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NASA NTRS2014

Miniature Filament Eruptions and their Reconnections in X-Ray Jets: Evidence for a New Paradigm

We investigate the onset of approximately10 random X-ray jets observed by Hinode/XRT. Each jet was near the limb in a polar coronal hole, and showed a ``bright point'' in an edge of the base of the jet, as is typical for previously-observed X-ray jets. We examined SDO/AIA EUV images of each of the jets over multiple AIA channels, including 304 Ang, which detects chromospheric emissions, and 171, 193, and 211 Ang, which detect cooler-coronal emissions. We find the jets to result from eruptions of miniature (size less than approximately 10 arcsec) filaments from the bases of the jets. Much of the erupting-filament material forms a chromospheric-temperature jet. In the cool-coronal channels, often the filament appears in absorption and the jet in emission. The jet bright point forms at the location from which the miniature filament is ejected, analogous to the formation of a standard solar flare in the wake of the eruption of a typical larger-scale chromospheric filament. Thus these X-ray jets and their bright points are made by miniature filament eruptions. They are evidently produced the same way as an on-disk coronal jet we observed in Adams et al. (2014); that on-disk jet had no obvious emerging magnetic field in its base. We conclude that, for many jets, the standard idea of X-ray jets forming from reconnection between emerging flux and preexisting coronal field is incorrect. ACS and RLM were supported by funding from NASA/LWS, Hinode, and ISSI.

Sterling, Alphonse C.↗

NASA NTRS2016

Suppression of Heating of Coronal Loops Rooted in Opposite Polarity Sunspot Umbrae

EUV observations of active region (AR) coronae reveal the presence of loops at different temperatures. To understand the mechanisms that result in hotter or cooler loops, we study a typical bipolar AR, near solar disk center, which has moderate overall magnetic twist and at least one fully developed sunspot of each polarity. From AIA 193 and 94 Å images we identify many clearly discernible coronal loops that connect plage or a sunspot of one polarity to an opposite-polarity plage region. The AIA 94 Å images show dim regions in the umbrae of the spots. To see which coronal loops are rooted in a dim umbral area, we performed a non-linear force‐free field (NLFFF) modeling using photospheric vector magnetic field measurements obtained with the Heliosesmic Magnetic Imager (HMI) onboard SDO. The NLFFF model, validated by comparison of calculated model field lines with observed loops in AIA 193 and 94 Å, specifies the photospheric roots of the model field lines. Some model coronal magnetic field lines arch from the dim umbral area of the positive-polarity sunspot to the dim umbral area of a negative-polarity sunspot. Because these coronal loops are not visible in any of the coronal EUV and X-ray images of the AR, we conclude they are the coolest loops in the AR. This result suggests that the loops connecting opposite polarity umbrae are the least heated because the field in umbrae is so strong that the convective braiding of the field is strongly suppressed.

sunspots↗

NASA NTRS2016

Suppression of Heating of Coronal Loops Rooted in Opposite Polarity Sunspot Umbrae

sunspot↗

NASA NTRS2019

An Evolving Broad Iron Line from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124

We present a spectral study of the ultraluminous Be/X-ray transient pulsar Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER) observations during the system's 2017–2018 giant outburst. The 1.2–10 keV energy spectrum of the source can be approximated with an absorbed cutoff power-law model. We detect strong, luminosity-dependent emission lines in the 6–7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddington regime, is seen to evolve into a broad Fe-line profile in the super-Eddington regime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex. An asymmetric broad-line profile, peaking at 6.67 keV, is possibly due to Doppler effects and gravitational redshift. The 1.2–79 keV broadband spectrum from Nuclear Spectroscopic Telescope Array (NuSTAR) and NICER observations at the outburst peak is well described by an absorbed cutoff power law plus multiple Gaussian lines and a blackbody component. Physical reflection models are also tested to probe the broad iron-line feature. Depending on the mass accretion rate, we found emission sites that are evolving from ~5000 km to a range closer to the surface of the neutron star. Our findings are discussed in the framework of the accretion disk and its implication on the magnetic field, the presence of optically thick accretion curtain in the magnetosphere, jet emission, and the massive, ultrafast outflow expected at super-Eddington accretion rates. We do not detect any signatures of a cyclotron absorption line in the NICER or NuSTAR data.

Gaurava K. Jaisawal↗

NASA NTRS2019

SOFIA Far-infrared Imaging Polarimetry of M82 and NGC 253: Exploring the Supergalactic Wind

We present far-infrared polarimetry observations of M82 at 53 and 154μm and NGC 253 at 89μm, which were taken with High-resolution Airborne Wideband Camera-plus(HAWC+)in polarimetry mode on the Stratospheric Observatory for Infrared Astronomy. The polarization of M82 at 53μm clearly shows a magnetic field geometry perpendicular to the disk in the hot dust emission. For M82 the polarization at 154μm shows a combination of field geometry perpendicular to the disk in the nuclear region, but closer to parallel to the disk away from the nucleus. The fractional polarization at 53μm(154μm)ranges from 7%(3%)off nucleus to 0.5%(0.3%)near the nucleus. A simple interpretation of the observations of M82 invokes a massive polar outflow, dragging the field along, from a region∼700pc in diameter that has entrained some of the gas and dust, creating a vertical field geometry seen mostly in the hotter(53μm)dust emission. This outflow sits within a larger disk with a more typical planar geometry that more strongly contributes to the cooler(154μm)dust emission. For NGC 253, the polarization at 89μm is dominated by a planar geometry in the tilted disk, with weak indication of a vertical geometry above and below the plane from the nucleus. The polarization observations of NGC 253 at 53μm were of a insufficient signal-to-noise ratio for a detailed analysis.

Galaxies↗

NASA NTRSDate not supplied

PRIMA: The PRobe Far-Infrared Mission for Astrophysics

PRIMA is a far-infrared observatory concept being developed to address timely and fundamental questions about the growths of galaxies and solar systems and their constituents. It will observe the build-up of heavy elements, dust, stars, and black holes in galaxies and their interrelationships, and trace the masses and water contents of protoplanetary disks to probe the growths of solar systems. The majority of observing time will be devoted to Guest Observer programs to enable the astrophysics community to identify and plan the most critical observations, with focused PI programs to address key science with rapid releases of data to inform community planning. PRIMA will have spectral, hyperspectral imaging, and polarimetric capabilities, enabled now for the first time by extraordinary progress in kinetic inductance detector (KID) array technology over the last two decades. The 2.0-meter telescope will be cooled to < 5 K to take maximum advantage of the KID sensitivities. For observations of atomic fine-structure lines, molecular lines, and solid-state emission and absorption bands, R = 200 spectral coverage will range from approximately 25 µm to 200 µm, with a high-resolution mode across the entire band that will have a spectral resolving power of a few thousand at 100 µm. R ~ 10 moderate-resolution (hyperspectral) imaging will range from 25 to 80 µm for rest-frame, mid- and far-infrared spectral energy distribution measurements to probe dust grain composition and disambiguate star formation and active galactic nuclei in galaxies. Polarimetric observations of large areas of Galactic molecular clouds and the Magellanic Clouds from 80 to ~200 µm will bridge between the large-scale polarimetry of the interstellar medium from cosmic microwave background observatories and protostellar disk-scale interferometric observations to probe magnetic fields at the critical scales at which clouds collapse to form stars. An overview of PRIMA’s basic design and capabilities will be presented. Other posters in this session describe PRIMA science, technology, and instrumentation in greater depth.

Jason Glenn↗

DOE OSTI2015

ANALYSIS OF MAGNETOROTATIONAL INSTABILITY WITH THE EFFECT OF COSMIC-RAY DIFFUSION

We present the results obtained from the linear stability analysis and 2.5 dimensional magnetohydrodynamic (MHD) simulations of magnetorotational instability (MRI), including the effects of cosmic rays (CRs). We took into account the CR diffusion along the magnetic field but neglected the cross-field-line diffusion. Two models are considered in this paper: the shearing box model and differentially rotating cylinder model. We studied how MRI is affected by the initial CR pressure (i.e., energy) distribution. In the shearing box model, the initial state is uniform distribution. Linear analysis shows that the growth rate of MRI does not depend on the value of the CR diffusion coefficient. In the differentially rotating cylinder model, the initial state is a constant angular momentum polytropic disk threaded by a weak uniform vertical magnetic field. Linear analysis shows that the growth rate of MRI becomes larger if the CR diffusion coefficient is larger. Both results are confirmed by MHD simulations. The MHD simulation results show that the outward movement of matter by the growth of MRI is not impeded by the CR pressure gradient, and the centrifugal force that acts on the concentrated matter becomes larger. Consequently, the growth rate of MRI is increased. On the other hand, if the initial CR pressure is uniform, then the growth rate of the MRI barely depends on the value of the CR diffusion coefficient.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Photometric Properties of Network and Faculae Derived from HMI Data Compensated for Scattered Light

We report on the photometric properties of faculae and network, as observed in full-disk, scattered-light-corrected images from the Helioseismic Magnetic Imager. We use a Lucy–Richardson deconvolution routine that corrects an image in less than one second. Faculae are distinguished from network through proximity to active regions. This is the first report that full-disk observations, including center-to-limb variations, reproduce the photometric properties of faculae and network observed previously only in sub-arcsecond-resolution; small field-of-view studies, i.e. that network, as defined by distance from active regions, exhibit higher photometric contrasts. Specifically, for magnetic flux values larger than approximately 300 G, the network is brighter than faculae and the contrast differences increase toward the limb, where the network contrast is about twice the facular one. For lower magnetic flux values, network appear darker than faculae. Contrary to reports from previous full-disk observations, we also found that network exhibits a higher center-to-limb variation. Our results are in agreement with reports from simulations that indicate magnetic flux alone is a poor proxy of the photometric properties of magnetic features. We estimate that the contribution of faculae and network to Total Solar Irradiance variability of the current Cycle 24 is overestimated by at least 11%, due to the photometric properties of network and faculae not being recognized as different. This estimate is specific to the method employed in this study to reconstruct irradiance variations, so caution should be paid when extending it to other techniques.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

An accretion disk-outflow model for hysteretic state transition in X-ray binaries

We suggest a model of the advection-dominated accretion flow (ADAF) with magnetically driven outflows to explain the hysteretic state transition observed in X-ray binaries (XRBs). The transition from a thin disk to an ADAF occurs when the mass accretion rate is below a critical value. The critical mass accretion rate for the ADAF can be estimated by equating the equilibration timescale to the accretion timescale of the ADAF, which is sensitive to its radial velocity. The radial velocity of thin disks is very small, which leads to the advection of the external field in thin disks becoming very inefficient. ADAFs are present in the low/hard states of XRBs, and their radial velocity is large compared with the thin disk. The external field can be dragged inward efficiently by the ADAF, so a strong large-scale magnetic field threading the ADAF can be formed, which may accelerate a fraction of gas in the ADAF into the outflows. Such outflows may carry away a large amount of angular momentum from the ADAF, which significantly increases the radial velocity of the ADAF. This leads to a high critical mass accretion rate, below which an ADAF with magnetic outflows can survive. Our calculations show that the critical luminosity of the ADAF with magnetic outflows can be one order of magnitude higher than that for a conventional ADAF, if the ratio of gas to magnetic pressure β∼4 in the disk. This can naturally explain the hysteretic state transition observed in XRBs.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Economic assessment for recycling critical metals from hard disk drives using a comprehensive recovery process

Since the 2011 price spike of rare earth elements (REEs), research on permanent magnet recycling has blossomed globally to reduce future REE criticality. Hard disk drives (HDDs) have emerged as one feasible feedstock for recovering valuable REEs such as praseodymium, neodymium, and dysprosium. However, current processes for recycling e-waste only focus on certain metals due to feedstock and metal price uncertainties. In addition, some believe that recycling REEs is unprofitable. To shed some light on the economic viability of REE recycling from HDDs, this paper combines techno-economic information of a hydrometallurgical process with end-of-life HDD availability in a simulation model. Results showed that adding REEs to HDD recycling was profitable given current prices. As a result, recovered REEs could meet up to 5.1% rest of world (excluding China) magnet demand. Aluminum, gold, copper scrap and REEs were the primary main revenue streams from HDD recycling.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗

NASA NTRS1989

Numerical simulations of magnetized jets

The present axisymmetric numerical simulations of light hypersonic jets allow unmagnetized jets and jets carrying a dynamically important magnetic field to be contrasted. After decelerating a weakly magnetized jet through a series of weak, oblique shocks, a Mach disk and a strong annular shock are encountered near the outer edges of the contact discontinuity separating the shocked fluid from the shocked ambient gas. Upon passing the annular shock, the gas quickly expands and enters a backflowing cocoon surrounding the jet. The overall speed of advance of the jet is reduced; matter near the jet axis which passes through the terminal Mach disk accumulates in a plug, and gas is discharged into the cocoon by the intermittent shedding of vortices. When magnetic stresses dominate, however, the jet is rapidly decelerated via a Mach disk and strong annular shock.

Lind, Kevin R.↗

DOE OSTI2024

Magnetic Flux Plays an Important Role during a Black Hole X-Ray Binary Outburst in Radiative Two-temperature General Relativistic Magnetohydrodynamic Simulations

Abstract Black hole (Bh) X-ray binaries cycle through different spectral states of accretion over the course of months to years. Although persistent changes in the Bh mass accretion rate are generally recognized as the most important component of state transitions, it is becoming increasingly evident that magnetic fields play a similarly important role. In this article, we present the first radiative two-temperature general relativistic magnetohydrodynamics simulations in which an accretion disk transitions from a quiescent state at an accretion rate of M ̇ ∼ 10 − 10 M ̇ Edd to a hard-intermediate state at an accretion rate of M ̇ ∼ 10 − 2 M ̇ Edd . This huge parameter space in mass accretion rate is bridged by artificially rescaling the gas density scale of the simulations. We present two jetted BH models with varying degrees of magnetic flux saturation. We demonstrate that in “standard and normal evolution” models, which are unsaturated with magnetic flux, the hot torus collapses into a thin and cold accretion disk when M ̇ ≳ 5 × 10 − 3 M ̇ Edd . On the other hand, in “magnetically arrested disk” models, which are fully saturated with vertical magnetic flux, the plasma remains mostly hot with substructures that condense into cold clumps of gas when M ̇ ≳ 1 × 10 − 2 M ̇ Edd . This suggests that the spectral signatures observed during state transitions are closely tied to the level of magnetic flux saturation.

Astronomy & Astrophysics↗

NASA NTRS1995

Coronal structures deduced from photospheric magnetic field and He I lambda 10830 observations

The National Solar Observatory synoptic program provides an extensive and unique data base of high-resolution full-disk observations of the line-of-sight photospheric magnetic fields and of the He I lambda 10830 equivalent width. These data have been taken nearly daily for more than 21 years since 1974 and provide the opportunity to investigate the behavior of the magnetic fields in the photosphere and those inferred for the corona spanning on the time scales of a day to that of a solar cycle. The intensity of structures observed in He I lambda 10830 are strongly modulated by overlying coronal radiation; areas with low coronal emission are generally brighter in He I lambda 10830, while areas with high coronal emission are darker. For this reason, He I lambda 10830 was selected in the mid-1970's as way to identify and monitor coronal holes, magnetic fields with an open configuration, and the sources of high-speed solar wind streams. The He I lambda 10830 spectroheliograms also show a wide variety of other structures from small-scale, short-lived dark points (less than 30 arc-sec, hours) to the large-scale, long-lived two 'ribbon' flare events that follow the filament eruptions (1000 arc-sec, days). Such structures provide clues about the connections and changes in the large-scale coronal magnetic fields that are rooted in concentrations of magnetic network and active regions in the photosphere. In this paper, what observations of the photospheric magnetic field and He I lambda 10830 can tell us about the short- and long-term evolution of the coronal magnetic fields will be discussed, focussing on the quiet Sun and coronal holes. These data and what we infer from them will be compared with direct observations of the coronal structure from the Yohkoh Soft X-ray Telescope.

Harvey, Karen L.↗

NASA NTRS1994

Magnetocentrifugally driven flows from young stars and disks. 3: Numerical solution of the sub-Alfvenic region

We construct steady, axisymmetric, numerical models of the sub-Alfvenic regions of cool winds driven by the X-celerator mechanism. We find that smooth acceleration to speeds of the order of the escape speed accompanied by substantial collimation of the flow can be achieved within the Alfven surface, located characteristically at several stellar radii. We apply the nondimensional results to winds which emerge from the equator of magnetized YSOs rotating at breakup and from circumstellar disks interacting with the magnetospheres of slowly rotating T Tauri stars. Stellar magnetic fields of kilogauss strength can drive wind mass-loss rates of 10(exp -6) solar mass/year from rapidly accreting YSOs and 10(exp -8) solar mass/year from slowly accreting T Tauri stars. The X-celerator mechanism can accelerate winds from these systems to velocities of hundreds of km/sec within a few stellar radii in the case of embedded YSOs and approximately 10 stellar radii in the case of T Tauri stars.

Najita, Joan R.↗

DOE OSTI2015

Shear dynamo, turbulence, and the magnetorotational instability

The formation, evolution, and detailed structure of accretion disks remain poorly understood, with wide implications across a variety of astrophysical disciplines. While the most pressing question – what causes the high angular momentum fluxes that are necessary to explain observations? – is nicely answered by the idea that the disk is turbulent, a more complete grasp of the fundamental processes is necessary to capture the wide variety of behaviors observed in the night sky. This thesis studies the turbulence in ionized accretion disks from a theoretical standpoint, in particular focusing on the generation of magnetic fields in these processes, known as dynamo. Such fields are expected to be enormously important, both by enabling the magnetorotational instability (which evolves into virulent turbulence), and through large-scale structure formation, which may transport angular momentum in different ways and be fundamental for the formation of jets. The central result of this thesis is the suggestion of a new large-scale dynamo mechanism in shear flows – the “magnetic shear-current effect” – which relies on a positive feedback from smallscale magnetic fields. As well as being a very promising candidate for driving field generation in the central regions of accretion disks, this effect is interesting because small-scale magnetic fields have historically been considered to have a negative effect on the large-scale dynamo, damping growth and leading to dire predictions for final saturation amplitudes. Given that small-scale fields are ubiquitous in plasma turbulence above moderate Reynolds numbers, the finding that they could instead have a positive effect in some situations is interesting from a theoretical and practical standpoint. The effect is studied using direct numerical simulation, analytic techniques, and novel statistical simulation methods. In addition to the dynamo, much attention is given to the linear physics of disks and its relevance to turbulence. This is studied using nonmodal stability theory, which both provides a highly intuitive connection between global domains and the commonly studied shearing box, and suggests that transient linear growth can often be more important than spectral instability. These realizations motivate the use of the quasi-linear models that are applied extensively throughout the turbulence and dynamo studies later in the thesis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2024

From Feast to Famine: A Systematic Study of Accretion onto Oblique Pulsars with 3D GRMHD Simulations

Disk-fed accretion onto neutron stars can power a wide range of astrophysical sources ranging from X-ray binaries, to accretion-powered millisecond pulsars, ultraluminous X-ray sources, and gamma-ray bursts. A crucial parameter controlling the gas–magnetosphere interaction is the strength of the stellar dipole. In addition, coherent X-ray pulsations in many neutron star systems indicate that the star's dipole moment is oblique relative to its rotation axis. Therefore, it is critical to systematically explore the 2D parameter space of the star's magnetic field strength and obliquity, which is what this work does, for the first time, in the framework of 3D general-relativistic magnetohydrodynamics. If the accretion disk carries its own vertical magnetic field, this introduces an additional factor: the relative polarity of the disk and stellar magnetic fields. We find that depending on the strength of the stellar dipole and the star–disk relative polarity, the neutron star's jet power can either increase or decrease with increasing obliquity. For weak dipole strength (equivalently, high accretion rate), the parallel polarity results in a positive correlation between jet power and obliquity, whereas the antiparallel orientation displays the opposite trend. For stronger dipoles, the relative-polarity effect disappears, and jet power always decreases with increasing obliquity. The influence of the relative polarity gradually disappears as obliquity increases. Highly oblique pulsars tend to have an increased magnetospheric radius, a lower mass accretion rate, and enter the propeller regime at lower magnetic moments than aligned stars.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

The emergence of negative superhumps in cataclysmic variables: Smoothed particle hydrodynamics simulations

Negative superhumps are believed to arise in cataclysmic variable systems when the accretion disk is tilted with respect to the orbital plane. Slow retrograde precession of the line-of-nodes results in a signal—the negative superhump—with a period slightly less than the orbital period. Previous studies have shown that tilted disks exhibit negative superhumps, but a consensus on how a disk initially tilts has not been reached. Analytical work by Lai (1999, ApJ, 524, 1030) suggests that a magnetic field on the primary can lead to a tilt instability in a disk when the dipole moment is offset in angle from the spin axis of the primary and when the primary’s spin axis is, itself, not aligned with the angular momentum axis of the binary orbit. However, Lai did not apply his work to the formation of negative superhumps. In this paper, we add Lai’s model to an existing smoothed particle hydrodynamics code. Using this code, we demonstrate the emergence of negative superhumps in the “light curve” for a range of magnetic dipole moments. We show that the period deficits calculated from these negative superhumps match those in simulations using manually tilted disks. When positive superhumps appear (q≲0.33), we show that the period excesses calculated from these signals are also consistent with previous results. Using examples, we show that the disks are tilted, though the tilt varies periodically, and that they precess in the retrograde direction. The magnetic fields found to lead to the emergence of negative superhumps lie in the kilogauss regime.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1981

Assessment of disk MHD generators for a base load powerplant

Results from a study of the disk MHD generator are presented. Both open and closed cycle disk systems were investigated. Costing of the open cycle disk components (nozzle, channel, diffuser, radiant boiler, magnet and power management) was done. However, no detailed costing was done for the closed cycle systems. Preliminary plant design for the open cycle systems was also completed. Based on the system study results, an economic assessment of the open cycle systems is presented. Costs of the open cycle disk conponents are less than comparable linear generator components. Also, costs of electricity for the open cycle disk systems are competitive with comparable linear systems. Advantages of the disk design simplicity are considered. Improvements in the channel availability or a reduction in the channel lifetime requirement are possible as a result of the disk design.

Chubb, D. L.↗