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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 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Radial evolution of spectral characteristics of magnetic field fluctuations at proton scales

This paper addresses the investigation of the character and the radial evolution of magnetic fluctuations within the dissipation range, right after the high-frequency spectral break, employing observations by Messenger and Wind of the same fast wind stream during a radial alignment. The same event has already been considered in literature to show, for the first time, that the high-frequency break separating the fluid from the kinetic regime moves to lower frequency as the wind expands. The present work aims to analyze the nature of the high-frequency magnetic fluctuations beyond the spectral break and show that their character is compatible with left-hand, outward-propagating, ion cyclotron waves and right-hand kinetic Alfvén waves. It is also shown that the low-frequency limit of these fluctuations follows the radial evolution of the spectral break, which also reflects in the behavior of their intermittency character. Finally, the total power and the compressive character of these two wave populations are analyzed and compared as a function of the heliocentric distance, leading us to conclude that the overall picture is in favor of a radial decrease.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Implications of turbulence for jets in core-collapse supernova explosions

We show that turbulence in core collapse supernovae (CCSNe) which has been shown recently to ease shock revival might also lead to the formation of intermittent thick accretion disks, or accretion belts, around the newly born neutron star (NS). The accretion morphology is such that two low density funnels are formed along the polar directions. The disks then are likely to launch jets with a varying axis direction, i.e., jittering jets, through the two opposite funnels. The energy contribution of jets in this jittering-jets mechanism might result in an explosion energy of E{sub exp} ≳ 10{sup 51} erg, even without reviving the stalled shock. We strengthen the jittering-jets mechanism as a possible explosion mechanism of CCSNe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A steady-state picture of solar wind acceleration and charge state composition derived from a global wave-driven MHD model

The higher charge states found in slow (<400 km s{sup −1}) solar wind streams compared to fast streams have supported the hypothesis that the slow wind originates in closed coronal loops and is released intermittently through reconnection. Here we examine whether a highly ionized slow wind can also form along steady and open magnetic field lines. We model the steady-state solar atmosphere using the Alfvén Wave Solar Model (AWSoM), a global MHD model driven by Alfvén waves, and apply an ionization code to calculate the charge state evolution along modeled open field lines. This constitutes the first charge state calculation covering all latitudes in a realistic magnetic field. The ratios O{sup +7}/O{sup +6} and C{sup +6}/C{sup +5} are compared to in situ Ulysses observations and are found to be higher in the slow wind, as observed; however, they are underpredicted in both wind types. The modeled ion fractions of S, Si, and Fe are used to calculate line-of-sight intensities, which are compared to Extreme-ultraviolet Imaging Spectrometer (EIS) observations above a coronal hole. The agreement is partial and suggests that all ionization rates are underpredicted. Assuming the presence of suprathermal electrons improved the agreement with both EIS and Ulysses observations; importantly, the trend of higher ionization in the slow wind was maintained. The results suggest that there can be a sub-class of slow wind that is steady and highly ionized. Further analysis shows that it originates from coronal hole boundaries (CHBs), where the modeled electron density and temperature are higher than inside the hole, leading to faster ionization. This property of CHBs is global and observationally supported by EUV tomography.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Accretion-powered pulsations in an apparently quiescent neutron star binary

Accreting millisecond X-ray pulsars (AMXPs) are an important subset of low-mass X-ray binaries (LMXBs) in which coherent X-ray pulsations can be observed during occasional, bright outbursts (X-ray luminosity L{sub X}∼10{sup 36} erg s{sup −1}). These pulsations show that matter is being channeled onto the neutron star’s magnetic poles. However, such sources spend most of their time in a low-luminosity, quiescent state (L{sub X}≲10{sup 34} erg s{sup −1}), where the nature of the accretion flow onto the neutron star (if any) is not well understood. Here we report that the millisecond pulsar/LMXB transition object PSR J1023+0038 intermittently shows coherent X-ray pulsations at luminosities nearly 100 times fainter than observed in any other AMXP. We conclude that in spite of its low luminosity, PSR J1023+0038 experiences episodes of channeled accretion, a discovery that challenges existing models for accretion onto magnetized neutron stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

DIRECT OBSERVATIONS OF MAGNETIC RECONNECTION OUTFLOW AND CME TRIGGERING IN A SMALL ERUPTING SOLAR PROMINENCE

We examine a small prominence eruption that occurred on 2014 May 1 at 01:35 UT and was observed by the Interface Region Imaging Spectrometer (IRIS) and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory. Pre- and post-eruption images were taken by the X-ray Telescope (XRT) on Hinode. Pre-eruption, a dome-like structure exists above the prominence, as demarcated by coronal rain. As the eruption progresses, we find evidence for reconnection between the prominence magnetic field and the overlying field. Fast flows are seen in AIA and IRIS, indicating reconnection outflows. Plane-of-sky flows of 300 km s{sup −1} are observed in the AIA 171 A channel along a potentially reconnected field line. IRIS detects intermittent fast line of sight flows of 200 km s{sup −1} coincident with the AIA flows. Differential emission measure calculations show heating at the origin of the fast flows. Post-eruption XRT images show hot loops probably due to reconfiguration of magnetic fields during the eruption and subsequent heating of plasma in these loops. Although there is evidence for reconnection above the prominence during the eruption, high spatial resolution images from IRIS reveal potential reconnection sites below the prominence. A height–time analysis of the erupting prominence shows a slow initial rise with a velocity of 0.4 km s{sup −1} followed by a rapid acceleration with a final velocity of 250 km s{sup −1}. Brightenings in IRIS during the transition between these two phases indicate the eruption trigger for the fast part of the eruption is likely a tether-cutting mechanism rather than a break-out mechanism.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ENERGY DISSIPATION IN MAGNETIC NULL POINTS AT KINETIC SCALES

We use kinetic particle-in-cell and MHD simulations supported by an observational data set to investigate magnetic reconnection in clusters of null points in space plasma. The magnetic configuration under investigation is driven by fast adiabatic flux rope compression that dissipates almost half of the initial magnetic field energy. In this phase powerful currents are excited producing secondary instabilities, and the system is brought into a state of “intermittent turbulence” within a few ion gyro-periods. Reconnection events are distributed all over the simulation domain and energy dissipation is rather volume-filling. Numerous spiral null points interconnected via their spines form null lines embedded into magnetic flux ropes; null point pairs demonstrate the signatures of torsional spine reconnection. However, energy dissipation mainly happens in the shear layers formed by adjacent flux ropes with oppositely directed currents. In these regions radial null pairs are spontaneously emerging and vanishing, associated with electron streams and small-scale current sheets. The number of spiral nulls in the simulation outweighs the number of radial nulls by a factor of 5–10, in accordance with Cluster observations in the Earth's magnetosheath. Twisted magnetic fields with embedded spiral null points might indicate the regions of major energy dissipation for future space missions such as the Magnetospheric Multiscale Mission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE ROTATION PERIOD AND MAGNETIC FIELD OF THE T DWARF 2MASSI J1047539+212423 MEASURED FROM PERIODIC RADIO BURSTS

Periodic radio bursts from very low mass stars and brown dwarfs simultaneously probe their magnetic and rotational properties. The brown dwarf 2MASSI J1047539+212423 (2M 1047+21) is currently the only T dwarf (T6.5) detected at radio wavelengths. Previous observations of this source with the Arecibo observatory revealed intermittent, 100%-polarized radio pulses similar to those detected from other brown dwarfs, but were unable to constrain a pulse periodicity; previous Very Large Array (VLA) observations detected quiescent emission a factor of ∼100 times fainter than the Arecibo pulses but no additional events. Here we present 14 hr of VLA observations of this object that reveal a series of pulses at ∼6 GHz with highly variable profiles, showing that the pulsing behavior evolves on time scales that are both long and short compared to the rotation period. We measure a periodicity of ∼1.77 hr and identify it with the rotation period. This is just the sixth rotation period measurement in a late T dwarf, and the first obtained in the radio. We detect a pulse at 10 GHz as well, suggesting that the magnetic field strength of 2 M 1047+21 reaches at least 3.6 kG. Although this object is the coolest and most rapidly rotating radio-detected brown dwarf to date, its properties appear continuous with those of other such objects, suggesting that the generation of strong magnetic fields and radio emission may continue to even cooler objects. Further studies of this kind will help to clarify the relationships between mass, age, rotation, and magnetic activity at and beyond the end of the main sequence, where both theories and observational data are currently scarce.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

GRAND MINIMA AND EQUATORWARD PROPAGATION IN A CYCLING STELLAR CONVECTIVE DYNAMO

The 3D MHD Anelastic Spherical Harmonic code, using slope-limited diffusion, is employed to capture convective and dynamo processes achieved in a global-scale stellar convection simulation for a model solar-mass star rotating at three times the solar rate. The dynamo-generated magnetic fields possesses many timescales, with a prominent polarity cycle occurring roughly every 6.2 years. The magnetic field forms large-scale toroidal wreaths, whose formation is tied to the low Rossby number of the convection in this simulation. The polarity reversals are linked to the weakened differential rotation and a resistive collapse of the large-scale magnetic field. An equatorial migration of the magnetic field is seen, which is due to the strong modulation of the differential rotation rather than a dynamo wave. A poleward migration of magnetic flux from the equator eventually leads to the reversal of the polarity of the high-latitude magnetic field. This simulation also enters an interval with reduced magnetic energy at low latitudes lasting roughly 16 years (about 2.5 polarity cycles), during which the polarity cycles are disrupted and after which the dynamo recovers its regular polarity cycles. An analysis of this grand minimum reveals that it likely arises through the interplay of symmetric and antisymmetric dynamo families. This intermittent dynamo state potentially results from the simulation’s relatively low magnetic Prandtl number. A mean-field-based analysis of this dynamo simulation demonstrates that it is of the α-Ω type. The timescales that appear to be relevant to the magnetic polarity reversal are also identified.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

TEMPORAL ANALYSIS OF DISSIPATIVE STRUCTURES IN MAGNETOHYDRODYNAMIC TURBULENCE

Energy dissipation is highly intermittent in turbulent plasmas, being localized in coherent structures such as current sheets. The statistical analysis of spatial dissipative structures is an effective approach to studying turbulence. In this paper, we generalize this methodology to investigate four-dimensional spatiotemporal structures, i.e., dissipative processes representing sets of interacting coherent structures, which correspond to flares in astrophysical systems. We develop methods for identifying and characterizing these processes, and then perform a statistical analysis of dissipative processes in numerical simulations of driven magnetohydrodynamic turbulence. We find that processes are often highly complex, long-lived, and weakly asymmetric in time. They exhibit robust power-law probability distributions and scaling relations, including a distribution of dissipated energy with power-law index near −1.75, indicating that intense dissipative events dominate the overall energy dissipation. We compare our results with the previously observed statistical properties of solar flares.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

NUMERICAL SIMULATIONS OF CORONAL HEATING THROUGH FOOTPOINT BRAIDING

Advanced three-dimensional (3D) radiative MHD simulations now reproduce many properties of the outer solar atmosphere. When including a domain from the convection zone into the corona, a hot chromosphere and corona are self-consistently maintained. Here we study two realistic models, with different simulated areas, magnetic field strength and topology, and numerical resolution. These are compared in order to characterize the heating in the 3D-MHD simulations which self-consistently maintains the structure of the atmosphere. We analyze the heating at both large and small scales and find that heating is episodic and highly structured in space, but occurs along loop-shaped structures, and moves along with the magnetic field. On large scales we find that the heating per particle is maximal near the transition region and that widely distributed opposite-polarity field in the photosphere leads to a greater heating scale height in the corona. On smaller scales, heating is concentrated in current sheets, the thicknesses of which are set by the numerical resolution. Some current sheets fragment in time, this process occurring more readily in the higher-resolution model leading to spatially highly intermittent heating. The large-scale heating structures are found to fade in less than about five minutes, while the smaller, local, heating shows timescales of the order of two minutes in one model and one minutes in the other, higher-resolution, model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

LIGHT BRIDGE IN A DEVELOPING ACTIVE REGION. I. OBSERVATION OF LIGHT BRIDGE AND ITS DYNAMIC ACTIVITY PHENOMENA

Light bridges, the bright structures that divide the umbra of sunspots and pores into smaller pieces, are known to produce a wide variety of activity events in solar active regions (ARs). It is also known that the light bridges appear in the assembling process of nascent sunspots. The ultimate goal of this series of papers is to reveal the nature of light bridges in developing ARs and the occurrence of activity events associated with the light bridge structures from both observational and numerical approaches. In this first paper, exploiting the observational data obtained by Hinode, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory, we investigate the detailed structure of the light bridge in NOAA AR 11974 and its dynamic activity phenomena. As a result, we find that the light bridge has a weak, horizontal magnetic field, which is transported from the interior by a large-scale convective upflow and is surrounded by strong, vertical fields of adjacent pores. In the chromosphere above the bridge, a transient brightening occurs repeatedly and intermittently, followed by a recurrent dark surge ejection into higher altitudes. Our analysis indicates that the brightening is the plasma heating due to magnetic reconnection at lower altitudes, while the dark surge is the cool, dense plasma ejected from the reconnection region. From the observational results, we conclude that the dynamic activity observed in a light bridge structure such as chromospheric brightenings and dark surge ejections are driven by magnetoconvective evolution within the light bridge and its interaction with the surrounding magnetic fields.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SUN-LIKE MAGNETIC CYCLES IN THE RAPIDLY ROTATING YOUNG SOLAR ANALOG HD 30495

A growing body of evidence suggests that multiple dynamo mechanisms can drive magnetic variability on different timescales, not only in the Sun but also in other stars. Many solar activity proxies exhibit a quasi-biennial (∼2 year) variation, which is superimposed upon the dominant 11 year cycle. A well-characterized stellar sample suggests at least two different relationships between rotation period and cycle period, with some stars exhibiting long and short cycles simultaneously. Within this sample, the solar cycle periods are typical of a more rapidly rotating star, implying that the Sun might be in a transitional state or that it has an unusual evolutionary history. In this work, we present new and archival observations of dual magnetic cycles in the young solar analog HD 30495, a ∼1 Gyr old G1.5 V star with a rotation period near 11 days. This star falls squarely on the relationships established by the broader stellar sample, with short-period variations at ∼1.7 years and a long cycle of ∼12 years. We measure three individual long-period cycles and find durations ranging from 9.6 to 15.5 years. We find the short-term variability to be intermittent, but present throughout the majority of the time series, though its occurrence and amplitude are uncorrelated with the longer cycle. These essentially solar-like variations occur in a Sun-like star with more rapid rotation, though surface differential rotation measurements leave open the possibility of a solar equivalence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

New modeling of galactic proton modulation during the minimum of solar cycle 23/24

During the recent prolonged solar minimum of cycle 23/24, the PAMELA detector measured 27-day averaged Galactic proton energy spectra over the energy range that is important for solar modulation. By comparing these spectra to computed spectra from a three-dimensional model that contains all of the important heliospheric modulation processes, the recent minimum can be studied in detail from a modulation perspective. This was done by setting up a realistic heliosphere in the model, and reproducing a representative selection of seven intermittent PAMELA spectra, separated by approximately six months, from 2006 July to 2009 December. Additionally, a new very local interstellar proton spectrum was constructed using measurements below 600 MeV from Voyager 1, taken beyond the heliopause, combined with PAMELA and AMS-02 measurements above 30 GeV at the Earth. As a result of the extreme minimum modulation conditions that governed the recent solar minimum, the highest ever Galactic cosmic ray spectrum at Earth was observed by PAMELA at the end of 2009. It was found that, apart from the self-consistent changes in the heliospheric current sheet and the heliospheric magnetic field over time, additional increases in the mean free paths during this period were required below ∼4 GV in order to reproduce the intensities observed by PAMELA.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ENERGY DISSIPATION PROCESSES IN SOLAR WIND TURBULENCE

Turbulence is a chaotic flow regime filled by irregular flows. The dissipation of turbulence is a fundamental problem in the realm of physics. Theoretically, dissipation ultimately cannot be achieved without collisions, and so how turbulent kinetic energy is dissipated in the nearly collisionless solar wind is a challenging problem. Wave particle interactions and magnetic reconnection (MR) are two possible dissipation mechanisms, but which mechanism dominates is still a controversial topic. Here we analyze the dissipation region scaling around a solar wind MR region. We find that the MR region shows unique multifractal scaling in the dissipation range, while the ambient solar wind turbulence reveals a monofractal dissipation process for most of the time. These results provide the first observational evidences for intermittent multifractal dissipation region scaling around a MR site, and they also have significant implications for the fundamental energy dissipation process.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A snowflake divertor: a possible solution to the power exhaust problem for tokamaks

This paper summarizes recent progress in the theory of a snowflake divertor, a possible path to reduce both steady-state and intermittent heat loads on the divertor plates to an acceptable level. The most important feature of a SF divertor is the presence of a large zone of a very weak poloidal magnetic field around the poloidal field (PF) null. Qualitative explanation of a variety of new features characteristic of a SF divertor is provided based on simple scaling relations. The main part of the paper is focused on the concept of spreading of the heat flux by curvature-driven convection near the PF null. References to experimental results from the NSTX and TCV tokamaks are provided.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2018

Clogging and transport of driven particles in asymmetric funnel arrays

In this paper, we numerically examine the flow and clogging of particles driven through asymmetric funnel arrays when the commensurability ratio of the number of particles per plaquette is varied. The particle-particle interactions are modeled with a soft repulsive potential that could represent vortex flow in type-II superconductors or driven charged colloids. The velocity-force curves for driving in the easy flow direction of the funnels exhibit a single depinning threshold; however, for driving in the hard flow direction, we find that there can be both negative mobility where the velocity decreases with increasing driving force as well as a reentrant pinning effect in which the particles flow at low drives but become pinned at intermediate drives. This reentrant pinning is associated with a transition from smooth one-dimensional flow at low drives to a clogged state at higher drives that occurs when the particles cluster in a small number of plaquettes and block the flow. When the drive is further increased, particle rearrangements occur that cause the clog to break apart. We map out the regimes in which the pinned, flowing, and clogged states appear as a function of plaquette filling and drive. Finally, the clogged states remain robust at finite temperatures but develop intermittent bursts of flow in which a clog temporarily breaks apart but quickly reforms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Thermal creep and the skyrmion Hall angle in driven skyrmion crystals

In this study, we numerically examine thermal effects on the skyrmion Hall angle for driven skyrmions interacting with quenched disorder. We identify a creep regime in which motion occurs via intermittent jumps between pinned and flowing states. Here the skyrmion Hall angle is zero since the skyrmions have time to relax into equilibrium positions in the pinning sites, eliminating the side-jump motion induced by the Magnus force. At higher drives we find a crossover to a viscous flow regime where the skyrmion Hall angle is finite and increases with increasing drive or temperature. Finally, our results are in agreement with recent experiments which also show a regime of finite skyrmion velocity with zero skyrmion Hall angle crossing over to a viscous flow regime with a skyrmion Hall angle that increases with drive.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetic fluctuations and the spin–orbit interaction in Mott insulating CoO

Motivated by the presence of an unquenched orbital angular momentum in CoO, a team at Chalk River, including a recently hired research officer Roger Cowley, performed the first inelastic neutron scattering experiments on the classic Mott insulator. Despite identifying two magnon modes at the zone boundary, the team was unable to parameterise the low energy magnetic excitation spectrum below TN using conventional pseudo-bosonic approaches, instead achieving only qualitative agreement. It would not be for another 40 years that Roger, now at Oxford and motivated by the discovery of the high-Tc cuprate superconductors, would make another attempt at the parameterisation of the magnetic excitation spectrum that had previously alluded him at the start of his career. Upon his return to CoO, Roger found a system embroiled in controversy, with some of its most fundamental parameters still remaining undetermined. Faced with such a formidable task, Roger performed a series of inelastic neutron scattering experiments in the early 2010s on both CoO and a magnetically dilute structural analogue Mg 0.97 Co 0.03 O. These experiments would prove instrumental in the determination of both single-ion and cooperative magnetic parameters for CoO. Both these sets of parameters would eventually be used in a spin–orbit exciton model, developed by his longtime friend and collaborator Bill Buyers, to successfully parameterise the complex spectrum that both measured at Chalk River almost 50 years prior. The story of CoO is of one that has come full circle, one filled with both spectacular failures and intermittent, yet profound, little victories.

36 MATERIALS SCIENCE↗