Changes In Geomagnetic Field Research Articles

Comparison of cosmogenic radionuclide production and geomagnetic field intensity over the last 200 000 years

The production rate of cosmogenic radionuclides such as 10Be or 14C is known to vary as a function of the geomagnetic field intensity. It should, therefore, be possible to extract a record of palaeofield intensity from the deposition record of these radionuclides in marine or terrestrial sediments and ice cores. Field intensity variations, however, are not the only factor that has influenced the cosmogenic radionuclide records. In the case of 14C, variations of the global carbon cycle, caused by reorganization of the ocean circulation patterns from the last glacial to the present interglacial, are superimposed. 10Be is not affected by these variations because it is not part of the carbon cycle, but its deposition rates in marine sediments vary as a function of lateral sediment redistribution and boundary scavenging intensity. A global stacked record of 10Be deposition rates, corrected for sediment redistribution by normalizing to 230Thex, was shown to remove most of the disturbances, and provides a record of 10Be production rate variations over the last 200 000 years, which translates into geomagnetic field intensity variations. This dataset is compared with palaeofield intensities reconstructed from marine sediments by palaeomagnetic methods, from variations in atmospheric 14C/12C derived from independent calibrations of 14C ages, such as U/Th dating and tree ring chronology, and from 36Cl and 10Be fluxes in polar ice cores. Potential influences of the Earth’s orbital parameters and insufficient correction for orbitally triggered climate variations on the palaeointensity reconstructions are assessed. It is argued that the palaeointensity records derived from marine sediments are not significantly affected by these factors.

Piezomagnetic potentials due to an inclined rectangular fault in a semi-infinite medium

Analytical solutions for the piezomagnetic potentials are derived for strike-slip, dip-slip and tensile-opening fault motions with arbitrary dip and strike angles, so as to be applicable in various types of earthquakes. These solutions are expressed as the composition of elementary functions which are identical to the magnetic potentials produced by magnetic dipoles, quadrupoles and octupoles distributed on the fault plane and other planes. Therefore, the geomagnetic field changes due to the piezomagnetic effect are expressed by the superposition of the fields produced by these equivalent sources. Examples of calculated results show characteristic features for various types of fault motions as follows: (1) the pattern of the geomagnetic field changes becomes significantly different depending on the strike direction, although the maximum amplitude is almost the same for all directions; (2) the geomagnetic field change reaches a maximum at a dip angle of 90° for strike-slip and tensile-opening fault motions and at 45° for dip-slip fault motion.

Paleosecular variation during sequential geomagnetic reversals from Hawaii

Paleomagnetic directions measured from 175 flows in four volcanic sequences on the island of O'ahu (Hawaii) cover about 0.3 m.y. of geomagnetic field changes and include detailed records of the successive Gilbert-Gauss (3.57 Ma) and Lower Mammoth reversals (3.33 Ma). The dominant pattern of the directional changes is the presence of large inclination variations with increasing amplitude and steep values during the transitional periods. The morphology of these recurring features during different transitions is reminiscent of the characteristics of the non-dipole field inferred from studies of paleosecular variation in the Pacific. These observations are consistent with transitional mechanisms yielding the emergence of non-dipole components with characteristics similar to those during periods of stable polarity. The three different records of the lower Mammoth reversal show that clusters of transitional pole positions are not reproducible between parallel sections and are thus linked to local eruptions and not to long-lived states of the transitional field.

Regional orthogonal models of the geomagnetic field changes over the Far East

Two spatial-temporal models of the geomagnetic field secular variations in a time interval from 1970.0 to 1995.0 were constructed for the Far East region using the technique of regional modeling. First model covers a rectangular area, which includes Kamchatka, Sakhalin, the Kuril and Japanese Islands, the Ohotsk and the Japan Seas. Data from Russian, Japanese, and partially, Chinese observatories, and repeat stations were used for its construction. For filling of no data areas (the seas) the global models were used. Second model covers the area of the Japanese Islands and, partially, surrounding seas. Data from the Japanese magnetic observatories and first class repeat station network were utilized for its construction. We used natural orthogonal components method for obtaining temporal and spatial functions of the models. The accuracy of obtained models is much better than that of global models.

Isolated sleep paralysis, vivid dreams and geomagnetic influences: II.

This report describes a test of the hypothesis that significant changes in the ambient geomagnetic field are associated with altered normal nighttime dream patterns. Specifically, it was predicted that there would be a greater incidence of isolated sleep, paralysis or vivid dreams with abrupt rises and falls of geomagnetic activity. The author's (JC) and a second subject's (KC) daily reports of dream-recall were analyzed in the context of daily fluctuations of geomagnetic activity (K indices). Two analyses of variance indicated (i) significantly higher geomagnetic activity three days before a recorded isolated sleep paralysis event and (ii) significantly lower geomagnetic activity three days before an unusually vivid dream took place. Conversely, geomagnetic activity did not fluctuate significantly for randomly selected days. Testing a large sample over time is required for confirmation and extension of this work.

Standard geomagnetic observatory data in tectonomagnetism: case study related to the M 5.7 Timisoara, Romania, earthquake

There has recently been much discussion of large-scale interactions of fault zones and the influence of large-scale processes in the preparation and triggering of earthquakes. As a consequence, an official recommendation was issued to set up observational networks at regional scale. In this context, the existing network of standard geomagnetic observatories might play a more important role in future tectonomagnetic studies. The data from standard geomagnetic observatories are basically not appropriate for the detection of small-magnitude and, in most cases, spatially very localized geomagnetic field changes. However, their advantage is a continuity in a long-time period which enables the study of regional tectonomagnetic features and long-term precursory changes. As the first step of a more extensive study aimed at examining the features of observatory data for this purpose, a three-year data set from five European observatories has been analyzed. Some common statistical procedures have been applied along with a simple difference technique and multivariate linear regression to define local geomagnetic field changes. The distribution of M ³ 4.5 earthquakes in Europe, in a corresponding period, was also taken into account. No pronounced field variation, related in time to the M 5.7 Timisoara (Romania) earthquake on July 12, 1991, was found at Grocka observatory at about 80 km from the earthquake epicenter. However, an offset in level of the differences in declination which include Grocka observatory, not seen in the case of differences between other observatories, could be associated with a possible regional effect of the M 4.8 earthquake which occurred in September 1991 at about 70 km SE from Grocka.

An Evaluation of a Regional Orthogonal Field Model for Canada with Respect to the Canadian Geomagnetic Reference Field

Regional spatial-temporal models of the geomagnetic field have been produced by a variety of techniques. Recently, spherical cap harmonic analysis has gained wide acceptance as a means of modelling the magnetic field over a portion of the globe. Time variations of the field are commonly represented by polynomials or trigonometric series. The Canadian Geomagnetic Reference Field for 1995 (CGRF) is an example of a model produced in this manner. There are, however, other techniques that may offer advantages for some applications. The method of natural orthogonal components is one of these. To demonstrate this method, a spatial-temporal model of the geomagnetic field changes over Canada (ROM-Canada) has been produced using North American observatory annual means and Canadian repeat station data. These data were fitted with rectangular Legendre polynomials using natural orthogonal components as the temporal functions. Overall, total-intensity first-differences computed from the CGRF and ROM-Canada model the observed first-differences with comparable accuracy. ROM-Canada appears to be somewhat more accurate in the centre of the modelling area, but less accurate near the edges. Moreover, the ROM-Canada models of the vector components are less accurate than the CGRF.

An Attempt to Detect a Long-Term Change of Geomagnetic Total Force at Fixed Observation Stations on the Sea Floor.

Since 1987 we have been working with the establishment of the sea floor observation stations in order to detect long-term geophysical changes. Study of the changes of geomagnetic field is one of the most important works to be carried out on the station. The sea floor observation station is a fixed and nonmagnetic platform installed firmly on the sea floor, which is easily accessible by manned submersibles. We have installed three such observation stations in the Sagami Bay which is close to Tokyo. The operation and observation were carried out with the aid of the Japanese Submersible SHINKAI 2000 every year. Measurements were made four times in the last four years. Duration of the measurement was from 1 hour to 25 hours depending on the conditions of the submersible. The equipment used to measure the geomagnetic total force is a proton magnetometer specially designed for the sea floor. The sensor is a toroidal coil dipped totally in Kerosene oil which is pressure-balanced by means of Beryllium-Copper bellows. The magnetometer is installed on the platform exactly in the same fashion every time. Transportation and installation are made by the submersible using manipulators. By comparing the data from the sea floor with those of the land geomagnetic stations, we confirmed the feasibility of detecting changes of geomagnetic total force on the sea floor.

The magnetic and electric fields induced by superparamagnetic magnetite in honeybees

Hairs on the abdomen of honeybees contain dendrites and a rod and ring structure composed of black particles, presumed to be superparamagnetic (SPM) magnetite. The rod and ring were divided into compartments and each compartment approximated by a dipole. The magnetic fields were calculated at a point P at various locations for a change of the external geomagnetic field from zero to 0.5 G in 0.1 s. The magnetite amplifies the external field at the rod/ring-dendrite interface. The induced electric field and potential difference for a small circular area are in the order of 10?7 V/m and 10?13 V respectively. Mechanisms are proposed for amplifying the electric fields in the dendrite and in an integrating nerve fibre. A hypothesis is developed for associative learning of visual and magnetic stimuli. If magnetic and visual inputs are associated in the ganglion and in the brain, very small changes of either magnetic or visual inputs could be perceived. A bee could sequentially follow the images associated with magnetic gradients on a cloudy day and find the food source.

ＳＱＵＩＤセンサを用いた地磁気計測システムの開発と都市ノイズの測定

Geomagnetic field changes are expected prior to volcanic eruptions because temperature and pressure changes associated with magma flows in surrounding rock give rise to temporal magnetic anomalies. It was planned to monitor temporal magnetic changes by a SQUID (Superconducting QUantum Interference Device) sensor in the vicinity of the Sakurajima volcano in an attempt to study the relation between volcanic activity and variations in the geomagnetic field. This paper describes the development of the geomagnetic field sensing system using the SQUID sensor and the measurement of electromagnetic environmental noise generated in Kagoshima city.

A paradox in volcanomagnetism: Disagreement between analytical and numerical estimates of geomagnetic changes due to an underground pressure nucleus.

The reason why numerical and analytical estimates of geomagnetic changes due to an underground pressure nucleus lead to considerably different results has been an unsolved enigma in volcanomagnetism. In particular, the two estimates manifest completely different characteristics of geomagnetic change when the depth of the Curie point isotherm is taken as a parameter even though they are based on the same stress field produced by a point source of dilatation.This paper aims to clarify the reason for this disagreement by using a numerical approach to evaluate changes in the surface geomagnetic field produced by stress-induced magnetization inside a pressurized sphere.The comparison of two numerical models subjected to the same stress field made it clear that the disagreement between numerical and analytical models is caused by the stress-induced magnetization inside the source sphere. The analytical estimate made so far cannot be applied to a case for which the interior of the sphere is at high temperature. It is still applicable, however, to a case in which the temperature inside an inflation sphere is below the Curie point.

Computer modelling of geomagnetic anomalies associated with block-faulting

The finite element method was used in a computer model to calculate stress distributions in a segment of the upper crust responding to dip-slip movements on a buried vertical fault. This structural style is characteristic of block-faulting. Based on the theory of the piezo-magnetic effect, changes in the local geomagnetic field associated with stress-induced changes in rock susceptibility and remanent magnetization were simulated. Model results show that regions of uplift and subsidence are marked by positive and negative magnetic anomalies respectively, while the amplitudes are controlled by the magnitude of the displacements on the basement fault. This result is important because of its bearing on the interpretation of some components of geomagnetic anomalies in terms of crustal movements, especially in tectonically active parts of the Earth.

Temporal 10Be and 14C Variations: A Tool for Paleomagnetic Research

Temporal variations of cosmogenic radionuclide atmospheric concentrations can be caused by such global phenomena as solar activity and geomagnetic field changes as well as atmospheric circulation processes. These causes can be distinguished by the comparison of several isotope records corresponding to the same time period. We discuss a possibility for reconstructing the geomagnetic moment during the last 30,000 years from the comparison of 10Be and 14C concentrations in terrestrial archives. The results agree with conventional paleomagnetic data and promise to enrich our knowledge of geomagnetic field variations and reversals.

Earthquake precursors in Japan: Precursor time and detectability

Earthquake precursor data amounting to 418 in number accumulated in Japan are analyzed. Most data are obtained under the nation-wide earthquake prediction programme. The precursor time ( T) versus main shock magnitude ( M) relationship is different for different groups of precursors. A linear relation such as: log T = a + bM where a and b are constants, seems to hold for precursor disciplines such as land deformation revealed by geodetic survey and tide-gauge observation, ground tilt observed by a water-tube tiltmeter, anomalous seismic activity and geomagnetic field change. Although a similar relationship seems to hold for the disciplines of earth-currents, radon concentration, underground water etc., the precursor time is substantially smaller for these disciplines than for the former disciplines. The above characteristics of precursor time suggests that precursors closely connected to the dilatancy generation seem to be observed at an earlier stage of seismogenetic activity; precursors probably associated with diffusion of underground water into the dilatant region seem to follow. For a number of precursor disciplines such as foreshock, ground tilt observed by a pendulum tiltmeter, resistivity by a variometer and so on, no dependence of T on M is found. The relationship between M and the logarithmic epicentral distance ( D) measured in kilometers is different from discipline to discipline. Relying on empirical and theoretical studies of coseismic strain step, it turns out that precursors found by geodetic and geomagnetic observations seem to reflect a crastal strain of the order of 10 −7-10 −6. Precursors obtained by a resistivity variometer seem to represent a strain of the order of 10 −9−10 −8. Precursors of other disciplines seem to correspond to a strain of 10 −8−10 −7. It is clear that precursors can be detected further from the epicenter as the main shock magnitude becomes larger. In some favourable cases, a precursor can be observed even where the epicentral distance is several times larger than that of the epicentral area.

Electrotelluric forerunners to earthquakes in Kamchatka

Electrotelluric field (ETF) data observed at Shipunski-station (SPN), Kamchatka, were studied in detail for a period of 10 months, from August 1982 to June 1983. During this time interval seven large earthquakes, M ≥ 5.5, took place in the vicinity of Cape Shipunski, within 180 km of the SPN-station. A large earthquake, M = 5.7, took place on November 14, 1982. This event was preceded by a period of seismic quiescence of about 3 months and it took place at an epicentral distance of 74 km from SPN, in the oceanic part of the lithosphere. A striking change in the electrotelluric potential (ETP) data for November 8 is observed, 6 days before the occurrence of the earthquake. The change in the ETP is about 200 mV/km at a signal-to-noise ratio of roughly 4. The independent parallel line at SPN reveals the same anomalous ETP change, thus excluding instrumental and/or electrode effect. The possibility of a local source can be excluded after consideration of electric power sources in the area. On the other hand the ETF anomaly is rather similar to anomalies observed in Greece prior to the Kefallina earthquake of January 17, 1983. The ETF data are correlated with geomagnetic recordings from a station at Petropavlovsk (PRT), located about 90 km from Cape Shipunski. From the correlation of the two fields we conclude that the changes in the geomagnetic field cannot produce the sudden changes in the electric field if the electric conductivity is constant. Spectral analysis of the ETF data shows that the anomalies contain predominant periods of 24 h, 12 h, 8 h and 6 h, thus resembling the typical main spectral components of geomagnetic field changes. In spite of the apparent independent behaviour of the electric and geomagnetic fields during the anomalous period, we propose that electromagnetic induction is the primary cause of the electric anomaly, accompanied by rapid changes in the transfer functions between the two fields. This means changes in the electromagnetic induction after large, rapid changes of the resistivity in the fractured zone of the forthcoming earthquakes, initiated by the percolation of an interstitial fluid at a critical state of the fracturing process.

Magnetic and electric signals precursory to earthquakes: An analysis of japanese data.

Magnetic and electric data of earthquake precursor amounting to 61 in number so far obtained in Japan are analyzed. The relations between main shock magnitude (M), precursor time (T) and epicentral distance (D) are studied for precursors of five subdisciplines g (geomagnetic field change), earth currents (e), resistivity measured by electrodes having a long distance interval (R), resistivity measured by a resistivity variometer (r) and electromagnetic radiation (w).Linear relations between log T and M are obtained for subdisciplines g, e and R. T for the latter two subdisciplines is smaller than that for subdiscipline g for a fixed M. Such a difference can be understood by taking stress build-up and dilatancy-diffusion processes in the earth's crust into consideration.M-log D relations suggest that a subdiscipline g precursor represents a premonitory crustal strain of the order of 10-7-10-6, while that for subdiscipline r a strain of 10-9-10-8. Precursors for other subdisciplines represent a strain of 10-8-10-7. It is remarkable that precursors are sometimes observed at a point of which the epicentral distance is several times as large as that of epicentral area.

Geomagnetic field changes in the Kopaonik thrust region, Yugoslavia, in the period 1980-1986.

Geomagnetic field changes in the Kopaonik thrust region, Yugoslavia, in the period 1980-1986.

Quiet Ionospheric Currents and Earth Conductivity Profile Computed from Quiet-time Geomagnetic Field Changes in the Region of Australia

Equivalent ionospheric source currents representing the quiet-day geomagnetic field variations were established for a half-sector. of the Earth that included Australia. The analysis used a spherical harmonic separation of the external and internal fields for the extremely quiet conditions existing in 1965. Month-by-month behaviour of the current system indicated a clockwise vortex source with a maximum of 12.8xl04 A in January and a minimum of 4.4xl04 A in June. The focus location shifted from about -32�5� geomagnetic latitude in summer to about - 30.0� in winter. The separated external and internal field coefficients were used to estimate the Earth's upper mantle electrical conductivity (J" at a depth d from about 250 to 350 km as (J" =0�00067 exp(O� 012 d) S m - I and from about 350 to 550 km as (J" = 0�0014exp(0�0088d) Sm- I .

Crcadian rhythms ? endogenous or exogenous?

A recent paper (Martin and Martin 1987) proposes that the 24-h behavioural rhythmicity of honeybees is not controlled by an endogenous (i.e. circadian) clock but merely exogenously by, in particular, the bees' responses to diel changes in the local geomagnetic field. The evidence derives from experiments involving the transplantation of the mushroom bodies from bees trained to feed at noon in Seattle, USA, into untrained bees tested at Wurzburg, West Germany. This and earlier evidence is here re-examined and shown to be consistent with the more accepted explanation of endogenous circadian rhythmicity — while acknowledging the probability that bees do use local magnetic time cues as entraining Zeitgebers.

An automatic system for data logging and verification of multi-channel, multi-size geophysical data

William Gilbert announced in 1600 that the Earth is a great magnet. Since that time, numerous methods and techniques have been developed to utilise the many properties of the Earth’s natural magnetic field. One such geophysical technique utilises the time changes in the natural geomagnetic field, which occur with frequencies from kilohertz to periods of the order of one day, to investigate the electrical properties of the Earth. When time changes with periods of the order of several seconds to one day are used, the technique is referred to as the magnetotelluric (MT) method. The MT method is a way of determining the distribution of the Earth’s electrical conductivity structure from measurements of the naturally occurring time changes in the electric and magnetic fields on the ground surface. For a complete description of the geo-electromagnetic field, five orthogonal field components must be recorded. The five data components are referred to as X, Y, H, D and Z, all being functions of time. The first two data components are the potential differences between two points on the Earth’s surface. X is a measure of the induced electric field in the north-south direction, Y is a measure of the electric field in the east-west direction. H, D and Z represent the three orthogonal components of the magnetic field. H is a measure of the magnetic field in the north-south direction, D is a measure in the east-west direction and Z is the vertical component. In practice, a sixth data channel is also recorded. This channel is temperature which is used to correct the instruments response to thermal effects. The six-channel MT instrumentation produces analogue output of the field values. These voltages are sampled at a discrete rate that defines the sampling interval simultaneously on all six channels. A discrete six-channel time series, suitable for computer analysis, is thus formed. The goal of each field MT system is to obtain such data over a long continuous period of time (say six months) to act as a data base for subsequent analysis. The data base will consist of time variations of the Earth’s electric and magnetic fields recorded every 5 s. Five such field systems are arranged in a spatial array, giving 30 data channels of simultaneous magnetotelluric data. Such an MT-based field monitoring system is to be set up in western Turkey, a region which is seismically active and earthquake prone. The purpose of the experiment is to use the recorded data base to investigate stress-induced changes in the crust of the region which may be detected by the fields recorded.