1,300 results for 1900, Journal article

Linear transport of solar wind fluctuations
Oughton, Sean; Matthaeus, William H. (1995)
Journal article
University of WaikatoNumerical solutions for the linear transport of solar wind fluctuations are presented. The model used takes into account the effects of advection, expansion, and wave propagation, as well as the recently illuminated effects of (nonWKB) “mixing” terms. The radial evolution of the fluctuating kinetic and magnetic energies and of the cross helicity is computed, and it is demonstrated that in appropriate limits the solutions converge to the WKB forms. In more general cases, however, where the fluctuations consist of a superposition of various types of turbulence, mixing leads to solutions which differ substantially from those predicted by WKB theory. The degree of mixing shows considerable dependence on the nature of the turbulence, giving rise to varying levels, at 1 ∼ AU, of the ratio of “inward” and “outward” fluctuation energies and the ratio of kinetic and magnetic fluctuation energies. The transport properties described here may provide a partial explanation for the observed decrease of cross helicity with increasing heliocentric distance in the solar wind.
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Anisotropic threedimensional MHD turbulence
Matthaeus, William H.; Ghosh, Sanjoy; Oughton, Sean; Roberts, D. Aaron (1996)
Journal article
University of WaikatoDirect spectral method simulation of the threedimensional magnetohydrodynamics (MHD) equations is used to explore anisotropy that develops from initially isotropic fluctuations as a consequence of a uniform applied magnetic field. Spectral and variance anisotropies are investigated in both compressible and incompressible MHD. The nature of the spectral anisotropy is consistent with the model of Shebalin et al. [1983] in which the spectrum broadens in the perpendicular wavenumber direction, the anisotropy being greater for smaller wavenumbers. Here this effect is seen for both incompressible and polytropic compressible MHD. In contrast, the longitudinal (compressive) velocity fluctuations remain isotropic. Variance anisotropy is observed for low plasma beta compressible MHD but not for incompressible MHD. Solar wind observations are qualitatively consistent with both variance and spectral anisotropies of the type discussed here.
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Magnetic helicity in magnetohydrodynamic turbulence with a mean magnetic field
Stribling, Troy; Matthaeus, William H.; Oughton, Sean (1995)
Journal article
University of WaikatoA computational investigation of magnetic helicity of the fluctuatingmagnetic fieldHm in ideal and freely decaying three‐dimensional (3‐D) magnetohydrodynamics (MHD) in the presence of a uniform mean magnetic field is performed. It is shown that for ideal 3‐D MHDHm, which is a rugged invariant in the absence of a mean magnetic field [Frisch et al., J. Fluid Mech. 77, 796 (1975)], decays from its initial value and proceeds to oscillate about zero. The decay of Hm is shown to result from the presence of a new ‘‘generalized’’ helicity invariant, which includes contributions from the uniform magnetic field. The loss of invariance of Hm will diminish the effects of inverse transfer of Hm on freely decaying turbulence. This is demonstrated in a discussion of the selective decay relaxation process.
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Evolution of energycontaining turbulent eddies in the solar wind
Matthaeus, William H.; Oughton, Sean; Pontius, Duane H.; Zhou, Ye (1994)
Journal article
University of WaikatoPrevious theoretical treatments of fluidscale turbulence in the solar wind have concentrated on describing the state and dynamical evolution of fluctuations in the inertial range, which are characterized by power law energy spectra. In the present paper a model for the evolution of somewhat larger, more energetic magnetohydrodynamic (MHD) fluctuations is developed by analogy with classical hydrodynamic turbulence in the quasiequilibrium range. The model is constructed by assembling and extending existing phenomenologies of homogeneous MHD turbulence, as well as simple twolengthscale models for transport of MHD turbulence in a weakly inhomogeneous medium. A set of equations is presented for the evolution of the turbulence, including the transport and nonlinear evolution of magnetic and kinetic energy, cross helicity, and their correlation scales. Two versions of the model are derived, depending on whether the fluctuations are distributed isotropically in three dimensions or restricted to the twodimensional plane perpendicular to the mean magnetic field. This model includes a number of potentially important physical effects that have been neglected in previous discussions of transport of solar wind turbulence. Numerical solutions are shown for several cases of interest that demonstrate the advantages of this approach. We suggest that this model may prove useful in studies of solar wind heating and acceleration, as well as in describing the response of interplanetary turbulence to wave energy injected by pickup ions and planetary upstream waves.
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The influence of a mean magnetic field on threedimensional magnetohydrodynamic turbulence
Oughton, Sean; Priest, Eric R.; Matthaeus, William H. (1994)
Journal article
University of WaikatoBuilding on results from twodimensional magnetohydrodynamic (MHD) turbulence (Shebalin, Matthaeus & Montgomery 1983), the development of anisotropic states from initially isotropic ones is investigated numerically for fully threedimensional incompressible MHD turbulence. It is found that when an external d.c. magnetic field (B₀) is imposed on viscous and resistive MHD systems, excitations are preferentially transferred to modes with wavevectors perpendicular to B₀). The anisotropy increases with increasing mechanical and magnetic Reynolds numbers, and also with increasing wavenumber. The tendency of B₀ to inhibit development of turbulence is also examined.
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Transport theory and the WKB approximation for interplanetary MHD fluctuations
Matthaeus, William H.; Zhou, Ye; Zank, Gary P.; Oughton, Sean (1994)
Journal article
University of WaikatoAn alternative approach, based on a multiple scale analysis, is presented in order to reconcile the traditional WKB approach to the modeling of interplanetary fluctuations in a mildly inhomogeneous largescale flow with a more recently developed transport theory. This enables us to compare directly, at a formal level, the inherent structure of the two models. In the case of noninteracting, incompressible (Alfvén) waves, the principle difference between the two models is the presence of leadingorder couplings (called “mixing effects”) in the nonWKB turbulence model which are absent in a WKB development. Within the context of linearized MHD, two cases have been identified for which the leading order nonWKB “mixing term” does not vanish at zero wavelength. For these cases the WKB expansion is divergent, whereas the multiplescale theory is well behaved. We have thus established that the WKB results are contained within the multiplescale theory, but leading order mixing effects, which are likely to have important observational consequences, can never be recovered in the WKB style expansion. Properties of the higherorder terms in each expansion are also discussed, leading to the conclusion that the nonWKB hierarchy may be applicable even when the scale separation parameter is not small.
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Phenomenology for the decay of energycontaining eddies in homogeneous MHD turbulence
Hossain, Murshed; Gray, Perry C.; Pontius, Duane H.; Matthaeus, William H.; Oughton, Sean (1995)
Journal article
University of WaikatoWe evaluate a number of simple, one‐point phenomenological models for the decay of energy‐containing eddies in magnetohydrodynamic(MHD) and hydrodynamicturbulence. The MHDmodels include effects of cross helicity and Alfvénic couplings associated with a constant mean magnetic field, based on physical effects well‐described in the literature. The analytic structure of three separate MHDmodels is discussed. The single hydrodynamic model and several MHDmodels are compared against results from spectral‐method simulations. The hydrodynamic model phenomenology has been previously verified against experiments in wind tunnels, and certain experimentally determined parameters in the model are satisfactorily reproduced by the present simulation. This agreement supports the suitability of our numerical calculations for examining MHDturbulence, where practical difficulties make it more difficult to study physical examples. When the triple‐decorrelation time and effects of spectral anisotropy are properly taken into account, particular MHDmodels give decay rates that remain correct to within a factor of 2 for several energy‐halving times. A simple model of this type is likely to be useful in a number of applications in space physics, astrophysics, and laboratory plasma physics where the approximate effects of turbulence need to be included.
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Properties of massloading shocks: 1. Hydrodynamic considerations
Zank, Gary P.; Oughton, Sean (1991)
Journal article
University of WaikatoThe onedimensional hydrodynamics of flows subjected to mass loading are considered anew, with particular emphasis placed on determining the properties of massloading shocks. This work has been motivated by recent observations of the outbound Halley bow shock (Neubauer et al., 1990), which cannot be understood in terms of simple hydrodynamical or magnetohydrodynamical descriptions. By including mass injection at the shock, we have investigated the properties of the RankineHugoniot conditions on the basis of a geometric formulation of the entropy condition. Such a condition, which is more powerful than the usual thermodynamical formulation, serves to determine those solutions to the RankineHugoniot conditions which correspond to a physically realizable downstream state. On this basis a concise theoretical description of hydrodynamic massloading shocks is obtained. We show that massloading shocks have more in common with combustion shocks than with ordinary nonreacting gas dynamical shocks. It is shown that for decelerated solutions to the RankineHugoniot conditions to exist, the upstream flow speed u0 must satisfy u0 > ucrit > cs, where cs is the sound speed. Besides the usual supersonicsubsonic transition, massloading fronts can also admit a decelerating supersonicsupersonic transition, the structure of which consists of a sharp decrease in the flow velocity preceding a recovery and an increase in the final downstream flow speed. We suggest the possibility that such structures may describe the inbound Halley bow shock (Coates et al., 1987a). Both parallel and oblique shocks are considered, the primary difference being that oblique shocks are subjected to a shearing stress due to mass loading. It is conjectured that such a shearing may destabilize the shock.
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Selective decay and coherent vortices in twodimensional incompressible turbulence
Matthaeus, William H.; Stribling, W.; Martinez, Daniel; Oughton, Sean; Montgomery, David (1991)
Journal article
University of WaikatoNumerical solution of twodimensional incompressible hydrodynamics shows that states of a nearminimal ratio of enstrophy to energy can be attained in times short compared with the flow decay time, confirming the simplest turbulent selective decay conjecture, and suggesting that coherent vortex structures do not terminate nonlinear processes. After all possible vortex mergers occur, the vorticity attains a particlelike character, suggested by the latetime similarity of the streamlines to Ewald potential contours.
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Reply to “Comment on ‘Evolution of energycontaining turbulent eddies in the solar wind’ by W. H. Matthaeus, S. Oughton, D. H. Pontius Jr., and Y. Zhou”
Matthaeus, William H.; Oughton, Sean; Pontius, Duane H.; Zhou, Ye (1995)
Journal article
University of WaikatoIn their preceding comment on our paper [Matthaeus et al., 1994] (hereinafter reffered to as paper 1), Tu and Marsh [this issue] object vehemently to what they perceive as undeserved criticism of the class of solar wind turbulence models developed by Tu and coworkers[Tu et al., 1984, Tu, 1987, 1988]. Our intent was not to slight the valuable contributions made in those groundbreaking theories for incorporating turbulence into models for spatial transport. Rather, we wished to point out certain shortcomings in existing theories and suggest how they might be improved.
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Massloading and parallel magnetized shocks
Zank, Gary P.; Oughton, Sean; Neubauer, F. M.; Webb, G. M. (1991)
Journal article
University of WaikatoRecent observations at comets GiacobiniZinner and Halley suggest that simple nonreacting gas dynamics or MHD is an inappropriate description for the bow shock. The thickness of the observed (sub)shock implies that massloading is an important dynamical process within the shock itself, thereby requiring that the RankineHugoniot conditions possess source terms. This leads to shocks with properties similar to those of combustion shocks. We consider parallel magnetized shocks subjected to massloading, describe some properties which distinguish them from classical MHD parallel shocks, and establish the existence of a new kind of MHD compound shock. These results will be of importance both to observations and numerical simulations of the cometsolar wind interaction.
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Ion parallel viscosity and anisotropy in MHD turbulence
Oughton, Sean (1996)
Journal article
University of WaikatoWe report on results from direct numerical simulation of the incompressible three dimensional magnetohydrodynamic (MHD) equations, modified to incorporate viscous dissipation via the strongly anisotropic ionparallel viscosity term. Both linear and nonlinear cases are considered, all with a strong background magnetic field. It is found that spectral anisotropy develops in almost all cases, but that the contribution from effects associated with the ionparallel viscosity is relatively weak compared with the previously reported nonlinear process. Furthermore, and in contrast to this earlier work, it is suggested that when B₀ is large, the anisotropy will develop and persist for many largescale turnover times even for nondissipative runs. Resistive dissipation is found to dominate over viscous even when the resistivity is several orders of magnitude smaller than the ion parallel viscosity. A variance anisotropy effect and anisotropy dependence on the polarization of the fluctuations are also observed.
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Phenomenology of hydromagnetic turbulence in a uniformly expanding medium
Matthaeus, William H.; Zank, Gary P.; Oughton, Sean (1996)
Journal article
University of WaikatoA simple phenomenology is developed for the decay and transport of turbulence in a constantspeed, uniformly expanding medium. The fluctuations are assumed to be locally incompressible, and either of the hydrodynamic or nonAlfvénic magnetohydrodynamic (MHD) type. In order to represent local effects of nonlinearities, a simple model of the KaármáDryden type for locally homogeneous turbulent decay is adopted. A detailed discussion of the parameters of this familiar onepoint hydrodynamic closure is given, which has been shown recently to be applicable to nonAlfvénic MHD as well. The effects of the largescale flow and expansion are incorporated using a twoscale approach, in which assumptions of particular turbulence symmetries provide simplifications. The derived model is tractable and provides a basis for understanding turbulence in the outer heliosphere, as well as in other astrophysical applications.
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Properties of massloading shocks, 2. Magnetohydrodynamics
Zank, Gary P.; Oughton, Sean; Neubauer, F. M.; Webb, G. M. (1992)
Journal article
University of WaikatoThe onedimensional magnetohydrodynamics of shocked flows subjected to significant mass loading are considered. Recent observations at comets GiacobiniZinner and Halley suggest that simple nonreacting MHD is an inappropriate description for active cometary bow shocks. The thickness of the observed cometary shock implies that mass loading represents an important dynamical process within the shock itself, thereby requiring that the RankineHugoniot condition for the mass flux possess a source term. In a formal sense, this renders massloading shocks qualitatively similar to combustion shocks, except that mass loading induces the shocked flow to shear. Nevertheless, a large class of stable shocks exist, identified by means of the Lax conditions appropriate to MHD. Thus massloading shocks represent a new and interesting class of shocks, which, although found frequently in the solar system, both at the head of comets and, under suitable conditions, upsteam of weakly magnetized and nonmagnetized planets, has not been discussed in any detail. Owing to the shearing of the flow, massloading shocks can behave like switchon shocks regardless of the magnitude of the plasma beta. Thus the behavior of the magnetic field in massloading shocks is significantly different from that occurring in nonreacting classical MHD shocks. It is demonstrated that there exist two types of massloading fronts for which no classical MHD analogue exists, these being the fast and slow compound massloading shocks. These shocks are characterized by an initial deceleration of the fluid flow to either the fast or the slow magnetosonic speed followed by an isentropic expansion to the final decelerated downstream state. Thus these transitions take the flow from a supersonic to a supersonic, although decelerated, downstream state, unlike shocks which occur in classical MHD or gasdynamics. It is possible that such structures have been observed during the GiottoHalley encounter, and a brief discussion of the appropriate Halley parameters is therefore given, together with a short discussion of the determination of the shock normal from observations. A further interesting new form of massloading shock is the “slowintermediate” shock, a stable shock which possesses many of the properties of intermediate MHD shocks yet which propagates like a slow mode MHD shock. An important property of massloading shocks is the large parameter regime (compared with classical MHD) which does not admit simple or stable transitions from a given upstream to a downstream state. This suggests that it is often necessary to construct compound structures consisting of shocks, slip waves, rarefactions, and fast and slow compound waves in order to connect given upstream and downstream states. Thus the Riemann problem is significantly different from that of classical MHD.
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Notice from the Editor
Selby, Michael J. (1971)
Journal article
University of WaikatoNotice from the Editor from Volume 5, Number 2, 1971 of Earth Science Journal.
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Coverpage and Contents
Waikato Geological Society (1971)
Journal article
University of WaikatoCoverpage and Contents from Volume 5, Number 2, 1971 of Earth Science Journal.
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Dynamic equilibrium in applied geomorphology: Two case studies
Douglas, Ian (1971)
Journal article
University of WaikatoEngineering works and agricultural activity which change the relationship between rainfall and river flow lead to modifications of river channels with attendant erosion and deposition problems. In the Swiss Jura Lakes area, the natural flooding of the River Aare became such an acute problem by the midnineteenth century that extensive engineering works were carried out to alleviate flooding. The land thus reclaimed became a valuable agricultural asset, but the fall of the water table following removal of the annual flood risk, led to a fall in the level of the land as peat was changed into humus. Renewed flooding occurred. The natural readjustment following the first series of flood alleviation works reproduced the original problem and a second series of engineering works has had to be undertaken to remedy the situation. On the Belgian coast, harbour construction and the spread of buildings over the sand dunes have resulted in severe beach erosion in the eastern seaside resorts. Extensive engineering works have had to be undertaken to restore the beach. These examples illustrate how man's challenges to nature are often recurrent phenomena, and how the alteration of one aspect of the physical environment may lead to a succession of readjustments. Each phase of engineering activity may be considered a break in natural equilibrium, and each period of natural erosion or deposition a trend towards a new equilibrium.
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Dust from Australia A reappraisal
Healy, T.R. (1970)
Journal article
University of WaikatoThis paper reviews the meteorological events of October 1928 associated with severe duststorms in Australia and subsequent transport of dust to New Zealand. In the light of contemporary knowledge of the jet streams, and from reappraisal of the original synoptic charts, reported meteorological conditions and press reports pertaining to these duststorms, it is postulated that for dust to be deposited upon New Zealand within 24 hours, of duststorms in Australia it presumably travelled via the jet stream region of the' middle and upper troposphere.
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Coverpage and Contents
Waikato Geological Society (1968)
Journal article
University of WaikatoCoverpage and Contents from Volume 2, Number 1, 1968 of Earth Science Journal.
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Chronology of fans and terraces in the Galatea Basin
Pain, C.F.; Pullar, W.A. (1968)
Journal article
University of WaikatoAirborne volcanic ash beds are used to date fans and terraces in the Galatea Basin and to outline the depositional history of this part of the Rangitaiki Valley. The basin is interpreted as a faultangle depression formed by a downwarped sheet of ignimbrite and an upthrusted block of greywacke which forms the Ikawhenua Range. It is from this range that much of the detritus has been derived to fill the basin, deposited mainly in the form of fans and terraces. The larger fans cover a wide area and their surfaces are older than the Rotoma eruption of c. 8000 years B.P. The widespread occurrence of these fans indicates a major erosion interval between c. 11,000 and c. 8,000 years ago. The younger fans are distributed in a particular order with fans of the PreTaupo surface north of the Horomanga Stream and those of the Pre and PostKaharoa surfaces south of the same stream. This ordered distribution of the younger fans suggests a climatic control of fan building. Aggradation and degradation phases in the Rangitaiki and Whirinaki Rivers have formed a pronounced meander trough containing terraces of the PreTaupo, PreKaharoa, and PostKaharoa surfaces. The terrace of the PreKaharoa surface, largely of Taupo Pumice alluvium, is the most common. Degradation, however, is controlled by a local base level at the ignimbrite rapids on the Rangitaiki River just north of the Galatea Basin.
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