Showing papers on "Dipole model of the Earth's magnetic field published in 1988"

Magnetic clouds and force-free fields with constant alpha

Abstract: Magnetic clouds observed at 1 AU are modeled as cylindrically symmetric, constant alpha force-free magnetic fields. The model satisfactorily explains the types of variations of the magnetic field direction that are observed as a magnetic cloud moves past a spacecraft in terms of the possible orientations of the axis of a magnetic cloud. The model also explains why the magnetic field strength is observed to be higher inside a magnetic cloud than near its boundaries. However, the model predicts that the magnetic field strength profile should be symmetric with respect to the axis of the magnetic cloud, whereas observations show that this is not generally the case.

Giotto magnetic-field results on the boundaries of the pile-up region and the magnetic cavity

Abstract: The application of the minimum-variance-technique to the magnetic field measurements obtained during the Giotto encounter at comet P/Halley has led to the following results on the inner plasma boundaries in the magnetoplasma surrounding the comet: (a) Modelling the boundary of the magnetic pile-up region firstly by a half-sphere cylinder combination and secondly by a paraboloid with the axes of both surfaces aligned with the aberrated solar wind has led to stand-off distances towards the aberrated solar wind of 143 000 km and 215 000 km, respectively These can be considered as extreme case without flaring and with strong flaring, respectively

Coronal Magnetic Fields Produced by Photospheric Shear

Abstract: The magneto-frictional method is used for computing force free fields to examine the evolution of the magnetic field of a line dipole, when there is relative shearing motion between the two polarities. It found that the energy of the sheared field can be arbitrarily large compared with the potential field. It is also found that it is possible to fit the magnetic energy, as a function of shear, by a simple functional form.

Where do field lines go in the quiet magnetosphere

Abstract: The state of knowledge concerning the global pattern of geomagnetic field lines is reviewed. Sources of information on that pattern include (1) magnetic-field models, derived directly from magnetic data or indirectly from generally observed properties and from physics; (2) the tracing of magnetospheric features (e.g., polar cusps or the inner edge of the plasma sheet); (3) matching of magnetic flux; and (4) analysis of magnetic fields. Field-line structure inside about 8 earth radii is known fairly well, but beyond that, especially in the tail, the situation becomes rather uncertain and variable. Two particularly difficult problems are the linkage between open field lines and the interplanetary field and the field-line structure of the quiescent magnetosphere following periods of prolonged northward Bz.

Magnetospheric Convection in the Nondipolar Magnetic Field of Uranus

Abstract: A method for determining the magnetospheric convection electric field using simple analytic approximations under the assumption of uniform ionospheric conductivity is described, and applied to Uranus. Magnetic field models including quadrupole and octupole moments are used to determine the shape of the polar caps and the mapping of the electric field and parallel currents between ionosphere and magnetosphere. The asymmetry in the magnetic field models between the northern and southern hemispheres leads to the inclusion of currents between the hemispheres in order to satisfy the assumption of equipotential magnetic field lines. The results show that the quadrupole moment of the Uranian magnetic field strongly influences magnetospheric convection, and that a significant octupole moment will further alter the flow pattern. Even with these modifications the basic flow is sunward in the inner magnetosphere as inferred previously. The total current which flows along field lines between the two hemispheres due to the asymmetry of the magnetic field is comparable in magnitude to that of the region 1 current system. Time dependent calculations including a self-consistent electric field show that ring current shielding of the electric field is important and may have formed the most prominent features in the plasma observations made by Voyager 2. The effectiveness of the shielding can be influenced by the magnetic field model. Other features in the data are characteristic of substorm injection, and the model has been used to show that a combination of plasma injection and electric field shielding may be applicable to the interpretation of the Voyager 2 data.

On the motion of a charge in a magnetic field

Abstract: The motion of a charge in a uniform magnetic field can be solved easily using an 'extra' constant of the motion which the author calls the Landau vector. Various physical situations are considered.

Toward a time-dependent magnetic reconnection model

Abstract: Magnetic field reconnection plays a crucial role in determining the topology of magnetic fields in cosmic plasmas and also provides an efficient way for the conversion of magnetic energy into kinetic energy. Much work has been done on this topic in many interrelated fields of interest by many researchers. The author's primary field of interest is magnetospheric physics, in particular, magnetic reconnection processes in the dayside magnetopause and the magnetotail. Understanding of these processes has improved greatly over the past 3 decades, but there is still much work to be done.

Lower bound for the electrical conductivity of the Earth's outer core

Abstract: The electrical conductivity of the Earth's outer core is an important physical quantity controlling the hydromagnetic process within the liquid outer core which is responsible for the creation and maintenance of the Earth's magnetic field. The conventional method, based on electromagnetic induction by external magnetic field variations, is not applicable to the core because of their insufficient penetration depth. Here we propose an alternative method based on non-dipole magnetic field data, from which fluid velocity fields in the outer core are estimated, and show that a lower bound can be imposed on the electrical conductivity of the outer core from the consideration of the possibility of dynamo action sufficient to maintain the Earth's magnetic field. The conclusion is that the electrical conductivity is larger than about 1×105 S/m. This conclusion is consistent with the results of recent experiments made for possible core material.

Statistic study on asymmetric propagation of flare-associated interplanetary shock waves

Abstract: On the basis of the comprehensive studies of solar flare-associated geophysical effects (geomagnetic disturbances and the Forbush decreases of cosmic rays), multi-spacecraft observations of solar wind and interplanetary scintillation (IPS) observations, we have furthered our knowledge of the flare-shock wave propagation in the interplanetary space. The main results are that the flare-shock wave is usually an asymmetric propagation related to the flare normal, the direction of the fastest propagation velocity in longitudes tends to deflect towards the interplanetary spiral magnetic field and that in latitudes approaches heliospheric, current sheet, the dynamic parameters (V, P, N, T, β, etc.) and the. shock wave energy. E, are higher in the southeast than in the northwest of the flare shock wave, and conversely, the magnetic field is weaker in the southeast than in the northwest. Based on the results, we proposed a 3-dimensional asymmetrical model of flare-shock wave propagationst. This model is consistent with recent statistical studies obtained by analysing a great deal of spacecraft observation data, and may naturally explain the asymmetric distribution of geomagnetical effects in company with the parent flare positions. The results of this paper could help develop the propagation theory of shock waves and the prediction of the solar-terrestrial disturbance events.

Magnetic Effects on Solar p-Modes

Abstract: Both analytical and numerical studies of the effect of a magnetic field at the base of the convection zone on p-modes are presented. It is argued that the recently reported changes in the low degree p-mode frequencies, from 1980 to 1984, may result from corresponding changes in the magnetic field strength. A lower limit of some 5 × 105 − 106 gauss is implied for the field strength at the base of the convection zone.

On the structure of the interplanetary magnetic field under non-stationary boundary conditions

Abstract: The structure of the interplanetary magnetic field (IMF) is investigated theoretically in a kinematic approximation under frozen-in conditions in the solar wind and non-stationary boundary conditions. As an example, a time-dependent model of the IMF, created in the case of a change of the general magnetic field of the Sun represented by the dipole term, is analyzed. Very simple assumptions as to the field of velocities in the solar wind are made. The results show the formation of zero IMF points of two types(O, X). Points of theO-type are formed and move radially in the equatorial plane. They are surrounded by magnetic clouds with loops of lines of force. Points of theX-type are formed and move radially above the poles.

Ionospheric scintillations and in-situ measurements at an auroral location in the European sector

Abstract: : The orbiting HiLat satellite offered a unique opportunity for studying the ionospheric scintillation parameters in relation to the in-situ measurements of ionization density, drift velocity, field-aligned current, and particle precipitation during the sunspot minimum period. This paper discusses the results of such a morphological study based on observations at the auroral oval station of Tromso, Norway. The dynamics of the spatial and temporal extent of this region are illustrated in the invariant latitude/magnetic local time grid. The geometrical enhancement of scintillations observed during the alignment of the propagation path with the local magnetic L-shell is shown to be the most consistent and conspicuous feature of scintillations in the nighttime auroral oval. The steepening of phase spectral slope in this region is indicative of the presence of L-shell aligned sheet-like irregularities at long scale lengths. The seasonal variational of total electron content (TEC) determined from the differential Doppler measurements of HiLat transmissions is discussed in relation to the in-situ density measurements at 830 km. The results are also utilized to illustrate the dependence of ionospheric structure parameters on short-term variability of solar activity during the sunspot minimum period. Special effort is made to illustrate that the joint study of scintillation/TEC and in-situ parameters provides an insight into the nature of magnetospheric coupling with the high latitude ionosphere. Keywords: Amplitude and phase scintillations; Total electron content; In-situ density; Auroral oval; Sunspot cycle variation; Irregularity anisotropy; Reprints.

Dipole axially symmetric, external field components in the analysis of the geomagnetic field.

Abstract: Ground measurements of the geomagnetic field contain components from external sources and associated currents induced in the Earth. Some of the external sources, namely ring, magnetotail, and boundary (Chapman-Ferraro) currents, produce relatively uniform fields in the vicinity of the Earth, which approximately parallel the dipole axis of the Earth. These fields are, in part, time-varying on a scale of a few minutes to a few days, but as long as the solar wind blows, they must also contain a quasi-steady component with a time scale of a few tens of days to several years. We develop correction equations to take into account the time-varying part of such axially symmetric external fields using the Dst, index. These equations are then applied to the inclination values Im obtained from the Project MAGNET data for the interval 1957-1966. Even after Dst corrections are made on the difference angle Im-Ic, where c denotes the inclination computed from internal terms only, a residual field remains. We believe this residual field to be evidence for the existence of a steady part of the field that cannot be accounted for by A corrections, and we estimate this residual field, assuming that it is axially symmetric. The steady field thus calculated has an equatorial value of about 40nT, which corresponds to the external spherical-harmonic coefficient g0e1 of the same order and positive sign. The positive sign implies the steady component in the vicinity of the Earth is direction southward.

Geomagnetic parameters derived from an analytic description of the Earth's magnetic field.

Abstract: A recent geomagnetic model containing 720 trend and trigonometric coefficients represents each spherical harmonic coefficient as a continuous function of time which exactly duplicates the IGRF coefficients at the nine IGRF epochs (1945 to 1985 at five year intervals) and allows a reasonable prediction five years beyond the last epoch. Use of this model permits time derivatives of magnetic elements and other derived quantities to be taken so that a detailed look at several geomagnetic parameters vs time can be obtained. It is shown that from 1945 to 1985 the quadrupole field contributed 2.6 times more power to the secular variation field on the surface of the Earth than the dipole field. It is also shown that the root-mean-squared surface field shows a strong periodicity which may be related to the solar cycle. This apparent solar-cycle effect diminishes with time as more satellite data are used indicating that an appreciable part of it may be caused by noisy data and a poor distribution of data for a couple of decades following World War II.

Magnetic Dipole Excitation of a Long Conductor in a

Abstract: Formulations for the excitation of currents on an infinitely long conductor by electric or magnetic dipoles of arbitrary orientation are presented. The conductor can be either insulated or hare to model ungrounded or grounded conductors. Specific calculations are pre- sented for a vertical magnetic dipole source because this source pro- duces the appropriate horizontal polarization and could be used in a borehole-to-borehole configuration. Numerical results for the induced current and secondary magnetic field indicate that long conductors produce a strong anomaly over a broad frequency range. The second- ary magnetic field decays slowly in the direction of the conductor and eventually becomes larger than the dipole source field.

Geomagnetic Field Energy as a Source of Thrust

Abstract: : This study determines analytic solutions to the interactive net force between the dipole created by a current loop and a non-uniform expanse of the geomagnetic field A simple dipole approximation is used for the geomagnetic field and it is assumed that the current loop can be maintained in any desired orientation Analytic solutions are found for four different orientations: the current loop parallel to the earth's surface, the current loop perpendicular to the earth's surface, the current loop dipole parallel to the instantaneous geomagnetic field, and an inertial orientation A hypothetical satellite current loop system is discussed and the action-at-a-distance magnetic interaction force is compared to aerodynamic drag and solar radiation pressure The magnetic force is also compared to the force that some space thrusters produce In terms of lifetime impulse or total impulse deliverable over one orbit, the magnetic force compares favorably to many thrusters in use today or under consideration Keywords: Magnetic forces; Space propulsion; Geomagnetic field; Theses

The magnetostatic dipole inversion problem in remote sensing

Abstract: The static magnetic dipole inversion problem is related to the problem of location and identification of compact ferrous objects. It is shown that the inversion of the dipole field or gradient is a necessary first step toward determining object location and identity. Several recursive and nonrecursive methods of determining dipole moment and location from field or gradient measurements are described and compared. It is shown that given dipole moment estimates, it is possible to determine the identity of the dipole source in practical situations by pattern recognition.

Comments, with reply, on "An MHD model for the Earth's magnetic field with spatially dependent electrical conductivity" by I. Alexeff and J.R. Roth

Abstract: It is shown that the MHD model for the Earth's magnetic field recently proposed by Alexeff and Roth (ibid., vol.PS-14, no.6 pp.862-4, Dec. 1986) does not work as a self-excited dynamo but has to be considered as an amplification mechanism for a field due to causes different from fluid motions. The authors reply that they agree with D. Radler's comments, and that they had explicitly stated the point in question in the latter part of their paper. They also give two additional points to consider. >