Vertical Density Structure Research Articles

Photon bubbles - Overstability in a magnetized atmosphere

The formation of 'photon bubbles' in a convectively stable scattering atmosphere supported against gravity entirely by radiation pressure is studied by means of linear stability theory. A simple model is developed for the 2D structure of a plasma mound formed by laminar accretion onto the magnetic poles of a neutron star, in which upward photon diffusion balances downward photon advection with the plasma. It is shown that the vertical pressure and density structure is the same as in an isothermal atmosphere. Application of the stability theory to this model suggests photon bubbles would form in a polar accretion mound under the conditions expected in accretion-powered pulsars within a few tenths of a millisecond. Because long-wavelength modes have the largest rise speeds, eventual dominance by a few large bubbles is suggested, and possible connections between bubble formation and short-time variability in accretion-powered pulsars is discussed, as well as a possible connection of the photon bubble phenomenon to the rapid time variability observed in the Rapid Burster and in quasi-period oscillator sources.

Solar cycle effects on the structure of the electron density profiles in the dayside ionosphere of Venus

Since 1975, United States and Soviet Venus‐orbiting spacecraft have carried out more than 190 radio occultation measurements of the vertical electron density structure of the dayside ionosphere of Venus: 148 of these have been produced by the Pioneer Venus orbiter between 1979 and 1989; Venera 9 and 10 provided 13 measurements in 1975; and Venera 15 and 16 made 31 measurements in 1983. These measurements were made at various times of the solar cycle, including solar maximum and solar minimum. It has therefore been possible to observe the changes in the electron density structure of the dayside ionosphere of Venus that were brought about by changing solar activity. Some of the resulting observations are as follows: (1) The ionopause height is generally low for values of the solar zenith angle below about 50° regardless of the phase in the solar cycle. (2) At solar maximum, and at times of intermediate solar activity, the ionopause height for solar zenith angles greater than about 50° is highly variable, ranging from a minimum of about 200 km to a maximum of more than 1000 km. (3) At times of solar minimum the great majority of all ionopause heights for all solar zenith angles are uniformly low, lying between 200 and 300 km, with the notable exception of three Venera 9 and 10 measurements. The compressed nature of the Venus ionosphere at solar minimum is most likely produced by permeation of the ionosphere by the solar wind magnetic field, which occurs when the solar wind dynamic pressure exceeds the ionospheric plasma pressure. This happens at times of solar maximum at low solar zenith angles and when the solar wind dynamic pressure is exceptionally high. At solar minimum the Venus ionosphere, like the Martian ionosphere at all phases of the solar cycle, does not appear to have sufficient thermal pressure to balance the incident solar wind pressure.

Determination of Precipitation Profiles from Airborne Passive Microwave Radiometric Measurements

This study presents the first quantitative retrievals of vertical profiles of precipitation derived from multispectral passive microwave radiometry. Measurements of microwave brightness temperature (Tb) obtained by a NASA high-altitude research aircraft are related to profiles of rainfall rate through a multichannel piecewise-linear statistical regression procedure. Statistics for Tb are obtained from a set of cloud radiative models representing a wide variety of convective, stratiform, and anvil structures. The retrieval scheme itself determines which cloud model best fits the observed meteorological conditions. Retrieved rainfall rate profiles are converted to equivalent radar reflectivity for comparison with observed reflectivities from a ground-based research radar. Results for two case studies, a stratiform rain situation and an intense convective thunderstorm, show that the radiometrically derived profiles capture the major features of the observed vertical structure of hydrometer density.

Pattern of planktonic bioluminescence in the northern Sargasso Sea: Seasonal and vertical distribution

The vertical structure of bioluminescence potential (BPOT) and flash density (FD) were measured on five cruises to the northern Sargasso Sea in 1987 and 1988. Depth-integrated (0 to 150 m) BPOT did not vary seasonally, remaining within the range 9 to 15 × 1015 photons m−2 in all months sampled. Conversely, depth-integrated FD was significantly higher (> 2 × 105 flashes m−2) during winter (November and March) than during summer (< 9 × 104 flashes m−2 in May and August). The vertical patterns of BPOT and FD were well correlated within a single profile, more highly so in summer than in winter. Despite intracruise variability in the vertical pattern of BPOT and FD, there were clear summer-winter differences in the vertical distribution of BPOT and FD. During winter, BPOT and FD were maximal and relatively uniform throughout the surface mixed layer; for example in November they declined sharply within the thermocline at 130 to 150 m. During summer, BPOT and FD were greatest (12 to 25 × 1013 photons m−3 and 600 to 1 200 flashes m−3, respectively) at subsurface depths. Commonly in summer, the upper depth limit of high BPOT and FD occurred at the base of the surface mixed layer (10 to 40 m) and the lower depth limit was located at the base of the subsurface fluorescence maximum (usually at 100 to 120 m).

Flare heating and ionization of the low solar chromosphere. I - Inversion methods for MG I 4571 and 5173

Various methods of inverting solar Mg I 4571 and 5173 spectral line observations are examined to find the best method of using these lines to calculate the vertical temperature and electron density structure around the temperature minimum region. Following a perturbation analysis by Mein (1971), a Fredholm integral equation of the first kind is obtained which can be inverted to yield these temperature and density structures as a function of time. Several inversion methods are tested and compared. The methods are used to test data as well as to a subset of observations of these absorption lines taken on February 3, 1986 before and during a solar flare. A small but significant increase is found in the temperature and a relatively large increase in the electron density during this flare. The observations are inconsistent with heating and ionization by an intense beam of electrons and with ionization by UV photoionization of Si I.

The vertical structure of density and turbidity currents: Theory and observations

A simple numerical model is developed that successfully simulates observations of small‐scale, laboratory, density currents flowing down inclines of constant slope. The model results suggest that laboratory determinations of the bulk Richardson number have been biased by molecular processes but that determinations of the entrainment coefficient are probably applicable to large‐scale currents, and even to turbidity currents in which the gravitational driving force is provided by suspended sediment. The entrainment coefficient is a function of the gradient Richardson number Ri above the velocity maximum in both density and turbidity currents; Ri is itself a strong function of the bottom slope but not of the sediment settling velocity or the roughness of the bottom boundary. Therefore for a given bottom slope the entrainment coefficient is uniquely defined. The bulk Richardson number Ri0, on the other hand, is additionally a function of the drag coefficient, which depends on both the Reynolds number and the bottom roughness. However, Ri0 is not a strong function of the sediment settling velocity as long as the current is energetic enough to maintain sediment in suspension.

Hydrogen line emission from optically thin accretion disks

The results of a non-LTE accretion disk model for the calculation of hydrogen line emission from regions of a disk which are optically thin in the continuum are presented. Each model calculated would apply to only one annulus radially outward in a real accretion disk. The vertical density structure in the disk is treated properly, and the transfer of line radiation is done using the escape probability formalism. The results indicate that the observed Balmer decrements are functions of the density in the disk. Flat Balmer decrements from the disk as a whole are still expected, however, as the decrements are near unity as the disk becomes optically thick in the continuum. Line profiles are presented and show that a double-peaked structure is expected even for disks viewed face-on. 30 references.

Water-column chlorophyll in an oligotrophic environment: correction for the sampling depths and variations of the vertical structure of density, and observation of a growth period

The chlorophyll content of a water column ( WCC ), which is commonly used as an index of the phytoplankton abundance, is affected by the choice of the sampling depths and by the variations of the vertical structure of density. For instance, the thickness of the water layer, between two sigma- t values, which contains the deep chlorophyll maximum, can vary with internal waves. The resulting noise often dominates the mesoscale variations of the observed water-column chlorophyll ( OWCC ). Sigma- t dependent statistics (mean, standard deviation) of the chlorophyll concentration are computed using the observations at 29 casts from a 22-day-long fixed station in an oligotrophic environment at 15•S, 173°E. For each cast, these statistics, the sampling depths, and the water density at these sampling depths, allow the estimation of a station-dependent ‘expected water-column chlorophyll’ ( EWCC ). The ratio of EWCC to the overall likelihood of WCC during the fixed station (i.e. the mean of all OWCC ) is a measure of the effect of sampling and variable density structure at each cast. When this effect is removed, the noise in WCC estimates decreases significantly. The time variations of WCC during the fixed station then show a trend with relatively high values during the first days, followed by a 12-day-long period with low values. A regular increase occurred from 1 October, which was accompanied by high carbon fixation rates and was mainly due to an increase of the chlorophyll concentration between the surface and the deep chlorophyll maximum. New production during this active phase was estimated to be 535 mgC m −2 day −1 , corresponding to 62% of the total production. Breaking of internal waves which were recorded at the beginning of the growth phase and vertical mixing of nutrients can explain the observation.

Hydrodynamic ejection of intrinsically bipolar flows

A general mechanism is presented for generating pressure-driven winds, which are intrinsically bipolar, from objects undergoing disk accretion. The energy liberated in a boundary-layer shock as disk matter impacts the central object makes the vertical density structure of the boundary layer go out of hydrostatic equilibrium by large factors. The resultant expansion in the direction of the density gradient, i.e., perpendicular to the disk, converts accretion energy into P dV work and is shown to be capable of ejecting a fraction β ~ 10−2–10−1 of the accreted mass from the system. The ejection of these flows, accelerated within [Formula: see text], can account for the jets observed from pre-main-sequence stars and cataclysmic variables. Unless an analogous boundary-layer shock develops during accretion onto massive black holes, however, this mechanism does not seem to be applicable to jets from galaxies.

Some numerical experiments on the withdrawal of magma from crustal reservoirs

Compositional heterogeneity in the form of continuous or discontinuous chemical and thermal gradients in lava and/or pyroclastic flows is very common. An understanding of the dynamics of magma withdrawal is essential to palinspastic reconstruction of intensive variable gradients in magma reservoirs. Important parameters governing the extent of subterranean magma mixing triggered by an eruption include the vertical structure of density and viscosity within the chamber, the discharge, the size and shape of the chamber, and whether eruption takes place along a sublinear fissure, a ring fracture, or a central vent. A numerical model has been set up to study isoviscous magma withdrawal from a central vent conduit as a function of the Reynolds number, the reservoir to conduit width ratio, reservoir aspect ratio (width/depth), and differing kinematic boundary conditions. Both open (magma recharge) and closed (caldera collapse) system behavior are considered. Finite difference solutions to the vorticity transport and Poisson equations enable determination of vorticity, stream function, and velocity fields as a function of time. The most petrologically significant output is the stream function (particle trajectories) and evacuation isochron diagrams. An evacuation isochron represents the locus of points within the chamber such that magma parcels along a given isochron arrive at the bottom of the volcanic conduit concurrently. Open systems evolve toward a time invariant state (fully developed flow). Spin‐up times depend on chamber aspect ratio (Br/Dr), reservoir/conduit width ratio (Br/Bc), and Reynolds number (Re). Br, Dr, and Bc represent chamber half‐width, depth, and conduit half‐width, respectively. Spin‐up times are relatively small (1/10 to 1/5) fractions of typical eruption durations. The shape and orientation of evacuation isochrons (EI) depend on Re (increasing Re decreases withdrawal depth) and geometric factors (increasing Br/Bc at constant Re and Br/Dr or decreasing Br/Br at constant Br/Bc and Re increases withdrawal depth). A significant amount of roofward magma can remain untapped in a chamber even for long duration eruptions. Systems driven by caldera collapse also involve juxtaposition of roofward and deep‐seated magma during the course of an eruption. Relative to the magma recharge (open system) situation, EI's are laterally elongated. The extent of vertical mixing is thus smaller although still significant in this case. Maximum withdrawal depths vary monotonically in both cases. There is excellent qualitative agreement between predictions based on the numerical experiments and Fe‐Ti oxide temperatures for a thermally zoned ash flow deposit south of Mono Lake in eastern California (Bishop Tuff).

Classification and dynamic simulation of the vertical density structure of lakes1

Field data from two lakes of widely differing geometry and size are analyzed in terms of four nondimensional numbers which allow the principal mixing processes in each lake to be identified. The numbers are based on basin geometry, density stratification, wind stress, and rates of inflow and outflow. The procedure highlights the differences in the dynamics of the two lakes and allows assessment of the validity of the assumption of one‐dimensionality. The result is that both lakes were dominated by one‐dimensional, but different processes. The dynamics of the epilimnion of the smaller lake were dominated by stirring from surface wind and cooling, whereas shear at the pycnocline was also significant in the larger lake. In neither case did the effects of the earth’s rotation, inflow, or outflow generate significant horizontal gradients. A one‐dimensional numerical model (DYRESM) was used to simulate the vertical temperature and salinity structures of both lakes over lengthy periods, with good results. The model is based on the parameterization of the important physical processes in a framework of horizontal layers of variable thickness and was applied in both lakes without alteration. The interpretative power of the model is demonstrated by examination of the formation and erosion of a thermal inversion in the larger lake.

Vertical density and temperature structure over Northern Europe

During the Energy Budget Campaign, several profiles of the density and temperature of the upper atmosphere were obtained. The measurements were made using rocket-borne instrumentation launched from ESRANGE, Sweden and Andoya Rocket Range, Norway during November and December, 1980. The techniques included meteorological temperature sondes, passive falling spheres, accelerometer instrumented falling spheres, density gauges, mass spectrometers and infrared emission experiments. The instruments provided data within the altitude range from 20 km to 150 km. The measurements were made during periods which have been grouped into three categories by level of geomagnetic activity. Analysis has been made to compare the results and to examine the oscillations and fluctuations in the vertical profiles for scales ranging between hundreds of meters and tens of kilometers. Most of the features observed fit qualitatively within the range expected for internal gravity waves. The geomagnetic storm conditions may be associated with enhanced wave activity and heating observed in the lower thermosphere.

Water Column Stability and Photosynthetic Capacity of Estuarine Phytoplankton: Long-Term Relationships

In estuaries, the intensity of vertical mixing due to tides varies over a broad range of time scales. In the St. Lawrence Estuary, Canada, increased stability of the water column has 2 effects on the phytoplankton: (1) a physiological effect in the form of a light-dependent increase in the photosynthetic capacity; (2) a population response in the form of an increase in chlrophyll a variability. Phytoplankton cells must be able to adjust their metabolic activities to highly fluctuating environmental conditions. Previous experimental studies demonstrate that they can adjust rapidly to changing light intensities through variations in chlorophyll content, thus allowing utilization of available light with maximum efficiency (Owens et al., 1978; Riper et al., 1979; Falkowski, 1980; Harris, 1980). If physiological adjustment by the phytoplankton is faster than changes in environmental conditions, the cells continuously adjust their metabolic activities to the new conditions (Vincent, 1980); if not, the cells can only adjust to mean environmental conditions (Savidge, 1979; Falkowski, 1980). An example of such a physiological adjustment is that of the photosynthetic capacity of phytoplankton which should respond to fluctuating environmental conditions, since it is influenced by light conditions to which the cells were previously exposed (Prezelin et al., 1977; Prezelin and Sweeney, 1977, 1978). In estuaries, the intensity of vertical mixing due to the tides varies over a broad range of frequencies: monthly (M,), fortnightly (Mf) and semidiurnal (M2) tidal harmonics, and slack waters. Vertical mixing not only affects nutrient and oxygen redistribution in the ' Contribution to the program of GIROQ (Groupe interuniversitaire de recherches ocCanographiques du QuBbec) Centre Champlain des Sciences de la mer, Sciences et lev6s ockaniques, C.P. 15 500, 901 Cap Diamant, Qukbec, Canada G1K 7Y7 O Inter-Research/Printed in F. R. Germany water column (Webb and D'Elia, 1980), but also the distribution of plankton organisms in the mixed layer. Since tidally-induced vertical mixing is highly prevalent in estuaries, they provide a unique experimental opportunity to study the effects of vertical mixing on phytoplankton. In the St. Lawrence Estuary, Quebec, a research program (ECOVARIATE) was conducted on estuarine variability between 1975 and 1977. Results on phytoplankton were pubhshed by Fortier et al. (1978), Frechette and Legendre (1978), Demers et al. (1979), Demers and Legendre (1979), Fortier and Legendre (1979), Lafleur et al. (1979), Roy and Legendre (1980), Demers and Legendre (1981), and Frechette and Legendre (in press). In the present study, the influence of varying degrees of vertical mixing on estuarine phytoplankton is examined, both at individual (physiological) and population (distributional) levels, over a 2-month period. During summer 1975, sampling was conducted at an anchor station (1 m depth) in the Middle St. Lawrence Estuary (the portion between the Saguenay Fjord and Quebec City). This station was occupied during 14 to 18 June, 5 to 12 July, and 1 to 8 August. Hourly measurements were made of chlorophyll a concentration per volume (B), photosynthetic capacity (under saturating light intensity of 375 ~ E i n m-2 S' , in an Hawaiian-type incubator: Doty and Oguri, 1959) normalized for chlorophyll a (E,,), and the vertical density structure of the water column. For more informations about the Methods, see Demers and Legendre (1979, 1981). The difference between the densities at 5 m and 35 m depths (Aa,) was used as an index of the vertical stability, in order to include the whole mixed layer. Under conditions of intense vertical mixing (spring tides), Demers and Legendre (1979) have found circadian variations in the photosynthetic capacity of Mar. Ecol. Prog. Ser. 7: 337-340, 1982 St. Lawrence Estuary phytoplankton. The intense vertical mixing causes all the cells in the mixed layer to experience similar average light conditions (Demers and Legendre, 1981). In order to eliminate these 24-h variations from the long-term trend studied here, the photosynthetic capacity was integrated over periods of 24 h. In addition, mean values of Ao, over 24 h were also computed. A linear increase of daily photosynthetic capacity was found with increasing mean daily density gradient (Fig. 1A). T h ~ s linear relationship is independent of the sampling dates, since the various conditions of stability were encountered at different times during the 2month sampling period. There is a n apparent linear relationship of both chlorophyll a concentrations and photosynthetic capacity with the density gradient (Table 1). However, when partial correlations are comTable 1. Coefficients of linear correlation (upper right, v = 13) and of partial correlation (lower left, v = 12) between daily photosynthetic capacity, daily mean chlorophyll a concentrations, and daily mean density gradient (index of vertical stability of the water column)

On the characteristics of astronomical refraction in the northern hemisphere

The astronomical refraction mainly depends on the vertical structure or the height profile of atmospheric density. Concerning the vertical structure of atmospheric density, various hypotheses were presented during the last century. Among them Newton's hypothesis of equal temperature and Ivory's one that temperature diminishes at a uniform rate with height appear to represent the height profile of temperature approximately in the lower part of the stratosphere and in the troposphere respectively.

An Investigation of the Occurrence of Oceanic Turbulence with Respect to Finestructure

Abstract Data obtained from the cycling mode of a towed system operated in the North Pacific are used to investigate the relationship of small-scale mixing to the finestructure which seems to be such a common characteristic of vertical profiles of oceanic water properties. The signal from a high-frequency response platinum-film thermometer is used, with the local mean vertical temperature gradient, to produce variables proportional to heat flux and an eddy diffusivity for heat. Along with measured values for the local vertical salinity and density gradients, this information is used to examine some questions of interest in the general understanding of turbulence and finestructure. First, it is shown that in regions where the vertical density structure is distinctly layered, there is no noticeable tendency for mixing to occur preferentially on “sheets,” the high-density-gradient regions which separate “layers of lower density gradient. Instead, mixing events seem to occur at random with respect to the den...

Dispersion and Flushing of Pollutants in Estuaries

The fate of a pollutant introduced into an estuary depends upon the relative density of the effluent and the receiving water, the vertical density structure of the estuary, the strength of the tidal currents, the nontidal circulation pattern, and the intensity of turbulent diffusion. Effluents having lower density than the receiving waters, and discharged from an outfall on the bottom, ascend to the surface as a buoyant plume, entraining diluting water en route. Tidal currents advect the polluted volume as an oscillating plume, and turbulent diffusion leads to further dilution of the pollutant. Numerical solutions to time dependent theoretical advection-diffusion equations can be used to depict the probable distribution of a pollutant in a tidal segment of the estuary centered on the outfall. Exchange of pollutant between tidal segments in the estuary and the ultimate flushing of the pollutant from the estuary are best treated using a two-dimensional segmented mathematical model of the estuary.

TIDES IN HOOGHLY RIVER

Summary The Hooghly is a tidal river and Calcutta one of the most important ports of India is situated on it. The tidal effect stretches to over 175 miles length up to Nabadwip where two nonperennial rivers Bhagirathi and Jalangi, both taking off at different sites from the Gangesmeet and discharge their rain water (freshets) into the Hoogly all through the summer months. Other rivers join it also downstream. Annual tide tables are published for three places on the river viz Sagar, Diamond Harbour and Garden Reach. In this paper lunitidal intervals, durations of rise and fall, mean high water, mean low water, mean tide level and mean sea level planes and mean ranges have been shown to go through periodic seasonal changes. It has been shown that different tidal planes have changed differently in the intrior of the river indicating changes in tidal regimes and in the river bed. Whereas variations in yearly sea levels since 1882 have not been appreciable at Sagar, these at Garden Reach have changed considerable at indicating sometimes an improvement in the channel, other times worsening of the channel. Four 19 yearly cycles of mean sea-level at Garden Reach have been analysed to give phaselags different from Zero and large variations in amplitudes. To carry this study further one period of 19 years each for Sagar, Madras and Vishakhapatnam on East Coast and 4 periods each for Bombay and Aden in the Arabian Sea have also been analysed to see the possibility of nodal variations being constant in amplitude and phase over large areas of the ocean. 19-yearly cycles in the differences between MTL & MSL have been examined for Bombay, Madras and Garden Reach. The effects of freshets on various planes have been analysed and the curves of annual freshets and MTLs at Garden Reach are found to run closely parallel. The variations in yearly high water and law water planes above MTLs of the corresponding years after corrections for the longitude of the moons' node have been found to be inappreciable. Theoretical extreme high water and low water planes have been found to be more extreme than actually recorded planes which are very near M.H.W.S. and M.L.W.S. planes indicating there are no predominant effects of winds and storms on the tides in the river. The mean ranges at Garden Reach have been found to be steadily increasing. Theoretical effects of increase in range and lowering of MSL on MHW and MLW plane has been found to agree with actual values. Monthly average temperatures and salinities show distinct annual cycles. High temperatures occur at the same times as low salinities and high mean sea levels. Vertical density structure may be responsible for a large part of annual variation in the sea level at Sagar which is a region of particular interest on account of opposite influences of seasonal monsoons, drainage of rivers and oceanic currents. Seasonal variations in average monthly pressure have been related to variations in mean sea levels and it is found that as the pressures decrease, the height of MSL increases. Tide in Hooghly has been found to be progressive type of wave. Rates of travel of tidal streams outside Sagar and rates of streams in the river have been given also, stating the effects of freshets on tidal streams. Bores in Hooghly have been examined. Shapes of tidal curves and profile of the bore at Garden Reach have been drawn. Rates of propagation of the bore from Doodsons' formulae have been compared with actual values and a remarkable agreement has been found. The rise of tide in 10 minutes and 30 minutes which is the time taken for the steep rise to disappear has been correlated with range of tide and a useful table has been included. Tides in Hooghly are predicted by applying harmonic shallow water corrections to open sea primary predictions at Sagar. As the freshets are unpredictable an approximate relationship has been worked out between the departures of actual freshets from num freshets of the years of analyses and departures of actual from predicted tides. From a knowledge of actual freshets the predictions can be corrected almost, daily. A large number of tables and only the essential graphs have been included to illustrate the contents.

A Theory of Vertical Structure of Density in the Ocean

A Theory of Vertical Structure of Density in the Ocean