Showing papers on "Dipole model of the Earth's magnetic field published in 1992"

Time-dependent mapping of the magnetic field at the core-mantle boundary

Abstract: We consider the problem of constructing a time-dependent map of the magnetic field at the core-mantle boundary. We use almost all the available data from the last 300 years to produce two maps, one for the period 1690-1840 and the other for 1840-1990. We represent the spatial dependency of the field using spherical harmonics, the time dependency using a cubic B-spline basis, and seek the smoothest solutions compatible with the observations. We argue that, for observations from permanent magnetic observatories, the most efficient strategy is to use the first differences of annual means; for satellite data, the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field. The resulting model fits the observatory data better than any previous model. The resulting time-dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies.

Particle scattering and current sheet stability in the geomagnetic tail during the substorm growth phase

Abstract: The particle scattering and current sheet stability features in the geomagnetic tail during the phase of substorm growth were investigated using Tsyganenko's (1989) magnetic field model. In a study of four substorm events which were observed both in the high-altitude nightside tail and in the auroral ionosphere, the model magnetic field was adjusted to each case so as to represent the global field development during the growth phase of the substorms. The model results suggest that the auroral brightenings are connected with processes taking place in the near-earth region inside about 15 earth radii. The results also suggest that there is a connection between the chaotization of the electrons and the auroral brightenings at substorm onset.

Magnetic field, reconnection, and particle acceleration in extragalactic jets

Abstract: Extra-galactic radio jets are investigated theoretically taking into account that the jet magnetic field is dragged out from the central rotating source by the jet flow. Thus, magnetohydrodynamic models of jets are considered with zero net poloidal current and flux, and consequently a predominantly toroidal magnetic field. The magnetic field naturally has a cylindrical neutral layer. Collisionless reconnection of the magnetic field in the vicinity of the neutral layer acts to generate a non-axisymmetric radial magnetic field. In turn, axial shear-stretching of reconnected toroidal field gives rise to a significant axial magnetic field if the flow energy-density is larger than the energy-density of the magnetic field. This can lead to jets with an apparent longitudinal magnetic field as observed in the Fanaroff-Riley class II jets. In the opposite limit, where the field energy-density is large, the field remains mainly toroidal as observed in Fanaroff-Riley class I jets. Driven collisionless reconnection at neutral layers may lead to acceleration of electrons to relativistic energies in the weak electrostatic field of the neutral layer. A simple model is discussed for particle acceleration at neutral layers in electron/positron and electron/proton plasmas.

The ionospheric effects of a weak intrinsic magnetic field at Mars

Abstract: An improved model of the Martian ionosphere which allows the magnetic field to have any direction in the horizontal plane is presented, as well as results of calculations for several different intrinsic magnetic field strengths and directions. When the solar wind dynamic pressure exceeds the Martian ionospheric thermal pressure, the plasma motion is weakly downward throughout the ionosphere for the case of no intrinsic magnetic field, but when the intrinsic and induced fields are in opposite directions, the plasma flow tends to converge toward the current sheet. As a consequence of this convergence, the plasma density is somewhat enhanced near the current sheet, which is located near an altitude of 170 km. The ionosphere above an altitude of about 190 km is not significantly affected by the existence of an intrinsic field as weak as 60 nT.

Intensity variations in the interplanetary magnetic field measured by Voyager 2 and the 11‐year solar cycle modulation of galactic cosmic rays

Abstract: New evidence is presented to support the hypothesis that the 11-yr solar cycle modulation of galactic cosmic rays is caused by strong diffusion inside long-lived merged interaction regions. To test this hypothesis, the 1D force-field approximation of the cosmic ray modulation equation is solved. It is assumed that a constant solar wind speed convects magnetic field compressions and rarefactions unchanged through a model heliosphere. The result is a reasonable simulation of the integrated high-energy cosmic ray intensity profile from about 1982 to mid-1989. This period encompasses both the full recovery portion of the last profile from about 1982 to mid-1989. This model responds to the Voyager 2 magnetic field data by correctly timing the beginning of the new modulation cycle in late 1987. It is concluded that the present hypothesis is consistent with the results of this simulation.

Uses and limitations of the Tsyganenko magnetic field models

Abstract: The Tsyganenko models contain 'modules' for two external current systems, the tail and ring currents; the former is the more detailed. No specific modules, however, cover the magnetopause and the Birkeland current system. The effects of these currents are represented by an all-purpose 'polynomial', and the resulting formulas involve about 30 parameters that specify the model. The dependence on magnetospheric indices is also simplified.

Electric field fluctuations and possible dynamo effects in the solar wind

Abstract: Magnetohydrodynamic fluctuations in different kinds of solar wind have been investigated. Electric field fluctuation spectra have been obtained from the observed velocity and magnetic field fluctuations. The mean electromotive force e, generated by the turbulent motion of the solar wind plasma and field, turns out to depend upon the nature and Alfvenicity of the fluctuations. Dynamo theory predicts a linear relationship between e and the mean magnetic field Bo. Correlation studies carried out with the intention to establish this so-called alpha effect have given negative results.

Mapping the Auroral Oval into the Magnetotail Using Dynamics Explorer Plasma Data

Abstract: The mapping of the nightside auroral oval into the magnetotail is investigated using DE-1 and -2 plasma data in combination with the Tsyganenko '89 and the Hilmer-Voigt'91 magnetic field models. In an attempt to clarify the relationship between magnetospheric regions and the BPS and CPS, data from the Low Altitude Plasma Instrument (LAPI) are used to determine the invariant latitude of the polar cap boundary, the poleward edge of the discrete aurora, the discrete-diffuse (BPS/CPS) auroral boundary, and the equatorward edge of the diffuse aurora. When these field lines are traced into the magnetotail, both models indicate that the region of discrete aurora (BPS) maps to an extended region (usually>20 RE in width) of the main portion of the plasma sheet outside the more dipolar, inner plasma sheet (CPS). Since BPS field lines map to such a wide range of neutral sheet crossing distances, we propose a less confusing and more descriptive terminology in which the term “BPS” is divided into two regions: the Plasma Sheet Boundary Layer (PSBL) and the “IPS”, for Isotropic (Boundary) Plasma Sheet.Auroral zone-magnetotail topology is further examined by using data from the High Altitude Plasma Instrument (HAPI) to determine values of electron number density and temperatures at 1° intervals across the auroral zone. The corresponding field lines are traced into the magnetotail to a distance of-15RE and compared with magnetotail (10-25RE) mean plasma sheet ion densities and temperatures for low- and mid-activity levels from HUANG and FRANK (1986). We find a very good correspondence between the density and temperature measurements mapped from DE-1 altitudes and those measured in the mid-tail, suggesting that both models successfully reproduce the average magnetospheric configuration at this distance. The mean of the ion temperatures mapped with the Hilmer-Voigt model were slightly higher than those reported from the ISEE data, indicating that the model may be slightly too stretched. A significant region (between-5-7 RE) of overlap between the hotter ring current and the cooler, fresher plasma sheet is observable at DE-1; these hotter electrons are so anisotropic (with a trapped pancake distribution) that they are not observable at the 1000km altitude of DE-2.

Modeling the Global Positioning System Signal Propagation Through the Ionosphere

Abstract: Based on realistic modeling of the electron density of the ionosphere and using a dipole moment approximation for the Earth's magnetic field, one is able to estimate the effect of the ionosphere on the Global Positioning System (GPS) signal for a ground user. The lowest order effect, which is on the order of 0.1-100 m of group delay, is subtracted out by forming a linear combination of the dual frequencies of the GPS signal. One is left with second- and third-order effects that are estimated typically to be approximately 0-2 cm and approximately 0-2 mm at zenith, respectively, depending on the geographical location, the time of day, the time of year, the solar cycle, and the relative geometry of the magnetic field and the line of sight. Given the total electron content along a line of sight, the authors derive an approximation to the second-order term which is accurate to approximately 90 percent within the magnetic dipole moment model; this approximation can be used to reduce the second-order term to the millimeter level, thus potentially improving precise positioning in space and on the ground. The induced group delay, or phase advance, due to second- and third-order effects is examined for two ground receivers located at equatorial and mid-latitude regions tracking several GPS satellites.

High resolution observations of the magnetic and velocity field of simple sunspots

Abstract: We have observed the disk passage of relatively simple round sunspots using a narrowband filter and a large format CCD detector and have created magnetograms, Dopplergrams, and continuum images nearly simultaneously. In addition the spectral resolution of the filter (88,000) allows the construction of “spectra” for all points in the field of view. The changing patterns of the line of sight magnetic and flow fields with position around the spots and on the solar disk allowed us to infer that the mean inclination of the magnetic field increases from 45–50 ° to 70–75° across the penumbra and that there is a fluctuation of the inclination angle about the mean of about ±18 °. The scale of the inclination fluctuations is the order of the size of the penumbral fibrils. The variation in inclination is large enough that substantial amounts of magnetic field are parallel to the solar surface from the mid to outer penumbra. We also find that the Evershed flow tends to occur in the regions where the magnetic field is horizontal. This suggests that the Evershed flow is confined to the regions of horizontal fields.

Structure of the Magnetopause Current Layer at the Subsolar Point

Abstract: Attention is given to a 1D electromagnetic particle simulation model developed for the magnetopause current layer between the shocked solar wind and the dipole magnetic field at the subsolar point, which was extended to include the IMF in the solar wind. Interaction of the solar wind with the vacuum dipole field as well as the dipole field filled with a low-density magnetospheric plasma are investigated. The width and structure of the magnetopause current layer are found to differ markedly depending on the direction of the IMF. When the IMF is pointing southward, the current layer between the solar wind and the dipole field is narrow, and the magnetic field has a single ramp structure caused by the reflection of the solar wind at that point. The current layer becomes several times wider and the magnetic field develops a multiple ramp structure when the IMF is northward. Comparisons of these simulation results with the recent spacecraft data of the magnetopause crossing near the subsolar point are provided.

Magnetic cloud observations by the helios spacecraft

Abstract: A possible interpretation for the observed characteristics of an interplanetary magnetic cloud is the passage of a magnetic flux rope. For simplification the flux rope might be considered as a cylindrically symmetric structure with the magnetic field lines being directed parallel to the axis at its center and circular at its outer edges. Near the center of this flux rope the magnetic field strength would be strongest. The minimum variance technique was applied to several magnetic clouds observed by the Helios spacecraft between 0.3 and 1 AU in order to determine the orientations of the magnetic flux rope axis. The calculated orientations are examined with respect to the global solar wind stream structure, the surrounding solar wind flow, the radial distance to the sun and their solar origin.

Magnetic clouds: comparison between spacecraft measurements and theoretical magnetic force-free solutions

Abstract: Several theoretical magnetic force-free solutions of magnetic clouds (cylindrical, spheroidal) and their differences are discussed. The cloud diameters following from the fit are generally larger for spherical model than for cylindrical one. From the plasma data we try to estimate cloud boundaries as an independent check and compare them with model cloud boundaries in the magnetic field data.

Magnetic diffusion and flare energy buildup

Abstract: Photospheric motion shears or twists solar magnetic fields to increase magnetic energy in the corona, because this process may change a current-free state of a coronal field to force-free states which carry electric current. This paper analyzes both linear and nonlinear two-dimensional force-free magnetic field models and derives relations of magnetic energy buildup with photospheric velocity field. When realistic data of solar magnetic field (B0 almost-equal-to 10(3) G) and photospheric velocity field (V(max) almost-equal-to 1 km s-1) are used, it is found that 3-4 hours are needed to create an amount of free magnetic energy which is of the order of the current-free field energy. Furthermore, the paper studies situations in which finite magnetic diffusivities in photospheric plasma are introduced. The shearing motion increases coronal region, while the photospheric diffusion reduces the energy. The variation of magnetic energy in the coronal region, then, depends on which process dominates.

Comment [on “Solar wind control of the magnetopause shape, location, and motion” by D. G. Sibeck, R. E. Lopez, and E. C. Roelof]

Abstract: The methodology employed in the paper 'Solar wind control of the magnetopause shape, location, and motion' by Sibeck et al. (1991), which quantifies the magnetospheric response to solar wind dynamic pressure and interplanetary magnetic field variations, is commented on, with emphasis on how the shape and position of the magnetopause boundary can be specified, within some uncertainty, for particular simultaneous values of the solar wind pressure in the magnetic field. The reply of Sibeck et al. included a data set of 1821 magnetopause crossings, each associated with an hourly averaged solar wind dynamic pressure and/or north-south component of the interplanetary magnetic field. A least squares fit to an ellipsoid of revolution to subsets of the data was performed.

Multifractals in the solar wind

Abstract: Multifractals have been observed in the solar wind in several contexts The velocity fluctuations observed by Voyager 2 near 8 AU have the structure of intermittent turbulence which has multifractal scaling symmetry The velocity fluctuations in corotating streams at 1 AU and near 6 AU also have multifractal structure, and the structure evolves significantly between 1 AU and 6 AU Multifractal scaling has also been observed in the magnetic field strength, density and temperature in recurrent streams at 1 AN and in large-scale fluctuations the magnetic field strength at 25 AU

A theory of the magnetic field from current monopoles

Abstract: A method for calculating the magnetic fields of volume currents by using point current sources (monopoles) is derived. A model is developed for a point source that is consistent with Maxwell’s equations. An arbitrary arrangement of point sources in a conducting volume is shown to generate no magnetic field outside the conductor if the volume surface coincides with an equipotential surface of the sources. This allows such sources to be added to any problem without affecting the magnetic field, and simplifies the calculation of the magnetic field generated by an arbitrary point source in both spherical and semi‐infinite volume conductors. The magnetic field generated by current monopoles located within these volume conductors is then derived. The formulation greatly simplifies the analysis of the magnetic field generated by higher‐order current sources, notably dipoles, in these volumes.

Alfvén wave propagation in the solar atmosphere and models of mhd turbulence in the solar wind

Abstract: The propagation of Alfven waves along a purely radial magnetic field in the solar atmosphere is discussed, with particular emphasis on the role of the Alfvenic critical point in determining the transmission of the waves into the wind Models for the evolution of Alfvenic turbulence are compared to the low-frequency limit of the linear equations

Magnetic field in a turbulent galactic disk

Abstract: A simple kinematic model has been applied to simulate the evolution of the interstellar magnetic field permanently twisted by turbulent gas motions accompanied by effects of the field diffusion. The magnetic field was found to develop well-ordered twisted structures over the whole gas parcel analyzed. This field configuration has a preferred sense of twisting dependent on the helicity of the gas flow. The model of synchrotron emission and polarization of M31-like galaxy containing magnetic field structures taken from our simulations reproduces quite well its basic observed radio and polarization properties.

Energy dissipation by alfvén waves propagating in an inhomogeneous magnetic field

Abstract: The evolution of incompressible MHD disturbance propagating in an inhomogeneous magnetic field is studied by means of numerical simulations. Small scales are generated in the perturbation which is dissipated within few Alfven times. This phenomenon could be relevant in the problem of coronal heating.

The magnetic field in the heliosheath

Abstract: The interplanetary magnetic field (IMF) behaves in a reasonably well-understood manner between the Sun and the heliospheric termination shock. At the shock, the azimuthal field is amplified by a factor of four (for a strong shock) and undergoes secular amplification in the heliosheath until the flow is fully turned into the downstream direction and has reached its asymptotic state in the distant heliotail. This amplification may lead to important MHD effects that can cause the shock to be closer to the Sun than otherwise expected. Here we further examine whether there are important MHD effects in the heliosheath. We do this by calculating the kinematic compression of the magnetic field in the heliosheath using an analytic incompressible flow model of the dynamics downstream of the shock. We conclude that it is likely that MHD effects are important in the heliosheath in a narrow cone about the upstream direction.

Magnetic field of the solar wind: a stationary kinematic solution with a finite electric conductivity

Abstract: The stationary models by E.N.Parker for the magnetic field in the solar wind and by K.H.Schatten for the “source surface” in the solar corona are generalized. The configuration of the interplanetary magnetic field, stretched by the expanding corona, depends on the magnitude of the electrical conductivity o of the solar wind plasma. Knowing the main empirical features of the field configuration, one may estimate the phenomenological value of o. The estimates show that the electrical conductivity should be approximately 10 13 times smaller than that calculated by Spitzer.