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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Simulations of coronal disconnection events

The lack of evidence for magnetic disconnection of coronal mass ejections (CMEs) from the sun has long been a puzzle, as it implies a buildup of the interplanetary magnetic field (IMF) magnitude over time. Such a buildup is ruled out by observations. Magnetic reconnection above helmet streamer configurations could provide a mechanism for maintaining the observed relative constancy of the IMF (McComas et al., 1989). McComas et al. (1991) showed observational evidence of reconnection above a streamer. This interpretation is investigated using time-dependent MHD simulations. The opening of new magnetic flux on the sun (as might occur in a CME or other transient event) is modeled as an increase in magnetic flux at the poles of a simulated corona. It is found that this perturbation can in fact cause reconnection above an equatorial helmet streamer, and the resultant density signature is similar to the observations of McComas et al. (1991).

Linker, Jon A.↗
NASA NTRS1993

Solar magnetic cycle

Using NSO/KP magnetograms, the pattern and rate of the emergence of magnetic flux and the development of the large-scale patterns of unipolar fields are considered in terms of the solar magnetic cycle. Magnetic flux emerges in active regions at an average rate of 2 x 10(exp 21) Mx/day, approximately 10 times the estimated rate in ephemeral regions. Observations are presented that demonstrate that the large-scale unipolar fields originate in active regions and activity nests. For cycle 21, the net contribution of ephemeral regions to the axial dipole moment of the Sun is positive, and is of opposite sign to that of active regions. Its amplitude is smaller by a factor of 6, assuming an average lifetime of ephemeral regions of 8 hours. Active regions larger than 4500 Mm(sup 2) are the primary contributor to the cycle variation of Sun's axial dipole moment.

Harvey, Karen L.↗
DOE OSTI2015

CONFINED FLARES IN SOLAR ACTIVE REGION 12192 FROM 2014 OCTOBER 18 TO 29

Using the observations from the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory, we investigate 6 X-class and 29 M-class flares occurring in solar active region (AR) 12192 from October 18 to 29. Among them, 30 (including 6 X- and 24 M-class) flares originated from the AR core, and the other 5 M-flares appeared at the AR periphery. Four of the X-flares exhibited similar flaring structures, indicating they were homologous flares with an analogous triggering mechanism. The possible scenario is that photospheric motions of emerged magnetic fluxes lead to shearing of the associated coronal magnetic field, which then yields a tether-cutting favorable configuration. Among the five periphery M-flares, four were associated with jet activities. The HMI vertical magnetic field data show that the photospheric fluxes of opposite magnetic polarities emerged, converged, and canceled with each other at the footpoints of the jets before the flares. Only one M-flare from the AR periphery was followed by a coronal mass ejection (CME). From October 20 to 26, the mean decay index of the horizontal background field within the height range of 40–105 Mm is below the typical threshold for torus instability onset. This suggests that a strong confinement from the overlying magnetic field might be responsible for the poor CME production of AR 12192.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2004

The Effects of Differential Rotation on the Magnetic Structure of the Solar Corona: MHD Simulations

Coronal holes are magnetically open regions from which the solar wind streams. Magnetic reconnection has been invoked to reconcile the apparently rigid rotation of coronal holes with the differential rotation of magnetic flux in the photosphere. This mechanism might also be relevant to the formation of the slow solar wind, the properties of which seem to indicate an origin from the opening of closed magnetic field lines. We have developed a global MHD model to study the effect of differential rotation on the coronal magnetic field. Starting from a magnetic flux distribution similar to that of Wang et al., which consists of a bipolar magnetic region added to a background dipole field, we applied differential rotation over a period of 5 solar rotations. The evolution of the magnetic field and of the boundaries of coronal holes are in substantial agreement with the findings of Wang et al.. We identified examples of interchange reconnection and other changes of topology of the magnetic field. Possible consequences for the origin of the slow solar wind are also discussed.

Lionello, Roberto↗
DOE OSTI2022

Semiclassics with ’t Hooft flux background for QCD with 2-index quarks

We study quantum chromodynamics including the two-index symmetric or anti-symmetric quark (QCD(Sym/ASym)) on small $\mathbb{R}^2$× T 2 with a suitable magnetic flux. We first discuss the ’t Hooft anomaly of these theories and claim that discrete chiral symmetry should be spontaneously broken completely to satisfy the anomaly matching condition. The T 2 compactification with the magnetic flux preserves the ’t Hooft anomaly, and the 2d effective theory is constrained by the same anomaly of 4d QCD(Sym/ASym). We demonstrate the spontaneous breakdown of chiral symmetry using the dilute gas of center vortices, which confirms the prediction of the ’t Hooft anomaly. We also find that each vacuum maintains the charge conjugation symmetry, and this gives affirmative support for the nonperturbative large-N orientifold equivalence between QCD(Sym/ASym) and $\mathcal{N}$ = 1 supersymmetric SU(N) Yang-Mills theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1990

Relations between the photospheric magnetic field and the emission from the outer atmospheres of cool stars. II - The C IV 1550 A doublet

Solar and stellar data are used to determine the relationship between C IV 1548 + 1551 A and magnetic flux densities. C IV SMM-UVSP spectroheliograms of solar quiet regions and plages are analyzed. Typical distribution functions of C IV flux densities over both quiet and active solar regions are presented. The C IV emission is shown to be dependent on magnetic flux density.

Schrijver, C. J.↗
NASA NTRS1991

The solar activity cycle

This review emphasizes observations of photospheric magnetic flux during cycle 21 (1976-1986) and how these measurements have been used to model the cyclic variability of the heliospheric magnetic field. Indices of solar activity are discussed in terms of their potential to figure in theoretical or empirical models. Other recent data, such as measurements of large-scale surface flows and information on the sun's internal rotation from helioseismology, as well as the magnetic flux observations, are considered in the context of Babcock's phenomenological model of the solar cycle.

Rabin, Douglas M.↗
NASA NTRS2005

Intermittent Turbulence and SOC Dynamics in a 2-D Driven Current-Sheet Model

Borovsky et al. have shown that Earth's magnetotail plasma sheet is strongly turbulent. More recently, Borovsky and Funsten have shown that eddy turbulence dominates and have suggested that the eddy turbulence is driven by fast flows that act as jets in the plasma. Through basic considerations of energy and magnetic flux conservation, these fast flows are thought to be localized to small portions of the total plasma sheet and to be generated by magnetic flux reconnection that is similarly localized. Angelopoulos et al., using single spacecraft Geotail data, have shown that the plasma sheet turbulence exhibits signs of intermittence and Weygand et al., using four spacecraft Cluster data, have confirmed and expanded on this conclusion. Uritsky et al., using Polar UVI image data, have shown that the evolution of bright, nightside, UV auroral emission regions is consistent with many of the properties of systems in self-organized criticality (SOC). Klimas et al. have suggested that the auroral dynamics is a reflection of the dynamics of the fast flows in the plasma. sheet. Their hypothesis is that the transport of magnetic fludenergy through the magnetotail is enabled by scale-free avalanches of localized reconnection whose SOC dynamics are reflected in the auroral UV emission dynamics. A corollary of this hypothesis is that the strong, intermittent, eddy turbulence of the plasma sheet is closely related to its critical dynamics. The question then arises: Can in situ evidence for the SOC dynamics be found in the properties of the plasma sheet turbulence? A 2-dimensional numerical driven current-sheet model of the central plasma sheet has been developed that incorporates an idealized current-driven instability with a resistive MHD system. It has been shown that the model can evolve into SOC in a physically relevant parameter regime. Initial results from a study of intermittent turbulence in this model and the relationship of this turbulence to the model's known SOC dynamics will be discussed.

Klimas, A. J.↗
NASA NTRS1996

Latitudinal Dependence of the Radial IMF Component - Interplanetary Imprint

Ulysses measurements have confirmed that there is no significant gradient with respect to heliomagnetic latitude in the radial component, B(sub r,), of the interplanetary magnetic field. There are two processes responsible for this observation. In the corona, the plasma beta is much less than 1, except directly above streamers, so both longitudinal and latitudinal (meridional) gradients in field strength will relax, due to the transverse magnetic pressure gradient force, as the solar wind carries magnetic flux away from the Sun. This happens so quickly that the field is essentially uniform by 5 solar radius. Beyond 10 solar radius, beta is greater than 1 and it is possible for a meridional thermal pressure gradient to redistribute magnetic flux - an effect apparently absent in Ulysses and earlier ICE and Interplanetary Magnetic Physics (IMP) data. We discuss this second effect here, showing that its absence is mainly due to the perpendicular part of the anisotropic thermal pressure gradient in the interplanetary medium being too small to drive significant meridional transport between the Sun and approx. 4 AU. This is done using a linear analytic estimate of meridional transport. The first effect was discussed in an earlier paper.

Suess, S. T.↗
DOE OSTI2016

Spin-orbit coupling, electron transport and pairing instabilities in two-dimensional square structures

Rashba spin-orbit effects and electron correlations in the two-dimensional cylindrical lattices of square geometries are assessed using mesoscopic two-, three- and four-leg ladder structures. Here the electron transport properties are systematically calculated by including the spin-orbit coupling in tight binding and Hubbard models threaded by a magnetic flux. These results highlight important aspects of possible symmetry breaking mechanisms in square ladder geometries driven by the combined effect of a magnetic gauge field spin-orbit interaction and temperature. The observed persistent current, spin and charge polarizations in the presence of spin-orbit coupling are driven by separation of electron and hole charges and opposite spins in real-space. The modeled spin-flip processes on the pairing mechanism induced by the spin-orbit coupling in assembled nanostructures (as arrays of clusters) engineered in various two-dimensional multi-leg structures provide an ideal playground for understanding spatial charge and spin density inhomogeneities leading to electron pairing and spontaneous phase separation instabilities in unconventional superconductors. Such studies also fall under the scope of current challenging problems in superconductivity and magnetism, topological insulators and spin dependent transport associated with numerous interfaces and heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

A plasma β transition within a propagating flux rope

We present a 2.5 dimensional magnetohydrodynamic simulation of a magnetic flux rope (FR) propagating in the heliosphere and investigate the cause of the observed sharp plasma β transition. Specifically, we consider a strong internal magnetic field and an explosive fast start, such that the plasma β is significantly lower in the FR than in the sheath region that is formed ahead. This leads to an unusual FR morphology in the first stage of propagation, while the more traditional view (e.g., from space weather simulations like Enlil) of a pancake-shaped FR is observed as it approaches 1 AU. We investigate how an equipartition line, defined by a magnetic Weber number, surrounding a core region of a propagating FR, can demarcate a boundary layer where there is a sharp transition in the plasma β. The substructure affects the distribution of toroidal flux, with the majority of the flux remaining in a small core region that maintains a quasi-cylindrical structure. We quantitatively investigate a locus of points where the kinetic energy density of the relative inflow field is equal to the energy density of the transverse magnetic field (i.e., effective tension force). The simulation provides compelling evidence that at all heliocentric distances the distribution of toroidal magnetic flux away from the FR axis is not linear, with 80% of the toroidal flux occurring within 40% of the distance from the FR axis. Thus, our simulation displays evidence that the competing ideas of a pancaking structure observed remotely can coexist with a quasi-cylindrical magnetic structure seen in situ.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

HOT ELECTROMAGNETIC OUTFLOWS. II. JET BREAKOUT

We consider the interaction between radiation, matter, and a magnetic field in a compact, relativistic jet. The entrained matter accelerates outward as the jet breaks out of a star or other confining medium. In some circumstances, such as gamma-ray bursts (GRBs), the magnetization of the jet is greatly reduced by an advected radiation field while the jet is optically thick to scattering. Where magnetic flux surfaces diverge rapidly, a strong outward Lorentz force develops and radiation and matter begin to decouple. The increase in magnetization is coupled to a rapid growth in Lorentz factor. We take two approaches to this problem. The first examines the flow outside the fast magnetosonic critical surface, and calculates the flow speed and the angular distribution of the radiation field over a range of scattering depths. The second considers the flow structure on both sides of the critical surface in the optically thin regime, using a relaxation method. In both approaches, we find how the terminal Lorentz factor and radial profile of the outflow depend on the radiation intensity and optical depth at breakout. The effect of bulk Compton scattering on the radiation spectrum is calculated by a Monte Carlo method, while neglecting the effects of internal dissipation. The peak of the scattered spectrum sits near the seed peak if radiation pressure dominates the acceleration, but is pushed to a higher frequency if the Lorentz force dominates. The unscattered seed radiation can form a distinct, low-frequency component of the spectrum, especially if the magnetic Poynting flux dominates.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2012

Multiscale Dynamics of Solar Magnetic Structures

Multiscale topological complexity of the solar magnetic field is among the primary factors controlling energy release in the corona, including associated processes in the photospheric and chromospheric boundaries.We present a new approach for analyzing multiscale behavior of the photospheric magnetic flux underlying these dynamics as depicted by a sequence of high-resolution solar magnetograms. The approach involves two basic processing steps: (1) identification of timing and location of magnetic flux origin and demise events (as defined by DeForest et al.) by tracking spatiotemporal evolution of unipolar and bipolar photospheric regions, and (2) analysis of collective behavior of the detected magnetic events using a generalized version of the Grassberger-Procaccia correlation integral algorithm. The scale-free nature of the developed algorithms makes it possible to characterize the dynamics of the photospheric network across a wide range of distances and relaxation times. Three types of photospheric conditions are considered to test the method: a quiet photosphere, a solar active region (NOAA 10365) in a quiescent non-flaring state, and the same active region during a period of M-class flares. The results obtained show (1) the presence of a topologically complex asymmetrically fragmented magnetic network in the quiet photosphere driven by meso- and supergranulation, (2) the formation of non-potential magnetic structures with complex polarity separation lines inside the active region, and (3) statistical signatures of canceling bipolar magnetic structures coinciding with flaring activity in the active region. Each of these effects can represent an unstable magnetic configuration acting as an energy source for coronal dissipation and heating.

Magnetic fields↗
DOE OSTI2016

THE POSSIBLE IMPACT OF L5 MAGNETOGRAMS ON NON-POTENTIAL SOLAR CORONAL MAGNETIC FIELD SIMULATIONS

The proposed Carrington-L5 mission would bring instruments to the L5 Lagrange point to provide us with crucial data for space weather prediction. To assess the importance of including a magnetograph, we consider the possible differences in non-potential solar coronal magnetic field simulations when magnetograph observations are available from the L5 point, compared with an L1-based field of view (FOV). A timeseries of synoptic radial magnetic field maps is constructed to capture the emergence of two active regions from the L5 FOV. These regions are initially absent in the L1 magnetic field maps, but are included once they rotate into the L1 FOV. Non-potential simulations for these two sets of input data are compared in detail. Within the bipolar active regions themselves, differences in the magnetic field structure can exist between the two simulations once the active regions are included in both. These differences tend to reduce within 5 days of the active region being included in L1. The delayed emergence in L1 can, however, lead to significant persistent differences in long-range connectivity between the active regions and the surrounding fields, and also in the global magnetic energy. In particular, the open magnetic flux and the location of open magnetic footpoints, are sensitive to capturing the real-time of emergence. These results suggest that a magnetograph at L5 could significantly improve predictions of the non-potential corona, the interplanetary magnetic field, and of solar wind source regions on the Sun.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1989

Characterization of hollow cathode, ring cusp discharge chambers

An experimental study into the effects of changes in such physical design parameters as hollow cathode position, anode position and ring cusp magnetic field configuration and strength on discharge chamber performance, is described. The results are presented in terms of comparative plasma ion energy cost, extracted ion fraction and ion beam profile data. Such comparisons are used to demonstrate specific means by which changes in these design parameters induce changes in performance, i.e., through changes in the loss rates of primary electrons to the anode, of ions to discharge chamber walls or of ions to cathode and anode surfaces. Results show: (1) the rate of primary electron loss to the anode decreases as the anode is moved downstream of the ring cusp toward the screen grid, (2) the loss rate of ions to hollow cathode surfaces are excessive if the cathode is located upstream of a point of peak magnetic flux density on the discharge chamber centerline, and (3) the fraction of the ions produced that are lost to discharge chamber walls and ring magnet surfaces is reduced by positioning the magnet rings so the plasma density is uniform over the grid surface and so there are no steep magnetic flux density gradients near the walls through which ions can be lost by Bohm diffusion. The uniformity of the plasma density at the grids can also be improved by moving the point of primary electron injection into the discharge chamber off of the chamber centerline. Other results show the discharge chamber losses decrease when a filament cathode is substituted for a hollow cathode to the extent of the hollow cathode operating power. When plasma ion energy cost is determined in such a way that the cost of operating the hollow cathode is subtracted out, the performance using either electron source is similar.

Vaughn, Jason A.↗
NASA NTRS1976

Flux-flow voltages during guided flux collapse from hollow superconducting cylinders

Voltages across diametrically opposite contact pairs on the outer surface of superconducting Nb tubes are found to depend dramatically upon the spatial configuration of the voltage-measuring leads relative to the positions of the moving magnetic flux lines. Experiments have been conducted to study these voltages for different wall thicknesses and for a variety of arrangements of the leads when flux in the hole and the wall of the Nb tube is made to exit or enter, completely or partially, by applying heat at a narrow strip along its length. Using the critical-state concept, a model for the change of flux and the resulting electric fields in the Nb tube on application of a heat pulse is presented. The resulting time-dependent and time-integrated voltages are calculated in excellent quantitative agreement with the experimental results. These results show that the flux-flow voltages across two contact points on a superconductor arise from the generation of an induced electric field over a chosen path in the superconductor between the contact points and from a change of magnetic flux through the surface bounded by the measuring leads and the chosen path in the superconductor.

Khanna, S. M.↗