Showing papers on "Earth's magnetic field published in 1977"

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.

Coronal holes and high-speed wind streams

Abstract: Coronal holes, regions of unusually low density and low temperature in the solar corona, are identified as Bartel's M regions, i.e., sources of high-speed wind streams that produce recurrent geomagnetic variations. Throughout the Skylab period the polar caps of the sun were coronal holes, and at lower latitudes the most persistent and recurrent holes were equatorial extensions of the polar caps. The holes rotated 'rigidly' at the equatorial synodic rate. They formed in regions of unipolar photospheric magnetic field, and their internal magnetic fields diverged rapidly with increasing distance from the sun. The geometry of the magnetic field in the inner corona seems to control both the physical properties of the holes and the global distribution of high-speed wind streams in the heliosphere. Phenomenological models for the birth and decay of coronal holes have been proposed.

Observation of an ionospheric acceleration mechanism producing energetic (keV) ions primarily normal to the geomagnetic field direction

Abstract: O/sup +/ ions with energies of approximately 1 keV have been observed flowing upward out of the ionosphere with a pitch angle distribution having a minimum along the magnetic field direction and maxima in about 130/sup 0/--140/sup 0/ range. The measurements were obtained with an energetic ion mass spectrometer experiment on the satellite 1976-65B at an altitude of about 7600 km in the northern dayside polar cusp. The data are interpreted as resulting from a mechanism which accelerates ambient ionospheric ions in a direction perpendicular to the geomagnetic field.

A region of intense plasma wave turbulence on auroral field lines

Abstract: This report presents a detailed study of the plasma wave turbulence observed by HAWKEYE-1 and IMP-6 on high latitude auroral field lines and investigates the relationship of this turbulence to magnetic field and plasma measurements obtained in the same region.

Magnetic holes in the solar wind

Abstract: An analysis is presented of high resolution interplanetary magnetic field measurements from the magnetometer on Explorer 43 which showed that low magnetic field intensities in the solar wind at 1 AU occur as distinct depressions or 'holes'. These magnetic holes are new kinetic-scale phenomena, having a characteristic dimension on the order of 20,000 km. They occurred at a rate of 1.5/day in the 18-day time span (March 18 to April 6, 1971) that was analyzed. Most of the magnetic holes are characterized by both a depression in the absolute value of the magnetic field, and a change in the magnetic field direction; some of these are possibly the result of magnetic merging. However, in other cases the magnetic field direction does not change; such holes are not due to magnetic merging, but might be a diamagnetic effect due to localized plasma inhomogeneities.

Triggering of substorms by solar wind discontinuities

Abstract: The probability of triggering of polar substorms by a large-scale magnetospheric compression associated with a discontinuity in the solar wind has been examined statistically using ground magnetogram data, AE index data and satellite geomagnetic data on 125 sudden storm commencements observed during 1967-1970. The triggering probability was found to depend on the amplitude of the sudden storm commencement and on the degree of preceding AE activity. In almost all cases the triggering occurred when the B-Z component of the interplanetary magnetic field was negative or decreasing during the 30 min before the passage of the discontinuity. Transient geomagnetic responses with a time scale of Alfven wave propagation in the polar cap also depend on interplanetary magnetic field conditions.

Near-bottom magnetic anomalies, asymmetric spreading, oblique spreading, and tectonics of the Mid-Atlantic Ridge near lat 37°N

Abstract: A detailed study of the magnetic anomalies of the Mid-Atlantic Ridge crest near lat 37 °N (FAMOUS) was conducted using a deeply towed instrument package. The most recent expression of the accreting plate boundary in rift valley 2 is an alternating series of linear central volcanoes and depressions that are marked by a sharp maximum in crustal magnetization only 2 to 3 km wide. Spreading in the FAMOUS area is highly asymmetric, with rates of 13.4 mm/yr to the east and 7.0 mm/yr to the west. At 1.7 m.y. B.P., the sense of asymmetry reverses in direction, with spreading faster to the west; this results in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Spreading in the FAMOUS area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate-boundary system. Spreading has been stably oblique for at least 8 m.y., even through a change in spreading direction. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been observed and may be due to old crust left behind or to recording of a geomagnetic field event. Crustal magnetization decays to 1/e its initial value in less than 0.6 m.y. The rapid decay may be facilitated by very intense crustal fracturing observed in the inner floor. Crustal magnetic sources may be approximated (mathematically) by a uniformly magnetized layer 700 m thick. Magnetic studies indicate that over 90 percent of the extrusive volcanism occurs within the rift inner floor and is extremely rare in the rift mountains. Magnetic anomaly transition widths vary from 1 to 8 km with time and appear to reflect a time-varying median-valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths), or it has a narrow inner floor and wide terraces — the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present rift valley 2 has a narrow inner floor and volcanic zone (1 to 3 km), whereas rift valley 3 is at the opposite end of the cycle with a wide inner floor and volcanic zone (9 to 11 km).

On the high correlation between long‐term averages of solar wind speed and geomagnetic activity

Abstract: Six-month and yearly averages of solar-wind speed from 1962 to 1975 are shown to be highly correlated with geomagnetic activity as measured by averages of the Ap index. On the same time scale the correlation between the southward component of the interplanetary magnetic field and geomagnetic activity is poor. Previous studies with hourly averages gave opposite results. The better correlation with the southward component on an hourly time scale is explained by its large variation compared with the relatively constant solar-wind speed. However, on a yearly time scale the magnitude of the variations in both parameters are about the same. This problem can be solved by invoking an energy transfer mechanism which is proportional to the first power of the southward component and a higher power of the solar-wind speed.

Seasonal and solar cycle variations of the zonal mean circulation in the thermosphere

Abstract: The seasonal and solar cycle variations of the zonal mean circulation (longitudinal or, equivalently, diurnal average) and the latitudinal variation of temperature in the earth's thermosphere are calculated by using a numerical model of the zonally symmetric thermosphere. The heat and momentum sources that drive the thermospheric circulation are solar EUV and UV heating, high-latitude heating primarily due to auroral processes, and a momentum source that results from the correlation of diurnal variations of wind and ion drag. The calculations show that the thermospheric circulation is mainly driven by heating due to the absorption of solar electromagnetic energy. However, it is modulated by a high-latitude heat source associated with auroral processes and related to geomagnetic activity. The seasonal variation of the thermosphere is characterized by a persistent solstice type of circulation that undergoes an abrupt transition within a week or two of equinox depending upon geomagnetic activity. The temperature difference from summer pole to winter pole is smaller and the circulation weaker during solar minimum than during solar maximum. To obtain agreement between the calculated and observed latitudinal variation of temperature and circulation, it is necessary to assume a high-latitude energy input during solstice of 2 × 1018 ergs s−1 for average solar maximum conditions and 4.5 × 1017 ergs s−1 for average solar minimum conditions.

Geomagnetic activity: dependence on solar wind parameters.

Abstract: Current ideas about the interaction between the solar wind and the earth's magnetosphere are reviewed. The solar wind dynamic pressure as well as the influx of interplanetary magnetic field lines are both important for the generation of geomagnetic activity. The influence of the geometry of the situation as well as the variability of the interplanetary magnetic field are both found to be important factors. Semi-annual and universal time variations are discussed as well as the 22-year cycle in geomagnetic activity. All three are found to be explainable by the varying geometry of the interaction. Long term changes in geomagnetic activity are examined.

Anomalies in the time‐averaged paleomagnetic field and their implications for the lower mantle

Abstract: The proposal that the time-averaged paleomagnetic field is not strictly that of a geocentric axial dipole but that of an axial dipole displaced slightly northward from the geocenter is examined in terms of spherical harmonic expansions. Standard procedures for spherical harmonic expansions such as are applied to the present instantaneous geomagnetic field are not necessarily applicable to paleomagnetic data. A technique is proposed that enhances the time-averaging process that is a necessary part of determining the paleomagnetic field. This involves analyzing the inclination anomaly ΔI around latitude strips to determine the zonal harmonics and the declination anomaly ΔD around longitude sectors to determine the first nonzonal harmonics. The technique is applied to a carefully selected data set covering the last 5 m.y. All data are divided into normal or reversed sets providing 266 land-based points (171 normal, 95 reversed) and 100 deep-sea core results with inclination data only (50 normal, 50 reversed). These data demonstrate clearly that the time-averaged field is not simply that of a geocentric axial dipole and also that it is different for the normal and reversed fields. With respect to the present field the zonal harmonics of the time-averaged field are reduced less than the nonzonal harmonics, the indication being that westward and/or eastward drift of the nondipole field is dominant over 5 m.y. The magnitude of the second zonal harmonic suggests that on the average, paleomagnetic poles could be in error by about 3° when calculated by the usual geocentric axial dipole assumption. The data show clear asymmetries between the northern and southern hemispheres and possibly between the oceanic (Pacific) and continental hemispheres. Possible explanations of these asymmetries include variations in topography and/or temperature at the core-mantle interface. Large-scale asymmetries in the boundary conditions at this interface could affect the magnetic field through alteration of the convection pattern in the core. The asymmetries correlate with other global asymmetries such as the distribution of continents and oceans and the locations of subduction zones. They also apparently correlate with seismic velocity anomalies observed in the lower mantle. All these may reflect large-scale inhomogeneities related to dynamic processes that have occurred in the lower mantle. Although all the Gaussian coefficients determined change sign when the axial dipole field reverses, there are significant differences between the normal and reversed fields, suggesting that the distribution of sources for these fields is not identical. This is consistent with the cyclonic convection models for reversal of the geomagnetic field proposed by Parker and Levy.

Geomagnetic excursions and their paleomagnetic record

Abstract: The paleomagnetic record of recent lake and marine sediments contains anomalous magnetic directions which have been interpreted as large-scale fluctuations of the geomagnetic field (excursions). In view of the many ways in which isolated distortions of the paleomagnetic recording process can arise, proof of the existence of an excursion must depend on consistent results from within a given lake or marine environment as well as from adjacent sedimentary environments. A critical review of the proposed excursions reveals that although some excursions probably did occur, there is not yet sufficient evidence to confirm the existence of many of the claimed excursions. When the existence of regional or global excursions is established, additional information will become available on the nature of the geomagnetic field, on the use of excursions as time-stratigraphic horizons, and on possible correlations between geomagnetic activity and climatic change.

Solar-Terrestrial Physics

Abstract: Magnetic observatories monitor continuously the geomagnetic field at various latitudes and longitudes of the Earth. With the advent of rockets and satellites, it has been studied also in space.

Studies of solar magnetic fields. V - The true average field strengths near the poles

Abstract: An estimate of the average magnetic field strength at the poles of the Sun from Mount Wilson measurements is made by comparing low latitude magnetic measurements in the same regions made near the center of the disk and near the limb. There is still some uncertainty because the orientation angle of the field lines in the meridional plane is unknown, but the most likely possibility is that the true average field strengths are about twice the measured values (0–2 G), with an absolute upper limit on the underestimation of the field strengths of about a factor 5. The measurements refer to latitudes below about 80°.

Thermal interaction of the core and the mantle and long‐term behavior of the geomagnetic field

Abstract: The effects of temperature changes at the earth's core-mantle boundary on the velocity field of the core are analyzed It is assumed that the geomagnetic field is maintained by thermal convection in the outer core A model for the thermal interaction of the core and the mantle is presented which is consistent with current views on the presence of heat sources in the core and the properties of the lower mantle Significant long-term variations in the frequency of geomagnetic reversals may be the result of fluctuating temperatures at the core-mantle boundary, caused by intermittent convection in the lower mantle The thermal structure of the lower mantle region D double prime, extending from 2700 to 2900 km in depth, constitutes an important test of this hypothesis and offers a means of deciding whether the geomagnetic dynamo is thermally driven

Evidence of magnetospheric cusp proton acceleration by magnetic merging at the dayside magnetopause

Abstract: We have analyzed the energy-latitude dispersion of protons observed in the low-altitude dayside cusp on two orbits of the AE-C satellite in an attempt to reconstruct the velocity distribution of the source plasma. On the assumption that the source distribution has the form of a convecting bi-Maxwellian we have inferred the source values of V/sub parallel/(field-aligned flow velocity), T/sub parallel/(temperature parallel to the magnetic field) and (T/sub parallel /Tperpendicular/) n, where n is the number density. The inferred source parameters are consistent with a single-particle model of acceleration and injection through magnetic merging at the dayside magnetopause, provided that n is significantly smaller than the nominal fluid dynamic magnetosheath values and the ratio (T/sub parallel /Tperpendicular/) is significantly less than 1. Both of these conditions are expected as consequences of the magnetopause depletion layer effect described by Zwan and Wolf (1976) and Crooker and Siscoe (1977).

Polarity Transition Records and the Geomagnetic Dynamo

Abstract: The Parker-Levy approach to reversals of the geomagnetic field predicts meridional transitional paths of the virtual geomagnetic pole (VGP) which pass either through the site of observation or through its antipode, depending upon the site location and the sense of the polarity transition. Comparison with the most detailed transitional VGP path records presently available gives some indication of the above behavior as predicted by the Parker-Levy model. Discrepancies may be due to complexities in the distribution of cyclonic convection cells in the core not considered in the formal mathematical treatment. The predicted variation in transitional field intensity experienced at any given site also is compatible with several reported transition records.

Evidence for the control of Pc 3,4 magnetic pulsations by the solar wind velocity

Abstract: The amplitude of Pc 3,4 magnetic pulsations at Calgary is shown to increase as the solar wind velocity increases The pulsation amplitude dependence on solar wind velocity can be understood in terms of the Kelvin-Helmholtz instability at the magnetopause boundary

Use of the earth's magnetic field by orienting cave salamanders ( Eurycea lucifuga )

Abstract: Two groups ofEurycea lucifuga were trained to move bidirectionally within separate training corridors by alternately supplying moisture to limestone-filled compartments located at each end. Both corridors were aligned horizontally along the magnetic North-South axis of the earth. One corridor was enclosed within a cube coil which rotated the magnetic field horizontally 90° clockwise, so that the group contained in this corridor moved perpendicular to the North-South axis of the magnetic field. The other corridor was in the normal earth's field so that this second group moved parallel to the horizontal North-South component of the magnetic field. Testing involved releasing both groups in the center of a cross-shaped testing assembly made up of the two training corridors. The two groups were confined together in a release device for 60 min before release. In several tests the two groups were significantly oriented (P<0.05) along the appropriate trained axes with respect to the normal or altered magnetic field. Movements in these tests did not reflect a consistent response to any other potential source of orientation cues (P<0.001). It is concluded that cave salamanders are able to (1) perceive the earth's magnetic field and (2) exhibit a directional response with respect to a magnetic field which indicates a learned relationship to the environment.

Explorer 45 and Imp 6 observations in the magnetosphere of injected waves from the Siple Station VLF transmitter

Abstract: Results are reported for an experiment in which VLF waves from a transmitter in Antarctica were injected into the magnetosphere along geomagnetic field lines and detected near the magnetic equatorial plane by high-altitude spacecraft. The purpose of this experiment was to conduct a controlled in situ study of VLF wave-particle interactions and to determine the propagation characteristics of the injected waves in the magnetosphere, the regions where VLF emissions are produced, and the effective volume of the magnetosphere illuminated by the transmitter. The results indicate that: (1) the bulk of the satellite receptions occurred during periods of quieting following magnetic disturbances, (2) receptions generally occurred inside the plasmapause, (3) the spacecraft detected predominantly unducted waves, (4) the injected signals could illuminate a large volume of the magnetosphere, and (5) VLF emissions were triggered by nonducted transmitter pulses.

The magnetic field of Mercury

Abstract: The geomagnetic field, suitably scaled down and parameterized, is shown to give a very good fit to the magnetic field measurements taken on the first and third passes of the Mariner 10 space probe past Mercury. The excellence of the fit to a reliable planetary magnetospheric model is good evidence that the Mercury magnetosphere is formed by a simple, permanent, intrinsic planetary magnetic field distorted by the effects of the solar wind. The parameters used for a best fit to all the data are (depending slightly on the choice of data) 2.44--2.55 for the ratio of Mercury's magnetic field strength at the subsolar point to that of the earth's subsolar point field (this results in a dipole moment of 170 ..gamma..R/sub M//sup 3/ (R/sub M/ is Mercury Radius), i.e., 2.41 x 10/sup 22/G cm/sup 3/ in the same direction as the earth's dipole), approx.-113 ..gamma..R/sub M//sup 4/ for the planetary quadrupole moment parallel to the dipole moment, 10degree--17degree for the tilt of the planet dipole toward the sun, 4.5degree for the tilt of the dipole toward dawn, and 2.5degree--7.6degree aberration angle for the shift in the tail axis from the planet-sun direction because of the planet's orbital velocity. The rmsmore » deviation overall for the entire data set compared with the theoretical fitted model for the magnetic field strength was 17 ..gamma.. (approx.4% of the maximum field measured). If the data from the first pass that show presumed strong time variations are excluded, the overall rms deviation for the field magnitude is only 10 ..gamma.. (approx.2.5% of the maximum field measured).« less

A pictorial comparison of interplanetary magnetic field polarity, solar wind speed, and geomagnetic disturbance index during the sunspot cycle

Abstract: Observations of interplanetary magnetic field polarity, solar wind speed, and geomagnetic disturbance index (C9) during the years 1962–1975 are compared in a 27-day pictorial format that emphasizes their associated variations during the sunspot cycle. This display accentuates graphically several recently reported features of solar wind streams including the fact that the streams were faster, wider, and longer-lived during 1962–1964 and 1973–1975 in the declining phase of the sunspot cycle than during intervening years (Bame et al., 1976; Gosling et al., 1976). The display reveals strikingly that these high-speed streams were associated with the major, recurrent patterns of geomagnetic activity that are characteristic of the declining phase of the sunspot cycle. Finally, the display shows that during 1962–1975 the association between long-lived solar wind streams and recurrent geomagnetic disturbances was modulated by the annual variation (Burch, 1973) of the response of the geomagnetic field to solar wind conditions. The phase of this annual variation depends on the polarity of the interplanetary magnetic field in the sense that negative sectors of the interplanetary field have their greatest geomagnetic effect in northern hemisphere spring, and positive sectors have their greatest effect in the fall. During 1965–1972 when the solar wind streams were relatively slow (500 km s-1), the annual variation strongly influenced the visibility of the corresponding geomagnetic disturbance patterns.

Electric and magnetic fields in the high-latitude magnetosphere

Abstract: The configuration of high-latitude electric and magnetic fields is reviewed. Various results suggest that high-latitude magnetic field lines from the outermost regions of the dayside magnetosphere converge toward a point near the noon meridian. Plasma flows, the midday cusp, and a dawn-dusk electric field across the polar cap are characterized. The electric fields associated with plasma flows produce Hall currents on the polar cap which vary with sector structure. Some evidence indicates that polar cap convection may reverse during intervals of strong northward interplanetary field. It is concluded that most observations are consistent with an open field magnetosphere model.

On propagation direction of ring current proton ULF waves observed by ATS 6 at 6.6 RE

Abstract: From June 11 to September 16, 1974, the NOAA low-energy proton detector on board the ATS 6 satellite observed 71 cases of ultralow-frequency oscillations of proton flux intensities. The oscillation periods varied from 40 s to 6 min, and the events were observed most frequently during moderate geomagnetic conditions. The flux oscillations occurred at various local times, yet almost two thirds of the events were detected in the near-dusk region of the magnetosphere. For a majority of the events in this set a substantial phase shift in flux oscillation was detected between different energy channels and/or between two oppositely oriented detector telescopes. The phase shift is mainly due to the finite gyroradius effect of the protons gyrating in the geomagnetic field. By examining this finite gyroradius effect on the perturbed particle distribution function associated with the wave in a nonuniform magnetic field, the propagation direction of the wave from particle observations made by a single spacecraft is determined

Reverse polarity field‐aligned currents at high latitudes

Abstract: The magnetometer on the Isis 2 satellite is used to study a number of magnetic field perturbations which are interpreted as being due to field-aligned currents whose directions are opposite to the direction usually observed. The perturbations are observed mainly on the dawn side of the earth and can be interpreted as a result of two adjacent and oppositely directed current sheets in which the equatorward current is into the atmosphere and the poleward current out of the atmosphere. These perturbations are observed only at times when the interplanetary magnetic field (IMF) has a strong northerly component, and they are found at latitudes above those usually associated with field-aligned currents. The locations of the magnetic field perturbations are compared with simultaneous particle measurements, and the results are discussed in relation to what might be expected from field line merging models.

Geomagnetic storms and electric fields in the equatorial ionosphere

Abstract: LARGE variations of the geomagnetic field observed at ground level during a geomagnetic storm are generally attributed to various magnetospheric changes following the arrival of the solar plasma ejected during the solar flare. Using the direct measurements of equatorial electric field during a geomagnetic storm, it is shown here that the large decrease in the field observed near the dip equator is due to the reversal of the equatorial electrojet current. This event is caused by the imposition of an additional westward electric field on the equatorial ionosphere which was originated by the interaction of solar wind with the interplanetary magnetic field.

The Mean Magnetic Field of the Sun: Method of Observation and Relation to the Interplanetary Magnetic Field,

Abstract: The mean solar magnetic field as measured in integrated light has been observed since 1968. Since 1970 it has been observed both at Hale Observatories and at the Crimean Astrophysical Observatory. The observing procedures at both observatories and their implications for mean field measurements are discussed. A comparison of the two sets of daily observations shows that similar results are obtained at both observatories. A comparison of the mean field with the interplanetary magnetic polarity shows that the IMF sector structure has the same pattern as the mean field polarity.