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In the first kilometers of the subsurface, temperature anomalies due to heat conduction processes rarely exceed 20–30 ∘C. When fault zones are sufficiently permeable, fluid flow may lead to much larger thermal anomalies, as evidenced by the emergence of thermal springs or by fault-related geothermal reservoirs. Hydrothermal convection triggered by buoyancy effects creates thermal anomalies whose morphology and amplitude are not well known, especially when depth- and time-dependent permeability is considered. Exploitation of shallow thermal anomalies for heat and power production partly depends on the volume and temperature of the hydrothermal reservoir. This study presents a non-exhaustive numerical investigation of fluid flow models within and around simplified fault zones, wherein realistic fluid and rock properties are accounted for, as are appropriate boundary conditions. 2D simplified models point out relevant physical mechanisms for geological problems, such as “thermal inheritance” or pulsating plumes. When permeability is increased, the classic “finger-like” upwellings evolve towards a “bulb-like” geometry, resulting in a large volume of hot fluid at shallow depth. In simplified 3D models wherein the fault zone dip angle and fault zone thickness are varied, the anomalously hot reservoir exhibits a kilometer-sized “hot air balloon” morphology or, when permeability is depth-dependent, a “funnel-shaped” geometry. For thick faults, the number of thermal anomalies increases but not the amplitude. The largest amplitude (up to 80–90 ∘C) is obtained for vertical fault zones. At the top of a vertical, 100 m wide fault zone, temperature anomalies greater than 30 ∘C may extend laterally over more than 1 km from the fault boundary. These preliminary results should motivate further geothermal investigations of more elaborated models wherein topography and fault intersections would be accounted for.

International audience; This paper investigated variations of quiet time ionospheric irregularities over the African equatorial ionization anomaly using the rate of change of total electron content index. Irregularities were quantified in terms of percentage occurrence and examined along with parameter of the anomaly, mainly its strength and the asymmetry of the crests as well as equatorial electric field derived from the real‐time equatorial electric field model and meridional wind obtained from the horizontal wind model. Irregularities occurred from 19:30 to 03:00 LT with a time difference of 1 hr between both crests. The highest occurrences were registered in April: 91.67%, 75.00%, and 96.43% for the northern crest, trough, and southern crest, respectively. Seasonally, stronger anomaly (>20 total electron content unit) in addition to the highest equatorial electric field value in the dusk sector corresponded with the equinoctial higher occurrence rate in both hemispheres, while stronger meridional wind and farthest crests location accounted for the least occurrence in winter. The summer occurrence rate was favored by reduced meridional wind, smaller crests location, and late time of prereversal enhancement at the magnetic equator. There was a significant asymmetry in irregularities over the crests in both hemispheres with stronger and greater occurrence rate over the southern crest. Also, irregularities strength and occurrence rate were similar over the northern crest and trough in summer. In addition to dusk‐sector activity, irregularities occurred during postmidnight in summer. Simultaneous variations of irregularities at the crests and trough also highlighted the contribution of nonequatorial processes to their formation at the crests.

International audience; HIGHLIGHTS-Soil analyses and airborne measurements are compared to each other.-In 47% of cases, both the datasets are compatible in small-and large-dimension variations.-Despite the general consistency of data, soil analyses are more heterogeneous.-Spatial-correlation within the variable could be a witness of hotspots in low-contaminated zones. ABSTRACT Following Fukushima nuclear disaster, several data gathering campaigns surveyed the radionuclide propagation in the environment. However, the acquired datasets do not have the same sampling dimension. For example, the airborne measurements are some sort of averaging over a circular field of view, beneath the sensor; while the soil analyses are much more punctual. The objective of this work is to compare the soil samples and an airborne survey to investigate whether these two datasets reflect the same spatial patterns or not. This is prerequisite for combining the multiresolution data to create and update the contamination map in a post-accidental situation. The analyses were performed on square tiles of 20 km side to study large-and small-dimension variations in 137 Cs concentration. The former was modelled by fitting a plane (called trend) to the georeferenced data points; and the latter was modelled by computing the difference (called residual) between the trend and the initial data. Dip direction and dip angle of trends as well as minimum spatial correlation distance and anisotropy of residuals were computed for both the soil and airborne datasets and compared. Dip directions are compatible in 73% of the tiles and dip angles are generally close. Anisotropy directions are compatible in 49% of the tiles and minimum spatial correlation distances are significantly more marked for the airborne dataset. The soil samples and airborne measurements are therefore more in agreement in large-dimension (trend) rather than in small-dimension (residual) variations. More generally, both the datasets allow highlighting the main contamination plumes distinguishable because of high concentration values. The airborne dataset yet appears to be more powerful to quantify spatial correlations, which could be linked to the contamination mechanisms.

To explain the possible destabilization of a two-dimensional magnetic equilibrium such as the near-Earth magnetotail, we developed a kinetic model describing the resonant interaction of electromagnetic fluctuations and bouncing electrons trapped in the magnetic bottle. A small-$\unicode[STIX]{x1D6FD}$approximation (i.e., the plasma pressure is lower than the magnetic pressure) is used in agreement with a small field line curvature. The linearized gyro-kinetic Vlasov equation is integrated along the unperturbed particle trajectories, including cyclotron and bounce motions. The dispersion relation for drift-Alfvèn waves is obtained through the plasma quasi-neutrality condition and Ampere’s law for the parallel current. It has been found that for a quasi-dipolar configuration ($\text{L}\sim 8$corresponds to the set of the Earth’s magnetic field lines, crossing the Earths magnetic equator at 8 Earth radii), unstable electromagnetic modes may develop in the current sheet with a growth rate of the order of a few tenths of a second provided that the typical scale of density gradient slope responsible for the diamagnetic drift effects is over one Earth radius or less. This instability growth rate is large enough to destabilize the current sheet on time scales of 2–4 minutes as observed during substorm onset.

International audience; The largest compilation of seismological and gravimetry data are combined in a sequential inversion to shed light on unresolved crustal structures in Northern Taiwan and Southernmost Ryukyu area. Our density and seismic velocity models reveal a NW–SE trending Transfer Zone (TZ) between 23.25°N and 24°N. This primary TZ delimits major changes in the Eurasian Plate (EP) slab geometry and dynamics, as well as first order crustal structure variations. North of this TZ, the EP slab is breaking off from north to south as a result of (1) the extreme EP bending favouring plastic deformation, (2) the stretching of the plate caused by the downward pull of the oceanic part of the slab versus the buoyancy of continental part of the plate still deforming in the Taiwan orogen and (3) the indentation of the EP slab by the western slab edge of the Philippine Sea Plate (PSP).The change of EP slab dip angle is clearly observed in the density profiles. The subvertical northernmost tip of EP slab allows asthenospheric upwelling through slab tear and subsequent invasion of the region beneath the Longitudinal Valley. This process is migrating southward up to the TZ, which marks the transition to a different rheological behaviour. The combination of deep arc-continent collision, clockwise asthenospheric toroidal flow and westward propagation of the southern Okinawa rift results in a 100° clockwise rotation of the northernmost tip of the Central Range. The eastward extrusion of this crustal block (∼90 km in diameter) overlays and deflects the PSP slab.The indentation of the EP by the PSP (Luzon Arc) thus results in complex lithospheric accommodation processes and structures.

Observations of Jovian broadband kilometric (bKOM) radiation and ultraviolet (UV) auroras were acquired with the Waves and Juno‐UVS instruments for ∼2 hr over the northern and southern polar regions during Juno's perijoves 4, 5, and 6 passes (PJ4, PJ5, and PJ6). During all six time periods, Juno traversed auroral magnetic field lines connecting to the UV main auroral ovals, matching the estimates of bKOM radio source footprints. The localized bKOM radio sources for the PJ4 north pass map to magnetic field lines having distances of 10 to 12 Jovian radii (R J) at the magnetic equator, whereas the extended bKOM radio sources for the other events map to field lines extending to 20–61 R J. We found the peak bKOM intensities during Juno's potential radio source crossings show positive, negative, and no correlations with the UV main oval brightness and color ratio. Only the positive correlations suggest wave‐particle energy transport., Key Points First simultaneous observations of Jovian broadband kilometric radio source footprints and ultraviolet auroras were carried out using JunoJovian broadband kilometric radio source footprints correspond to the UV main oval locationsLink of Jovian radio intensity during Juno's near‐source crossing with both brightness and color ratio of the UV main oval was investigated

International audience; A temporary onshore‐offshore seismic network deployed during the 2‐year period of the Observación SISmológica en ECuador project provides a detailed and well‐focused image of the seismicity for magnitudes as low as 2.1 at the Central Ecuadorian subduction zone. During this 2‐year experiment, the shallow and locked subduction patch shows little evidence of background seismicity that instead occurred downdip of the coupled patch at ~20‐km depth. In this region, seismicity is possibly controlled by the crustal faults bounding the sedimentary basin of Manabí and the rheology of the upper plate. The dip angle of the interplate contact zone, defined by a smooth interpolation through the hypocenters of thrust events, is consistent with a progressive increase from 6° to 25° from the trench to 20‐km depth. Offshore, a seismic swarm, concomitant with a slow‐slip event rupturing the highly coupled subduction megathrust, highlights the reactivation of secondary active faults within the thickened crust of the subducting Carnegie Ridge at the leading edge of a large oceanic seamount.

This paper presents the first regional model of the magnetic field of Mercury developed with mathematical continuous functions. The model has a horizontal spatial resolution of about 830 km at the surface of the planet, and it is derived without any a priori information about the geometry of the internal and external fields or regularization. It relies on an extensive dataset of the MESSENGER’s measurements selected over its entire orbital lifetime between 2011 and 2015. A first order separation between the internal and the external fields over the Northern hemisphere is achieved under the assumption that the magnetic field measurements are acquired in a source free region within the magnetospheric cavity. When downward continued to the core-mantle boundary, the model confirms some of the general structures observed in previous studies such as the dominance of zonal field, the location of the North magnetic pole, and the global absence of significant small scale structures. The transformation of the regional model into a global spherical harmonic one provides an estimate for the axial quadrupole to axial dipole ratio of about g 2 0 / g 1 0 = 0.27 . This is much lower than previous estimates of about 0.40. We note that it is possible to obtain a similar ratio provided that more weight is put on the location of the magnetic equator and less elsewhere.

International audience; Vertical velocities obtained from uplifted river terrace dating near mountain fronts are commonly converted into overthrusting slip rates assuming simple geometry of the fault at depth. However, the lack of information on the dip angle of these shallow structures can lead to misinterpretation in the accommodation of convergence, and thus to erroneous conclusions on the transfer of shortening to the emergent thrust faults. Here, to assess the impact of fault geometry, we focus on the eastern Himalayan region in the south Central Bhutan, where the topographic frontal thrust (TFT) has been already documented by GPS, paleoseismic, geomorphic and geological studies. This study is based on high-resolution near-surface geophysical investigations, including electrical resistivity, seismic and gravity measurements. Using a similar stochastic inversion approach for all data sets, new quantitative constraints on both fault geometry and petrophysical parameters are obtained to image shallow depths, in the upper ca. 80 m. The combined results from both surface observations and geophysical measurement provide a TFT geometry that is dipping northwards with a shallow angle at the top (0–5 m), steeply dipping in the middle (5–40 m) and flattening at deeper depths (>40 m). Together, our new constraints on the fault geometry allow us to estimate a minimum overthrusting slip rate of 10 ± 2 mm yr−1, which is only a part of the ca. 17 mm yr−1 GPS convergence. This suggests that, in the study area, significant deformation partitioning on several faults including TFT and the Main Boundary Thrust cannot be ruled out. More importantly, assuming constant slip rate, the obtained dip angle variations lead to uplift rate changes with distance to the TFT. This underlines that taking into account uplift rate from terrace dating only at the front location and assuming a constant dip angle fault geometry based on surface observations may significantly bias the slip rate estimates.

This work aims at studying the fracture initiation pressure of a horizontal wellbore drilled in an anisotropic poroelastic medium based on an analytical model. Using the complex potential approach, the closed-form solution of stresses in the surrounding formation is deduced as a function of the orientation in respect with stratification and the pressure inside wellbore. The analytical solution is developed for the steady state flow in a permeable formation due to the difference between pressures applied at well wall and the pore pressure at the far-field. Once the stress distribution on the borehole wall is obtained, it is possible, by using a known anisotropic tensile strength criterion for the rock, to infer the explicit expression of the fracture initiation pressure as a function of different parameters, such as the anisotropic poroelastic properties, the anisotropic initial stresses, the bedding dip angle and the anisotropic tensile strength of the formation. The validation of the analytical is conducted by comparing it with the numerical results using the finite element method . Finally, through the parametric study , we highlight the influence of different factors on the fracture initiation pressure.

International audience; Stress concentration is a direct result of stress redistribution around an excavation, that may lead to rock bursting under specific geomechanical conditions. This paper presents a case-study of a rockburst that took place in the shaft station area of the Provence coal mine in Southern France. The mined coal seam has a 2.5 m thickness and a 10° dip angle. The rockburst occurred in 1993 at the shaft station level, where it is surrounded by several longwall panels that were excavated between 1984 and 1994. The area of the shaft station is at 1000 m depth. A very thin layer of stiff limestone occupied the middle of the exploited coal seam. A large-scale finite difference numerical model of the mine has been constructed by using FLAC3D. The model simulates the area of the shaft with its irregular pillars and the longwall panels excavated between 1984 and 1993. The excavations were performed in two steps. Firstly, the galleries of the shaft station area were excavated in order to determine their effect on the failed pillar. Then, the longwall panels were excavated year by year to detect the stress and strain energy increments induced on the pillars. The origin of the rockburst was analysed based on different rockburst criteria. The results show that the vertical stress increased in the shaft station pillars due to excavation of longwall panels. In addition, we found that the small pillars have higher burst tendency than the large ones. Finally, the Burst Potential Index (BPI) was found to be able to estimate the pillar burst tendency based on the energy storage rate (ESR), however, this criterion (BPI) is based on calculating the stress and the energy changes in the vertical direction only.

International audience; Fracture properties are important in carbonate reservoir characterization, as they are responsible for a large part of the fluid transfer properties at all scales. It is especially true in tight rocks where the matrix transfer properties only slightly contribute to the fluid flow. Open fractures are known to strongly affect seismic velocities, amplitudes and anisotropy. Here, we explore the impact of fracture evolution on the geophysical signature and directional V-p anisotropy of fractured carbonates through diagenesis. For that purpose, we studied a meter-scale, parallelepiped quarry block of limestone using a detailed structural and diagenetic characterization, and numerous V-p measurements. The block is affected by two en-echelon fracture clusters, both being formed in opening mode (mode 1) and cemented, but only one being reactivated in shear. We compared the diagenetic evolution of the fractures, which are almost all 100% filled with successive calcite cements, with the P-wave velocities measured across this meter-scale block of carbonate, which recorded the tectonic and diagenetic changes of a South Provence sedimentary basin. We found that a directional V-p anisotropy magnitude as high as 8-16% correlates with the reactivated fractures' cluster dip angle, which is explained by the complex filling sequence and softer material present inside the fractures that have been reactivated during the basin's tectonic inversion. We show that although a late karstification phase preferentially affected these reactivated fractures, it only amplified the pre-existing anisotropy due to tectonic shear. We conclude that V-p anisotropy measurements may help to identify the fracture sealing/opening processes associated with polyphased tectonic history, the anisotropy being independent of the current stress state. This case shows that velocity anisotropies induced by fractures resulted here from a cause that is different from how these features have often been interpreted: selective reactivation of sealed fractures clusters rather than direction of currently open ones. (C) 2017 Elsevier B.V. All rights reserved.

International audience; Data-based modeling of the magnetic field originating in the Earth’s ionosphere is challenging due to the multiple timescales involved and the small spatial scales of some of the current systems, especially the equatorial electrojet (EEJ) that flows along the magnetic dip equator. The Dedicated Ionospheric Field Inversion (DIFI) algorithm inverts a combination of Swarm satellite and ground observatory data at mid- to low latitudes and provides models of the solar-quiet (Sq) and EEJ magnetic fields on the ground and at satellite altitude. The basis functions of these modelsare spherical harmonics in quasi-dipole coordinates and Fourier series describing the 24-, 12-, 8- and 6-h periodicities, as well as the annual and semiannual variations. A 1-D conductivity model of the Earth and a 2-D conductivity model of the oceans and continents are used to separate the primary ionospheric field from its induced counterpart. First results from the DIFI algorithm confirm several well-known features of the seasonal variability and westward drift speed of the Sq current systems. They also reveal a peculiar seasonal variability of the Sq field in the Southern hemisphere and a longitudinal variability reminiscent of the EEJ wave-4 structure in the same hemisphere. These observations suggest that the Sq and EEJ currents might be electrically coupled, but only for some seasons and longitudes and more so in the Southern hemisphere than in the Northern hemisphere.

International audience; We detected several thousand deep englacial icequakes on Glacier d’Argentiere (Mont-Blanc Massif) between 30 March and 3 May 2012. These events have been classified in eight clusters. Inside each cluster, the waveforms are similar for P-waves and S-waves, although the time delay between the P-waves and S-waves vary by up to 0.03 s, indicating an extended source area. Although these events were recorded by a single accelerometer, they were roughly located using a polarization analysis. The deepest events were located at a depth of 130 m, 60 m above the ice/bed interface. The clusters are separated in space. The largest cluster extends over about 100 m. For this cluster, the strike of the rupture plane is nearly parallel to the direction of the open crevasses, and the dip angle is 56◦. Deep icequakes occur in bursts of activity that last for a few hours and are separated by quiet periods. Many events occurred on 28-29 April 2012, during the warmest days, when snow melting was likely important. The distribution of events in time and space obeys a power law, as also observed for earthquakes, but with larger exponents.The polarity of the P-waves for all of the events is consistent with tensile faulting. Finally, between 25 April and 3 May, we observed a gliding harmonic tremor with a fun- damental resonance frequency that varied between 30 Hz and 38 Hz, with additional higher frequency harmonics. During this time we also observed shallow hybrid events with high-frequency onsets and a monochromatic coda. These events might be produced by the propagation of fractures and the subsequent flow of water into the fracture. The strongest resonance was observed just after a strong burst of deep icequakes, and during an un- usually warm period when the snow height decreased by 60 cm in one week. The res- onance frequency shows a succession of several sharp decreases and phases of progressive increases. One of the strongest negative steps of the resonance frequency on 28 April coincides with a burst of deep icequakes. These events appear to be associated with the propagation of fractures, which can explain the decrease in the resonance frequency. Finally, we observed an acceleration of glacier flow on 29 April and suggested that melt-water had reached the ice/bed interface. These observations suggest that deep icequakes are due to hydraulic fracturing, and that they can be used to track fluid flow inside glaciers.

Archeomagnetic field models may provide important insights to the geodynamo. Here we investigate the existence and mobility of reversed flux patches (RFPs) in an archeomagnetic field model. We introduce topological algorithms to define, identify, and track RFPs. In addition, we explore the relations between RFPs and dipole changes and apply robustness tests to the RFPs. In contrast to previous definitions, patches that reside on the geographic equator are adequately identified based on our RFPs definition. Most RFPs exhibit a westward drift and migrate toward higher latitudes. Undulations of the magnetic equator and RFPs oppose the axial dipole moment (ADM). Filtered models show a tracking behavior similar to the nonfiltered model, and surprisingly new RFPs occasionally emerge. The advection and diffusion of RFPs have worked in unison to yield the decrease of the ADM at recent times. The absence of RFPs in the period 550–1440 A.D. is related to a low in intermediate degrees of the geomagnetic power spectrum. We thus hypothesize that the RFPs are strongly dependent on intermediate spherical harmonic degrees 4 and above.

In this paper, we report on the night-time equatorial F-layer height behaviour at Korhogo (9.2° N, 5° W; 2.4° S dip lat), Ivory Coast, in the West African sector during the solar minimum period 1995–1997. The data were collected from quarter-hourly ionograms of an Ionospheric Prediction Service (IPS) 42-type vertical sounder. The main focus of this work was to study the seasonal changes in the F-layer height and to clarify the equinox transition process recently evidenced at Korhogo during 1995, the year of declining solar flux activity. The F-layer height was found to vary strongly with time, with up to three main phases. The night-to-night variability of these morphological phases was then analysed. The early post-sunset slow rise, commonly associated with rapid chemical recombination processes in the bottom part of the F layer, remained featureless and was observed regardless of the date. By contrast, the following event, either presented like the post-sunset height peak associated with the evening E × B drift, or was delayed to the midnight sector, thus involving another mechanism. The statistical analysis of the occurrence of these events throughout the solar minimum period 1995–1997 revealed two main F-layer height patterns, each characteristic of a specific season. The one with the post-sunset height peak was associated with the northern winter period, whereas the other, with the midnight height peak, characterized the northern summer period. The transition process from one pattern to the other took place during the equinox periods and was found to last only a few weeks. We discuss these results in the light of earlier works.

International audience; Magnetosonic waves are commonly observed in the vicinity of the terrestrial magnetic equator. It has been proposed that within this region they may interact with radiation belt electrons, accelerating some to high energies. These wave-particle interactions depend upon the characteristic properties of the wave mode. Hence, determination of the wave properties is a fundamental part of understanding these interaction processes. Using data collected during the Cluster Inner Magnetosphere Campaign, this paper identifies an occurrence of magnetosonic waves, discusses their generation and propagation properties from a theoretical perspective, and utilizes multispacecraft measurements to experimentally determine their dispersion relation. Their experimental dispersion is found to be in accordance with that based on cold plasma theory.

International audience; The eastward equatorial electric field (EEF) in the E region ionosphere drives many important phenomena at low latitudes. We developed a method of estimating the EEF from magnetometer measurements of near-polar orbiting satellites as they cross the magnetic equator, by recovering a clean signal of the equatorial electrojet current and modeling the observed current to determine the electric field present during the satellite pass. This algorithm is now implemented as an official Level-2 Swarm product. Here we present first results of EEF estimates from nearly a year of Swarm data. We find excellent agreement with independent measurements from the ground-based coherent scatter radar at Jicamarca, Peru, as well as horizontal field measurements from the West African Magnetometer Network magnetic observatory chain. We also calculate longitudinal gradients of EEF measurements made by the A and C lower satellite pair and find gradients up to about 0.05 mV/m/deg with significant longitudinal variability.

International audience; We numerically investigate the effect of thermal pressurization (TP) on fault branch behaviour during dynamic rupture propagation, a situation likely to occur during large earthquakes at subduction interfaces. We consider a 2-D mode II (in-plane) rupture in an infinite medium that propagates spontaneously along a planar main fault and encounters an intersection with a pre-existing branching fault. The fault system is subjected to uniform external stresses. We adopt the values used by Kame et al. We use the 2-D boundary integral equation method and the slip-weakening friction law with a Coulomb failure criterion, allowing the effective normal stress to vary as pore pressure changes due to TP. We reveal that TP can alter rupture propagation paths when the dip angle of the main fault is small. The rupture propagation paths depend on the branching angle when TP is not in effect on either of the faults, as described by Kame et al. However, the dynamic rupture processes are controlled more by TP than by the branching angle. When TP is in effect on the main fault only, the rupture propagates along the main fault. It propagates along the branch when TP is in effect on both faults. Finally, we considered the case where there is a free surface above the branch fault system. In this case, the rupture can propagate along both faults because of interaction between the free surface and the branch fault, in addition to TP on the main fault.

Generation of extremely/very low frequency (ELF/VLF) chorus waves in Earth's inner magnetosphere has received increased attention recently because of their significance for radiation belt dynamics. Though past theoretical and numerical models have demonstrated how rising-tone chorus elements are produced, falling-tone chorus element generation has yet to be explained. Our new model proposes that weak-amplitude falling-tone chorus elements can be generated by magnetospheric reflection of rising-tone elements. Using ray tracing in a realistic plasma model of the inner magnetosphere, we demonstrate that rising-tone elements originating at the magnetic equator propagate to higher latitudes. Upon reflection there, they propagate to lower L-shells and turn into oblique falling tones of reduced power, frequency, and bandwidth relative to their progenitor rising tones. Our results are in good agreement with comprehensive statistical studies of such waves, notably using magnetic field measurements from THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft. Thus, we conclude that the proposed mechanism can be responsible for the generation of weak-amplitude falling-tone chorus emissions.

International audience; We present a study of wave normal angles (θk) of whistler mode chorus emission as observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) during the year 2008. The three inner THEMIS satellites THA, THD, and THE usually orbit Earth close to the dipole magnetic equator (±20°), covering a large range of L shells from the plasmasphere out to the magnetopause. Waveform measurements of electric and magnetic fields enable a detailed polarization analysis of chorus below 4 kHz. When displayed in a frequency-θk histogram, four characteristic regions of occurrence are evident. They are separated by gaps at f/fc,e≈0.5 (f is the chorus frequency, fc,e is the local electron cyclotron frequency) and at θk∼40°. Below θk∼40°, the average value for θk is predominantly field aligned, but slightly increasing with frequency toward half of fc,e (θk up to 20°). Above half of fc,e, the average θk is again decreasing with frequency. Above θk∼40°, wave normal angles are usually close to the resonance cone angle. Furthermore, we present a detailed comparison of electric and magnetic fields of chorus rising and falling tones. Falling tones exhibit peaks in occurrence solely for θk>40° and are propagating close to the resonance cone angle. Nevertheless, when comparing rising tones to falling tones at θk>40°, the ratio of magnetic to electric field shows no significant differences. Thus, we conclude that falling tones are generated under similar conditions as rising tones, with common source regions close to the magnetic equatorial plane.

International audience; Rapid and accurate source characterizations of large, shallow subduction earthquakes are key for improved tsunami warning efforts. We assess the quality of source parameters of large magnitude (Mw ≥ 7.5) shallow subduction earthquakes of the past 20 yr determined using SCARDEC, a recent fully automated broad-band body-wave source inversion technique for the fast estimation of the moment magnitude, depth, focal mechanism and source time functions of global events. We find that SCARDEC source parameters agree well with those reported in the global centroid moment tensor (GCMT) catalogue, with only the fault dip angle showing a tendency for steeper SCARDEC dip values than GCMT. We investigate this discrepancy through independent validation tests of the source models by: (i) testing how well they explain data not used in their construction, notably low-frequency normal mode data; and, (ii) assessing the data fit using 3-D forward modelling tools more sophisticated than those used to build the source models; specifically, we use a spectral element method for a 3-D earth model. We find that SCARDEC source parameters explain normal mode data reasonably well compared to GCMT solutions. In addition, for the 3-D earth model used in our experiments, SCARDEC dip angles explain body-wave data similarly or slightly better than GCMT. Moreover, SCARDEC dip angles agree well with results from individual earthquake studies in the literature and with geophysical constraints for different subduction zones. Our results show that SCARDEC is a reliable technique for rapid determinations of source parameters of large (Mw ≥ 7.5) subduction earthquakes. Since the SCARDEC method provides realistic source time functions allowing the fast identification of long-duration tsunami earthquakes, it is complementary to existing methods routinely used for earthquake monitoring and suitable for ocean-wide tsunami warning purposes

International audience; In continental collision, most of the convergence between the two involved plates is accommodated on a major thrust system at crustal to lithospheric scale, which thermal state depends on critical parameters such as thrust rate, initial thermal properties of involved lithosphere units or local processes such as shear heating. In active orogens the deeper part of such thrust systems is only imaged by geophysics and their thermal state is inferred from modeling. The Norwegian Caledonides offer a rare exhumed thrust system : the Jotun Basal Thrust (JBT), that allows the direct determination of its thermal envelope through Raman Spectroscopy of Carbonaceous Material (RSCM). Data were collected in the Alum shales formation, an organic carbon-rich unit of Cambro-Ordovician age along which the basal decollement of the JBT accommodated a significant portion of the relative movement between the Caledonian nappe stack and Baltica during the Scandian collision and its later collapse. Maximum temperature mapping within this unit shows isotherms almost cylindrically grading from 320 C in the southeast to 500 C in the northwest in the trailing end of the nappe stack. Based on Bt+Chl+Grt+Phg equilibrium, we estimate that the trailing end reached 500 C at 1.2 0.1 GPa pressure. 2D thermo-kinematic modeling constrained with these new natural data and timing considerations for the Scandian collision were used to understand the geodynamic significance of this maximum temperature envelope. By testing the influence of the geometry, velocity and extensional reactivation duration, our models indicate that (1) peak temperatures were diachronous along the JBT and were reached during extensional reactivation in its shallowest parts, (2) thrust rate and dip angle must have remained low for the JBT and (3) the Scandian Caledonides represent a relatively cold orogenic wedge compared to other mountain belts of the same size.

International audience; Most Ground Penetrating Radar (GPR) measurements are performed on nearly flat areas. If strongly dipping reflections and/or diffractions are present in the GPR data, a classical migration processing step is needed in order to determine the geometries of shallow structures. Nevertheless, a standard migration routine is not suitable for GPR data collected on areas showing a variable and large topographic relief. To account for the topographic variations, the GPR data are, in general, corrected by applying static shifts instead of using an appropriate topographic migration which would place the reflectors at their correct locations with the right dip angle. In this article we present an overview of Kirchhoff's migration and show the importance of topographic migration in the case where the depth of the target structures is of the same order as the relief variations. Examples of synthetic and real GPR data are shown to illustrate the efficiency of the topographic migration.

International audience; The 2011 Mw 9.1 Tohoku-Oki earthquake had unusually large slip (over 50 m) concentrated in a relatively small region, with local stress drop inferred to be 5-10 times larger than that found for typical megathrust earthquakes. Here we conduct a detailed analysis of foreshocks and aftershocks (Mw 5.5-7.5) sampling this megathrust zone for possible clues regarding such differences in seismic excitation. We find that events occurring in the region that experienced large slip during the Mw 9.1 event had steeper dip angles (by 5-10°) than the surrounding plate interface. This discrepancy cannot be explained by a single smooth plate interface. We provide three possible explanations. In Model I, the oceanic plate undergoes two sharp breaks in slope, which were not imaged well in previous seismic surveys. These break-points may have acted as strong seismic barriers in previous seismic ruptures, but may have failed in and contributed to the complex rupture pattern of the Tohoku-Oki earthquake. In Model II, the discrepancy of dip angles is caused by a rough plate interface, which in turn may be the underlying cause for the overall strong coupling and concentrated energy-release. In Model III, the earthquakes with steeper dip angles did not occur on the plate interface, but on nearby steeper subfaults. Since the differences in dip angle are only 5-10°, this last explanation would imply that the main fault has about the same strength as the nearby subfaults, rather than much weaker. A relatively uniform fault zone with both the main fault and the subfaults inside is consistent with Model III. Higher resolution source locations and improved models of the velocity structure of the megathrust fault zone are necessary to resolve these issues.

International audience; HP/UHP and LT metamorphic units that commonly occur in the inner parts of mountain belts result from the subduction of continental and oceanic material, most often exhumed prior to continental collision. The prograde pressure-temperature history of HP-UHP rocks strongly depends on the convergence rate and on the subduction zone geometry. The maximum pressure recorded provides a proxy for the depth of shearing off and stacking of HP metamorphic nappes. A 2-D thermal model of continental subduction at lithospheric scale is used to compute the length and pressure peak of detached HP metamorphic units as a function of the slab dip angle and the convergence rate. Model results are applied to the metamorphic nappe pile of the inner Alps. A mean convergence rate of 1 cm/year during the subduction of the Briançonnais terrane is indicated by the paleogeographic reconstructions between 46 and 38 Ma. On this basis, the available petrological data and lengths of metamorphic units are used to compute the variations of the slab dip angle. The slab dip angle is shown to increase, from the northeast to the southwest, along the Alpine arc with estimated values of 20° for Suretta, 30-45° for Monte Rosa and Gran Paradiso, and 60° for Dora Maira. From Eocene to Oligocene times, the increase in slab dip angle is controlled by changes of buoyancy, due to the spatial configuration of the Valaisan trough and the incoming of crustal material within the subduction zone

The evening pre-reversal vertical drift velocity enhancement (PRE) constitutes an important seeding mechanism for the generation of F region irregularities. Ion density and drift measurements from ROCSAT-1 and DMSP satellites are used to examine the correlation of longitudinal/seasonal (l/s) variations in the evening pre-reversal vertical drift velocity at the magnetic equator in the topside ionosphere and the plasma bubble (PB) occurrence probability. The analysis performed for three years 2000–2002 (solar maximum), provides consistent evidence as the ground observations that the equatorial PB occurrence is dependent on and increases approximately linearly with PRE, and the l/s variations of PRE play an important role in the global l/s distribution of PB occurrence. The solstitial evening PRE and equatorial PB occurrence show similar longitudinal variations: During June solstice, two peaks appear in the African and Pacific longitude sectors, and two minimums are observed in the Indian and American regions; During December solstice, the situation is approximately opposite. The equinoctial longitudinal effects are comparably small. It is concluded that the large-scale l/s variations of equatorial PB occurrence can be closely related to the l/s variations of PRE.

We study equatorial night-time F layer behaviour from quarter-hourly ionograms at Korhogo/Ivory Coast (9.2° N, 5° W, dip lat. −2.4°) during local Spring March–April 1995, declining solar flux period, according to the magnetic activity. The height and thickness of the F-layer are found to vary intensely with time and from one day to the next. At time of the equinox transition, by the end of March, a net change of the nightly height-time variation is observed. The regime of a single height peak phase before 22 March changes to up to three main F-layer height phases after 30 March, each associated to a dominant mechanism. The first phase is identified to the post-sunset E×B pulse, the second phase associated to a change in the wind circulation phenomenon and the third one attributed to pre-sunrise phenomena. The influence of the magnetic activity is identified by the increase in the second peak amplitude. After the 21 April magnetic-equinox period, the height-time morphology becomes more irregular suggesting meridional wind abatement. The initiation, the growth and the maintenance of ESF are explored in relation to these nightly variations. The Rayleigh-Taylor instability is clearly identified as main precursor phenomenon. This is followed by the P-type (F-layer peak spread) structures, the whole with no specific dependence on the magnetic activity and on the F-layer phases, in contrast to further I and F-type (Inside and Frequency spread) ESFs. We discuss our results in the light of recent advanced experiments in Peru and the pacific.

Context. The horizontal photospheric flows below and around a filament are one of the components in the formation and evolution of filaments. Few studies exist because they require multiwalength time sequences at high spatial resolution. Aims. Our objective is to measure the horizontal photospheric flows associated with the evolution and eruption of a filament. Methods. We present observations obtained in 2004 during the international JOP 178 campaign which involved eleven instruments both in space and at ground based observatories. We use TRACE WL, DOT and DST observation to derive flow maps which are then coaligned with intensity images and with the vector magnetic field map obtained with THEMIS/MTR. Results. Several supergranulation cells cross the Polarity Inversion Line (PIL) and can transport magnetic flux through the PIL, in particular parasitic polarities. We present a detailed example of the formation of a secondary magnetic dip at the location of a filament footpoint. Large-scale converging flows, which could exist along the filament channel and contribute to its formation, are not observed. Before the filament's eruptive phase, we observe both parasitic and normal polarities being swept by a continuously diverging horizontal flow located in the filament gap. The disappearance of the filament initiates in this gap. Such purely horizontal motions could lead to destabilization of the filament and could trigger the sudden filament disappearance.

During moderate magnetic storms, an electron channel (300–1100 keV) of the NOAA satellite has shown sudden electron flux enhancements in the inner radiation belt. After examinating the possibility of contamination by different energetic particles, we conclude that these electron flux enhancements are reliable enough to be considered as natural phenomena, at least for the cases of small to moderate magnetic storms. Here, we define small and moderate storms to be those in which the minimum Dst ranges between −30 and −100 nT. The electron flux enhancements appear with over one order of magnitude at L~2 during these storms. The enhancement is not accompanied by any transport of electron flux from the outer belt. Statistical analysis shows that these phenomena have a duration of approximately 1 day during the period, starting with the main phase to the early recovery phase of the storms. The flux enhancement shows a dawn-dusk asymmetry; the amount of increased flux is larger in the dusk side. We suggest that this phenomenon could not be caused by the radial diffusion but would be due to pitch-angle scattering at the magnetic equator. The inner belt is not in a stationary state, as was previously believed, but is variable in response to the magnetic activity.

International audience; Results of a statistical study of equatorial noise emissions are presented. These electromagnetic emissions are observed in the inner magnetosphere in the vicinity of the geomagnetic equator at frequencies below the lower hybrid frequency. We use the data recorded by four Cluster spacecraft during years 2001-2003. The data set was processed in three steps. In the first one, we have selected the data with a nearly linear polarization (ellipticity less than 0.2), corresponding to the known properties of the equatorial noise. Secondly, we have found parameters of a Gaussian model of the frequency-averaged power-spectral density of those selected waves as a function of the geomagnetic latitude. Finally, we have analyzed the data in the latitudinal interval defined by the width of the Gaussian model. Our results show that most intensity peaks of equatorial noise occur exactly at the magnetic equator. Incidental deviations are most probably caused by problems in determination of the true magnetic equator, which is shown by using different magnetic field models. We have estimated the plasma number density at the observation points using the cold plasma theory. These estimates are, within experimental errors, close to the values obtained from the spacecraft potential data measured by the EFW instrument.

This paper investigates the features of pre-earthquake ionospheric anomalies in the total electron content (TEC) data obtained on the basis of regular GPS observations from the IGS network. For the analysis of the ionospheric effects of the 26 December 2004 Indonesian earthquake, global TEC maps were used. The possible influence of the earthquake preparation processes on the main low-latitude ionosphere peculiarity – the equatorial anomaly – is discussed. Analysis of the TEC maps has shown that modification of the equatorial anomaly occurred a few days before the earthquake. For 2 days prior to the event, a positive effect was observed in the daytime amplification of the equatorial anomaly. Maximal enhancement in the crests reached 20 TECU (50–60%) relative to the non-disturbed state. In previous days, during the evening and night hours (local time), a specific transformation of the TEC distribution had taken place. This modification took the shape of a double-crest structure with a trough near the epicenter, though usually in this time the restored normal latitudinal distribution with a maximum near the magnetic equator is observed. It is assumed that anomalous electric field generated in the earthquake preparation zone could cause a near-natural "fountain-effect" phenomenon and might be a possible cause of the observed ionospheric anomaly.

International audience; The east-vergent Chucal thrust system, on the east side of the Chapiquiña-Belén ridge in the Western Cordillera, was continuously or almost continuously active for ?18 m.y. (2.7 Ma). Contractional activity deformed late Oligocene tuffaceous, fluvial, or distal alluvial deposits of the uppermost Lupica Formation; fluvial and lacustrine deposits of the Miocene Chucal Formation; tuffaceous and coarse fluvial deposits of the Quebrada Macusa Formation; and the lower part of the westernmost, latest Miocene??Pliocene, essentially lacustrine Lauca Formation. It controlled the paleogeographic and paleoenvironmental conditions in which these units were deposited. More humid conditions on the east side of the Chapiquiña-Belén ridge favored the development of an abundant mammal fauna and flora. The deformation is characterized by the Jaropilla thrust fault and the Chucal anticline, which is east of the fault. Deformation on the Chucal anticline began before the deposition of the Chucal Formation and was controlled by a blind thrust fault. The west flank has a nearly constant dip (45 50°) to the west and nearly continuous stratigraphic units, whereas on the east flank, the dip angle is variable, diminishing away from the axis, and the stratigraphic units are discontinuous. The anticline growth on this flank caused the development of three observable progressive unconformities. Deformation was particularly rapid during the deposition of the ?600 m thick Chucal Formation (between the 21.7±0.8 Ma old uppermost Lupica Formation and the 17.5±0.4 Ma old base of the Quebrada Macusa Formation, a 4 m.y. period). The deformation rate decreased during the deposition of both (1) the ?200 m thick Quebrada Macusa Formation (between the 17.5±0.4 Ma age of its basal deposits and the ?11 Ma age of its uppermost levels, a 7 m.y. period) and (2) the lower Lauca Formation (between the ?11 Ma age of the upper Quebrada Macusa Formation and the 2.3±0.7 Ma old Lauca ignimbrite, which is intercalated within its middle part). We interpret the contractional deformation to be associated with tectonic activity that led to the uplift of the Altiplano; however, paleobotanical evidence does not indicate any major altitude changes during the time period considered here but rather suggests that rapid uplift took place after the deposition of the Quebrada Macusa Formation.

In this paper we analyze the vertical structure of the magnetotail current sheet for a few intervals during which Cluster repeatedly crosses the neutral sheet due to fast flapping motion. They include fast flows with episodes of flow reversals. The curl, gradient and divergence of the magnetic field are estimated using linear gradient/curl estimator techniques. The reconstruction of the electric current and magnetic field profiles are performed by integration of the estimated gradient scales. We found examples of thin current sheets, (half-thickness of 1000 km) with the current density maximum at the magnetic equator ( B x = 0 ) , as well as examples of off-center or bifurcated current sheets.

Solutions for ultra-low frequency (ULF) wave fields in the frequency range 1–100mHz that interact with the Earth's ionosphere in the presence of oblique background magnetic fields are described. Analytic expressions for the electric and magnetic wave fields in the magnetosphere, ionosphere and atmosphere are derived within the context of an inductive ionosphere. The inductive shielding effect (ISE) arises from the generation of an "inductive" rotational current by the induced part of the divergent electric field in the ionosphere which reduces the wave amplitude detected on the ground. The inductive response of the ionosphere is described by Faraday's law and the ISE depends on the horizontal scale size of the ULF disturbance, its frequency and the ionosphere conductivities. The ISE for ULF waves in a vertical background magnetic field is limited in application to high latitudes. In this paper we examine the ISE within the context of oblique background magnetic fields, extending studies of an inductive ionosphere and the associated shielding of ULF waves to lower latitudes. It is found that the dip angle of the background magnetic field has a significant effect on signals detected at the ground. For incident shear Alfvén mode waves and oblique background magnetic fields, the horizontal component of the field-aligned current contributes to the signal detected at the ground. At low latitudes, the ISE is larger at smaller conductivity values compared with high latitudes. Key words. Ionosphere (ionosphere-magnetosphere interactions; electric fields and currents; wave propagation)

The four Cluster Wideband (WBD) plasma wave receivers occasionally observe electromagnetic triggered wave emissions at and near the plasmapause. We present the remarkable cases of such observations. These triggered emissions consist of very fine structured VLF risers, fallers and hooks in the frequency range of 1.5 to 3.5 kHz with frequency drifts for the risers on the order of 1 kHz/s. They appear to be triggered out of the background whistler mode waves (hiss) that are usually observed in this region, as well as from narrowband, constant frequency emissions. Occasionally, identical, but weaker, emissions are seen to follow the initial triggered emissions. When all the Cluster spacecraft are relatively close (

Historic, as well as teleseismically, and locally recorded seismicity of the subduction margin of Copiapo, northern Chile is analysed. An on- and off-shore temporary network was deployed during 2 months in the region in order to obtain a detailed definition of the Wadati–Benioff zone. An average dip angle of 20° along the subduction thrust interplate contact was determined; this value is similar to that observed along the Chilean margin, suggesting that there are no variations in the dip angle of the Nazca plate for depths less than 50–60 km. Locally recorded data do not provide seismic evidence to determine whether subhorizontal subduction exists north of latitude 28°S. Teleseismically recorded events exhibit a low rate of occurrence of inland earthquakes, with a tendency for epicentres to be located off-shore; in contrast, locally recorded seismicity reveals a homogeneous epicentral distribution, without a predominance of off-shore events. Along the subduction thrust contact there is a reverse stress regime with the greatest compression axis (σ1) horizontal and oriented perpendicular to the strike of the trench. Events located above the main decollement are in a normal stress regime with the minimum compressive stress axis σ3 horizontally oriented parallel to the convergence direction. The events located below the coupled interplate contact also lie in a normal faulting regime but the σ3 axis plunges almost parallel to the subducting plate. The intermediate depth fault plane solutions exhibit a substantial scatter, suggesting a complicated faulting regime similar to that observed in previous studies of northern Chile. They are associated with a normal faulting stress regime in which σ3 is oriented in a N14°E direction parallel to the strike of the trench. High resolution bathymetric data and seismicity located above the thrust contact suggest that seamounts are being subducting.

[1] We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9–2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm kyr−1, respectively. For the older part of the record (>1.2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference δ18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic δ18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30–60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Reunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190–1215 (Cobb Mountain Subchron), 1255, 1472–1480, 1567–1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.

We study compressional pulsations in Pc5 frequency range observed in the dawn-side at distances of about 10 RE , close to the magnetic equator. We use data obtained during two events of conjunctions between Equator-S and Geotail: 1000–1700 UT on 9 March 1998, and 0200–0600 UT on 25 April 1998. In both events, pulsations are observed after substorm activity. The pulsations are antisymmetric with respect to the equatorial plane (even mode), and move eastward with phase velocity close to plasma velocity. The pulsations tend to be pressure balanced. We also discuss possible generation mechanisms of the pulsations.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; MHD waves and instabilities; plasma sheet)

International audience; The penetration of disturbance electric fields from the polar region to the magnetic equator on the dayside of the Earth is examined with geomagnetic data on May 27, 1993. First, we examine a dayside equatorial disturbance that followed the rapid recovery of magnetic activity from a storm and that has the characteristics of overshielding caused by persistent region-2 field-aligned currents. It lasted *0 3 hours. Second, we analyze a series of fluctuations with periods of 25-75 min, to determine the variations of amplitude and phase with magnetic latitude and magnetic local time. The fluctuations were highly coherent at all latitudes between the magnetic equator and the auroral zone, but the coherency decreasedin the polar cap. A northward fluctuation at the equator during midday hours accompanied auroral zone fluctuations that were southward before noon, eastward around noon, and northward after noon. The amplitudes decreased away from the auroral zone toward midlatitudes but were amplified under the equatorial electrojet. No detectablep hased ifferencesa re found, indicating that any temporal lags which might be inducedb y persistencein the region-2f ield-alignedc urrentsa re less than i min for fluctuations having periods like those examined here. A synoptic inversion analysis of the high-latitude magnetic data to estimate the time-varying high-latitude electric potential patterns shows that fluctuations of the high-latitude east-west potential gradient tended to be concentrated around midday, where they were in phase with fluctuations in the midday east-west potential gradient at the magnetic equator.

International audience; We present the magnetostratigraphy of the Abadeh section (central Iran). Thermal demagnetization shows two magnetic components: one at low unblocking temperatures, which corresponds to a present day-field direction, and another at high temperatures. This second component, which contains both polarities, is interpreted as a primary magnetization acquired during sediment deposition at a paleolatitude of about 7.7øS. A sequence of 14 magnetic polarity zones is obtained in the intervals during the Upper Dzhulfian-Lower Changhsingian (Upper Permian) and during the Griesbachian (Lower Triassic). Combining data from south China, Canada, Pakistan, and Iran, we propose a composite (but still uncomplete) Upper Permian and Lower Triassic geomagnetic polarity timescale.

Huancayo is the only equatorial electrojet station where the daytime increase of horizontal geomagnetic field (H) is associated with a simultaneous increase of eastward geomagnetic field (Y). It is shown that during the counter electrojet period when ∆H is negative, ∆Y also becomes negative. Thus, the diurnal variation of ∆Y at equatorial latitudes is suggested to be a constituent part of the equatorial electrojet current system. Solar flares are known to increase the H field at an equatorial station during normal electrojet conditions (nej). At Huancayo, situated north of the magnetic equator, the solar flare effect, during nej, consists of positive impulses in H and Y and negative impulse in Z field. During counter electrojet periods (cej), a solar flare produces a negative impulse in H and Y and a positive impulse in Z at Huancayo. It is concluded that both the zonal and meridional components of the equatorial electrojet in American longitudes, as in Indian longitudes, flows in the same, E region of the ionosphere.Key words. Geomagnetism and paleomagnetism (dynamo theories) · Ionosphere (equatorial ionosphere; ionosphere disturbances)

International audience; We study the effect of a superficial layer overlying a half-space on the surface displacements created by a dip-slip dislocation on a planar rectangular fault using a 2-D finite element model. The effect of the density and Poisson’s ratio is negligible. On the other hand, the contrast in Young’s modulus between the top layer and the half-space below significantly affects the modelled coseismic displacements. The horizontal displacements are more sensitive than the vertical displacements to the existence of the top layer. Near the fault, a low-rigidity layer with a Young’s modulus of 10 GPa can increase the horizontal displacements by up to 40 per cent. An analytical 3-D half-space model is used to interpret this effect in terms of an ‘equivalent’ homogeneous model. The presence of a top layer can be partly simulated in a homogeneous half-space by perturbing the depth and the slip on the fault from their ‘real’ values to ‘equivalent’ values. For a given displacement field observed at the surface, the perturbations in fault depth and slip can reach 1km and 25 per cent respectively. The fault dip angle is not model-dependent. We conclude that for the accurate estimation of fault depth and slip from coseismic geodetic data, rigidity contrasts existing within the upper crust must be taken into account. Not taking into account the effect of an existing low-rigidity layer also leads to an underestimation of seismic moment release. This may in part be the cause of apparent discrepancies between seismic and geodetic moment releases in particular areas such as the Gulf of Corinth, Greece.

This letter presents some night-time observations of neutral wind variations at F2 layer levels near the dip equator, measured by the Fabry-Perot interferometer set up in 1994 at Korhogo (Ivory Coast, geographic latitude 9.25°N, longitude 355°E, dip latitude –2.5°). Our instrument uses the 630 nm (O1D) line to determine radial Doppler velocities of the oxygen atoms between 200 and 400 km altitude. First results for November 1994 to March 1995 reveal persistent eastward flows, and frequent intervals of southward winds of larger than 50 ms–1 velocity. Compared with the simultaneous ionospheric patterns deduced from the three West African equatorial ionosondes at Korhogo, Ouagadougou (Burkina-Faso, dip latitude +1.5°) and Dakar (Sénégal, dip latitude +5°), they illustrate various impacts of the thermospheric winds on F2 layer density: (1) on the mesoscale evolution (a few 103 km and a few 100 minutes scales) and (2) on local fluctuations (hundreds of km and tens of minutes characteristic times). We report on these fluctuations and discuss the opportunity to improve the time-resolution of the Fabry-Perot interferometer at Korhogo.Key words. Ionosphere (Equatorial ionosphere; Ionosphere-atmosphere interaction) · Meteorology and Atmospheric Dynamics (General circulation)

International audience; Preliminary paleointensity determinations using the Thelliers' method have been carried out on 141 samples of core HSDP (Hawaii Scientific Drilling Project), drilled into the Mauna Loa and Mauna Kea volcanoes and spanning the last 400 kyr. Rock magnetic investigations identify pseudo-single-domain magnetite as the main carrier of the natural reinanent magnetization (NRM) all along the core. In the Mauna Kea sequence-75% of the specimens yielded exploitable results with linear NRM/TRM (Thermoremanent Magnetization) plots and positive partial TRM (pTRM) checks generally up to the highest temperatures (550øC). A lower percentage of successful experiments (-50%) was observed for the Mauna Loa samples and the NRM/TRM diagrams are linear over a shorter temperature interval than for the Mauna Kea samples. These preliminary results, based on only one sample per flow, document a marked geomagnetic field intensity low at about 40 kyr probably related to the Laschamp event. They also suggest that the geomagnetic field at Hawaii might be characterized by large, sharp intensity fluctuations spaced by a few thousand years. The average value of the field intensity suggested by these results for the last 400,000 years is slightly higher than the present field at Hawaii.

A record of relative paleointensity from marine sediments in the equatorial Indian Ocean spanning the last 4 Ma completes a previous dataset from Ocean Drilling Program (ODP) Leg 138 sites in the equatorial Pacific [1]. The timescale was defined with a precision better than 20 kyr with different and independent methods. Most of the features present in the record from the equatorial Pacific are confirmed: the overall saw-tooth pattern is observed across every field reversal and the short-term fluctuations superimposed on the slow intensity decrease preceding the reversals appear to be similar at both sites. The global character of these features reinforces their interpretation in terms of changes in dipole field intensity and provides new important constraints on models of the geodynamo. A synthetic curve of field intensity changes during the last 4 Ma is proposed as a first paleointensity timescale for future stratigraphic studies.

International audience; The spectra of VLF/ELF emissions in the off-equatorial region in the outer magnetosphere are, on occagion, found to consist of upper and lower bands with a frequency gap between them. The Wave normal directions of the upper band VLF emissions have been determined by means of wave distribiution function analyses based first On the measurement of three magnetic field components only, and then with the additional use of an electric field component. It is found that the wave diStribUtion functions are composed of two peaks whose central wave normal angles 0 with respect to the magnetic field at a geomagnetic latitude of •, 17 ø are close to the local oblique resonance cone Ore s but whose values of 0 are always about 15ø-20 ø less than 0re s at a higher latitude of ,-, 26 ø. This observed wave normal behavior is compared with the theoretical prediction from direct ray tracing studies using the hypothesis deduced from a previous investigation that the waves are generated at the equator with wave normals close to 0re s. A combination of this comparison and inverse ray tracing in which the initial wave normal directions are those determined from the wave distribution analysis has yielded the result that the 0 values of the upper band VLF emissions are very close to 0re • at the equator and that the normalized frequency there, Acq, is above 0.5. Hence the upper band VLF emissions are identified as being "half-gyrofrequency" VLF emissions generated in the vicinity of the magnetic equator, with their •wave normals close to 0re •. The characteristics of propagation between the equatorial source region and the observing positions have been investigated by direct ray tracing. The generation mechanism is also discussed. 1.

Balmer alpha emissions of intensity larger than three times the nominal value have been recorded during five different nights at geomagnetic latitudes inferior to plus or minus 22 deg . They are correlated with sudden impulses in magnetograms. (auth)