Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “STELLAR ACTIVITY”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Testing photons' Bose-Einstein statistics with Compton scattering

It is an empirical question whether photons always obey Bose-Einstein statistics, but devising and interpreting experimental tests of photon statistics can be a challenge. The nonrelativistic cross section for Compton scattering illustrates how a small admixture {nu} of wrong-sign statistics leads to a loss of gauge invariance; there is a large anomalous amplitude for scattering timelike photons. Nevertheless, one can interpret the observed transparency of the solar wind plasma at low frequencies as a bound {nu}<10{sup -25} if Lorentz symmetry is required. If there is instead a universal preferred frame, the bound is {nu}<10{sup -14}, still strong compared with previous results.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Energetic Electrons Associated with Magnetic Reconnection in the Magnetic Cloud Boundary Layer

Here is reported in situ observation of energetic electrons ({approx}100-500 keV) associated with magnetic reconnection in the solar wind by the ACE and Wind spacecraft. The properties of this magnetic cloud driving reconnection and the associated energetic electron acceleration problem are discussed. Further analyses indicate that the electric field acceleration and Fermi-type mechanism are two fundamental elements in the electron acceleration processes and the trapping effect of the specific magnetic field configuration maintains the acceleration status that increases the totally gained energy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Fast Magnetic Reconnection in the Plasmoid-Dominated Regime

A conceptual model of resistive magnetic reconnection via a stochastic plasmoid chain is proposed. The global reconnection rate is shown to be independent of the Lundquist number. The distribution of fluxes in the plasmoids is shown to be an inverse-square law. It is argued that there is a finite probability of emergence of abnormally large plasmoids, which can disrupt the chain (and may be responsible for observable large abrupt events in solar flares and sawtooth crashes). A criterion for the transition from the resistive magnetohydrodynamic to the collisionless regime is provided.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Anisotropy of Imbalanced Alfvenic Turbulence in Fast Solar Wind

We present the first measurement of the scale-dependent power anisotropy of Elsasser variables in imbalanced fast solar wind turbulence. The dominant Elsasser mode is isotropic at lower spacecraft frequencies but becomes increasingly anisotropic at higher frequencies. The subdominant mode is anisotropic throughout. There are two distinct subranges exhibiting different scalings within what is normally considered the inertial range. The low Alfven ratio and the different scaling of the Elsasser modes suggests an interpretation of the observed discrepancy between the velocity and magnetic field scalings, the total energy is dominated by the latter. These results do not appear to be fully explained by any of the current theories of incompressible imbalanced MHD turbulence.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Resonance excitation of the magnetosphere by hydromagnetic waves incident from solar wind

The eigenfrequencies and eigenmodes of an MHD cavity in the front part of the magnetosphere and its excitation by monochromatic hydromagnetic waves incident onto the magnetosphere from solar wind are studied theoretically in the model of a plane-stratified plasma. The eigenmodes are damped due to both their absorption at the Alfven resonance points and their emission into solar wind through the magnetopause, which is partially transparent for the excited waves. It is shown that, due to the influence of the magnetospheric cavity, the pumping of the magnetosphere by the incident waves is resonance in character. The waves penetrate into the magnetosphere only if their frequencies lie in narrow spectral ranges near the eigenfrequencies of the cavity, the width of these ranges being on the order of the damping rate of the eigenmodes. Waves with other frequencies are almost completely reflected from the magnetopause.

58 GEOSCIENCES↗
NASA NTRS1988

Accretion disk modeling of AGN continuum using non-LTE stellar atmospheres

Active galactic nuclei (AGN) accretion disk spectra were calculated using non-LTE stellar atmosphere models for Kerr and Schwarzschild geometries. It is found that the Lyman limit absorption edge, probably the most conclusive observational evidence for the accretion disk, would be drastically distorted and displaced by the relativistic effects from the large gravitational field of the central black hole and strong Doppler motion of emitting material on the disk surface. These effects are especially pronounced in the Kerr geometry. The strength of the Lyman limit absorption is very sensitive to the surface gravity in the stellar atmosphere models used. For models at the same temperature but different surface gravities, the strength of the Lyman edge exhibits an almost exponential decrease as the surface gravity approach the Eddington limit, which should approximate the thin disk atmosphere. The relativistic effects as well as the vanishing of the Lyman edge at the Eddington gravity may be the reasons that not many Lyman edges in the rest frames of AGNs and quasars are found.

Sun, Wei-Hsin↗
DOE OSTI2025

Investigation of oscillations above the acoustic cut-off frequency for a sample of Kepler stars. II. Identifying temporal frequency shifts

ABSTRACT Stellar oscillations with frequencies greater than the acoustic cut-off frequency are not trapped within the star’s interior. Geometric interference between these high-frequency waves produces a peak-like structure in the power spectrum of the star, known as the pseudo-modes. In this study, we aim to analyse 51 stars previously identified (from a sample of 91 Kepler stars) to contain statistically significant evidence for pseudo-modes, to determine if the pseudo-mode frequencies vary in time. We obtained these frequency variations and the uncertainty by the resampled periodogram approach, where periodogram realisations were created from successive, overlapping 90-d time segments, and a cross-correlation function was generated between them. The computed shifts were compared against temporal p-mode frequency shifts and a stellar magnetic activity proxy, $S_{\mathrm{ph}}$. We found that for 16 per cent of the stars pseudo-mode frequency shifts were significantly anticorrelated with p-mode shifts, as is the case for the Sun. However, we also found pseudo-mode and p-mode shifts to be significantly correlated in-phase in 8 per cent of our sample. The remainder showed no significant correlation. We also searched for trends between the maximum variation of pseudo-mode frequency shifts and stellar parameters. Whilst we found no correlation between the maximum variation and stellar photospheric and chromospheric proxies, we found frequency variations to have a greater amplitude for cooler and slower rotating stars, which is in opposition to p-mode frequency shift variation. Temporal changes to pseudo-mode frequencies may be attributable to variations in stellar magnetic activity, and therefore can contribute to our understanding of stellar activity cycles.

Millson, L. J. (ORCID:0009000342542676)↗
NASA NTRS1998

The Fundamental Physical Processes Producing and Controlling Stellar Coronal/ Transition-Region/Chromospheric Activity and Structure

Our LTSA grant supports a long-term collaborative investigation of stellar activity. The project involves current NASA spacecraft and supporting ground-based telescopes, will make use of future missions, and utilizes the extensive archives of IUE, ROSAT, HST, and EUVE. Our interests include observational work (with a nonnegligible groundbased component); specialized processing techniques for imaging and spectral data; and semiempirical modeling, ranging from optically-thin emission measure studies to simulations of optically-thick resonance lines. Collaborations with our cool-star colleagues here in Boulder (at JILA and the High Altitude Observatory) provide access to even broader expertise, particularly on the solar corona, convection, and magnetohydrodynamic phenomena (including "dynamo" theories). The broad-brush of our investigation include the following: (1) where do coronae occur in the Hertzsprung-Russell diagram? (2) the winds of coronal stars: hot, cool, or both? (3) age, activity, rotation relations; (4) atmospheric inhomogeneities; and (5) heating mechanisms, subcoronal flows and flares. Our observation task has been to map the global properties of chromospheres and coronae in the H-R diagram and conduct detailed studies of key objects.

Ayres, Thomas R.↗
DOE OSTI2010

Scale invariance of normalized cross-helicity throughout the inertial range of solar wind turbulence

Solar wind measurements over the period from 1995 through 2006 are used to study fluctuations in the plasma bulk velocity and magnetic field over the range of magnetohydrodynamic (MHD) scales commonly referred to as the inertial range. Power spectra of the solar wind velocity, magnetic field, proton density, total energy (kinetic plus magnetic), and cross-helicity are analyzed for 176 time intervals in which the interplanetary magnetic field is restricted to a single magnetic sector. The data yield measurements of the normalized cross-helicity {sigma}{sub c}, the ratio of the cross-helicity spectrum to the energy spectrum, that span the entire inertial range at 1 AU and extend previous measurements by more than one decade in wavenumber. The results show that {sigma}{sub c} is approximately constant throughout the inertial range, independent of wavenumber, consistent with existing theoretical ideas. At the highest frequencies measured, |{sigma}{sub c}| is observed to decrease toward zero, however, this decrease is caused by measurement noise and is not a real physical effect. Other new results obtained in this study are (1) the average spectral index for the total energy--kinetic plus magnetic--is found to be closer to 3/2 than 5/3 with an average value 1.540{+-}0.033 for highly Alfvenic wind, in agreement with simulations of incompressible MHD turbulence with a strong ambient magnetic field; (2) the spectral index for the total energy is a function of |{sigma}{sub c}| that varies from roughly 5/3 when {sigma}{sub c}{approx_equal}0 to approximately 3/2 when |{sigma}{sub c}|{approx_equal}1; (3) the spectral index for the total energy exhibits a solar cycle dependence during solar cycle 23 with above average values (steeper spectra) in the early rising phase of the cycle and below average values (shallower spectra) during the declining phase of the cycle--the controlling parameter for this solar cycle dependence appears to be |{sigma}{sub c}|.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

The effect of spectral anisotropy of fast magnetosonic turbulence on the plasma heating at the proton kinetic scales

The energy transport associated with the fast magnetosonic turbulence in a low-beta plasma is one of the possible driving forces of the solar wind. Two different kinds of the turbulence spectra are discussed in this paper: a unidirectional spectrum of the fluctuations propagating away from the sun and a bidirectional spectrum composed of the sunward and antisunward modes. The evolution of the system is analyzed with the help of hybrid numerical simulations at the proton kinetic scales. It is shown that in the unidirectional case, the heating is always preferentially across the background magnetic field. It is this property that allows the solar wind plasma to be propelled away from the sun by the mirror force of the inhomogeneous magnetic field. Adding the counterpropagating modes reverses the sense of the energy deposition with the parallel heating now dominating the process. However, the parallel heating is produced mostly by fast modes propagating along the magnetic field. If these modes are eliminated, the heating becomes perpendicular again.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Wavenumber spectrum of whistler turbulence: Particle-in-cell simulation

The forward cascade of decaying whistler turbulence is studied in low beta plasma to understand essential properties of the energy spectrum at electron scales, by using a two-dimensional electromagnetic particle-in-cell (PIC) simulation. This simulation demonstrates turbulence in which the energy cascade rate is greater than the dissipation rate at the electron inertial length. The PIC simulation shows that the magnetic energy spectrum of forward-cascaded whistler turbulence at electron inertial scales is anisotropic and develops a very steep power-law spectrum which is consistent with recent solar wind observations. A comparison of the simulated spectrum with that predicted by a phenomenological turbulence scaling model suggests that the energy cascade at the electron inertial scale depends on both magnetic fluctuations and electron velocity fluctuations, as well as on the whistler dispersion relation. Thus, not only kinetic Alfven turbulence but also whistler turbulence may explain recent solar wind observations of very steep magnetic spectra at short scales.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Turbulence in Hall MHD Plasmas

The Hall MHD description of magnetized plasma is invoked in many space, laboratory and astrophysical plasmas when the characteristic frequency is bigger than the ion cyclotron frequency where the MHD description is inapplicable. In this paper, we present fully self consistent 3D simulations of compressible Hall MHD plasma to describe spectral features relevant to the solar wind plasma. We find that a k{sup -7/3} spectrum sets in for the fluctuations that are smaller than ion gyro radius. We further investigate scale dependent anisotropy led by nonlinear processes relevant to the solar wind plasma. Our work is important particularly in understanding the role of wave and nonlinear cascades in the evolution of the solar wind, structure formation at the largest scales.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

The modified two stream instability at nonmagnetic planets

We describe the role the modified two stream instability plays in the interaction of the solar wind with non-magnetized planets. The instability leads to the production of energetic electrons that can be responsible for the observed x-ray emission.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Electron Scale Solar Wind Turbulence: Cluster Observations and Theoretical Modeling

Turbulence at MagnetoHydroDynamics (MHD) scales of the solar wind has been studied for more than three decades, using data analyzes, theoretical and numerical modeling. However smaller scales have not been explored until very recently. Here, we review recent results on the first observation of cascade and dissipation of the solar wind turbulence at the electron scales. Thanks to the high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectra of turbulence up to {approx}100 Hz (in the spacecraft reference frame) and found two distinct breakpoints in the magnetic spectrum at 0.4 Hz and 35 Hz, which correspond, respectively, to the Doppler-shifted proton and electron gyroscales, f{sub {rho}p} and f{sub {rho}e}. Below f{sub {rho}p} the spectrum follows a Kolmogorov scaling f{sup -1.62}, typical of spectra observed at 1 AU. Above f{sub {rho}p} a second inertial range is formed with a scaling f{sup -2.3} down to f{sub {rho}e}. Above f{sub {rho}e} the spectrum has a steeper power law {approx}f{sup -4.1} down to the noise level of the instrument. Solving numerically the linear Maxwell-Vlasov equations combined with recent theoretical predictions of the Gyro-Kinetic theory, we show that the present results are fully consistent with a scenario of a quasi-two-dimensional cascade into Kinetic Alfven modes (KAW).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Saturn's magnetosphere: An example of dynamic planetary systems

Planetary magnetospheres are prime examples of interacting plasma regimes at different scales. There is the principal interaction with the solar wind that seems to be the main driver of the dynamics at Mercury and Earth. But these inner planet magnetospheres are relatively simple when compared to those of the outer planets which are primarily driven by planetary rotation and include internal plasma sources from various moons and rings, in addition to those from the planetary ionospheres and the solar wind. Io's volcanic source at Jupiter is a prime example, but now Enceladus at Saturn has joined the fray, while Titan is a surprisingly minor player despite its thick nitrogen atmosphere and its continued bombardment by energetic particles. Mass loading of plasma leads to interchange instability in the inner magnetospheres at both Jupiter and Saturn, while ionospheric slippage, among other processes, seems to contribute to a variable rotation period in the spin-aligned dipole field of Saturn, manifested in auroral kilometric radiation (SKR), components of the magnetic field itself, and the plasma periodicities measured at several energies. Through use of the ENA (energetic neutral atom) technique, it is now possible to observe bulk motions of the plasma and their connection to planetary auroral processes. Such imaging at Saturn by Cassini has revealed the location of a region of post-midnight acceleration events that seem to corotate with the planet and coincide with auroral brightening and SKR. Periodic injections of plasma have been identified and repeat at the Kronian rotation period of 10.8 hours. A semi-permanent but asymmetric ring current has also been imaged, located between the orbits of the satellites Rhea ({approx}9 RS) and Titan ({approx}20 R{sub S}), with a maximum at {approx}10{+-} 1R{sub S} and dominated by the hot (>3 keV) plasma component.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Droplet Charging Effects in the Space Environment

Several applications exist for transiting liquid droplets through the near-Earth space environment. Numerical results are presented for the charging of liquid droplets of trimethyl pentaphenyl siloxane (DC705) in three different plasma environments: ionosphere, auroral, and geosynchronous Earth orbit (GEO). Nominal and high geomagnetic activity cases are investigated. In general, high levels of droplet charging (>100 V) exist only in GEO during periods of high geomagnetic or solar activity. An experiment was conducted to assess the charging of silicon-oil droplets due to photoemission. The photoemission yield in the 120-200 nm wavelength range was found to be approximately 0.06.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

ACCELERATING WAVES IN POLAR CORONAL HOLES AS SEEN BY EIS AND SUMER

We present EIS/Hinode and SUMER/SOHO observations of propagating disturbances detected in coronal lines in inter-plume and plume regions of a polar coronal hole. The observation was carried out on 2007 November 13 as part of the JOP196/HOP045 program. The SUMER spectroscopic observation gives information about fluctuations in radiance and on both resolved (Doppler shift) and unresolved (Doppler width) line-of-sight velocities, whereas EIS 40'' wide slot images detect fluctuations only in radiance but maximize the probability of overlapping field of view between the two instruments. From distance-time radiance maps, we detect the presence of propagating waves in a polar inter-plume region with a period of 15-20 minutes and a propagation speed increasing from 130 {+-} 14 km s{sup -1} just above the limb to 330 {+-} 140 km s{sup -1} around 160'' above the limb. These waves can be traced to originate from a bright region of the on-disk part of the coronal hole where the propagation speed is in the range of 25 {+-} 1.3 to 38 {+-} 4.5 km s{sup -1}, with the same periodicity. These on-disk bright regions can be visualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbances with the same range of periodicity but with propagation speeds in the range of 135 {+-} 18 to 165 {+-} 43 km s{sup -1} only. A comparison between the distance-time radiance map of the two regions indicates that the waves within the plumes are not observable (may be getting dissipated) far off-limb, whereas this is not the case in the inter-plume region. A correlation analysis was also performed to find out the time delay between the oscillations at several heights in the off-limb region, finding results consistent with those from the analysis of the distance-time maps. To our knowledge, this result provides first spectroscopic evidence of the acceleration of propagating disturbances in the polar region close to the Sun (within 1.2 R/R{sub sun}), which provides clues to the understanding of the origin of these waves. We suggest that the waves are likely either Alfvenic or fast magnetoacoustic in the inter-plume region and slow magnetoacoustic in the plume region. This may lead to the conclusion that inter-plumes are a preferred channel for the acceleration of the fast solar wind.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

INHOMOGENEOUS NEARLY INCOMPRESSIBLE DESCRIPTION OF MAGNETOHYDRODYNAMIC TURBULENCE

The nearly incompressible theory of magnetohydrodynamics (MHD) is formulated in the presence of a static large-scale inhomogeneous background. The theory is an inhomogeneous generalization of the homogeneous nearly incompressible MHD description of Zank and Matthaeus and a polytropic equation of state is assumed. The theory is primarily developed to describe solar wind turbulence where the assumption of a composition of two-dimensional (2D) and slab turbulence with the dominance of the 2D component has been used for some time. It was however unclear, if in the presence of a large-scale inhomogeneous background, the dominant component will also be mainly 2D and we consider three distinct MHD regimes for the plasma beta {beta} << 1, {beta} {approx} 1, and{beta} >> 1. For regimes appropriate to the solar wind ({beta} << 1, {beta} {approx} 1), compared to the homogeneous description of Zank and Matthaeus, the reduction of dimensionality for the leading-order description from three dimensional (3D) to 2D is only weak, with the parallel component of the velocity field proportional to the large-scale gradients in density and the magnetic field. Close to the Sun, however, where the large-scale magnetic field can be considered as purely radial, the collapse of dimensionality to 2D is complete. Leading-order density fluctuations are shown to be of the order of the sonic Mach number O(M) and evolve as a passive scalar mixed by the turbulent velocity field. It is emphasized that the usual 'pseudosound' relation used to relate density and pressure fluctuations through the sound speed as {delta}{rho} = c {sup 2} {sub s{delta}}p is not valid for the leading-order O(M) density fluctuations, and therefore in observational studies, the density fluctuations should not be analyzed through the pressure fluctuations. The pseudosound relation is valid only for higher order O(M{sup 2}) density fluctuations, and then only for short-length scales and fast timescales. The spectrum of the leading-order density fluctuations should be modeled as k {sup -5/3} in the inertial range, followed by a Bessel function solution K {sub {nu}}(k), where for stationary turbulence {nu} = 1, in the viscous-convective and diffusion range. Other implications for solar wind turbulence with an emphasis on the evolution of density fluctuations are also discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗