Journal of Geophysics
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Parameters of the auroral electrojet from magnetic variations along a meridian
Get PDFThe parameters of the auroral electrojet are described by using an ionospheric sheet current model. Data from a line of magnetometers between 62 and 67 degrees magnetic latitude are the input for the computation of the parameters during electrojet activity. The parameters are depicted in the parameter-time diagram which furnishes mainly the following information: 1. Applicability of the two dimensional current model. 2. Latitudinal extent of the electrojets as a function of time. 3. Location of centre of the current as a function of time. 4. Current flow direction.Two case studies are carried out. The first comprises DMSP (Defense Meteorological Satellite Program) data, the second makes use of data collected simultaneously by the Scandinavian Twin Auroral Radar Experiment (STARE). The results of the parameter-time diagram and the DMSP as well as the STARE data are in good agreement.
ARK: https://n2t.net/ark:/88439/y059691
Permalink: https://geophysicsjournal.com/article/257
 
Energetics of the Earth's core
Get PDFThe energy supplied to generate the Earth's magnetic field must ultimately result in heat flowing across the core-mantle boundary and through the Earth's surface. If the liquid core is stirred by thermal convection then only a small fraction of the total heat is dissipated in the electric currents, and in order to explain the observed field at least 1011 watt and probably 1013 watt of the Earth's surface heat flux must originate deep inside the core. If the core is cooling and there is concomitant chemical differentiation, a large amount of gravitational energy is released. This energy, unlike the heat released, is completely dissipated in the electric currents and enables the same magnetic field to be generated with a much lower heat flux. Chemical differentiation is therefore favoured as the energy source for the dynamo. The importance of gravitational settling depends on the density jump at the inner core boundary and on the stratification parameter in the outer core, both of which can, in principle, be determined seismologically.
ARK: https://n2t.net/ark:/88439/y027224
Permalink: https://geophysicsjournal.com/article/64
 
The upper mantle under Western Europe inferred from the dispersion of Rayleigh modes
Get PDFA stacking technique is applied to measure phase velocities of the fundamental and several higher Rayleigh modes over an array of long period stations located in Western Europe. The higher mode dispersion has been measured for periods between 25 and 100 s and for phase velocities up to 7.5 km/s. Using Backus-Gilbert inversion, a detailed model for the shear wave velocity in the upper mantle under the array is obtained. The low velocity zone is located between 150 and 230 km depth and is not very pronounced, but it is preceded by a rise in S velocity around 120 km depth. Strong velocity gradients are found at depths of 360 and 520 km. A good fit to the data can only be obtained if a zone of low density is assumed at a depth of 220 km or there about. As yet little can be said about depth, shape and extent of this zone, but the magnitude of the density drop implies a chemical or mineralogical stratification. A mechanism based on eclogite fractionation (Press, 1969) appears to be a likely candidate as the cause for such a gravitationally unstable stratification.
ARK: https://n2t.net/ark:/88439/y010885
Permalink: https://geophysicsjournal.com/article/161
 
Crustal structure of the Rhenish Massif and adjacent areas; a reinterpretation of existing seismic-refraction data
No full textMost of the existing seismic-refraction profiles in the Rhenish Massif/Rhenohercynian zone of Western Germany have been jointly reinterpreted using traveltime and amplitude information. The general pattern of observed phases can be divided into three types; each type corresponds to a distinct kind of velocity structure. Type I: Throughout the central Rhenish Massif and the adjacent Hessische Senke a strong P-phase reflection from the crust-mantle boundary is recorded in regions where no major volcanic features are crossed by the lines of seismic observations. The average crustal thickness is 28-29 km, the average crustal velocity (excepting sediments) is 6.2-6.3 km/sec, and the crust is nearly homogeneous. This structure is here referred to as the Rhenohercynian crustal model. Type II: Beneath the southern part of the Rhenish Massif and two areas in the northeast and southeast some structure within the crust is evident. Both an intracrustal and the Moho discontinuities are evidenced by strong reflected phases, the Moho reflection being the stronger one. Along the profiles crossing major volcanic features such as Vogelsberg and central Westerwald, but not beneath the eastern Eifel, the M-discontinuity is heavily disrupted or "smeared" and an intermediate intracrustal boundary at about 20 km depth forms the main reflector for seismic waves. Beneath this boundary the velocity increases gradually from about 7 km/sec to upper-mantle velocities. Type III: For profiles crossing the northern Rhine Graben area as well as for a line from the Siebengebirge through the Rhenish Massif to the north, east of the Lower Rhine basin, the observed phases indicate only one major seismic boundary at a depth of about 23 km where the velocity increases rapidly to 7.3 km/sec. Below this boundary the velocity increases gradually with depth reaching 8 km/sec at 27-28 km. The occurrence of types I, II, and III can be roughly correlated with tectonic setting. The Pn phase is recorded with variable success and disappears completely on a profile passing the eastern Eifel volcanics, but is clear on the lines through Vogelsberg and central Westerwald. The petrographic differences between these volcanics appear such to be reflected in the behaviour of the seismic waves. Cross sections and areal views are used to display the variations in crustal and upper mantle velocity structure.
ARK: https://n2t.net/ark:/88439/y002726
Permalink: https://geophysicsjournal.com/article/180
 
Three dimensional seismic velocity anomalies in the lithosphere
Get PDFThis paper discusses cruxes of the method for inverting the P-time residual data introduced by Aki et al. (1976a) and summarizes the results obtained by the method on 3-dimensional seismic velocity anomalies in the lithosphere under several seismic arrays around the world. The velocity anomalies at shallow depths correlate well with geologic features in young, active areas such as California, Hawaii, and Yellowstone, but the correlation is not apparent in old, stable areas such as eastern Montana and Norway. Significant small scale (20~50 km) lateral inhomogeneity is observed everywhere to the depth of 100~150 km, with the minimum estimate of root mean square fluctuation about 3%. The lithosphere-asthenosphere boundary seems to manifest itself as change in the roughness of anomaly pattern or in the trend of anomaly.
ARK: https://n2t.net/ark:/88439/y086778
Permalink: https://geophysicsjournal.com/article/218
 
Finite-difference modelling for P-pulse propagation in elastic media with arbitrary polygonal surface
Get PDFThe applicability of the finite-difference methods has been limited in most cases to simple geometric shapes. The problem of introducing boundary conditions into the scheme has usually restricted the models to structures in which the boundaries are parallel to the coordinates. Recently, several investigators have studied the effect of prominent topographic features on seismic signals. Most deal with SH waves. The behaviour of a P-SV pulse in media with prominent irregular surfaces is yet almost unknown. The difficulty of the last problem relative to the SH case lies in the vectorial form of the equation of motion and the more complicated boundary conditions. In the present work a technique is proposed for simulating the P-SV wave propagation in a two-dimensional half-space with an arbitrary polygonlike topography. This technique has been applied to compute seismograms due to a P-pulse on surfaces of ridges and canyons. The incident pulse is amplified at the crest of mountains and at the upper corners of canyons. The magnitude of amplification is a function of the steepness of the topographic structure and can increase by 50 % compared to a flat surface under the same conditions. The maximum attenuation computed at the bottom of a canyon was 25 %. It can be concluded that the influence of prominent topographic features on the incident P-pulse is similar to that on incident SH waves, which was computed in previous investigations.
ARK: https://n2t.net/ark:/88439/y066050
Permalink: https://geophysicsjournal.com/article/279
 
Three-dimensional seismic ray tracing
Get PDFTwo methods for tracing seismic rays between 2 given end points through three dimensional, continuously varying velocity structures are available. This paper describes and compares them for problems of practical interest and for analytical ray paths through an idealized velocity structure. One method involves "shooting" the ray from one point with a given starting direction and then modifying this starting direction until the ray emerges at the desired target, while the other method involves "bending" an initial path between the end points until it satisfies the principle of stationary time. For most of the models investigated, "bending" is computationally faster than "shooting" by a factor of 10 or more. The "bending" method can be modified to deal with discontinuities in the velocity model, and can also be adapted for use in conjunction with a table of distances as a function of ray parameter when the three dimensional anomaly influences only a small fraction of the total ray path. The geometrical spreading effect on the amplitude of the ray may be retrieved easily from the "bending" solution.
ARK: https://n2t.net/ark:/88439/y041002
Permalink: https://geophysicsjournal.com/article/133
 
Joint magnetometer array and radar backscatter observations of auroral currents in Northern Scandinavia
No full textAs a contribution to the International Magnetospheric Study the University of Munster has installed an array of 32 Gough-Reitzel type magnetometers located mostly in Northern Scandinavia. Also for the IMS, the Max Planck Institute for Aeronomy at Lindau is operating the Scandinavian Twin Auroral Radar Experiment (STARE) which consists of two nearly identical backscatter radars located near Trondheim (Norway) and Sauvamaki (Finland). For a weak isolated substorm on October 7, 1976 the spatial structure of the electron density irregularities observed by the Trondheim-radar and the equivalent current distribution derived from the magnetic measurements have been compared. A good correspondence has been found between the location and magnitude of the maxima of the horizontal magnetic disturbance and the radar backscatter amplitude for an eastward electrojet. For most of the comparison there appeared also to be good agreement between the direction of the equivalent current and the direction antiparallel to the line-of-sight irregularity drift. This supports the idea that the backscatter irregularities are caused by current driven plasma instabilities and that it is possible to determine auroral ionospheric currents with the backscatter radar technique. However, during periods of enhanced electron precipitation, differences between the drift directions given by the two methods were observed.
ARK: https://n2t.net/ark:/88439/y099827
Permalink: https://geophysicsjournal.com/article/250
 
Precursors to P'P' and upper mantle discontinuities
Get PDFIn this paper the origin of precursors to P' P' with lead times greater than 50 s is investigated. Good NORSAR records of P'650P' and P'400P' for these arrivals as well as corresponding slowness estimates are presented. These phases are interpreted as done by others in terms of underside reflections from discontinuities or sharp transition zones in the upper mantle. An extensive search of 5 years of NORSAR records did not produce any significant evidence on P' P' precursor arrivals with lead times greater than 50 s other than those mentioned above.
ARK: https://n2t.net/ark:/88439/y030873
Permalink: https://geophysicsjournal.com/article/255
 
Scattered Waves in the Coda of P
Get PDFThis paper presents a survey of the development and use of first order elastic scattering theory in seismology. The various methods used to provide expressions for scattered waves from variations in structure are shown to lead to a single scattering formula. A ray theory approximation for the incident and scattered waves provides a simple formula from which the radiation patterns of different types of scatterer can be derived. As an illustration, the solution for a homogeneous 'average' structure is given in detail. The statistical properties of the signal in time are clearly related to those of the scatterers in space and, in particular, the correlation time of the signal is related to the correlation distance of the scatterers. The paper ends with a discussion of the possible use of first order (weak scattering) theory in cases when the scattered signals are large.
ARK: https://n2t.net/ark:/88439/y038723
Permalink: https://geophysicsjournal.com/article/88