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All sourcesNASA NTRSDOE OSTI
NASA NTRS2017

Solar Magnetic Activity and Solar-Stellar Connections

Prediction of solar magnetic activity on various temporal scales is a fundamental element of space weather, which requires a wide range of theoretical and observational expertise in solar phenomena from the deep interior to the corona. Historical observations have revealed many features of cyclic variations of the solar activity; but these data are dramatically insufficient to draw a physical picture of global magnetic field evolution. New observational data, currently available from space missions and ground-based observatories, provide us with detailed information about solar dynamics and magnetism. However, because of the relatively short duration of data series and the great variety of data types and quality, it is challenging to assimilate these data in theoretical models and make reliable forecasts. The recent unexpectedly weak solar activity cycles, as well as observations of rotational and magnetic topology transitions in solar-type stars, suggest that the Sun and its magnetic dynamo are currently in a very interesting evolutionary stage. This could relate to the difficulty in getting a model of the Sun to produce solar-like – rather than anti-solar-like – differential rotation, to reproduce the rotation profile obtained from helioseismology, and to predict solar activity cycles.

magnetic↗
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 NTRS1997

Gamma-Ray Bursts and Relativistic MHD around Coalescing Neutron Stars

In this proposal we performed the first relativistic hydrodynamic simulations of the binary neutron stars near coalescence. Calculations were performed in three spatial dimensions and were supplemented with magnetohydrodynamic simulations of the magnetic reconnection as the neutron star fluid responds to the relativistic forces of the binary. The ultimate goal of the proposed work was to develop a theoretical model for the temporal and spectral evolution of cosmological gamma-ray bursts produced during the magnetic reconnection. This is the first time such hydrodynamic calculations were performed and some surprising results were obtained. The neutrons stars were observed to compress and heat in the simulations. They could even collapse to black holes many orbits before coalescence. The question then arose as to whether this strange phenomenon could power a gamma-ray burst. The bulk of computing effort in this proposal was therefore devoted to an analysis of the implications of this possible new gamma-ray burst paradigm. The initial focus of the work performed under this proposal was therefore to utilize (3+1) dimensional and spherical numerical general relativistic hydrodynamics to study the origin, evolution, and parametric sensitivity of the collapse instability. We derived modified conditions of hydrostatic equilibrium for stars in the curved space of quasi-static orbits. We followed the magnetic field evolution for approximately one orbit (10 msec). The magnetic field energy was observed to exponentiate with an e-folding time of about 1 msec. Thus, the field should build up very quickly to a magnitude such that reconnection and back reaction of the fluid inhibits further growth.

Mathews, Grant J.↗
NASA NTRS1989

On the possibility of detecting weak magnetic fields in variable white dwarfs

It is suggested that 'weak' magnetic fields of strengths less than 10 to the 6th G may be detectable in some variable white dwarfs. Weak fields can cause subtle changes in the Fourier power spectra of these stars in the form of 'splitting' in frequency of otherwise degenerate signals. Present-day observational and analysis techniques are capable of detecting these changes. It is suggested suggested, by listing some well-studied candidate stars, that perhaps the magnetic signature of splitting has already been observed in at least one object and that the difficult task of intensive measurements of weak fields should now be undertaken of those candidates.

Jones, Philip W.↗
DOE OSTI2017

Magnetar-like X-Ray Bursts Suppress Pulsar Radio Emission

Rotation-powered pulsars and magnetars are two different observational manifestations of neutron stars: rotation-powered pulsars are rapidly spinning objects that are mostly observed as pulsating radio sources, while magnetars, neutron stars with the highest known magnetic fields, often emit short-duration X-ray bursts. Here, we report simultaneous observations of the high-magnetic-field radio pulsar PSR J1119−6127 at X-ray, with XMM-Newton and NuSTAR , and at radio energies with the Parkes radio telescope, during a period of magnetar-like bursts. The rotationally powered radio emission shuts off coincident with the occurrence of multiple X-ray bursts and recovers on a timescale of ∼70 s. These observations of related radio and X-ray phenomena further solidify the connection between radio pulsars and magnetars and suggest that the pair plasma produced in bursts can disrupt the acceleration mechanism of radio-emitting particles.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Lower bound for the neutrino magnetic moment from kick velocities induced at the birth of neutron stars

We show that the neutrino chirality flip, which can take place in the core of a neutron star at birth, is an efficient process to allow neutrinos to anisotropically escape, thus providing a to induce the neutron star kick velocities. The process is not subject to the no-go theorem since although the flip from left- to right-handed neutrinos happens at equilibrium, the reverse process does not take place given that right-handed neutrinos do not interact with matter and therefore detailed balance is lost. For simplicity, we model the neutron star core as being made of strange quark matter. We find that the process is efficient when the neutrino magnetic moment is not smaller than [Formula: see text], where [Formula: see text] is the Bohr magneton. When this lower bound is combined with the most stringent upper bound, which uses the luminosity data obtained from the analysis of SN 1987A, our results set a range for the neutrino magnetic moment given by [Formula: see text]. The obtained kick velocities for natal conditions are consistent with the observed ones and span the correct range of radii for typical magnetic field intensities.

Physics↗
NASA NTRS1989

Relations between the photospheric magnetic field and the emission from the outer atmospheres of cool stars. I - The solar Ca II K line core emission

Observations of a solar active region complex and its surroundings are used to establish a quantitative relation between the Ca II K line core intensity and magnetic flux density. The Ca II K line core intensity is transformed to a Ca II H + K line core flux density to facilitate a comparison of solar and stellar data. A new absolute calibration for the Mount Wilson Ca II H + K fluxes for G-type dwarfs is derived. The minimum Ca II K flux, found in the centers of supergranulation cells in quiet regions on the sun, is identical to the minimum flux that is observed for solar-type stars. An expression is presented for the nonlinear trend between the Ca II H + K line core excess flux density and the absolute value of the magnetic flux density. Models that explain the nonlinearity of the mean trend and the large intrinsic scatter about it are discussed. The solar data define a relation that is similar to the relation between stellar hemisphere-average magnetic flux densities and Ca II H + K excess flux densities.

Schrijver, C. J.↗
NASA NTRS1996

Imaging Polarized Dust Emission in Star Formation Regions with the OVRO MM Array

We present OVRO interferometric observations of linearly polarized emission from magnetically aligned dust grains which allow the magnetic field geometry in nearby star formation regions to be probed on scales ranging from 100 to 3000 AU. Current results include observations of the young stellar objects NGC1333/IRAS 4A, IRAS 16293-2422 and Orion IRc2-KL.

Akeson, Rachel↗
DOE OSTI2016

ASTEROSEISMIC SIGNATURES OF EVOLVING INTERNAL STELLAR MAGNETIC FIELDS

Recent asteroseismic analyses indicate the presence of strong ( B ≳ 10{sup 5} G) magnetic fields in the cores of many red giant stars. Here, we examine the implications of these results for the evolution of stellar magnetic fields, and we make predictions for future observations. Those stars with suppressed dipole modes indicative of strong core fields should exhibit moderate but detectable quadrupole mode suppression. The long magnetic diffusion times within stellar cores ensure that dynamo-generated fields are confined to mass coordinates within the main-sequence (MS) convective core, and the observed sharp increase in dipole mode suppression rates above 1.5 M {sub ⊙} is likely explained by the larger convective core masses and faster rotation of these more massive stars. In clump stars, core fields of ∼10{sup 5} G can suppress dipole modes, whose visibility should be equal to or less than the visibility of suppressed modes in ascending red giants. High dipole mode suppression rates in low-mass ( M ≲ 2 M {sub ⊙}) clump stars would indicate that magnetic fields generated during the MS can withstand subsequent convective phases and survive into the compact remnant phase. Finally, we discuss implications for observed magnetic fields in white dwarfs and neutron stars, as well as the effects of magnetic fields in various types of pulsating stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Measurements of the chiral magnetic effect with background isolation in 200 GeV Au+Au collisions at STAR

Using two novel methods, pair invariant mass (m inv ) and comparative measurements with respect to reaction plane (Ψ RP ) and participant plane (Ψ PP ), we isolate the possible chiral magnetic effect (CME) from backgrounds in 200 GeV Au+Au collisions at STAR. The invariant mass method identifies the resonance background contributions, coupled with the elliptic flow (ν 2 ), to the charge correlator CME observable (Δγ). At high mass (m inv > 1.5 GeV/c 2 ) where resonance contribution is small, we obtain the average Δγ magnitude. In the low mass region (m inv > 1.5 GeV/c 2 ), resonance peaks are observed in Δγ(m inv ). An event shape engineering (ESE) method is used to model the background shape in m inv to extract the potential CME signal at low m inv . In the comparative method, the Ψ RP is assessed by spectator neutrons measured by the ZDCs, and the Ψ PP by the 2 nd -harmonic event plane measured by the TPC. The ν 2 is stronger along Ψ PP and weaker along Ψ RP ; in contrast, the magnetic field, mainly from spectator protons, is weaker along Ψ PP and stronger along Ψ RP . As a result, the Δγ measured with respect to Ψ RP and Ψ PP contain different amounts of CME and background, and can thus determine these two contributions. It is found that the possible CME signals with background isolation by these two novel methods are small, on the order of a few percent of the inclusive Δγ measurements.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1977

Binary X-ray pulsars

Recent progress in the study of the pulse profiles of the nine known binary X-ray pulsars is discussed. The sources considered include SMC X-1, Her X-1, Cen X-3, A0535+26, GX1+4, 3U 0900-40, A1118-61, GX301-2, and 3U 0352+30. Sample X-ray profiles for these sources are provided, and general features exhibited by the profiles are identified. It is noted that the apparent existence of a gap in the period distribution of X-ray pulsars, in the range from about 5 to 100 sec, seems to be genuine within obvious statistical limitations and is probably not an observational effect. Two explanations for the production of X-ray pulsations are examined: one requiring a temporal modulation of mass accretion onto the surface of a neutron star and another requiring the funnelling of matter along magnetic field lines to the neutron star's surface.

Rappaport, S.↗
NASA NTRS1981

No evidence of rings around Neptune

The results of two observations of stellar occultations of Neptune to determine if the planet has a ring system are reported. The sightings were made from Mt. Stromlo, Mauna Kea, and Cerro Tololo, noting that an equatorial ring would subtend only two arcsec of view. An upper accretion limit was defined to set the region around Neptune where rings, rather than satellites, could form. The intensities of the starlight from the two selected stars were recorded by photometers on magnetic tape during the occultation period. One of the stars did not occult, but passed through the entire region where a ring system might be present. No definitive evidence for rings was found, although an optical depth for a Neptunian ring was calculated at 0.07, with a width of more than 5 km and a radius of 31,400 km.

Elliot, J. L.↗
NASA NTRS1985

The modelling of latitudinal gradients in the solar wind in the outer solar system

A steady, axisymmetric, quasi-radial, global model is developed for thermally driven stellar winds with embedded magnetic fields. The asymptotic, linear results are presented for 0(1) latitudinal variations in the radial magnetic field, mass-loss rate, and radial velocity of the wind. The MHD equations are solved for the latitudinal dependence of the rotational velocity and magnetic field. They are driven by the meridional flows that develop naturally from internal magnetic stresses. Most flows open flux tubes in the stars equatorial plane, redistributing mass and magnetic flux as a function of stellar latitude. The plasma spins up to conserve angular momentum in fields and plasma.

Nerney, S.↗
NASA NTRS2013

XIPE the X-Ray Imaging Polarimetry Explorer

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 in the 210 keV band in 105 s for pointed observations, and 0.6 for an X10 class solar flare in the 1535 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin 14.7 arcmin. The spectral resolution is 20 at 6 keV and the time resolution is 8 s. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut fr extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 of the net observing time.

Astronomy X-ray Polarimetry↗
DOE OSTI2014

Magnetohydrodynamics and deep mixing in evolved stars. I. Two- and three-dimensional analytical models for the asymptotic giant branch

The advection of thermonuclear ashes by magnetized domains emerging near the H shell was suggested to explain asymptotic giant branch (AGB) star abundances. Here we verify this idea quantitatively through exact MHD models. Starting with a simple two-dimensional (2D) geometry and in an inertia frame, we study plasma equilibria avoiding the complications of numerical simulations. We show that below the convective envelope of an AGB star, variable magnetic fields induce a natural expansion, permitted by the almost ideal MHD conditions, in which the radial velocity grows as the second power of the radius. We then study the convective envelope, where the complexity of macroturbulence allows only for a schematic analytical treatment. Here the radial velocity depends on the square root of the radius. We then verify the robustness of our results with 3D calculations for the velocity, showing that for both studied regions the solution previously found can be seen as a planar section of a more complex behavior, in which the average radial velocity retains the same dependency on the radius found in 2D. As a final check, we compare our results to approximate descriptions of buoyant magnetic structures. For realistic boundary conditions, the envelope crossing times are sufficient to disperse in the huge convective zone any material transported, suggesting magnetic advection as a promising mechanism for deep mixing. The mixing velocities are smaller than for convection but larger than for diffusion and adequate for extra mixing in red giants.

79 ASTRONOMY AND ASTROPHYSICS↗