Showing papers in "Journal of geomagnetism and geoelectricity in 1968"
TL;DR: In this article, the effect of directional stress on the remanent magnetization of magnetite bearing rocks and nickel polycrystallites was investigated and a nearly reversible change in intensity was found when the direction of compression is perpendicular to that of TRM.
Abstract: Experiments to test the effect of directional stress on the remanent magnetization of magnetite bearing rocks and nickel polycrystallites are described. The TRM of volcanic rocks under repeated compression of less than 1kb was found to reveal a nearly reversible change in intensity. The intensity increases when the direction of compression is perpendicular to that of TRM and decreases when the compression is parallel, in accordance with the theory of magnetoelasticity. Stability of IRM and PRM induced in previously workhardened nickel rods under compression is compared with that of TRM. TRM of a nickel polycrystallite under compression changes in a manner similar to that of volcanic rocks. However, no reversible change was found for IRM, even the compressional stress did not exceed a few hundred bars. The results, combined with the magnetic susceptibility of rocks under compression, can be used for estimation of seismo-magnetic effects.
69 citations
TL;DR: In this paper, changes in the magnetization curves, magnetic susceptibility, and remanent magnetization of magnetite and titaniferous magnetite under uniaxial compression are experimentally examined in a systematic way.
Abstract: Changes in the magnetization curves, magnetic susceptibility, and remanent magnetization of magnetite and titaniferous magnetite under uniaxial compression are experimentally examined in a systematic way. The uniaxial compression results in a reduction of the intensity of magnetization and the magnetic susceptibility of both single crystals and assemblages of fine grains of titaniferous magnetite.However, a removal of the compression from the samples in presence of a magnetic field results in an abrupt increase of magnetization, the resultant intensity of magnetization being larger than the ordinary magnetization without the effect of compression in the same field. The majority of the increased magnetization remains as the remanent magnetization after removal of the field. Thus, the isothermal remanent magnetization produced by removing uniaxial compression in a magnetic field increases with increasing magnitude of the uniaxial compression.
48 citations
TL;DR: In this article, the effect of electromagnetic core-mantle coupling on small changes in the length of day and the geographical location of the pole is treated by a simplified model which neglects the time required for magnetic diffusion through the lower mantle, and approximates the poloidal field by the centred dipole.
Abstract: The effect of electromagnetic core-mantle coupling on small changes in the length of day and the geographical location of the pole is treated by a simplified model which neglects the time required for magnetic diffusion through the lower mantle, and approximates the poloidal field by the centred dipole. The resulting formulae for the time constants of the coupling are more meaningful physically than, and give nearly the same numerical values as, those derived previously by more rigorous methods. It is shown that, despite the complexity of the electromagnetic interactions at the coremantle boundary, the two kinds of perturbation in the Earth's rotation can be treated independently. Simple expressions are given for the electromagnetic restoring torque on the accelerating mantle. The effect of the geomagnetic coupling on the diurnal wobble is discussed, and concluded to be negligible.
36 citations
TL;DR: In this article, a method of determining the magnetospheric plasma density distribution, using long period geomagnetic micropulsations (period 45-600sec), is presented.
Abstract: A new method is presented of determining the magnetospheric plasma density distribution, using long period geomagnetic micropulsations (period 45-600sec.). Long period micropulsations are assumed to be caused by resonant oscillations of geomagnetic lines of force in the magnetosphere. Eliminating the time dependent factor from the wave equations leads to an ordinary differential equation that depends on a spatial parameter only. With certain boundary conditions assigned beforehand (which may be determined by observation), the differential equation uniquely determines the ‘spatial form’ of the oscillation if the plasma density distribution is known. A distribution is determined so that the spatial form is that of the fundamental and of the second harmonic, since these modes are the most likely to be excited. Results are compared with other investigations-good agreement is obtained with those from the IMP II satellite.
33 citations
TL;DR: In this article, a large number of whistlers recorded during the magnetic storm of 13th to 15th January, 1967, have been presented to illustrate the formation of ducts corresponding to path latitudes of 24°, 27°, and 31° and the VLF ducts formation during the main phase of magnetic storm was attributed to the enhanced electron density produced in the F2-region heights and above upto about 2 earth's radii.
Abstract: Sonagrams of a large number of whistlers recorded during the magnetic storm of 13th to 15th January, 1967, have been presented to illustrate the formation of ducts corresponding to path latitudes of 24°, 27°, and 31°and the VLF ducts formation during the main phase of the magnetic storm has been attributed to the enhanced electron density produced in the F2-region heights and above upto about 2 earth's radii. A correlation between the Kp-index and the whistler occurrence during the storm has been drawn and the enhanced whistler activity has been explained in terms of additional VLF ducts produced for whistler mode of propagation.From a large number of whistlers recorded during the storm, it is inferred, contrary to the idea of Smith, that though the formation of ducts can take place in much less than 30 minutes, once the duct has formed it can persist for at least 2 to 3 days.It is found that usually whistlers have pure gliding tones during quiet days while most of the whistlers recorded during the disturbed days show a large amount of diffuseness, which has been attributed to the propagation of VLF waves within a wide area of the duct. This is in contrast to the idea of Crouchley and Finn who show that diffuseness increases with dispersion.From the diffuseness of sonagrams of the whistlers, the effective width of the VLF ducts for the waves of 5Kc/s have been calculated and are found to vary from 15km to 25km for normal whistlers and 40 to 180km for those recorded during the magnetically disturbed days.
33 citations
TL;DR: In order to trace the secular variation of the geomagnetic field in historic time, the documents of ancient aurorae can be utilized as discussed by the authors, which can be reasonably supposed to describe the events of auroral appearance.
Abstract: In order to trace the secular variation of the geomagnetic field in historic time, the documents of ancient aurorae can be utilized. In China, Korea and Japan, there are a number of valuable records, which can be reasonably supposed to describe the events of auroral appearance. After a comparison of these descriptions with those in Europe, the archaeo-secular variation in the geomagnetic field can be inferred. A preliminary examination of the auroral appearance on the same day in the Occident and Orient suggests that the geomagnetic dipole axis might have been inclined towards China around 11-12th centuries AD.
33 citations
TL;DR: In this paper, an upper limit of temperature needed to erase thermally in the laboratory any viscous magnetization acquired by a rock at or near room temperature was predicted. But this upper limit was not observed for all except four of the 123 specimens investigated.
Abstract: Secondary natural magnetizations have been examined by thermally demagnetizing a variety of rock types of mainly Tertiary age from Britain, India and Iceland. The conclusion is that the secondary magnetizations are most likely to be viscous magnetizations slowly acquired isothermally in the earth's magnetic field. A theory is developed which predicts an upper limit of temperature needed to erase thermally in the laboratory any viscous magnetization acquired by a rock at or near room temperature. This upper limit is near to 300°C, and is observed to be correct for all except four of the 123 specimens investigated.It also appears that if, during laboratory heating, the Curie point of a specimen rises due to oxidation before complete demagnetization of the NRM, then the NRM direction may be retained by the specimen unchanged, and only disappears at the new, higher, Curie point.
31 citations
TL;DR: By measuring the natural remanent magnetization of potteries from Bolivia and Ecuador, the intensity of the ancient geomagnetic field was compared with that of the present as mentioned in this paper, and it was concluded that the intensities of 2000 B.C. and 0 A. D. were nearly equal to the present intensity, and that of 700 A.D. was about 1.5 times as large as the present one.
Abstract: By measuring the natural remanent magnetization of potteries from Bolivia and Ecuador, the intensity of the ancient geomagnetic field was compared with that of the present. These potteries cover the period of about 2000 B. C. to 1200 A. D. It was concluded that the intensities of 2000 B. C. and 0 A. D. were nearly equal to the present intensity, and that of 700 A. D. was about 1.5 times as large as the present one. In this study Thellier's stepwise heating method was used to estimate the intensities of the ancient geomagnetic field. Some samples exhibited anomalous shapes of thermal demagnetization curves of natural remanent magnetization. An experimental examination to clarify the origin of these anomalies indicated that some samples might have been reheated at a temperature somewhat below the highest Curie temperature sometime after the production of the potteries. A criterion to select useful samples was postulated on the basis of these experimental results and only the proper samples were used for the present purpose.
28 citations
TL;DR: In this paper, three magnetic classes of ocean basalts are defined on the basis of thermomagnetic and microscopic analysis, and the transformation mechanisms are described and the characteristics of each class are tabled.
Abstract: Three magnetic classes of ocean basalts are defined on the basis of thermomagnetic and microscopic analysis. Moderate heating of Class I basalts results in an anomalous increase in Curie point, magnetization intensity, the ratio TRM to NRM, and magnetic susceptibility. The same thermal treatment of Class II rocks results in an anomalous decrease in all magnetic parameters except the Curie point. Class III rocks are stable. Several Class I and II ocean basalts possess the property of self reversal. Class I and II rocks contain metastable magnetic minerals and readily transform into stable Class III rocks when heated. The transformation mechanisms are described and the characteristics of each class are tabled. It is concluded that in situ cores are necessary to discover the composition of the ocean crust, and the distribution of the different magnetic classes of ocean basalts within the ocean crust. Otherwise, little faith can be placed in any interpretation of ocean magnetic anomalies.
25 citations
24 citations
TL;DR: Paleomagnetism and K-Ar ages of two successive lava flows from Kita-Matsuura, northwestern Kyushu were studied in attempting to estimate the time required for the geomagnetic field to switch from one polarity to another as mentioned in this paper.
Abstract: Paleomagnetism and K-Ar ages of two successive lava flows from Kita-Matsuura, northwestern Kyushu were studied in attempting to estimate the time required for the geomagnetic field to switch from one polarity to the other. Both lava flow successions record normal and reversed polarities. K-Ar ages of these flows range from 7m. y. to 10m. y. However, because of inconsistensies within a flow and among successive flows, K-Ar ages should be regarded as indicating approximate ages of these lava flows. Hence, it was not possible to determine the transition time for the geomagnetic field switching.
TL;DR: In this paper, the authors examined the natural remanent magnetization of Pleistocene andesite and scoria samples collected from Usami Volcano Izu Penninsula and found that the magnetic properties of these rocks show significant departure from that of the present geomagnetic field in Japan.
Abstract: The natural remanent magnetization of Pleistocene andesite and scoria samples collected from Usami Volcano Izu Penninsula was examined by paleomagnetic methods and found to be stable. The directions of the stable magnetization of these rocks show significant departure from that of the present geomagnetic field in Japan A hypothesis based on geological field evidence that the strata in the sampled area have been tilted sometime after the formation of the rocks, can account for this fairly well. Using these samples with stable remanences, the intensity of the Pleistocene geomagnetic field (normal and reversed) in Japan has been determined by means of Thellier's metood. Many samples show some irreversible changes in magnetic properties when they are heated above certain critical temperatures. The intensity estimates were obtained from the linear relation of Jn-Jt in the temperature range where irreversible changes did not occur. The intensity of the Pleistocene geomagnetic field determined in the present study is very similar to the present field intensity in Japan (0.46oe), the mean values being 0.61±0.61oe for reversed samples and 0.55±0.16oe for normal ones.
TL;DR: This article showed that the self-reversal of submarine basalts is caused by an exchange interaction between two magnetic components with a Curie temperature of 300°C and there exist momentarily two magnetic component.
Abstract: Three of eight submarine basalts from various localities of the Pacific show self-reversal of TRM when they are heated in air at about 300°C and cooled to room temperature in the geomagnetic field. As the sample is heated to 300°C, the original ferrimagnetic component with a Curie temperature of 250°C is gradually transformed to another component with a Curie temperature of 300°C and there exist momentarily two magnetic components. Magnetic interaction between these two components seems to be responsible for selfreversal. The fact that TRM produced in an external field of 2600 Oe reverses at low temperatures indicates that the self-reversal phenomenon is caused by an exchange interaction.
TL;DR: In this article, a technique for determining the theoretical uniaxial stress required to impose magnetization rotation and dominate the arrangement of a pattern of magnetic domains is proposed, which involves an appropriate balance of magnetocrystalline and magnetoelastic anisotropy energies.
Abstract: For consideration of magnetomechanical effects in rock magnetism, it would be desirable to have a criterion for the influence of directed stress on magnetization. A technique is outlined for determining the theoretical uniaxial stress required to impose magnetization rotation and dominate the arrangement of a pattern of magnetic domains. It involves an appropriate balance of magnetocrystalline and magnetoelastic anisotropy energies. Calculations for magnetite agree with reported experimental results obtained from domain observations. Results for nickel further support the method as a quantitative measure for determining the stress sensitivity of a magnetic crystal.
TL;DR: The large coercivities of some ilmenohematite minerals are probably due to magnetostrictive effects associated with exsolution, as reported in this paper, where the authors show.
Abstract: Large coercivities of some ilmenohematite minerals are probably due to magnetostrictive effects associated with exsolution.
TL;DR: In this paper, the results of the thermomagnetic investigation of two natural pyrrhotites of intermediate chemical composition indicate the coexistence of a ferrimagnetic phase of estimated composition Fe0.875S to Fe 0.88S (Tc≈300°C) and an antiferromagnetic phase which becomes Ferrimagnetic above 180°C due to vacancy ordering.
Abstract: The results of the thermomagnetic investigation of two natural pyrrhotites of intermediate chemical composition indicate the co-existence of (1) a ferrimagnetic phase of estimated composition Fe0.875S to Fe0.88S (Tc≈300°C) and (2) an antiferromagnetic phase which becomes ferrimagnetic above 180°C due to. vacancy ordering (Tc between 250 and 275°C). The second phase comprises 70 to 90% of the individual specimens, and is less Fe deficient than the composition of the whole samples. These conclusions were largely confirmed by the results of the application of magnetite powder to etched polished surfaces of the samples. Changes in thermomagnetic properties of both phases are observed after continued heating of the specimens above about 250°C. Heating of the specimens to 500-600°C probably results in homogenization of the specimens as the points of inflection below 300°C on the J(T) curves disappear.
TL;DR: A few geological evidences indicate that the Kirizumi group is Pliocene as discussed by the authors, and the rocks of both the Hanamagari volcano and the Kii-mei group are calc-alkali andesites with common phenocrysts of plagioclase, hypersthene, augite, and opaque minerals.
Abstract: The northeastern part of Nagano Prefecture and western part of Gumma Prefecture belong to the extensive Miocene-to-Quaternary volcanic province of the \"Fossa Magna\" in central Japan (Fig. 1). A partly destroyed Quaternary strato-volcano, Hanamagari (Aramaki, 1963) lies on the boundary between Nagano and Gumma Prefectures and is underlain by a thick pile of volcaniclastic materials and lava flows. This pile is collectively called the Kirizumi group (Iijima et al., 1958; Aramaki, 1963). The Kirizumi group is more or less flat-lying and made up of essential tuff, lapilli tuff, tuff breccia, lava flow, and smaller amounts of sandstone, shale, conglomerate, etc. A few geological evidences indicate that the Kirizumi group is Pliocene. The rocks of both the Hanamagari volcano and the Kirizumi group are calc-alkali andesites with common phenocrysts of plagioclase, hypersthene, augite, and opaque minerals (Aramaki, 1963). Within the drainage area of the Kirizumi River, both the Hanamagari and Kirizumi
TL;DR: In this paper, an empirical method was used to calculate the Curie temperatures of spinel materials containing Fe2+ and Fe3+ ions to within 20% for titanomagnetites.
Abstract: An empirical method is used to calculate the Curie temperatures of spinel materials containing Fe2+ and Fe3+ ions. The approach taken is to select three simple spinel oxides of known cation distribution and Curie temperature containing the paramagnetic ions Fe2+ and Fe3+. In this way the relative strengths of the Fe3+—Fe3+, Fe3+—Fe2+ and Fe2+—Fe2+ AB interactions are calculated. For titanomagnetites the calculated Curie temperatures agree with the observed values to within 20%. A Curie temperature of 770°C is calculated for maghemite. This is higher than the Curie temperature of magnetite. Similarly an increase in the Curie temperatures of titanomagnetites is predicted on oxidation. This is in agreement with experiment. Finally the method is applied to the ratio of the BB/AB interactions in the spinel structure. This predicts that the apparent ferrimagnetic P-type behaviour of titanium rich titanomagnetites is not explained by the Neel theory. However the behaviour of titanium rich oxidized titanomagnetites is shown to be of the Neel P-type.
Journal Article•
TL;DR: In this article, the growth of a magnetic field given to the zero-field state of a Bullard-Gellman dynamo model is studied, and a steady state seems to be reached within a time comparable with that of free decay of the magnetic field, say several thousand years, for the earth's core.
Abstract: Growth of a magnetic field given to the zero-field state of a Bullard-Gellman dynamo model is studied. Reaction of electromagnetic origin to the fluid motion is completely ignored. A steady state seems to be reached within a time comparable with that of free decay of the magnetic field, say several thousand years, for the earth's core. A more complete study, in which an equation of motion is taken into account, will be made in the second paper.
Abstract: A theory of one of several possible mechanisms responsible for micropulsations is presented. It is shown that hydromagnetic waves generated by an instability of the interface between solar wind and magnetosphere are transmitted through the magnetosphere and received on the surface of the earth as micropulsations. The bandwidth of the micropulsations essentially determined by the source mechanism is found to be a few tenths of a second to hundreds of seconds. The important role of the path in the magnetosphere, including that in the outer exosphere in determining the transmission characteristics of micropulsations is analyzed. The crucial role of the slightest variation of the path length in the outer magnetosphere (due to variations in the solar wind) in masking out the resonances of lower exosphere and upper ionosphere is discussed. The resonances in transmission due to the path in outer magnetosphere are seen to be more likely than those of the lower exosphere and upper ionosphere.
TL;DR: In this paper, two basalt samples with homogeneous titanomagnetites as the only ferrimagnetic component, have been investigated, to find out about the influence of uniaxial compression on coercive force and saturation remanence at different temperatures and on thermoremanent magnetization.
Abstract: Two basalt samples with homogeneous titanomagnetites as the only ferrimagnetic component, have been investigated, to find out about the influence of uniaxial compression on coercive force and saturation remanence at different temperatures and on thermoremanent magnetization. The samples have been investigated in natural state and after a heat treatment of 30h at 700 in air. Caused by this heat treatment the original homogeneous titanomagnetites are transformed into an inhomogeneous mineral aggregate. The natural samples show a strong dependence of all measured values on uniaxial compression: At room temperature and a compression of 1 kbar for example, the coercive force increases by about 100% and the saturation remanence decreases by about 50%. These variations are reversible. The behaviour of thermoremanence is distinctly different depending upon whether uniaxial compression and applied magnetic field are parallel or perpendicular to each other. In the first case a decrease of about 50%, in the second case an increase of about 50% is observed. In contrast to these results the heat treated samples show only a very slight dependence of their magnetic parameters on uniaxial compression.
TL;DR: Theoretical interpretotions are given on the basis of calculations made for emission rates for various excitation mechanisms, also using results of simultaneous measurements of electron density and electron temperature as mentioned in this paper.
Abstract: Rocket results are presented on visible and ultraviolet dayglow observed in the altitude range of 80-350km, for different values of solar zenith distance. Theoretical interpretotions are given on the basis of calculations made for emission rates for various excitation mechanisms, also using results of simultaneous measurements of electron density and electron temperature.
TL;DR: In this paper, the Pc 1 events with amplitudes over -1/2γ recorded at College, Alaska, over a 6 1/2 year interval, were studied.
Abstract: The Pc 1 events with amplitudes over -1/2γ recorded at College, Alaska, over a 6 1/2 year interval, were studied. Their general characteristics are described. Sonagram displays and polarizations are similar to those for smaller events, but there is typically more ionospheric cosmic noise absorption accompanying the large events, and the large events are usually observed at only one station. It is proposed that large amplitudes observed on the ground are at least partially due the presence of unusually welldefined wave guides of enhanced ionization along magnetospheric field lines.