Showing papers on "Earth's magnetic field published in 1969"

Standing Alfvén waves in the magnetosphere

Abstract: Transverse LF oscillations in geomagnetic field observed during January 1967, noting possibility of being second harmonic of magnetospheric standing Alfven waves

Geomagnetic Variations and the Electrical Conductivity of the Upper Mantle

Abstract: Summary The electrical conductivity of the upper mantle can be determined by comparing the measured response of the Earth to magnetic variations of all frequencies with the theoretical response of particular conductivity distributions. On the basis of a limited amount of data the response has been estimated at frequencies in the range 0.003 to 0.25 c day−1. In this range of the geomagnetic spectrum, line spectra at frequencies of 1 and 2 c yr−1 and 1, 2, and 3 cycles per 27 days can be used. Investigations of the continuum spectrum show that it also occurs on a worldwide scale, and must correspond to a real geophysical process. Meaningful estimates of the response can therefore be made over the whole of the frequency range considered. The entire magnetic variation spectrum in the range 2 c yr−1 to 0.25 c day−1 appears to be generated by fluctuations in the strength of the ring current, and a P10 spherical harmonic adequately describes the variation of the magnetic field over the surface of the Earth.

An improved model equatorial electrojet with a meridional current system

Abstract: Construction of model equatorial electrojet by spherical harmonic expansion for geomagnetic field

Pressure balance between solar wind and magnetosphere

Abstract: An evaluation of the subsolar pressure balance between the solar wind and the geomagnetic field shows that the average proton density of the quiet-day solar wind (300–400 km/sec) should be between 6 and 10 p/cm³. Even during storm times the proton density should always be between 2 p/cm³ and 70 p/cm³. The relation between the interplanetary solar-wind parameters and the stagnation pressure is reviewed. The subsolar geomagnetic field, including the quiet-day ring current field, is evaluated as a function of subsolar distance. The quiet-day ring current is based on the dipole gradient-drift motion of the low-energy protons observed by Davis and Williamson and by Frank. This quiet-day ring current has a magnetic moment of 0.26 ME and produces a 41-γ decrease at the earth's surface. A geomagnetic field normalization of observed boundary distances is also proposed to remove the effects of the dipole tilt to the solar wind.

Dynamo currents, winds, and electric fields

Abstract: After a brief presentation of Sq and L electric current systems deduced from geomagnetic data, electric conductivities and wind models in the dynamo region are discussed. It is then shown that the solar negative-mode thermal diurnal tide and the lunar semidiurnal gravitational tide having a phase shift with altitudes produce the best fit Sq and L current systems, respectively. Distributions of electrostatic fields are computed, and electromagnetic drift speeds in the ionospheric F region are also examined. These calculated values agree well with observed results.

Physics of the earth.

Abstract: Preface 1. Origin and history of the Solar System 2. Composition of the Earth 3. Radioactivity, isotopes and dating 4. Isotopic clues to the age and origin of the Solar System 5. Evidence of the Earth's evolutionary history 6. Rotation, figure of the Earth and gravity 7. Precession, wobble and rotational irregularities 8. Tides and the evolution of the lunar orbit 9. The satellite geoid, isostasy and post-glacial rebound 10. Elastic and inelastic properties 11. Deformation of the crust: rock mechanics 12. Tectonics 13. Convective and tectonic stresses 14. Kinematics of the earthquake process 15. Earthquake dynamics 16. Seismic wave propagation 17. Seismological determination of Earth structure 18. Finite strain and high pressure equations of state 19. Thermal properties 20. The surface heat flux 21. The global energy budget 22. Thermodynamics of convection 23. Thermal history 24. The geomagnetic field 25. Rock magnetism and paleomagnetism 26. Alternative energy sources and natural climate variations: some geophysical background Appendix A. General reference data Appendix B. Orbital dynamics (Kepler's laws) Appendix C. Spherical harmonic functions Appendix D. Relationships between elastic moduli of an isotropic solid Appendix E. Thermodynamic parameters and derivative properties Appendix F. An Earth model: mechanical properties Appendix G. A thermal model of the Earth Appendix H. Radioactive isotopes Appendix I. A geological time scale 2004 Appendix J. Problems References Index.

Causes of sudden commencements and sudden impulses.

Abstract: Worldwide changes in earth magnetic field determined by examination of magnetic field and interplanetary plasma data for solar wind near earth

Conjugacy of visual auroras during magnetically quiet periods

Abstract: The spatial and temporal conjugacy of auroras at high latitudes has been investigated by flying two NC-135 jet aircraft, equipped with all-sky and narrow-field cameras, along magnetically conjugate paths crossing the northern and southern auroral zones along the dipole 256° meridian. The flights were centered about magnetic midnight on March 12, 14, and 16, 1967. Magnetic activity was very low throughout this period; nevertheless, auroras were observed between dp latitudes 66° and 71° during each flight. A visual examination of all the data and a quantitative analysis of selected film frames indicate that, within the limits of error in the measurements and aircraft navigation, all the observed auroras were conjugate according to the GSFC 12/66 model of the main geomagnetic field. In most cases when the relative positions of aircraft and aurora were the same in both hemispheres, the conjugate auroras exhibited remarkably detailed similarity (point conjugacy) in their shape, intensity, and in their spatial and temporal variations. There is a tendency for the southern auroras to be located slightly eastward of the northern conjugate auroras. Such a displacement appears to be consistent with a slight asymmetric distortion of the main geomagnetic field due to solar wind pressure and earth dipole-sun geometry at the date and time of the measurements. These results suggest that, during magnetically very quiet times, auroras located at dp latitudes up to 71° are caused by a symmetrical injection of electrons and protons near the equatorial plane and onto magnetic field lines that are closed, stable, and relatively undistorted by external magnetic fields. Preliminary results from similar conjugate flights conducted during March 1968, a magnetically disturbed period, are briefly described. A visual inspection of these data has revealed substantial deviations in auroral conjugacy and a considerably more complex behavior of particle precipitation than that occurring during magnetically quiet periods.

The shape of the tilted magnetopause

Abstract: A model for the determination of the shape of the magnetopause is developed that permits the inclusion of cases where the solar wind is directed obliquely toward the geomagnetic dipole axis. The solar wind is assumed to have no motions perpendicular to its (constant) velocity vector (zero temperature approximation) and to be free of magnetic fields. The region within the magnetopause is characterized as having only one magnetic source and as being free of plasma. The procedure used here is in many ways similar to the self-consistent field method developed by Mead and Beard to calculate the shape of the magnetopause when the solar wind flow is perpendicular to the dipole axis. They were able to find the surface shape (for perpendicular incidence) at one point at a time. In the present study, however, it is necessary to determine the position of the surface at several points at a time. The geocentric distance to the subsolar point is found to be largest for perpendicular incidence of the solar wind on the dipole axis. The cross sections of the tail of the magnetopause given by the model are not cylindrical but elongated in the direction perpendicular to the ecliptic plane. Generally, the boundary is very dependent upon the wind-dipole angle in the region of the neutral points but exhibits very little dependence on this angle in ‘equatorial’ regions.

Sudden commencement associated discontinuities in the interplanetary magnetic field observed by IMP 3

Abstract: The magnetic field measurements made by the magnetic field experiment on the IMP 3 (Explorer 28) spacecraft have been examined at the time of geomagnetic s.s.c. events. Thirty-six such events occurred while IMP 3 was in the interplanetary medium during 1965, 1966 and 1967 and have been analysed. Of these events 8 must have been tangential discontinuities, 2 are either tangential discontinuities or rotational discontinuities and 26 are possible shock waves. (Two of these 26 events have been shown by other authors to be shocks.) These 26 possible shocks have similar magnetic signatures: an increase of 20 % or more in the magnetic field magnitude and a relatively small (always less than 90°) change in direction. The larger s.s.c. events were more likely to be caused by possible shocks while the smaller events were often associated with tangential discontinuities. The orientation of the discontinuity surfaces of the 26 possible shocks shows a preference to be aligned somewhere between a direction perpendicular to the sun-earth line and a direction tangent to the local spiral angle of the magnetic field. It was possible to associate solar flares with 14 of the 26 possible shock events. Of these 14, a reliable orientation was deduced for 8 events. By considering the orientation of these 8 events in relation to the position of the parent flares on the solar disk it is suggested that a typical shock front propagating out from the sun has a radius of curvature less than but of the order of 1 AU.

The Occasional Reversal of the Geomagnetic Field

Abstract: Cyclonic convective cell fluctuation and nonuniform core rotation effects on reversal of geomagnetic field, investigating fossil magnetism

VLF measurements of the Poynting flux along the geomagnetic field with the Injun 5 satellite

Abstract: Poynting flux direction for proton whistlers determined from Injun 5 observations, obtaining data on source region and propagation in ionosphere

The geomagnetic tail.

Abstract: Satellite observations of geomagnetic tail in magnetosphere near midnight meridian plane, discussing formation, shape, plasma sheet and models

Non-Dipole Behaviour During an Upper Miocene Geomagnetic Polarity Transition in Oregon

Abstract: Summary The palaeomagnetism of a section of 71 successive lavas from southeastern Oregon is presented. A geomagnetic polarity change from reversed to normal polarity which took place during the extrusion of the section 15 million years ago has allowed a palaeomagnetic examination of the non-dipole field during a period of diminished dipole intensity. During the polarity transition, there existed an apparent easterlydrifting non-dipole component. Together with an independent determination of the palaeo-intensity on one lava (Coe 1967) and other recently presented data (Goldstein et al. 1968) an application of Irving & Ward's(1964) geomagnetic field model to the data suggests a maximum dipole intensity as low as two per cent of the present value for a period of not less than 100 years during the transition, outside of which the data are consistent with some present-day ratios of non-dipole to dipole field intensity. A very short period of diminished relative dipole intensity occurred during the final stages of the polarity change, when the virtual geomagnetic pole returned to very low latitudes, without any polarity change actually occurring. Because of a great similarity with an independently presented result, it is suggested that this ‘rebound effect’ may reflect a fundamental aspect of the mechanism of at least some polarity changes. Comparison with estimates of the duration of some more recent geomagnetic polarity transitions suggests that the section may have accumulated in less than 50000 years. The lavas of normal polarity, which comprise most of the section, do not show any preferred association with lower or higher oxidation states.

On the Polar‐Wander Path for Australia during the Cenozoic

Abstract: Summary Detailed palaeomagnetic measurements potassium-argon and (K-Ar) dating have been carried out on continuous lava sequences in the Nandewar, Liverpool and Barrington Volcanoes of New South Wales. Consistent K-Ar ages in each case indicate that the volcanoes were built rapidly at 17.5±0.3 m.yr (Nandewar), 33.7±0.7m.yr (Liverpool) and 51.6±0.7 m.yr (Barrington), where the errors quoted are the standard deviations. Most of the 150 flows investigated show stable directions of magnetization after magnetic cleaning. Several field reversals are noted and in one instance a series of nine flows records the transition zone from reversed to normal polarity. The frequency and timing of the reversals are consistent with the Cenozoic geomagnetic time scale deduced from the interpretation of marine magnetic anomalies in terms of the sea-floor spreading hypothesis around 17 and 34 m.yr, but not at 52 m.yr. It appears that the extrapolations made in its derivation may no longer be valid at that time. The three new precisely determined palaeomagnetic poles add detail to the Cenozoic polar-wander path for Australia. A steady movement of the South Pole at about 0.45° per m.yr (∼ 5 cm yr−l) is indicated for the past 100 m.yr. This movement is not consistent with a simple northward movement of Australia as inferred from sea-floor spreading data. It indicates in addition a westward excursion of the South Pole, which is attributed to a component of polar wandering superimposed upon the continental drift.

Geomagnetic data and core motions

Abstract: Backus has observed that infinite core conductivity implies the vanishing of the time derivative of the magnetic flux through any patch on the core surface bounded by a 'null-flux curve', on which the radial magnetic field vanished. Field model GSFC (12/66) is consistent with this criterion only if features with scales smaller than angular degree 7 are important or if the dipole time derivatives are deleted. Deleting the dipole is reasonable if the dipole decays in its 3rd radial mode or core conductivity is 4 x 4$^4$ mho/m. If the data admit infinite conductivity, Backus has also shown that there is an infinite-dimensional affine space of 'eligible' surface velocity fields which will produce the observed secular variation from the observed geomagnetic field, but that at any point on any null-flux curve all eligible flows have the same component normal to the curve. Using only secular variation harmonic coefficients with angular degrees 2 to 6, we obtain velocity components normal to the null-flux curves which are compatible with primarily latitude-dependent westward drift, but not with the velocity field recently proposed by Kahle, Ball & Vestine (1967b).

Grid values of total magnetic intensity igrf-1965.

Abstract: : As a result of the combined efforts of various groups within the International Association of Geomagnetism and Aeronomy, an International Geomagnetic Reference Field was agreed upon in October 1968. The IGRF, consisting of 80 spherical harmonic coefficients, provides a common datum suitable for various applications in the analysis of the earth's magnetic field such as the removal of trend from detailed geomagnetic surveys. Included in this publication are: Tables of total magnetic intensity (F) and annual change (F) at every 2 degree grid intersection of latitude and longitude for the entire earth, small scale magnetic charts of F and F, and a computer program to derive all the magnetic components and rates of annual change from the IGRF coefficients. (Author)

Paleomagnetism of the Kau Volcanic Series, Hawaii

Abstract: Paleomagnetic studies of 54 superposed lavas from the northwest wall of Mokuaweoweo, Mauna Loa, Hawaii, show that changes in direction of the geomagnetic field were very small during the time of extrusion of these lavas. After reasonable estimates are made for the time involved (a period of several thousands of years, occurring sometime after 10,000 years ago), it is tentatively concluded that secular variation due to nondipole field changes in the central Pacific Ocean is very small and can be described by an angular deviation for virtual poles of only 0.05 radians (3°). In combination with other Hawaiian paleomagnetic studies representing longer time intervals, this leads to an estimated angular deviation of 0.20 radians (12°) for ‘wobble’ of the main dipole during the Brunhes magnetic polarity epoch.

FURTHER EVIDENCE ON THE CORRELATION BETWEEN TRANSVERSE FLUCTUATIONS IN THE INTERPLANETARY MAGNETIC FIELD AND Kp.

Abstract: Geomagnetic variability relations to interplanetary magnetic field transverse fluctuations, discussing data from Mariner flights

Hydromagnetic shocks in the solar wind

Abstract: The Rankine-Hugoniot relations are applied to shock-like discontinuities measured by both magnetic field and plasma instruments on the satellite Explorer 34 between May 30, 1967 and Jan. 11, 1968. Shock normals were either determined from the magnetic field observations, or from the times of occurrence of the discontinuity at Explorers 33, 34 and 35. The Rankine-Hugoniot relations are obeyed to the accuracy of the observations, and the values of shock velocities, density ratios, and Mach numbers indicate that at 1 AU the typical interplanetary shock is not strong, although all the events studied caused geomagnetic impulses.