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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Ice ages and geomagnetic reversals

There have been speculations on the relationship between climatic cooling and polarity reversals of the earth's magnetic field during the Pleistocene. Two of the common criticisms on this relationship have been the reality of these short duration geomagnetic events and the accuracy of their dates. Champion et al. (1988) have reviewed recent progress in this area. They identified a total of 10 short-duration polarity events in the last 1 Ma and 6 of these events have been found in volcanic rocks, which also have K-Ar dates. Supposing that the speculated relationship between climatic cooling and geomagnetic reversals actually exist, two mechanisms that assume climatic cooling causes short period magnetic reversals will be investigated. These two methods are core-mantle boundary topography and transfer of the rotational energy to the core.

Wu, Patrick↗
DOE OSTI2014

Enhancement of exchange bias and training effect in ion-beam sputtered Fe{sub 46}Mn{sub 54}/Ni{sub 81}Fe{sub 19} bilayers

We present a remarkable enhancement by 300% of the exchange-bias field at room temperature, without affecting the coercivity value, via optimum magnetic annealing (250 °C/3 kOe) in ion-beam sputtered FeMn(30 nm)/NiFe(10 nm) bilayers. This specific behavior has been attributed to a higher degree of γ-FeMn(111) orientation that offers more interfacial FeMn moments to get pinned with the moments of the adjacent NiFe layer. Unlike the absence of training effect at room temperature, a pronounced training effect and an accompanying magnetization reversal asymmetry are evidenced upon field cooling below 50 K due to the presence of biaxial exchange induced anisotropy across the interdiffused FeMn/NiFe interface. The present findings not only have technological significance but also are of relevance to the understanding of interfacial spin disorder and frustration in these exchange-biased systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

OBSERVED VARIABILITY OF THE SOLAR Mg II h SPECTRAL LINE

The Mg ii h and k doublet are two of the primary spectral lines observed by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These lines are tracers of the magnetic and thermal environment that spans from the photosphere to the upper chromosphere. We use a double-Gaussian model to fit the Mg ii h profile for a full-Sun mosaic data set taken on 2014 August 24. We use the ensemble of high-quality profile fits to conduct a statistical study on the variability of the line profile as it relates the magnetic structure, dynamics, and center-to-limb viewing angle. The average internetwork profile contains a deeply reversed core and is weakly asymmetric at h2. In the internetwork, we find a strong correlation between h3 wavelength and profile asymmetry as well as h1 width and h2 width. The average reversal depth of the h3 core is inversely related to the magnetic field. Plage and sunspots exhibit many profiles that do not contain a reversal. These profiles also occur infrequently in the internetwork. We see indications of magnetically aligned structures in plage and network in statistics associated with the line core, but these structures are not clear or extended in the internetwork. The center-to-limb variations are compared to predictions of semi-empirical model atmospheres. We measure a pronounced limb darkening in the line core that is not predicted by the model. The aim of this work is to provide a comprehensive measurement baseline and preliminary analysis on the observed structure and formation of the Mg ii profiles observed by IRIS.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1991

On the position of the near-earth neutral sheet - A comparison of magnetic model predictions with empirical formulas

A detailed comparison of the near-earth neutral sheet position according to several approximations has been made. In particular, two empirical formulas for the neutral sheet location, recently derived from AMPTE/CCE data, have been compared with the corresponding positions derived from Tsyganenko's magnetic field models. Cylindrical coordinates referenced to the geodipole have been used, and the position of the neutral sheet has been identified by the reversal of the radial component of the magnetic field. The analysis reveals that, in the region X(GSM) = -9 to -5 earth radii, abs. value of Y(GSM) not greater than 5 earth radii, close agreement exists between the empirical formulas and the Tsyganenko models. Furthermore, the Tsyganenko models provide a representation for the neutral sheet position in the transition region between near-earth and distant tail models.

Peredo, Mauricio↗
NASA NTRS1982

Energetic protons accelerated at corotating shocks - Pioneer 10 and 11 observations from 1 to 6 AU

Vector helium magnetometer and proton telescope data are used to examine the relationship between low energy proton increases and corotating interaction regions (CIRs). A general correlation is noted between the maximum CIR field intensity and the maximum proton count rate. The minimum proton flux, located between the two proton maxima, appears to be correlated with the maximum field strength of the CIR. The evidence presented strongly supports shock acceleration as the primary source of the 1-MeV protons. A schematic figure incorporating many of the features deduced in the present study is given to illustrate the relationship between energetic protons, forward and reverse shocks, and the interplanetary magnetic field structure. The predictions of various theories and mechanisms for interplanetary nucleon acceleration are discussed in light of the experimental results presented.

Tsurutani, B. T.↗
DOE OSTI2013

Field-driven sense elements for chirality-dependent domain wall detection and storage

A method for locally sensing and storing data of transverse domain wall chirality in planar nanowire logic and memory systems is presented. Patterned elements, in close proximity to the nanowires, respond to the asymmetry in the stray field from the domain wall to produce a chirality-dependent response. When a bias field is applied, a stray field-assisted reversal of the element magnetization results in a reversed remanent state, measurable by scanning electron microscopy with polarization analysis (SEMPA). The elements are designed as triangles with tips pointing toward the nanowire, allowing the shape anisotropy to be dominated by the base but having a portion with lower volume and lower energy barrier closest to the domain wall. Micromagnetic modeling assists in the design of the nanowire-triangle systems and experiments using SEMPA confirm the importance of aspect ratio and spacing given a constant bias field magnitude.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1972

Satellite measurements of high latitude convection electric fields.

This paper reviews the first results of satellite experiments to measure magnetospheric convection electric fields using the double-probe technique. The earliest successful measurements were made with the low-altitude (680-2530 km) polar orbiting Injun-5 spacecraft. The Injun-5 results are compared with the initial findings of the electric field experiment on the polar orbiting OGO-6 satellite. Electric field measurements from the OGO-6 satellite have substantiated many of the initial Injun-5 observations with improved accuracy and sensitivity. The OGO-6 detector revealed the persistent occurrence of anti-sunward convection across the polar cap region at velocities not generally detectable with the Injun-5 experiment. The OGO-6 observations also provided information indicating that the location of the electric field reversal shifts equatorward during periods of increased magnetic activity. The implications of the electric field measurements for magnetospheric and auroral structure are summarized, and a list of specific recommendations for improving future experiments is presented.

Cauffman, D. P.↗
DOE OSTI2014

EMAPS: An Efficient Multiscale Approach to Plasma Systems with Non-MHD Scale Effects

Using Discrete-Event Simulation (DES) as a novel paradigm for time integration of large-scale physics-driven systems, we have achieved significant breakthroughs in simulations of multi-dimensional magnetized plasmas where ion kinetic and finite Larmor radius (FLR) and Hall effects play a crucial role. For these purposes we apply a unique asynchronous simulation tool: a parallel, electromagnetic Particle-in-Cell (PIC) code, HYPERS (Hybrid Particle Event-Resolved Simulator), which treats plasma electrons as a charge neutralizing fluid and solves a self-consistent set of non-radiative Maxwell, electron fluid equations and ion particle equations on a structured computational grid. HYPERS enables adaptive local time steps for particles, fluid elements and electromagnetic fields. This ensures robustness (stability) and efficiency (speed) of highly dynamic and nonlinear simulations of compact plasma systems such spheromaks, FRCs, ion beams and edge plasmas. HYPERS is a unique asynchronous code that has been designed to serve as a test bed for developing multi-physics applications not only for laboratory plasma devices but generally across a number of plasma physics fields, including astrophysics, space physics and electronic devices. We have made significant improvements to the HYPERS core: (1) implemented a new asynchronous magnetic field integration scheme that preserves local divB=0 to within round-off errors; (2) Improved staggered-grid discretizations of electric and magnetic fields. These modifications have significantly enhanced the accuracy and robustness of 3D simulations. We have conducted first-ever end-to-end 3D simulations of merging spheromak plasmas. The preliminary results show: (1) tilt-driven relaxation of a freely expanding spheromak to an m=1 Taylor helix configuration and (2) possibility of formation of a tilt-stable field-reversed configuration via merging and magnetic reconnection of two double-sided spheromaks with opposite helicities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Pressure-driven reconnection and quasi periodical oscillations in plasmas

This paper presents a model for an ohmically heated plasma in which a feedback exists between thermal conduction and transport, on one side, and the magneto-hydro-dynamical stability of the system, on the other side. In presence of a reconnection threshold for the magnetic field, a variety of periodical or quasi periodical oscillations for the physical quantities describing the system are evidenced. The model is employed to interpret the observed quasi periodical oscillations of electron temperature and perturbed magnetic field around the so called “Single Helical” state in the reversed field pinch, but its relevance for other periodical phenomena observed in magnetic confinement systems, especially in tokamaks, is suggested.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

A multi-frame soft x-ray pinhole imaging diagnostic for single-shot applications

For high energy density magnetized target fusion experiments at the Air Force Research Laboratory FRCHX machine, obtaining multi-frame soft x-ray images of the field reversed configuration (FRC) plasma as it is being compressed will provide useful dynamics and symmetry information. However, vacuum hardware will be destroyed during the implosion. We have designed a simple in-vacuum pinhole nosecone attachment, fitting onto a Conflat window, coated with 3.2 mg/cm{sup 2} of P-47 phosphor, and covered with a thin 50-nm aluminum reflective overcoat, lens-coupled to a multi-frame Hadland Ultra intensified digital camera. We compare visible and soft x-ray axial images of translating ({approx}200 eV) plasmas in the FRX-L and FRCHX machines in Los Alamos and Albuquerque.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS2007

Alfven Waves in the Solar Wind, Magnetosheath, and Outer Magnetosphere

Alfven waves Propagating outward from the Sun are ubiquitous in the solar wind and play a major role in the solar wind-magnetosphere interaction. The passage of the waves generally occurs in the form of a series of discrete steepened discontinuities, each of which results in an abrupt change in the interplanetary magnetic field direction. Some orientations of the magnetic field permit particles energized at the Earth's bow shock to gain access to the foreshock region immediately upstream from the Earth's bow shock. The thermal pressure associated with these particles can greatly perturb solar wind plasma and magnetic field parameters shortly prior to their interaction with the Earth's bow shock and magnetosphere. The corresponding dynamic pressure variations batter the magnetosphere, driving magnetopause motion and transient compressions of the magnetospheric magnetic field. Alfven waves transmit information concerning the dynamic pressure variations applied to the magnetosphere to the ionosphere, where they generate the traveling convection vortices (TCVs) seen in high-latitude ground magnetograms. Finally, the sense of Alfvenic perturbations transmitted into the magnetosheath reverses across local noon because magnetosheath magnetic field lines drape against the magnetopause. The corresponding change in velocity perturbations must apply a weak torque to the Earth's magnetosphere.

Sibeck, D. G.↗
DOE OSTI2019

Comparing the anomalous Hall effect and the magneto-optical Kerr effect through antiferromagnetic phase transitions in Mn 3 Sn

In the non-collinear antiferromagnet Mn 3 Sn, we compare simultaneous measurements of the anomalous Hall effect (AHE) and the magneto-optical Kerr effect (MOKE) through two magnetic phase transitions: the high-temperature paramagnetic/antiferromagnetic (AF) phase transition at the Néel temperature ( T N ≈ 420 K) and a lower-temperature incommensurate magnetic ordering at T 1 ≈ 270 K. While both the AHE and MOKE are sensitive to the same underlying symmetries of the AF non-collinear spin order, we find that the transition temperatures measured by these two techniques unexpectedly differ by approximately 10 K. Moreover, the applied magnetic field at which the AF order reverses is significantly larger when measured by MOKE than when measured by AHE. Furthermore these results point to a difference between the bulk and surface magnetic properties of Mn 3 Sn.

36 MATERIALS SCIENCE↗
NASA NTRS1981

The cosmic ray electron/proton intensity ratio between 1965 and 1979

In experimental investigations, differences have been found regarding the modulation of low-energy electrons and protons. These differences cannot be explained on the basis of conventional theory. Evenson et al. (1979) have attempted to relate the differences in the modulation of electrons and protons to the solar field reversal of 1969-1971. The analyses have been inconclusive due to lack of a long time base and to a scarcity of reliable electron data. The present study represents an extension of he investigation of Evenson et al. (1979) to the time period 1965-1979. New data obtained in 1977-1979 are added, 1965 electron data are reexamined, and the time behavior of the helium/proton ratio is considered. It is concluded that there was a systematic shift in the intensity ratio of low-energy electrons to protons in the time period around the reversal of the solar polar magnetic field, leading to a clear difference in the values of the ratio for the two consecutive solar minima 1965 and 1976.

Evenson, P.↗
NASA NTRS1986

Hyper-resistivity produced by tearing mode turbulence

Tearing mode turbulence produces a hyperresistivity or effective anomalous electron viscosity. The hyperresistivity is calculated for the mean magnetic field quasi-linearly, and for long-wavelength modes using the direct interaction approximation. The hyperresistivity accounts for current relaxation in reversed-field pinch experiments, and gives a magnetic fluctuation scaling of S to the -1/3. It causes enhanced tearing mode growth rates in the turbulent phase of tokamak disruptions. In astrophysics, it limits magnetic energy growth because of the dynamo effect, and may explain rapid reconnection phenomena such as solar flares.

Strauss, H. R.↗
DOE OSTI2014

Predicting the sun's polar magnetic fields with a surface flux transport model

The Sun's polar magnetic fields are directly related to solar cycle variability. The strength of the polar fields at the start (minimum) of a cycle determine the subsequent amplitude of that cycle. In addition, the polar field reversals at cycle maximum alter the propagation of galactic cosmic rays throughout the heliosphere in fundamental ways. We describe a surface magnetic flux transport model that advects the magnetic flux emerging in active regions (sunspots) using detailed observations of the near-surface flows that transport the magnetic elements. These flows include the axisymmetric differential rotation and meridional flow and the non-axisymmetric cellular convective flows (supergranules), all of which vary in time in the model as indicated by direct observations. We use this model with data assimilated from full-disk magnetograms to produce full surface maps of the Sun's magnetic field at 15 minute intervals from 1996 May to 2013 July (all of sunspot cycle 23 and the rise to maximum of cycle 24). We tested the predictability of this model using these maps as initial conditions, but with daily sunspot area data used to give the sources of new magnetic flux. We find that the strength of the polar fields at cycle minimum and the polar field reversals at cycle maximum can be reliably predicted up to 3 yr in advance. We include a prediction for the cycle 24 polar field reversal.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1982

The flux ejection dynamo with small diffusivity. I - Basic properties. II - Illustrative examples

In the first part of this paper, the flux ejection dynamo in an infinitely broad convective layer of finite depth is treated in the limit of electrical conductivity, where the convective layer upper surface is open to empty space while the lower surface is closed to the passage of fluid and field. The idealized convective overturning used allows an exact description of the symmetry and distribution of the magnetic field carried with the fluid. The total flux of the mean horizontal field grows linearly with the number of convective cycles. A discussion is given of the boundary conditions to be applied to an astronomical body whose flux ejection dynamo operates at its surface. In the second part, examples are presented which illustrate the consequences of reverse flux ejection from the surface of a convective layer of conducting fluid. Reverse flux ejection generally has the opposite effect of magnetic buoyancy, burying the fields rather than bringing them through the surface. Reverse flux injection at the surface of an alpha-omega dynamo profoundly alters the character of the solutions of dynamo equations.

Parker, E. N.↗
DOE OSTI2013

Reversible collisionless magnetic reconnection

Reversible magnetic reconnection is demonstrated for the first time by means of gyrokinetic numerical simulations of a collisionless magnetized plasma. Growth of a current-driven instability in a sheared magnetic field is accompanied by magnetic reconnection due to electron inertia effects. Following the instability growth, the collisionless reconnection is accelerated with development of a cross-shaped structure of current density, and then all field lines are reconnected. The fully reconnected state is followed by the secondary reconnection resulting in a weakly turbulent state. A time-reversed simulation starting from the turbulent state manifests that the collisionless reconnection process proceeds inversely leading to the initial state. During the reversed reconnection, the kinetic energy is reconverted into the original magnetic field energy. In order to understand the stability of reversed process, an external perturbation is added to the fully reconnected state, and it is found that the accelerated reconnection is reversible when the deviation of the E × B streamlines due to the perturbation is comparable with or smaller than a current layer width.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1993

Flow downstream of the heliospheric terminal shock - The magnetic field on the heliopause

Modeling the kinematic magnetic field in the solar wind beyond the terminal shock shows that a ridge of magnetic pressure is produced just inside the heliopause. This ridge is sufficiently large that it will cause the layer immediately inside the heliopause to thicken, pushing the heliopause outward and slightly affecting its position relative to the terminal shock. However, the ridge is far too thin to cause an important change in the distance of the terminal shock from the sun. We show that these conclusions are a simple consequence of geometrical arguments for incompressible, steady, laminar flows. Moreover, the heliopause magnetic field originates on the terminal shock near the substagnation point. Consequently, the heliospheric current sheet field reversals are painted onto the inside surface of the heliopause. Alternate magnetic polarity strips will be oppositely directed relative to the interstellar magnetic field, implying that reconnection inevitably occurs on a fine some near the nose of the heliosphere. This suggests that the heliopause is a leaky, diffuse surface.

Nerney, Steven↗