Showing papers on "Dipole model of the Earth's magnetic field published in 1974"

A quantitative model for the potential resulting from reconnection with an arbitrary interplanetary magnetic field

Abstract: A quantitative three-dimensional model is proposed for the electric potential arising from magnetopause reconnection, in which several approximations are made concerning configuration of the magnetosheath flow, limitations on the magnitude of the reconnection speed, and the geometry of the problem. These approximations are such that the model yields an upper limit for the potential. The magnitude of the polar cap ionospheric electric field computed from this model is larger than that measured on balloons by an average factor of about 3, and the model reproduces the temporal variations of the experimental data. It is concluded that magnetopause reconnection is a highly efficient process that is probably the dominant mechanism driving polar cap convection and supplying energy to the magnetosphere. It seems that the most efficient way for the solar wind to pass the magnetospheric obstacle is by magnetopause reconnection.

The inversion of magnetic anomalies in the presence of topography

Abstract: The inversion of magnetic anomalies in terms of an irregular layer of magnetized material is studied, and an efficient procedure for constructing solutions is developed. Even when magnetic orientation and layer thickness are known, the solution is not unique because of the existence of a magnetization (called the magnetic annihilator) that produces no observable magnetic field. We consider an example of near-bottom marine data and discuss methods for overcoming the problem of nonuniqueness.

Magnetic Field Observations near Mercury: Preliminary Results from Mariner 10.

Abstract: Results are presented from a preliminary analysis of data obtained near Mercury on 29 March 1974 by the NASA-GSFC magnetic field experiment on Mariner 10. Rather unexpectedly, a very well-developed, detached bow shock wave, which develops as the super-Alfvenic solar wind interacts with the planet, has been observed. In addition, a magnetosphere-like region, with maximum field strength of 98 gammas at closest approach (704 kilometers altitude), has been observed, contained within boundaries similar to the terrestrial magnetopause. The obstacle deflecting the solar wind flow is global in size, but the origin of the enhanced magnetic field has not yet been uniquely established. The field may be intrinsic to the planet and distorted by interaction with the solar wind. It may also be associated with a complex induction process whereby the planetary interior-atmosphere-ionosphere interacts with the solar wind flow to generate the observed field by a dynamo action. The complete body of data favors the preliminary conclusion that Mercury has an intrinsic magnetic field. If this is correct, it represents a major scientific discovery in planetary magnetism and will have considerable impact on studies of the origin of the solar system.

A quantitative model of the magnetospheric magnetic field

Abstract: Quantitative representations of the magnetic fields associated with the magnetopause currents and the distributed currents (tail and quiet time ring currents) have been developed. These fields are used together with a dipole representation of the main field of the earth to model the total vector magnetospheric magnetic field. The model is based on quiet time data averaged over all ‘tilt angle’ values. The weak field in the equatorial region of the inner magnetosphere and the tail field structure are included in the model. The depressed field region in the inner magnetosphere is especially important for the accurate modeling of several observed particle and magnetic field phenomena. The field representation is analytic and is given in Cartesian coordinates with power series and exponential terms. It is valid from the subsolar magnetosphere to beyond lunar orbit. This series expansion allows the magnetic field to be modeled accurately over an extended region of the magnetosphere and permits the representation of the field in the same region as that of its source currents. The model has been tested by using it to calculate several observed magnetospheric particle and field properties. The latitude cutoffs for solar cosmic rays and the trapping boundary of low-energy particles computed from the model agree well with observations. Model calculations also yield field line shapes in agreement with barium cloud observations. A simplified version of the model that can be used out past geosynchronous orbit is also presented.

Magnetic field of Jupiter and its interaction with the solar wind

Abstract: Jupiter's magnetic field and its interaction with the magnetized solar wind were observed with the Pioneer 10 vector helium magnetometer. The magnetic dipole is directed opposite to that of the earth with a moment of 4.0 gauss R_J^3 (R_J, Jupiter radius), and an inclination of 15° lying in a system III meridian of 230°. The dipole is offset about 0.1 R_J north of the equatorial plane and about 0.2 R_J toward longitude 170°. There is severe stretching of the planetary field parallel to the equator throughout the outer magnetosphere, accompanied by a systematic departure from meridian planes. The field configuration implies substantial plasma effects inside the magnetosphere, such as thermal pressure, centrifugal forces, and differential rotation. As at the earth, the outer boundary is thin, nor diffuse, and there is a detached bow shock.

Observations of moon-plasma interactions by orbital and surface experiments

Abstract: Extensive magnetic field observations together with crucial plasma measurements by the Explorer 35 lunar orbiter and Apollo surface and orbital experiments have established the basic nature of the moon's interaction with the solar wind and interplanetary magnetic field. The effective absorption of the incident solar wind by the moon creates a plasma void or cavity behind the moon. The cavity-associated magnetic signature is characterized by an enhancement in magnetic field magnitude B within the cavity as compared with the mean level of B in the surrounding interplanetary plasma and dips or decreases in B near the cavity boundaries with the solar wind. Apollo particle and field measurements on the lunar surface have provided evidence of a regional interaction of the highly conducting solar wind with lunar remanent magnetic fields. Simultaneous plasma and magnetic field data, from the spectrometer and the lunar surface magnetometer at the Apollo 12 location, show the compression of the local remanent field by large solar wind and magnetosheath plasma dynamic pressures.

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)

Equilibrium and stability of force-free magnetic field

Abstract: Force-free magnetic fields (f.f.f) are considered as the first approximation of magnetic hydrodynamic equations in the case when the energy of the field exceeds the thermal energy of the medium. Such a relation of energies takes place in the upper atmosphere of the Sun in active regions.

The Magnetospheric Boundary

Abstract: Magnetic fields deflect charged particles and the fields are somewhat impervious to charged particles not already trapped within them Thus, as we should expect, the Earth’s magnetic field makes a bubble in the supersonic plasma flowing outward from the Sun and called the solar wind This bubble in which the Earth’s magnetic field is largely confined and compressed is called the magnetosphere and the boundary between the Earth’s magnetic field on the inside and the turbulent interplanetary plasma and magnetic field on the outside is called magnetopause The magnetopause is very thin when compared to its radius of curvature (about 10−3 to 10−5) and can be regarded as infinitesimally thin Within this thin boundary layer electric currents flow which are created by the magnetically deflected plasma The effect of these currents is to cancel or terminate the geomagnetic field outside the magnetosphere and to augment the geomagnetic dipole field inside since the effect of the outside pressure is to compress the geomagnetic field

The effect of the east-west component of the interplanetary magnetic field on magnetospheric convection as deduced from magnetic perturbations at high latitudes

Abstract: The variations of the horizontal and vertical components of the ground magnetic field at high latitudes are analyzed in order to separate the perturbations related to the east-west component By of the interplanetary magnetic field from other perturbations. At invariant latitude Λ ≳ 80° the perturbations of the vertical component are characterized by a daytime extremum that is maximum when By is negative and minimum when By is positive. This effect becomes inverted when Λ ≲ 80° and disappears when Λ ≲ 75°. The currents that are equivalent to the additional horizontal perturbations due to the orientation of By are maximum on the day side and follow a rather circular flow pattern, which in the southern hemisphere is westward in the case where By is positive and eastward in the case where By is negative. In the northern hemisphere these average directions are reversed symmetrically. An asymmetric magnetospheric convection pattern may be deduced from these results.

A model of the distributed magnetospheric currents

Abstract: A model of the quiet time ring current and the tail currents has been developed for quiet magnetic conditions and perpendicular incidence of the solar wind on the main (dipolar) field of the earth. Previous work of several authors has shown that these distributed currents flow throughout the magnetosphere and that they are produced by the drift of charged particles in the nonuniform magnetic field B and by plasma diamagnetism. The currents are modeled from a few thousand kilometers above the earth's surface to distances of over 200 RE back in the tail of the magnetosphere. This procedure allows the computation of the magnetic field BD, associated with these distributed currents to beyond lunar orbit. The currents are chosen so that BD correctly models observed field features in the inner magnetosphere and the magnetotail. Although the tail currents can be formed continuously by the entry of magnetosheath particles into the equatorial flanks of the tail even during quiet times, the ring currents must be supplied at irregular (substorm or storm) intervals when dynamic processes are operative. These currents make significant contributions to the earth's surface field variations (e.g., in Sq and Dst). The formation of the tail currents implies that the magnetosphere is open (in the equatorial regions of the tail) to low-energy charged particles. This model work also suggests that when magnetospheric particles and fields are considered self-consistently, it is not possible to produce a magnetically closed magnetosphere. The static magnetic field associated with this distributed current model exhibits a neutral line beyond the orbit of the moon. The inclusion of the field associated with these distributed currents in a model of the total magnetic field should yield better agreement between observed magnetospheric particle and field phenomena and model calculations.

Models of the earth's electric field

Abstract: Detailed models of the electric field of the magnetosphere are derived in several stages. For all, the conductivity along field lines is assumed to be high enough to ensure the vanishing of E B everywhere except in the ionosphere. At first the rotation of the earth is ignored completely and a simple model is constructed which fits certain observed properties. Next, the rotation of the earth is taken into account, but the field is assumed to be that of a magnetic dipole rotating around its symmetry axis. This allows the concept of the electric potential to be retained, which permits the derivation of interesting properties including the use of a conjugate potential which paces the drift of charged particles in the field. Finally, the general case involving asymmetrical rotation is briefly discussed.

A unified theory on radiation of a vertical electric dipole above a dissipative earth

Abstract: Analytical expressions are derived for the electromagnetic field associated with a vertical electric dipole (VED) above a dissipative earth. The scattered field is shown to consist of a direct contribution from a perfect image source, and a correction due to the finite conductivity of earth which is expressible in terms of an incomplete Hankel function. The resultant expression readily reduces to the well-known asymptotic solutions in both the space-wave and ground-wave regions. Validity of the space-wave expression in the far zone is found to be dependent upon the observation angle as well as the refractive index of earth. In the near field, alternative approximate formulas in closed form are derived from the incomplete Hankel function expression for different elevation angles. These formulas are shown to yield accurate results whenever the distance from the image source is greater than about 0.2λc where λc is the effective wavelength (or skin depth) of earth. For distance smaller than 0.2λc, however, an analytical expression based upon a quasistatic approximation is generally applicable and has been discussed elsewhere.

Lunar magnetic field: permanent and induced dipole moments.

Abstract: Apollo 15 subsatellite magnetic field observations have been used to measure both the permanent and the induced lunar dipole moments Although only an upper limit of 13 x 10 to the 18th gauss-cubic centimeters has been determined for the permanent dipole moment in the orbital plane, there is a significant induced dipole moment which opposes the applied field, indicating the existence of a weak lunar ionosphere

The Instability of Strong Magnetic Fields in Stellar Interiors

Abstract: There has been discussion of the possibility of resolving the solar neutrino dilemma with a sufficiently strong magnetic field (500 MG) in the solar interior to relieve the gas pressure by some 10% or more. The time in which magnetic buoyancy will bring a strong field to the surface is examined and is found to be less than 100 m.y. No possibility is seen for retaining a suitably strong magnetic field in the solar interior.

A comment on the detection of closed magnetic structures in the solar wind

Abstract: It has recently been suggested that the large scale structure of the interplanetary magnetic field can be deduced solely from solar wind speed measurements. Here it is emphasized that, in addition to speed measurements, direct measurements of the interplanetary field and indirect diagnostics such as measurements of the solar wind kinetic temperature and galactic and solar energetic particle modulations and anisotropics are required to distinguish between open and closed magnetic structures in the solar wind.

A model of the open magnetosphere

Abstract: The Chapman-Ferraro image method is extended to construct an idealized model of the open magnetosphere that responds to a change of the interplanetary field direction as well as to a change of the field magnitude or of the solar wind momentum flux. The magnetopause of the present model is an infinite plane surface having a normal field component distribution that is consistent with the merging theory. An upper limit on the inward displacement of the magnetopause following a southward turning of the interplanetary field is obtained. The results are in fair agreement with a single event reported by Aubry et al. (1971). The model determines the field configuration and the total magnetic flux connecting the magnetosphere to interplanetary space.

Power spectra of the interplanetary magnetic field, 0.7-1.6 AU

Abstract: Power spectra of the fluctuations in the interplanetary magnetic field have been obtained from a number of spacecraft. Russell (1972) and Childers and Russell (1972) have recently reviewed published power spectra of the interplanetary field. In the present paper the computation of power spectra in the frequency range from .0000116 to .0000296 Hz, corresponding to periods from 1 day to 5.86 min, is described. The data used for these spectra were taken during the Mariner 4 and Mariner 5 missions and cover a radial distance from 0.7 to 1.6 AU.

Theoretical and observational analysis of spacecraft fields

Abstract: In order to investigate the nondipolar contributions of spacecraft magnetic fields a simple magnetic field model is proposed. This model consists of randomly oriented dipoles in a given volume. Two sets of formulas are presented which give the rms-multipole field components, for isotropic orientations of the dipoles at given positions and for isotropic orientations of the dipoles distributed uniformly throughout a cube or sphere. The statistical results for an 8 cu m cube together with individual examples computed numerically show the following features: Beyond about 2 to 3 m distance from the center of the cube, the field is dominated by an equivalent dipole. The magnitude of the magnetic moment of the dipolar part is approximated by an expression for equal magnetic moments or generally by the Pythagorean sum of the dipole moments. The radial component is generally greater than either of the transverse components for the dipole portion as well as for the nondipolar field contributions.

The effect of the conducting earth on the micropulsation-generating capability of a ground-based current loop

Abstract: It has been suggested that the magnetic field produced by a ground-based current loop carrying a sinusoidally varying current can be used to disturb the lower ionosphere and to produce ultra low frequency hydromagnetic waves, i.e., artificial geomagnetic micropulsations. The magnetic field and hence the effectiveness of the loop as a micropulsation generator are reduced by the presence of induced earth currents in the conducting ground. However, for an average continental location it is shown that the reduction in the magnetic field at E region height directly above a horizontal current loop is less than 20% for frequencies close to 1 Hz. Other changes in the magnetic field, such as a change in its orientation, also appear to be small at 1 Hz. At lower micropulsation frequencies the effect of earth currents on the magnetic field of the loop can probably be neglected completely. Thus we conclude that the presence of the conducting earth is unlikely to impair seriously the micropulsation-generating capability of the horizontal current loop on the ground. The losses caused by earth currents increase rapidly for frequencies just above the micropulsation range, and much greater power would be required if an attempt were made to generate the higher frequencies.

On the solar magnetic 'monopole'

Abstract: A superposed epoch analysis shows that the Sun's spurious magnetic monopole varies like the solar declination. This indicates that the monopole is caused by instrumental effects.

Electric and magnetic fields measured during a sudden impulse

Abstract: The electric field in the ionosphere and the magnetic field at the earth's surface in the mid-latitude region were both measured during a sudden impulse. Ionospheric conductivities deduced from this data were consistent with expectations, thus suggesting that the fluctuations in the magnetic field at the earth's surface were caused by overhead ionospheric currents that were driven by an electric field associated with the sudden impulse.