The ground magnetic field disturbance caused by ionospheric currents can be represented by equivalent currents placed to the ionospheric plane. Equivalent currents provide valuable information about the ionospheric electrodynamics, and thus they can be used, for example, in studies of space weather, ionosphere‐magnetosphere coupling, and the magnetotelluric source effect. We derive equivalent currents by using the spherical elementary current system method. The applicability of the method for the Baltic Electromagnetic Array Research (BEAR) magnetometer array is validated by means of synthetic ionospheric current models and by investigating the goodness of the fit between the modeled and measured ground magnetic field. The applicability of the method for the sparser International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network is also proved. In addition, the combination of the elementary current system method and the complex image method, used for the calculation of the induced electromagnetic fields on ground, is introduced, and the combination of the methods is tested by using geoelectric field data from the BEAR project. Our special interest is in the effects that rapidly varying ionospheric currents have on technological conductor systems at the surface of the Earth due to geomagnetically induced currents. Comparison between equivalent currents and the time derivative vector of the horizontal magnetic field emphasizes the importance of small‐scale structures.

Previous interpretations of magnetotelluric data from the vicinity of the German Deep Drilling Project (KTB) revealed two major structures: a midcrustal layer of increased conductivity and large, regional extent and a highly anisotropic upper crust. Nevertheless, a satisfactory combination of both structures explaining all measurements has not yet been achieved, mostly due to incomplete and qualitatively poor data. Simplified superposition of both structures could not yield an explanation of the observations. On the basis of a carefully processed new data set we apply different modeling approaches to verify the existence of both structures. Models calculated with a two‐dimensional modeling program, which allows for general anisotropy, as well as a full three‐dimensional code show that an anisotropic upper crust is overlaying a regional east‐west striking high‐conductivity structure. Nevertheless, this continuous conductive midcrustal layer with a conductance decreasing from north to south must be replaced by a quasi‐anisotropic one, at least in the region of the KTB. The final model may still be oversimplified, considering the complexity of the true but unknown geology in this particular area, but it demonstrates which major electrically effective structures could be resolved.

Using data from a continuously operating two-station magnetotelluric (MT) array in central California we have computed robust remote reference MT transfer functions (TFs) for each day in the 2 yr period 1996-1997. Typical deviations of the daily estimates from the overall long-term average TF ranged from 2-3 per cent for periods of less than 300 s to about 10 per cent at a period of 2000 s. Day-to-day deviations were largely random, and exhibited little temporal correlation or lone-term trend. There is some evidence for small-frequency independent variations in impedance amplitudes, suggestive of subtle slow changes in near-surface distortion. However, there was no clear seasonal component to this signal, as might be expected if hydrologic changes in the near surface were the cause. Comparison of estimated error bars to TF variability showed that for periods between 10 and 100 s (where coherent noise sometimes biased TF estimates) the standard asymptotic theory for the robust estimator yielded error bars that were too small by as much as a factor of two. At longer and shorter periods these standard error bars were consistent with the actual precision of the TF estimates. We also considered the reliability of error bars computed with two variants on the jackknife approach. For the first approximate scheme we computed the weights for the robust TF estimates once with all data, followed by application of the jackknife to the final weighted least-squares estimate. We show that for this 'fixed-weight' jackknife, variances can be given in closed form even for the remote reference case. Fixed-weight jackknife error bars were larger than those computed in the conventional fashion, but still significantly underestimated the true variability in the 10-100 s bias band, and were systematically too large at other periods. We also tried a subset deletion jackknife, applying the full robust procedure with contiguous subsets of data deleted. Provided large subsets (5 per cent, or approximately 1 hr) were deleted, this approach yielded significantly more realistic error bars in the bias band. However, error bars at periods outside the bias band now significantly overestimated the actual day-to-day variability of TF estimates. The jackknifed error bars were thus always more conservative, though not necessarily more reliable.

In the paper, DC trains and Pc3s: Source effects in mid-latitude geomagnetic transfer functions by Egbert et al., Geophysical Research Letters 27 [1] 25-28, Figures 2 and 4 were inadvertantly transposed. They appear correctly below with their captions.

Magnetotelluric exploration has been used to image along strike variations in the electrical resistivity structure of the San Andreas Fault at Parkfield, California. A low resistivity wedge extending to a depth of several kilometers is continuous over a horizontal distance of 8 km. The base of the wedge is coincident with the shallowest microearthquakes. A change in the electrical and fluid connection of the San Andreas Fault with a low resistivity zone in the Franciscan formation is observed along the Parkfield segment.

Magnetotelluric (MT) data from two sites 150 and 300 km southeast of San Francisco, California (geomagnetic dipole latitude: 43 degrees, L approximately 1.9) show that the usual MT assumption of spatially uniform external magnetic fields is violated to a significant degree in the period range 10–30 s. Inter‐station transfer functions exhibit large systematic temporal variations which are consistent with a combination of two distinct sources: electromagnetic noise due to the San Francisco Bay Area Rapid Transit (BART) DC electric railway, and Pc3 geomagnetic pulsations. There is a suggestion in the data that some of the Pc activity may actually be excited by BART.

The interpretation of magnetotelluric (MT) data collected in the vicinity of the German Continental Drilling Project (KTB) has revealed two major structures in the Earth's crust. First, a large-scale mid-crustal layer of enhanced conductivity is required to fit the regional observations of the anomalous vertical magnetic field. Second, the MT impedances of the area close to the KTB, namely the crustal unit of the Zone Erbendorf-Vohenstrauss (ZEV), support a model with a highly anisotropic upper to middle crust. The extremely high values of anisotropy derived from the MT measurements exceed the values of anisotropy observed on rock samples in the laboratory by an order of magnitude and therefore exclude intrinsic features as a source for the observed anisotropy. The regional mid-crustal conductor is in agreement with Central Europe-wide observations of high-conductivity layers in the crust, while the proposed model of well-conducting, subvertical dykes within a resistive host rock explains the MT observations and agrees with the steeply dipping seismic reflectors, the downhole and core observations from the drilling and the model of the tectonic development of this area. However, the superposition of both structures leaves some unanswered questions.

Magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic are and forearc regions of northern Chile between 19.5 degrees and 22 degrees S to study the electrical conductivity structures of this active continental margin. The instruments used covered a very broad period range from 10(-4) s to approx. 2 x 10(4) s and thus enabled a resolution of deep as well as shallow structures.In this paper we focus on the interpretation of data from an east-west profile crossing Chile from the Pacific coast to the Western Cordillera at 20.5 degrees S. A decomposition of the impedance tensors using the Groom-Bailey decomposition scheme shows that a two-dimensional interpretation is possible. The resulting regional strike direction is N9 degrees W. Two-dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the Andes, The final model shows a generally high resistivity in the forearc and a very good conductor below the Precordillera. Unlike earlier models from areas further south, a good conductor is not observed below the magmatic are itself. This correlates with the so-called Pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas. (C) 1997 Published by Elsevier Science B.V.

The German Continental Deep Drilling Program (KTB) drilled two holes through crystalline rocks which are rich in both high-salinity fluids and graphite accumulated along shear zones. Analyses of a large number of borehole measurements yield models for the electrical resistivity of the upper and middle crust in the vicinity of the KTB holes. High observed resistivity, of more than 10(5) Omega m in the lowermost part of the 9000 m deep main hole, in a rather ''wet'' crust, indicates that effective mechanisms exist to cut down connections between fluid accumulations and therefore that fluids are not the likely cause of high-conductivity anomalies. On the other hand, graphite accumulations appear to be connected along shear lineaments over hundreds of meters or more. Structural, mineralogical, and geochemical studies suggest a tectonic model which explains the deposition of graphite as the relic and witness of a shearing process that occurred during the late Variscan (Upper Carboniferous) thrusting. This process took place while this part of the crust resided at temperatures between 240 degrees and 380 degrees C, Subsequent independent reverse faulting lifted this part to the Earth's surface. Our conclusion is that the KTB case indicates how high electrical conductivities in the upper crust, which originated from the middle to lower crust, are caused by graphite accumulations, rather than by fluids, and that these anomalies are related to shearing processes, Such graphite accumulations may exist elsewhere and may be of relevance in the context of present-day midcrustal conductors.

The application of a magnetotelluric (MT) tensor decomposition scheme to the long period responses of the BC87 data set yields an almost two-dimensional regional conductivity structure with a strike of N45-degrees-E. The main feature in the MT responses of the observed region are diverging apparent resistivity curves and phase differences.1-D modelling and 2-D inversion of different combinations of these MT parameters suggest the existence of a strongly anisotropic layer in the middle to lower crust. Based on the 1-D estimates for resistivities, and using the geometry from the 2-D inversion, a 2-D forward model is derived which includes a strongly anisotropic layer at 30 km depth with the two principal resistivities of 300 OMEGAm and 5 OMEGAm. This model explains mainly the phases of the impedance tensor at longer periods. The apparent resistivities are affected by small-scale, near surface inhomogeneities.