Hotspots,mantle convection and plate tectonics: A synthetic calculation

A model is developed that unifies vigorous hotspots with global-scale mantle convection and plate tectonics. The convection dynamics are assumed to generate flow patterns that emerge as closely packed polygonal cells in approaching the asthenosphere, and whose geometry is completely determined by a defining set of vigorous hotspots. Overlying viscously coupled rigid plates are driven with unique velocities (Euler vectors) at which the area integral of the shear forces is zero; these velocities are dynamically stable. The computed plate velocities, resulting from convection based on 15 hotspots, are compared with the velocities of plate motion models AM1-2 (Minster andJordan, 1978) and HS-NUVEL1 (Gripp andGordon, 1990), which combine transform fault geometries, magnetic anomalies and seismic data. The comparison shows a striking agreement for a majority of the plates. Geophysical implications of this numerical exercise are discussed.     

Forecasting thunderstorms and hailstorms by means of temperature and dew-point anomalies on a time-section chart

Summary Vertical time-section charts for temperature and dew-point anomalies have been plotted. The temperature anomaly decreasing upwards give convection according to Byers and Braham [1]²). The coincidence of temperature anomalies with central minimum and of dew-point anomalies with central maximum, or of upward decreasing trend in temperature anomalies and upward increasing trend in dewpoint anomalies, correspond to the convection associated with greater moisture influx, thus representing the case of a thunderstorm. The mentioned anomalies have been found to occur in such a way that cells with central maximum are followed by those with central minimum, and vice versa. Thus simultaneous occurrences of upward decreasing anomalies in temperature, i.e. occurrence of cells with central minimum of temperature, and upward increasing anomalies in dew-point, i.e. occurrence of cells with central maximum, can be observed at any station, hence the occurrences of thunderstorms at that station can be predicted. Moreover, the two kinds of anomalies can give the idea (along with that of convection, of coming of the synoptic situation having dry air aloft and moist air below, which is the ideal condition) for the occurrence of a hailstorm (Fawbush andMiller [2]).     

EM induction in the Vancouver Island region: 3D numerical,analogue model,and field site results

Electromagnetic induction in the Vancouver Island region for a uniform inducing source field for 300 s period is investigated with the aid of three-dimensional (3-D) numerical and analogue model results and field site measurements. The thin sheet numerical model, based on the subducting Juan de Fuca plate analogue model ofDosso et al., consists of a 5km thick non-uniform thin sheet (comprising the lateral conductivity contrasts arising from the land, the varying depth ocean, and the sediment) underlain by a four-layer conductive structure. The four-layer conductive structure beneath the non-uniform thin sheet simulates the effect of the Juan de Fuca plate subducting Vancouver Island. To examine the effects of the ocean channel depth between Vancouver Island and the British Columbia (Canada) mainland, numerical results were obtained for two channel depths (0 and 600 m). The results indicate that the channel plays an important role in the geomagnetic response in the central and inner coastal regions of Vancouver Island. The general agreement of the 3-D numerical model induction arrows with the analogue model and field site induction arrows for 300 s supports the premise of a layered conductive substructure dipping at a small angle, at most, beneath Vancouver Island.Lithoprobe Publication No. 311.     

Small-scale magnetic buoyancy and magnetic pumping effects in a turbulent convection

We determine the nonlinear drift velocities of the mean magnetic field and nonlinear turbulent magnetic diffusion in a turbulent convection. We show that the nonlinear drift velocities are caused by three kinds of the inhomogeneities; i.e., inhomogeneous turbulence, the nonuniform fluid density and the nonuniform turbulent heat flux. The inhomogeneous turbulence results in the well-known turbulent diamagnetic and paramagnetic velocities. The nonlinear drift velocities of the mean magnetic field cause the small-scale magnetic buoyancy and magnetic pumping effects in the turbulent convection. These phenomena are different from the large-scale magnetic buoyancy and magnetic pumping effects which are due to the effect of the mean magnetic field on the large-scale density stratified fluid flow. The small-scale magnetic buoyancy and magnetic pumping can be stronger than these large-scale effects when the mean magnetic field is smaller than the equipartition field. We discuss the small-scale magnetic buoyancy and magnetic pumping effects in the context of the solar and stellar turbulent convection. We demonstrate also that the nonlinear turbulent magnetic diffusion in the turbulent convection is anisotropic even for a weak mean magnetic field. In particular, it is enhanced in the radial direction. The magnetic fluctuations due to the small-scale dynamo increase the turbulent magnetic diffusion of the toroidal component of the mean magnetic field, while they do not affect the turbulent magnetic diffusion of the poloidal field.     

Flow past spheres and spheroids in the presence of a toroidal magnetic field

Summary Stewartson [1]²) has considered the inviscid flow past a sphere in the presence of a uniform magnetic field andMurray andLudford [2] have investigated a similar problem in which the magnetic field originates from an axially symmetric dipole field situated at the centre of the sphere. In connection with the study of earth's magnetic field, the toroidal part of this field plays a dominant part. This gives rise to the importance of studying the effect of a toroidal magnetic field on flows past different bodies of revolution; specially past spheres and spheroids. In the present note inviscid flows past a sphere, and a spheroid, are considered, for the case of a toroidal magnetic field originating in the fluid. In the case of the sphere the field inside the sphere consists of an electric dipole directed along the axis of symmetry together with a uniform electric field which produces a uniform current along the axis. In the case of the spheroid, the field inside it is due to an electric dipole and quadrupole directed along the axis of symmetry, together with a uniform electric field which produces a uniform current along this axis.     

Solar wind effects on ionospheric convection: a review

The solar wind, magnetosphere, and ionosphere are intrinsically coupled through magnetic field lines. The electrodynamic state of the high-latitude ionosphere is controlled by several geophysical processes, such as the location and rate of magnetic reconnection at the magnetopause and in the magnetotail, and the energisation and precipitation of solar wind and magnetospheric plasmas. Amongst the most observed ionospheric manifestation of solar wind/magnetospheric processes are the convection bursts associated with the so-called flux transfer events (FTEs), magnetic impulse events (MIEs), and travelling convection vortices (TCVs). Furthermore, the large-scale ionospheric convection configuration has also demonstrated a strong correspondence to variations in the interplanetary medium and substorm activity. This report briefly discusses the progress made over the past decade in studies of these transient convection phenomena and outlines some unsettled questions as well as future research directions.     

Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction

Convection in the Earth's core is driven much harder at the bottom than the top. This is partly because the adiabatic gradient steepens towards the top, partly because the spherical geometry means the area involved increases towards the top, and partly because compositional convection is driven by light material released at the lower boundary and remixed uniformly throughout the outer core, providing a volumetric sink of buoyancy. We have therefore investigated dynamo action of thermal convection in a Boussinesq fluid contained within a rotating spherical shell driven by a combination of bottom and internal heating or cooling. We first apply a homogeneous temperature on the outer boundary in order to explore the effects of heat sinks on dynamo action; we then impose an inhomogeneous temperature proportional to a single spherical harmonic Y ₂² in order to explore core-mantle interactions. With homogeneous boundary conditions and moderate Rayleigh numbers, a heat sink reduces the generated magnetic field appreciably; the magnetic Reynolds number remains high because the dominant toroidal component of flow is not reduced significantly. The dipolar structure of the field becomes more pronounced as found by other authors. Increasing the Rayleigh number yields a regime in which convection inside the tangent cylinder is strongly affected by the magnetic field. With inhomogeneous boundary conditions, a heat sink promotes boundary effects and locking of the magnetic field to boundary anomalies. We show that boundary locking is inhibited by advection of heat in the outer regions. With uniform heating, the boundary effects are only significant at low Rayleigh numbers, when dynamo action is only possible for artificially low magnetic diffusivity. With heat sinks, the boundary effects remain significant at higher Rayleigh numbers provided the convection remains weak or the fluid is stably stratified at the top. Dynamo action is driven by vigorous convection at depth while boundary thermal anomalies dominate in the upper regions. This is a likely regime for the Earth's core.     

Two-dimensional electric field measurements in the ionospheric footprint of a flux transfer event

Line-of-sight Doppler velocities from the SuperDARN CUTLASS HF radar pair have been combined to produce the first two-dimensional vector measurements of the convection pattern throughout the ionospheric footprint of a flux transfer event (a pulsed ionospheric flow, or PIF). Very stable and moderate interplanetary magnetic field conditions, along with a preceding prolonged period of northward interplanetary magnetic field, allow a detailed study of the spatial and the temporal evolution of the ionospheric response to magnetic reconnection. The flux tube footprint is tracked for half an hour across six hours of local time in the auroral zone, from magnetic local noon to dusk. The motion of the footprint of the newly reconnected flux tube is compared with the ionospheric convection velocity. Two primary intervals in the PIFs evolution have been determined. For the first half of its lifetime in the radar field of view the phase speed of the PIF is highly variable and the mean speed is nearly twice the ionospheric convection speed. For the final half of its lifetime the phase velocity becomes much less variable and slows down to the ionospheric convection velocity. The evolution of the flux tube in the magnetosphere has been studied using magnetic field, magnetopause and magnetosheath models. The data are consistent with an interval of azimuthally propagating magnetopause reconnection, in a manner consonant with a peeling of magnetic flux from the magnetopause, followed by an interval of anti-sunward convection of reconnected flux tubes.     

Tremblements de terre et anomalies magnétiques

Résumé On a déjà souvent signalé que le champ magné ique terrestre s'est trouvé modifié de fa?on durable à la suile de mouvements séismiques. L'auteur rappelle plusieurs exemples récents d'anomalies magnétiques mesurées au Japon à la suite d'importants séismes (Kato). L'anomalie magnélique duBassin de Paris se déforme dans le temps en liaison possible avec des déplacements magmatiques et une activité séismique avortée. Le séisme du 11 juin 1938 en Belgique montre d'intéressants rapports avec l'anomalie magnétique belge. Les cartes isoporiques mondiales présentent des relations avec les cartes de séismicité (Omer). L'auteur pense que séismes et déformations du champ magnétique terrestre sont plus ou moins étroitement liés et que peut être une réaction atomiqueen chaine peut expliquer les séismes orogéniques et leurs répliques.

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Summary It was yet often noted that the earth's magnetic field has been steadily modified on the result of seismic earth movements. The author cites many recent examples of magnetic anomalies measured in Japan after major earthquakes (Kato). The ?Bassin de Paris?'s magnetic anomaly is modified during a certain time in a possible connection with magmatic removals and with some seismic activity. The earthquake of June 11 th., 1938 in Belgium shows some very interesting connections with the belgian magnetic anomaly. Suggestive relationships are noted between changes in the earth's secular magnetic field and areas of seismicity (Omer). The author thinks that the earthquakes and the deformations of the earth's magnetic field are more or less closely connected one with another, and it may perhaps be that the atomic energy (chain reactions) may explain some orogenic earthquakes and their aftershocks.

     

Il primo problema fondamentale della Geodesia ampliato nel campo di Somigliana

Sommario Dopo aver richiamato l'ampliamento ad un campo potenziale spaziale delle formule diLegendre che risolvono il primo problema fondamentale della Geodesia, l'A. applica tali formule al caso del campo diSomigliana, che ammette l'ellissoide come una delle superfici equipotenziali della famiglia; tale applicazione rappresenta la più naturale estensione ad un campo tridimensionale delle classiche formule ellissoidiche superficiali, e si accorda con l'adozione internazionale del campo diSomigliana per tutte le speculazioni della Geodesia dinamica.

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Summary After having recalled the extension to a spatial potential field ofLegender's formulae solving the first fundamental problem in Geodesy, the author applies such formulae toSomigliana's field, which admits the ellipsoid as one of the equipotential surfaces of the family. Such application represents the most natural extension to a three-dimensional field of the classical ellipsoidal surface formulae, and agrees with the international adoption ofSomigliana's field in dynamic Geodesy.

     

Reduced heat transfer in saltwater by a magnetic field: do oceans have a “geomagnetic brake”?

Seawater is the major heat transporter in our global environment, covering more than two-thirds of the surface of the earth. With an average salinity of ～3.5%, it is a moderate electric conductor, which is permanently in motion by thermal and hydrodynamic forces. A magnetic field exerts a Lorentz force on seawater that principally influences both the dissipation of turbulence and the flow properties by magnetic friction. Here we show by experiments on laboratory scale that convection in seawater is slowed down by an external static magnetic field and leads to a reduced heat flux resulting in an increased or decreased heat content in the volume in response to influx or drain of heat, respectively. Experimentally, the application of a vertical magnetic field of 60 mT reduces the convective heat transport on the liquid-air surface within in 5 min by about 8% perpendicular to the field and up to 14% parallel to it. The effect is strongly correlated with the magnetic interaction parameter of the system, which relates the magnetic to the viscous volume force. In the natural environment, the geomagnetic field is omnipresent. It is weaker by about three orders of magnitude compared with the magnetic field applied in the experiments. It has, however, an undisturbed and long-lasting impact on the convection, at low Reynolds numbers, in the large body of water in the deeper ocean below the mixed layer. There are no investigations regarding a possible contribution of this effect to natural saltwater flows, neither by proxy experiments nor by model calculations. The data presented raise the possibility that convective heat transport in the sea could be always slowed down by the geomagnetic field to a certain extent, besides it could be modulated by the geomagnetic secular variation on relatively short timescales like decades.     

Response of the convecting high-latitude F layer to a powerful HF wave

A numerical model of the high-latitude ionosphere, which takes into account the convection of the ionospheric plasma, has been developed and utilized to simulate the F-layer response at auroral latitudes to high-power radio waves. The model produces the time variations of the electron density, positive ion velocity, and ion and electron temperature profiles within a magnetic field tube carried over an ionospheric heater by the convection electric field. The simulations have been performed for the point with the geographic coordinates of the ionospheric HF heating facility near Tromso, Norway, when it is located near the midnight magnetic meridian. The calculations have been made for equinox, at high-solar-activity, and low-geomagnetic-activity conditions. The results indicate that significant variations of the electron temperature, positive ion velocity, and electron density profiles can be produced by HF heating in the convecting high-latitude F layer.     

On the propagation of plane waves in a porous visco-elastic material containing a fluid and deformed by a magnetic field

Summary In the present note, an attempt has been made to investigate the wave-motions in a poro-viscoelastic medium traversed by a static magnetic field, by using the equations ofMaxwell, and those ofBiot.     

Propagation of magneto-elastic waves in a perfectly conducting plate extending to infinity in vacuum

Summary The effects of a uniform external magnetic field on the propagation of waves in a homogeneous, infinitely conducting flat plate with free boundaries have been studied. It has been found that in general all the three types of waves —P, SV andSH waves—are coupled and the influence may be more pronounced in coupling the symmetric and antisymmetric types of motions in every mode.When the magnetic field is parallel to the plane faces and transverse to the direction of wave propagation, the shear wave polarized parallel to the field is purely elastic whereas the coupledP andS V waves are magnetoelastic and exhibit dispersion strikingly similar to the non-magnetic case, provided the electro-magnetic radiation into the surrounding free space is neglected.The results reported in an earlier communication [1]²) are also confirmed.     

Sunspots

Abstract

Some examples of research on structure and formation of sunspots are briefly recollected in historical sequence. They relate to many facets of sunspots, first: magnetic inhibition of convection, the conjecture of a fiat penumbra, the stratification beneath the umbra, the observable magnetic profile, the Evershed effect as syphon flow, the concept of a magnetopause; next: cooling by Alfven waves, evolution and stability, the “bright ring”, the observed change of umbra brightness with the phase of the sunspot cycle, the hypothetical cluster of separate flux strands underneath the umbra, the profile of the magnetopause, the structure of the penumbra and the inclination of its field and finally: the concept of a deep penumbra with volume currents, exchange convection and the concept of a second current sheet separating umbra and penumbra.

Of course, the rigorous theoretical modeling of local magnetoconvection is an essential tool for our understanding of all these processes. I do not deal with it here, but the reader has a fascinating review of magnetoconvection already in his hands (Weiss, 1991).     

地幔对流与岩石层板块的相互耦合及影响──（Ⅰ）球腔中的自由热对流

板块运动是地幔对流的主要证据之一．同时，作为地球动力系统中一个相对独立部分，板块自身的存在和运动对地幔内部物质的流动形态有巨大影响．地幔内部的流动由两部分组成：一是由内部非绝热温度差异造成的自由对流解；另一部分是由在地表运动的板块所激发．作为系列工作的第一部分，本文研究球腔中的自由热对流问题．得到了对地幔对流研究有实际意义的下边界为自由、上边界为刚性情况下的临界瑞利数值，不同的瑞利数时球腔内流场和温度场的分布形态等．     

The effects of IMF and convection on thermal ion outflow in magnetosphere-ionosphere coupling

We study the influence of the interplanetary magnetic field (IMF) and convection electric field on the rate and destination of polar wind and other thermal (low-energy) ion outflows, and its resulting effects on magnetosphere–ionosphere coupling, using single-particle trajectory simulations in conjunction with ion velocity distribution measurements on Akebono and IMF and ionospheric convection data. We find that the ions preferentially feed the dusk sector of the plasma sheet when the IMF is duskward (B_y>0), and are more evenly distributed in the plasma sheet when the IMF is dawnward. The flow of oxygen ions originating from the noon or dusk sectors of the polar cap has a higher probability of reaching the magnetosphere and beyond compared with that from the dawn or midnight sectors, due to the increased centrifugal acceleration associated with the larger magnetic field curvature near noon and the increased convection electric field in the dusk sector. The flow is enhanced and confined to lower L-shells at times of strongly southward IMF, compared with that at times of northward IMF. The outflow rate to both the plasma sheet and the magnetotail correlates strongly with the ion temperature. As a result, the IMF and the convection electric fields affect both the overall magnitude and the detailed distribution of mass transfer from the ionosphere to the magnetosphere in magnetosphere–ionosphere coupling.     

The onset of magnetoconvection at large Prandtl number in a rotating layer II. Small magnetic diffusion

Abstract

This paper develops further a convection model that has been studied several times previously as a very crude idealization of planetary core dynamics. A plane layer of electrically-conducting fluid rotates about the vertical in the presence of a magnetic field. Such a field can be created spontaneously, as in the Childress-Soward dynamo, but here it is uniform, horizontal and externally-applied. The Prandtl number of the fluid is large, but the Ekman, Elsasser and Rayleigh numbers are of unit order. In Part I of this series, it was also supposed that the ratio thermal diffusivity diffusivity/magnetic diffusivity is O(1), but here we suppose that this ratio is large. The character of the solution is changed in this limit. In the case of main interest, when the layer is confined between electrically-insulating no-slip walls, the solution is significantly different from the solution when the mathematically simpler, illustrative boundary conditions also considered in Part I are employed. As in Part I, attention is focussed on the onset of convection as the temperature difference applied across the layer is increased, and on the preferred mode, i.e., the planform and time-dependence of small amplitude convection.     

Convection in a rotating magnetic system and Taylor's constraint

Abstract

A system is considered in which electrically conducting fluid is contained between two rigid horizontal planes and bounded laterally by a circular cylinder. The fluid is permeated by a strong azimuthal magnetic field. The strength of the field increases linearly with distance from the vertical axis of the cylinder, about which the entire system rotates rapidly. An unstable temperature gradient is maintained by heating the fluid from below and cooling from above. When viscosity and inertia are neglected, an arbitrary geostrophic velocity, which is aligned with the applied azimuthal magnetic field and independent of the axial coordinate, can be superimposed on the basic axisymmetric state. In this inviscid limit, the geostrophic velocity which occurs at the onset of convection is such that the net torque on geostrophic cylinders vanishes (Taylor's condition). The mathematical problem which describes the ensuing marginal convection is nonlinear, and was discussed previously for the planar case by Soward (1986). Here new features are isolated which result from the cylindrical geometry. New asymptotic solutions are derived valid when Taylor's condition is relaxed to include viscous effects.