Showing papers on "Dipole model of the Earth's magnetic field published in 1990"

Modeling solar force-free magnetic fields

Abstract: A class of nonlinear force-free magnetic fields is presented, described in terms of the solutions to a second-order, nonlinear ordinary differential equation. These magnetic fields are three-dimensional, filling the infinite half-space above a plane where the lines of force are anchored. They model the magnetic fields of the sun over active regions with a striking geometric realism. The total energy and the free energy associated with the electric current are finite and can be calculated directly from the magnetic field at the plane boundary using the virial theorem. In the study of solar magnetic fields with data from vector magnetographs, there is a long-standing interest in devising algorithms to extrapolate for the force-free magnetic field in a given domain from prescribed field values at the boundary. The closed-form magnetic fields of this paper open up an opportunity for testing the reliability and accuracy of algorithms that claim the capability of performing this extrapolation. The extrapolation procedure as an ill-posed mathematical problem is discussed. 22 refs.

Midday auroral breakup events and related energy and momentum transfer from the magnetosheath

Abstract: Combined observations by meridian-scanning photometers, all-sky auroral TV camera and the EISCAT radar permitted a detailed analysis of the temporal and spatial development of the midday auroral breakup phenomenon and the related ionospheric ion flow pattern within the 71°–75° invariant latitude radar field of view. The radar data revealed dominating northward and westward ion drifts, of magnitudes close to the corresponding velocities of the discrete, transient auroral forms, during the two different events reported here, characterized by IMF |BY/BZ| 2, respectively (IMF BZ between −8 and −3 nT and BY > 0). The spatial scales of the discrete optical events were ∼50 km in latitude by ∼500 km in longitude, and their lifetimes were less than 10 min. Electric potential enhancements with peak values in the 30–50 kV range are inferred along the discrete arc in the IMF |BY/BZ| 2 case. Joule heat dissipation rates in the maximum phase of the discrete structures of ∼ 100 ergs cm−2 s−1 (0.1 W m−2) are estimated from the photometer intensities and the ion drift data. These observations combined with the additional characteristics of the events, documented here and in several recent studies (i.e., their quasi-periodic nature, their motion pattern relative to the persistent cusp or cleft auroral arc, the strong relationship with the interplanetary magnetic field and the associated ion drift/E field events and ground magnetic signatures), are considered to be strong evidence in favour of a transient, intermittent reconnection process at the dayside magnetopause and associated energy and momentum transfer to the ionosphere in the polar cusp and cleft regions. The filamentary spatial structure and the spectral characteristics of the optical signature indicate associated localized ˜1-kV potential drops between the magnetopause and the ionosphere during the most intense auroral events. The duration of the events compares well with the predicted characteristic times of momentum transfer to the ionosphere associated with the flux transfer event-related current tubes. It is suggested that, after this 2–10 min interval, the sheath particles can no longer reach the ionosphere down the open flux tube, due to the subsequent super-Alfvenic flow along the magnetopause, conductivities are lower and much less momentum is extracted from the solar wind by the ionosphere. The recurrence time (3–15 min) and the local time distribution (∼0900–1500 MLT) of the dayside auroral breakup events, combined with the above information, indicate the important roles of transient magnetopause reconnection and the polar cusp and cleft regions in the transfer of momentum and energy between the solar wind and the magnetosphere.

Global Configuration of a Magnetic Cloud

Abstract: A magnetic cloud associated with a 2N flare on January 1, 1978 was observed by IMP-8, Helios A, Helios B, and Voyager 2 The variation of the magnetic field observed at each spacecraft is represented to good approximation by Lundquist's solution for a cylindrically symmetric force-free magnetic field with constant alpha A least-squares fit of Lundquist's solution to the data from each spacecraft gives the local orientation of the axis of the magnetic cloud The times of the estimated boundaries of the magnetic cloud at each spacecraft, together with the speeds of the boundaries and the spacecraft position, give the positions of the boundaries at a given time From these results the magnetic cloud is determined to resemble a flux rope whose minor radius is approximately 015 AU at 1 AU, and whose radius of curvature at 1 AU is approximately 1/3 AU

Viking magnetic and electric field observations of Pc 1 waves at high latitudes

Abstract: Magnetic and electric field fluctuations in the Pc 1 frequency range (0.2-5 Hz) have been observed by the polar-orbiting Viking satellite. The fluctuations, interpreted here as electromagnetic ion cyclotron (EMIC) waves, were observed during 21 of 450 orbits surveyed between 0900 and 1400 MLT, near 3 R{sub E} geocentric altitude, and at invariant latitudes from 59{degree} to 77{degree}. The frequency structure of the waves is investigated, using spectral analysis and by determining the distribution of the wave frequency as a function of invariant latitude. At invariant latitudes from 59{degree} to 72{degree}, EMIC waves were observed in the frequency range below the equatorial He{sup +} gyrofrequency, while from 70{degree} to 77{degree} invariant latitude, EMIC waves were observed in the frequency range above the equatorial He{sup +} gyrofrequency. This latitude structure of the wave frequency is discussed in terms of the linear growth rate dependence of the waves on the heavy ion density, ion anisotropy, and ion energy. The propagation characteristics of these waves were also investigated, using minimum variance analysis and polarization analysis, and by estimating the Poynting flux based on the observed magnetic and electric field. The waves had Poynting vectors directed downward toward Earth and reached magnitudes between 0.01more » and 0.1 erg/cm{sup 2} s. The polarization of the waves was found to vary between linear, left-hand, and right-hand as a function of time or latitude. This variation is interpreted as the structure of spatially localized Pc 1 waves at high latitudes above the ionosphere.« less

Line element for efficient computation of the magnetic field created by thin iron plates

Abstract: The problem of computing the magnetic anomaly created by iron ships in the Earth's field is considered. From a modeling point of view, one of the major characteristics of this problem is the external and uniform magnetic source (the Earth's field). A good way of taking this into account is to simulate infinity and to use the reduced scalar potential as the state function. The major difficulty in modeling ships is the presence of sophisticated 3-D geometries due to the disproportion between the global size of a ship and the thickness of the iron plates. The modeling of the thin iron plates is examined, and it is shown that an efficient method is to use line elements in 2-D and surface elements in 3-D. The authors present the finite-element formulations used, applied to scalar potentials (reduced or total). The formulations using line elements have been successfully applied, even to a magnetic region with small permeability values. Results in 2-D and comparisons with analytical solutions and actual measurements are presented. >

Periodic auroral forms and geomagnetic field oscillations in the 1400 MLT region

Abstract: The UV images obtained with the Viking satellite often show bright features which resemble “beads” or “pearls” aligned in the east–west direction between noon and 1800 MLT (Lui et al., 1989). Viking acquired a series of 25 UV images during a 28-min period on July 29, 1986, which showed a distinctive series of periodic bright features in this region. Magnetic field and hot plasma measurements obtained by Viking confirm that the UV emissions are colocated with the field line projection of an upward-flowing region 1 Birkeland current and precipitating energetic (∼200 eV) electrons. These observations support previous work (see the review by Evans, 1985), which concludes that this region is the ionospheric projection of the cusp and low-latitude boundary layer. The magnetic field and electric field measurements show transverse oscillations (predominantly in the westward and southward components, respectively) with a nearly constant period of about 3.5 min from 67° invariant latitude equatorward up to the location of the large-scale Birkeland current system near 76° invariant latitude. The electric field oscillations lead the magnetic field oscillations by about a quarter-period. We interpret the observed oscillations as standing Alfven waves driven at a frequency near the local resonance frequency by a large-scale wave in the boundary layer. We propose that the energy flux of the precipitating low-energy electrons in this afternoon region is modulated by this boundary wave and produces the periodic UV emission features. These observations may be interpreted in terms of a boundary wave drifting tailward with a speed of about 0.12 MLT hour/min (1.8° longitude/min or about 800 m/s in the ionosphere) and with a wavelength of about 0.35 MLT hour (5.3° of longitude or about 160 km in the ionosphere). The results of this study support the view that large-scale oscillations of magnetospheric boundaries, possibly associated with the Kelvin-Helmholtz instability, can modulate currents, particles, and auroral forms.

Transient reconnection: Search for ionospheric signatures

Abstract: The concept of magnetic reconnection originated with the suggestion by Giovanelli [1946] that particles could be energized during solar flares near nulls in the magnetic field. Hoyle [1949] subsequently proposed that such a process could also act at nulls between the geomagnetic field and the interplanetary magnetic field (IMF) to generate the energized particles responsible for auroral displays. However, the idea of the interconnection of the two magnetic fields, as we know it today, was first presented by Hoyle's student, Dungey [1953, 1961].

Zeeman-doppler imaging: A new option for magnetic field study of Ap and solar-type stars

Abstract: In the task of studying stellar magnetic fields, polarimetric methods have been intensively used in Ap stars. But the observational material classically used to reconstruct stellar magnetic structures (average longitudinal magnetic field as a function of rotational phase) is not rich enough in spatial information to derive geometries more complex than centered or decentered dipoles. In solar-type stars, all evidences of activity recently detected on their surfaces (starspots, flares, ...) indicate they are most likely magnetic stars. But polarimetric methods have always failed in these stars, probably due to the complex magnetic topologies encountered which even prevented until now a simple detection (Borra, Edwards, and Mayor, 1984). With the Zeeman broadening measurement technique proposed by Robinson (1980), no reliable results can be derived for rapid rotators, which are otherwise presumed to be the best candidates for magnetic detections. Once more, if temperature inhomogeneity charts are already available for solar-type stars (Vogt, 1987), spatial information on their magnetic distributions has conversely not yet been obtained. The new option, recently proposed by Semel (1989) and qualified by Donati, Semel, and Praderie (1989), is based on the rotational modulation study of a rapid rotator Stokes parameter V(λ), obtained with both high spectral resolution R, and high signal-to-noise ratio S/N. Since the magnetic information used refers to localized strips on the stellar disc (as a consequence of the star rotation), multipolar structures can thus be resolved. A new instrumentation and observing procedure have been defined for ZDI, in order to obtain very high S/N data. The method has been successfully tested on two bright magnetic Ap stars: a magnetic detection was obtained on ɛ UMa and a 15-point phase coverage of α 2 CVn is available for the reconstruction of complete 2D abundance and magnetic mappings of its photosphere. Concerning solar-type stars, a numerical simulation was carried out in order to determine the observational constraints required for the detection of ‘typical’ magnetic field similar to those reported in slow rotators with the Robinson method (Saar, 1988). The specifications needed are S/N ≥ 400 per 40 mA pixel and R ∼- 6 × 104.

Residual magnetic dipole cancellation system for satellites

Abstract: The magnitude and direction of the residual magnetic field or dipole of an orbiting spacecraft is estimated from the attitude-perturbing effects of its interaction with the magnetic field of the heavenly body about which it orbits. The measurements are performed over a significant portion of an orbit. The estimate uses h, the total spacecraft momentum in the body frame, and its body angular rate ω. The Earth's magnetic flux density B is determined either by magnetometer measurements, or by recourse to stored historical and ephemeris information.

Structure of interplanetary magnetic field near the sun

Abstract: The interplanetary structures of the magnetic field and the current near the sun are analyzed by MHD computer simulation for the case of the magnetic dipole configuration on the photosphere. It is shown that the solar wind in the closed field region corotates with the sun. The azimuth magnetic field is formed in the open region but remains very weak in the closed region. The poloidal current, which associates with the azimuth magnetic field, is found to come into and go out of the sun in the open and closed regions, respectively.

Electron-gas theory of the chemical shift

Abstract: We construct a gauge-invariant approximation of the energy density of an interacting electron gas in the presence of a non-uniform magnetic field. The interaction is approximated by a slowly varying local potential. The magnetic field is a superposition of a constant field B and that due to a magnetic dipole μ, thus making the energy density suitable for direct calculation of chemical-shift tensors of, for example, interacting closed-shell systems. Unlike a similar theory for the magnetic susceptibility, the field-dependent exchange energy does not diverge.

On the continuum intensity-magnetic field relation along the decay phase of sunspots

Abstract: We present continuum intensity-magnetic field distributions for a decaying sunspot. It is shown that a very simple model accounts for the observed correlation. The Wilson depression is determined.

Toroidal field generation and magnetic field relaxation in a conical‐theta‐pinch‐generated configuration

Abstract: The configurations generated by conical theta pinches (CθP’s) of angles 10° and 18° were studied experimentally. The hydrogen plasma density and temperature were approximately 1–3×1015 cm−3 and 5 eV, respectively. The magnetic field structure was measured by four three‐dimensional magnetic probes. The magnetic helicity and energy of the configuration were calculated directly from the measured magnetic field through algorithms that were independent of the chosen coordinate and configuration axis. The magnetic fluxes were calculated after the axis was located. Single‐fluid model equations predict that the toroidal field can be generated by effects that are normally neglected in the simple Ohm’s law, of which the Hall effect is the most important. The average toroidal field generation was measured to vary quadratically with the cone angle of the CθP, and this field could be accounted for by the Hall effect alone. The time histories of the magnetic helicity and energy decay and poloidal to toroidal flux conve...

Transient electromagnetic field of a vertical magnetic dipole above a plane conducting Earth

Abstract: The pulsed electromagnetic radiation from a vertical magnetic dipole above a conducting Earth is investigated theoretically. The modified Cagniard method is used to derive closed form expressions for the magnetic Hertzian vector anywhere above the conducting Earth. Numerical results are presented for the magnetic Hertzian vector for different points of excitation and observation above the Earth, as well as for different values of the Earth's material parameters, namely, permittivity and electrical conductivity.

Oscillations of a dipole in a magnetic field: An experiment

Abstract: The small oscillations of a parallelepidal magnet along the axis of a circular coil carrying an electric current have been analyzed theoretically and experimentally. The study of this system (a particular case of a magnetic dipole in motion in a nonuniform magnetic field) involves basic ideas from both mechanics and electromagnetism, and the equipment used in the experiment is very common in an undergraduate laboratory. Comparison with the experiment shows that a very simple theoretical approach gives good results. It is also shown how the introduction of some refinements in the physical model can improve the agreement between theory and experiment, though the theoretical analysis becomes more involved in this case. The use of the principle of superposition to calculate magnetic fields is emphasized throughout the article.

A two-dimensional kinematic dynamo model of the ionospheric magnetic field at Venus

Abstract: The results of a high-resolution, two-dimensional, time dependent, kinematic dynamo model of the ionospheric magnetic field of Venus are presented. Various one-dimensional models are considered and the two-dimensional model is then detailed. In this model, the two-dimensional magnetic induction equation, the magnetic diffusion-convection equation, is numerically solved using specified plasma velocities. Origins of the vertical velocity profile and of the horizontal velocities are discussed. It is argued that the basic features of the vertical magnetic field profile remain unaltered by horizontal flow effects and also that horizontal plasma flow can strongly affect the magnetic field for altitudes above 300 km.

Radar and satellite observations of the storm time cleft

Abstract: During the magnetic storm of February 8–9, 1986, the region of strong ion convection in the vicinity of the dayside cusp expanded equatorward into the field of view of the Millstone Hill radar at lower mid-latitudes. High-speed (>1.5 km s−1) poleward ion flows were found at latitudes as low as 60° invariant latitude, at least 10° lower than the typical cleft/cusp position for moderately disturbed (Kp>4) magnetospheric conditions. The ion velocity pattern responded promptly to changes in the interplanetary magnetic field By direction. The large-scale two-dimensional convection pattern across the dayside was well resolved using radar azimuth scan data at Millstone Hill, thus enabling us to place the fine-scale radar/satellite observations of the storm time cusp and cleft in the context of the large-scale pattern. We present a detailed comparison of radar and DMSP F7 satellite observations in the prenoon sector during a period of Kp > 7, to examine the low-altitude signatures of various plasma regions in the vicinity of the cusp. The combination of particle precipitation, magnetic field perturbation, radar measurements of ion heating, and convection consistently suggests the unusual low-latitude position of cusp at 65° invariant latitude. Boundary plasma sheet particles were observed to coincide with a narrow region of magnetic-field-aligned currents, and with antisunward convection flows at the equatorward edge of the cleft. This may be an indication of the presence of a viscous cell. The radar and satellite observations indicate that the storm time cleft and cusp at ionospheric altitudes retain the general characteristics revealed in their average configuration. Particle and field signatures of the plasma sheet, plasma sheet boundary layer, low-latitude boundary layer, cusp, and mantle are identified at unusually low magnetic latitude in the 9–10 magnetic local time sector during this event.

Angular momentum transport and magnetic fields in the solar interior

Abstract: The possible mechanisms of angular momentum transport in convectively stable regions of a star are reviewed, with emphasis on transport by magnetic torques. The strength and configuration of the field in such layers is quite uncertain, because it is not known if the field can reach a dynamically stable configuration. A lower limit to the field strength is obtained by assuming that the field is always dynamically unstable, and decaying at the (rotation modified) dynamical time scale. The present field in the sun would then be of the order 1G, with poloidal and toroidal components of similar strength. The differential rotation in the core, if due only to the solar wind torque, would be very small for this field strength, and instead would more likely be governed by magnetic coulpling to the differential rotation of the convection zone. If small scale hydrodynamic transport mechanisms are present, their properties would also be influenced by a field of this strength.

The dipole tilt angle effect on the magnetosphere of Neptune – A laboratory simulation

Abstract: Recent observations of Neptune by Voyager 2 reveal that the dipole tilt angle is almost 50 degrees – close to that of Uranus. When comparing the magnetospheres of planets, it is therefore important to understand configurations of magnetic field lines with large dipole tilt angles. This paper deals with the laboratory characteristics of Neptune's magnetosphere whose dipole tilt angle dominates the configuration of its magnetic field lines and particle intrusions from interplanetary space. The dipole tilt angle has a considerable effect on the formation of the asymmetrical structure of the field lines and the cylindrical tail current sheet in the magnetosphere. The experiment also shows the effect of the x-component of the interplanetary Magnetic field to form an asymmetrical magnetospheric structure.

Atmospheric Effects on the Earth’s Rotation

Abstract: The atmosphere as the outer volatile region of the Earth is directly exposed to solar radiation. It is also connected to the solar wind via interaction with the geomagnetic field and the magnetospheric plasma. The atmospheric wind blowing over the rough surface can exchange angular momentum with the solid Earth. On a short-term scale (days to years), fluctuations of the angular momentum vector give rise to variations in the length of the day (LOD) and changes in the polar motion. Secular variations of the LOD may be partly due to a solar gravitational torque acting on the thermally excited semidiurnal density bulge on the ground. The frictional force between the magnetopause and the solar wind may be capable of braking the Earth’s rotation. In this paper, I estimate the various effects of atmospheric motions which may induce short-term and long-term changes in the Earth’s rotation.

Analysis of the expected resolution of the electron‐cyclotron emission crossed‐sightline magnetic field diagnostic

Abstract: The local value of the magnetic field B(r) inside the plasma in magnetic fusion experiments is a quantity of considerable interest. Given information about plasma density and temperature it can be used to calculate the poloidal magnetic field, or given a zero current device (or the current profile) the local value of beta. The time history B(,t) can be used to infer the level of magnetic fluctuations in the plasma. The poloidal magnetic field and the level of magnetic fluctuations are particularly interesting quantities, since they are difficult to measure, and are both important for understanding particle and energy transport in magnetic fusion experiments. In fact, to date, no measurement of the local value of the total magnetic field or its time variation has been made in a high temperature plasma. Previous measurements have been confined to relatively cold plasmas where probes could be used, or have inadequate resolution to distinguish the value of the field with plasma present from its vacuum value. ...

Coriolis emf as the real generator of solar magnetic field and a new model of the reversal of the general solar magnetic field

Abstract: Generation of general solar magnetic field as proposed by Cowling (1945, 1953) and Drobyshevski (1977) from the Coriolis emf is reconsidered and a new mechanism of the reversal of teh general solar magnetic field is proposed.

Current collection model on tether satellite system

Abstract: Results are reported from numerical simulations of current collection by a tethered satellite moving in the earth magnetic field. The focus is on the instruments of the Research on Orbital Plasma Electrodynamics (ROPE) experiment planned for the TSS-1 mission (scheduled launch 1991). The derivation of the governing equations is outlined, and the results of simulations with and without the geomagnetic field are presented in graphs. It is predicted that beam-beam interaction will occur when no magnetic effect is present, especially when the instrument bias is lower than the local potential; with the magnetic effect, there should be complex multiple-stream interactions, magnetic-field shielding covering 30 percent or more of the satellite surface when the B field is greater than 0.35 G, and nearly complete collection of the electrons by the instrument arm.