Showing papers on "Dipole model of the Earth's magnetic field published in 1984"

Correlated low‐frequency electric and magnetic noise along the auroral field lines

Abstract: Dynamics Explorer 1 measurements of intense low-frequency electric and magnetic noise observed at low altitudes over the auroral zone are described. The intensity of both the electric and magnetic fields decreases rapidly with increasing frequency. Most of the energy is at frequencies below the O(+) cyclotron frequency, and some evidence is found for a cutoff or change in spectral slope near that frequency. The magnetic to electric field ratio decreases rapidly with increasing radial distance and also decreases with increasing frequency. The polarization of the electric field in a plane perpendicular to the earth's magnetic field is essentially random. The transverse electric and magnetic fields are closely correlated, with the average Poynting flux directed toward the earth. The total electromagnetic power flow associated with the noise is substantial. Two general models are discussed to interpret these observations, one based on static electric and magnetic fields imbedded in the ionosphere and the other based on Alfven waves propagating along the auroral field lines.

Auroral zone electric fields from DE-1 and -2 at magnetic conjunctions. Progress Report

Abstract: Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and 4,500 km during magnetic conjunctions. The measured electric fields are usually perpendicular to the magnetic field lines. The north-south meridional electric fields are projected to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields are measured by both DE-1 and DE-2 show that the large-scale electric field is the same at both altitudes, as expected. Superimposed on the large-scale fields, however, are small-scale features with wavelengths less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high and low altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small-scale electric field variations are associated with field-aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE-1.

First measurements of supersonic polar wind in the polar magnetosphere

Abstract: Measurements from the Retarding Ion Mass Spectrometer on the Dynamics Explorer have, for the first time, revealed a supersonic polar wind along polar cap field lines. The observations reported were obtained on the nightside (22:30 to 23:30 MLT) from 65 to 81 deg invariant latitude and at altitudes near 2 earth radii. Fitting the data using a thin-sheath model gives a range of temperatures of 0.1 to 0.2 eV with corresponding flow velocities of 25 to 16 km/s over the estimated range of spacecraft potential of +3 to +5 V. For these values the Mach number ranged from 5.1 to 2.6 (with a most likely value of 3). Characteristics of the H(+) flow are in general agreement with those predicted by 'classical' polar wind theory, but high variabilty of the He(+)/H(+) ratio was observed.

Structure and evolution of the large scale solar and heliospheric magnetic fields

Abstract: Structure and evolution of large scale photospheric and coronal magnetic fields in the interval 1976-1983 were studied using observations from the Stanford Solar Observatory and a potential field model. The solar wind in the heliosphere is organized into large regions in which the magnetic field has a componenet either toward or away from the sun. The model predicts the location of the current sheet separating these regions. Near solar minimum, in 1976, the current sheet lay within a few degrees of the solar equator having two extensions north and south of the equator. Soon after minimum the latitudinal extent began to increase. The sheet reached to at least 50 deg from 1978 through 1983. The complex structure near maximum occasionally included multiple current sheets. Large scale structures persist for up to two years during the entire interval. To minimize errors in determining the structure of the heliospheric field particular attention was paid to decreasing the distorting effects of rapid field evolution, finding the optimum source surface radius, determining the correction to the sun's polar field, and handling missing data. The predicted structure agrees with direct interplanetary field measurements taken near the ecliptic and with coronameter and interplanetary scintillation measurements which infer the three dimensional interplanetary magnetic structure. During most of the solar cycle the heliospheric field cannot be adequately described as a dipole.

Time scales for the decay of induced large‐scale magnetic fields in the Venus ionosphere

Abstract: Observations made with the aid of a magnetometer on the Pioneer Venus Orbiter have shown large-scale horizontal magnetic fields in the dayside ionosphere of Venus. According to Cloutier and Daniell (1981), the observed magnetic structures may be quasi-steady features produced by an ionospheric current system driven by solar wind interaction. Russell et al. (1983) have suggested that the altitude profiles of the horizontal field on different orbits exhibit a pattern which can be interpreted as phases in the temporal evolution of an initial state in which the ionosphere was permeated with magnetosheath-like fields. The present investigation is concerned with the argument in favor of a temporal versus spatial explanation for some of the observed field structure. A calculation indicates that the diffusion time for ionospheric fields is long enough to justify attributing the observed fields to the 'memory' of the Venus ionosphere in certain regions.

The evolution of large-scale magnetic fields in the ionosphere of Venus

Abstract: Large-scale magnetic fields are often observed in the ionosphere of Venus by the magnetometer on the Pioneer Venus Orbiter, especially near the subsolar point or when the solar wind dynamic pressure is high. An equation for the time evolution of the magnetic field is derived which includes both a term representing the time rate of change of the field due the convection of magnetic flux by plasma motions and a magnetic diffusion/dissipation term. The ionospheric plasma velocities required by these equations were obtained by numerically solving the momentum equation. Numerical solutions to the magnetic field equation indicate that large-scale magnetic fields, which are not being actively maintained, decay with time scales ranging from tens of minutes to several hours. The vertical convection of magnetic flux enables magnetic field structures deep within the ionosphere to persist longer than would otherwise be expected. This vertical convection also explains the shape of these structures.

Distribution of solar wind angular momentum between particles and magnetic field: Inferences about the Alfvén critical point from Helios observations

Abstract: We first discuss theoretically the relative importance and the behavior of the two basic terms adding to the total angular momentum flux, the angular momentum of the particles (electrons, protons and alpha particles) and of the magnetic field stresses, respectively. Second, we analyze these two quantities with respect to their dependence on heliocentric distance by using the Helios 1 and 2 plasma and interplanetary magnetic field observations between 0.3 and 1 AU classified according to low-speed ( 600 km s−1) solar wind for the 1975–1976 epoch. Applying now these results as well as various combinations of the constants of motion for the solar wind (such as the total angular momentum flux, the mass flux, and the magnetic flux) and their observational constraints, as deduced earlier by Marsch and Richter (1984), we finally present various methods (1) to derive the values of several characteristic solar wind plasma and magnetic field parameters at the Alfven critical points, (2) to estimate their locations above the solar surface, and (3) to obtain the radial slope of the associated solar wind velocity profiles for the three solar wind classes separately.

HF ray tracing at high latitudes using measured meridional electron density distributions

Abstract: In many physical cases, small- and intermediate-scale (1–100 m) electron density irregularities in the ionospheric E and F regions are elongated in the direction of the earth's magnetic field. As a result of this elongation or field alignment, radar studies directed toward the analysis of backscattered signals from ionospheric irregularities require that the radar wave vector propagate perpendicular to the magnetic field in the irregularity region. At low and middle latitudes, orthogonality may be achieved at any radar operating frequency, but at higher latitudes this geometry becomes increasingly difficult. Above 60° invariant latitude it is almost impossible to achieve normality with straight line propagation. In order to observe the wealth of ionospheric irregularities in the auroral zone and polar cap E and F regions, one must operate radars at HF frequencies where refraction affects wave propagation and aids one in achieving the orthogonality condition. However, refraction from large-scale (1–100 km) ionospheric irregularity structure also increases the complexity in accurately determining the location of the scattering volume. In this paper we use a modified version of an existing HF ray tracing program to determine with high precision the HF propagation paths in a realistic ionosphere. In this analysis the ionosphere is simulated by a two-dimensional array of electron density as a function of geomagnetic latitude and altitude. This analysis is then applied to real measurements obtained from a meridian scan of the Chatanika incoherent scatter radar, and we show how this electron density distribution affects propagation from a simulated HF radar located at Anchorage, Alaska. The variations of the propagation conditions are also studied as one moves the density pattern in the direction of the Anchorage radar.

The magnetic field of Mars: Implications from gas dynamic modeling

Abstract: On January 21, 1972, the Mars 3 spacecraft observed a variation in the magnetic field during its periapsis passage over the dayside of Mars that was suggestive of entry into a Martian magnetosphere. Original data and trajectory of the spacecraft have been obtained (Dolginov, 1983) and an attempt is made to simulate the observed variation of the magnetic field by using a gas dynamic simulation. In the gas dynamic model a flow field is generated and this flow field is used to carry the interplanetary magnetic field through the Martian magnetosheath. The independence of the flow field and magnetic field calculation makes it possible to converge rapidly on an IMF orientation that would result in a magnetic variation similar to that observed by Mars 3. There appears to be no need to invoke an entry into a Martian magnetosphere to explain these observations.

On the diagnostic of magnetic fields in sunspots through the interpretation of stokes parameters profiles

Abstract: The inversion routine proposed by Aueret al. (1977), for the determination of vector magnetic fields from Stokes profiles, has been generalized to include magneto-optical and damping effects. Synthetic profiles have then been generated from a sunspot model atmosphere accounting for the depth variation of the relevant physical parameters such as the magnetic field amplitude, inclination angle, etc...., each variation being considered one at a time. Alfven waves and magnetic inhomogeneities over the field of view have also been considered. These synthetic profiles have been presented to the inversion routine. The results of the fits show that the magnetic field amplitude and direction are always recovered with good accuracy when these quantities are constant in the model atmosphere, and, in those cases where te magnetic field vector is supposed to vary monotonically with optical depth, the values recovered are always intermediate between the values corresponding to the top and bottom of the atmosphere. Moreover, we found that the differences between synthetic and best-fit profiles are able to characterize, in many cases, the particular physical situation considered.

Oscillating dynamo magnetic field in the presence of an external nondynamo field - The influence of a solar primordial field

Abstract: Dynamo magnetic fields are self-excited and, once started, can perpetrate themselves with no outside source of magnetic flux, as long as the necessary fluid motions persist. Such dynamo fields behave completely independently of the field's overall polarity. In the presence of an external field of separate origin this polarity symmetry of the dynamo states is broken; the dynamo states become asymmetric with respect to polarity. In this paper a calculation is performed of the characteristics of a spherical shell dynamo in the presence of a fossil magnetic field penetrating into the dynamo from below. The asymmetric periodic states are found as a function of the strength of underlying fossil field. Applying these results to the sun, there appears to be no evidence of any intense large-scale primordial magnetic flux, having either dipole-like or quadrupole-like symmetry about the sun's equator, penetrating into the convection zone from the sun's radiative core. Indeed, the calculations indicate, even on the basis of the presently crude observations, that any such primordial field must have an intensity smaller than a few gauss.

A comparison of the observed mid-latitude magnetic disturbance fields with those reproduced from the high-latitude modeling current system

Abstract: Magnetic disturbance fields at six mid-latitude stations are computed at 5-min intervals for March 17, 18, and 19, 1978. The three-dimensional current system responsible for the fields was determined on the basis of magnetic records from the six International Magnetospheric Study meridian chains of stations above invariant latitude 50° through a numerical modeling method developed by Kamide et al. (1981a). Then each ionospheric high-latitude current segment is assumed to be connected to two (upward and downward) field-aligned currents along the dipole field lines and a closure current in the equatorial plane of the magnetosphere. None of the magnetic records from the six mid-latitude stations were used in determining the high-latitude modeling current system. It is shown that the middle-latitude magnetic disturbance fields thus constructed can reproduce fairly well the observed fields (both the H and D components), indicating that the method developed by Kamide et al. (1981a) is fairly reliable. Further, in agreement with earlier studies, the field-aligned current segments have, in general, the largest contribution to both the H and D components in the middle latitude. The differences between the observed and computed magnetic fields are discussed in detail.

On the Electromagnetic Fields Induced by Oceanic Internal Waves

Abstract: Model spectra for the electric and magnetic fields induced by oceanic internal waves are obtained by combining Green function solutions to the two electromagnetic modal equations with the Garrett-Munk kinematic description of the internal wave field. The poloidal magnetic mode is dominant at frequencies above 3f, where f is the local Coriolis frequency, and self and mutual induction are not important over this range. The toroidal magnetic mode is increasingly important at frequencies below 3f and is sensitive to the conductivity structure below the seafloor for nearinertial frequencies. The moored electric field is shown to be largely a measure of the local velocity field at high frequencies. The vertical electric field is sensitive to the horizontal velocity field, while the horizontal electric field primarily reflects the vertical velocity field and is quite small at the seaearth and sea-air interfaces. The magnetic field is a measure of the spatially averaged velocity field and is dominated by the poloidal magnetic mode. Electromagnetic boundary effects reduce the horizontal magnetic spectrum by decades at the seafloor and sea surface. At the seafloor and sea surface, internal wave-induced magnetic fields are within an order of magnitude of their externally induced counterparts, while in the ocean's interior, internal waves are probably the largest source of magnetic signals in the period range one day to one hour. The internal wave-induced electric field is not measurable except in the vertical component.

Chromospheric and coronal Alfvénic oscillations in non-vertical magnetic fields

Abstract: We generalize previous studies of Alfvenic oscillations in the solar atmosphere to geometries in which the background magnetic field is not parallel to the gravitational acceleration. A uniform but inclined field produces only subtle changes in the mathematics, and virtually no changes to the coronal energy flux, over previous vertical field studies. We show that simple, two-layer models agree remarkably well with more sophisticated, multi-layer calculations. In addition, we derive several useful and accurate analytic results with which we highlight many features and parameter dependences. We also study a model with a spreading field geometry. For low magnetic fields (∼- 10 G) the corona still appears WKB to the oscillations and we do not find any significant deviations from the uniform field calculations. This is not the case for higher magnetic fields in active regions (∼- 3000 G) where we confirm previous results which suggest an order of magnitude increase in the coronal flux. Again, we derive useful analytic approximations.

Magnetohydrodynamic modelling of interplanetary disturbances between the sun and earth

Abstract: A time-dependent, nonplanar, two-dimensional magnetohydrodynamic computer model is used to simulate a series, separately examined, of solar flare-generated shock waves and their subsequent disturbances in interplanetary space between the sun and the earth's magnetosphere. The 'canonical' or ansatz series of shock waves include initial velocities near the sun over the range 500 to 3500 km/s. The ambient solar wind, through which they propagate, is taken to be a steady state homogeneous plasma (that is, independent of heliolongitude) with a representative set of plasma and magnetic field parameters. Complete sets of solar wind plasma and magnetic field parameters are presented and discussed. Particular attention is addressed to the MHD model's ability to address fundamental operational questions vis-a-vis the long-range forecasting of geomagnetic disturbances. These questions are: (1) will a disturbance (such as the present canonical series of solar flare shock waves) produce a magnetospheric and ionospheric disturbance, and, if so, (2) when will it start, (3) how severe will it be, and (4) how long will it last? The model's output is used to compute various solar wind indices of current interest as a demonstration of the model's potential for providing 'answers' to these questions.

K‐coronameter observations and potential field model comparison in 1976 and 1977

Abstract: Results of determining the shape and location of the heliospheric current sheet from a potential field model and from K-coronameter observations are compared Interplanetary magnetic field polarities as observed by IMP 8, Helios 1 and 2, and Voyager 2 spacecraft were used to test the two methods over the period May 1976 to August 1977 throughout 18 Carrington rotations The computed heliospheric current sheets from both methods had a quasi-stationary four-sector structure and very similar shapes Agreement between interplanetary magnetic field polarity and the results from the potential field model was found on 79 percent of the days, while agreement between the interplanetary field polarity and the polarities derived from the K-coronameter data was found on 87 percent of the days

On the absence of critical levels in the solar atmosphere

Abstract: We consider the propagation of linear oscillations in a stratified atmosphere permeated by an oblique but nearly horizontal uniform magnetic field. Our results do not show any of the strong characteristics associated with critical level singularities in the exactly horizontal field case. This suggests that critical levels may be of more mathematical interest than physical relevance in the solar atmosphere where the planar horizontal field approximation is seldom adequate.

Dynamics of hydromagnetic clouds from powerful solar flares

Abstract: At present two complementary approaches to calculations of flare-generated interplanetary disturbances are beeing developed. One of them is based on Parker's model with one end of every field line rooted at the Sun and the other end somewhere in outer space (Wu et al., 1983). Another approach is based on Morrison's closed-cloud hypothesis. In this paper the dynamics of flare-generated hydromagnetic oblate clouds in the solar corona and interplanetrary space is developed. Magnetic buoyancy, gravity, and momentum loss due to interactions with the interplanetary medium are taken into account. If the magnetic fields, number density, and velocity of the external plasma are known, the equations allow a simple solutions for variations of the velocity and characteristic lengths of the clouds during their motion between the Sun and the Earth. Two theoretical velocity profiles are calculated and compared with an experimental one.

Solar noise storms coordinated observations: May 16 24, 1981

Abstract: We present the main results obtained from the coordinated observations of solar radio noise storms organized by C.E.S.R.A. in May 1981. They concern the structure and polarization of radio sources, and the relation with the photospheric magnetic field. A model of coronal magnetic field, accounting for the observations, is briefly discussed.

Computation of solar magnetic fields from photospheric observations

Abstract: The observational difficulties of obtaining the magnetic field distribution in the chromosphere and corona of the Sun has led to methods of extending photospheric magnetic measurements into the solar atmosphere by mathematical procedures. A new approach to this problem presented here is that a constant alpha force-free field can be uniquely determined from the tangential components of the measured photospheric flux alone. The vector magnetographs now provide measurements of both the solar photospheric tangential and the longitudinal magnetic field. This paper presents derivations for the computation of the solar magnetic field from these type of measurements. The fields considered are assumed to be a constant alpha force-free fields or equivalent, producing vanishing Lorentz forces. Consequently, magnetic field lines and currents are related by a constant and hence show an identical distribution. The magnetic field above simple solar regions are described from the solution of the field equations.

Self-interaction of an electric or magnetic dipole in the Schwarzschild space-time

Abstract: The authors determine the electromagnetic field generated by an electric or magnetic dipole held fixed in the Schwarzschild space-time and that is assumed to be point-like. Besides a divergent field at the position of the dipole, there is a finite field which may be considered as an 'external' field which exerts on the dipole a force and a torque for which the authors give the exact expressions from the covariant derivative of the energy-momentum tensor of electromagnetism.

Ion propagation in the magnetosheath

Abstract: The transport of test ions through the magnetosheath depends upon the macroscopic structure defined by the average magnetic and flow fields, B0 and U, and the fluctuating fields, δE and δB, encountered by the particles In this paper an approximate model is used to generate the macroscopic configuration: it satisfies the jump conditions at the bow shock, treats the magnetopause as a tangential discontinuity (B0 and U tangent to the magnetospheric boundary), and uses intermediate field amplitudes and orientations that resemble the more precise results of gas dynamic simulations Equations of motion for a fictitious ensemble average particle describe the ion propagation, with the effects of the inhomogeneous turbulence phenomenologically incorporated into a friction tensor whose components reflect the coupling of the ions to the bulk plasma along and across the magnetic field B0 Application to the solar wind releases of lithium ions by the Active Magnetospheric Particle Tracer Explorers mission illustrates the approach: we follow the guiding center trajectory of the ensemble average “particle” for several upstream solar wind and interplanetary magnetic field conditions, different locations of the release site, and quiet and disturbed magnetosheath turbulence regimes Implications of the results for the release strategy are discussed

Earth's magnetic field perturbations as the possible environmental impact of the conceptualized solar power satellite

Abstract: It is predicted that the earth's magnetic field can be significantly perturbed locally by the microwave beam transmitted from the conceptualized solar power satellite (SPS) at a frequency of 2.45 GHz with incident power density of 230 W/m² at the center of the beam. The simultaneous excitation of earth's magnetic field fluctuations and ionospheric density irregularities is caused by the thermal filamentation instability of microwaves with scale lengths greater than a few hundred meters. Earth's magnetic field perturbations with magnitudes (∼ a few tens of gammas) comparable to those in magnetospheric substorms can be expected. Particle precipitation and airglow enhancement are the possible, concomitant ionospheric effects associated with the microwave-induced geomagnetic field fluctuations. Our present work adds earth's magnetic field perturbations as an additional effect to those such as ionospheric density irregularities, plasma heating, etc., that should be assessed as the possible environmental impacts of the conceptualized solar power satellite program.

Magnetic Flux Tube in a Stratified Atmosphere Under the Influence of the Vertical Magnetic Field

Abstract: The magnetohydrostatic equilibrium of a magnetic flux tube in a homogeneous gravitational and vertical magnetic field is studied. Gas pressure and density are presented explicitly as a function of the external magnetic field. The influence of the external magnetic field on the magnetic and thermodynamic structures is illustrated by two exact solutions. The possible applications to sunspot and facular modeling are discussed.

Magnetic structure of the distant geotail from -60 to -220 earth radii - ISEE-3

Abstract: ISEE-3 magnetic-field measurements in the region of the geomagnetic tail from -80 to -220 earth radii are reported and discussed A well-ordered field structure is found, comprising two 7-8-nT lobes separated by a plasma sheet, an embedded neutral sheet with significant By fields, and an intermittent plasma-sheet boundary layer with 5-nT-amplitude (peak-to-peak) electromagnetic waves The plasma-sheet Bz distribution changes from principally northern orientation near the earth to an approximately equal north-south distribution at 200-220 earth radii These findings are considered to be in general agreement with magnetic-reconnection models of the magnetosphere, with reconnection either throughout the region observed (in tearing-mode or plasmoid-formation models) or at a constant (about 220-earth-radii) or variable (40-80 to 220-earth-radii) X line (in X-line models)

Magnetic fields and thermal structure of solar plasmas

Abstract: The connection between the magnetic field geometry and the thermal properties of solar coronal structures is investigated. Gravitational effects, that can modify the spatial dependence of the thermodynamical quantities, but have no influence on the plasma-field interaction, are omitted to simplify the problem. A series of two-dimensional models is constructed, in which a strong coupling between the magnetic field shear and the thermal structure of the loop is clearly pointed out.

The effect of rotation on the luminosity of magnetic stars

Abstract: The effect of rotation and a general magnetic field on the luminosity, radius, and effective temperature of the upper Main-Sequence stars has been investigated using a perturbation analysis. The magnetic field profile prevailing inside the star is assumed to have both poloidal and toroidal components. The case of constant as well as differential rotation is admitted. Model calculations indicate that these stellar parameters modify considerably as a result of coupling between rotation and the magnetic field.