Earth's Main Field Research Articles

A magnetic investigation of a tectonic problem: The propagating rift, Galapagos 95°30′W

The propagation of an oceanic rift is an important tectonic problem, with a bearing on the reorganization of plate motion and on the early opening of oceanic basins. At the propagating rift at 95°30′W near the Galapagos Islands, we can use magnetic methods to determine the tectonic origin of a set of important sea floor features. The observed 27 km offset between the axes of the propagating rift and the dying rift presents us with an ideal situation, in which the oceanic crust created by the opposing systems has been magnetized in opposite directions. The normally magnetized crust of the propagating rift tip penetrates into older crust, which created when the earth's main field was reversed. A combined Deep Tow and Sea Beam investigation at 95°30′W on the Cocos-Nazca spreading center has revealed the crustal contact between the propagating rift and the dying rift systems. The inherent magnetic labelling of the crust has been recovered by performing inversions on the gridded representations of the observed magnetic field and bathymetry, working in the Fourier domain. The result is a gridded rock magnetization distribution. The inversion of the surface data covers a large area, 6000 km 2, and demonstrates close agreement with magnetization amplitudes of rock samples at existing dredge sites. In general, the propagating rift process appears to be much more orderly than the dying rift process. The magnetic polarity transition widths are narrower, and the boundaries have fewer undulations than the dying rift, which appears to be quite episodic in behavior. The average propagation rate is 52 mm/yr, compared to the average spreading half-rate of 29 mm/yr. The locations of the boundaries suggest that the acceleration to the normal spreading rate on the propagation rift requires about 250, 00 years. The inversion of the Deep Tow data, near the sea floor, provides a high resolution definition of the tip of the propagation rift, at 2°38.1t'N, 95°30.0′W.

Near bottom magnetic profile across the Red Sea

Results are presented from a high precision geophysical profile made at an altitude of about 100 m above the sea floor with the Deep Two instrument package, crossing the Red Sea at 17°30′N. The emphasis is on the analysis and interpretation of the magnetic field, including an inversion which removes the distortions due to bathymetry and the orientation with respect to the earth's main field vector. The spreading rates are determined precisely and found to be highly asymmetric: only 5 mm yr-1 to the east and up to 10 mm yr-1 to the west. We conclude that the axis of spreading is located on a volcanic ridge, rather than on the axial graben, based on the presence of a zone of high magnetization. The magnetization high (40 Am-1) is about twice as great as found on the Mid-Atlantic Ridge with the same instrument and analysis. The quality of the recording of the magnetic anomalies in the oceanic crust is much greater than expected for such a low spreading rate.

Very long solar periods and the radiocarbon record

The ∼ 200‐year periodicity in the time variations of atmospheric radiocarbon is shown to extend over the entire 8500‐year La Jolla record and appears to be associated with a longer period between about 1500 and 2000 years via amplitude, frequency, or phase modulation, or some combination of these; but the statistical certainty of the source and form of the modulation is hampered by the low signal/noise ratio of the two periods. Autocorrelations of the La Jolla sequence show that the record violates even weak stationarity; although the 200‐year period is well ordered in time, the appearance of other periods may be more sporadic. Significant cross correlation between the very long period and the modulation envelope about the neighborhood of the 200‐year line suggests an identity between the two manifestations of the long period, and modulation by a nonlinearity satisfying the rule that it have at least one nonvanishing derivative of odd order and n > 2. Commensurate segments of the La Jolla, Belfast, and Groningen radiocarbon records confirm the existence between 3900 B.C. and 3200 B.C. of other periods, particularly 150 and 300 years. A likely source of these periodic changes in the radiocarbon record is the sun because the source of the variations is time dependence of the cosmic ray flux on the atmosphere. Further evidence is the recently reported correlation of radiocarbon and bristlecone pine growth ring variations and the lack of observational evidence to support the very large change in the earth's main field required for a geomagnetic explanation, especially of the ∼200‐year period. If it can be confirmed that the sun is the source, the very long periods suggest as one possibility that the core of the sun is the ultimate source, though multiple convective zone dynamo eigenmodes are an equally conjectural possibility. An alternate source for the longer period is pressure variations of the local arm of the galaxy, but this model is not favored.

A regional magnetic field model around Japan at the epoch 1980.0 and its comparison with world magnetic field models MGST(4/81) and IGRF 1980.

The Hydrographic Department of the Maritime Satety Agency conducted an airborne magnetic survey from 1979 to 1980 to derive a regional field model around Japan at the epoch 1980.0. The regional field model, consisting of 3rd-degree polynomials, was compared with the models MGST (4/81) and IGRF 1980 for the earth's main geomagnetic field over the world. The comparison of the regional field model and MGST (4/81) or IGRF 1980 revealed the following differences:1) MGST (4/81) model approximates the actual magnetic field better than IGRF 1980, insofar as the RMS residuals for the region around Japan are concerned. The RMS residuals from MGST (4/81) are 18.1nT, 44.3nT, 47.3nT, and 42.3nT for the total intensity F and three components X, Y, Z respectively, whereas the residuals from IGRF 1980 are 23.6nT, 46.6nT, 67.6nT, and 50.4nT in the same order.2) Large negative residuals of the geomagnetic total intensity in the area of the Japan Sea and Okhotsk Sea (amounting to -300nT from the IGRF 1965 or IGRF 1975) are thought to be attributable to the insufficient representation of the core-field in these IGRF models. If MGST (4/81) is used as a reference model field, the residual map shows a positive deviation along the outer region of Island Arc and a negative one in the marginal sea. This indicates the existence of a significant long-wavelength magnetic anomaly around Japan.

Determination of Depth to Hydrocarbon Maturation Temperature from Magnetic Data, Bligh Water, Fiji: ABSTRACT

The depth to the temperature at which rocks lose their magnetization (Curie point depth) can be calculated by estimating the average depth to the bottom of the magnetized bodies which make up the crust. Using the method of spectral analysis on magnetic data, as suggested by Bhattacharyya and Leu, we have calculated the depth to the Curie point temperature in Bligh Water, Fiji. Measurements of the magnetic susceptibility of rocks versus temperature at field strengths approaching that of the earth's main field suggest that the effective Curie point temperature of the magnetic crust is about 500°C. Using this temperature with the calculated Curie point depth and reasonable ocean-bottom temperatures, we have prepared a crustal-thermal-gradient map of the Bligh Water basin. The thermal gradients thus calculated are in reasonable agreement with gradients determined from conventional oceanographic heat-flow measurements in surrounding deep-water regions. The results for Bligh Water, Fiji, suggest that hydrocarbon maturation temperatures are probably reached in the relatively shallow lower to middle Miocene reefs. We are presently analyzing marine magnetic data over the Fiji Plateau west of Fiji, where an active back-arc spreading center is postulated. The results of this investigation will provide a direct comparison between thermal gradients calculated from Curie point data and marine heat-flow measurements, as well as provide insight into the regional geophysics of the area. End_of_Article - Last_Page 447------------

Polar cap geomagnetic field responses to solar sector changes

I made a computerized analysis of digitized magnetograms from Alert, Thule, Resolute Bay, Mould Bay, and Godhavn for 1965 and from Thule and Vostok for 1967 to determine the characteristic features of the day-to-day geomagnetic field variations related to the interplanetary solar sector field direction. Higher invariant latitude stations showed the sector effects most clearly. A sector-related phase shift in the characteristic diurnal variation of the field occurred principally for the dayside vertical geomagnetic component. The amplitude of this diurnal variation was related to Ap and could not be used to identify the sector direction. The quiet nighttime level of field Z component rose and fell on days when the interplanetary magnetic field was directed toward or away from the sun, respectively. When a station's base level field was determined from quiet magnetospheric conditions by using days with low values of Dst and AE indices, the mean field level of the Z component for the whole day increased or decreased (often over 100 ..gamma..) from this level as the solar sector direction was toward or away, respectively. With respect to the earth's main field direction the souther polar station field level changes were opposite those at the northern stations. Thismore » level shift corresponded with the two solar field directions during the summer months at polar stations for about 70% of the days in 1965 and 88% of the days in 1967. In 1967 the standoff locations of the magnetopause and magnetoshock boundaries were abotu 1 R/sub E/ more distant from the earth for the average toward sector days than for the away sector days. (AIP)« less

The role of solar local time in polar cap magnetic variations

The role of the earth's main field in controlling the morphology of magnetic disturbance is usually accounted for by use of invariant latitude and MLT (magnetic local time) as a coordinate system. Magnetic disturbance from ionospheric currents is also controlled by ionospheric conductivity. At high latitudes, where SLT (solar local time) can be very different from MLT, the use of only MLT can give misleading results. In particular, those diurnal magnetic variations in the polar cap that change characteristics between interplanetary magnetic sectors have a tendency to peak near noon SLT rather than noon MLT at Alert, where noon MLT and noon SLT differ by more than 10 hours. Because of the sparsity of magnetic observatories it is not possible to completely separate SLT and MLT effects.

Correspondence of solar field sector direction and polar cap geomagnetic field changes for 1965

World Data Center A magnetic tapes containing 2.5-min digitizations of 1965 magnetograms from observatories at Godhavn, Resolute Bay, and Thule were used to determine the daily average residual variation of geomagnetic field after removal of the Sq change and earth's main field contribution to the field. A correlation coefficient of only --0.6 was found beiween the Thule Z component and the Godhavn H component. If days when the average H residual magnitude at Godhavn was greater than 10 gamma were used, the negative values correspond to solar sector fields toward the sun and the positive values corresponded to solar sector fields away from the sun for 61% of the days. By further restricting the data to summer months the agreement rose to 70% of the days. Our own automated method was used to evaluate Svalgaard's visual techniques. (auth)

Calculations of magnetic variations induced by internal ocean waves

Previous theoretical and experimental work has shown that the motion of an ocean swell across the earth's main field induces magnetic signals of detectable magnitude. Here we present calculations giving the magnetic field induced by internal waves whose periods fall in the range 1–30 min. Solutions for the induced field, both within and above the ocean, are derived for a two-layer ocean of finite depth. The magnitude of the field associated with internal waves is less important than that induced by the shorter-period surface waves, but under certain conditions it may become quite significant. For example, internal waves of about 10-meter amplitude and 5½-min period 100 meters below the surface of a deep ocean whose layer densities are in the ratio 1.01 induce magnetic fields of the order of 1γ at 50 meters above and 300 meters below the surface.

The paleomagnetism of deep sea sediments

Paleomagnetic measurements made on deep sea sediments are described. The main features to be seen in the pattern of NRM directions in the sediment cores are: (1) Very few of the cores show reversals of NRM direction, but the position of the latest reversal in those which have a reversal correlate well in time. (2) There is evidence for a wobble of the earth's main dipole field with a period between 105 and 106 years. The NRM in these sediments is thought to be stable.

The Dynamo Model, Field Reversal, and Polar Wandering

The convective flow conducting heat from the inner core of the earth to the mantle may follow a jet-stream-like pattern in which trapped segments of fluid form cylindrical rotating columns—as observed in laboratory studies in cylindrical rotating vessels. This suggests a very simple hydromagnetic version of the dynamo theory in which bundles of magnetic lines of force twisted by the rotation of the outer columns play a predominant role. The westward drift of the convection pattern is a reaction to the eastward drift which the jet stream imparts to the inner core. This drift bends the bundles of force lines in such a way that the twisting provides the curl H (near the equator) which supports the earth's main field. The dynamo action tends to produce a main dipole moment parallel to the axis of rotation and effects that are intrinsically fairly small are proposed to account for the present inclination between them. One such effect is inhomogeniety of the temperature distribution and conductivity of the mantle due to convection related to continental configuration. Another arises from irregular kinking of the almost axial bundles of lines of force threading through the solid core and twisted by its rotation relative to the mantle. It is thus plausible that magnetic and rotational axes have remained nearly parallel so that paleomagnetism approximately traces the course of the kinematic poles. Field reversal and continental drift are also briefly discussed.

Marine Geomagnetic Anomalies

The critieron for separating the earth's main field from magnetic anomalies of crustal origin is reviewed. A concise resume of the nature of magnetic anomalies over the Mid-Atlantic Ridge, the anomalies caused by great elongated but unidentified bodies and the magnetic anomalies over seamounts is presented.

The World Magnetic Survey and the Earth's Interior

The present World Magnetic Survey is described and discussed in relation to defining the earth's main field and secular change. Some effects dependent on geomagnetic noise levels affecting the survey are indicated. Difficulties incident to the extrapolation of faulty surface-field data to the surface of the core are noted. The extrapolated results of previous surveys are used to indicate the possible principal descriptive features of surface fluid motions that are derived for the outer core.

The world magnetic survey

The mathematical and graphical description of the earth's main field has been, and is, a “data limited” problem. The World Magnetic Survey (WMS) is an endeavor to minimize this limitation by rapidly and comprehensively blanketing the earth with magnetic field measurements. Satellite surveys, which will play a key role in the WMS, are the principal topic of this paper. Existing magnetic field descriptions, the expected results from new surveys, and the methods of obtaining these results with the POGO satellite are emphasized. It is anticipated on the basis of extrapolation from Vanguard 3 results and other considerations that a factor of 10 improvement will be obtained. This means that the average errors of 1 to 3 percent now present in field charts and spherical harmonic descriptions should be reduced to 0.1 to 0.3 percent as a result of the survey.

Some Comments on the Paper by A. T. Price, ‘Magnetic Disturbance and the Earth's Main Field’

Some Comments on the Paper by A. T. Price, ‘Magnetic Disturbance and the Earth's Main Field’

Magnetic disturbance and the Earth's main field

An examination is made of J. S. Chatterjee's recent suggestion that the earth's magnetic field has been gradually built up by magnetic storms which induce unidirectional current in the highly conducting core because the semiconducting mantle acts as a rectifier. It is shown that, even if rectification of current in the mantle does occur, it is most unlikely that magnetic storms or other transient variations have any lasting effect on the earth's main field. Some comments are made on Chatterjee's arguments and on a note by T. Rikitake.

A field experiment with a rubidium-vapor magnetometer

A field experiment in the vicinity of a small deposit of massive magnetite has served to illustrate the nature of the results obtainable with a rubidium-vapor magnetometer in a natural micropulsation field superimposed upon a natural nonuniform unidirectional magnetic field. The ultimate purpose of this type of experiment is to explore the feasibility of using this instrument to measure the ratio of induced to remanent magnetism for a buried natural ferromagnet. The results indicate that (1) magnetic ‘amplification’ of micropulsations occurs in the vicinity of ferromagnetic masses, (2) a change in the direction of either the unidirectional earth's main field or the micropulsation field leads to a change in the component of the micropulsation field recorded, (3) stability of the magnetometer platform must be constant to within 0.01 mm, and (4) the signal-to-noise ratio of the magnetometer tends to degenerate. This last effect can be minimized by appropriate orientation of the detection heads, but it is a phenomenon which occurs only under the rare condition of an outcropping massive magnetite body.

Note on the direction of high auroral arcs

The relationships between geomagnetic micropulsations and pulsations in auroral luminosity are considered. Several features are found to be compatible with the concept of drainage of trapped radiation in the region of the outer Van Allen radiation belt in the western hemisphere. Certain homogeneous auroral arcs noted by Stormer at a height of 200 km in central Norway have a mirror point for trapped particles computed to be below ground level in the south Indian Ocean, a region likely to need surveys of the earth's main field. The magnetic-field gradients near the night-time electro jets near the northern and southern auroral zones will change the sign of drift of auroral particles, so that electrons spiraling up or down the geomagnetic-field lines may drift westward in some regions and eastward in others. The gradients of the transient electrojet fields may help to explain many of the complex features of auroral forms, when aurora are formed by incoming charged particles.

THE RELATIONSHIPS BETWEEN THE SECULAR CHANGE AND THE NON-DIPOLE FIELDS

Using airborne, ground, and repeat magnetic observations in Canada compiled in the isomagnetic and isoporic charts for epoch 1955.0, the drift and decay contributions of the non-dipole field to the observed secular variation have been estimated. The drift rates which produce the minimum residual secular variation were found to be unusually small. It is then confirmed, using the longitude displacement method and isomagnetic data only, that the westward drift of the non-dipole field in recent years in Canada is significantly smaller than the world-wide average. These results clearly demonstrate the large local fluctuations which occur in westward drift.The two different methods were applied to obtain relationships between the Gaussian coefficients in the spherical harmonic analyses of the earth's main field and the secular variation. Calculations show that both methods give the accepted world-wide average value of westward drift, that one half of the world-wide secular variation is produced by westward drift, and in general decay terms are unimportant.

Geomagnetic secular change during past epochs

The present paper concerns the description of geomagnetic secular change during the past four decades, and its preliminary interpretation in relation to the origin of the Earth's main field and its secular change. The work is based mainly on extensive magnetic surveys over land and sea over the past 40 years, carried out by or in cooperation with the Department of Terrestrial Magnetism, Carnegie Institution of Washington, with final reductions financed by the United States Navy.