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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

OBSERVATIONS OF MAGNETIC FLUX-ROPE OSCILLATION DURING THE PRECURSOR PHASE OF A SOLAR ERUPTION

Based on combined observations from the Interface Region Imaging Spectrograph (IRIS) spectrometer with the coronal emission line of Fe xxi at 1354.08 Å and SDO /AIA images in multiple passbands, we report the finding of the precursor activity manifested as the transverse oscillation of a sigmoid, which is likely a pre-existing magnetic flux rope (MFR), that led to the onset of an X class flare and a fast halo coronal mass ejection (CME) on 2014 September 10. The IRIS slit is situated at a fixed position that is almost vertical to the main axis of the sigmoid structure that has a length of about 1.8 × 10{sup 5} km. This precursor oscillation lasts for about 13 minutes in the MFR and has velocities in the range of [−9, 11] km s{sup −1} and a period of ∼280 s. Our analysis, which is based on the temperature, density, length, and magnetic field strength of the observed sigmoid, indicates that the nature of the oscillation is a standing wave of fast magnetoacoustic kink mode. We further find that the precursor oscillation is excited by the energy released through an external magnetic reconnection between the unstable MFR and the ambient magnetic field. It is proposed that this precursor activity leads to the dynamic formation of a current sheet underneath the MFR that subsequently reconnects to trigger the onset of the main phase of the flare and the CME.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Observation of half-quantum flux in the unconventional superconductor β-Bi2Pd

Magnetic flux quantization is one of the defining properties of a superconductor. We report the observation of half-integer magnetic flux quantization in mesoscopic rings of superconducting β-Bi 2 Pd thin films. The half-quantum fluxoid manifests itself as a π phase shift in the quantum oscillation of the superconducting critical temperature. This result verifies unconventional superconductivity of β-Bi 2 Pd and is consistent with a spin-triplet pairing symmetry. Our findings may have implications for flux quantum bits in the context of quantum computing.

phonons, thermal conductivity, thermoelectric, spi↗
DOE OSTI2016

ENSEMBLE EMPIRICAL MODE DECOMPOSITION OF THE MAGNETIC FIELD OF THE SUN AS A STAR

The ensemble empirical mode decomposition (EEMD) analysis is utilized to extract the intrinsic mode functions (IMFs) of the solar mean magnetic field (SMMF) observed at the Wilcox Solar Observatory of Stanford University from 1975 to 2014, and then we analyze the periods of these IMFs as well as the relation of IMFs (SMMF) with some solar activity indices. The two special rotation cycles of 26.6 and 28.5 days should be derived from different magnetic flux elements in the SMMF. The rotation cycle of the weak magnetic flux element in the SMMF is 26.6 days, while the rotation cycle of the strong magnetic flux element in the SMMF is 28.5 days. The two rotation periods of the structure of the interplanetary magnetic field near the ecliptic plane are essentially related to weak and strong magnetic flux elements in the SMMF, respectively. The rotation cycle of weak magnetic flux in the SMMF did not vary over the last 40 years because the weak magnetic flux element derived from the weak magnetic activity on the full disk is not influenced by latitudinal migration. Neither the internal rotation of the Sun nor the solar magnetic activity on the disk (including the solar polar fields) causes the annual variation of SMMF. The variation of SMMF at timescales of a solar cycle is more related to weak magnetic activity on the full solar disk.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1991

Magnetic reconnection in solar flares

The magnetic energy stored in the corona is the only plausible source for the energy released during large solar flares. During the last 20 years most theoretical work has concentrated on models which store magnetic energy in the corona in the form of electrical currents, and a major goal of present day research is to understand how these currents are created, and then later dissipated during a flare. Another important goal is to find a flare model which can eject magnetic flux into interplanetary space. Although many flares do not eject magnetic flux, those which do are of special importance for solar-terrestrial relations since the ejected flux can have dramatic effects if it hits the Earth's magnetosphere. Three flare models which have been extensively investigated are the emerging-flux model, the sheared-arcade model, and the magnetic-flux-rope model. All of these models can store and release magnetic energy efficiently provided that rapid magnetic reconnection occurs. However, only the magnetic-flux-rope model appears to provide a plausible mechanism for ejecting magnetic flux into interplanetary space.

Forbes, T. G.↗
NASA NTRS2012

An Exploration of the Emission Properties of X-Ray Bright Points Seen with SDO

We present preliminary results of a study of X-ray Bright Point (XBP) EUV emission and its dependence on other properties. The XBPs were located using a new, automated XBP finder for AlA developed as part of the Feature Finding Team for SDO Computer Vision. We analyze XBPs near disk center, comparing AlA EUV fluxes, HMI LOS magnetic fields, and photospheric flow fields (derived from HMI data) to look for relationships between XBP emission, magnetic flux, velocity fields, and XBP local environment. We find some evidence for differences in the mean XBP temperature with environment. Unsigned magnetic flux is correlated with XBP emission, though other parameters play a role. The majority of XBP footpoints are approaching each other, though at a slight angle from head-on on average. We discuss the results in the context of XBP heating.

Saar, S. H.↗
DOE OSTI2021

Regulation of the normalized rate of driven magnetic reconnection through shocked flux pileup

Magnetic reconnection is explored on the Terrestrial Reconnection Experiment (TREX) for asymmetric inflow conditions and in a configuration where the absolute rate of reconnection is set by an external drive. Magnetic pileup enhances the upstream magnetic field of the high-density inflow, leading to an increased upstream Alfvén speed and helping to lower the normalized reconnection rate to values expected from theoretical consideration. In addition, a shock interface between the far upstream supersonic plasma inflow and the region of magnetic flux pileup is observed, important to the overall force balance of the system, thereby demonstrating the role of shock formation for configurations including a supersonically driven inflow. Despite the specialized geometry where a strong reconnection drive is applied from only one side of the reconnection layer, previous numerical and theoretical results remain robust and are shown to accurately predict the normalized rate of reconnection for the range of system sizes considered. This experimental rate of reconnection is dependent on system size, reaching values as high as 0.8 at the smallest normalized system size applied.

Physics↗
NASA NTRS1987

Comparative plasma tails of Venus and comets

A review of current models of solar wind flow in the plasma tails of weakly magnetized bodies is presented. Measurements conducted by the Mariner 5 spacecraft, the Veneras 9 and 10, and the PVO orbiters, in the Venus tail, and with the ICE spacecraft in the tail of Comet Giacobini-Zinner, reveal common plasma properties which suggest that similar physical processes are operative. Most notable is the observation of decreased flow velocities and enhanced plasma temperatures in the vicinity of their plasma tails. In Venus, the measured velocity and temperature fields are consistent with the effects of frictional forces between the mass-loaded ionosheath flow and the ionosphere along the (magnetic) polar regions of the ionopause. It is argued that similar conditions exist at a cometary ionopause and that the distribution of magnetic fluxes in a cometary tail is controlled by the entry of plasma fluxes from the (magnetic) polar regions of the comet's ionospheric obstacle. This question is further addressed in connection with the two-step shape of the magnetic profile measured across the tail of the comet. It is suggested that the low intensity outer increases of the magnetic lobes are associated with the draping of the interplanetary magnetic field lines around the comet's ionospheric obstacle, and that the higher intensity increases seen in the inner regions of the magnetic lobes are due to an additional compression of magnetic fluxes produced by the entry of plasma particles into the tail.

Perezdetejada, H.↗
DOE OSTI2015

Critical decay index at the onset of solar eruptions

Magnetic flux ropes are topological structures consisting of twisted magnetic field lines that globally wrap around an axis. The torus instability model predicts that a magnetic flux rope of major radius R undergoes an eruption when its axis reaches a location where the decay index −d(lnB{sub ex})/d(lnR) of the ambient magnetic field B{sub ex} is larger than a critical value. In the current-wire model, the critical value depends on the thickness and time evolution of the current channel. We use magnetohydrodynamic simulations to investigate whether the critical value of the decay index at the onset of the eruption is affected by the magnetic flux rope’s internal current profile and/or by the particular pre-eruptive photospheric dynamics. The evolution of an asymmetric, bipolar active region is driven by applying different classes of photospheric motions. We find that the critical value of the decay index at the onset of the eruption is not significantly affected by either the pre-erupitve photospheric evolution of the active region or the resulting different magnetic flux ropes. As in the case of the current-wire model, we find that there is a “critical range” [1.3–1.5], rather than a “critical value” for the onset of the torus instability. This range is in good agreement with the predictions of the current-wire model, despite the inclusion of line-tying effects and the occurrence of tether-cutting magnetic reconnection.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

Extension of the Parker bound on the flux of magnetic monopoles

The Parker bound on the flux of magnetic monopoles is extended and strengthened. A new limit on the survival and growth of a small galactic seed field is obtained which rules out the possibility that monopoles much lighter than 10 exp 17 GeV/sq c can provide the closure density of the universe.

Adams, Fred C.↗
DOE OSTI2023

AMR Magnetic Sensors in Flux Expulsion Studies

Magnetic flux expulsion properties of the superconducting material such as bulk niobium, widely used for the radio-frequency cavity fabrication, substantially affect the performance characteristics of the cavities. The quality factor of the SRF resonators can be significantly compromised due to the presence of the trapped flux vortices causing additional RF energy losses in the material. Large number of experiments have been carried out by different research groups to establish the correlation of the flux trapping in niobium cavities with the presence of impurities in niobium as well as various surface treatment methods. Majority of these experiments utilize commercially available cryogenic fluxgate magnetic sensors to measure the field before and after the niobium transition to the superconducting state to quantify the amount of flux trapped. One disadvantage of the typically used fluxgates is the size of the sensing volume. As an example, the Barting-ton F and G type cryogenic fluxgates have a sensing core length of about 30mm, which is comparable to the curvature radius of the cavity walls and hence the magnetic field lines curvature radius after the expulsion. Thus, the measured field value needs to be corrected to account for the sensor effective averaging over the sensing volume. In case of the sharper geometries, for instance if the flux expulsion to be measured on the edge of the rectangular niobium flat sheet with a thickness of ~5mm, the use of the fluxgate would be impractical.

Netepenko, A.↗
DOE OSTI2017

Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. III. Twist Number Method

We study the writhe, twist, and magnetic helicity of different magnetic flux ropes, based on models of the solar coronal magnetic field structure. These include an analytical force-free Titov–Démoulin equilibrium solution, non-force-free magnetohydrodynamic simulations, and nonlinear force-free magnetic field models. The geometrical boundary of the magnetic flux rope is determined by the quasi-separatrix layer and the bottom surface, and the axis curve of the flux rope is determined by its overall orientation. The twist is computed by the Berger–Prior formula, which is suitable for arbitrary geometry and both force-free and non-force-free models. The magnetic helicity is estimated by the twist multiplied by the square of the axial magnetic flux. We compare the obtained values with those derived by a finite volume helicity estimation method. We find that the magnetic helicity obtained with the twist method agrees with the helicity carried by the purely current-carrying part of the field within uncertainties for most test cases. It is also found that the current-carrying part of the model field is relatively significant at the very location of the magnetic flux rope. This qualitatively explains the agreement between the magnetic helicity computed by the twist method and the helicity contributed purely by the current-carrying magnetic field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Inferring the Heliospheric Magnetic Field Back through Maunder Minimum

Recent solar conditions include a prolonged solar minimum (2005–2009) and a solar maximum that has not fully recovered in terms of the Heliospheric Magnetic Field (HMF) strength when compared to the previous maximum values. These anomalies may indicate that we are entering an era of lower solar activity than observed at other times during the space age. We study past solar grand minima, especially the Maunder period (1645–1715) to gain further insight into grand minima. We find the timescale parameters associated with three processes attributed to the magnetic flux balance in the heliosphere using chi-square analysis. We use HMF time series reconstructed based on geomagnetic data and near-Earth spacecraft measurements (OMNI) data to find the fundamental timescales that influence heliospheric field evolution through conversion or opening of magnetic flux from coronal mass ejections (CMEs) into the ambient heliospheric field, removal or loss of the ambient heliospheric field through magnetic reconnection, and interchange reconnection between CME magnetic flux and ambient heliospheric magnetic flux. We also investigate the existence of a floor in the heliospheric magnetic flux, in the absence of CMEs, and show that a floor ⩽1.49nT is sufficient to successfully describe the HMF evolution. The minimum value for the HMF at 1 au in the model-predicted historic record is 3.13 ± 0.35 nT. Our model results favorably reproduce paleocosmic data and near-Earth spacecraft measurements data and show how the HMF may evolve through periods of extremely low activity.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2007

The Cosmic Battery Revisited

We reinvestigate the generation and accumulation of magnetic flux in optically thin accretion flows around active gravitating objects. The source of the magnetic field is the azimuthal electric current associated with the Poynting-Robertson drag on the electrons of the accreting plasma. This current generates magnetic field loops which open up because of the differential rotation of the flow. We show through simple numerical simulations that what regulates the generation and accumulation of magnetic flux near the center is the value of the plasma conductivity. Although the conductivity is usually considered to be effectively infinite for the fully ionized plasmas expected near the inner edge of accretion disks, the turbulence of those plasmas may actually render them much less conducting due to the presence of anomalous resistivity. We have discovered that if the resistivity is sufficiently high throughout the turbulent disk while it is suppressed interior to its inner edge, an interesting steady-state process is established: accretion carries and accumulates magnetic flux of one polarity inside the inner edge of the disk, whereas magnetic diffusion releases magnetic flux of the opposite polarity to large distances. In this scenario, magnetic flux of one polarity grows and accumulates at a steady rate in the region inside the inner edge and up to the point of equipartition when it becomes dynamically important. We argue that this inward growth and outward expulsion of oppositely-directed magnetic fields that we propose may account for the approx. 30 min cyclic variability observed in the galactic microquasar GRS1915+105.

Contopoulos, Ioannis↗
NASA NTRS1990

The structure of photospheric flux tubes

Basic physical mechanisms for producing the observed intense magnetic flux tubes in the solar photosphere are reviewed. The mechanism of flux expulsion by convective cells can concentrate magnetic flux up to the equipartition field strength, which is only about 200 G at the solar surface for the observed granular convection. Other mechanisms that partially evacuate the flux tube are needed to produce further concentration of magnetic flux to the observed values of 1000-1500 G. Two such mechanisms are discussed: concentration by convective collapse of a vertical flux tube in the superadiabatic layer just below the solar surface, and concentration by a siphon flow in an arched, isolated flux tube.

Thomas, John H.↗
DOE OSTI2016

GENESIS OF INTERPLANETARY INTERMITTENT TURBULENCE: A CASE STUDY OF ROPE–ROPE MAGNETIC RECONNECTION

In a recent paper, the relation between current sheet, magnetic reconnection, and turbulence at the leading edge of an interplanetary coronal mass ejection was studied. We report here the observation of magnetic reconnection at the interface region of two interplanetary magnetic flux ropes. The front and rear boundary layers of three interplanetary magnetic flux ropes are identified, and the structures of magnetic flux ropes are reconstructed by the Grad–Shafranov method. A quantitative analysis of the reconnection condition and the degree of intermittency reveals that rope–rope magnetic reconnection is the most likely site for genesis of interplanetary intermittency turbulence in this event. The dynamic pressure pulse resulting from this reconnection triggers the onset of a geomagnetic storm.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

The CMS Magnetic Field Measuring and Monitoring Systems

This review article describes the performance of the magnetic field measuring and monitoring systems for the Compact Muon Solenoid (CMS) detector. To cross-check the magnetic flux distribution obtained with the CMS magnet model, four systems for measuring the magnetic flux density in the detector volume were used. The magnetic induction inside the 6 m diameter superconducting solenoid was measured and is currently monitored by four nuclear magnetic resonance (NMR) probes installed using special tubes outside the barrel hadron calorimeter. Two more NRM probes were installed at the faces of the tracking system. The field inside the superconducting solenoid was precisely measured in 2006 in a cylindrical volume of 3.448 m in diameter and 7 m in length using 10 three-dimensional (3D) B-sensors based on the Hall effect. These B-sensors were installed on each of the two propeller arms of an automated field-mapping machine. In addition to these measurement systems, a system for monitoring the magnetic field during the CMS detector operation has been developed. Inside the solenoid in the horizontal plane, 4 3D B-sensors were installed at the faces of the tracking detector. Twelve 3D B-sensors were installed on the surfaces of the flux-return yoke nose disks. Seventy 3D B-sensors were installed in the air gaps of the CMS magnet yoke. A specially developed flux loop technique was used for the most complex measurements of the magnetic flux density inside the steel blocks of the CMS magnet yoke. The flux loops are installed in 22 sections of the flux-return yoke blocks. The areas enclosed by these coils varied from 0.3 to 1.59 m$^{2}$ in the blocks of the barrel wheels and from 0.5 to 1.12 m$^{2}$ in the blocks of the yoke endcap disks. The development of these systems and the results of the magnetic flux density measurements across the CMS magnet are presented and discussed in this review article.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1995

Geometry of interplanetary magnetic clouds

Two dimensional magnetohydrodynamic simulations are presented of the distortion of a magnetic flux rope that is being accelerated through ambient solar wind plasma. The flux rope magnetic field has an axial component parallel to the solar wind field and an azimuthal component, which lies in the simulation plane. As the flux rope moves through the solar wind plasma, vortices form on its trailing edge and couple strongly to its interior. If the flux rope azimuthal field is weak, it deforms into an elongated banana-like shape a few Alfven transit times. A strong azimuthal field component tends to inhibit this distortion. If the flux rope is taken to model a magnetic cloud, it is suggested that the shape of the cloud at 1 AU is determined by its distortion in the inner solar wind. Distortion timescales beyond 1 AU are estimated as many days. It is estimated that effective drag coefficients somewhat greater than unity are appropriate for modelling flux rope propagation.

Cargill, P. J.↗