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Results for “INTERSTELLAR MAGNETIC FIELDS”

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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Magnetized thermal conduction fronts

The evolution of planar thermal conduction fronts in the presence of a dynamically weak, but otherwise self-consistent, magnetic field is considered. The field is assumed to be connected and untangled. In the diffusion limit for the thermal conductivity, these fronts exhibit self-similar behavior, even in the presence of a field. The role of the field is restricted to channeling the heat flux along its lines of force, and it enters into the problem as a dimensionless angle variable. 'Combing' (or opening) of insulating field lines by the evaporative flow is explicitly demonstrated. Unless the field is nearly perpendicular to the front normal in the hot gas, insulating effects are not profound. Self-similarity breaks down if the front becomes saturated, and under certain conditions magnetized saturated conduction fronts cannot propagate: the solution characteristics of the wave equation form caustics. The physical resolution is the advent of two-fluid (nonlocal) heating. Such Coulomb-heated fronts are expected to be relatively rare in typical astrophysical systems. The large-scale effects of a magnetic field on cloud evaporation in the interstellar medium are briefly discussed, and it is suggested that these fields preclude the presence of time-independent evaporative solutions. Thermal interfaces may then continue to evolve until radiative cooling halts their development; large tracts of warm 10,000 K gas may result.

Balbus, S. A.↗
NASA NTRS1988

The polarization of the far-infrared radiation from the Galactic center

The first detection of linear polarization of the far-infrared (100-micron) radiation from the about 3-pc-diameter dust ring surrounding the galactic nucleus is reported. The percentage of polarization is between 1 and 2 percent at the three measured positions. It is argued that the polarized radiation is produced by thermal emission from elongated interstellar grains oriented by the local magnetic field. The dust ring is optically thin at 100 microns; therefore the observations sample dust through the entire depth of the cloud and are free of confusing effects due to embedded sources, scattering, or selective absorption. These data provide the first information about the configuration of the magnetic field in the dust ring.

Werner, M. W.↗
NASA NTRS1991

Stripped interstellar gas in cluster cooling flows

It is suggested that nonlinear perturbations which lead to thermal instabilities in cooling flows might start as blobs of interstellar gas which are stipped out of cluster galaxies. Assuming that most of the gas produced by stellar mass loss in cluster galaxies is stripped from the galaxies, the total rate of such stripping is roughly 100 solar masses/yr, which is similar to the rates of cooling in cluster cooling flows. It is possible that a substantial portion of the cooling gas originates as blobs of interstellar gas stripped from galaxies. The magnetic fields within and outside of the low-entropy perturbations may help to maintain their identities by suppressing both thermal conduction and Kelvin-Helmholtz instabilities. These density fluctuations may disrupt the propagation of radio jets through the intracluster gas, which may be one mechanism for producing wideangle-tail radio galaxies.

Soker, Noam↗
NASA NTRS1991

Cosmic rays in the Small Magellanic Cloud

The SMC is investigated in order to determine whether this galaxy is in dynamic quasi-stable equilibrium between the expansive pressure of the hot cosmic ray gas, the magnetic fields, and the kinetic motion of interstellar gas and the gravitational attraction of matter. It is argued that the cosmic ray density level is three to five times smaller than that which would exist if the galaxy were in approximate dynamic equilibrium and cosmic ray sources were adequate. The scale of coupling between the cosmic rays and matter in the projected dimension that can be observed is less than 0.5 kpc, which is rather small to be consistent with a stable cosmic ray distribution. The observed matter distribution and the calculated cosmic ray density distribution are used to determine the expected gamma ray intensity distribution.

Sreekumar, P.↗
NASA NTRS1994

Shock interactions with magnetized interstellar clouds. 1: Steady shocks hitting nonradiative clouds

We study the interaction of a steady, planar shock with a nonradiative, spherical, interstellar cloud threaded by a uniform magnetic field. For strong shocks, the sonic Mach number scales out, so two parameters determine the evolution: the ratio of cloud to intercloud density, and the Alfven Mach number. We focus on the case with initial field parallel to the shock velocity, though we also present one model with field perpendicular to the velocity. Even with 100 zones per cloud radius, we find that the magnetic field structure converges only at early times. However, we can draw three conclusions from our work. First, our results suggest that the inclusion of a field in equipartition with the preshock medium can prevent the complete destruction of the cloud found in the field-free case recently considered by Klein, McKee, & Colella. Second, the interaction of the shock with the cloud can amplify the magnetic field in some regions up to equipartition with the post-shock thermal pressure. In the parallel-field case, the shock preferentially amplifies the parallel component of the field, creating a 'flux rope,' a linear structure of concentrated magnetic field. The flux rope dominates the volume of amplified field, so that laminar, rather than turbulent, amplification is dominant in this case. Third, the presence of the cloud enhances the production of X-ray and synchrotron emission. The X-ray emission peaks early, during the initial passage of the shock over the cloud, while the synchrotron emission peaks later, when the flow sweeps magnetic field onto the axis between the cloud and the main shock.

Low, Mordecai-Mark Mac↗
DOE OSTI2011

The flow of interstellar dust through the solar system: the role of dust charging

Interstellar dust can enter the solar system through the relative motion of the Sun with respect to the Local Interstellar Cloud. The trajectories of the dust through the solar system are not only influenced by gravitation and solar radiation pressure forces, but also by the Lorentz forces due to the interaction of the interplanetary magnetic field with the charged dust particles. The interplanetary magnetic field changes on two major time scales: 25 days (solar rotation frequency) and 22 years (solar cycle). The short-term variability averages out for regions that are not too close (>{approx}2 AU) to the Sun. This interplanetary magnetic field variability causes a time-variability in the interstellar dust densities, that is correlated to the solar cycle.In this work we characterize the flow of interstellar dust through the solar system using simulations of the dust trajectories. We start from the simple case without Lorentz forces, and expand to the full simulation. We pay attention to the different ways of modeling the interplanetary magnetic field, and discuss the influence of the dust parameters on the resulting flow patterns. We also discuss the possibilities of using this modeling for prediction of dust fluxes for different space missions or planets, and we pay attention to where simplified models are justified, and where or when a full simulation, including all forces is necessary. One of the aims of this work is to understand measurements of spacecraft like Ulysses, Cassini and Stardust.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE IBEX RIBBON AND THE PICKUP ION RING STABILITY IN THE OUTER HELIOSHEATH. II. MONTE-CARLO AND PARTICLE-IN-CELL MODEL RESULTS

The nearly circular ribbon of energetic neutral atom (ENA) emission discovered by NASA’s Interplanetary Boundary EXplorer satellite ( IBEX ), is most commonly attributed to the effect of charge exchange of secondary pickup ions (PUIs) gyrating about the magnetic field in the outer heliosheath (OHS) and the interstellar space beyond. The first paper in the series (Paper I) presented a theoretical analysis of the pickup process in the OHS and hybrid-kinetic simulations, revealing that the kinetic properties of freshly injected proton rings depend sensitively on the details of their velocity distribution. It was demonstrated that only rings that are not too narrow (parallel thermal spread above a few km s{sup −1}) and not too wide (parallel temperature smaller than the core plasma temperature) could remain stable for a period of time long enough to generate ribbon ENAs. This paper investigates the role of electron dynamics and the extra spatial degree of freedom in the ring ion scattering process with the help of two-dimensional full particle-in-cell (PIC) kinetic simulations. A good agreement is observed between ring evolution under unstable conditions in hybrid and PIC models, and the dominant modes are found to propagate parallel to the magnetic field. We also present more realistic ribbon PUI distributions generated using Monte Carlo simulations of atomic hydrogen in the global heliosphere and examine the effect of both the cold ring-like and the hot “halo” PUIs produced from heliosheath ENAs on the ring stability. It is shown that the second PUI population enhances the fluctuation growth rate, leading to faster isotropization of the solar-wind-derived ring ions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The effect of new interstellar medium parameters on the heliosphere and energetic neutral atoms from the interstellar boundary

We present new results from three-dimensional simulations of the solar wind interaction with the local interstellar medium (LISM) using recent observations by NASA's Interstellar Boundary EXplorer (IBEX) mission estimates of the velocity and temperature of the LISM. We investigate four strengths of the LISM magnetic field, from 1 to 4 μG, and adjust the LISM proton and hydrogen densities so that the distance to the termination shock (TS) in the directions of the Voyager spacecraft is just below 90 AU, and the density of hydrogen at the TS is close to 0.09 cm{sup –3} in the nose direction. The orientation of the magnetic field is chosen to point toward the center of the ribbon of enhanced energetic neutral atom (ENA) flux seen in the IBEX data. Our simulations show that the plasma and neutral properties in the outer heliosheath vary considerably as a function of the LISM magnetic field strength. We also show that the heliotail points downwind in all cases, though its structure is strongly affected by the external magnetic field. Comparison and consistency between the simulated ENA flux and the circularity of the ribbon as measured by IBEX are most consistent with a LISM magnetic field strength aligned with the center of the ribbon and a magnitude in the range 2.5-3 μG.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Applying the Velocity Gradient Technique in NGC 1333: Comparison with Dust Polarization Observations

Magnetic fields (B-fields) are ubiquitous in the interstellar medium (ISM), and they play an essential role in the formation of molecular clouds and subsequent star formation. However, B-fields in interstellar environments remain challenging to measure, and their properties typically need to be inferred from dust polarization observations over multiple physical scales. In this work, we seek to use a recently proposed approach called the velocity gradient technique (VGT) to study B-fields in star-forming regions and compare the results with dust polarization observations in different wavelengths. The VGT is based on the anisotropic properties of eddies in magnetized turbulence to derive B-field properties in the ISM. We investigate that this technique is synergistic with dust polarimetry when applied to a turbulent diffused medium for the purpose of measuring its magnetization. Specifically, we use the VGT on molecular line data toward the NGC 1333 star-forming region ( 12 CO, 13 CO, C 18 O, and N 2 H + ), and we compare the derived B-field properties with those inferred from 214 and 850 μm dust polarization observations of the region using Stratospheric Observatory for Infrared Astronomy/High-Resolution Airborne Wide-band Camera Plus and James Clerk Maxwell Telescope/POL-2, respectively. We estimate both the inclination angle and the 3D Alfvénic Mach number M A from the molecular line gradients. Crucially, testing this technique on gravitationally bound, dynamic, and turbulent regions, and comparing the results with those obtained from polarization observations at different wavelengths, such as the plane-of-sky field orientation, is an important test on the applicability of the VGT in various density regimes of the ISM. We in general do not find a close correlation between the velocity gradient inferred orientations and the dust inferred magnetic field orientations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EVIDENCE FOR A SHOCK IN INTERSTELLAR PLASMA: VOYAGER 1

Voyager 1 (V1) observed electron plasma oscillations preceding a jump by a factor of 1.4 in the magnetic field intensity B near the end of 2012. The frequency of the electron plasma oscillations gives an electron density n{sub e} = 0.05 cm{sup –3}, which implies that V1 was immersed in plasma from the interstellar medium. The last day on which plasma oscillations were observed is day 332, 2012, and the jump in the B was centered on day 335, 2012 after a data gap in the wave data. The close association between the electron plasma oscillations and the jump in B suggests a causal connection, such as that frequently observed between electron plasma oscillations and interplanetary shocks at 1 AU. Based on the observed parameters and the smooth profile of B(t), the jump in B appears to be associated with a weak, subcritical, laminar, low beta, quasi-perpendicular, resistive, collisionless shock. However, the width of the jump is of the order of 10{sup 4} times that expected for such a stationary shock at 1 AU. The large width of the jump in B might be the result of differences between the structure of shocks in the interstellar medium and the plasma near 1 AU. Alternatively, the subcritical resistive shock might have decayed during a few days after producing the plasma waves, leaving a broad profile in B(t) without significantly changing ambient parameters. Another possibility is that the jump in B is a pressure wave.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Alignment of the magnetic field in star-forming regions and why it might be difficult to observe

ABSTRACT Magnetic fields are an important component of the interstellar medium (ISM) and exhibit strongly varying field strengths and a non-trivial correlation with the gas density. Its dynamical impact varies between individual regions of the ISM and correlates with the orientation of the field with respect to the gas structures. Using high-resolution magneto-hydrodynamical simulations of the ISM, we explore the connection between the orientation of the field and the dynamical state of the gas. We find that the onset of gravitational instability in molecular gas above a density of $\rho \sim 10^{-21}\, \mathrm{g\, cm}^{-3}$$(n\sim 400\, \mathrm{cm}^{-3})$ coincides with an alignment of the magnetic field lines and the gas flow. At this transition, the gradient of the density changes from mainly perpendicular to preferentially parallel to the field lines. A connection between the three-dimensional alignment and projected two-dimensional observables is non-trivial, because of a large dispersion of the magnetic field orientation along the line of sight. The turbulent correlation lengths can be small compared to the typical integration lengths. As a consequence, the small-scale signal of the orientation can sensitively depend on the line of sight or the dynamical state of the cloud can fluctuate stochastically or be completely averaged out. With higher spatial resolution more small-scale structures are resolved, which aggravates the link between magneto-hydrodynamical quantities and projected observables.

Girichidis, Philipp (ORCID:0000000293009914)↗
DOE OSTI2022

Reynolds number dependence of Lagrangian dispersion in direct numerical simulations of anisotropic magnetohydrodynamic turbulence

Large-scale magnetic fields thread through the electrically conducting matter of the interplanetary and interstellar medium, stellar interiors and other astrophysical plasmas, producing anisotropic flows with regions of high-Reynolds-number turbulence. It is common to encounter turbulent flows structured by a magnetic field with a strength approximately equal to the root-mean-square magnetic fluctuations. In this work, direct numerical simulations of anisotropic magnetohydrodynamic (MHD) turbulence influenced by such a magnetic field are conducted for a series of cases that have identical resolution, and increasing grid sizes up to $2048^3$ . The result is a series of closely comparable simulations at Reynolds numbers ranging from 1400 up to 21 000. We investigate the influence of the Reynolds number from the Lagrangian viewpoint by tracking fluid particles and calculating single-particle and two-particle statistics. The influence of Alfvénic fluctuations and the fundamental anisotropy on the MHD turbulence in these statistics is discussed. Single-particle diffusion curves exhibit mildly superdiffusive behaviours that differ in the direction aligned with the magnetic field and the direction perpendicular to it. Competing alignment processes affect the dispersion of particle pairs, in particular at the beginning of the inertial subrange of time scales. Scalings for relative dispersion, which become clearer in the inertial subrange for a larger Reynolds number, can be observed that are steeper than indicated by the Richardson prediction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1992

On the virial theorem for turbulent molecular clouds

An Eulerian, rather than Lagrangian, form of the virial theorem is derived for a turbulent, magnetized cloud embedded in a steady, turbulent, low-density intercloud medium. The role of turbulent pressure in cloud confinement is clarified, and it is shown that, in the absence of a magnetic field, a cloud can be at a somewhat lower pressure than the intercloud medium. Simple forms for the magnetic term in the virial equation are obtained. Radiation pressure is considered; its effects are relatively small under average conditions in the interstellar medium. Under typical conditions, external pressure and magnetic fields are shown to have a relatively small effect on virial estimates of the mass of self-gravitating clouds.

Mckee, Christopher F.↗
DOE OSTI2015

ENERGETIC PARTICLE ANISOTROPIES AT THE HELIOSPHERIC BOUNDARY. II. TRANSIENT FEATURES AND RIGIDITY DEPENDENCE

In the preceding paper, we showed that large second-order anisotropies of heliospheric ions measured by the Voyager 1 space probe during the August 2012 boundary crossing event could be explained by a magnetic shear across the heliopause preventing particles streaming along the magnetic field from escaping the inner heliosheath. According to Stone et al., the penetration distance of heliospheric ions into the outer heliosheath had a strong dependence on the particle’s Larmor radius. By comparing hydrogen, helium, and oxygen ions with the same energy per nucleon, these authors argued that this effect must be attributed to larger cyclotron radii of heavier species rather than differences in velocity. We propose that gradient drift in a nonuniform magnetic field was the cause of both the large second-order anisotropies and the spatial differentiation based on the ion’s rigidity. A latitudinal gradient of magnetic field strength of about 10% per AU between 2012.7 and 2012.9 could have provided drift motion sufficient to match both LECP and CRS Voyager 1 observations. We explain the transient intensity dropout observed prior to the heliocliff using flux tube structures embedded in the heliosheath and magnetically connected to interstellar space. Finally, this paper reports a new indirect measurement of the plasma radial velocity at the heliopause on the basis of the time difference between two cosmic-ray telescopes measuring the same intensity dropout.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

Flow downstream of the heliospheric terminal shock: Magnetic field line topology and solar cycle imprint

The topology of the magnetic field in the heliosheath is illustrated using plots of the field lines. It is shown that the Archimedean spiral inside the terminal shock is rotated back in the heliosheath into nested spirals that are advected in the direction of the interstellar wind. The 22-year solar magnetic cycle is imprinted onto these field lines in the form of unipolar magnetic envelopes surrounded by volumes of strongly mixed polarity. Each envelope is defined by the changing tilt of the heliospheric current sheet, which is in turn defined by the boundary of unipolar high-latitude regions on the Sun that shrink to the pole at solar maximum and expand to the equator at solar minimum. The detailed shape of the envelopes is regulated by the solar wind velocity structure in the heliosheath.

Nerney, Steven↗
DOE OSTI2017

Theoretical Description of GRB 160625B with Wind-to-ISM Transition and Implications for a Magnetized Outflow

GRB 160625B, one of the brightest bursts in recent years, was simultaneously observed by Fermi and Swift satellites, and ground-based optical telescopes in three different events separated by long periods of time. In this paper, the non-thermal multiwavelength observations of GRB 160625B are described and a transition phase from wind-type-like medium to interstellar medium (ISM) between the early (event II) and the late (event III) afterglow is found. The multiwavelength observations of the early afterglow are consistent with the afterglow evolution starting at ∼150 s in a stellar wind medium, whereas the observations of the late afterglow are consistent with the afterglow evolution in ISM. The wind-to-ISM transition is calculated to be at ∼8×10{sup 3} s when the jet has decelerated, at a distance of ∼1 pc from the progenitor. Using the standard external shock model, the synchrotron and synchrotron self-Compton emission from reverse shock is required to model the GeV γ-ray and optical observations in the early afterglow, and synchrotron radiation from the adiabatic forward shock to describe the X-ray and optical observations in the late afterglow. The derived values of the magnetization parameter, the slope of the fast decay of the optical flash, and the inferred magnetic fields suggest that Poynting flux-dominated jet models with arbitrary magnetization could account for the spectral properties exhibited by GRB 160625B.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1976

Nonhomologous contraction and equilibria of self-gravitating, magnetic interstellar clouds embedded in an intercloud medium: Star formation. II - Results

The paper studies the equilibrium of self-gravitating isothermal models of interstellar clouds with a frozen-in magnetic field linked smoothly to the field of a hot tenuous intercloud medium. Equilibrium states are determined which can be reached by clouds contracting nonhomologously from a spherical uniform initial state. Three free parameters characterize the problem: a dimensionless initial radius related to the Jeans length of the cloud, the initial ratio of the magnetic and gas pressures in the cloud, and the initial ratio of the intercloud and cloud pressures. The dependence of the solutions on each of these parameters is investigated. It is found that: (1) the frozen-in field causes the cloud to become oblate with its major axis normal to the field lines; (2) the flattening increases as the magnetic-field strength, gravitational forces, or intercloud pressure increases; (3) increasing intercloud pressure eventually leads to gravitational collapse; and (4) the cloud can reach equilibrium only if its radius does not exceed some critical value. The observed inefficiency of the star-formation process within massive clouds is examined and explained in terms of magnetic phenomena in a collapsing cloud.

Mouschovias, T. C.↗
NASA NTRS1979

'Coronae' of rotating interstellar clouds

This letter considers differential rotation of cool interstellar clouds in the presence of internal magnetic fields, and shows that because of the relative ineffectiveness of field dissipation within the clouds, magnetized gas experiences buoyant forces. The resulting field loops emerge from the cloud and dissipate their energy by field reconnection. The consequent heating is sufficient to produce relatively hot (T approximately 10,000 K) 'coronae' about the clouds.

Rosner, R.↗