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An apparent paradox is discussed, arising from the contrast between an inferred constant mantle viscosity profile and theoretical and experimental rheological flow laws, which predict a mantle viscosity function varying strongly as a function of both temperature and pressure. One can explain the paradox by a particular choice of material parameters, but then mantle temperatures (computed adiabatically) are too low; increasing the temperature by inserting compensatory thermal boundary layers is considered to be dynamically unfeasible, again because of the flow law. We consider this an impasse, and to resolve it, we suggest that old dogmas concerning boundary layers and adiabats need to be critically re-examined, to understand their basis. When this is done, we find that the observed constant viscosity is, in effect, demanded by the interplay of the rheology with the convective process, the mantle temperature is not necessarily adiabatic, and some form of layering effect may be expected, although the ideas presented here are virtually independent of the precise dynamical style of the convective motion. A consequence of these results is that explanations and extrapolations taken from constant-viscosity convection models are, a priori, unjvstifiable. (Specifically, a constant viscosity mantle is a fundamental consequence of the state of flow together with the fluid parameters and rheology: it is not a passive coincidence, which may then be used to deduce the flow state, etc.)
ARK: https://n2t.net/ark:/88439/y014655
Permalink: https://geophysicsjournal.com/article/17
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Observation of auroral hiss at Jupiter by Voyager I has been suggested as being directly related to regions of parallel electric field and auroral particle precipitation. The effect of a parallel electric field on whistler mode propagation in Jupiter's magnetosphere has been studied. The dispersion relation for whistler mode waves in an isothermal one-component electron plasma in the presence of a parallel electric field has been used to study the growth of whistler mode waves propagating in the Jovian magnetosphere. The growth rates have been computed by using the observed plasma parameters at 5.6 Rj . The growth rate, which is found to be a maximum in the equatorial magnetosphere, is reduced to zero in the absence of the electrostatic field. This has lead us to conclude that, in the case of isothermal magnetosplasma, the growth rate is induced by the electrostatic field.
ARK: https://n2t.net/ark:/88439/y007466
Permalink: https://geophysicsjournal.com/article/16
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On the basis of stress accumulation between two large earthquakes the maximum strain rate can be estimated. But the strain rates cannot be used to estimate directly the magnitude of the tectonic processes. The estimates of stress rates and horizontal crustal movement values are bigger than the real values in nature. Due to the significant differences among stations situated in the same region it seems that secular rates are modified or increased to a considerable extent by local conditions of different type, which are not connected to tectonic activity.
ARK: https://n2t.net/ark:/88439/y092577
Permalink: https://geophysicsjournal.com/article/15
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The relevance of charge exchange processes for the evaporation of neutral gases out of planetary gravitational fields has been known for quite a long time. For the planetary escape, however, collisions of the escaping constituent with both neutral and ionized atmospheric species operate in building up the escape flux. For the Earth, hydrogen collisions with O-atoms and O+- and H+- ions lead to about comparable contributions to the hydrogen escape at heights below 800 km, whereas at larger heights only charge exchange collisions need to be considered. In the present work, the lower region is therefore described by particle and energy flux conservation concepts, whereas in the upper region free-flight kinetic concepts are used. We give solutions for the height profiles of the hydrogen density, temperature and escape flux. The inclusion of charge exchange collisions increases the scale height, and consequently a smaller decrease of the hydrogen density is given. The hydrogen temperature, however, is reduced by including this effect. The charge exchange processes contribute about 70% to the total hydrogen escape which is of the order of a few 107 atoms/cm2 s at the plasma pause.
ARK: https://n2t.net/ark:/88439/y085188
Permalink: https://geophysicsjournal.com/article/14
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Complicated radially symmetric models of the seismic velocity structure at the base of the mantle (Bullen's D" region) and the uppermost outer core have been inferred from analyses of the waveforms and relative amplitudes of S, SKS and ScS phases. Using radially symmetric structure, it has been difficult to construct physically realizable models of the rheology of D" that simultaneously satisfy P and S amplitudes and slownesses in the core shadow. These data are reviewed in the light of an increasing body of evidence that the structure of D" is characterized by heterogeneities having a broad spectrum of scale lengths. Depending on the region and range interval of D" sampled, S waveforms can be found that support either a radially simple or complex model of D". The complex models have one or more first-order discontinuities in velocity. The particle motion measured by three-component recordings of some S + ScS waveforms is consistent with a discontinuous increase in S velocity 250--300 km above the core-mantle boundary. The observed particle motion in these examples cannot readily or alternatively be explained by either general anisotropy or by strong lateral velocity gradients in D". Sufficient variability in S waveforms and travel times exists, however, that any radially symmetric model having a strong degree of complexity should be accepted with caution until all of the competing effects of lateral heterogeneity and possible anisotropy in D" are fully investigated. The distribution and scale lengths of heterogeneities in D" may account for regional differences in the properties of D" inferred from waveform data, including features that mimic intrinsic attenuation and anisotropy.
ARK: https://n2t.net/ark:/88439/y072899
Permalink: https://geophysicsjournal.com/article/13
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Pure-path upper-mantle models appropriate for tectonic, shield and old ocean have been recently presented by Grand and Helmberger. This was accomplished by modeling a rather restricted data set of S and SS triplication waveforms as well as the beginning portion of the Love waves. A much larger data set of S, SS and SSS, etc. (multibounce S-wave triplications) with a mixture of tectonic paths is available. In particular, events usually occur at tectonic margins and are recorded on stable continents. We present results of modeling these observations for laterally varying structure, essentially along a profile from California to Greenland. The models are allowed to be locally dipping with the lithosphere thickening with age at the expense of a dwindling low-velocity zone. Lateral variation does not appear to be required for depths greater than 400 km along this particular profile. The best-fitting model has a large increase in lithospheric thickness near the Rocky Mountain Front, roughly an increase of 75 km in thickness over a horizontal distance of 400 km or less. The low-velocity zone, with a velocity of 4.4 km/s, is replaced by a much faster upper 300 km with velocities near 4.7 km/s or a 7% overall increase. The one-way travel time jumps by roughly 4 s across this boundary, which compares reasonably well with the direct S residuals obtained from deep earthquake data although the latter data show large scatter.
ARK: https://n2t.net/ark:/88439/y069500
Permalink: https://geophysicsjournal.com/article/12
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Air pressure lows and highs with periods of some days and seasonal variations of mean air pressure can be regarded as loading functions on the Earth's surface. They result in quasi-periodic surface deformations. The influence of such displacements on the results and the accuracy of VLBI experiments (Very Long Baseline Interferometry) is investigated by simulation calculations for the transatlantic Effelsberg-Haystack baseline. Different models for the time dependence of air-pressure-induced station displacements are considered. It is shown that today's standard VLBI data analysis, including model parameters for clock- and atmospheric-refraction effects, is not able to correct the measurements for atmospheric loading effects. It leads to erroneous baseline vectors. Hence, for every geodetic VLBI experiment, the amount by which (1) the local air pressure at the station and (2) the mean air pressure in a surrounding area of 2,000 km radius has changed during the experiment should be tested. These two values give an estimate of the resulting vertical displacments by the use of a regression formula. The corresponding delay-time corrections have to be applied to the VLBI data. Most of the radiotelescopes participating in geodetic VLBI experiments are situated in regions with small seasonal variations of the station position. However, an increasing VLBI accuracy and an expanded and denser VLBI network will also require the consideration of seasonal displacements.
ARK: https://n2t.net/ark:/88439/y053411
Permalink: https://geophysicsjournal.com/article/8
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As a common mechanism for the various volcanic phenomena on Earth we point out a severe disobeyance of Poincare's (von Zeipel's) theorem: hot, gas-rich, high-pressure "fingers" (diatremes) can grow out of the boundary layer above a molten domain and thrust their way up from the asthenosphere toward the surface. The isobars of a planet or moon can look like the surface of a bed of nails. Linear arrays of high-pressure diatremes can drive continental motion. Moreover, we hold the tidal torque responsible for magnetic dynamo action.
ARK: https://n2t.net/ark:/88439/y049522
Permalink: https://geophysicsjournal.com/article/7
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Deep seismic reflection profiling of the continental crust has found world-wide evidence for prominent reflections from the middle and lower crust. This paper presents evidence for a correlation between the depth to the zone of pronounced reflectivity (the 'reflective lower crust') and the surface heat flow. The highly reflective zone appears to be shallower beneath regions with higher heat flow, suggesting that one condition for the development of the reflective zone is the existence of a sufficiently high temperature in the crust. The data presented in this paper suggest that the highly reflective zone is generally developed only at temperatures higher than 300°-400 °C. This correlation of reflectivity with heat flow implies that crustal reflectivity must be variable on the same time scales on which crustal heat flow is variable. This constraint appears to favour an origin for the lower crustal reflectivity related either to ductile strain banding or to free fluids that may be transient on geologic time scales, rather than to compositional layering or multiple igneous intrusions that are relatively permanent features of the crust.
ARK: https://n2t.net/ark:/88439/y039633
Permalink: https://geophysicsjournal.com/article/6
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In magnetotelluric studies time variations of the horizontal telluric and magnetic field components at the earth's surface are compared to get detailed information of the electrical conductivity structure of the earth's interior. The development of conductivity models from the data demands the thorough estimation of the transfer functions in the frequency domain between the Fourier transforms of the recorded time series. The analytical presentation of the estimated transfer functions allows an individual selection of a number of frequencies for further investigation. Larsen's presentation (Larsen, 1975, 1980) of the transfer functions by single polynomials demands a complicated calculation of confidence limits. Therefore, the transfer functions are presented here by the sum of polynomials which fulfil an orthogonality criterion. The orthogonality criterion allows a rather simple estimation of the frequency-dependentconfidence limits of the transfer functions. The polynomial method is applied to a 100-day record of the magnetic and telluric field variations near Gottingen. As the telluric field is usually partially disturbed during such a long time interval, the polynomial method is extended to treat telluric time series with missing data. The comparison of the smooth polynomial transfer functions with band-averaged estimates yields a good correlation between the estimates as well as between their confidence intervals.
ARK: https://n2t.net/ark:/88439/y026544
Permalink: https://geophysicsjournal.com/article/4
