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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

A survey of long-term interplanetary magnetic field variations

Interplanetary-magnetic-field data from the IMP-10, IMP-A, and Heos spacecraft have been merged into a composite data set spanning the period from 1963 to 1974. Consideration of the mutual consistency of the individual data sets reveals agrement typically to within 0.2 gamma. Analysis of the composite data set reveals the following: (1) although the yearly averaged magnitudes of all field vectors show virtually no solar-cycle variation, the yearly averaged magnitudes of positive- and negative-polarity field vectors show separate solar-cycle variations consistent with variations in the average azimuthal angles of positive- and negative-polarity field vectors; (2) there is no solar latitude dependence of long-time average field magnitudes; (3) field vectors parallel to the earth-sun line are on the average 1 gamma less in magnitude than field vectors perpendicular to this line; and (4) the solar latitude-dependent dominant polarity effect exhibits a complex sign reversal in the period from 1968 to 1971 and a measure of symmetry in 1972 through 1974 not found in earlier data.

King, J. H.↗
NASA NTRS1976

The August 1972 solar-terrestrial events - Interplanetary magnetic field observations

Interplanetary-magnetic-field measurements made by Pioneers 9 and 10, HEOS 2, and Explorer 41 during early August 1972 are reviewed. It is noted that the two Pioneers were nearly radially aligned during the flare events, with Pioneer 9 at a distance of 0.78 AU from the sun and Pioneer 10 at a distance of 2.2 AU. The data obtained by Pioneer 9, Pioneer 10, and the two near-earth satellites are analyzed separately, and the major flare-associated shocks are identified. An attempt is made to identify corresponding shocks at the different locations and to determine their propagation velocities in the region between 0.8 and 2.2 AU. It is found that there was an obvious tendency for the average shock velocities to decrease with increasing radial distance from the sun and that the local velocities at the Pioneer locations were significantly smaller than the appropriate average values. A comparison of these local velocities indicates that there was a large deceleration of the shocks between the sun and some distance within 0.8 AU but little, if any, deceleration beyond that distance. A plot of average shock velocities from the sun to 1.0 AU as a function of longitude separation between the flares and Pioneer 9 is shown to suggest a pronounced deviation of the shock fronts from spherical symmetry.

Smith, E. J.↗
NASA NTRS1972

Fine structure of solar magnetic fields.

Extension of the deduction of magnetic fields from chromospheric structure to active regions and transverse fields. Fields independently predicted by these rules from a high-resolution H alpha filtergram are compared with a high-resolution magnetogram. The H alpha method has the advantage over conventional magnetograms that it shows transverse fields and relates the fields to the real sun. It has the disadvantage that higher spatial resolution is required and that it is difficult and time consuming in very complicated regions. The response of the chromosphere to magnetic fields is most consistent. Vertical field is invariably marked by bright plage, with brightness roughly proportional to the field strength (except for sunspots). All dark fibrils mark transverse fields and are parallel to field lines. All polarity changes are marked by dark fibrils, which may be transverse fibrils perpendicular to the field boundary, or filaments (prominences) which connect more distant points, and in which the field lines run nearly parallel to the boundary. The asymmetry between preceding and following polarity found by Veeder and Zirin (1970) does not exist; it was due to the low resolution of the Mount Wilson magnetograms.

Zirin, H.↗
NASA NTRS1992

The magnetic field investigation on the Ulysses mission - Instrumentation and preliminary scientific results

A fundamental feature of the heliosphere is the three-dimensional structure of the interplanetary magnetic field. The magnetic field investigation on Ulysses, the first space probe to explore the out-of-ecliptic and polar heliosphere, aims at determining the large-scale features and gradients of the field, as well as the heliolatitude dependence of interplanetary phenomena so far only observed near the ecliptic plane. The Ulysses magnetometer uses two sensors, one a Vector Helium Magnetometer, the other a Fluxgate Magnetometer. Onboard data processing yields measurements of the magnetic field vector with a time resolution up to 2 vectors/second and a sensitivity of about 10 pT. Since the switch-on of the instrument in flight on 25 October 1990, a steady stream of observations has been made, indicating that at this phase of the solar cycle the field is generally disturbed: several shock waves and a large number of discontinuities have been observed, as well as several periods with apparently intense wave activity. The paper gives a brief summary of the scientific objectives of the investigation, followed by a detailed description of the instrument and its characteristics. Examples of wave bursts, interplanetary shocks and crossings of the heliospheric current sheet are given to illustrate the observations made with the instrument.

Balogh, A.↗
NASA NTRS1987

The steady state toroidal magnetic field at the core-mantle boundary

Recent measurements indicate that the strength of the toroidal magnetic field at Earth's core-mantle boundary is comparable in strength to the poloidal field - 5 to 10 gauss. Calculations are given to show that this is an inevitable result of the external boundary condition on the core, in which the mantle electrical conductivity is several orders of magnitude lower than that of the core. The measurements are shown to imply that the internal core magnetic field is in the range of several hundred gauss. Thus the measurements imply that the Earth's core contains a strong toroidal magnetic field. They also support the idea that Earth's dynamo, and by implication, other planetary magnetic fields, involves efficient toroidal magnetic field generation through strong differential rotation.

Pearce, S. J.↗
NASA NTRS2004

The Magnetic Field in the Outer Heliosphere

One of the great achievements of Parker was the prediction that the solar magnetic field would be drawn into Archimedian spirals as it is carried away from the Sun by the solar wind. This prediction has been amply confirmed by many in situ measurements in the intervening four decades. But, Parker made his prediction for a solar wind that expands into infinite space while we now know that the local interstellar medium (LISM) is far from empty and, in fact, confines the solar wind to a finite volume, known as the heliosphere, that extends to approximately 100 AU in the upstream direction (the solar system is moving through the LISM). Voyagers 1/2, presently at -80 AU, are approaching the upstream boundaries of the heliosphere and returning data on the properties of the magnetic field. This is important for understanding how galactic cosmic rays (GCRs) reach the Earth. Voyagers show that the IMF at 10-80 AU behaves much as Parker predicted - with two important exceptions. This is not surprising since the field is essentially passively advected by the solar wind out to 80 AU. But, new models say that nearer the heliosphere boundaries the field plays a major role in the solar wind-LISM interaction. However, of the many physical ingredients that constitute the outer heliosphere, the magnetic field poses some of the most interesting and difficult numerical modeling problems. Presently, only a few results have been published and much remains to be done. Here I will summarize the expected and measured behavior of the magnetic field at 80 AU. Then I will describe modeling predictions beyond 80 AU: magnetic "tornadoes", polarity envelopes, the Axford-Cranfill effect, inner and outer magnetic walls and more. I will also list what I believe to be important new modeling objectives. Finally, I will speculate on what is happening with the magnetic field near the nose of the heliosphere. My conclusion is that models of GCR modulation rarely incorporate even crudely realistic magnetic fields so it is a wonder that they are as successful as they are and no surprise that there are still important discrepancies between GCR modulation observations and the models.

Suess, S. T.↗
NASA NTRS1971

The stability of sunspot magnetic fields and the origin of solar flares.

The steady motion in sunspot magnetic regions is considered for both current-free and force-free configurations. The sufficient condition for stability is obtained in the presence of both external current-free and force-free magnetic fields and a steady motion. It is shown that the pattern of such steady motion is most important in triggering an instability of sunspot magnetic fields, both for the current-free and force-free configuration. When there is no steady motion, the current-free configuration of sunspot magnetic fields is always stable, whereas the stability in the case of force-free magnetic fields is connected with the configuration. The onset of a solar flare seems to be associated with an instability connected to the steady motion within the sunspot magnetic regions.

Sakurai, K.↗
NASA NTRS1996

Semiconductor crystal growth in crossed electric and magnetic fields: Center Director's Discretionary Fund

A unique growth cell was designed in which crossed electric and magnetic fields could be separately or simultaneously applied during semiconductor crystal growth. A thermocouple was inserted into an InSb melt inside the growth cell to examine the temperature response of the fluid to applied electromagnetic fields. A static magnetic field suppressed time-dependent convection when a destabilizing thermal field was applied. The simultaneous application of electric and magnetic fields resulted in forced convection in the melt. The InSb ingots grown in the cell were polycrystalline. An InGaSb crystal, 0.5 cm in diameter and 23-cm long, was grown without electromagnetic fields applied. The axial composition results indicated that complete mixing in the melt occurred for this large aspect ratio.

Mazuruk, K.↗
NASA NTRS1987

Nonlinear force-free magnetic fields: Calculation and applicatin to astrophysics

The problem concerned in this work is that of calculating magnetic field configurations in which the Lorentz force (vector)j x (vector)B is everywhere zero, subject to specified boundary conditions. The magnetic field is represented in terms of Clebsch variables in the form (vector)B = del alpha x del beta. These variables are constant on any field line. The most appropriate choice of boundary conditions is to specify the values of alpha and beta on the bounding surface. It is proposed that the field lines move in the direction of local Lorentz force and relax towards a force-free field configuration. This concept leads to an iteration procedure for modifying the variables alpha and beta that tends asymptotically towards the force-free state. This method is first applied to a simple problem in two rectangular dimensions; the calculation shows that the convergence of magnetic field energy to a minimum state (force-free) is close to exponential. This method is then applied to study some astrophysical force-free magnetic fields, such as the structures and evolution of magnetic fields of rotating sunspots and accretion disks. The implication of the results, as related to the mechanisms of solar flares, extragalactic radio sources and radio jets, are discussed.

Yang, Wei-Hong↗
NASA NTRS1992

The magnetic field in the heliosheath

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.

Suess, S. T.↗
NASA NTRS1992

The effect of a magnetic field gradient on anode double layers

In experiments on anode (ionization) double layers in nonuniform magnetic fields it has been noted that the magnetic field gradient seems to stabilize the double layer position. This effect is further investigated in a Q machine in which the magnetic field geometry could be varied. It is found that the position of the double layers, along the axis of the device, could be controlled by changing the magnetic geometry. This effect is accounted for in a physical model which takes into account the effect of ion reflection by the magnetic mirror force in the region of magnetic field nonuniformity. This model is also able to account for variation of the double layer position when the neutral gas pressure is varied.

Song, B.↗
NASA NTRS1994

Long-term evolution of crustal neutron star magnetic fields

We have derived an analytic solution to the asymptotic behavior of dipolar magnetic fields that are generated in the crusts of neutron stars. We show that if the conductivity is due to impurity scattering, as expected for late stages of evolution, the surface field strength at the magnetic pole declines with the power law B(sub p) approximately = (t/t(sub 0))(exp -2/3). The results are shown to be qualitatively consistent with detailed numerical calculations. These latter results are consistent with some recent analyses of pulsar statistics and the magnetic fields of several binary pulsars with white dwarf companions whose ages have been determined. The dependence of the surface magnetic field on spin period of the pulsar is derived.

Urpin, V. A.↗