Showing papers on "Dipole model of the Earth's magnetic field published in 1979"

Generation of Field Aligned Current During Substorm

Abstract: A mechanism for the generation of field aligned currents during magnetospheric substorms is presented. The energy which is injected to the geomagnetic tail is converted into plasma flow energy in solar and anti-solar directions. When the flow meets the inner magnetosphere, a viscous interaction occurs. This interaction creates time-increasing vorticities which produce field aligned currents upward in the pre-midnight sector and downward in the post-midnight sector.

Inversion of satellite magnetic anomaly data

Abstract: A method of finding a first approximation to a crustal magnetization distribution from inversion of satellite magnetic anomaly data is described. Magnetization is expressed as a Fourier series in a segment of spherical shell. Input to this procedure is an equivalent source representation of the observed anomaly field. Instability of the inversion occurs when high frequency noise is present in the input data, or when the series is carried to an excessively high wave number. Preliminary results are given for the United States and adjacent areas.

Relationships between auroral particle distributions and magnetic field perturbations associated with field-aligned currents

Abstract: Simultaneous measurements of magnetic fields and soft particle distributions in the topside auroral ionosphere reveal the relationships between field-aligned currents and primary and secondary auroral particles. In the evening MLT sector the region 2 field-aligned currents, which are directed into the ionosphere, are found to extend on the average 2.4° of invariant latitude equatorward of the low-latitude boundary of 1-keV electron precipitation from the magnetospheric plasma sheet. This current system is found to lie within the mid-latitude total ion trough, as defined by its topside density depletion. This is a region predominantly void of particles between 5 eV and 15 keV. The current carriers are inferred to be upward flowing cold ionospheric electrons. At higher latitudes in the premidnight sector the region 1 field-aligned current system lies within the region of particle precipitation and terminates at its high-latitude boundary coincidently with the termination of energized precipitating particle fluxes. The primary reversal from downward to upward flowing field-aligned currents may be quite sharp or may occur over several degrees of latitude. The current reversal is not necessarily associated with any particular particle precipitation structure.

Mars in the solar wind

Abstract: Recent spacecraft observations and model calculations of the solar wind interaction (SWI) at Mars have shown that the particle pressure is not large enough to stand off the solar wind unless the electron temperature is 4 times the ion temperature in the ionosphere, but the additional pressure is presumably magnetic, provided by a planetary magnetic field In addition, a planetary field corresponding to a surface field of about 20 gammas, and a dipole moment of about 8 x 10 to the 21st gauss per cu cm are implied Calculations indicate that 60-70% of the pressure is supplied by the magnetic field, and 30-40% by the ionosphere Thus, the SWI at Mars is unique, being an interaction both with the atmosphere and with the planetary magnetic field

Singular lines of one-dimensional force-free magnetic field

Abstract: The behavior of adiabatically slow deformations of the force-free field is investigated. Using the linear approximation it is shown that for a rather wide class of boundary perturbations of one-dimensional force-free field there appear singular magnetic force lines or surfaces. Hence the problem of quasi-steady deformation of frozen-in magnetic field has no solution. Relating to the problem of magnetic field in the solar corona it means that there will appear discontinuities (current sheets), when the magnetic field is deformed, for example, due to photospheric motion.

A simple model of the magnetosphere

Abstract: A phenomenological magnetic field model for the earth's magnetosphere is constructed from a dipole field and a uniform field directed sunward in the northern hemisphere and antisunward in the southern hemisphere. The properties of this simple model are compared with those of several other quantitative models. The present model is found to be more suitable for calculations than some other simple models in cases where the distant (>13 Re) magnetotail configuration is important. Moreover, this model is easily adaptable to changes in the field geometry and to the description of magnetotail asymmetries.

An extended investigation of Helios 1 and 2 observations: The interplanetary magnetic field between 0.3 and 1 AU

Abstract: Helios 1 and 2 spacecraft allowed a detailed investigation of the radial dependence of the interplanetary magnetic field components between 0.3 and 1 AU. The behavior of the radial component is in a very good agreement with Parker's model (approximately equal to the inverse square of the heliocentric distance) and the azimuthal component also shows a radial dependence which is close to theoretical predictions (approximately equal to the inverse of the heliocentric distance). Experimental results for the normal component and for the field magnitude are consistent with those from previous investigations. The relative amplitude of the directional fluctuations with periods less than 12 hr is essentially independent of heliocentric distance, while their power decreases approximately as the inverse cube of the heliocentric distance without any appreciable difference between higher and lower velocity regimes.

Instability of a thin magnetic tube in the solar atmosphere

Abstract: The static equilibrium of a thin vertical magnetic tube embedded in the solar atmosphere is shown to be dynamically unstable against the fundamental mode of perturbation having no nodes in the vertical displacement. The instability has its origin in the convection zone, and the eigenfunction is extended further up in the stable upper layers by the magnetic field which guides the displacement mainly in the longitudinal direction. It is suggested that the downdraft observed in the solar network structure is a finite amplitude consequence of this instability. The overtone modes are found to be stable.

The magnetotail magnetic field

Abstract: The magnetic vector potential for points throughout the magnetosphere has been obtained by integrating over a model magnetotail current system. Parameterized functions of x, y, z have then been fit to the potential at these points. The curl of the resultant vector potential function matches the computed magnetic field closely. The final expression for the field presented in this work is a good representation of the field from an arbitrarily thick (or thin) current sheet which diminishes antisolarward from the earth and returns on the surface of the magnetosphere.

Electromagnetic induction effects at an ocean coast

Abstract: This paper reviews recent progress in model calculations towards understanding electromagnetic induction effects at ocean coasts. Early models consisted of two adjacent quarter-spaces of different conductivity, whereas the newer models simulate the ocean with a very thin sheet of a perfect conductor placed on top of a uniform Earth medium. The inducing field is assumed to arise from a monochromatic plane wave incident vertically from above. With any of these models one succeeds at once in explaining the occurrence of large vertical magnetic fields when the inducing electric field is polarized parallel to the coast (E-polarization), thereby also confirming the highly directional character of the coast effects as discovered a few years before by Parkinson. Another important step was made when, first numerically, then analytically, the behavior of the horizontal component of the magnetic field at the surface was rigorously calculated. For H-polarization (inducing magnetic field parallel to shore) this horizontal surface field is uniform, but is not so for E-polarization. Indeed, it has now been shown that the surface field can vary appreciably close to the coast, particularly on the ocean side of shore. With E-polarization, very large currents flow in the ocean, parallel to shore, with the result that the effect of the coast is felt at very large distances over both land and sea. Under H-polarization induction the range of the coast effect is very much shorter, in fact almost an order of magnitude shorter over the land and even reducing to zero at the surface of the perfectly conducting model ocean. The magnetic fields at the ocean floor have also been calculated, which should be of interest in the rapidly expanding field of marine survey and prospecting.

The closed model of the Earth's magnetosphere

Abstract: The existence of large-scale motions within the earth's magnetosphere and that of a long magnetotail were predicted in 1960 as results of a hypothetical frictional interaction between the solar wind and the geomagnetic field. The boundary layer model of this interaction involves the flow of magnetosheath plasma in a magnetospheric boundary layer. The flow is across magnetic field lines, and so the layer must be polarized, with a space charge field nearly balancing the induction field . The space charge tends to discharge through the ionosphere, thus providing some magnetic and related activity as well as the Lorentz frictional force. This closed magnetosphere model has been largely neglected in favor of the reconnection model but is now strongly supported by observational results and their interpretation as follows. (1) The evidence for the reconnection model, increasing activity with a southward interplanetary field and invasion of the polar caps by flare particles, is shown to be equally compatible with the closed field model. (2) The magnetotail grows by the motions of closed flux tubes through the dawn and dusk meridians, a process which depends on the nature of the boundary between magnetosphere and magnetosheath plasmas and perhaps also on the solar wind dynamo. Both of these features depend, in turn, on the direction of the interplanetary magnetic field. (3) Closed field lines entering the tail may be stretched to a few tens of earth radii and then contract back to the corotating magnetosphere. Others enter the long tail and are stretched to hundreds of earth radii and so are pervious to fast solar particles. (4) A new model of the magnetospheric substorm involves the entry of closed field lines into the tail and their rapid return to the corotating magnetosphere. The return is due, first, to the release of their trapped plasma as it becomes electrically polarized and, second, to mounting magnetic and plasma stresses in the inflated magnetotail.

Magnetic contour maps at the core-mantle boundary.

Abstract: Contour maps of the northward, eastward, and downward components of the main geomagnetic field and of the field intensity at the surface of earth's liquid core are presented and compared with similar maps at earth's surface. The mantle is assumed to be an insulator for purposes of the extrapolation. Maximum values of the individual field components are found to be amplified by factors of order 10-20 and the maximum field intensity by 12 over the value at earth's surface and the increase in small scale structure is evident. By comparing maps of places where Br=0 at different truncation levels, we find that even for a recent magnetic model which best fits the data quite well, contours of the field extrapolated to the core are quite sensitive to truncation level. This suggests a need to develop nonlinear data-fitting procedures if one's interest is in precise location of field contours rather than in estimating field values.

Interior magnetohydrodynamic structure of a rotating relativistic star

Abstract: We study the interior magnetohydrodynamic structure of a rotating stationary axisymmetric neutron star. We assume the fluid is ideal, infinitely conducting, and flows only azimuthaly. We justify this assumption by considering in detail the superfluid physics in the interior. We obtain some of our results by taking a certain limit of previously discovered magnetohydrodynamic conservation laws. We show that the angular velocity, electric and magnetic potentials, and the red-shifted chemical potential are constant on magnetic surfaces. We demonstrate that the absence of meridional circulation implies the vanishing of the toriodal magnetic field. This clashes with previous arguments from the probable evolution of the magnetic field during the collapse to the neutron star. We solve completely Maxwell's equations for the distribution of magnetic field strength, and we show that the magnetic surfaces are the equipotentials of a simple geometrical invariant. With neglect of gravitational effects the magnetic field must be uniform in the interior in accordance with the Deutsch model, but at variance with numerous other models which have been proposed for ordinary stars. Gravitation causes the magnetic surfaces to flare out toward the polar regions and enhances the central field as compared to the polar field. The star must bemore » charged; the charge distribution depends on the magnetic field strength and on the angular velocity relative to the local inertial frames.« less

Transient electromagnetic fields of a vertical magnetic dipole on a two-layer earth

Abstract: The transient fields of a vertical magnetic dipole on a two-layer earth model are expressed in analytical form using two different approaches. In the first, the fields in the time domain are obtained as the inverse Laplace transforms of derived full wave time-harmonic solutions, while in the second, the concept of natural frequencies of the stratified earth is utilized. Comparison with a previously obtained approximate solution reveals that the latter is the late time part of the present solution. Important features in the waveforms of the surface fields due to step and pulsed current excitations are demonstrated by a variety of numerical examples. These features provide diagnostic means of sensing the earth's stratification, overburden thickness, and the ratio of conductivities of the layers.

The control of the magnetopause by the interplanetary magnetic field

Abstract: The solar wind dynamic pressure determines the “zeroth-order” location of the earth’s magnetopause. However, the normal stresses of the solar wind dynamic pressure are also accompanied by tangential stresses which erode the magnetopause from its equilibrium position and transport magnetic flux into the magnetotail. It is clear that the tangential stress on the magnetopause is at least in part controlled by the southward component of the interplanetary magnetic field. When the interplanetary magnetic field turns from northward to southward, the magnetopause moves in toward the earth, the polar cusp moves equator-ward, and the polar cap increases in size, as does the diameter of the magnetotail. Since particle observations show that the polar cap magnetic field is directly connected to the interplanetary magnetic field, this observation of magnetopause erosion is an unambiguous demonstration that the process of reconnection is occurring. However, it does not elucidate the physical mechanisms by which such reconnection occurs. The study of the physical processes at the magnetopause and their control by the IMF is actively being investigated on the ISEE mission. Initial results indicate that when the magnetosheath magnetic field is southward the connection takes place in a series of flux transfer events capable of transporting 1016 Mx or more per hour.

Possible displacement of mercury's dipole

Abstract: Earlier attempts to model the Hermean magnetospheric field based on a planet-centered magnetic multipole field have required the addition of a quadrupole moment to obtain a good fit to space vehicle observations. In this work we obtain an equally satisfactory fit by assuming a null quadrupole moment and least squares fitting of the displacement of the planetary dipole from the center of the planet. We find a best fit for a dipole displacement from the planet center of 0.033 RM away from the solar direction, 0.025 RM toward dawn in the magnetic equatorial plane, and 0.189 RM northward along the magnetic dipole axis, where RM is the planet radius. Therefore the presence of a magnetic quadrupole moment is not ruled out. The compressed dipole field more completely represents the field in the present work than in previous work where the intrinsic quadrupole field was not included in the magnetopause surface and field calculations. Moreover, we have corrected a programing error in previous work in the computation of dipole tilt λ away from the sun. We find a slight increase for the planet dipole moment of 190 γ RM³ and a dipole tilt angle λ of only 1.2° away from the sun. All other parameters are essentially unchanged.

Planetary magnetic fields

Abstract: A dimensional analysis, based on a simple model, indicates that at present both Venus and Mercury have a fluid core and an intrinsic magnetic field, but that this is not the case for Mars nor for the Moon.

Spatial distribution of large-scale solar magnetic fields and their relation to the interplanetary magnetic field

Abstract: The spatial organization of the observed photospheric magnetic field, as well as its relation to the polarity of the interplanetary field, have been studied using high resolution magnetograms from Kitt Peak National Observatory. Systematic patterns in the large scale field have been found to be due to contributions from both concentrated flux and more diffuse flux. It is not necessary to assume, as has often been done in previous studies, that there is a weak background solar magnetic field causing the large-scale patterns in the photosphere, although the existence of such a field cannot be excluded. The largest scale structures in the photosphere correspond to the expected pattern at the base of a warped heliomagnetic equator. The polarity of the photospheric field, determined on various spatial scales, correlates with the polarity of the interplanetary field, with the most significant correlation due to mid-latitude fields. However, because the interplanetary field is likely to be rooted in concentrated photospheric regions, rather than across an entire polarity region, both the strength and polarity of the field are important in determining the interplanetary field. Thus studies of the interplanetary field which are based on either instrumental or numerical averaging of fields in the solar photosphere are subject to serious inherent limitations. Analyses based on several spatial scales in the photosphere suggest that new flux in the interplanetary medium is often due to relatively small photospheric features which appear in the photosphere up to one month before they are manifest at the Earth. The evolution of the over-all photospheric pattern may be due to individual sub-patterns which have slightly different rotation properties and which alternate in their relative dominance of the interplanetary medium.

Motion of solar cosmic rays in the coronal magnetic field

Abstract: Trajectories of solar cosmic rays have been calculated in a static ninth-order coronal magnetic field. It is found that as a result of field curvature and gradients, protons drift across the field lines at a rate of up to 200 γβ2 deg hr−1. These drift rates are of the same order as, but somewhat smaller than, empirically derived rates. Localized enhancements of magnetic field have been inserted into the ninth-order field in order to model (in a highly idealized manner) the effects of the small-scale magnetic features which give rise to X-ray bright points. The motions of the particles in the presence of these scattering centers can be parameterized approximately by a cross-field diffusion coefficient. Our estimates of this coefficient, although crude, overlap with empirical values which have been deduced over a wide range of energies.

on the Three-Dimensional Structure of the Solar Magnetic Field in Interplanetary Space

Abstract: The three-dimensional structure of the solar magnetic field in the interplanetary space is inferred from a theoretical point of view. We use the magnetic field produced by a magnetic dipole rotating obliquely in vacuum. The correction for the presence of a plasma surrounding the Sun is taken into account in terms of a phenomenological approximation.

The Origin of the Earth's Magnetic Field

Abstract: A theory is developed and a model described for a homogeneous axisymmetric generator of the geomagnetic field, based on the Nernst effect associated with a radially outward flow of heat from heat sources within the core region of the earth across an initial meridional magnetic field. The thermomagnetic e. m. f. drives a system of two azimuthal current shells in the core region, one nested inside the other, with the currents flowing in opposite directions. The current shells slowly expand radially. As the outer shell decays a new current shell develops inside the inner shell. The resultant magnetic field near and beyond the Earth’s surface approximates to a dipole field that undergoes repeated reversals. A rough estimate of the required magnitude of the Nernst coefficient indicates that the effect could be large enough to drive the generator. The generator does not violate Cowling’s theorem because the temperature gradient, which plays a part analogous to fluid velocity in conventional homogeneous fluid dynamo theory, has a non-zero divergence in regions where heat is being evolved.