation in the nuclear charge-to-mass ratio in the crust will also result in a Q22 sufficient to halt spin-up from accretion even in the absence of a lateral temperature gradient. We also derive a general relation between the stresses and strains in the crust and the maximum quadrupole moment they can generate. We show, under quite general conditions, that maintaining a Q22 of the magnitude necessary to balance the accretion torque requires a dimensionless strain at near-Eddington accretion rates, of order the breaking strain of conventional materials. This leads us to speculate that accreting neutron stars reach the same equilibrium spin because they all are driven to the maximum Q22 that the crust can sustain.

Baroclinically driven estuarine flow: A perturbation approach

The residual circulation and density in a shallow estuary of constant depth and width and flowing at high Rayleigh number (Ra) and small internal Froude number (Fr), based on riverine flow (U∞) and the horizontal density variation, are studied. Criteria under which direct tidal effects on the residual flow can be neglected are established. The effects of varying the bottom boundary condition on the velocity field from free slip to no slip are examined using a linear bottom stress boundary condition. The governing equations are nondimensionalized, expanded in a power series in the small parameter, and the zeroth‐ and first‐order equations governing the density field are solved; these solutions yield the zeroth‐ and first‐order results for the velocity field. The equation governing the density field at lowest order is nonlinear and is solved numerically. The results show that for sufficiently large Ra, the mean transport of salt is balanced by baroclinic advection near the mouth and by horizontal turbulent transport farther upstream. For these large Ra the horizontal structure as well as the vertical structure of the flow is strongly affected by the bottom boundary condition with the salinity disturbances determined under the free‐slip condition extending twice as far upstream as the no‐slip results. For smaller Ra, though still large compared to 1, the mean transport is balanced by horizontal mixing throughout the system, and the bottom boundary condition does not affect the horizontal structure. The penetration distance of oceanic waters, with the internal Fr fixed, increases as Ra increases above 24, reaches a maximum upstream penetration distance of Kh/U∞ when Ra reaches 65, and decreases as Ra increases further. The ratio of entrained oceanic waters to riverine inflow is inversely proportional to the internal Fr to the 2/3 power for very large Ra and becomes proportional to Ra as the latter decreases.

A coupled three-basin sea level model for the Baltic Sea

Sea level variations in the Baltic Sea on time-scales 2 days and longer are studied using a coupled three-basin quasi-stationary model. The three large sub-basins (Baltic proper, Bothnian Sea and Bothnian Bay) are coupled in series by two short and narrow straits. A third strait connects the Baltic proper with the Kattegat. Currents are forced through the connecting straits by the sea level difference. The mean sea levels in the sub-basins change due to freshwater supply and the water exchanges through the straits. Wind stress and horizontal variations of air pressure and density maintain sea-level slopes in the sub-basins. With the model, 85% of the total variance of the recorded sea levels can be explained. The unexplained variance is partitioned among different period bands, and for periods longer than 2 weeks 20% or less of the variance is unexplained. For shorter periods, 30–60% of the variance is unexplained. The relative importance of different forcings is also investigated. For periods longer than a few months the forcing provided by the sea level variations in the Kattegat is dominating but horizontal variations of air pressure and density also contribute. For short periods (2 days–2 weeks) the wind stress is most important, especially during winter. Estimated barotropic water transports between the subbasins are investigated, and it is found that major Baltic outflows have the same intensity and magnitude as major Baltic inflows.

On the effects of rotation on fluid motions in cylindrical containers of various shapes and topological characteristics

Abstract Previous theoretical and laboratory studies of mechanically driven fluids in general rotation relative to an inertial frame have shown that there is a special class of flows for which the (Eulerian) flow field u ( r , t ) relative to the rotating frame of reference is unaffected by gyroscopic (Coriolis) forces, and therefore remains the same for all values of the rotation vector Ω . (Here t denotes time and r the position of a general point R in a reference frame attached to the rotating apparatus.) Such flows occur when (a) Ω is independent of time t ; (b) u ( r , t ) is independent of the coordinate z (say) parallel to Ω , (c) the fluid has constant density and is therefore ‘barotropic’ (i.e. no density variations on horizontal surfaces) and (d) the topology of the cross-section of the (cylindrical) container, in planes z = constant, is such that the bounding surfaces can support the concomitant field of (kinematic) pressure P 1 satisfying ▿ P 1 + 2 Ω × u = 0 Condition (d) is equivalent to the requirement that any fluid sources or siks within the system be multipole in character, but not monopole. In the present study the ‘baroclinic’ case is treated, where buoyancy forces due to the action of gravity (and centripetal forces) on horizontal density variations have to be taken into account. These include investigations of flows due entirely to buoyancy forces, such as thermal convection in fluids in rotating cylindrical containers of various shapes and topological characteristics subject to horizontal temperature gradients. The implications for the impressed temperature field of the mathematical requirements that the fields of kinematic pressure P 1 and density ϱ ϑ (where ϱ denotes the mean density) be everywhere single-valued are guiding such investigations and facilitating the interpretation of their findings. The investigations include laboratory studies, reported elsewhere, of convection in a rotating fluid annulus with a circular cross-section blocked by a radial barrier, where it is found inter alia that advective heat transfer is virtually independent of | Ω | over a wide range of conditions. They also include (as yet unpublished) studies of thermal convection in rotating systems with topologically triply connected cross-sections which can be rendered doubly or simply connected by the insertation of suitable barriers.

Mean sea‐level topography in the Baltic sea determined by oceanographic methods

The long‐term mean sea level in the Baltic Sea is investigated using the coupled three‐basin model constructed by Carlsson (1997). The model is forced by the observed sea level in the Kattegat, the freshwater supply, horizontal air pressure and density gradients, and the wind stress. Both the seasonal variations and the slope of the mean sea level are investigated and compared with the results of another oceanographic model (Lisitzin, 1962) and a geodetic model (Ekman & Mäkinen, 1996). In the geodetic model an unofficial height system, NH60, is used, and one part of the investigation is to find out whether this height system is useful for oceanographers. The estimated mean sea level difference between the northern and the southern parts of the Baltic Sea are: 17.1 cm (the present model), 26 cm (Lisitzin's model), and 18.3 cm (the geodetic model). It is concluded that the mean sea level difference between the northern and southern parts of the Baltic Sea is due mainly to horizontal variations of density and air pressure, and that the height system NH60 is suitable for oceanographie applications.

On horizontal density variation in randomly-formed short-fibre wood composite boards

Abstract A mathematical model for characterizing horizontal density variation of a short-fibre wood composite panel is derived in terms of local point density variance V(D), point autocorrelation function η(Z,λ,ω) and variance of density averages between finite sampling zones V(Ď). The point density variance is the upper-bound of zone density variance as zone size become infinitely small. Assuming random mat formation, the model predicts how the horizontal density variation changes with fibre dimensions and other processing parameters. The zone density variance V(Ď) as a monotonically decreasing function of zone size is determined by global panel density, compaction ratio (panel-to-wood density ratio), thickness ratio (panel-to-element thickness ratio), and point-to-point density correlation function which, in turn, is a function of element width and length. According to the model prediction, V(Ď) is affected by element width, length and zone size in a highly interactive manner. While increasing width and length and decreasing zone size generally cause an increase in V(Ď), the extent to which V(Ď) changes with one parameter depends on the magnitude of others. The predictive results on how the zonal density variation is affected by the processing parameters are graphically presented and discussed.

A model of the effects of horizontal density gradients on mixing processes in the oceanic surface layer

A diurnal Eulerian model of the upper mixed layer incorporating the effects of horizontal density gradients is developed based on that of Janowitz and Kamykowski [1991] and is utilized to study the effects of wind‐driven cross‐isopycnal transport on mixing processes in the layer. Four runs of the model are conducted for a constant horizontal density gradient of 2.5 × 10−5 kg m−4 with wind directions at 0°, 90°, 180°, and 270° to this gradient. Two runs are conducted with half this density gradient and wind directions 90° to the left and right of the gradient and one run with no density gradient. In all cases the model is first run with a constant heat loss rate of 150 W m−2 through the sea surface for 11 hours to simulate nighttime cooling and to provide initial conditions for daytime runs. The model is then run for 24 hours with a constant wind speed of 7 m s−1 and a maximum solar radiation of 900 W m−2 during the first 13 hours of the 24‐hour simulation. The results show that the shallowest mixed layer occurs when the wind direction is 90° to the left of the density gradient and the deepest layer occurs with the wind direction 90° to the right of the gradient. After 24 hours, the ratio of the deepest to the shallowest mixed layer depth is 2.35 for the stronger gradient and 1.83 for the weaker gradient. These results are explained by the effect of vertical shear of the wind‐driven advection of horizontal density variations, which changes the vertical density gradient and can destabilize or stabilize the mixed layer. The results indicate that wind‐driven, cross‐isopycnal transport can have significant effects on mixing processes in the layer. Dimensionless parameters are developed to estimate the relative importance of cross‐isopycnal, wind‐driven transport on mixed layer properties.

Evidence of in situ diel vertical migration of a red-tide microplankton species in R�a de Vigo (NW Spain)

The migration capacity of red-tide species in the natural environment was studied at a station in the Ria de Vigo (Rias Bajas, NW Spain) over a 24 h period in September 1991. The Ria de Vigo, where red tides are frequent, normally shows a positive estuarine circulation and is subjected to seasonal upwelling and downwelling phenomena. A marked diel pattern was observed for five species that are capable of causing red tides (Ceratium furca, Scrippsiella trochoidea, Dinophysis acuminata, Mesodinium rubrum, and Eutreptiella sp.). Such diel behaviour could be clearly advantageous in a stratified environment where light and nutrients are often in two separate layers. Active movement enables species such as dinoflagellates and some ciliates to exploit high levels of irradiance at the surface during the day and to take up nutrients in deeper layers at night. Patchy distribution of phytoplankton, shear-induced horizontal dispersion and density variations were considered, but none of them accounted for the vertical changes observed. Vertical migration is thought to be one of the mechanisms that could promote blooms in nutrient-depleted surface layers.

The evolution of deformation and topography of high elevated plateaus: 1. Model, numerical analysis, and general results

A temperature dependent viscoplastic flow model of continental lithosphere is used to investigate the evolution of deformation and topography of high elevated plateaus. Such plateaus are products of both continent‐continent collision (Tibetan Plateau) and ocean‐continent collision (the Altiplano) and develop in the overriding continental plate. In this study we emphasize the mechanically simpler case of oceanic collision, because it does not involve mass transfer between the two plates. The lithosphere is deformed in response to tectonic and buoyancy forces. The tectonic forces arise from subduction of an oceanic plate (or underthrusting of continental lithosphere) that horizontally indents and vertically shears the overriding lithosphere. The buoyancy forces arise in response to horizontal density variations and tend to relax existing topography or thick crust. The time evolution of the deformation and topography is investigated using a finite element technique that solves for the flow field in the overriding lithosphere. The model produces dynamically supported near‐trench topography and inland mountain topography that is isostatically supported by a thick crust. A finite region of localized deformation, thick crust, and high topography develops only if the model includes a horizontal thermal perturbation or an initially thick crust; however, only thermally perturbed lithosphere generates a plateau topography. The shape and size of the calculated plateau depend on the wavelength of the thermal perturbation, Grashof number, and density contrast between the crust and mantle. The time evolution of the deformation shows a significant change in the deformation pattern as the high elevated plateau evolves. During early stages, compressional deformation of the crust and mantle are localized in the thermally perturbed weak zone. At later stages, as the crust thickens, buoyancy forces of larger magnitude resist further thickening of the crust and the locus of compressional crustal deformation migrates inland. This migration does not affect the location of the mantle deformation, which remains in the thermally weak region, but it is accompanied by a significant shear deformation in the weak lower crust. The separation of the crustal locus from the mantle locus of deformation emphasizes the importance of vertically dependent deformation in the formation of high elevated plateaus. This demonstrates the limitations of models that ignore changes of deformation with depth, such as plane stress or thin sheet models.

Dynamically supported trench topography

A viscous flow model of a subduction zone is used to calculate the near‐trench deformation and topography of the overriding plate in response to tectonic and buoyancy forces. The tectonic force, which is associated with global plate motion, arises from subduction of a cold oceanic lithosphere that shears the overriding lithosphere along the contact between the two plates. The buoyancy force arises in response to horizontal density variations and tends to relax the existing topography. The time evolution of the near‐trench topography is investigated via a finite element technique that solves for the flow field in the overriding plate. The results indicate that the near‐trench topography approaches a steady state configuration, in which the upper surface topography produces a buoyancy force that balances the tectonic force induced by the subducting slab. The model predicts that the steady state trench depth increases with the subduction rate, and hence explains the correlation between trench depth and subduction rate observed by Grellet and Dubois (1982). Transition from one steady state trench topography to another adds or removes material from the overriding plate; this may result from a change in subduction rate, angle of subduction, or subduction of a seamount. During a transition to a deeper steady state configuration, the near‐trench stress field has an extensional component, which agrees with the observed extension in regions of tectonic erosion. Similarly, during a transition to a shallower steady state configuration, the near‐trench stress field has a compressional component, which agrees with the observed compression of accretionary wedges. Hence, the model explains tectonic erosion and accretion at trenches as transitional features that indicate a change from one steady state configuration to another.

Tidal and buoyancy‐driven currents in Chignecto Bay, Bay of Fundy

Circulations in the Chignecto Bay system (upper part of the Bay of Fundy) were studied using current meter moorings, tidal and diagnostic models, and salinity and suspended sediment concentration measurements. The current meter observations revealed that (1) the tides behaved basically as standing waves (tidal current leads tidal elevation by about 90°); (2) tidal currents were typically of the order of 1 m s−1 and were almost rectilinear; (3) residual currents were up to 0.17 m s−1 and the circulation pattern was quite complex; and (4) a two‐layer residual circulation with inflow in the upper layer and outflow in the lower layer existed at the entrance to the bay. Most of the residual currents in the upper part of the bay were driven by tidal rectification. The two‐layer residual circulation at the bay entrance was examined using a diagnostic model. It was found that the circulation was probably induced by horizontal density variations. Near the entrance to Cumberland Basin, Shepody Bay, and Chignecto Bay, the cross‐channel variation of surface suspended sediment concentration was consistent with the observed residual currents from current meter moorings and the estimated currents from the diagnostic model. The cross‐channel variation of salinity near the entrance to Chignecto Bay was also consistent with the observed and computed residual currents.

Observations of small eddies in the Beaufort Sea

The horizontal and vertical variations of velocity, temperature, density, and turbulent kinetic energy dissipation in the central Beaufort Sea were measured with helicopter‐borne expendable current profilers and conductivity‐temperature‐depth units. Almost all velocity differences larger than 0.05 m s−1 could be consistently interpreted as being due to nearly circular eddies with a diameter of 10–20km, a thickness of 50–500 m, peak velocities of 0.07–0.3 m s−1, and little expression at the top of the ocean. Four such eddies were found. Two anticyclones were found and accurately sampled. Two cyclones were found but were only poorly sampled; both were beneath and close to one of the anticyclones. Relative vorticity could be computed for two anticyclones and one cyclone. Within experimental error, the two anticyclones, and all historical Beaufort Sea anticyclones, had a relative vorticity of −ƒ and thus zero potential vorticity at the center. The cyclone had a central relative vorticity of about 0.5ƒ and thus a different value of potential vorticity than the surrounding fluid. One anticyclonic eddy (B) was sampled seven times over a period of 23 days. Its measured velocity and density fields were in approximate gradient wind balance. Its central core contained values of temperature, nitrate, and silicate that were different from those outside the core at the same density. It moved eastward at about 0.02 m s−1 through the surrounding fluid. Measurements of turbulent kinetic energy dissipation were almost always less than the noise level of the instrument (about 10−9 W kg−1), corresponding to a mean dissipation much less than this and a minimum decay time due to turbulence much longer than 0.3 year. The nondimensional strength (Rossby number) and shape (Burger number) of anticylcone B are very similar to those of other Beaufort Sea anticyclones and similar to those of small mid‐latitude eddies. Each of the eddies observed here was centered on a temperature maximum associated with one of the warm water masses that advect into the Arctic Ocean. The two anticyclones were centered on temperature maxima originating in the Pacific Ocean, whereas the two cyclones were centered on temperature maxima from the Atlantic Ocean. This is consistent with the hypothesis that the eddies are formed as these water masses enter the Arctic Ocean.

A Two-Level Model of a Thermally Forced Ocean Basin

Abstract Some simple solutions (mostly analytic) are presented for the large-scale baroclinic response to thermal forcing on a mid-latitude beta-plane. Surface heat flux is parameterized as (TA–TT)/tau;, with atmospheric temperature TA prescribed as a function of latitude, varying ocean surface temperature TT, and equilibration time τ. For long times (decades) benthic forcing is included, using a similar representation. The model allows horizontal density variations at each level. When there are no meridional boundaries there is only a local response to the forcing. A geostrophic baroclinic zonal flow is driven by the north–south temperature gradient, but it has no associated advection or divergence effects. This picture is greatly changed when east and/or west coasts are added. Kelvin waves pass information rapidly (about 200 km day−1) along coasts, and Rossby waves travel slowly offshore, most effectively from the cut with speed c ≈ 1 km day−1. For spin-up problems (e.g., the response to a change in for...

A suspended sediment front in the Severn Estuary

A zone of marked local gradient in the regional suspended solids field has been located along the axis of the Severn Estuary. This front occupies virtually the same position on ebb and flood and on spring and neap tides, although the amplitude of the gradient fluctuates. Thermohaline fronts have been described in coastal waters1,2 suspended sediment fronts associated with the discharge of major rivers into the sea are common3–5 whilst fronts caused by convergence of secondary circulations within estuaries have also been reported6,7. The previous work describes features with significant differences in the salinity or temperature fields in which any suspended sediment contrasts present are incidental and do not, of themselves, produce large horizontal density variations. In contrast, in the Severn, strong thermo-haline gradients in the vicinity of the suspended solids front have not been detected. The mechanisms for the formation and maintenance of the front are not yet known.

The atmosphere of Jupiter: An analysis of the Voyager radio occultation measurements

Coherently related S (2.3 GHz) and X band (8.4 GHz) signals transmitted from Voyager 1 and 2 have been used to probe the Jovian atmosphere during occultations of the spacecraft by Jupiter. The observations have yielded profiles in height of the gas refractivity, molecular number density, pressure, temperature, and microwave absorption in the troposphere and stratosphere of Jupiter at latitudes ranging from 0° to about 70°S. The data cover a pressure range from 1000 to 1 mbar over a height interval of 160 km. At the 1000‐mbar level, the temperature was 165±5 K, and the lapse rate was equal to the adiabatic value of 2.1 K/km, within the resolution of the measurements. The ammonia abundance in this region of the atmosphere was about 0.022±0.008%, in approximate agreement with the value derived from cosmic abundance considerations. The tropopause, which was detected near the 140‐mbar level, had a temperature of 110 K. Above the tropopause, the temperature increased with increasing altitude, reaching 160±20 K in the 10‐ to 1‐mbar region of the stratosphere. Significant horizontal density variations were detected in the stratosphere. This may imply a nonuniform temperature and aerosol distribution across the Jovian disk or high‐ and low‐pressure regions due to local atmospheric dynamics. The zenoid or gravity equipotential surface which best fits the 100‐mbar isobaric surface has an equatorial radius of 71,541±4 km and a polar radius of 66,896±4 km.

Seismotectonics of northeastern United States and adjacent Canada

Data for local earthquakes recorded by a network of stations in northeastern United States and adjacent Canada were analyzed to study the seismicity, the relationship between earthquakes and known faults, the state of stress, and crustal and upper mantle velocity structure. In addition, portable seismographs were deployed in the field to study aftershocks. As a result, accurate locations for about 364 local earthquakes (2 ≤ mb ≤ 5) and 22 focal mechanism solutions were determined. A comparison of the spatial distribution of these events (1970–1979) with historical earthquakes (1534–1959) reveals that seismic activity in the northeast is relatively stationary in space: those areas that have had little or no seismicity historically are relatively aseismic today, whereas the historically active areas are also active today. The instrumental locations, historical seismicity, and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces. (1) The Adirondack‐western Quebec province is a northwesterly trending zone of seismic activity, about 200 km wide and at least 500 km long, extending from the SE Adirondacks into western Quebec, Canada. Thrust faulting on planes striking NNW to NW appears to predominate, and the inferred axis of maximum horizontal compression is largely uniform and trends WSW, nearly parallel to the calculated absolute plate motion of North America. Little or no seismicity is found where anorthosite outcrops at the surface. Correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep NE or SW gradient in Bouguer anomalies. This zone does not appear to extend southeastward to Boston, as proposed by some workers. (2) The Appalachian province is a northeasterly trending zone of seismic activity extending from northern Virginia to New Brunswick, Canada. Highangle reverse or thrust faulting on N to NE trending planes appears to predominate. The western margin of this province, however, appears to be relatively aseismic. We attribute this relative lack of activity to one or more of the following: the presence of igneous activity postdating rifting of Africa from North America, the occurrence of intense metamorphism during the Acadian orogeny which may have annealed preexisting faults, and the predominance of ductile as opposed to brittle deformation in the geologic past. The inferred axis of maximum horizontal compression along the eastern margin of the Appalachians is rather uniform and trends W‐WNW, almost perpendicular to the magnetic lineations offshore. We suggest that this W‐WNW compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers.

Pioneer Venus Orbiter Neutral Gas Mass Spectrometer Experiment

The Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) is designed to measure the vertical and horizontal density variations of the major neutral constituents in the upper atmosphere of Venus. The mass spectrometer sensor includes a retarding potential ion source, hyperbolic quadrupole rod analyzer, and electron multiplier detector. The supporting electronic system consists of hybrid integrated circuits to reduce weight and power. The ONMS instrument was launched aboard the Pioneer Venus Orbiter on May 20, 1978, and turned on in orbit around Venus on December 4, 1978. It has operated flawlessly for over a Venus year (243 earth days) and has returned data of the composition of the major constituents in the Venus atmosphere between the altitudes of 150 and 350 km.