Engineering topics

Rosner, R.

Publications and source records attributed to Rosner, R..

At least 55 records · Page 3

NASA NTRS1985

On magnetohydrodynamic thermal instabilities in magnetic flux tubes

The stability of current-driven filamentary modes in magnetic flux tubes embedded in a plane-parallel atmosphere in LTE and in hydrostatic equilibrium is discussed. Within the tube, energy transport by radiation only is considered. The dominant contribution to the opacity is due to H- ions and H atoms (in the Paschen continuum). A region in the parameter space of the equilibrium configuration in which the instability is effective is delimited, and the relevance of this process for the formation of structured coronae in late-type stars and accretion disks is discussed.

Massaglia, S.↗

NASA NTRS1985

The equilibrium structure of thin magnetic flux tubes. I

A model atmosphere within a thin magnetic flux tube that is embedded in an arbitrarily stratified medium is presently constructed by solving the radiative transfer equation in the two-stream approximation for gray opacity, under the assumption that the magnetic field is sufficiently strong to warrant the neglect of both thermal conduction and convective diffusion; energy inside the flux tube therefore being transported solely by radiation. The structure of the internal atmosphere is determined on the basis of the hydrostatic and radiative equilibrium conditions of the tube embedded in an external atmosphere. The gas temperature along the axis of the tube is computed, and the geometry of the flux tube is determined on the basis of magnetic flux conservation and total pressure equilibrium.

Ferrari, A.↗

NASA NTRS1985

On the solution topologies of polytropic winds

Steady polytropic wind flows are studied by examining their solution topologies. The problem of whether the degeneracy of the continuous and standing shocked solutions in the isothermal case persist in the more general case of a polytropic expanding atmosphere is addressed. The effects of departures from spherical symmetry and nonthermal momentum addition are considered within the context of the new class of magnetospheric models proposed by Low and Tsinganos (1985). The flow near the boundary of a polar coronal hole is considered for the specific case of a magnetosphere in which this boundary asymptotically approaches the equator at large heliocentric distances.

Bailyn, C.↗

NASA NTRS1985

The X-ray spectra and the rotation-activity connection of RS CVN binaries

Results from a survey of RS CVn binaries which were observed with the Imaging Proportional Counter (IPC) on board the Einstein Observatory are presented. Spectral analyses of the IPC pulse height spectra show that the coronae of RS CVn binaries always contain hot gas with temperatures 10 million K, similar to active late-type main sequence stars, and that at least 2 temperature components are necessary to account for the higher quality IPC spectra (when absorption is unimportant). It is argued that these bimodal temperature distributions indicate true distributions of emission measure vs temperature that are continuous (just as is the case for magnetically-confined coronal plasma loops observed on the Sun). It is shown that none of the derivable X-ray characteristics of RS CVn binaries depend on rotation period, implying that claims of period-activity relationships in RS CVn binaries are unfounded.

Majer, P.↗

NASA NTRS1985

Einstein X-ray survey of the Pleiades - The dependence of X-ray emission on stellar age

The data obtained with two pointed observations of 1 deg by 1 deg fields of the Pleiades region have been analyzed, and the results are presented. The maximum-likelihood X-ray luminosity functions for the Pleiades G and K stars in the cluster are derived, and it is shown that, for the G stars, the Pleiades X-ray luminosity function is significantly brighter than the corresponding function for Hyades G dwarf stars. This finding indicates a dependence of X-ray luminosity on stellar age, which is confirmed by comparison of the same data with median X-ray luminosities of pre-main sequence and local disk population dwarf G stars. It is suggested that the significantly larger number of bright X-ray sources associated with G stars than with K stars, the lack of detection of M stars, and the relatively rapid rotation of the Pleiades K stars can be explained in terms of the onset of internal differential rotation near the convective envelope-radidative core interface after the spin-up phase during evolution to the main sequence.

Micela, G.↗

NASA NTRS1985

Magnetohydrodynamic thermal instabilities in cool inhomogeneous atmospheres

The stability of magnetic loops to current-driven filamentation instabilities is investigated. The unperturbed atmosphere is assumed to be composed of an (upper) isothermal optically thin low-density portion and a (lower) higher-density portion which is in radiative equilibrium; in both cases, the atmosphere is in hydrostatic equilibrium, so that gravitational stratification is taken into account. In order to provide specific equilibrium conditions for evaluation of the dispersion relation, conditions appropriate for the surface of a solar-type star are adopted; i.e., a fairly low temperature (T = 5000 K) appropriate for a 'precoronal' state associated, for example, with magnetic flux emerging from photospheric levels under the action of magnetic buoyancy. A linear stability analysis is performed, and numerical results show that physically plausible current densities, which would be generated by typical loop-footpoint motions, are effective in driving MHD instabilities in such a plasma. The instability growth rates are strongly dependent on the assumed current density distribution and on the density scale height.

Bodo, G.↗

NASA NTRS1985

Propagation of nonlinear, radiatively damped longitudinal waves along magnetic flux tubes in the solar atmosphere

For solar magnetic flux tubes three types of waves are compared: longitudinal MHD tube waves, acoustic tube waves propagating in the same tube geometry but with rigid walls and ordinary acoustic waves in plane geometry. It is found that the effect of the distensibility of the tube is small and that longitudinal waves are essentially acoustic tube waves. Due to the tube geometry there is considerable difference between longitudinal waves or acoustic tube waves and ordinary acoustic waves. Longitudinal waves as well as acoustic tube waves show a smaller amplitude growth, larger shock formation heights, smaller mean chromospheric temperature but a steeper dependence of the temperature gradient on wave period.

Herbold, G.↗

NASA NTRS1985

An Einstein Observatory X-ray survey of main-sequence stars with shallow convection zones

The results of an X-ray survey of bright late A and early F stars on the main B-V sequence between 0.1 and 0.5 are presented. All the stars were observed with the Einstein Observatory for a period of at least 500 seconds. The survey results show significantly larger X-ray luminosities for the sample binaries than for the single stars. It is suggested that the difference is due to the presence of multiple X-ray sources in binaries. It is shown that the X-ray luminosities for single stars increase rapidly with increasing color, and that the relation Lx/Lbol is equal to about 10 to the -7th does not hold for A stars. No correlation was found between X-ray luminosity and projected equatorial rotation velocity. It is argued on the basis of the observations that X-ray emission in the sample stars originated from coronae. The available observational evidence supporting this view is discussed.

Schmitt, J. H. M. M.↗

NASA NTRS1985

The X-ray corona of Procyon

X-ray emission from the nearby system Procyon A/B (F5 IV + DF) was detected, using the IPC (Imaging Proportional Counter) on board the Einstein Observatory. Analysis of the X-ray pulse height spectrum suggests that the observed X-ray emission originates in Procyon A rather than in the white dwarf companion Procyon B, since the derived X-ray temperature, log T = 6.2, agrees well with temperatures found for quiescent solar X-ray emission. Modeling Procyon's corona with loops characterized by some apex temperature Tmax and emission length scale L, it is found that Tmax is well constrained, but L, and consequently the filling factor of the X-ray emitting gas, are essentially unconstrained even when EUV emission from the transition region is included in the analysis.

Schmitt, J. H. M. M.↗

NASA NTRS1985

On stellar X-ray emission

Stellar X-ray astronomy represents an entirely new astronomical discipline which has emerged during the past five years. It lies at the crossroads of solar physics, stellar physics, and general astrophysics. The present review is concerned with the main physical problems which arise in connection with a study of the stellar X-ray data. A central issue is the extent to which the extrapolation from solar physics is justified and the definition (if possible) of the limits to such extrapolation. The observational properties of X-ray emission from stars are considered along with the solar analogy and the modeling of X-ray emission from late-type stars, the modeling of X-ray emission from early-type stars, the physics of stellar X-ray emission, stellar X-ray emission in the more general astrophysical context, and future prospects.

Rosner, R.↗

NASA NTRS1984

Temporal evolution of the solar wind and the formation of a standing shock

The temporal evolution of the solar wind from one steady state to another is explored when momentum deposition produces multiple critical points in the flow. It is shown that the wind always evolves in time to a new steady state compatible with the solution of the steady state equation of motion. However, for the same initial state and identical asymptotic momentum deposition rate, the temporal evolution pattern of the wind depends on the detailed time history of momentum addition and is therefore not unique. This feature plays an important role in the particular case when multiple (three in this study) steady states exist for identical boundary conditions; each one of these solutions is thus shown to be physically accessible. The details of the temporal evolution pattern of the wind reveal the formation of a shock discontinuity whenever the flow becomes supersonic at a critical point upstream from the initial critical point. If the flow remains supersonic at that inner critical point, the shock can become a standing one, depending on the strength and the temporal history of momentum addition. The results of this study indicate that the time scale required for the solar wind to evolve between steady states is of the order of 30-60 hours. Furthermore, the results also reveal the interesting and novel phenomenon that a standing shock is likely to develop in the inner solar wind flow within this time frame, in particular, in coronal hole regions with rapidly diverging geometries.

Habbal, S. R.↗

NASA NTRS1984

Equilibrium model of thin magnetic flux tubes

The existence of a physically realizable domain in which approximations that lead to a self consistent solution for flux tube stratification in the solar atmosphere, without ad hoc hypotheses, is proved. The transfer equation is solved assuming that no energy transport other than radiative is present. Convective motions inside the tube are assumed to be suppressed by magnetic forces. Only one parameter, the plasma beta at tau = 0, must be specified, and this can be estimated from observations of spatially resolved flux tubes.

Bodo, G.↗

NASA NTRS1984

The appearance of magnetic flux on the surfaces of the early main-sequence F stars

Available chromospheric, transition region, and coronal observations of the early main-sequence F stars are examined in order to find that while these objects exhibit enhanced levels of magnetic field-related radiative emissions, significant inhomogeneities in surface activity are not present. This phenomenon is discussed within the context of the calculations published by Schmitt and Rosner (1983) for the production of flux ropes of various spatial scales at a given rotation rate at the bottom of a stellar convection zone. It is found that the spatial scales and area contrast of emergent magnetic flux in these stars that, as a class, are characterized by rapid rotation and thin convection zones are substantially reduced relative to that of the sun.

Giampapa, M. S.↗

NASA NTRS1984

The overshoot region at the bottom of the solar convection zone

The extent and thermal stratification of the region of convective overshoot underneath the convection zone of the sun are investigated. The phenomenon of convective overshoot in general is discussed, and some of the modal and model approaches to studying it are briefly reviewed. A detailed theoretical description of the motion of plumes in a stably stratified medium is given, leading to a 'derivation' of the plume equations from the hydrodynamic equations. Entrainment is discussed, and it is shown how the plume equations can be used to compute convective overshoot in the sun. The limitations of the plume model are addressed, arguing that a thin boundary layer must exist which separates convective and radiative regions. The results of numerical integrations of the plume equations, as applied to the region of convective overshoot underneath the solar convective zone, are discussed.

Schmitt, J. H. M. M.↗

NASA NTRS1984

On the topological stability of magnetostatic equilibria

The topological stability of MHD equilibria is investigated by exploring the formal analogy, in the ideal MHD limit, between the topology of magnetic lines of force in coordinate space and the topology of integral surfaces of one- and two-dimensional Hamiltonian systems in phase space. It is demonstrated that in an astrophysical setting, symmetric magnetostatic equilibria satisfying the ideal MHD equations are exceptional. The principal result of the study is that previous infinitesimal perturbation theory calculations can be generalized to include finite-amplitude and symmetry-breaking effects. The effect of the ergodicity of perturbed symmetric equilibria on heat dispersal in magnetically dominated plasmas is discussed.

Tsinganos, K. C.↗

NASA NTRS1984

A wind-type model for the generation of astrophysical jets

Wind-type solutions for the generation of astrophysical jets from active galactic nuclei and stellar sources, such as those associated with SS 433 and protostellar objects, are discussed. Acceleration, collimation, and morphology are consistently interpreted in terms of a flow starting from the galactic or stellar core inside the 'throat' of a thick accretion disk.

Ferrari, A.↗

NASA NTRS1983

Multiple transonic solutions and a new class of shock transitions in solar and stellar winds

The steady isothermal solar wind equations are shown to admit, under certain circumstances, mutliple transonic solutions when, for example, momentum deposition gives rise to multiplee critical points in the flow. These multiple solutions consist of a continuous solution and solutions which involve shock transitions between critical solutions. The ambiguity arising from the multiplicity of the solutions can be resolved by following the time evolution of a wind profile with one critical point. Results of the numerical integration of the time-dependent equations with momentum addition show that each of these multiple solutions is physically accessible and depends on the rate of change of momentum deposition. These results suggest that standing shocks are likely to be present in the inner solar wind flow.

Habbal, S. R.↗

NASA NTRS1983

Formation of standing shocks in stellar winds and related astrophysical flows

Stellar winds and other analogous astrophysical flows can be described, to lowest order, by the familiar one dimensional hydrodynamic equations which, being nonlinear, admit in some instances discontinuous as well as continuous transonic solutions for identical inner boundary conditions. The characteristics of the time dependent differential equations of motion are described to show how a perturbation changes profile in time and, under well defined conditions, develops into a stationary shock discontinuity. The formation of standing shocks in wind type astrophysical flows depends on the fulfillment of appropriate necessary conditions, which are determined by the conservation of mass, momentum and energy across the discontinuity, and certain sufficient conditions, which are determined by the flow's history.

Tsinganos, K.↗