Showing papers on "Dipole model of the Earth's magnetic field published in 1978"

Determination of constant α force-free solar magnetic fields from magnetograph data

Abstract: At first it is shown that a magnetic field being force-free, i.e. satisfying ▽ × B = αB, with α = constant (α ≠ 0) in the whole exterior of the Sun cannot have a finite energy content and cannot be determined uniquely from only one magnetic field component given at the photosphere. Then the boundary value problem for a semi-infinite column of arbitrary cross section is solved by a Green's function method.

Magnetohydrodynamics of the Earth's Dynamo

Abstract: is generally accepted that the earth's magnetic field is generated by motions in the liquid part of the earth's core, but the details of this process are still unresolved. In this article an attem pt is made to outline the

Magnetic field perturbations in the dayside cleft and their relationship to the IMF

Abstract: Magnetometers on the Isis 2 satellite have been used to study large-scale perturbations in the geomagnetic field in the region of the dayside cleft (or cusp). Relatively large perturbations, in a plane perpendicular to the main field direction, are observed on most satellite passes; on the average the perturbations near noon are larger than those observed in the dawn and dusk sectors. The peak perturbations tend to lie in two relatively narrow ranges of direction, pointing approximately eastward or westward depending on the sign of By, the y component of the interplanetary magnetic field (IMF). In general, for this northern hemisphere data set, positive By corresponds to an eastward perturbation, and negative By to a westward perturbation. The invariant latitude of the perturbation is correlated with Bz, the z component of the IMF, while the magnitude of the perturbations is more strongly correlated with By than with Bz. The perturbations are discussed briefly in terms of the tilt of the field direction relative to a model field, and it is suggested that this is related to the direction of convection in the magnetosphere. On the basis of this interpretation the predominant convection direction in the cleft is approximately either eastward or westward.

The interplanetary modulation and transport of Jovian electrons

Abstract: Based on simultaneous measurements by Pioneer 11 of the 3-6 MeV Jovian electron flux, interplanetary magnetic field magnitude, and solar wind speed, the interplanetary transport of energetic particles is studied. It is found that corotating interaction regions (CIR's) greatly inhibit electron transport across the average field direction. Cross-field transport is also influenced by the degree of compression of the solar wind since CIR's are areas of compressed solar wind plasma. The propagation of Jovian electrons is studied by a model that includes the effects of CIR's. The model tests whether or not the three-dimensional convection-diffusion theory adequately describes the cross-field transport of electrons. The model is also valid for Jovian electron observations from earth-orbiting satellites. The model may be further applied to 1 AU from the sun where it is found that the cross-field diffusion of electrons explains why Jovian electrons are detected at the earth even during periods when the interplanetary magnetic field does not connect the earth directly to Jupiter.

The measurement of solar magnetic fields

Abstract: Solar activity is basically caused by the interaction between magnetic fields, solar rotation and convective motions. Detailed mapping of the Sun's rapidly varying magnetic field helps one to understand the mechanisms of solar activity. Observations in recent years have revealed unexpected and intriguing properties of solar magnetic fields, the explanation of which has become a challenge to plasma physicists. The review deals primarily with how the Sun's magnetic field is measured, but it also includes a brief review of the present observational picture of the magnetic field, which is needed to understand the problems of how to properly interpret the observations.

Bifurcation of force-free solar magnetic fields: A numerical approach

Abstract: Numerical calculations of two-dimensional force-free fields as models of solar active regions are presented. For a given ‘toroidal’ component of the photospheric magnetic field two branches of solutions are numerically obtained which merge at the critical point of maximum allowed toroidal magnetic field. Depending on boundary conditions magnetic islands may or may not form. The results are discussed with respect to their relevance to the flare process.

Small‐scale transverse magnetic disturbances in the polar regions observed by Triad

Abstract: Small-scale transverse magnetic field disturbances are often observed with the magnetometer experiment on board the Triad satellite at latitudes poleward of the large-scale field-aligned current regions associated with auroral phenomena (Iijima and Potemra, 1976b). In contrast with the magnetic variations associated with the large-scale auroral field-aligned currents the polar cap magnetic variations are smaller in amplitude (∼100 γ) and show variations over smaller latitude ranges (∼0.2°). The small-scale variations are transverse to the main geomagnetic field and therefore explainable by field-aligned currents, but these currents are not necessarily sheets aligned in the geomagnetic east-west direction as the auroral field-aligned currents often are. The small-scale disturbances occur most frequently in the morning sector (between approximately 0300 and 0900 MLT) up to ∼82° invariant latitude during a wide range of Kp. The amplitudes of the polar magnetic variations show little correlation with Kp, but they are observed more frequently at lower invariant latitudes during disturbed geomagnetic conditions. The frequency of occurrence of the small-scale magnetic variations is correlated with the azimuthal direction of the interplanetary magnetic field (IMF). They are observed in the north polar cap twice as often during periods when the IMF is directed away from the sun than when it is directed toward the sun. The situation is reversed in the southern hemisphere, where the small-scale variations are observed more than twice as often during periods when the IMF direction is toward the sun compared with periods when the IMF is away from the sun. There is no obvious dawn-dusk asymmetry of the occurrence distribution of the polar magnetic variations which is related to the IMF. These observations contribute to the mounting evidence for the direct connnection of earth magnetic field lines to those of the solar wind and the ready access of charged particles to the polar regions.

On the reality of potential magnetic fields in the solar corona.

Abstract: Global magnetic field calculations, using potential field theory, are performed for Carrington rotations 1601–1610 during the Skylab period. The purpose of these computations is to quantitatively test the spatial correspondence between calculated open and closed field distributions in the solar corona with observed brightness structures. The two types of observed structures chosen for this study are coronal holes representing open geometries and theK-coronal brightness distribution which presumably outlines the closed field regions in the corona. The magnetic field calculations were made using the Adams-Pneuman fixed-mesh potential field code based upon line-of-sight photospheric field data from the KPNO 40-channel magnetograph. Coronal hole data is obtained from AS&E's soft X-ray experiment and NRL's Heii observations and theK-coronal brightness distributions are from HAO'sK-coronameter experiment at Mauna Loa, Hawaii. The comparison between computed open field line locations and coronal holes shows a generally good correspondence in spatial location on the Sun. However, the areas occupied by the open field seem to be somewhat smaller than the corresponding areas of X-ray holes. Possible explanations for this discrepancy are discussed. It is noted that the locations of open field lines and coronal holes coincide with the locations ofmaximum field strength in the higher corona with the closed regions consisting of relatively weaker fields. The general correspondence between bright regions in theK-corona and computed closed field regions is also good with the computed neutral lines lying at the top of the closed loops following the same general ‘warped’ path around the Sun as the maxima in the brightness. One curious feature emerging from this comparison is that the neutral lines at a given longitude tend systematically to lie somewhat closer to the poles than the brightness maxima for all rotations considered. This discrepancy in latitude increases as the poles are approached. Three possible explanations for this tendency are given: perspective effects in theK -coronal observations, MHD effects due electric currents not accounted for in the analysis, and reported photospheric field strengths near the poles which are too low. To test this latter hypothesis, we artificially increased the line-of-sight photospheric field strengths above 70° latitude as an input to the magnetic field calculations. We found that, as the polar fields were increased, the discrepancy correspondingly decreased. The best agreement between neutral line locations and brightness maxima is obtained for a polar field of about 30 G.

Structure of current sheets in magnetic holes at 1 AU. [regions of low magnetic field intensity in the solar wind

Abstract: Current density profiles in several types of interplanetary magnetic holes have been calculated using high-resolution Imp 6 magnetic field data (12.5 vector measurements/s), assuming that the currents flow in planar sheets and that the magnetic field varies only in the direction normal to the sheet. The planarity was verified in four holes which were observed by two suitably spaced spacecraft. The structure of the current sheets ranges from very simple in some holes to very complex in others. Four types of simple magnetic holes are discussed, in which B varies nearly monotonically on each side of the hole. In two of the holes, B varies in intensity but not in direction as a result of currents normal to B. In the other two holes, B changes in both magnitude and direction as a result of currents both normal and parallel to B. The observed structures are found to be qualitatively consistent with the models of Burlaga and Lemaire, which are based on self-consistent solutions of Vlasov's equation and Maxwell's equations. Examples of complex irregular magnetic holes are also presented, and they are shown to contain multiple current sheets in which currents flow parallel to one another at various angles with respect to B. There is no model of such magnetic holes at present.

Nonaxial electric field gradients in amorphous rare earth alloys

Abstract: The authors show that a model of magnetic amorphous rare earth alloys with nonaxial electric field gradients is equivalent to the simple model with axial crystal fields -DJz(i)2 and D positive, when the rare earth moment is large and z(i) is a local axis varying randomly from site to site. For small values of J, the axial-field model is less justifiable, especially for non-Kramers ions. They discuss the consequences of nonaxial field gradients for some magnetic and nonmagnetic properties.

On the apparent source depth of planetary magnetic fields

Abstract: Two simple assumptions regarding the ratios of the strengths of the field contributions of the multipole moments of the terrestrial magnetic field at its effective source depth are used to examine the consistency between the apparent source depths for the magnetic fields of Mercury and Jupiter and the present understanding of their interior structure. Both fields are consistent with the present understanding. However, the comparison would be facilitated by further measurements of the magnetic fields at both planets, especially at Mercury.

Magnetic field in the primitive solar nebula

Abstract: Carbonaceous chondrites have apparently been magnetized in their early history in magnetic fields with intensities of 0.1 to 10 G, but the origin of the magnetizing field has remained obscured. It is suggested that the magnetic field recorded in the remanence of carbonaceous chondrites may have been produced by a self-excited hydromagnetic dynamo in the gaseous preplanetary nebula from which the solar system is thought to have formed. Recently computed models for the evolution of the preplanetary nebula, consisting of turbulent and differentially rotating gaseous disks with characteristic radial scales of several AU, are used to demonstrate the feasibility of this hypothesis. The maximum field intensity that might be realized by the dynamo production process is estimated to be as high as 1 to 10 G, taking into account two dynamical mechanisms that limit the strength of the field (the Coriolis force and ambipolar diffusion).

Electromagnetic subsoil prospecting process using an asymptotic low frequency range

Abstract: In a sub-soil prospecting process a vertical magnetic field transmitter dipole and a receiver dipole are placed on the ground, away from each other. The receiver dipole is oriented to detect a radial horizontal magnetic field relative to the transmitter dipole. At low frequencies, the magnetic field detected assumes asymptotic values, and by measuring these values an apparent alternating current resistivity of the sub-soil is deduced. By varying the transmitter-receiver distance, and by simultaneously using a direct current electric prospecting process, a sub-soil model is determined which is less ambiguous than those supplied by electric or induction prospecting alone.

On the relationship of ∼3 mHz (Pc5) electric, magnetic, and particle variations

Abstract: The first evidence is presented of associated magnetic field, electric field, and electron precipitation fluctuations from ground-based and balloon-based measurements. The magnetic field fluctuations are produced by magnetohydrodynamic waves of approximately 5 min period in the earth ’s magnetosphere. The electric, magnetic, and particle variations are observed to be in close phase relationship when the measurements are made near the resonance region of the magnetospheric waves. When the resonance region is not close to the location of the electric field observations no clear wave-associated electric field or particle variations are observed. The observed wave frequency is ∼3 times lower than the fundamental Alfven resonance frequency. This may arise from a Doppler shift of the resonant frequency by a diamagnetic plasma drift. The height-integrated ionospheric Pedersen conductivity deduced from the simultaneous electric and magnetic field variations corresponds to approximately 30 mho.

Correlated electric field and low-energy electron measurements in the low-altitude polar cusp

Abstract: Correlated electric field and low-energy electron measurements are presented for two passes of Hawkeye 1 through the south polar cusp at 2000-km altitude during local morning. In one case the electric field reversal coincides with the boundary of detectable 5.2keV electron intensities and the equatorward boundary of the cusp. In the other case the electric field reversal and the 5.2 keV electron trapping boundary coincide, but the equatorward edge of the cusp as determined from the presence of 180 eV electron intensities is 5 degrees invariant latitude equatorward of the electric field reversal. It is concluded that in the second case, electron intensities associated with the polar cusp populate closed dayside field lines, and hence the corresponding equatorward edge of these electron intensities is not always an indicator of the boundary between closed dayside field lines and polar cap field lines.

Rocket‐borne measurements of particles and ion convection in dayside aurora

Abstract: As part of Operation Periquito, two sounding rockets were launched into regions of particle precipitation at invariant latitude 76°N and magnetic local time 13 hours from Cape Parry, Northwest Territories, Canada, in November 1975 On one of the flights, which did not cross any well-defined auroral arcs, the electron spectra were always soft, the thermal ion drift velocity was variable but generally westward at 500 m/s, and energetic particles (>70 keV) were always well above cosmic ray background, indicating a region of closed magnetic field lines The other flight crossed two auroral arcs Electrons with soft (mean energy of 200 eV) power law spectra were observed on the equatorward and poleward sides, whereas intense 1-keV precipitation was measured within the arcs The mean drift velocity of thermal ions was seen to change from 1500 m/s westward, parallel to the boundary, on the equatorward side to ∼200 m/s southward on the poleward side of the arcs Energetic (E > 70 keV) particle measurements showed that the high-latitude boundary was well poleward of the auroral arcs (Heikkila and Winningham's ‘cleft’ precipitation and ion convection velocity reversals), which indicates that the entire region was on closed magnetic field lines Comparison of these features with those from midnight aurora shows that the two are qualitatively similar, strongly suggesting that dayside and nightside arcs are different aspects of the same process No support was seen for the idea that direct penetration of magnetosheath particles is responsible for the dayside aurora

Plasma electron observations in the wake of Venus

Abstract: Two plasma regimes were observed in the Venus wake, one characterized by negative-going anisotropies accompanied by high bulk speeds and low magnetic variance, and the other characterized by positive-going anisotropies accompanied by low bulk speeds and high magnetic variance. The two modes alternate in time in a stochastic bimodal fashion as interplanetary magnetic field discontinuities propagate past Venus and switch on the positive anisotropies. The positive mode probably represents enhanced planetary ion pickup which results in instabilities, the damping of which energizes the electrons in the distant wake.

Ionospheric pulse time dispersion including effects of Earth's magnetic field

Abstract: For frequencies above 100 MHz, the distortion due to ionospheric dispersion has been evaluated in the past for the assumption that the earth's magnetic field can be neglected. It is shown that for noncircularly polarized signals, conditions can exist at even L -band where the ionosphere distortion due to the presence of the earth's magnetic field can be significant. A simple expression relating the Faraday rotation to the ionospheric dispersion due to the earth's magnetic field is given. An example is presented which shows that the Faraday rotation can be as much as 145\deg at L -band.

The magnetic field transfer in the solar convective zone

Abstract: The large-scale azimuth magnetic field is pumping to the bottom of the solar convective zone due to the diamagnetic action of turbulent conductive fluids. When the field at the bottom is of about 103 G, an equilibrium is established between diamagnetic pumping and buoyancy. If, in addition to the density gradient, an additional anisotropy exists (for instance, due to rotation), another mechanism of the magnetic field transfer appears, the efficiency of which greatly depends on the magnitude of the anistropy parameter.

Earth's electric field

Abstract: The earth becomes charged during thunderstorm activity and discharges through the weak conducting atmosphere. Balloon and rocket studies infer that a high altitude electric field penetrates virtually unattenuated through the atmosphere, at least as far as balloon heights. The field has two primary sources. At low and mid latitudes, interaction between the earth's magnetic field and the neutral wind creates electric fields. At latitudes above 60/sup 0/, the high altitude electrical structure is dominated by the interaction between the solar wind and the earth's magnetic field. The auroral light is emitted by atmospheric atoms and molecules excited by electrons with potentials of many thousands volts. The potentials are induced by the solar wind. Recent satellite data shows that the electrons get this energy by passing through a localized electric field about 6000 km above the auroral zone. Several rocket and satellite experiments used to study the earth's electric field are discussed. (SC)

Comparisons of measured and calculated potential magnetic fields.

Abstract: Photospheric line-of-sight and transverse magnetic field data obtained with the Marshall Space Flight Center vector magnetograph system for an isolated sunspot are described A study of the linear polarization patterns and of the calculated transverse field lines indicates that the magnetic field of the region is very nearly potential The Hα fibril structures of this region as seen in high resolution photographs corroborate this conclusion Consequently, a potential field calculation is described using the measured line-of-sight fields together with assumed Neumann boundary conditions; both are necessary and sufficient for a unique solution The computed transverse fields are then compared with the measured transverse fields to verify the potential field model and assumed boundary values The implications of these comparisons on the validity of magnetic field extrapolations using potential theory are discussed

Dependence of cosmic-ray transport on the orientation of the solar magnetic dipole

Abstract: In this paper we discuss the dependence of cosmic-ray transport in the interplanetary space on the polarity configuration of the solar and interplanetary magnetic fields. The diffusion mechanism is not only related to the irregularities present in the interplanetary magnetic field, but also to its regular structure, because of the curvature and gradient drifts of the particles. Hence diffusion is affected by the polarity reversals of the interplanetary magnetic dipole which are related to the reversals of the solar dipole. As a consequence a phase reversal of both the annual and semi-annual variations in the cosmic-ray intensity occurs. This effect has been studied throughout the period 1954–1976, by analysing the Climax neutron monitor data. In addition, the latitudinal distribution of the average solar activity itself in this period changes simultaneously to the polarity reversals of the solar polar fields.

IMF structures between 0.3 and 1 A.U. A comparison of two-spacecraft observations

Abstract: Interplanetary-magnetic-field observations by Helios 1 and IMP 8 have been compared for a period of time (corresponding to four solar rotations) in which the radial and latitudinal separations between spacecraft, respectively, ranged between 0 and 0.69 AU and 0 and 14.5 degrees. The correspondence between macrostructural features is good even when the observing spacecraft are located at opposite sides of the solar equatorial plane. Unipolar regions of the interplanetary magnetic field are confirmed as steady-state, corotating structures and are also mostly consistent with a quite regular extension of the field polarities of the observed coronal holes. There are, however, interesting examples of recurrent unipolar regions which are not associated with the recurrent pattern of high-velocity streams. A comparison of two field enhancements suggests that these structures do not experience significant modifications with the heliocentric distance.

Magnetic variations excited by a radial magnetic dipole located in the earth's core

Abstract: Theoretical formulae are derived for computing a variable magnetic field, excited by a harmonically oscillating radial magnetic dipole (RMD), located eccentrically at the surface of the Earth's core. By numerical computations, using a three-layered conductivity model of the Earth, it is proved that the field due to this source, computed for the surface of the Earth, is relatively weak in comparison to the field of a stationary magnetic dipole, provided the period of the changes is less than 500 years. The “zone of influence” of the RMD at the Earth's surface is also determined and on its basis a number of conclusions were drawn with respect to representing the non-dipole part of the geomagnetic field by means of the RMD system in the Earth's core.