Momentum Gain Research Articles

A unidimensional model of comet ionosphere structure

We have considered the one‐dimensional continuity and momentum equations for the plasma created in the expanding coma of a comet near the Sun. For an inverse square dependence of neutral density we have solved the continuity equation in the presence of radial plasma flow. For distances large compared to the size of the nucleus, our distribution falls off as r−1 with a velocity dependent coefficient which reduces to the photochemical steady state case in the limit of vanishing radial plasma outflow. This density distribution is used in the treatment of the momentum equation from which an analytic expression for the magnetic field configuration is derived in the presence of outflow, photoionization, dissociative recombination, plasma fluid pressure, and friction between the ions and neutrals. The magnetic field thus derived can be shown to have a maximum value which we fix to be attained at the point corresponding to the point shown by the Giotto observations and find that for a Halley type comet there will be a region sunward of the nucleus from which the magnetic field is excluded, primarily by the effect of ion neutral collisions, which is consistent with the Giotto observations. We have performed the calculations for both Halley and Giacobini‐Zinner type comets. In the field‐free region we find that the dominant terms in the momentum equation balancing the magnetic pressure gradient are the ion neutral friction and the net mass loading momentum gain. For large distances the field reduces to a constant field. The scale size of field diffusion as a result of finite conductivity, brought about by electron neutral collisions, is found to be vanishingly small.

The expansion of the cathode spot plasma in vacuum arc discharges

A stationary one-dimensional two-fluid model for the expansion and corresponding acceleration of the dense cathode spot plasma in vacuum arc discharges is evaluated. The equations exhibit a saddle point at the ion sound velocity. For an acceleration to supersonic velocities, the effect of Joule heating due to electron–ion collisions has to compensate for the effect of geometrical dilution at the sonic point. As a consequence, a transsonic flow is possible for a specific ion starting velocity only. This model shows that large final ion velocities observed in experiments are mainly caused by the directed friction force due to electron–ion collisions. A small potential hump has only a minor effect for the momentum gain of the ions.

Hierarchical clustering - Angular momentum versus density

The growth of angular momentum in a general clustering scenario is analyzed and its correlation with the primordial density perturbation delta<SUB>0</SUB> is calculated. High-density peaks are found to evolve into low-angular momentum systems. The basic mechanism behind this anticorrelation is that anisotropic structure, which leads to angular momentum gain, is statistically uncorrelated with the primordial perturbation. The collapse time, on the other hand, is correlated with it. This time scale is the one that determines the angular momentum of a bound object. Three cases are studied, and the correlation of J with delta<SUB>0</SUB> is calculated: (1) Lagrangian sphere (quasi-linear evolution); (2) Eulerian sphere (second order in delta); (3) arbitrary Lagrangian volume (first order in delta).

Band-crossing phenomena in 167, 168Hf and 170, 171W

High-spin states in 167,168Hf and 170, 171W have been excited by the reactions 159 Tb( 14 N, x n) 173 − x Hf and 155 Gd( 20 Ne, x n) 175 − x W . The yrast bands have been observed up to J π = 49 2 + in 167Hf, J π = 28 + in 168Hf, J π = 45 2 + in 171W and J π = 22 + in 170W. Both even-even nuclei display a strong backbend around J π = 14 +, whereas 167Hf shows an upbend at ω ∼- 0.33 MeV and 171W exhibits a progressive gain in aligned angular momentum above ω ∼- 0.26 MeV. At even higher rotational frequencies, bridges have been found in the central valleys of the γ-γ correlation matrices at ω ∼- 0.42 and 0.52 MeV for the Hf isotopes and ω ∼- 0.45 and 0.47 MeV for the W isotopes. Deduced moments of inertia for the Hf isotopes using the correlation data show a smooth increase up to about the rigid-sphere value at ω ∼- 0.5 MeV. The data are satisfactorily accounted for by cranked shell-model calculations. In particular, a qualitative interpretation is given for the experimentally observed systematic difference in the strength of the interaction potential for the N = 95 and N = 97 isotopes of Hf and W.

Self-similar magnetohydrodynamics. I - The gamma = 4/3 polytrope and the coronal transient

view Abstract Citations (104) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Self-similar magnetohydrodynamics. I - The gamma = 4/3 polytrope and the coronal transient Low, B. C. Abstract The full set of ideal magnetohydrodynamic (MHD) equations for a gamma = 4/3 polytrope admits self-similar solutions which can be derived by analytic methods. An axisymmetric magnetic field in a stratified stellar atmosphere is assumed. The solutions admit a large variety of magnetic structures, including those in the form of loops, moving through the corona with large scale coherence and sharp small scale features. The Lagrangian velocity of an individual transient feature is found to accelerate or decelerate according to a positive or negative gain in momentum by the plasma in a self-similar distribution of the Lorentz force, the pressure gradient, and the gravitational force. In both cases, the net force decreases with time so that, at large radial distances, the motion becomes inertial. The physical implications are discussed, arguring in favor of the transient beginning as a fully nonlinear MHD motion that ejects both magnetic field and plasma out of the gravitational bond of the sun. Publication: The Astrophysical Journal Pub Date: March 1982 DOI: 10.1086/159790 Bibcode: 1982ApJ...254..796L Keywords: Magnetohydrodynamics; Polytropic Processes; Solar Corona; Stellar Atmospheres; Coronal Loops; Gamma Ray Astronomy; Gravitational Effects; Hydrodynamic Equations; Magnetic Field Configurations; Solar Limb; Stellar Magnetic Fields; Transient Response; Astrophysics full text sources ADS |

Inelastic scattering of He atoms from Cu(001) at low temperature with energy analysis

A LiF crystal analyzer has been used to measure phonon creation events when a low energy He beam scatters from a Cu(001) surface at low temperature (16 °K). As only phonon creation events were observed, the inelastic scattering was subspecular. Atom parallel momentum gain and loss events were observed and both bulk and surface modes contributed to the inelastic scattering. An elastic component of scattering was observed over a wide angular range.

The cranking model applied to Yb bands and band crossings

In-beam γ-ray spectroscopy experiments have been performed on 160, 161, 162Yb in Copenhagen and on 160Yb in Oak Ridge. Fifteen rotational sequences (bands) were observed, most of them beyond I = 20. Thirteen band crossings (upbends or backbends) were found, and these are classified according to the frequency and the gain in aligned angular momentum in the crossing. Six negative-parity side bands in 160,162Yb and the i 13 2 band in 161Yb experience a similar crossing around ω = 0.36 MeV/ h ̵ , a value intermediate to the frequencies of the first and second yrast backbends in 160Yb. Cranking model calculations are used to suggest quasiparticle assignments for the bands and to explain the nature of the three basic types of band crossings observed in our measurements.

Frequency and Alignment Classification of Multiple Band Crossings

Experiments on $^{160}\mathrm{Yb}$ indicate three negative-parity bands up to $Ig~21\ensuremath{\hbar}$. All three experience a band crossing around a frequency $\ensuremath{\hbar}\ensuremath{\omega}=0.36$ MeV, a value intermediate to those of the first and second yrast backbends. The ${i}_{\frac{13}{2}}$ band in $^{161}\mathrm{Yb}$ is observed up to ${\frac{49}{2}}^{+}$; it also has a strong upbend at this intermediate frequency. The fingerprint of a band crossing based upon the crossing frequency and gain in aligned angular momentum is demonstrated and the results compared to cranking-model calculations to suggest quasiparticle assignments for the bands.

Cross sections for ionization of metastable rare-gas atoms (Ne*, Ar*, Kr*, Xe*) and of metastableN2*, CO* molecules by electron impact

Cross sections for the collisional ionization of the metastable atoms Ne*, Ar*, Kr*, and Xe* by electrons with impact energy E in the range 6 < E < 250 eV are determined in the Born and binary-encounter approximations. For low energies epsilon of ejection, the s-d and s-f bound-free transitions dominate the associated form factor, while transitions to continuum states with progressively higher angular momentum gain importance with increasing epsilon. While up to nine partial waves are normally sufficient for convergence of the bound-free form factor at a given energy epsilon and momentum change K of the ejected electron, as many as 30 are required for those K in the vicinity of the Bethe ridge at large epsilon. These properties are the origin of the overall closeness obtained between the Born and binary-encounter cross sections. Also inner-shell ionization, as described by the binary-encounter treatment, becomes increasingly important as the target atom becomes more complex. Cross sections for ionization of metastable N/sub 2/* and CO* are also determined. Good agreement with available measurements (for Ne* and Ar*) is obtained. (AIP)

Angular Momentum Loss and the Origin of Cataclysmic Binaries

A two-stage mechanism is proposed whereby detached dwarf systems like V471 Tau may evolve into cataclysmic binaries. The two stages are (1) angular momentum loss from the secondary due to braking by a stellar wind linked with magnetic fields, and (2) angular momentum gain by the secondary from the orbital motion, due to tidal friction. This combination may be able to remove angular momentum from short-period binaries on a timescale of ~109 yr.

Interference of Wing and Multipropellers

A systematic procedure is presented for the analysis of the interference of a wing with multipropellers. The assumptions are made that the radius of the propeller and the chord are of the same order and both are much smaller than the span. Using the chord to span ratio as the small expansion parameter, the local two-dimensional solution for the sectional lift to angle-of-attack relationship is uncoupled from the outer three-dimensional solution in the spirit of Prandtl's lifting line theory. Details of the nonuniform stream behind the propellers enter into the local sectional analysis. For the outer solution, the planform of the wing reduces to the lifting line and the stream behind the propellers reduces to a thin sheet of a jet carrying the sectional momentum gained across the propellers and supporting a pressure difference across the sheet. By representing the thin sheet of a jet as a vortex sheet, an integral equation is obtained. When the momentum gain across the propellers is not too large, the integral equation can be further simplified. Several numerical examples are presented to demonstrate the versatility of the analysis.

Diurnal wind variations in Antarctica

AbstractFor the three Antarctic ice shelf stations considered (Norway Station, Maudheim, and Halley Bay) it is shown that from spring to autumn a systematic diurnal variation of the mean vector wind at the surface exists. Different factors that may be responsible for this variation are discussed. The results seem to agree well with the assumption that in areas far from the ice front (the seaward edge of the ice shelf) the main cause is the larger net gain of momentum in the lowest layers during the middle of the day, while closer to the ice front there is an additional effect of the varying contrast between the air temperature over the sea and over the ice shelf.

Laboratory Simulation of Wake Effects on Second and Third Thermals in a Series

Abstract Individual thermals rising through the atmosphere may encounter the wake of one or more thermals that started their rise at an earlier time. Laboratory simulations show that the growth rate of second and third thermals in a series is enhanced relative to that of a solitary thermal. The enhancement is due to a gain in momentum from the wake of a preceding thermal, and equations are developed for predicting the amount of enhancement as a function of the delay time between thermals. Agreement with experimental data is good for first and second thermals. The third thermal was found to have a growth rate very similar to that of a second thermal. Theory agrees to the extent that only a slight increase is predicted, and this amount could have been lost within the experimental error. Thus, it appears that by the time the third thermal in a series occurs, an equilibrium condition has been reached as far as the thermal growth rate and velocity field are concerned.

Intensity-dependent gain in momentum of electrons in electromagnetic waves

Intensity-dependent gain in momentum of electrons in electromagnetic waves

Some ideas about the mechanism causing two-phase critical flow

While the theory concerning critical discharges of single-phase flow, in particular of gases, is well known, the direct opposite is true for two-phase mixtures. The question still remains to be answered whether critical flow occurs for two-phase flow with the same implications that it carries for single-phase flow. A theoretical analysis, which is supported by experimental data, is presented in this paper and indicates that the implications for single-phase and two-phase critical flow differ significantly. While single-phase flow is restricted by so-called sonic choking, the extra freedom available in a two-phase flow system, the slip ratio, restricts the maximum flow in the latter. Furthermore, two-phase critical flow is achieved when the slip ratio takes on a value such that the rate of momentum gain by convection per unit volume becomes identical in each fluid phase.

On the nature of the ionisation of a molecule by an α-particle

No experiments have hitherto been made to determine the source of the energy which appears when ionisation takes place. The energy involved may be accounted for in two ways (1), it may be derived from the potential energy of the atom which is set free by a “trigger” action of the ionising agent, or (2), it may be energy directly absorbed from the ionising agent which is transformed into the energy of the ionised molecule. If the energy of the ionising agent is kinetic energy, which is the case with the α -particle, an experiment can be made to decide whether any of the energy of ionisation can be accounted for on the second hypothesis. If an α -particle is losing energy during its motion, its loss of the second hypothesis. If an α -particle is losing energy during its motion, its loss of momentum must be equal to the gain of momentum of the electrons ejected from molecules by α -particles should have on the whole a motion in the same direction as the α -particles. These considerations led the writer to undertake some experiments on the direction of motion of electrons when ionisation is produced by α -particles.