Showing papers on "Earth's magnetic field published in 1974"

Spatial Power Spectrum of the Main Geomagnetic Field, and Extrapolation to the Core

Abstract: Summary The spatial ‘power’ spectrum of the main geomagnetic field has been estimated for harmonics up to n= 500. It is shown to consist of two components, long wavelengths being dominated by fields originating in the core, and short wavelengths by fields originating in the crust; the cross-over occurs at n≥ 11, a wavelength ≤ 3600 km. The core field is often approximated by a set of spherical harmonic coefficients. It is shown that at present main field coefficients for n≤ 9, and secular variation coefficients for n≤ 6, are not known with significant accuracy. Estimates are made of the standard deviations of the IGRF coefficients, and the standard deviation of the IGRF field deduced. This field is known to about 0.5 per cent at the surface but only to about 10 per cent at the core. Its time variation is known only to about 20 per cent at the surface, and is very uncertain at the core.

Incoherent scatter measurements of E region conductivities and currents in the auroral zone

Abstract: Data taken by incoherent scatter radar have been used to investigate ionospheric conductivities and electrical currents. During quiet days, the conductivities appear to vary in a way consistent with ionization arising from solar EUV radiation. In the evening hours, enhancements in the northward electric field are found to precede small increases in the conductivities. Strong enhancements of the Hall conductivity relative to the Pedersen conductivity occur during negative bays when the electric field is in a southwestward direction. The ionospheric currents calculated in the geomagnetic east-west direction are in good agreement with the H component measured by a nearby magnetometer; this result indicates that the current causing the ground level magnetic fluctuations is a broad horizontal sheet current. The north-south ionospheric current, however, consistently disagrees with the observed D component in a manner that cannot easily be explained unless currents parallel to the earth's magnetic field are present.

Mapping the local time‐universal time development of magnetospheric substorms using mid‐latitude magnetic observations

Abstract: Contour maps of mid-latitude geomagnetic perturbations of both north-south and east-west field components have been created to study the variability of magnetospheric substorm development. The maps were obtained by subtracting quiet day variations from observations along a longitudinal chain of observatories; this procedure created smooth interpolations of the resulting local time profiles at equal increments of universal time. The results were then contoured in the local time-universal time plane. To isolate magnetic perturbations during substorm expansions, ‘normalized’ maps were created by first subtracting the local time profile at the onset of the expansion from all later profiles. Examples of substorm variability have been chosen to demonstrate multiple onsets and movement of bay activity either eastward or westward from the meridian of initial onset. The local time profile obtained from the normalized map at the peak of the expansion represents the mid-latitude effects of the final substorm current systems. These profiles match the magnetic effects of a simple model in which a portion of the tail current is diverted through the auroral ionosphere via field lines. These profiles are further used to compute substorm parameters that include the meridian of onset, the meridian of maximum bay development, the magnitude, and extent of the event. Such quantification allows for better description of substorm features than has previously been available.

The origin and propagation of chorus in the outer magnetosphere

Abstract: Wave normals of chorus in the outer magnetosphere have been determined for the first time from data obtained with the Ogo 5 search coil magnetometer. These measurements combined with simultaneous information concerning geomagnetic field, plasma density, and the electron energy and pitch angle distributions provide a consistent picture of the generation, propagation, and subsequent damping of chorus in agreement with theory. Specifically, the data are consistent with chorus generation within 25 deg of the equatorial plane on the dayside and within 2 deg on the nightside. Chorus is generated by a Doppler-shifted cyclotron resonance with electrons between 5 and 150 keV but only when the pitch angle distribution is peaked at 90 deg to the local magnetic field and the anisotropy exceeds a critical value.

Geomagnetic reversals and crustal spreading rates during the Miocene

Abstract: Statistical analysis of the geomagnetic time scale suggests that the rate of reversals was anomalously low during the Miocene. To determine whether undetected reversals actually occurred in the Miocene, 14 magnetic profiles from a survey of the northeast Pacific by NOAA are analyzed by signal-averaging techniques. The data suggest that during the time period 7.3 to 22.7 m.y., eight short-wavelength anomalies are present that are not predicted by previously published time scales. Anomaly 5 in particular, originally considered a major polarity epoch, is actually composed of numerous shorter events. Additional data, both from the east Pacific rise and the Indian-Antarctic ridge, corroborate these observations. A statistical study of the spreading rates of all the data indicates apparent accelerations of the order of 30 km/m.y.2 for the Pacific, Indian, and antarctic tectonic plates. These abrupt changes in spreading rate are considered artifacts due to errors in the published time scale, and a new Miocene time scale is derived on the assumption of constant spreading rates for the three plates. The new time scale contains 18 additional reversals and 14 short events ranging from 10,000 yr to 0.1 m.y. in length. The widths of the transition zones from normal to reversely magnetized crust are determined to be 6 km for the northeast Pacific data, which is consistent with a recently proposed two-layer crustal model.

Normal fluctuations in the earth's magnetic field influence pigeon orientation

Abstract: In an attempt to determine whether naturally occurring fluctuations in the earth's magnetic field influence homing pigeons' initial bearings, three series of test releases (1970, 1972, 1973) at a site 45.7 miles north of the loft were conducted under an experimental design that controlled for most extraneous variables. The mean bearings for each series showed a significant inverse correlation with the K index of magnetic activity, i.e. the bearings were more to the left when K was high. In a single series of releases at a site 43.6 miles west of the loft, the means again showed a significant inverse correlation with K. Since most of the magnetic fluctuations in all four series were less than 70 gamma, it is concluded that the sensitivity of pigeons to magnetic cues probably approaches that already demonstrated for honeybees. A brief discussion of Lamotte's (1974) paper concerning the effect of bar magnets on initial orientation is appended.

Field-aligned scattering from a heated region of the ionosphere—Observations at HF and VHF

Abstract: Radio scattering measurements have been made of a region of the ionosphere above the Institute for Telecommunication Sciences' ionospheric heating facility at Platteville, Colorado. Strong scattering was observed at frequencies in the HF and VHF bands. The scattering has been shown to originate from electron density fluctuations that are highly elongated in the direction of the geomagnetic field. Radar scattering cross sections of 70 to 80 dbsm are measured in the HF and low VHF bands. The scattering cross section decreases at a rate of 20 to 30 db per octave at the high end of the VHF band. The scattering irregularities are produced by operation of the heating facility with o-mode polarization at frequencies that reflect within the ionosphere. The strength of the scattered energy decreases at a rate of at least 10 db per degree for observation geometries that do not match the geometry for specular reflections from long cylinders aligned with the geomagnetic field. The spatial distribution of irregularities has been estimated for both E- and F-region heating. The temporal properties of the scattering have also been determined including the frequency spectrum of the scattered signals. The processes by which ionospheric heating produces these irregularities are not yet fully explained. Several potentially useful applications of radio scattering from such irregularities are discussed.

Magnetism and the interior of the Moon

Abstract: The application of lunar magnetic field measurements to the study of properties of the lunar crust and deep interior is reviewed. Following a brief description of lunar magnetometers and the lunar magnetic environment, measurements of lunar remanent fields and their interaction with the solar plasma are discussed. The magnetization induction mode is considered with reference to lunar magnetic permeability and iron abundance calculations. Finally, electrical conductivity and temperature calculations from analyses of poloidal induction, for data taken in both the solar wind and in the geomagnetic tail, are reviewed.

On the cause of geomagnetic storms

Abstract: A study of the solar wind and interplanetary magnetic field during four geomagnetic storms shows that strong southward interplanetary magnetic fields are associated with the development of each main phase. Weak southward magnetic fields do not necessarily lead to an increase in the ring current even though such southward fields persist. Such behavior is consistent with the existence of a threshold for the initiation of strengthening of the ring current. These data also suggest that if the threshold is exceeded and the southward field attains a new constant level, Dst will decrease until some saturation value is reached. This apparent threshold for the interplanetary magnetic field and the saturation level for Dst can be explained by a ring current injection process, which is a function of the interplanetary magnetic (or electric) field, and by a ring current dissipation process, which is a function of the strength of the ring current.

Plasma wave turbulence at distant crossings of the plasma sheet boundaries and the neutral sheet

Abstract: The orbit of IMP 7 frequently takes the spacecraft across the neutral sheet in the geomagnetic tail at a downstream distance of about 35 earth radii. This preliminary report discusses the detection of strong low-frequency electromagnetic signals and moderate electrostatic noise enhancements near local midnight. Data used were obtained during a substorm period on Oct. 28, 1972, when there were multiple crossings of the neutral sheet and the boundary of the plasma sheet.

Westward equatorial electrojet during daytime hours

Abstract: The phenomenon of the depression of the geomagnetic horizontal field during the daytime hours of magnetically quiet days at equatorial stations is described. These events are generally seen around 0700 and 1600 LT, being more frequent during the evening than the morning hours. The evening events are more frequent during periods of low solar activity and in the longitude region of weak equatorial electrojet currents. The latitudinal extent of the phenomenon is limited to the normal equatorial electrojet region, and on some occasions the phenomenon is not seen at both stations, separated by only a few hours in longitude. During such an event, the latitudinal profile of the geomagnetic vertical field across the equator is reversed, the ionospheric drift near the equator is reversed toward the east, the q type of sporadic E layer is completely absent, and the height of the peak ionization in the F2 region is decreased. It is suggested that these effects are caused by a narrow band of current flowing westward in the E region of the ionosphere and within the latitude region of the normal equatorial electrojet, due to the reversal of the east-west electrostatic field at low latitudes.

Spherical Harmonic Analyses of the Geomagnetic Field for Eight Epochs between 1600 and 1910

Abstract: Summary The data for epochs between 1600 and 1910 in the catalogue of Veinberg and Shibaev have been analysed and two sets of spherical harmonic models of the geomagnetic field have been derived. Two methods are used to overcome the shortage of intensity data for the earlier epochs. In the first set of models the ratios of the harmonic coefficients to gIo are determined using both D and I data. The second set uses assumed values of 8,’ for these epochs. In contrast to previous analyses based on this catalogue, the data are weighted according to the number of original observations contributing to the catalogue mean and allowance is made for the oblateness of the Earth. The dipolar nature of the field in the Pacific region is investigated and the positions of the geomagnetic poles are derived. The variation of the position of the eccentric dipole with time is studied.

Observations of interactions between interplanetary and geomagnetic fields

Abstract: Magnetospheric effects associated with variations of the north-south component of the interplanetary magnetic field are examined in light of recent recent experimental and theoretical results. Although the occurrence of magnetospheric substorms is statistically related to periods of southward interplanetary magnetic field, the details of the interaction are not understood. In particular, attempts to separate effects resulting directly from the interaction between the interplanetary and geomagnetic fields from those associated with substorms have produced conflicting results. The transfer of magnetic flux from the dayside to the nightside magnetosphere is evidenced by equatorward motion of the polar cusp and increases of the magnetic energy density in the lobes of the geomagnetic tail. The formation of a macroscopic X-type neutral line at tail distances less than 35 R sub E appears to be a substorm phenomenon.

A theory of latitude dependent geomagnetic micropulsations: The asymptotic fields

Abstract: The asymptotic temporal behavior of hydromagnetic waves in a model of the inner magnetosphere is shown to be characterized by guided modes. The basic hydromagnetic wave equation is derived and applied to a cylindrical model of the inner magnetosphere. This model offers a good representation of the spatially dependent field line frequencies present in a dipole field. The initial value problem for the symmetric toroidal mode is solved, and its singularities are treated by Fourier superposition. Singularities also are present in the asymmetric poloidal mode wave equation and are shown to be logarithmic. Fourier superposition leads to the solutions for the electric and magnetic fields, which are asymptotic in time. The results indicate decay of the poloidal modes and domination by the toroidal modes, i.e., field line control of the propagation. At the latitude at which the maximum amplitude of a particular frequency occurs, the wave becomes linearly polarized. The asymptotic micropulsation periods depend on the characteristic field line periods and are usually longer at higher latitudes. Undamped guided waves lead to some terms, such as change density and parallel current density, that increase linearly with time. The inclusion of loss mechanisms in the wave equation, e.g., conductivity, limits the guided modes and prevents such nonphysical effects from occurring.

The Effects of Geomagnetic Storms on Electrical Power Systems

Abstract: Geomagnetic field fluctuations produce spurious, quasi-dc currents in power systems. The currents enter through system grounding points, and are of sufficient magnitude to cause half-cycle saturation in power transformers. These currents have been recorded and are summarized, and recommendations made in the significant problem areas of system disturbances and equipment effects due to geomagnetic storms. The major problem areas include increased VAR requirements and shifts in VAR flow, system-voltage fluctuations, generation of harmonics, protective relayiing misoperations, and possible localized internal heating in transformers. Recommendations relating to these effects have Pot been published previously.

Outage of the l4 system and the geomagnetic disturbances of 4 august 1972

Abstract: An outage of the Plano, Illinois, to Cascade, Iowa, link of the L4 coaxial cable occurred at about 2240 UT on 4 August 1972 during a large geomagnetic storm. The available geomagnetic data measured in North America, as well as data received from two satellite instruments, are analyzed. These data show that, at the time of the L4 outage, the boundary of the magnetosphere was pushed to unusually low altitudes by a greatly enhanced solar wind. As a result, large, rapid changes of the earth's magnetic field strength were observed over North America. It is demonstrated that the field changes at about 2241 to 2242 UT were of such magnitude as to induce earth currents of sufficient strength to produce the L4 outage by causing a high-current shutdown of the system link. The geomagnetic disturbances that produced the shutdown were not of the auroral-electrojet type normally associated with disruptions of power systems.

Behaviour of the Earth's Palaeomagnetic Field from Small Scale Marine Magnetic Anomalies

Abstract: CERTAIN areas of the ocean crust exhibit a high resolution recording of the magnetic field history. A recent survey1 of the Gorda-Juan de Fuca Rise area in the North Pacific displays short wavelength (10 to 20 km), low amplitude (40 to 80 gamma) features superimposed upon the larger scale (30 to 200 km, 200 to 800 gamma) magnetic anomaly pattern of Heirtzler et al.2. Many of these small scale anomalies form lineations which are parallel to the major magnetic lineations of Heirtzler et al. Sequences of small scale anomalies form characteristic patterns within intervals previously thought to be of constant polarity. We have identified two of these patterns that we observed in the North Pacific on profiles from the South Pacific and South-east Indian Oceans. Because of their global distribution, we conclude that the small scale anomalies are due to time variations of the Earth's magnetic field. That is, these features record either short (less than 3 × 104 yr) polarity reversals or fluctuations in the intensity of the dipole moment, perhaps with periods greater than 3 × 104

Lightning-induced electric fields in the ionosphere

Abstract: In a recent paper we discussed the penetration of dc thundercloud electric fields into the ionosphere and magnetosphere (Park and Dejnakarintra, 1973). In the present paper we extend the analysis to the time-varying case and consider how electric field variations due to lightning are transmitted to ionospheric heights. We represent the thundercloud by a vertical dipole whose moment recovers exponentially with time following periodic lightning discharges. Electrical conductivity profiles are approximated by several piecewise exponential functions of altitude, and the anisotropy of the medium is taken into account above 70-km altitude. The source wave form is Fourier-analyzed, and electric fields at each harmonic frequency up to 10 Hz are mapped upward separately. The total electric field at any point in space is then obtained by adding all frequency components together. It is found that electric fields are transmitted upward more efficiently as the frequency increases. This compensates for the decrease with frequency in the amplitudes of the source spectrum, so that the electric field spectrum becomes fairly flat at large heights. In the time domain the recovery time following a lightning discharge decreases rapidly with altitude. At the 100-km level the electric field wave form appears as a sharp spike of ∼100-ms duration. The peak amplitude of the spike may be several hundred microvolts per meter across the geomagnetic field at night. In the daytime the corresponding amplitude is about an order of magnitude smaller because of larger conductivities. These fields should be investigated as a possible excitation source of geomagnetic pulsations in the magnetosphere.

Latitudinal cross section of the auroral electrojet and its relation to the interplanetary magnetic field polarity

Abstract: The latitudinal dependence of the auroral electrojet profile across the Alaska meridian chain of observatories on the polarity of the interplanetary magnetic field (IMF) is semiquantitatively examined. It is found that for polar magnetic substorms associated with a northward IMF and a southward IMF of a small magnitude (1) the auroral electrojet is confined to a rather narrow area in higher latitudes (∼70° in geomagnetic latitudes), (2) the total westward electrojet current intensity is smaller, and (3) the positive change in the low-latitude H component is smaller (<10γ) than the corresponding quantities of substorms associated with the southward IMF of a large magnitude. The correlation between the total electrojet intensity and the low-latitude H change (ΔH) is higher than that between the AE index and ΔH. It is suggested that the total electrojet current intensity across the midnight meridian is a better measure for the intensity of magnetospheric substorms than the AE index.

Tracing of high‐latitude magnetic field lines by solar particles

Abstract: ABS>Recent measurements of solar particles in the energy interval between hundreds of keV and a few MeV have shown that a direct connection exists between a portion of the high-latitude geomagnetic field and the interplanetary magnetic field. The access window for 300-keV solar protons that reach the center of the polar cap may be as near as 150 R/sub E/ of the downstream magnetotail. Solar protons that precipitate into the atmosphere at latitudes near the geomagnetic cutoff enter through the flanks of the magnetosphere and the nearby neutral sheet, possibly within 30 RE of the Earth. Comparison of the patterns of auroral particle precipitation with the zones of access of energetic solar electrons and protons indicates that a substantial fraction of the aurora originates on field lines connected to the interplanetary field. (auth)

Investigation of the Large Scale Palaeomagnetic Field over the past 25 million years. Eastward Shift of the Icelandic Spreading Ridge

Abstract: Summary An up-to-date appraisal of 291 late Tertiary and Quaternary palaeomagnetic pole positions shows that they cannot be explained by accepted rates and directions of rigid plate movements (although such movements are not disputed). The pole positions are thus believed to reflect in large part the behaviour of the geomagnetic field itself. The mean late Tertiary best-fitting field source is an axial dipole, displaced north of the equatorial plane by 325 ± 57 km. However, there has been a slow rate of change of this parameter during the late Tertiary. Icelandic data, when examined in detail, also show evidence of long-term geomagnetic field variations and further suggest that the hypothesis of an eastward shift of the Icelandic spreading ridge during the Pliocene is correct. A consequence of these long-term geomagnetic field variations, spanning at least several years, is that it may not be possible to define palaeomagnetic pole positions with an accuracy better than about 5°.

Alfvenic acceleration of solar wind helium and related phenomena. 1. Theory

Abstract: We present a new physical mechanism by which helium nuclei can be preferentially accelerated by Alfven waves in the solar wind. The mechanism works as follows. The acceleration of the solar wind by Alfven wave pressure is basically a δJ × δB force; but the wave-associated current δJ carried by a given plasma species depends, via the Lorentz transformation, on the bulk velocity of that species, and thus species that move at different bulk velocities experience different accelerations. In the solar wind this differential acceleration can for the most part be interpreted as an additional frictional interaction between protons and helium nuclei. Numerical estimates indicate that this interaction is important at 1 AU. It is expected that this additional friction will be able to account for the observed fact that helium nuclei almost never flow slower than protons in the solar wind; detailed numerical calculations are deferred to a later paper, however. Although our calculation is done for Alfven waves, we argue that the basic physics of our mechanism is applicable to a wide variety of waves, and we suggest that it may be important for cosmic rays, interstellar gas, comet tails, and the earth's (and Jupiter’) geomagnetic tail.

Palaeomagnetic Evidence for the Transitional Behaviour of the Geomagnetic Field

Abstract: Summary A definition of transition intervals based on the definition of Normal, Intermediate and Reversed pole positions proposed by Wilson, Dagley and MCormack is used to select data for comparison with two models of the transitional geomagnetic field. The data for twenty-three transition intervals reported by various authors are reviewed. The diversity of pole paths and common westerly trends lend support to the model in which the non-dipole field becomes dominant during the transition (Larson et al.). However, certain similarities and the sharp east-west changes of longitude support a purely dipole model with an independently inverting equatorial dipole but there is no common pole path nor even a preferred sector of the globe as suggested by Creer and Ispir. Neither model is preferred and a combination of a three-component dipole and non-dipole fields may be needed. Much evidence indicates that the field passes through a minimum during a transition and the observed magnitudes suggest that the nondipole field decreases also. A possible link between inversions and tectonic activity is suggested by the detail recorded in some transitions while the fact that there are more transitions from the reversed mode of the field than from the normal mode implies that the two modes are not equally stable.

High-latitude irregularities in the top side ionosphere based on Isis 1 Thermal Ion Probe data

Abstract: Thermal ion density irregularities at high latitudes have been investigated by using results from the spherical electrostatic analyzer flown on the polar-orbiting satellite Isis 1. The size of the irregularities is found to be highly variable, dimensions typically ranging from 1 to 140 km. Results from 3500 orbits in the top side ionosphere during the first year of operation have been examined. The daily and seasonal behavior of the equatorial boundary of the high-latitude irregularity zone and its dependence on magnetic activity have been established for the northern and southern hemispheres. The daily variation in the location of the boundary of the irregularity zone is found to be 14°±2° for any level of magnetic activity; the boundary is at the highest latitude near local noon. The irregularities consistently extend across the pole; there is no upper boundary. Systematic differences are found in the mean location of the equatorial boundary in the northern and southern hemispheres. In the south the boundary is on the average 4.8°±0.6° closer to the pole than in the north. Good agreement is found between the direct measurements of the irregularity boundary and scintillations, severe top side spread F and ≤300-eV electron precipitation boundaries, and trough and plasmapause positions on the night side. Significant differences found to exist between the irregularity boundary and plasmapause positions on the day side are considered to be due to their different origins.