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Results for “MAGNETIC FIELD CONFIGURATIONS”

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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Determination of plasma parameters from soft X-ray images for coronal holes /open magnetic field configurations/ and coronal large-scale structures /extended closed-field configurations/

In connection with high-quality solar soft X-ray images the 'quiet' features of the inner corona have been separated into two sharply different components, including the strongly reduced emission areas or coronal holes (CH) and the extended regions of looplike emission features or large-scale structures (LSS). Particular central meridian passage observations of the prominent CH1 on August 21, 1973, are selected for a quantitative study. Histogram photographic density distributions for full-disk images at other central meridian passages of CH 1 are also presented, and the techniques of converting low photographic density data to deposited energy are discussed, with particular emphasis on the problems associated with the CH data.

Maxson, C. W.↗
NASA NTRS1982

On perturbations of magnetic field configurations

The behavior of stationary equilibrium solutions to the MHD equations possessing a well-defined symmetry to perturbations lacking that symmetry is explored. Two distinct situations of astrophysical interest are considered: solutions of the magnetostatic equations and of the magnetoconvection equations. The results show that in these cases changes in solution topology are not accessible to small-parameter (epsilon) expansions, so that such expansions do not describe the full range of behavior. In particular, finite-amplitude perturbations can lead to new stationary solutions possessing different symmetries from the initial solution.

Rosner, R.↗
NASA NTRS1991

Magnetic field configurations associated with fast solar wind

The implications are considered of the observed inverse correlation between solar wind speed at earth and the expansion rate of the sun-earth flux tube as it passes through the corona. It is found that the coronal expansion rate depends critically on the large-scale photospheric field distribution around the footpoint of the flux tube. The smallest expansions occur in tubes that are rooted near a local minimum in the field. This suggests that the fastest wind streams originate from regions where large coronal holes are about to break apart and from the facing edges of adjacent like-polarity holes, whose field lines converge as they transit the corona. Predictions are made which follow from the above ideas.

Sheeley, N. R., Jr.↗
DOE OSTI2017

Lepton Acceleration in the Vicinity of the Event Horizon: Very High Energy Emissions from Supermassive Black Holes

Around a rapidly rotating black hole (BH), when the plasma accretion rate is much less than the Eddington rate, the radiatively inefficient accretion flow (RIAF) cannot supply enough MeV photons that are capable of materializing as pairs. In such a charge-starved BH magnetosphere, the force-free condition breaks down in the polar funnels. Applying the pulsar outer-magnetospheric lepton accelerator theory to supermassive BHs, we demonstrate that a strong electric field arises along the magnetic field lines in the direct vicinity of the event horizon in the funnels, that the electrons and positrons are accelerated up to 100 TeV in this vacuum gap, and that these leptons emit copious photons via inverse-Compton (IC) processes between 0.1 and 30 TeV for a distant observer. It is found that these IC fluxes will be detectable with Imaging Atmospheric Cherenkov Telescopes, provided that a low-luminosity active galactic nucleus is located within 1 Mpc for a million-solar-mass central BH or within 30 Mpc for a billion-solar-mass central BH. These very high energy fluxes are beamed in a relatively small solid angle around the rotation axis because of the inhomogeneous and anisotropic distribution of the RIAF photon field and show an anticorrelation with the RIAF submillimeter fluxes. The gap luminosity depends little on the 3D magnetic field configuration, because the Goldreich–Julian charge density, and hence the exerted electric field, is essentially governed by the frame-dragging effect, not by the magnetic field configuration.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Comparison between hybrid and fully kinetic models of asymmetric magnetic reconnection: Coplanar and guide field configurations

Magnetic reconnection occurring in collisionless environments is a multi-scale process involving both ion and electron kinetic processes. Because of their small mass, the electron scales are difficult to resolve in numerical and satellite data, it is therefore critical to know whether the overall evolution of the reconnection process is influenced by the kinetic nature of the electrons, or is unchanged when assuming a simpler, fluid, electron model. This paper investigates this issue in the general context of an asymmetric current sheet, where both the magnetic field amplitude and the density vary through the discontinuity. A comparison is made between fully kinetic and hybrid kinetic simulations of magnetic reconnection in coplanar and guide field systems. The models share the initial condition but differ in their electron modeling. It is found that the overall evolution of the system, including the reconnection rate, is very similar between both models. The best agreement is found in the guide field system, which confines particle better than the coplanar one, where the locality of the moments is violated by the electron bounce motion. It is also shown that, contrary to the common understanding, reconnection is much faster in the guide field system than in the coplanar one. Both models show this tendency, indicating that the phenomenon is driven by ion kinetic effects and not electron ones.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Toroidal–Poloidal Magnetic Field Generation in Neutron Stars by Spacetime Frame-dragging

Neutron star magnetic fields, particularly in magnetars, have strong poloidal and toroidal components, reaching magnitudes up to 10{sup 15} T and extending into the magnetosphere, where they cause twisted magnetic fields and surface currents. Here, we show that to a large extent neutron star toroidal and poloidal magnetic fields are due to frame-dragging of spacetime in the stellar exterior. In the explicit case of linearized Kerr metric we show that general relativistic Maxwell equations admit a mixed poloidal and toroidal magnetic field configuration in which the toroidal field strength is comparable with the poloidal magnetic field. Calculations are performed to show that the frame-dragging-induced magnetic field has the observed order of magnitude at the stellar surface, and exhibits the flare-like phenomenon observed in magnetars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EFFECT OF BACKGROUND MAGNETIC FIELD ON TURBULENCE DRIVEN BY MAGNETOROTATIONAL INSTABILITY IN ACCRETION DISKS

We investigate the background magnetic field dependence of the saturated state of a magnetorotational instability (MRI) in an accretion disk by performing three-dimensional magnetohydrodynamic simulations. We assume an unstratified disk by employing the local shearing box approximation. Three different uniform background magnetic field configurations are treated for a wide range of field intensities. These simulations indicate that the time variations of the turbulent stress and the magnetic energy are altered by the presence of a poloidal component of the background field. We find that the saturation amplitude of the turbulent stress and the magnetic energy are determined by both the poloidal and azimuthal components of the field. In particular, when the poloidal component has the same intensity, the obtained turbulent stress for {beta}{sub y0} Almost-Equal-To 200 becomes smaller than those for a purely poloidal field case. Despite the fact that the background field affects the MRI turbulence, the correlation between the obtained turbulent stress and the magnetic energy in the nonlinear stage is independent of the field topology. Our results indicate that the saturated turbulent stress has a stronger correlation with the power of the perturbed component of the magnetic field than with the power of the total magnetic field. These results suggest that both the intensity and the direction of the background magnetic field significantly affect the turbulent motion of the MRI in accretion disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Diagnosis of magnetic and electric fields of chromospheric jets through spectropolarimetric observations of H I Paschen lines

Magnetic fields govern the plasma dynamics in the outer layers of the solar atmosphere, and electric fields acting on neutral atoms that move across the magnetic field enable us to study the dynamical coupling between neutrals and ions in the plasma. In order to measure the magnetic and electric fields of chromospheric jets, the full Stokes spectra of the Paschen series of neutral hydrogen in a surge and in some active region jets that took place at the solar limb were observed on 2012 May 5, using the spectropolarimeter of the Domeless Solar Telescope at Hida observatory, Japan. First, we inverted the Stokes spectra taking into account only the effect of magnetic fields on the energy structure and polarization of the hydrogen levels. Having found no definitive evidence of the effects of electric fields in the observed Stokes profiles, we then estimated an upper bound for these fields by calculating the polarization degree under the magnetic field configuration derived in the first step, with the additional presence of a perpendicular (Lorentz type) electric field of varying strength. The inferred direction of the magnetic field on the plane of the sky approximately aligns to the active region jets and the surge, with magnetic field strengths in the range 10 G < B < 640 G for the surge. Using magnetic field strengths of 70, 200, and 600 G, we obtained upper limits for possible electric fields of 0.04, 0.3, and 0.8 V cm{sup –1}, respectively. This upper bound is conservative, since in our modeling we neglected the possible contribution of collisional depolarization. Because the velocity of neutral atoms of hydrogen moving across the magnetic field derived from these upper limits of the Lorentz electric field is far below the bulk velocity of the plasma perpendicular to the magnetic field as measured by the Doppler shift, we conclude that the neutral atoms must be highly frozen to the magnetic field in the surge.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

3D WHOLE-PROMINENCE FINE STRUCTURE MODELING

We present the first 3D whole-prominence fine structure model. The model combines a 3D magnetic field configuration of an entire prominence obtained from nonlinear force-free field simulations, with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along multiple fine structures within the 3D magnetic model. Through the use of a novel radiative transfer visualization technique for the Hα line such plasma-loaded magnetic field model produces synthetic images of the modeled prominence comparable with high-resolution observations. This allows us for the first time to use a single technique to consistently study, in both emission on the limb and absorption against the solar disk, the fine structures of prominences/filaments produced by a magnetic field model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE ROLE OF STREAMERS IN THE DEFLECTION OF CORONAL MASS EJECTIONS: COMPARISON BETWEEN STEREO THREE-DIMENSIONAL RECONSTRUCTIONS AND NUMERICAL SIMULATIONS

On 2009 September 21, a filament eruption and the associated coronal mass ejection (CME) were observed by the Solar Terrestrial Relations Observatory (STEREO) spacecraft. The CME originated from the southern hemisphere and showed a deflection of about 15 Degree-Sign toward the heliospheric current sheet (HCS) during the propagation in the COR1 field of view. The CME source region was near the central meridian, but no on-disk CME signatures could be seen from the Earth. The aim of this paper is to provide a physical explanation for the strong deflection of the CME observed on 2009 September 21. The two-sided view of the STEREO spacecraft allows us to reconstruct the three-dimensional travel path of the CME and the evolution of the CME source region. The observations are combined with a magnetohydrodynamic simulation, starting from a magnetic field configuration closely resembling the extrapolated potential field for that date. By applying localized shearing motions, a CME is initiated in the simulation, showing a similar non-radial evolution, structure, and velocity as the observed event. The CME gets deflected toward the current sheet of the larger northern helmet streamer due to an imbalance in the magnetic pressure and tension forces and finally gets into the streamer. This study shows that during solar minima, even CMEs originating from high latitude can be easily deflected toward the HCS, eventually resulting in geoeffective events. How rapidly they undergo this latitudinal migration depends on the strength of both the large-scale coronal magnetic field and the magnetic flux of the erupting filament.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

FIRST SIMULTANEOUS DETECTION OF MOVING MAGNETIC FEATURES IN PHOTOSPHERIC INTENSITY AND MAGNETIC FIELD DATA

The formation and the temporal evolution of a bipolar moving magnetic feature (MMF) was studied with high-spatial and temporal resolution. The photometric properties were observed with the New Solar Telescope at Big Bear Solar Observatory using a broadband TiO filter (705.7 nm), while the magnetic field was analyzed using the spectropolarimetric data obtained by Hinode. For the first time, we observed a bipolar MMF simultaneously in intensity images and magnetic field data, and studied the details of its structure. The vector magnetic field and the Doppler velocity of the MMF were also studied. A bipolar MMF with its positive polarity closer to the negative penumbra formed, accompanied by a bright, filamentary structure in the TiO data connecting the MMF and a dark penumbral filament. A fast downflow ({<=}2 km s{sup -1}) was detected at the positive polarity. The vector magnetic field obtained from the full Stokes inversion revealed that a bipolar MMF has a U-shaped magnetic field configuration. Our observations provide a clear intensity counterpart of the observed MMF in the photosphere, and strong evidence of the connection between the MMF and the penumbral filament as a serpentine field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Asymmetric kinetic equilibria: Generalization of the BAS model for rotating magnetic profile and non-zero electric field

Finding kinetic equilibria for non-collisional/collisionless tangential current layers is a key issue as well for their theoretical modeling as for our understanding of the processes that disturb them, such as tearing or Kelvin Helmholtz instabilities. The famous Harris equilibrium [E. Harris, Il Nuovo Cimento Ser. 10 23, 115–121 (1962)] assumes drifting Maxwellian distributions for ions and electrons, with constant temperatures and flow velocities; these assumptions lead to symmetric layers surrounded by vacuum. This strongly particular kind of layer is not suited for the general case: asymmetric boundaries between two media with different plasmas and different magnetic fields. The standard method for constructing more general kinetic equilibria consists in using Jeans theorem, which says that any function depending only on the Hamiltonian constants of motion is a solution to the steady Vlasov equation [P. J. Channell, Phys. Fluids (1958–1988) 19, 1541 (1976); M. Roth et al., Space Sci. Rev. 76, 251–317 (1996); and F. Mottez, Phys. Plasmas 10, 1541–1545 (2003)]. The inverse implication is however not true: when using the motion invariants as variables instead of the velocity components, the general stationary particle distributions keep on depending explicitly of the position, in addition to the implicit dependence introduced by these invariants. The standard approach therefore strongly restricts the class of solutions to the problem and probably does not select the most physically reasonable. The BAS (Belmont-Aunai-Smets) model [G. Belmont et al., Phys. Plasmas 19, 022108 (2012)] used for the first time the concept of particle accessibility to find new solutions: considering the case of a coplanar-antiparallel magnetic field configuration without electric field, asymmetric solutions could be found while the standard method can only lead to symmetric ones. These solutions were validated in a hybrid simulation [N. Aunai et al., Phys. Plasmas (1994-present) 20, 110702 (2013)], and more recently in a fully kinetic simulation as well [J. Dargent and N. Aunai, Phys. Plasmas (submitted)]. Nevertheless, in most asymmetric layers like the terrestrial magnetopause, one would indeed expect a magnetic field rotation from one direction to another without going through zero [J. Berchem and C. T. Russell, J. Geophys. Res. 87, 8139–8148 (1982)], and a non-zero normal electric field. In this paper, we propose the corresponding generalization: in the model presented, the profiles can be freely imposed for the magnetic field rotation (although restricted to a 180 rotation hitherto) and for the normal electric field. As it was done previously, the equilibrium is tested with a hybrid simulation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

SUBMILLIMETER POLARIZATION OBSERVATION OF THE PROTOPLANETARY DISK AROUND HD 142527

We present the polarization observations toward the circumstellar disk around HD 142527 by using Atacama Large Millimeter/submillimeter Array at the frequency of 343 GHz. The beam size is 0.″51 × 0.″44, which corresponds to the spatial resolution of ∼71 × 62 au. The polarized intensity displays a ring-like structure with a peak located on the east side with a polarization fraction of P = 3.26 ± 0.02%, which is different from the peak of the continuum emission from the northeast region. The polarized intensity is significantly weaker at the peak of the continuum where P = 0.220 ± 0.010%. The polarization vectors are in the radial direction in the main ring of the polarized intensity, while there are two regions outside at the northwest and northeast areas where the vectors are in the azimuthal direction. If the polarization vectors represent the magnetic field morphology, the polarization vectors indicate the toroidal magnetic field configuration on the main ring and the poloidal fields outside. On the other hand, the flip of the polarization vectors is predicted by the self-scattering of thermal dust emission due to the change of the direction of thermal radiation flux. Therefore, we conclude that self-scattering of thermal dust emission plays a major role in producing polarization at millimeter wavelengths in this protoplanetary disk. Also, this puts a constraint on the maximum grain size to be approximately 150 μ m if we assume compact spherical dust grains.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE LIGHT CURVE AND INTERNAL MAGNETIC FIELD OF THE MODE-SWITCHING PULSAR PSR B0943+10

A number of radio pulsars exhibit intriguing mode-switching behavior. Recent observations of PSR B0943+10 revealed correlated radio and X-ray mode switches, providing a new avenue for understanding this class of objects. The large X-ray pulse fraction observed during the radio-quiet phase (Q-mode) was previously interpreted as a result of changing obscuration of X-rays by dense magnetosphere plasma. We show that the large X-ray pulse fraction can be explained by including the beaming effect of a magnetic atmosphere, while remaining consistent with the dipole field geometry constrained by radio observations. We also explore a more extreme magnetic field configuration, where a magnetic dipole displaced from the center of the star produces two magnetic polar caps of different sizes and magnetic field strengths. These models are currently consistent with data in radio and X-rays and can be tested or constrained by future X-ray observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Multi-fluid Modeling of Magnetosonic Wave Propagation in the Solar Chromosphere: Effects of Impact Ionization and Radiative Recombination

In order to study chromospheric magnetosonic wave propagation including, for the first time, the effects of ion–neutral interactions in the partially ionized solar chromosphere, we have developed a new multi-fluid computational model accounting for ionization and recombination reactions in gravitationally stratified magnetized collisional media. The two-fluid model used in our 2D numerical simulations treats neutrals as a separate fluid and considers charged species (electrons and ions) within the resistive MHD approach with Coulomb collisions and anisotropic heat flux determined by Braginskiis transport coefficients. The electromagnetic fields are evolved according to the full Maxwell equations and the solenoidality of the magnetic field is enforced with a hyperbolic divergence-cleaning scheme. The initial density and temperature profiles are similar to VAL III chromospheric model in which dynamical, thermal, and chemical equilibrium are considered to ensure comparison to existing MHD models and avoid artificial numerical heating. In this initial setup we include simple homogeneous flux tube magnetic field configuration and an external photospheric velocity driver to simulate the propagation of MHD waves in the partially ionized reactive chromosphere. In particular, we investigate the loss of chemical equilibrium and the plasma heating related to the steepening of fast magnetosonic wave fronts in the gravitationally stratified medium.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MAGNETICALLY CONTROLLED ACCRETION FLOWS ONTO YOUNG STELLAR OBJECTS

Accretion from disks onto young stars is thought to follow magnetic field lines from the inner disk edge to the stellar surface. The accretion flow thus depends on the geometry of the magnetic field. This paper extends previous work by constructing a collection of orthogonal coordinate systems, including the corresponding differential operators, where one coordinate traces the magnetic field lines. This formalism allows for an (essentially) analytic description of the geometry and the conditions required for the flow to pass through sonic points. Using this approach, we revisit the problem of magnetically controlled accretion flow in a dipole geometry, and then generalize the treatment to consider magnetic fields with multiple components, including dipole, octupole, and split monopole contributions. This approach can be generalized further to consider more complex magnetic field configurations. Observations indicate that accreting young stars have substantial dipole and octupole components, and that accretion flow is transonic. If the effective equation of state for the fluid is too stiff, however, the flow cannot pass smoothly through the sonic points in steady state. For a multipole field of order l, we derive a general constraint on the polytropic index, n > l + 3/2, required for steady transonic flow to reach free-fall velocities. For octupole fields, inferred on surfaces of T Tauri stars, the index n > 9/2, so that the flow must be close to isothermal. The inclusion of octupole field components produces higher densities at the stellar surface and smaller areas for the hot spots, which occur at higher latitudes; the magnetic truncation radius is smaller (larger) for octupole components that are aligned (anti-aligned) with the stellar dipole. This contribution thus increases our understanding of magnetically controlled accretion for young stellar objects and can be applied to a variety of additional astrophysical problems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Slender Ca ii H Fibrils Mapping Magnetic Fields in the Low Solar Chromosphere

A dense forest of slender bright fibrils near a small solar active region is seen in high-quality narrowband Ca ii H images from the SuFI instrument onboard the Sunrise balloon-borne solar observatory. The orientation of these slender Ca ii H fibrils (SCF) overlaps with the magnetic field configuration in the low solar chromosphere derived by magnetostatic extrapolation of the photospheric field observed with Sunrise/IMaX and SDO/HMI. In addition, many observed SCFs are qualitatively aligned with small-scale loops computed from a novel inversion approach based on best-fit numerical MHD simulation. Such loops are organized in canopy-like arches over quiet areas that differ in height depending on the field strength near their roots.

79 ASTRONOMY AND ASTROPHYSICS↗