Thermally driven hydromagnetic instabilities in a non-constantly stratified layer

Summary The convection in a rapidly rotating electrically conducting, fluid horizontal layer of non-constant stratification, permeated by an inhomogeneous magnetic field, is studied. In this connection, a temperature model of the layer is constructed, which creates a structure such that part of the layer is unstably and a part stably stratified. The results obtained are applied to the conditions in the fluid Earth's core.

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¶rt;m u m aa mn¶rt; u¶rt;uma , m um nm mamuuau u a¶rt;um ¶rt;¶rt; aum n. uaa nu m mna ¶rt; nu¶rt;um uu ma mm, nu m am mamuuum mau, a am — mau. mam unm ¶rt; aaua n, nu¶rt;u u¶rt; ¶rt; u.

     

Boundary conditions in the hydromagnetic dynamo problem

au am nu¶rt;, nuau ¶rt;u¶rt;aum ¶rt;ua, a u ¶rt; ma aum u¶rt;uu aam ma¶rt;am . a a au ¶rt; uuauu u nm nmu a u m muna, mm m¶rt; ma a anma n u. uunua muau u m¶rt; nua [4, 5]. n¶rt; am nuam m¶rt; u, u¶rt;u u m¶rt;a a u a nm nu nu¶rt;um au m u m¶rt;.     

Asymmetry in a nearly symmetric dynamo

mam u¶rt;m uu aumuu a m n uamu ¶rt;u nmu umu ¶rt;ua a maum ¶rt;a amu u. a um naa nu nu naama umuu , au a um aumu m n aau umuu mau aum n. aam, m mum au ¶rt;-amu a ma mu aum u u n a aumu m n.     

Magnetohydrodynamic oscillations due to precessional motion

Summary The motion of a non-viscous liquid medium with an ideal electrical conductivity, enclosed in the ellipsoidal cavity of an envelope subject to precessional motion, is investigated. Inside the cavity there is a toroidal magnetic field. It is proved that the precessional motion of the envelope generates forced oscillations of the medium with a period of 24 hours and two different types of eigen oscillations. Expressions for computing the amplitudes and frequencies of all types of oscillations are derived. The maximum values of disturbance fields are determined for geophysically acceptable parameters.     

Magnetic and ionospheric storms

Summary The characteristic ionospheric storm variation of F₂ Layer ionisation at a temperate latitude station is described and compared with corresponding effects at other latitudes. The well known marked variability of the F₂ Layer critical frequency is attributed, in part, to the operation of the ionospheric storm mechanism. The lack of correlation between variations of F₂ ionisation at two sites, at a considerable distance apart, is due to the fact that ionospheric storm manifestation is found to vary, in sign and magnitude, with geomagnetic latitude, and is also subject to a diurnal control determined by local time.

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Zusammenfassung Die charakteristische Ionosphärensturmvariation der Ionisation der F₂-Schicht an einer Station der gemäßigten Breite wird beschrieben und mit entsprechenden Vorgängen in anderen Breiten verglichen. Die bekannte eindeutige Veränderlichkeit der kritischen Frequenz der F₂-Schicht wird teilweise der Wirkung des Mechanismus der Ionosphärenstürme zugeschrieben. Das Fehlen einer Korrelation zwischen den Variationen der F₂-Schicht an zwei in beträchtlicher Entfernung voneinander gelegenen Orten erklärt sich aus der Tatsache, daß das Auftreten der Ionosphärenstürme nach Vorzeichen und Ausmaß mit der geomagnetischen Breite variiert und außerdem einem ortszeitlich bedingten Tageseinfluß unterliegt.

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Résumé On décrit la variation la variation caractéristique d'ionisation de la couche F₂ lors des tempêtes ionosphériques observées à une station de latitude moyenne et on la compare aux phénomènes analogues à d'autres latitudes. La variabilité marquée et bien connue de la fréquence critique de la couche F₂ est partiellement attribuée au mécanisme des tempêtes ionosphériques. L'absence de corrélation entre les variations de la couche F₂ en deux endroits considérablement éloignés l'un de l'autre s'explique par le fait que l'apparition des tempêtes ionosphériques varie en signe et en intensité avec la latitude

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With 4 figures.

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Dedicated to Professor Dr.H. Benndorf on the occasion of his eightieth birthday.     

The need for international co-operation in the management of coastal environments

The coastal zones are areas of high biological productivity and intense human pressure. Environmental challenges arise from pollution and from urban and industrial development. Administrative, social and legal challenges centre upon the need for sound management of coastal zone resources, as an important component of national strategies for sustainability. A number of specific actions have been set out in Agenda 21, the principal product of the Earth Summit held in Rio de Janeiro in 1992. But these need to be carried forward within cross-sectoral, integrated coastal area management procedures. International action is also needed because marine ecosystems rarely coincide with national boundaries, and are affected by international economic, social and legal decisions.

International scientific co-operation on marine issues is already well established, and many regional action plans and Conventions have been adopted. Technological co-operation is less advanced. Such efforts need to be intensified, and continuously adapted. Sound plans for the future must be based on good science, critical economic evaluation of resources, sensitive evaluation of social and cultural factors and of the needs of local communities, evaluation of the risks of climate change, sea-level rise and other changes, and monitoring as a basis for continuing adaptation.     

Conditions of generation of a stationary dipole magnetic field

Summary With the aid of the selected spherical model the conditions of generation of a stationary magnetic dipole field in a fluid electrically conducting spherical region, bounded by an outer unconducting region, are derived. It is assumed that there exist turbulent motions in the conducting medium, as well as differential rotation. The induction MHD equation is solved under appropriate boundary conditions. It was found that a stationary dipole field may only be generated at a certain positive value of the coefficient G which characterizes the geometric and physical parameters of the conducting region. If this coefficient is zero, the amplitudes of the magnetic fields decrease exponentially with time.