Pure Dipole Field Research Articles

Pair production and pulsar cutoff in magnetized neutron stars with nondipolar magnetic geometry

We construct models of the open field zone in pulsars in which we assume space-charge limited, time independent, electron flow from the surface of a conducting magnetized neutron star with dipolar and quadrupolar components to the magnetic field. We find that the radius of curvature of the field can be much less than in a pure dipole field and can be sufficient to account quantitatively for the cutoff line in the P-P diagram if systematic departures from our basic assumptions about the outer magnetosphere occur (such as decreasing the maximum radius of the last closed field line at small P). We further investigate the effects on the cutoff line of three simple models of torque decay: ohmic decay of the dipolar magnetic field, magnetic field complication due to an MHD-secular instability suggested by Flowers and Ruderman, and alignment of the magnetic dipole result (e.g., that potential drops at low altitude in the nondipolar field might result in self consistent particle acceleration even in the aligned rotator). We suggest possible observational consequences to the existence of higher order components to the field (e.g., differences in the polarization angle versus pulse longitude relation).

A self‐consistent model of a corotating Jovian magnetosphere

A rotation‐dominated model of the Jovian magnetosphere is constructed in which the plasma current distribution is dynamically consistent with the magnetic field. We assume an ionospheric plasma source with no pitch angle scattering, magnetically aligned rotation, loss‐free radial transport through flux tube interchange diffusion, and a static balance between centrifugal force and magnetic stress. From these assumptions a dynamical equation is derived for a self‐consistent magnetic field configuration having a single adjustable parameter that is related to the plasma source strength. An iterative numerical method is used to solve this equation. The self‐consistent field resembles a radially distorted dipole field and does not resemble the flat ‘magnetodisc’ configuration that has often been inferred from the Pioneer data. The implication is that such a magnetodisc field would require an equatorial plasma source (for example, the Galilean satellites) rather than a source at the feet of the field lines. The centrifugal force opens the field at a radial distance approximately 10% greater than that expected for a pure dipole field for a given source strength. For expected source strengths this opening radius is found to lie in the range 40–75 RJ.

High Precision Superconducting Magnets

Window-frame type superconducting dipole magnets have demonstrated accuracy and predictability of fields which compare well with the best conventional accelerator-type magnets. Precision measurements were made on the two series powered 2 m long 4 T modules comprising the 80 bending magnet at BNL after two years of beam operation including several dozen beam induced quenches. The integral field of the two units is identical to measurement accuracy, ≤ 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> parts at ~ full aperture, for all multipoles except quadrupole. Random quadrupole terms of G/B = 6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> or 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> parts are present. A 1 m long 6 T model of more advanced design shows pure dipole field, at all field levels, to 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> parts, using a single correction coil with predetermined excitation. The field shape at ~ 0.5 T/sec rise rate is identical to dc. The advantages of the extended current sheet coil construction are discussed. Results will be compared with computer simulations of construction errors. Suitable conceptual designs of compact 6 T dipoles and of quadrupoles applied to a 1 km accelerator lattice will be presented.

Analysis of Brunhes Epoch Palaeomagnetic Data in Terms of Geocentric and Offset Axial Dipole Fields: Long-Term Flattening of the Dipole Field?

Hemispherical contrasts in the consistency of Brunhes epoch (t= 0·69–0 My) palaeomagnetic data with Wilson's offset dipole model earlier led to the suggestion that the Brunhes epoch geomagnetic field may best be represented by an unequal co-axial dipole pair. Models have now been derived employing major centred or offset geocentric dipoles to which have been added the effect of minor axial dipoles, offset to the core-mantle boundary. It is shown that the Brunhes epoch palaeomagnetic data require an offset dipole, but also two minor axial dipoles, which would be from one to four per cent of the major dipole moment, and of opposite polarities. Compared to that field produced by a pure dipole field, a slight flattening of the Brunhes epoch geomagnetic field is therefore required at the geographic poles, as is the case for the present geomagnetic field. Should such an effect be of long term, then diagnosis of longitudinal plate motion by analysis of palaeomagnetic inclinations would need to incorporate limitations caused by resulting departures from axial dipole configurations.

Design of Dipole Magnet with Circular Iron Shield

Complex variable function theory is used to formulate the magnetostatic problem of a rectangular current block of uniform current density located within a circular region bounded by infinite permeability iron. The magnetic field from the superposition of several blocks arranged to exhibit dipole symmetry is calculated using a generalization of Cauchy's integral formula. Subsequent multipole expansion yields expressions for the multipole coefficients which through the use of variable metric minimization permits block arrangements to be found for rather pure dipole fields. Application is made to superconducting dipoles for both a pancake arrangement and a shell arrangement of the individual blocks.