Showing papers on "Dipole model of the Earth's magnetic field published in 1986"

Magnetic fields at uranus.

Abstract: The magnetic field experiment on the Voyager 2 spacecraft revealed a strong planetary magnetic field of Uranus and an associated magnetosphere and fully developed bipolar masnetic tail. The detached bow shock wave in the solar wind supersonic flow was observed upstream at 23.7 Uranus radii (1 R(U) = 25,600 km) and the magnetopause boundary at 18.0 R(U), near the planet-sun line. A miaximum magnetic field of 413 nanotesla was observed at 4.19 R(U ), just before closest approach. Initial analyses reveal that the planetary magnetic field is well represented by that of a dipole offset from the center of the planet by 0.3 R(U). The angle between Uranus' angular momentum vector and the dipole moment vector has the surprisingly large value of 60 degrees. Thus, in an astrophysical context, the field of Uranus may be described as that of an oblique rotator. The dipole moment of 0.23 gauss R(3)(U), combined with the large spatial offset, leads to minimum and maximum magnetic fields on the surface of the planet of approximately 0.1 and 1.1 gauss, respectively. The rotation period of the magnetic field and hence that of the interior of the planet is estimated to be 17.29+/- 0.10 hours; the magnetotail rotates about the planet-sun line with the same period. Thelarge offset and tilt lead to auroral zones far from the planetary rotation axis poles. The rings and the moons are embedded deep within the magnetosphere, and, because of the large dipole tilt, they will have a profound and diurnally varying influence as absorbers of the trapped radiation belt particles.

Possible evidence of flux transfer events in the polar ionosphere

Abstract: We present magnetic field data from the cusp-latitude South Pole station that exhibit, under appropriate local time and interplanetary magnetic field conditions, the signature expected in the ionosphere from a flux transfer event (FTE) at the magnetopause. In particular, the model of multiple X-line reconnection at the magnetopause predicts field-aligned currents in helical flux tubes, with transverse magnetic fields propagating as Alfven waves toward the ionosphere. The distinctive magnetic signature at a polar cap magnetic station, particularly in the vertical component, can be used to infer the signs of the By and Bz components of the interplanetary magnetic field.

Auroral zone thermospheric dynamics: 1. Averages

Abstract: Forty-four nights of thermospheric neutral wind and temperature measurements were obtained with a ground-based Fabry-Perot spectrometer at College, Alaska (65° invariant latitude), mostly during high to moderate levels of solar activity. The spectrometer scanned the O I 15,867 K (6300 A) emission line yielding line profile measurements that originate from an altitude range between 170 and 280 km. The observed average wind direction is to the northwest in the early evening, changing to the south near midnight and then to the southeast in the morning. The overall wind pattern generally persists, but the wind speed increases with increasing geomagnetic activity. A change in the zonal wind direction occurs earlier in magnetic local time with increasing magnetic activity. The meridional wind pattern also shifts equatorward with the auroral oval during increasing magnetic activity. Consequently, the average wind pattern in the north during low geomagnetic activity is rather similar to the moderate activity average in the south, and the moderate geomagnetic activity average wind pattern in the north is similar to the high geomagnetic activity average in the south. The observed average thermospheric temperature is found to be governed by geomagnetic activity and the previous day's 10.7-cm solar flux.

Interaction between comet Halley and the interplanetary magnetic field observed by Sakigake

Abstract: During 10–12 March 1986, the magnetometer carried by the Japanese spacecraft Sakigake detected clear multiple crossings of the nearly horizontal heliospheric neutral sheet. As there was no apparent disconnection event (DE) in comet Halley's ion tail during 11–14 March, an interplanetary magnetic field (IMF)–comet interaction model with a quasi-parallel neutral sheet is proposed to explain why a DE did not occur. Enhancements of hydromagnetic (HM) waves were observed near the time of closest approach to comet Halley. Neutral particles of cometary origin are proposed to yield, at ∼7 × 106 km upstream of the comet, the ionized O+ (or H2O+) particles which excite the HM waves.

Magnetostrophic balance in planetary dynamos: Predictions for Neptune's magnetosphere

Abstract: With the purpose of estimating Neptune's magnetic field and its implications for nonthermal Neptune radio emissions, a new scaling law for planetary magnetic fields was developed in terms of externally observable parameters (the planet's mean density, radius, mass, rotation rate, and internal heat source luminosity). From a comparison of theory and observations by Voyager it was concluded that planetary dynamos are two-state systems with either zero intrinsic magnetic field (for planets with low internal heat source) or (for planets with the internal heat source sufficiently strong to drive convection) a magnetic field near the upper bound determined from magnetostrophic balance. It is noted that mass loading of the Neptune magnetosphere by Triton may play an important role in the generation of nonthermal radio emissions.

On the limits of energy transfer through dayside merging

Abstract: Empirical studies indicate that the rate of energy transfer to the magnetosphere increases as solar wind coupling parameters increase, up to a certain limit, and then the rate remains constant. The split separator line merging model undergoes the same behavior. When the model interplanetary magnetic field (IMF) magnitude exceeds a critical value, the field configuration in the Chapman-Ferraro current plane ceases to expose closed field lines to the solar wind. The critical value depends upon IMF orientation, stagnation pressure, and earth's dipole tilt angle. The absence of exposed closed field lines prevents closed-to-open flux transfer and, consequently, energy transfer. Further, the limit on energy transfer governed by the model dipole tilt angle affords the first explanation of both the semiannual and UT variations of geomagnetic activity completely in terms of merging.

Magnetized cosmological model

Abstract: The object of this paper is to investigate the behavior of the magnetic field in a cosmological model for perfect fluid distribution. The magnetic field is due to an electric current produced along thex axis. It is assumed that expansion (θ) in the model is proportional toσ11, the eigenvalue of the shear tensorσij. The behavior of the model when the magnetic field tends to zero and other physical properties are also discussed.

Magnetic field corrections to solar oscillation frequencies

Abstract: It is argued that the frequencies of both the solar p- and g-modes of oscillation are modified by a magnetic field. In particular, the decrease in p-mode frequencies is attributed to a magnetic field within the solar interior evolving over the solar cycle. Field strengths at the base of the convection zone of at least 500,000 G are required.

Stability of energy gaps under variations of the magnetic field

Abstract: It is proved that the location of the spectrum of the one-body Schrodinger operator is stable under small variations of the magnetic field. It is not supposed that the potential or the magnetic field vanishes at infinity. The potential is not supposed to be periodic so the results applies to crystalline and amorphous solids as well.

Effects of Three-Dimensional Heliospheric Structures on Cosmic-Ray Modulation

Abstract: The theory of cosmic-ray transport in the heliosphere contains four distinct physical processes — diffusion, convection, adiabatic cooling, and drifts. The last of these has only recently been evaluated. Extrapolation of present understanding of the regions near the heliospheric equator to high heliographic latitudes leads to the conclusion that particle drift in the large-scale magnetic field plays an important role in cosmic-ray modulation. The large-scale, three-dimensional structure of the interplanetary magnetic field is therefore very important in understanding cosmic rays. Several key observed modulation effects are summarized, each of which is a natural consequence of drift, but which requires special assumptions if drift plays no role. It is concluded that particle drifts play an important and possibly dominant role in transport in the heliosphere.

The decay of the mean solar magnetic field

Abstract: We have analyzed the effects that differential rotation and a hypothetical meridional flow would have on the evolution of the Sun's mean line-of-sight magnetic field as seen from Earth. By winding the large-scale field into strips of alternating positive and negative polarity, differential rotation causes the mean-field amplitude to decay and the mean-field rotation period to acquire the value corresponding to the latitude of the surviving unwound magnetic flux. For a latitudinally broad two-sector initial field such as a horizontal dipole, the decay is rapid for about 5 rotations and slow with a t −1/2 dependence thereafter. If a poleward meridional flow is present, it will accelerate the decay by carrying the residual flux to high latitudes where the line-of-sight components are small. The resulting decay is exponential with an e-folding time of 0.75 yr (10 rotations) for an assumed 15 m s−1 peak meridional flow speed.

Viscosity in the solar wind

Abstract: The effects of viscosity on a steady, radial, spherically symmetric solar wind with an embedded, non-radial magnetic field are reconsidered. The correct expression for the classical viscosity in the presence of a non-radial magnetic field is shown to be different from that used in the past, and a means of describing non-classical viscosity is presented. A physical interpretation of the classical and nonclassical descriptions of viscosity is provided, and observational inferences are used in discussing the nature and degree of viscous effects in the solar wind.

The relationship of the large-scale solar field to the interplanetary magnetic field - What will Ulysses find?

Abstract: Photospheric magnetic fields observed at the Wilcox Solar Observatory at Stanford together with a potential field model can be used to calculate the configuration of the interplanetary magnetic field (IMF). The IMF is organized into large regions of opposite polarity separated by a neutral sheet (NS). Small quadripolar warps in a roughly equatorial NS produce the four-sector structure commonly observed in the IMF at Earth near solar minimum. Soon after minimum the latitudinal extent of the NS increases substantially. Near maximum the structure is more complex, occasionally including multiple neutral sheets. The IMF structure simplifies and becomes very stable during the declining phase. Large-scale structures persist for up to two years during the entire cycle. From 1978 through 1983 the NS extended to at least 50°. Like coronal holes, the computed coronal field shows evidence of decreased differential rotation. Patterns in the inferred IMF polarity from solar cycles 16 – 21 are very similar. This suggests that structures observed by Ulysses during the coming cycle will be similar to those in the past.

An MHD Model for the Earth's Magnetic Field with Spatially Dependent Electrical Conductivity

Abstract: A simple model demonstrates that symmetrical fluid flow in the earth's core can maintain a steady-state magnetic field if the electrical conductivity is allowed to vary as a function of radius (temperature).

The Large-Scale Structure of the Heliospheric Magnetic Field

Abstract: The heliospheric magnetic field, embedded in the solar wind, originates in the highly non-uniform magnetic structures of the solar corona. Its source function is not easily discernible, but a brief description of the photospheric and coronal fields can give an indication of its temporal and spatial boundary conditions. Direct observations of the interplanetary magnetic field have only been made in and near the solar equatorial plane. These have confirmed Parker’s original model as a good first approximation. The most significant feature of the equatorial structure is the current sheet separating the two polarity regions on the sun and extending deep into interplanetary space. This feature has been successfully modelled and remains the major achievement among efforts to relate solar and coronal structures to interplanetary observations. On the other hand, the heliolatitude dependence of the structure of the field remains unknown. The paper concludes with a model list of questions which will only be answered by the in situ observations planned during the forthcoming Ulysses mission.

Solar Wind Control of Magnetospheric Configuration

Abstract: The solar wind exerts intimate control of the configuration of the magnetosphere by the reconnection of interplanetary magnetic field with the magnetospheric magnetic field. Thus the solar wind controls the energization of the magnetosphere proper including the ring current as well as the transport of magnetic flux to the tail and the consequent storage of energy there. The dynamics of the magnetopause are also controlled closely by the interplanetary magnetic field. The formation of flux transfer events, twisted tubes of flux across the dayside magnetopause, arise for southward interplanetary magnetic fields. Whether these flux tubes are connected to closed field lines or are completely open is not yet clearly resolved.

The determination of coronal potential magnetic fields using line-of-sight boundary conditions

Abstract: Previous efforts to construct solar coronal fields using surface magnetograph data have generally employed a least squares minimization technique in order to determine the spherical harmonic expansion coefficients of the magnetic scalar potential. Provided there is no source surface high up in the corona, we show that knowledge of the line-of-sight component of the surface magnetic field, B i = B r sin θ + B θ cos θ, is sufficient to uniquely determine the potential coronal magnetic field by an explicit construction of the magnetic scalar potential for an arbitrary B l (θ, φ).

Global Models of the Magnetic Field in Historical Times

Abstract: The earliest measurements of the Earth's magnetic field were of declination By 1600 these measurements were widespread, but widespread measurements of dip are available only after 1700, and of intensity only after 1832 Global models of the Earth's magnetic field for these very early times must therefore be augmented by indirect measurements of dip and intensity using paleomagnetic methodsSuppose that the declination is known everywhere and a potential field B′ (=ΔΨ′) has been found that matches it Then the ratio of the horizontal intensity of this field to the true value is found to be constant along the lines of constant Ψ′ Thus one measurement of horizontal intensity on each line is sufficient to determine the horizontal field, and hence the complete field, uniquelyContours of Ψ′ encircle the dip-poles, and so, for example, it would be sufficient to measure horizontal intensity along a single line joining the dip-poles to provide good global coverageThe quantity of paleomagnetic information needed for the horizontal intensity is quite small The Earth's field at present has only two dip-poles, and measurements at some 10 sites spaced approximately evenly in latitude, and approximately 50y apart in time, should be adequate

The boundary value problem of the solar force-free magnetic field with constant α and its analytical solution

Abstract: In this paper we present a physical model which uses boundary conditions which seem to correspond more appropriately to actual situations. A boundary value problem of solar force-free magnetic field with constant α has been specified to represent the discretely concentrated characteristics of the longitudinal magnetic field on the photosphere. A unique analytical solution for the problem is obtained by a more strict method in mathematical physics. The most distinctive feature of our method is to make the solution be the superposition of the fields of single sources which are described by the physical parameters of corresponding sunspots on the photosphere, such as their position, strength, decay rates and the extent of the same polarity. The solution enables us to make an analytical description of the configuration of the magnetic field in the chromosphere and corona, and to investigate more conveniently its development as the foot points on the photosphere evolve.

The magnetic field of the equatorial magnetotail from 10 to 40 earth radii

Abstract: A statistical study of IMP 6, 7, and 8 magnetotail magnetic field measurements near the equatorial plane reveals new information about various aspects of magnetospheric structure. More magnetic flux crosses the equatorial plane on the dawn and dusk flanks of the tail than near midnight, but no evidence is found for a dependence on the interplanetary magnetic field sector polarity. Field magnitudes within 3 earth radii of the equatorial plane near dawn are more than twice as large as those near dusk for Xsm = -20 to -10 earth radii. The frequency of occurrence of southward fields is greatest near midnight, and such fields are seen almost twice as often for Xsm = -20 to -10 earth radii as for Xsm beyond -20 earth radii. This latter result supports the idea that the midnight region of the tail between 10 and 20 is a special location where neutral lines are particularly apt to form. Such a neutral line will approach nearest the earth in the midnight and premidnight region, where substorms are thought to have their onset.

Two‐density model of the Earth

Abstract: A model of the Earth is discussed in which a spherical and homogeneous core is surrounded by the mantle, taken to be a spherical shell of constant density. Four pedagogical exercises are worked out using this model: the variation of gravitational acceleration with radius, the moment of inertia of the Earth, the pressure at the center of the Earth, and the tunneling time through the Earth. The results are compared to actual geophysical values and also to the predictions of the single‐density model.