Showing papers in "Journal of geophysics in 1985"

Upper- mantle cross-section from California to Greenland ( USA).

Abstract: Pure-path upper-mantle models appropriate for tectonic, shield and old ocean have been recently presented by Grand and Heimberger. 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 hQrizontal distance of 400 km or less. The low-velocity zone, with a velocity of 4.4 kmjs, 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.

On the origin of the cusp field-aligned currents

Abstract: On suggere que la partie du systeme de courants alignes situee vers l'equateur, nommee Region 1 est provoquee par un relâchement des courants associe aux discontinuites rotationnelles de la plasmapause. La partie situee vers le pole, nommee courants du cornet est due a la magnetopause de la queue. Ils sont donc associes a la composante By du champ interplanetaire

Some problems with S, SKS and ScS observations and implications for the structure of the base of the mantle and the outer core

Abstract: 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 coremantle 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

Large-scale studies of Pi-2's associated with auroral breakups

Abstract: On a utilise les enregistrements de 28 stations magnetometriques centrees sur 73,9°-36,9 °N pour analyser les correlations entre les Pi 2 generees par les sous-orages et le breakup auroral. On observe que les Pi 2 se presentent en meme temps que des arcs calmes brillants au voisinage du bord equatorial de la zone d'activite aurorale. On situe les Pi 2 plus a l'equateur de la discontinuite de Harang. Leurs intensite et polarisation indique qu'elles proviennent des zones de courants alignes

Evaluation of the effectiveness of theoretical model calculation in determining the plasmapause structure

Abstract: On a compare le modele de structure de la plasmapause obtenu d'apres la convection de champ electrique aux donnees de mesures de la densite electronique en periode d'orage et a des formules empiriques. Le modele de convection pourrait etre utilise pour les etudes d'interaction ondes-particules quand il n'existe pas de donnees sur les ondes siffleuses

Observation of kinetic Alfvén waves excited at substorm onset

Abstract: Les systemes de courants intenses observes simultanement par EISCAT et GEOS-2 sont expliques comme etant des ondes cinetiques d'Alfven. Ces ondes sont excitees sur le bord interne de la couche de plasma, en derive vers l'est, par une conversion de mode des ondes hydromagnetiques de surface

Stochastic ion acceleration by coherent electrostatic waves

Abstract: On a etudie la limite de l'acceleration des ions par des ondes elestrostatiques coherentes, basse frequence en utilisant la methode des surfaces de Poincare. On observe que les ondes cyclotron de l'hydrogene accelerent preferentiellement O + , alors que les ondes hybrides inferieures favorisent H +