Showing papers on "Dipole model of the Earth's magnetic field published in 1998"

The magnetic field of the earth : paleomagnetism, the core, and the deep mantle

Abstract: History of Geomagnetism and Paleomagnetism: Discovery of the Main Magnetic Elements. Fossil Magnetism and the Magnetic Field in the Past. Investigations of the External Magnetic Field. Origin of the Earth's Magnetic Field. The Present Geomagnetic Field: Analysis and Description from Historical Observations: Magnetic Elements and Charts. Spherical Harmonic Description of the Earth's Magnetic Field. Uniqueness and Other Mathematical Problems. Geomagnetic Secular Variation. The External Magnetic Field. Foundations of Paleomagnetism: Rock Magnetism. Magnetic Mineralogy. Paleomagnetic Directions and Poles. Paleointensity Methods. Age Determinations. The Recent Geomagnetic Field: Paleomagnetic Observations: Archeomagnetic Results. Analysis of Recent Lake Sediments. Geomagnetic Excursions. The Geomagnetic Power Spectrum. Reversals of the Earth's Magnetic Field: Evidence for Field Reversal. Marine Magnetic Anomalies. Analysis of Reversal Sequences. Polarity Transitions. The Time-Averaged Paleomagnetic Field: Geocentric Axial Dipole Hypothesis. Second-Order Terms. Variation in the Earth's Dipole Moment. Paleosecular Variation from Lavas (PSVL). Processes and Properties of the Earth's Deep Interior: Basic Principles: Seismic Properties of the Earth's Interior. Chemical and Physical Properties. Thermodynamic Properties of the Earth's Deep Interior. Thermal History Models. Non-dynamo Models for the Earth's Magnetic Field. Fluid Mechanics Fundamentals. Energy Sources. Introduction to Dynamo Theory: The Dynamo Problem. The Magnetic Induction Equation. The a and w Effects of Dynamo Theory. Waves in Dynamo Theory. Symmetries in Dynamo Theory. Theories for Geomagnetic Secular Variations and magnetic Field Reversals. Dynamo Theory: Vector Spherical Harmonics. Kinematic Dynamos. Cowling's Theorem and Other Constraints. Turbulence in the Core. Dynamo Waves. Dynamics of the Geodynamo. The Magnetic Fields of the Sun, Moon, and Planets: Origin of the Solar System. The Sun. The Moon. Meteorites. Magnetic Fields of the Planets. Geomagnetic Relevance. Examples of Synthesis: Fluid Velocities in the Core. Core-Mantle Coupling: Length of Day. Paleomagnetism and Dynamo Theory. Variations at the Core-Mantle Boundary and the Earth's Surface. Appendices. References. Subject Index.

The sub-Alfvénic interaction of the Galilean satellites with the Jovian magnetosphere

Abstract: Recent observations by the Galileo spacecraft and Earth-based techniques have motivated us to reconsider the sub-Alfvenic interaction between the Galilean satellites of Jupiter and the magnetosphere. (1) We show that the atomic processes causing the interaction between the magnetoplasma and a neutral atmosphere can be described by generalized collision frequencies with contributions from elastic collisions, ion pickup, etc. Thus there is no fundamental difference in the effect of these processes on the plasma dynamics claimed in the recent literature. For a magnetic field configuration including possible internal fields, we show that the sub-Alfvenic, low-beta interaction can be described by an anisotropically conducting atmosphere joined to an Alfven wing as one extreme case and the Jovian ionosphere as the other extreme case. (2) The addition of a small magnetic field of internal origin does not modify the general Alfven wing model qualitatively but only quantitatively. All magnetic moments discussed in the literature for In are small in this sense. For an aligned internal dipole and ambient Jovian magnetic field the interaction will be enhanced by focusing of the electric field. (3) A qualitative change occurs by the additional occurrence of closed magnetic field lines for larger internal magnetic fields as in the case of Ganymede. Here the focusing is even enhanced. (4) The first discussion of nonstationary plasma flows at the satellites shows that electromagnetically induced magnetic fields may play an important role if the satellite interiors are highly conducting. From the point of view of the external excitation, induction effects may be strong for Callisto, In, Europa, and Ganymede in order of decreasing importance. The magnetic field observations at the first Callisto encounter can be explained by these effects.

Cosmic magnetic fields in large scale filaments and sheets

Abstract: We consider the possibility that cosmic magnetic field, instead of being uniformly distributed, is strongly cor- related with the large scale structure of the universe. Then, the observed rotational measure of extra-galactic radio sources would be caused mostly by the clumpy magnetic field in cos- mological filaments/sheets rather than by a uniform magnetic field, which was often assumed in previous studies. As a model for the inhomogeneity of the cosmological magnetic field, we adopt a cosmological hydrodynamic simulation, where the field is passively included, and can approximately represent the real field distribution with an arbitrary normalization for the field strength. Then, we derive an upper limit of the magnetic field strength by comparing the observed limit of rotational measure with the rotational measure expected from the magnetic field geometry in the simulated model universe. The resulting upper limit to the magnetic field in filaments and sheets is Bfs < 1G which is 10 3 times higher than the previously quoted values. This value is close to, but larger than, the equipartition magnetic field strength in filaments and sheets. The amplification mech- anism of the magnetic field to the above strength is uncertain. The implications of such a strength of the cosmic magnetic field are discussed.

Complex image method for calculating electric and magnetic fields produced by an auroral electrojet of finite length

Abstract: . The electromagnetic field due to ionospheric currents has to be known when evaluating space weather effects at the earth's surface. Forecasting methods of these effects, which include geomagnetically induced currents in technological systems, are being developed. Such applications are time-critical, so the calculation techniques of the electromagnetic field have to be fast but still accurate. The contribution of secondary sources induced within the earth leads to complicated integral formulas for the field at the earth's surface with a time-consuming computation. An approximate method of calculation based on replacing the earth contribution by an image source having mathematically a complex location results in closed-form expressions and in a much faster computation. In this paper we extend the complex image method (CIM) to the case of a more realistic electrojet system consisting of a horizontal line current filament with vertical currents at its ends above a layered earth. To be able to utilize previous CIM results, we prove that the current system can be replaced by a purely horizontal current distribution which is equivalent regarding the total (=primary + induced) magnetic field and the total horizontal electric field at the earth's surface. The latter result is new. Numerical calculations demonstrate that CIM is very accurate and several magnitudes faster than the exact conventional approach. Key words. Electromagnetic theory · Geomagnetic induction · Auroral ionosphere

Magnetic cloud field intensities and solar wind velocities

Abstract: For the sets of magnetic clouds studied in this work we have shown that there is a general relationship between their magnetic fields strength and velocities. With a clear tendency that the faster the speed of the cloud the higher the magnetic field.

An analytic solar magnetic field model

Abstract: We describe a simple analytic model for the mag- netic field in the solar corona and interplanetary space which is appropriate to solar minimum conditions. The model com- bines an azimuthal current sheet in the equatorial plane with an axisymmetric multipole field representing the internal mag- netic field of the Sun. The radial component of the field filling interplanetary space is approximately monopolar at large helio- centric distances as observed. These open field lines connect to the polar regions of the Sun and define the polar coronal holes which are prevalent at solar minimum and which are the source of the fast solar wind. By including both dipole and a quadrupole terms at the origin it is possible to construct a good representa- tion of the coronal magnetic field in such conditions. We also note that the Parker spiral will be underwound relative to the case of the monopole because the open field lines emanate from solar latitudes in excess of 60.

Proton Acceleration near an X-Type Magnetic Reconnection Region

Abstract: We investigate the behavior of protons near an X-type magnetic reconnection region by numerical simulations. The magnetic field is taken to be hyperbolic and time stationary with a uniform electric field perpendicular to the magnetic field. We also study the effects of the magnetic field along the uniform electric field. We found from many parametric runs that the energy spectrum of accelerated protons near an X-type magnetic reconnection region is universal with a power-law spectrum E-γ, where the power-law index γ is about 2.0-2.2. The acceleration time of protons with the energy range of 1-20 MeV is very rapid and within ~102ω−1ci , which is much less than 1 s for solar flare plasmas. We compare our results with some observations of solar flares.

Spatial Distribution of Ionospheric Plasma and Field Structures in the High-Latitude F Region

Abstract: Ion density and velocity measurements from the Dynamics Explorer 2 (DE 2) spacecraft are used to obtain the average magnetic local time versus invariant latitude distribution of irregularities in the high-latitude F region ionosphere. To study the small-scale structure and its relationship to background conditions in the ionosphere, we have formed a reduced database using 2-s (approx. = 16 km) segments of the ion density and velocity data. The background gradients associated with each 2-s segment and the spectral characteristics, such as power at 6 Hz (approx. = 1.3 km) and spectral index, are among the reduced parameters used in this study. The relationship between the observed plasma structure and its motion is complex and dependent on the externally applied fields as well as locally generated plasma structure. The evolution of plasma structures also depends critically on the conductivity of the underlying ionosphere. Observations indicate an enhancement of irregularity amplitudes in two spatially isolated regions in both the ion density and the velocity. Convective properties seem to play a more important role in winter hemisphere where smaller-scale structures are maintained outside the source regions. (Delta)V irregularity amplitudes are enhanced in the cusp and the polar cap during northward interplanetary magnetic field regardless of season. The power in (Delta)V is usually higher than that associated with local polarization electric fields, suggesting that the observed structure in (Delta)N/N is strongly influenced by (Delta)V structure applied to large density gradients.

Electron precipitation accompanying Pc 5 pulsations observed by the DE satellites and at a ground station

Abstract: Using data from the polar orbiting Dynamic Explorer (DE) −1 and −2 satellites and a ground-based station, we investigated electron precipitation phenomena accompanying Pc 5 pulsations. DE-2 observed oscillatory disturbances in the magnetic and electric fields in the upper ionosphere at the geomagnetic footprint of the high altitude region in which transverse Pc 5 pulsations were detected by DE-1. DE-2 observed electrons precipitating into the ionosphere with energies of several keV to several tens of keV. These electrons were accelerated in the direction of the ambient magnetic field. When Pc 5 pulsations in the H-component and periodic variations of cosmic radio noise absorption (CNA pulsations) were observed at Syowa Station, DE-2 which was in geomagnetic conjunction with Syowa Station also observed oscillatory disturbances in the magnetic and electric fields. These oscillatory disturbances are caused by small-scale field-aligned currents each with width of 0.5°–1.4° invariant latitude. This suggests that Pc 5 pulsations have a small-scale resonance structure in the radial direction. The resonance structure has a small scale comparable to the ion acoustic gyroradius, then kinetic Alfven waves having electric fields parallel to the ambient magnetic field can arise. The parallel electric field generates a field-aligned potential drop of about 3–5 kV. Electrons accelerated by these kinetic Alfven waves would cause CNA pulsations, the phase of which leads that of the H-component of the Pc 5 pulsations by 90° in the southern hemisphere. This is consistent with the observations at Syowa Station.

Fractal properties of the IMF and the Earth's magnetotail field

Abstract: The time series of the interplanetary and Earth’s magnetotail magnetic fields are analyzed. The data subsets taken by the IMP 8 spacecraft outside and inside of the Earth’s magnetosphere show clearly the fractal properties: their “box dimensions” are found to be close to 1.7 and 1.5 respectively. It is shown that data gaps have a little effect on the results. The magnetic field increment distributions are found to be non-Gaussian, but self-affine.

Heliospheric magnetic field strength out to 66 AU: Voyager 1, 1978–1996

Abstract: We discuss Voyager 1 (V1) observations of the heliospheric magnetic field strength from 1978 through 1996. During this period the distance of V1 from the Sun increased from ≈3 AU to 66 AU and its heliographic latitude increased from ≈5°S to 33°N. The magnetic field strength profile observed by V1 is consistent with Parker's spiral field model when one considers (1) the solar cycle variation of the observed magnetic field strength at 1 AU, B1(t) (which is a measure of the source field strength) and (2) the latitudinal and solar cycle variations of the solar wind speed, V(t, θ). Both B1(t) and V(t, θ) make significant contributions to the variation of the magnetic field strength variations observed by V1. There is no evidence for a “magnetic flux deficit” increasing with distance from the Sun. There is a solar cycle variation of the magnetic field strength in the outer heliosphere, which will affect the modulation of cosmic rays.

The magnetic structure of B ≠ 0-reconnection

Abstract: Magnetic reconnection can be interpreted as a process in which the electromagnetic field is frozen into a four-velocity field in Minkowski space. For reconnection to occur the four-velocity field has to be a special type of stagnation flow. Prescribing this type of flow in a finite spatial domain allows the modelling of localized reconnection events and the investigation of examples of reconnection in regions without magnetic nulls. In the present contribution, we start with a simple twisted magnetic flux tube. Reconnection occuring along a part of the axis of the tube results in a structure of the magnetic field which is a superposition of a two-dimensional X-type magnetic field well-known from stationary 2D reconnection models, and a component resulting from the magnetic field parallel to the axis. For localized reconnection, the latter component of the magnetic field evolves in a non-trivial way. This evolution is important for the spatial variation of the parallel electric field integrated along the magnetic field lines. The integrated electric field gives an upper limit for the energy to which particles can be accelerated in a reconnection event and its distribution shows to be localized in very thin structures.

Generation of a magnetic field due to excited Q -balls

Abstract: We investigate phase transitions due to excited $Q$-balls. As excited $Q$-balls have angular momentum, a magnetic field can be generated if one considers gauged $Q$-balls. Based on the course of the phase transition we estimate the strength of the magnetic field and then we find that it might be the origin of observed magnetic fields of astrophysical objects such as galaxies and clusters of galaxies.

Mapping between the ionospheric and the tail electric fields in a time‐dependent Earth's magnetosphere

Abstract: In this paper we use the concept of the Faraday loop to connect the ionosphere and the tail along the geomagnetic field lines to study the coupling between ionospheric and magnetospheric electric fields. The formulation using the Faraday loop shows that the coupling consists of three contributions: One is the familiar mapping of the ionospheric field to the tail along equipotential magnetic field lines. The other two are parallel potential differences between the tail and the ionosphere distributed across the magnetic field lines and magnetic flux transport across the Faraday loop due to time evolution of the magnetic field, which gives rise to an inductive electric field perpendicular to the magnetic field. Application of this method requires a model for the ionospheric electric field, a model distribution of the parallel potential differences, and a time-evolving magnetic field model. In the present study the ionospheric electric field pattern is described using a statistical model. The parallel potential differences are modeled by Gaussian distributions in latitude as narrow longitudinally elongated ridges. The magnetic field model describes the time evolution of the geomagnetic field during the substorm growth phase. We show that the effects of parallel potential differences and the inductive fields are of the same order as the mapped ionospheric field, and hence they must be taken into account when the large-scale coupling is studied.

Substorm research moves toward a unifying framework

Abstract: The magnetosphere is the region of space that contains the geomagnetic field confined by the solar wind, which is the plasma flowing from the Sun at high speeds Figure 1 depicts the structure of the magnetosphere The magnetotail consists of open field lines in the tail lobe and closed field lines in the plasma sheet As shown in Figure 1, open field lines are connected with the interplanetary magnetic field so that one end of the open field line is on Earth while the other end of the field line is in the solar wind For the closed field line, both ends are anchored on Earth

Unusual features of the January 1997 magnetic cloud and their effect on optical dayside auroral signatures

Abstract: We study features of the January 1997 magnetic cloud and their effect on the optical aurora at 0630–0930 magnetic local time (MLT). WIND data suggest a thin plasma depletion layer (PDL) preceding the cloud, at whose outer edge the magnetic field rotates southwards, the proton temperature has a local maximum, and the dynamic pressure drops by a factor of 2. At the cloud's front boundary there is a further dynamic pressure drop, another localized temperature rise, and a ∼30° field shear. Prior to arrival of the southward rotation at Earth, the ∼08 MLT aurora is dominated by forms with intense 557.7 nm emission, presumably of boundary plasma sheet origin, located south of zenith (∼75° MLAT) and moving eastward. Minutes after the southward rotation reaches Earth, this emission is replaced by an auroral form which encroaches into the field-of-view at ∼ 73°MLAT, expands southward to ∼70° MLAT and westward, approaching ∼0800 MLT at 0600 UT. Its latitudinal positions satisfy a known relation between cusp latitude and interplanetary BZ, and its morphological and spectral properties are similar to those previously associated with an energy-latitude dispersion signature attributed to plasma transfer at an open low latitude boundary layer. We argue that this auroral precipitation at such far off-noon MLTs may be due to enhanced reconnection occasioned by the strongly southward field in the PDL/cloud. A POLAR pass across the open/closed field line boundary at 66° invariant latitude is consistent with these inferences.

Elementary Theoretical Forms for the Spatial Power Spectrum of Earth's Crustal Magnetic Field

Abstract: The magnetic field produced by magnetization in Earth's crust and lithosphere can be distinguished from the field produced by electric currents in Earth's core because the spatial magnetic power spectrum of the crustal field differs from that of the core field. Theoretical forms for the spectrum of the crustal field are derived by treating each magnetic domain in the crust as the point source of a dipole field. The geologic null-hypothesis that such moments are uncorrelated is used to obtain the magnetic spectrum expected from a randomly magnetized, or unstructured, spherical crust of negligible thickness. This simplest spectral form is modified to allow for uniform crustal thickness, ellipsoidality, and the polarization of domains by an periodically reversing, geocentric axial dipole field from Earth's core. Such spectra are intended to describe the background crustal field. Magnetic anomalies due to correlated magnetization within coherent geologic structures may well be superimposed upon this background; yet representing each such anomaly with a single point dipole may lead to similar spectral forms. Results from attempts to fit these forms to observational spectra, determined via spherical harmonic analysis of MAGSAT data, are summarized in terms of amplitude, source depth, and misfit. Each theoretical spectrum reduces to a source factor multiplied by the usual exponential function of spherical harmonic degree n due to geometric attenuation with attitude above the source layer. The source factors always vary with n and are approximately proportional to n(exp 3) for degrees 12 through 120. The theoretical spectra are therefore not directly proportional to an exponential function of spherical harmonic degree n. There is no radius at which these spectra are flat, level, or otherwise independent of n.

Initial polar magnetic field experiment observations of the low-altitude polar magnetosphere: Monitoring the ring current with polar orbiting spacecraft

Abstract: The equatorial ring current and the magnetopause current both contribute to the magnetic field at low altitudes over the polar cap. In this region the magnetic field is dominated by the Earth's internal field and is well described by empirical models. The average change in the magnetic field strength due to external sources (the residual of the observed magnetic field strength upon subtracting the International Geomagnetic Reference Field (IGRF) 95 internal field model) is typically a few tens of nanoTesla, or a few tenths of 1% of the total magnetic field over the polar cap at the altitude of the Polar spacecraft. It is easier to measure such small differences in the total field than in the vector components because the accuracy of the residuals in the vector magnetic field depends on the accuracy of the knowledge of spacecraft pointing which is generally less well known than the position of the spacecraft. In order to isolate the ring current effects on the total field we adjust the Dst index for the contributions of the magnetopause current using the solar wind dynamic pressure, and we adjust the observed field values for the same effect using the Tsyganenko [1996] model. After these adjustments the ring current strength as measured by the adjusted Dst index is well correlated with the residual of the field strength observed in the low-altitude polar region over a wide range of local times except when the spacecraft track is near the noon-midnight meridian. These comparisons, using more than a full year of Polar data, demonstrate that the Tsyganenko [1996] model together with the IGRF 1995 internal field model provides a good baseline for the magnetic field at the altitude of the Polar perigee passes (∼5000 km above the Earth's surface). Further they demonstrate that total field measurements from low-altitude polar orbiting spacecraft are potentially useful as monitors of the ring current when they cross the polar cap.

POLAR magnetic field observations at apogee during the January 1997 magnetic cloud event

Abstract: Magnetic field measurements from the POLAR satellite are presented for three dusk to dawn apogee passes during the January 1997 magnetic cloud event. The variability in the residual fields from one orbit to the next are investigated in terms of the changing solar wind and magnetospheric current systems. The observed fields are compared to the Tsyganenko 96 (T96_01) model which allows dynamic solar wind and Dst inputs. The model fields are found to generally agree with the observed fields with the greatest discrepancy occurring during periods of large dynamic pressure. An investigation of the contributions to the T96_01 model by the various solar wind and Dst inputs provides insight into the solar wind control of the high altitude magnetosphere above the polar cap. It is found that the ring current is responsible for the largest proportion of the observed magnetic field residuals while the interplanetary magnetic field and solar wind dynamic pressure control a smaller proportion primarily producing the shorter time scale variations in the observed fields.

SAMPEX observations of energetic electron precipitation in the dayside low‐latitude boundary layer

Abstract: We address the problem of the magnetic topology of the dayside low-latitude boundary layer by using conjugate observations from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and the Defense Meteorological Satellite Program Flight 10 (DMSP F10) satellites while they are in coplanar polar orbits in the 0900-2100 magnetic local time (MLT) meridian. In the region identified in the DMSP observations as the low-latitude boundary layer (LLBL), we observe energetic electron precipitation at SAMPEX in detectors measuring electrons in the energy ranges E > 150 keV and E > 1.05 MeV. This precipitation is in the region that would otherwise be identified as the polar cap. We find that the equatorward edge of the energetic electron precipitation coincides precisely with the equatorward edge of the LLBL as these boundaries move over a wide range of invariant latitude, We investigate the hypothesis that the energetic electron precipitation is due to pitch angle scattering in the magnetopause current layer of magnetospheric electrons on open drift paths and find that this hypothesis is reasonable. Additional evidence supporting this hypothesis is the dependence of the precipitating flux on the solar wind clock angle. A consequence of this hypothesis is that the low-latitude boundary layer in the 0900-1000 MLT sector lies completely on open magnetic field lines except, perhaps, when the interplanetary magnetic field is predominantly northward.

A current sheet model for the Earth’s magnetic field

Abstract: As an example in magnetostatics we consider the main magnetic field of the Earth and its current sources. The measured field on the surface is accurately given, in tables of the International Geological Reference Field, in terms of Gaussian coefficients. By applying Maxwell’s equations to these data we calculate the extended field, inside the Earth, and give graphical representations of it. We also construct a simple theoretical model of the source of the field, in which the field is the result of currents flowing on the surface of a sphere inside the Earth. The current sources which give the observed field are calculated in terms of vector spherical harmonics. The stream function and currents are displayed on a Mercator projection for a sphere whose radius is half the Earth’s radius. Interesting properties of vector operations on the Mercator plane are analytically and graphically described.

Rsfp predictions for transverse solar magnetic field distribution from solar neutrino data

Abstract: Even though the standard solar model (SSM) has been very successful in predicting the thermal and nuclear evolution of the Sun, it does not throw enough light on solar magnetic activity. In the absence of a generally accepted theory of solar dynamo, various general arguments have been put forth to constrain solar magnetic fields. In the absence of reliable knowledge of solar magnetic fields from available astrophysical data, it may be worthwhile to constrain the solar magnetic fields from solar neutrino observations assuming Resonant Spin-Flavor Precession (RSFP) to be responsible for the solar neutrino deficit. The configuration of solar magnetic field derived in this work is in reasonably good agreement with the magnetic field distribution proposed by Akhmedov et al. (Sov. Phys. JETP68, 250 (1989)). However, the magnetic field distribution in the radiation zone used by Pulido (Phys. Rep.211, 167 (1992)) is ruled out. The magnitude of the magnetic field in the radiation and convective zones of the Sun are very sensitive to the value chosen for the neutrino magnetic moment. However, any change in the value of neutrino magnetic moment does not affect the magnetic field distribution as it only scales the magnetic field strength at different points by the same amount.

An interpretation of the non-tidal secular variation in the earth rotation: The interaction between the solar wind and the earth's magnetosphere

Abstract: The long period variation of the earth rotation is generally explained by the tidal friction. The tidal friction, however, is not the only source to influence the earth rotation in long term. In this paper, by means of the interaction between the solar wind and the magnetosphere of the earth, the additional magnetic pressure will exist in the magnetic tail due to the crowding and sparseness of the magnetic lines in the consideration of the earth rotation, which could be considered as a source of effecting the long term variation of the earth rotation. It is shown in this paper that this mechanism can produce angular deceleration of the Earth rotation in the magnitude of ω = −1.7 × 10-22 s-2. This result might be a prompt to search for other sources in the secular variation of the rate of the Earth rotation variation further in order to regulate the observed result with the theoretical one.