Showing papers on "Earth's magnetic field published in 1970"

Anomalies of geomagnetic variations in the Southwestern United States

Abstract: A survey with temporary geomagnetic field stations (Askania Variographs) has been conducted in the southwestern United States in order to study local differences of geomagnetic variations. Anomalous large Z-variations, accompanied with a slight reduction of the variations in D and H, have been recorded along the California coastline for long-period (Sq) and short-period (bays, ssc's) variations. This Californian coastal anomaly has been interpreted as edge-effect of the Pacific Ocean and its effect upon the induction within the highly conductive substratum in the upper mantle, thereby allowing conclusions about the deep conductivity structure Less pronounced anomalous variations further inland in central California (Sierra anomaly) seem to be caused by conductivity differences in the continental surface layers. A third anomaly between Tuscon, Arizona, and Sweetwater, Texas (Texas anomaly), has been interpreted as a change in the deep conductivity structure which occurs along the eastern margin of the Rocky Mountains.

Vertical drift velocities and east‐west electric fields at the magnetic equator

Abstract: Incoherent scatter observations of vertical drifts taken at Jicamarca (2° dip) are presented. Vertical drifts are found to be nearly constant as a function of height. These vertical drifts can also be taken as a direct measurement of the east-west electric fields at the magnetic equator. Their daily and seasonal behavior is presented. The effect of geomagnetic activity is discussed.

Configuration of the geomagnetic tail during substorms

Abstract: Geomagnetic tail configuration during substorms from Imp 4 magnetic field and auroral index measurements

Latitude Dependence of the Angular Dispersion of the Geomagnetic Field

Abstract: Summary Changes in the direction of the Earth's magnetic field at a given site are produced in part by wobble of the main geomagnetic dipole, in part by fluctuations in the intensity and direction of the non-dipole field, and in part by changes in the intensity of the main dipole field. These three processes combine to produce an angular variance that is strongly latitude dependent. A method is presented for isolating the contribution due to variation with latitude of the average intensity of the non-dipole field. The aspect of geomagnetic secular variation most accessible to palaeomagnetic measurement is the angular dispersion of the field over long periods of time. The amount of the angular dispersion is determined by two factors. One is the angular wobble of the main geomagnetic field. The other is the time average of the ratio flF of the non-dipole field to the dipole field. This time average is of general geophysical interest because it provides a measure of the time average of the magnetohydrodynamic processes in the region of the Earth's core beneath the site where the palaeomagnetic samples were collected. The time average of the nondipole field may exhibit a longitude dependence and a latitude (or zonal) dependence. Existence of the former implies that lateral differences exist in the boundary conditions at the core-mantle interface. Lateral temperature variations (Cox & Doell 1964) or lateral differences in relief along the core-mantle interface (Hide 1966) may lead to longitudinal differences in the operation of the geomagnetic dynamo and hence to longitudinal differences in the time average of the non-dipole field. Even if the core-mantle boundary conditions were completely uniform, however, the angular dispersion of the geomagnetic field would still vary with latitude for two reasons. The first is that the Coriolis forces of the geomagnetic dynamo vary with latitude, hence it is reasonable that the nondipole field may also vary. The second is that because of the geometry of the vector addition of dipole and nondipole fields on a sphere, the geomagnetic angular dispersion will vary with latitude even if the non-dipole field is the same at all latitudes. The objective of the present study is to develop models that describe the latitude dependence of geomagnetic

Geometry of the geomagnetic tail.

Abstract: An analysis of magnetic field measurements in the geomagnetic tail from Explorers 33 and 35 during 1967–1968 has shown that there is a broad region of depressed field magnitude approximately 12 RE thick and centered on the neutral sheet. The solar magnetospheric Bz component is proportionately larger within that region than outside of it. Bz is found to decrease with distance from the earth but to be positive on the average within the depressed field region out to a distance of 70 RE, indicating that although for short periods of time the neutral line may be closer to the earth than the orbital distance of the moon, on the average it is beyond that distance. A negative Bz component was found in ⅔ of the measurements outside the depressed field region. This analysis has shown that the magnetotail field diverges from the tail axis with distance from the earth. This result, together with the Bz observations, supports a geometry in which for large |Zsm| the tail field is diverging in the Zsm as well as the Ysm direction, but is converging slightly toward the neutral sheet within the depressed field region. The combination of expanding tail and reconnection at the neutral sheet can account for the observed tail field magnitude gradient. The Explorer 33 and 35 measurements also indicate that the average angle between the tail axis and the earth-sun line is 3.1°±1.5°.

Kelvin-Helmholtz instability and the semiannual variation of geomagnetic activity

Abstract: Kelvin-Helmholtz instability at magnetopause, initiating semiannual variation of geomagnetic disturbances

A three‐dimensional model current system for polar magnetic substorms

Abstract: A model current system, in which magnetospheric and ionospheric sections are connected by currents flowing along the geomagnetic field lines, is proposed to represent the current system responsible for polar magnetic substorms. The magnetic perturbations from model current systems of this type are studied in terms of elementary loops, whose magnetic effects are evaluated numerically. The influence of currents induced in the ground is studied for a time-independent case, but it is found that such effects do not change the gross character of the perturbation pattern. Using a north-south chain of stations and an appropriate coordinate system, the model predictions are compared with the magnetic variations observed during some substorms. It is found that the model is capable of explaining the gross features of the magnetic perturbation pattern observed. However, at times there are minor deviations from the predictions, which suggest the presence of additional ionospheric and magnetospheric current systems.

The ionospheric response to internal gravity waves: 1. The F2 region response

Abstract: The F2 region ionospheric response to individual internal gravity waves is calculated as a function of the azimuth of wave propagation. This response is shown to be highly anisotropic, with the anisotropy itself depending on the wave parameters, the geomagnetic dip, and the prevailing ionization density gradient. Because the ionization density gradient changes with time of day and season, the ionospheric response and hence the traveling ionospheric disturbance statistics themselves should exhibit corresponding diurnal and seasonal variations. The physical basis for the anisotropy of the ionospheric response is discussed.

Inversion of the geomagnetic induction problem

Abstract: (Communicated by Sir Edward Bullard, F.R.S.-Received 26 February 1969) An algorithm has been found for inverting the problem of geomagnetic induction in a concentrically stratified Earth. It determines the (radial) conductivity distribution from the frequency spectrum of the ratio of internal to external magnetic potentials of any surface harmonic mode. The derivation combines the magnetic induction equation with the principle of causality in the form of an integral constraint on the frequency spectrum. This algorithm generates a single solution for the conductivity. This solution is here proved unique if the conductivity is a bounded, real analytic function with no zeros. Suggestions are made regarding the numerical application of the algorithm to real data. A number of investigators (Chapman i919; Lahiri & Price I939; Rikitake I950; E:ckhardt, Larner & Madden I963) have made estimates of the electrical conductivity of the Earth as a function of radius by studying natural geomagnetic variations. Varying magnetic fields of external origin (i.e. those generated by electric currents in the ionosphere) induce eddy currents in the conducting Earth, which in turn produce magnetic fields of internal origin. The magnetic fields observed at the Earth's surface can be mathematically separated (Chapman & Bartels I940) into those of external origin and those of internal origin. More precisely, one can calculate from the available experimental observations the ratio of internal and external magnetic potentials over a range of frequencies for a number of different spatial distributions of source field. This observed induction response ratio has been used in an indirect way in the work mentioned above; the calculated behaviour of different conductivity models has been compared with the behaviour of the real Earth, and the best fitting model selected as the solution. This method provides geophysically useful information, but it lacks the elegance of a direct method. More important, one does not know if an appreciably different but untried conductivity model also fits the experimental data well.

Novel correlations between global features of the earth's gravitational and magnetic fields.

Abstract: There is a significant correlation between the Earth's gravitational field and the non-dipole part of the geomagnetic field, provided the latter is displaced in longitude.

Geomagnetic Deep Sounding and Upper Mantle Structure in the Western United States

Abstract: Summary Magnetic field time variations were observed in September 1967, with a two-dimensional array of 42 three-component variometers between latitudes 36" and 43" N and longitudes 101" W and 116" W. Fourier analysis of a polar substorm and of a storm shows that the former has a smooth spectrum and the latter a complex spectrum with many maxima. Upper mantle conductivity structure can be seen qualitatively in the original variograms, but is far more sharply defined in maps of Fourier spectral component amplitudes and phases. A ridge of high conductivity runs at a depth no greater than 200 km under the Southern Rocky Mountains between the Great Plains and the Colorado Plateau, which marks a low-conductivity region within the Cordillera. A strong conductivity anomaly runs north-south along the Wasatch Front through central Utah, and indicates the presence of an upwelling of highly conductive material at depth no greater than 120 km along the edge of a step structure which brings the conductive mantle to shallower depth under the Basin and Range Province than under the Colorado Plateau. Long-period maps from the storm suggest a rise in the conductive mantle between the northsouth structures, from the Colorado Plateau southward to the Basin and Range. The daily variation shows the conductivity structures and indicates their great extent in depth. The geomagnetic deep sounding anomalies are found to be in excellent agreement with existing heat flow data, and this supports correlation of electrical conductivity with temperature. There is also good correlation with the available seismic velocity information for the upper mantle. 1. Introdaction Geophysical observations of several kinds indicate that the upper mantle of the Earth under North America is laterally inhomogeneous. Upper mantle seismic velocities of 8.0 km s-' or larger are characteristic of the eastern United States and the Great Plains Province, while velocities decrease to values of 7.9 km s-' or lower west of the Rocky Mountains (Herrin & Taggart 1962). A similar pattern is shown by travel-time anomalies of seismic waves at vertical incidence. P and S waves arrive early at stations in the eastern United States; late arrivals are predominant in the western United States (Cleary & Hales 1966; Doyle & Hales 1967; Hemn & Taggart 1968). As the differences between the P travel-time residuals and the gravity anomalies in the central and western U.S. cannot be explained by the Birch (1961) relation between velocity and density, Hales & Doyle (1967) suggested that tempera

Wave‐induced fluctuations in ionospheric electron content: A model indicating some observational biases

Abstract: By assuming that a plane atmospheric wave acts on a plane-stratified F layer in the presence of a constant geomagnetic field, we derive an expression for the resulting perturbations in total columnar electron content as a function of wave and ionospheric-layer parameters and of the orientation of the column through the ionosphere. Although idealized, the model not only correctly predicts the order of magnitude of the observed perturbations but also makes visible much of the physics that determines the magnitude of wave-induced total-content fluctuations observed by using satellite transmissions through the ionosphere. Geomagnetic constraints on ion motion and a tendency for total-content fluctuation to be greatest when the ground-satellite path lies in a surface of constant atmospheric wave phase combine to introduce strong experimental biases that favor the observation of some waves and prevent the observation of others. The effects of these biases must be fully removed before total-content measurements can yield the statistical properties of atmospheric waves at ionospheric heights that may be useful in locating wave sources or in characterizing the dynamic state of the ionosphere.

Hydromagnetic oscillations of the geomagnetic tail and plasma sheet

Abstract: Dispersion equations are derived for hydromagnetic oscillations of the geomagnetic tail and plasma sheet by representing the tail as a long cylindrical current-vortex sheet and the sheet as a slab of hot plasma surrounded by cold, magnetized plasma. It is shown that a cylindrical geomagnetic tail is stable if the speed of the hot gas flowing outside the cylinder is less than the Alfven speed in the tail, and that the characteristic period is about 13 min for disturbances with wavelengths of the order of the diameter. The model of the plasma sheet yields periods of 6 min for the sausage mode, and 3 min for the kink mode, for wavelengths of the order of the thickness of the sheet.

A Method of Inferring the Direction of Drift of the Geomagnetic Field from Paleomagnetic Data

Abstract: Runcorn has shown that a moving magnetic source in the earth's core can cause the north pole of the magnetic vector at a fixed observatory on the earth's surface to rotate clockwise (when viewed along the vector toward its north-seeking end) for a westward motion of the source and counter-clockwise for an eastward motion of the source. Examination of the terrestrial magnetic record, which contains evidence of such vector rotations at many observatories, reveals no obvious contradictions to Runcorn's rule. Numerical calculations based on radial dipole source models of the non-dipole field show the theory can also account for the magnitude of the effect observed in the terrestrial record and, together with the results of simple tests with a spherical compass and a magnetic dipole, these calculations indicate that exceptions to the rule are rare, if indeed they exist at all. The theory therefore presents a potentially powerful tool for the analysis of the geomagnetic secular variation from paleomagnetic and archaeomagnetic data obtained from a single locality. As an example, it is shown that paleomagnetic results from Pleistocene sediments in Japan indicate the non-dipole field may have been drifting westward around 50, 000 years ago.

Separation of lunar daily geomagnetic variations into parts of ionospheric and oceanic origin.

Abstract: Summary The mechanism for the production of lunar daily geomagnetic variations is discussed. Making reasonable assumptions, it is shown that it is possible to separate the observed variations into parts of ionospheric and oceanic origin. The method is applied to results from six observatories in the British Isles, and the Ocean dynamo part is found to make an important contribution to the total lunar daily variation in all three geomagnetic elements.

Ionospheric Winds in the Auroral Zone

Abstract: : Observations of the drift velocity of rocket-borne chemical releases have been used to determine neutral wind velocity and ion velocity in the altitude range 90-230 km at ESRANGE, Kiruna. The ion cloud drift measurements have demonstrated the orthogonality of the simultaneous magnetic perturbation vector at ground level and the ion drift vector in the F region of the ionosphere. This result implies that in the auroral zone Hall currents are predominantly responsible for that part of the total magnetic perturbation seen at ground level due to ionospheric currents. A comparison of neutral wind velocity profiles obtained under quiet and disturbed geomagnetic conditions has shown that in the auroral zone the observed west-east neutral wind component above about 120 km altitude is directly correlated with the mean south-north magnetic perturbation vector over a period of two-three hours prior to the observations. This correlation is due to acceleration of the neutral atmosphere above 120 km by ions drifting in a meridional electric field. Both the ground magnetic perturbation and the neutral acceleration are direct functions of the locally magnetic perturbation and the neutral acceleration are direct functions of the locally enhanced electric field and of the electron (and hence ion) density. (Author)

Geomagnetic Intensity: Changes during the Past 3000 Years in the Western Hemisphere.

Abstract: A series of archeomagnetic measurements have been carried out on archeologic materials from Arizona and Mexico which can be compared with results from Europe and Asia This comparison shows a westward drift of geomagnetic intensity at a rate of about 024 degree per year Furthermore, an apparent coincidence between changes in the earth's magnetic moment and changes in the production rate of radiocarbon is observed

Storm-related wave phenomena observed at the synchronous, equatorial orbit

Abstract: Quasi-sinusoidal fluctuations of magnetic field during geomagnetic storms measured by ATS 1 in synchronous equatorial orbit

Experimental Observation of Magnetic Field Effects on VLF Propagation at Night

Abstract: Computations of nighttime field intensity versus distance are made for a 23.4-kHz signal radiated from Hawaii and for propagation paths to Seattle, Ontario (California), Samoa, and Wake Island. The computations were made by using the waveguide computer program developed at the Naval Electronics Laboratory Center to obtain waveguide mode constants, each 2° of arc or 222 km along each path. An exponential electron-density profile defined by β = 0.5 km−1 and h′ = 85.5 was assumed, where β and h′ are defined by Wait [1964]. The resultant field was computed by using a WKB approximation to allow for the variation of mode constants along the paths. Experimental measurements of 23.4-kHz signals from NPM were made aboard an airplane as it flew along these propagation paths. (NPM are the call letters for the Lualualei Navy Radio Station in Hawaii.) Good agreement was obtained, between the theoretical calculations and experimental measurements, strongly supporting the validity of the theoretical approach used (the β = 0.5, h′ = 85.5 km profile assumed) and the conclusion that the increased attenuation observed for propagation to the south is an effect of the geomagnetic field.

A new analysis of geomagnetic depth-sounding data from western Canada

Abstract: Time varying geomagnetic fields observed in southern British Columbia and Alberta are analyzed using optimum "transfer function" techniques for periods from 4 to 240 min. Estimates of the frequency and spatial dependence of the "coast effect" are obtained. Inland anomalies are delineated and attributed to variations in conductivity in the upper-to-middle crust coupled with an abrupt east–west transition in deeper conductivity. Regional attenuation of the uncorrelated portion of the vertical magnetic component is indicative of high conductivity near the crust mantle transition zone under the western part of the Cordillera. The conclusion is in agreement with magnetotelluric observations and other geophysical data.

Diffusive entry of solar‐flare particles into geomagnetic tail

Abstract: Solar-flare protons often enter the magnetospheric tail by a slow diffusion-like process. Solar-flare electrons, on the other hand, seem to enter rapidly, as would be consistent with either fast diffusion or direct entry along geomagnetic field lines that are connected to interplanetary field lines. We argue that the experimental results for both protons and electrons can be fit entirely to a diffusion model. The basic diffusion model utilizes a magnetospheric structure in which the tail is described as being composed of filaments, each having slightly different plasma and field properties. We show that it is possible to obtain energy-independent access of protons into such a tail model, at least over a broad range of proton energies. Diffusion results from the fluctuating fields seen by the protons in their cyclotron motion. This diffusion mechanism is ineffective for the electrons. The electrons enter further down the tail where the tail becomes flattened and the filaments intermingle with the interplanetary field lines. Depending on the circumstances, the latter mechanism could be important for proton entry as well. Access to the distant tail regions should be sensitive to the direction of the interplanetary field relative to the tail field, which we suggest accounts for the observed north-south polar-cap asymmetry. Merging of geomagnetic and interplanetary field lines could also become an important asymmetry mechanism when the size of the filaments become sufficiently small.

Rapid auroral luminosity fluctuations and geomagnetic field pulsations

Abstract: The relationship of 4278 A emission fluctuations and rapid geomagnetic field variations during two active disturbance periods at Byrd, Antarctca, has been studied in detail. These quasi-sinusoidal pulsation data have been compared with the simultaneous field changes at the conjugate location of Great Whale River, Canada. Common spectral characteristics of the light and field were examined. A five-phototube array has been used to identify large-scale auroral pulsation events and to separate them for analysis of stationary and moving forms. For the stationary group of pulsations, the geomagnetic field changes occur about 1.3 sec later than the auroral luminosity. This delay increases with increasing E-region electron density. It is reasoned that quasi-trapped electron bouncing is not important in determining the pulsation periodicity and that the luminosity is an indicator of the ionospheric arrival of a hydromagnetic wave, which is there converted to an electromagnetic disturbance. Travel time for an electromagnetic pulse through a conducting medium is computed and is shown to be consistent with the observed delays.

On the structure of the inner magnetosphere

Abstract: Morphology of thermal and energetic particles in inner magnetosphere during geomagnetic disturbances and solar cycles

Ionospheric Storms at Midlatitudes

Abstract: The total electron content of the ionosphere often responds in a dramatic way to increases in geomagnetic activity By monitoring the VHF signals from the geostationary satellite ATS 3, it has been possible to study in detail the very pronounced increases in total content often found during the afternoon hours on the day of the commencement of a magnetic storm Comparisons with magnetic field data show that the enhancements in electron content coincide with increases in the total magnetic field This simultaneity suggests that, when the magnetosphere is compressed during the initial phase of a storm, the ionization stored in the magnetic tubes of force may be dumped into the topside of the F region Such a depletion of the protonosphere is in agreement with whistler measurements, which indicate that a contraction of the plasmasphere occurs during periods of increased magnetic activity