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At least 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
NASA NTRS2021

An Encounter with the Ion and Electron Diffusion Regions at a Flapping and Twisted Tail Current Sheet

We analyze data returned by the Magnetospheric Multiscale mission (MMS) constellation during a rapid (~1.5 s) traversal of a flapping and reconnecting current sheet (CS) in the near-Earth magnetotail (X ~ -20 RE). The CS was highly tilted, with its normal pointing strongly duskward. Its extreme thinness was confirmed by a curvature analysis of the magnetic field lines. The event was associated with a guide field of 8% of the reconnecting components. From the pitch angle distributions of low-energy electrons we infer that the crossing occurred earthward of the X-line. Traveling practically normal to the CS, MMS encountered an ion diffusion region (IDR) in which was embedded an electron diffusion region (EDR). IDR signatures included breaking of the ion frozen-in condition in the presence of Hall B and E fields. EDR signatures included a strong out of-plane current associated with a super-Alfvenic electron jet, evidence of positive energy transfer, and a temperature anisotropy (TeII > Te1) which disappeared at the field reversal. Derived scale sizes normal to the CS are: ~6.9 de (EDR) and ~0.4 di (IDR) (40 and 100 km). We estimate the average dimensionless reconnection rate as 0.077 + 0.050. The observations and inferences are supported by PIC numerical simulations. We find very good agreement in the reconnection rates. We also discuss the effects of asymmetries in the density, temperature and magnetic field strength on the Hall fields and length of the outflow jets. The event is associated with a substorm onset which began 7 min after the MMS observations.

C J Farrugia↗
DOE OSTI2026

Spin reorientations in structurally metastable, disordered, and hexagonal Cr 7 ⁢Te 8

Vapor deposited two-dimensional Cr 7 ⁢Te 8 displays unusual temperature dependent Hall effect properties, including a room-temperature anomalous Hall effect, sign reversals of the Hall resistivity on cooling, and a peak in the Hall resistivity at low temperatures. The two-dimensional Cr 7⁢ Te 8 heterostructures that form the basis of these measurements are hexagonal in structure. We study the magnetic and structural properties of bulk Cr 7 ⁢Te 8 synthesized by quenching from 1000⁢°C with the goal of relating the magnetic, structural, and electronic properties. This quenched phase is metastable, hexagonal, and displays different magnetic properties from the slow-cooled and more thermodynamically stable monoclinic phase. High-resolution x-ray diffraction of the quenched hexagonal phase finds a first-order transition to a lower symmetry monoclinic phase on heating above ∼550 K. Magnetic susceptibility measurements of the quenched hexagonal phase reveal ferromagnetic ordering above room temperature, along with the two distinct transitions at ∼220 K and ∼70 K. Through neutron diffraction studies, we find the ∼220 K anomaly is a spin reorientation transition of the ferromagnetically aligned magnetic moments and the ∼70K feature represents a transition from a high-temperature ferromagnet to a low-temperature antiferromagnet. We suggest that these magnetic transitions are related to changes in the unit-cell dimensions and are connected to the temperature-dependent Hall resisitivity studied in two-dimensional heterostructures. This implies a link between structural, magnetic, and electronic properties in the “pseudo” two-dimensional chromium tellurides.

Chemical synthesis↗
DOE OSTI2022

Edge spin transport in the disordered two-dimensional topological insulator $\mathrm{WTe_2}$

The spin conductance of two-dimensional topological insulators (2D TIs) is not expected to be quantized in the presence of perturbations that break the spin-rotational symmetry. However, the deviation from the pristine-limit quantization has yet to be studied in detail. In this paper, we define the spin current operator for the helical edge modes of a 2D TI and introduce a four-terminal setup to measure spin conductances. Using the developed formalism, we consider the effects of disorder terms that break spin-rotational symmetry or give rise to edge-to-edge coupling. We identify a key role played by spin torque in an out-of-equilibrium edge. We then utilize a tight-binding model of topological monolayer $\mathrm{WTe_2}$ and scattering matrix formalism to numerically study spin transport in a four-terminal 2D TI device. In particular, we calculate the spin conductances and characteristic spin decay length in the presence of magnetic disorder. In addition, we also study the effects of interedge scattering in a quantum point contact geometry. Here we find that the spin Hall conductance is surprisingly robust to spin symmetry-breaking perturbations, as long as time-reversal symmetry is preserved and interedge scattering is weak.

36 MATERIALS SCIENCE↗
NASA NTRS1982

Sunward convection in both polar caps

The storm of July 29, 1977 was chosen as an object of study for the Coordinated Data Analysis Workshop (CDAW), a part of the International Magnetospheric Study (IMS). The purpose of CDAW was to synthesize many diverse data sets in order to understand as fully as possible the magnetosphere's response to a variety of inputs, including a strong interplanetary shock, a time of strongly southward interplanetary magnetic field (IMF), and a time of strongly northward IMF. The July 29 event was particularly interesting because it contained effects of storm time compressions and of strongly southward and strongly northward IMF'S. The convection data appear to indicate reversed (sunward) convection in the polar cap in both the northern and southern hemispheres, although conjugacy cannot be established because the northern and southern passes were at different locations and different times

Reiff, P. H.↗
NASA NTRS1984

Electrodynamic Tethers. 1: Power Generator in LEO. 2: Thrust for Propulsion and Power Storage

An electrodynamic tether consists of a long insulated wire in space whose orbital motion cuts across lines of magnetic flux to produce an induce voltage that in typical low orbits averages about 200 v/km. Such a system should be capable of generating substantial electrical power, at the expense of IXB drag acting on its orbital energy. If a reverse current is driven against the induced voltage, the system should act as a motor producing IXB thrust. A reference system was designed, capable of generating 20 KW of power into an electrical load located anywhere along the wire at the expense of 2.6N (20,000 J/sec) drag on the wire. In an ideal system, the conversion between mechanical and electrical energy would reach 100% efficiency. In the actual system part of the 20 KW is lost to internal resistance of the wire, plasma and ionosphere, while the drag force is increased by residual air drag. The 20 KW PMG system as designed is estimated to provide 18.7 KW net power to the load at total drag loss of 20.4 KJ/sec, or an overall efficiency of 92%. Similar systems using heavier wire appear capable of producing power levels in excess of 1 Megawatt at voltages of 2-4 KV, with conversion efficiency between mechanical and electrical power better than 95%. The hollow cathode based system should be readily reversible from generator to motor operation by driving a reverse current using onboard power.

Mccoy, J. E.↗
NASA NTRS2007

Reversible Intercalation of Fluoride-Anion Receptor Complexes in Graphite

We have demonstrated a route to reversibly intercalate fluoride-anion receptor complexes in graphite via a nonaqueous electrochemical process. This approach may find application for a rechargeable lithium-fluoride dual-ion intercalating battery with high specific energy. The cell chemistry presented here uses graphite cathodes with LiF dissolved in a nonaqueous solvent through the aid of anion receptors. Cells have been demonstrated with reversible cathode specific capacity of approximately 80 mAh/g at discharge plateaus of upward of 4.8 V, with graphite staging of the intercalant observed via in situ synchrotron X-ray diffraction during charging. Electrochemical impedance spectroscopy and B-11 nuclear magnetic resonance studies suggest that cointercalation of the anion receptor with the fluoride occurs during charging, which likely limits the cathode specific capacity. The anion receptor type dictates the extent of graphite fluorination, and must be further optimized to realize high theoretical fluorination levels. To find these optimal anion receptors, we have designed an ab initio calculations-based scheme aimed at identifying receptors with favorable fluoride binding and release properties.

electrolytes↗
NASA NTRSDate not supplied

Reversion to the mean of the Solar Wind Input to the Magnetosphere

Earth’s magnetosphere responds to solar wind input over time scales ranging from minutes to days. On minute times scales, researchers seek to identify discontinuities in the solar wind that may trigger transient magnetospheric phenomena such as flux transfer events or substorm onsets. On hour time scales, researchers seek to determine the solar wind phenomena that drive the growth phase of geomagnetic substorms and the storage of magnetic flux within the magnetotail. On longer time scales, researchers seek to determine the solar wind features than propel geomagnetic storms through their initial, main, and recovery phases. Researchers must, perforce, rely upon spacecraft located near the L1 libration point for observations of the solar wind input into the magnetosphere over periods ranging from hours to days. Some of the hazards of doing this are well known. Features seen far upstream at locations away from the Sun-Earth line may never arrive at Earth due to structure in the solar wind transverse to the Sun-Earth line. Alternatively, they may arrive at unexpected times due to errors in calculating their orientation and velocity. Other hazards when predicting the magnetospheric response to varying solar wind conditions are less well understood. When observations of large solar wind drivers at spacecraft far upstream and off the Sun-Earth line are used to select events for study, the ‘reversion to the mean’ effect associated with spatial structure and errors in arrival timing for solar wind features at Earth demands that the magnetosphere will actually be subjected to and respond to solar wind drivers with much lower strengths. Consequently, the magnetospheric response must saturate for the highest solar wind drivers.

David Sibeck↗
NASA NTRS1976

Contributions of rock magnetism and paleomagnetism to recent geophysical advances

The origin of natural remanent magnetization (NRM) in rocks is discussed both in terms of types and carriers of NRM. The importance of the concept of pseudo-single domain (PSD) grains as carriers of stable remanences is underscored. Recent advances in rock magnetism and paleomagnetism have helped to understand (1) continental motions which took place in the first 4 billion years of the earth's life, (2) fine details of field fluctuations both during 'normal' times as well as during a geomagnetic field reversal, and (3) indicate the magnitudes of the fields present during the formation of the moon and of the early solar system.

Banerjee, S. K.↗
NASA NTRS1995

Satellite observations of zonal electric fields near sunrise in the equatorial ionosphere

We report here on a number of examples of anomalous enhancements of eastward electric fields near sunrise in the equatorial ionospheric F-region. These examples were selected from the data base of the equatorial satellite, San Marco D (1988), which measured ionospheric electric fields during a period of solar minimum. The eastward electric fields reported correspond to vertical plasma drifts. The examples studied here are similar in signature and polarity to the pre-reversal electric field enhancements seen near sunset from ground-based radar systems. The morphology of these sunrise events, which are observed on about 14% of the morning-side satellite passes, are studied as a function of local zonal velocity, magnetic activity, geographic longitude and altitude. The nine events studied occur at locations where the zonal plasma flow is generally measured to be eastward, but reducing as a function of local time and at satellite longitudes where the magnetic declination has the opposite polarity as the declination of the sunrise terminator.

Aggson, T. L.↗
NASA NTRS1995

Simultaneous prenoon and postnoon observations of three field-aligned current systems from Viking and DMSP-F7

The spatial structure of dayside large-scale field-aligned current (FAC) systems is examined by using Viking and Defense Meteorological Satellite Program-F7 (DMSP-F7) data. We focus on four events in which the satellites simultaneously observed postnoon and prenoon three FAC systems: the region 2, the region 1, and the mantle (referred to as midday region O) systems, from equatorward to poleward. These events provide the most solid evidence to date that the midday region O system is a separate and unique FAC system, and is not an extension of the region 1 system from other local times. The events are examined comprehensively by making use of a mulit-instrumental data set, which includes magnetic field, particle flux, electric field, auroral UV image data from the satellites, and the Sondrestrom convection data. The results are summarized as follows: (1) Region 2 currents flow mostly in the central plasma sheet (CPS) precipitation region, often overlapping with the boundary plasma sheet (BPD) at their poleward edge. (2) The region 1 system is located in the core part of the auroral oval and is confined in a relatively narrow range in latitude which includes the convection reversal. The low-latitude boundary layer, possibly including the outer part of the plasma sheet, and the external cusp are the major source regions of dayside region 1 currents. (2) Midday region O currents flow on open field lines and are collocated with the shear of antisunward convection flows with velocites decreasing poleward. On the basis of these results we support the view that both prenoon and postnoon current systems consist of the three-sheet structure when the disctortion ofthe convection pattern associated with interplanetary magnetic field (IMF) B(sub Y) is small and both morningside and eveningside convection cells are crescent-shaped. We also propose that the midday region O and a part of the region 1 systems are closely coupled to the same source.

Ohtani, S.↗
DOE OSTI2026

Structuring, stochastic behavior, and charge storage capacity of redox-active microemulsions formulated with mixtures of toluene and ionic liquid as oil phase

Oil/water microemulsions (µEs) are promising electrolytes for redox flow batteries (RFBs) as they simultaneously improve charge capacity and ionic conductivity. Here, we report the successful formulation of bicontinuous µEs where the oil phase is a solution of trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide ionic liquid in toluene with redox-active ferrocene. We examined the effect of supporting electrolyte anion (NO 3 – , Cl – , ClO 4 – ) on the structure, reactivity, transport, and electrolytic performance of redox-active µEs. Neutron scattering and nuclear magnetic resonance showed that the domain size increased as Cl – > NO 3 – > ClO 4 – while the ferrocene diffusion coefficient increased as NO 3 – > Cl – > ClO 4 – . Scanning electrochemical microscopy indicated anion-dependent current fluctuations during electrolysis, with ClO 4 – exhibiting the least high-frequency oscillations, which correlate to the highest charge and discharge capacity and reversibility. All ionic liquid containing systems improved the performance of toluene-based µEs, highlighting new design principles for these electrolytes.

diffusion↗
NASA NTRS1995

Magnetic observations during the recent declining phase of solar activity

Changes in the heliospheric magnetic field during the recent declining phase in solar activity are reviewed and compared with observations during past sunspot cycles. The study is based principally on data obtained by IMP-8 and Ulysses. The field magnitude is found to have increased during the declining phase until it reached a maximum value of 11.5nT in approximately 1991.5, approximately two years after sunspot maximum. The field of the sun's south pole became negative after a reversal in early 1990. The sector structure disappeared at Ulysses in April 1993 when the latitude of the spacecraft was -30 deg revealing a low inclination of the heliospheric current sheet. A large outburst of solar activity in March 1991 caused four Coronal Mass Ejections (CMEs) and numerious shocks at the location of Ulysses. Following a delay of more than a year, a series of recurrent high speed streams and Corotating Interaction Regions commenced in July 1992 which were observed by IMP-8, Ulysses and Voyager 2. In all these respects, the behavior of the magnetic field mimics that seen in the two earlier sunspot cycles. The comprehensive data set suggests a correlation between the absolute value of B and sunspot number. The major solar cycle variations in the radial component (and magnitude) of the field have been successfully reproduced by a recent model consisting of a tilted solar dipole, whose strength and tilt undergo characteristic changes over the sunspot cycle, and the heliospheric current sheet. The large outbursts of activity in mid-1972, mid-1982 and the first quarter of 1991 may represent a characteristic last 'gasp' of solar activity before the sun evolves to a different state. The recurrent high speed streams in 1973, 1984 and 1992 accompany the developemnt of large asymetrical polar coronal holes and the growth in intensity of the polar cap fields. After they endure for about one year, the polar coronal holes recede and the high speed streams are replaced by weaker streams more characteristic of solar minimum.

Smith, E. J.↗
DOE OSTI2023

Quantum spin nematic phase in a square-lattice iridate

Spin nematic is a magnetic analogue of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid. Particularly intriguing is a valence-bond spin nematic, in which spins are quantum entangled to form a multipolar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here, in this study, we establish a spin nematic phase in the square-lattice iridate Sr 2 IrO 4 , which approximately realizes a pseudospin one-half Heisenberg antiferromagnet in the strong spin-orbit coupling limit. Upon cooling, the transition into the spin nematic phase at T C approximate to 263 K is marked by a divergence in the static spin quadrupole susceptibility extracted from our Raman spectra and concomitant emergence of a collective mode associated with the spontaneous breaking of rotational symmetries. The quadrupolar order persists in the antiferromagnetic phase below TN approximate to 230 K and becomes directly observable through its interference with the antiferromagnetic order in resonant X-ray diffraction, which allows us to uniquely determine its spatial structure. Further, we find using resonant inelastic X-ray scattering a complete breakdown of coherent magnon excitations at short-wavelength scales, suggesting a many-body quantum entanglement in the antiferromagnetic state. Taken together, our results reveal a quantum order underlying the Neel antiferromagnet that is widely believed to be intimately connected to the mechanism of high-temperature superconductivity. We establish a spin nematic phase in the square-lattice iridate Sr 2 IrO 4 and find a complete breakdown of coherent magnon excitations at short-wavelength scales, suggesting a many-body quantum entanglement in the antiferromagnetic state.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Suppression of voltage-decay in Li 2 MnO 3 cathode via reconstruction of layered-spinel coexisting phases

Voltage decay, i.e., the voltage decrease during electrochemical cycling, has been a decade-long challenge for lithium-ion batteries. This issue not only leads to a substantial loss of energy density, but also raises challenges for the battery management system, hindering the commercial application of high capacity lithium-rich oxide. Here, we show that through a combination of electrochemical conditioning and thermal treatment, Li 2 MnO 3 , the parent compound of lithium-rich oxide, which typically displays severe voltage and capacity decay, could be converted into a new phase that essentially suppresses the voltage decay with improved capacity retention and rate performance. Furthermore, by combining atomic-sensitive nuclear magnetic resonance, differential electrochemical mass spectrometry and synchrotron-based resonant inelastic X-ray scattering, we disclose that treatment triggers the formation of three-coexisting phases, i.e., the lithium-rich layered, spinel and defect spinel phases, which enables improved reversibility of the oxygen redox activity and enhanced manganese redox reactions in the initial cycle. Our findings suggest the key role of the local structure in the voltage decay problem and provide insights for material optimizations towards lithium- and manganese-rich cathodes without the voltage decay.

25 ENERGY STORAGE↗
DOE OSTI2021

Direct measurements of the 3D plasma velocity in single-helical-axis RFP plasmas

The first local velocity measurements of helical equilibrium plasmas in the Reversed Field Pinch (RFP) Single Helical Axis (SHAx) state using a Charge Exchange Recombination Spectroscopy (CHERS) diagnostic are presented. Measurements show strong axisymmetric and non-axisymmetric flow, with n ¼ 5 components of flow related to the ðm; nÞ¼ð1; 5Þ dominant magnetic mode on the order of the axisymmetric flow in certain regions of the plasma, as well as significant n > 5 flow. Flow measurements are compared with NIMROD simulations of visco-resistive, single-fluid MHD in toroidal and cylindrical geometries with limited axial periodicity. Both measurements and the simulation with toroidal geometry show stronger inboard flows relative to the outboard flows, which is attributed to the toroidal geometry of the device. In the experiment, the n ¼ 5 component of flow is phase shifted from the reconnection-like flow pattern observed in the single-fluid simulations, possibly due to decoupling of the ion and electron fluids over much of the plasma. Finally, the strength of the helical angular flow shear relative to the critical shear necessary to disrupt nonlinear coupling between tearing modes is calculated around the helical magnetic axis. The shear in the measured flow is on the order of the theoretical critical threshold needed to nonlinearly decouple modes, but the measurement uncertainty in the gradient of the flow is large.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1975

In situ measurements of plasma drift velocity and enhanced NO/+/ in the auroral electrojet by the Bennett spectrometer on AE-C

Simultaneous measurements of ion composition and plasma drift velocity by the Bennett mass spectrometer on the Atmosphere Explorer-C satellite reveal a direct correlation between enhancements in NO(+) concentration and ion drift velocity in the southern auroral oval. Low altitude (137 to 250 km) data obtained between 1700 and 2400 hr magnetic local time on October 22, 1974, reveal a region of westward plasma flow at velocities up to 1.3 km/s between 62 and 68 deg invariant latitude, with corresponding NO(+) enhancements of up to a factor of 20. A narrow region of reverse flow at about 0.9 km/s was also measured. These drift observations are consistent with convective flow patterns derived from electric field measurements, and their correlation with NO(+) appears to support the suggestion that NO(+) enhancements would be expected in regions of drift owing to the dependence on ion energy of the reaction O(+) + N2 yields NO(+) + N.

Brinton, H. C.↗
DOE OSTI2025

Measurements of the polarization of several instabilities in the DIII-D tokamak

Recently, a method to infer the polarization of modes with frequencies much less than the ion cyclotron frequency was published [X.D. Du et al., Phys. Rev. Lett. 132 (2024) 215101]. The method uses measurements of electron temperature and density fluctuations δTₑ and δnₑ at the same spatial position to infer the local ratio of “acoustic polarization,” |δϕ ∥ |/(|δϕ ∥ |+|δψ|), where δϕ ∥ is the effective parallel potential and δψ is related to the parallel magnetic vector potential A ∥ . This paper summarizes key formulas, with emphasis on their range of validity, and elaborates on the workflow required to infer the acoustic polarization from experimental data. The drift-acoustic polarization of ellipticity-induced, toroidicity-induced, and reversed shear Alfvén eigenmodes is nearly zero, as expected for modes with predominately shear-Alfvénic polarization. The polarization of beta-induced Alfvén eigenmodes contains an acoustic component that increases with poloidal wave number. “Low frequency modes,” (instabilities that appear transiently when the minimum of the safety factor qₘᵢₙ passes through rational values) have large and highly variable acoustic polarization. In both experiment and simulation, fishbones have non-zero acoustic polarization that increases as the mode chirps down in frequency.

Alfven eigenmode↗
NASA NTRS1975

Field-aligned currents observed by the OGO 5 and Triad satellites

The existence of field-aligned currents in the polar cap boundary layer as a permanent feature of the magnetosphere is investigated. Magnetic field observations from Triad at 800 km altitude and from OGO 5 in the high-altitude magnetosphere are examined. Results indicate that in the morning half of the boundary layer, currents flow into the ionosphere, and that the current direction is reversed in the afternoon half of the layer. The Triad data further indicate that the net current is a maximum near 1500 MLT and that there may be a secondary maximum during early morning hours. According to the Isis 2 electron observations, the locations of these maximums of field-aligned net current roughly match those of two maximums in the isointensity contours for 150 ev electrons. It is proposed that the polar cap boundary current is driven by a current generator in the magnetotail, or ultimately in the solar wind. It is suggested that the large scale field-aligned currents in the polar cap boundary layer are associated with the dominance of protons on the morning side and of electrons on the afternoon side near the poleward edge of the precipitation zone along the auroral oval.

Sugiura, M.↗