Electrical Conductivity Of Earth Research Articles

Electromagnetic Induction in the Earth Using Long Period Geomagnetic Variations

Early studies of the Earth's electrical conductivity assumed, for mathematical simplicity, a spherically symmetric conductivity model. In more recent times, a number of studies have questioned this assumption. In particular, two studies of the long-period geomagnetic variation response of the Earth indicate departure from spherical symmetry.

Signals and noise in magnetic observatory annual means: Mantle conductivity and jerks

Geomagnetic temporal variations can yield valuable information on the electrical conductivity of Earth's mantle and on motion of core fluid. The external‐source signal in first differences of magnetic observatory annual means is primarily due to a degree‐one spherical harmonic closely aligned with Earth's magnetic dipole axis. The transfer function between the electromagnetically induced degree‐one internal Gauss coefficient (Schmidt seminormalized) and the inducing degree‐one external Gauss coefficient is 0.089 ± 0.020 with a phase shift of 0° ± 45° for a 2‐year period. This transfer function is consistent with a nearly insulating mantle and a highly conducting core for which the theoretical transfer function is 0.082 with no phase shift. The temporal power spectrum for noise in first differences of magnetic observatory annual means is approximately white. Third differences of annual means are primarily noise and degree‐one external‐source signal; thus, when the degree‐one external‐source signal is removed from annual means third differences, the root‐mean‐square residuals for a given field component and time interval at a given observatory are a good indicator of noise for the relevant component, observatory, and time interval. These rms residuals were used as weights for construction of spherical harmonic models of geomagnetic secular variation. European secular variation graphs for the 1962–1983 time interval exhibit prominent changes of slope (geomagnetic jerks) in the geomagnetic east component at approximately 1970 and 1978. The jerk of 1970 (but not 1978) is evident on the geomagnetic north and vertical components. The vertical component exhibits additional slope changes at approximately 1966 and 1975.

The magnetospheric disturbance ring current as a source for probing the deep earth electrical conductivity

Two current rings have been observed in the equatorial plane of the earth at times of high geomagnetic activity. An eastward current exists between about 2 and 3.5 earth radii (Re) distant, and a larger, more variable companion current exists between about 4 and 9 Re. These current regions are loaded during geomagnetic substorms. They decay, almost exponentially, after the cessation of the particle influx that attends the solar wind disturbance. This review focuses upon characteristics needed for intelligent use of the ring current as a source for induction probing of the earth’s mantle. Considerable difficulties are found with the assumption that Dst is a ring-current index.

The oceanological application of electromagnetic fields induced by synoptic currents

Summary. The determination of induced magnetic fields of synoptic ocean movements is considered in the case of horizontal uniformity of the bottom conductivity. The problem of calculation of hydrodynamic parameters is solved by means of induced fields. These appear to depend closely on the appearance of barotropic or baroclinic parts in the synoptic ocean movement. The different properties of fields from parts of the above permit us to separate hydrodynamic parameters of complex ocean movements into barotropic and baroclinic parts by means of magnetic measurements. In recent years there has been great interest in the study of synoptic ocean movement. In this paper we investigate the possibility of determining the values of the hydrodynamical parameters of such movement by means of induced magnetic fields. The magnetic field induced by synoptic currents depends on the spatial and time variation of the water velocity and on the Earth's electrical conductivity structure. In the P-plane approximation, the vertical structure of the water velocity is controlled by ocean density stratification (Monin, Kamenkovich & Kort 1974). The vertical structure of the phenomenon outlined above is described by the following boundary-value problem,

Electromagnetic Response Functions from Interrupted and Noisy Data

General methods for constructing reliable electromagnetic response functions from interrupted and noisy recordings of the naturally occurring electromagnetic variations are described. The aim is to be able to effectively utilize the 11 years of hourly mean electric and magnetic data (1932-42) from Tucson, Arizona, despite the many gaps and large extraneous values. This long-time series is extremely useful in helping one to focus on techniques that yield stable and reproducible response functions. The ultimate goal is to obtain accurate response functions in order to construct reliable earth electrical conductivity models and to determine the ocean fluid-induced part of the electromagnetic field. The Tucson electric field data is found to have diurnal and semidiurnal tidal frequencies most likely of oceanographic origin and a 6 d-1 peak of unknown origin.

Electric and Magnetic Fields Induced by Deep Sea Tides

A lunar semidiurnal variation (12·4206 h period) induced by tides in the deep ocean is demonstrated in (i) a month's observations of the horizontal electric field and magnetic declination at a sea floor site located 600 km off the California coast in 4·4 km of water, in (ii) a month's observations of the vertical magnetic field at a coastal site near Cambria, California, and in (iii) two years' observations of the vertical magnetic field at the island magnetic observatory on San Miguel, Azores. Comparison of the sea floor, coastal, and island observations with simultaneous continental magnetic observations permits an estimation of that part of the variation due to a lunar ionospheric oscillation. The observed oceanic induced variation at the sea floor and coastal sites are found to agree well with tidal induced fields computed for a model of the Earth's electrical conductivity and lunar semidiurnal tide. The model assumes (i) a flat semi-infinite ocean of uniform depth and conductivity, rotating at a uniform rate appropriate to the latitude of interest, (ii) non-conducting atmosphere, continent and upper mantle, and (iii) superconducting mantle at a uniform depth beneath the ocean. The observed oceanic induced vertical magnetic field at San Miguel is found to agree qualitatively with tidal induced fields computed for an island model. This model assumes (i) a uniformly conducting small circular island, (ii) a flat infinite ocean of uniform depth and conductivity, rotating at a uniform rate appropriate to the latitude of interest, (iii) a non-conducting atmosphere and upper mantle, and (iv) a superconducting mantle at a depth much greater than the size of the island.