Large-scale structure of magnetospheric plasma

Recent investigations of magnetospheric plasma structure are summarized under the broad categories of empirical models, transport across boundaries, formation, and dynamics of the plasma sheet. This report reviews work in these areas during the period 1991 to 1993. Fully three-dimensional empirical models and simulations have become important contributors to our understanding of the magnetospheric system. Some new structural concepts have appeared in the literature: the entry boundary and geopause, the plasma sheet region 1 vortices, the low-energy layer, the adiabaticity boundary or wall region, and a region in the tail to which we refer as the injection port. Traditional structural concepts have also been the subject of recent study, notably the plasmapause, the magnetopause, and the plasma sheet. Significant progress has been made in understanding the nature of plasma sheet formation and dynamics, but the acceleration of electrons to high energy remains somewhat mysterious.     

Results of concentration and mobility measurements of positively and negatively charged particles taken by a rocket-borne parachuted aspiration (Gerdien) probe in the height region from 72 to 39 km

Summary With a self-neutralizing, rocket-borne parachute aspiration probe concentration and mobility measurements of positively and negatively charged carriers were made between 72 and 40 km over Sardinia. Between 72 km and 60 km a heavy and a light group of positively charged particles were found. The heavy group was more abundant than the hight group in the greater heights. For the negatively charged particles only one mobility group was found which roughly corresponded to the mobility of the light group of the positively charged carriers. The probe was designed so that no electrons could enter the analyzer section of the aspiration system. By this the electron concentration was given by the difference in concentration between the positively and negatively charged particles.This paper was read byH. Dolezalek, in an abbreviated form supplied by the authors.     

Numerical modeling of the high-latitude F-layer anomalies

The results of a detailed numerical study of the behaviour of the convective polar ionosphere are presented. The developed theoretical model produces three-dimensional distribution of electron density, electron and ion temperature. The effects of auroral particle precipitation on the density and temperature structures are studied for winter and low solar activity conditions. The high-latitude ionospheric features, such as the tongue of ionization, the main trough the polar ionization peak, the auroral ionization peak, the high-latitude ionization hole, the tongue of electron temperature, the high latitude minima of electron, temperature, and the ion temperature hot spot are obtained from calculations.Numerically obtained results are used for determining the HF propagation paths in the polar ionosphere. The effects of ionospheric irregularities on high frequency ionospheric radio waves are investigated by using a three-dimensional ray-tracing computer program. Ray-path trajectories are presented for different values of the elevation angle of transmission. From our study, it was found that large-scale irregularity structures of the high-latitude ionosphere, in the presence of the earth's magnetic field, significantly affect high frequency radio wave propagation.     

Generators in atmospheric electricity

Summary The definition of a generator in atmospheric electricity is considered, and various phenomena are discussed as to whether they can be described as generators.     

A note on the general concept of wave breaking for Rossby and gravity waves

A recently proposed general definition of wave breaking is further discussed, in order to deal with some points on which misunderstanding appears to have arisen. As with surface and internal gravity waves, the classification of Rossby waves into breaking and not breaking is a generic classification based on dynamical considerations, and not a statement about any unique signature or automatically recognizable shape. Nor is it a statement about passive tracers uncorrelated with potential vorticity on isentropic surfaces. A strong motivation for the definition is that proofs of the nonacceleration theorem of wave, mean-flow interaction theory rely, explicitly or implicitly, on a hypothesis that the waves do not break in the sense envisaged.The general definition refers to the qualitative behaviour of a certain set of material contours, namely those, and only those, which would undulate reversibly, with small slopes, under the influence of the waves' restoring mechanism, in those circumstances for which linearized, nondissipative wave theory is a self-consistent approximation to nonlinear reality. The waves' restoring mechanism depends upon the basic-state vertical potential density gradient in the case of gravity waves, and upon the basic-state isentropic gradient of potential vorticity in the case of Rossby waves. In the usual linearized theory of planetary scale Rossby waves on a zonal shear flow, the relevant material contours lie along latitude circles when undisturbed.     

The joint influence of the Coriolis force and the stratification on stationary motions in the zonal model of equatorial atmosphere

Summary In the framework of a zonal model of the equatorial atmosphere, the joint influence of the Coriolis force and stratification on stationary motion is studied. In addition we assume a Boussinesq approximation and consider the equatorial region as a macrodisturbance localized round the Equator, against a background of an isothermal atmosphere with a zonal distribution. Exact analytical solutions of the system of equations (1)–(5) are obtained and analyzed. An interesting result is that the equatorial atmosphere could exist in different stationary states in each of them separately, as well as in a number of them simultaneously.     

Numerical study of non-linear storm-surges

Summary The problem of storm-surges is investigated in a rectangular basin, setting up a numerical hydrodynamical model using the shallow-water equations. The forcing exerted by the atmosphere is given in the idealized form of a constant wind blowing over a section of the basin. The case in which the window of wind is moving is considered and for a basin that has the same dimensions of the Adriatic Sea. The conditions that give the maximum surge at one edge of the basin are investigated.

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Riassunto Il problema relativo alle storm-surges viene studiato in un bacino rettangolare, costru endo un modello idrodinamico numerico ed usando le equazioni relative ad acqua poco profonda. Lo stress esercitato dall'atmosfera è dato nella forma idealizzata di un vento costante che soffia sopra una sezione del bacino. Si considera il caso in cui la finestra di vento è in moto e per un bacino che ha le medesime dimensioni del mare Adriatico. Si studiano le condizioni che provocano il massimo dell'innalzamento ad un estremo del bacino.

     

Seismic signals in geothermal areas of active volcanism: a case study from ‘La Fossa’, Vulcano (Italy)

Since the last eruption (1888–1890) volcanism at Vulcano, Aeolian Archipelago, southern Tyrrhenian Sea, has taken the form of persistent fumarolic activity. The gas-vapour phases of the geothermal systems are mainly discharged within two restricted areas about 1 km apart from each other, in the northern part of the island. These areas are La Fossa crater, and the beach fumaroles of the Baia di Levante. Fluids released at the two main fumarolic fields display quite different chemical and temperature characteristics, implying different origins. The local seismicity essentially takes the form of discrete shocks of shallow origin (depth1 km) at La Fossa, usually with energy < 10¹³ ergs. They are thought to be related to the uprise of pressurized hot gases and vapours discharged at the crater fumaroles. The present investigation points to the existence of two principal categories of seismic events (called M-shocks and N-shocks). These are short events (normally < 10 s). M-type shocks are thought to be due to resonance vibrations within the interior of the volcano, probably driven by the excitation of shock-waves within cavities deeply affected by deposition and alteration of self-sealant hydrothermal minerals. N-type events display features that resemble those of volcano-tectonic earthquakes, but have no recognizable S-phases. Here they are tentatively attributed to microfracturing of rocks which have been extensively hydrothermally altered. Results of the present study permit a preliminary conceptual model of the local shallow seismic processes in the framework of geochemical modelling of fumarolic activity and geological inferences from geothermal drilling.     

An electromagnetic prospecting instrument for subsurface conducting zones

Summary A portable electromagnetic prospecting unit comprising a transmitter, receiver and resolver, designed and constructed to operate with a vertical setup of coils at a single frequency of 1250 cps, measures the components of the magnetic field at the receiver station resolved in phase and at quadrature with field near the transmitter. This may be used to locate vertical or steeply dipping conductors down to a depth of about 0.6 times the maximum workable transmitter receiver separation of 120 meters, the accuracy of measurements of the resolved components being better than 2%. The variation of resolved components over a buried conductor indicates its position and depth.This paper is published by the kind permission of the Director General, Geological Survey of India.     

Recycling of magmatic clasts during explosive eruptions: estimating the true juvenile content of phreatomagmatic volcanic deposits

The juvenile content of phreatomagmatic deposits contains both first-cycle juvenile clasts derived from magma at the instant of eruption, and recycled juvenile clasts, which were fragmented and first ejected by earlier explosions during the eruption, but fell back or collapsed into the vent. Recycled juvenile clasts are similar to accessory and accidental lithics in that they contribute no heat to further magma: water interaction, but previously no effective criteria have been defined to separate them from first-cycle juvenile clasts. We have investigated componentry parameters (vesicularity, clast morphology and extent of mud-coating) which, in specific circumstances, can distinguish between first-cycle juvenile clasts, involved in only one explosion, and such recycled juvenile clasts. Phreatomagmatic fall deposits commonly show gross grainsize and sorting characteristics identical to deposits of purely dry or magmatic eruptions. However the abundance of non-juvenile clasts in pyroclastic deposits is a sensitive indicator of the involvement of external water. If this component is calculated including recycled juvenile clasts with accidental and accessory clasts the contrast is even more striking. Data from a Holocene maar deposit in Taupo Volcanic Zone, New Zealand, suggest that the first-cycle juvenile component of the deposits is less than one-third of that determined by simple juvenile:lithic:crystal componentry.     

Nuclear technique in tracing the cumuli clouds

Summary Within the scopes of Mount Olympus Cumulus Project (sponsored by NATO) which is currently in progress on the summit of Mount Olympus (Hellas), a new method has been applied, with non radioactive tracers, which would become radioactive afterwards by neutron irradiation at the Democritus (Athens) nuclear reactor.To this effect the solution of the tracers (various chemical compounds) was injected into the cumuli clouds by spraying it in the form of small droplets by means of an agricultural sprayer airblast nozzle type. After tracing, samples of the cloud were taken by means of a filter paper connected with a high volume air sampler.     

Plasma drift estimates from the Dynasonde: comparison with EISCAT measurements

Modern ionosondes make almost simultaneous measurements of the time rate of change of phase path in different directions and at different heights. By combining these Doppler measurements and angles of arrival of many such radar echoes it is possible to derive reliable estimates of plasma drift velocity for a defined scattering volume. Results from both multifrequency and kinesonde-mode soundings at 3-min resolution show that the Dynasonde-derived F-region drift velocity is in good agreement with EISCAT, despite data loss during intervals of blanketing by intense E-region ionisation. It is clear that the Tromsø Dynasonde, employing standard operating modes, gives a reliable indication of overall convection patterns during quiet to moderately active conditions.     

Thermomagnetic properties of some Late Mesozoic diabase dikes of South Spitsbergen

Summary Laboratory investigations of thermomagnetic properties of specimens from seven diabase dikes of South Spitsbergen have been performed. Studies of temperature variation of isothermal saturation remanence combined with Hopkinson effect measurements and investigations of low temperature remanence give information on the mineral composition of the magnetic phases present in the magnetic fraction of the diabases. The experiments show that the clean natural remanence of these dikes has been acquired at the time of their, consolidation in the Late Mesozoic, though three of them may have been partially remagnetized at the time of Tertiary orogeny. Since the dikes reveal several normal and reversed directions of clean natural remanence, it is concluded that they were formed at different times in the Late Mesozoic, even though some of them may have been subjected to secondary influences at a later date.     

Sudden ionospheric disturbances and flare activity in the course of the solar cycle no 20

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The Bad Step Tuff: a lava-like rheomorphic ignimbrite in a calc-alkaline piecemeal caldera,English Lake District

Lava-like tuffs are lithologically indistinguishable from lavas, and form part of a temperature-and composition-controlled continuum from low-grade tuffs (which are non-welded or slightly welded), through high-grade (densely welded) tuffs, extremely high-grade tuffs (which may be agglutinated right to their upper and lower contacts), to spatter-fed lava flows. In some high-grade tuffs, a component of nonparticulate flow may postdate emplacement and deposition, but in extremely high-grade tuffs non-particulate deformation normally occurs during emplacement and deposition. In such cases, syn-depositional non-particulate deformation (previously called primary welding) and non-particulate slumping (previously called secondary flowage) processes overlap and are continuous, one into the other, so that distinction between them and their resultant structures is unrealistic and inapplicable. Therefore the term rheomorphism should be used to embrace all types of non-particulate flow. The Bad Step Tuff is the most lava-like of a sequence of rheomorphic calc-alkaline rhyolitic ignimbrites emplaced during a climactic caldera-forming eruption episode in the English Lake District. It is a ponded sheet, 40 to 400 m thick, which comprises a basal crudely stratified heterolithic breccia, a thick flow-laminated and locally vesicular central part, which beomes increasingly flow-folded upwards, and an upper autobreccia. Despite an absence of vitroclastic textures within the main laminated part, field relations show it to be a tuff. Diagnostic criteria are (1) a gradation, within a lithophysal zone, from unambiguous vitroclastic matrix of the basal lithic breccia upwards into the central flow-laminated tuff; (2) only rate autobreccia at the base of the sheet but ubiquitous autobreccia at the top of the sheet; and (3) close textural similarity with localized, intensely rheomorphic parts of associated ignimbrites that widely display unequivocal vitroclastic textures where their rheomorphism is less marked. The extremely high-grade character of the Bad Step Tuff may reflect its proximal setting in a piecemeal-type caldera. High emplacement temperatures resulted from high-rate but low-velocity vent emission from fissures along numerous cross-cutting calderafloor faults, producing very low boil over eruption columns and proximal ponding.     

Perturbation of eigenfrequencies of a cylinder due to a ridge on the surface

Summary An infinite cylinder is assumed to have a small ridge on the surface. The effect of the ridge on the eigenfrequencies of flute mode of oscillation is obtained in the linear approximation. The usefulness of these results in seismological problems is indicated.     

On the excitation of short-term variations in the length of the day and polar motion

Variations in the distribution of mass within the atmosphere, and changes in the pattern of winds produce fluctuations in all three components of the angular momentum of the atmosphere on time-scales upwards of a few days. It, has been shown that variations in theaxial component of atmospheric angular momentum during the Special Observing Periods in the recent First GARP Global Experiment (FGGE, where GARP is the Global Atmospheric Research Programme) are well correlated with short-term changes in the length of the day. They are consistent with the total angular momentum of the atmosphere and solid Earth being conserved on short timescales (allowing for lunar and solar effects), without requiring significant angular momentum transfer between the Earth's liquid core and solid mantle on timescales of weeks or months. It has also been shown that fluctuations, in the equatorial components of atmospheric angular momentum make a major contribution to the observed wobble of the instantaneous pole of the Earth's rotation with respect to the Earth's crust. A necessary step in the investigation was a re-examination of the underlying theory of non-rigid body rotational dynamics and angular momentum exchange between the atmosphere and solid Earth. Since only viscous or topographic coupling between the atmosphere and solid Earth can transfer angular momentum, no atmospheric flow that everywhere satisfied inviscid equations (including, but not solely, geostrophic flow) could affect the rotation of a spherical solid Earth. New effective angular momentum functions were introduced in order to exploit the available data and allow for rotational and surface loading deformation of the Earth. A theoretical basis has now been established for future routine determinations of atmopheric, angular momentum fluctuations for the purpose of meteorological and geophysical research, including the assessment of the extent to which movements in the solid Earth associated with very large earthquakes contribute to the excitation of the Chandlerian wobble.     

A new dual frequency moving source-receiverem unit

Summary The paper describes a new portable highly sensitive dual-frequency electromagnetic prospecting unit designed for detailedem surveys based on moving source compensation method. Employing horizontal-horizontal coil profiling, the unit is capable of working on a source-receiver separation of 330 feet (ninety meters) on both frequencies of 500 cycles per sec and 1500 cycles per sec to an accuracy of ±1%. The presence of secondary field characterised by two related components in-phase and out-of-phase are measured accurately to define the conductor characteristics. An unique provision is made for compensating the phase-shifts in the presence of medium to poor conductors, such as conductive overburden or relatively disseminated ore mineralisation.This paper is published by the kind permission of the Director General, Geological Survey of India.     

Matrix approach to the solution of electromagnetic induction in a spherically layered earth

Summary Matrix formulae for the intensities of the M- and E-fields have been derived. They have been applied to express the apparent resistivity, the transfer function, as well as the frequency equation determining the frequencies of free motion of the M- and E-waves. A fast algorithm for computing the transfer function and the apparent resistivity has been suggested.

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A new aspect of condensation theory

This paper examines the effects of the mixing of dry air into a cloud top from the point of view of the droplet spectra. It is shown theoretically that the resulting cycling of the air up and down in the cloud, as seems to be the essential mechanism by which cumuli have been diluted to their observed liquid water mixing ratio, can double the largest drop radius and generate cloud parcels containing drops of all sizes up to this maximum. These changes in the droplet distribution with size occur by a process which is not greatly influenced by the cloud condensation nuclei or the details of droplet growth since maritime like spectra can develop in continental type cumuli. It shows that large numbers of cloud condensation nuclei should not have much effect in inhibiting the rainforming process by reducing coalescence growth. On the contrary, the controlling parameters which determine precipitation efficiency and times seem to be those which control the mixing.