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Results for “microturbulence”

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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

First results from the Goddard High-Resolution Spectrograph - Evidence for photospheric microturbulence in early O stars - Are surface gravities systematically underestimated?

GHRS spectra of two very hot stars provide evidence for the presence of microturbulence in their photospheres. In attempting to reproduce the observed spectra, theoretical models have been built in which the microturbulence is allowed to modify not only the Doppler line widths (classical 'spectroscopic' microturbulence), but also the turbulent pressure (thus mimicking a 'physical' turbulence). It is found that a corresponding modification of the temperature-pressure stratification influences the hydrogen and helium line profiles to the extent that the surface gravities of early O stars determined without considering microturbulence are too low by 0.1-0.15 dex. Thus, including microturbulence would reduce, or resolve completely, a long-standing discrepancy between evolutionary and spectroscopic stellar masses.

Hubeny, I.↗
NASA NTRS1973

Velocity fields in stellar atmospheres and the concept of microturbulence

In the line spectrum of a star, the discrepancies between observed and theoretical widths cannot be fully accounted for by Doppler broadening due to thermal motion. Microturbulence has been introduced to explain the occurrence of nonthermal broadening of the lines. Based on a layer concept of line formation, microturbulence can be defined as the effect of a small-amplitude oscillatory velocity field on opacity. It is shown that no part of the velocity field which is observed in the solar atmosphere can be responsible for microturbulence.

Worrall, G.↗
NASA NTRS1980

Microturbulence and Wesselink radii

Using the calibration of Bell and Parsons (1974), the effects of changes in microturbulence and surface gravity throughout the cycles of delta Cephei and eta Aquilae are estimated. When the changes in microturbulence are considered, Wesselink radii for these stars are reduced 20%.

Evans, N. R.↗
NASA NTRS1972

Velocity gradients and microturbulence in Cepheids

Variations of the microturbulent velocity with phase and height in the atmosphere were reported in classical Cepheids. It is shown that these effects can be understood in terms of variations of the velocity gradient in the atmospheres of these stars.

Karp, A. H.↗
NASA NTRS1973

Velocity gradients and microturbulence in Cepheids.

Variations of the microturbulent velocity with phase and height in the atmosphere have been reported in classical Cepheids. It is shown that these effects can be understood in terms of variations of the velocity gradient in the atmospheres of these stars.

Karp, A. H.↗
NASA NTRS1977

Microturbulence, systematic motions, and line formation in molecular clouds

Microturbulence and systematic motions are viewed as simplifying assumptions made to facilitate treatment of line formation in molecular clouds, and line intensities calculated in the two approximations are compared to estimate how uncertainties about the actual line-broadening mechanism affect the interpretation of molecular emission lines. For lines formed by two-level molecules in an isothermal homogeneous cloud, the alternative assumptions lead to peak and integrated line intensities which agree within the differences (up to a factor of 3) associated with the ignorance of cloud geometry. New multilevel calculations for CO in the same cloud model bear out the generality of this result. It follows that, within the geometrical uncertainties, the Sobolev (1960) approximation may be used confidently in the numerous applications for which this simple cloud model suffices.

White, R. E.↗
NASA NTRS1980

The role of microturbulence on collisionless reconnection

The linear, non-linear and anomalous transport properties associated with various microinstabilities driven by cross field currents in reconnecting geometries are reviewed. An assessment of their role in collisionless tearing based on analytic theory, computer simulations and experimental evidence, supports the dominant role of lower hybrid waves. The relevance of microturbulence on macroscopic stationary and time dependent models of merging is presented. It is concluded that a fluid-numerical simulation approach that includes (at each space and time step) the effects of anomalous transport in a self consistent manner, similar to the one used for laboratory collisionless shocks, represents the best method for studying and modeling the details of the reconnection process.

Papadopoulos, K.↗
NASA NTRS1987

Microturbulence in the upper photosphere of Alpha Persei (F5 Ib) derived from ultraviolet spectral observations

High-resolution ultraviolet spectra of the moderate supergiant Alpha Per (F5 Ib) were studied to determine the dynamic state of its upper photosphere. It was found that the line-of-sight microturbulent velocity component in the region of origin of the UV spectrum is about 5 km/s, and is slightly smaller than the value derived from the visual spectrum. This is ascribed to dissipation of mechanical energy between the higher and lower layers where, respectively, the ultraviolet and visual light lines originate. Between these two levels, which are one scale height apart, the mechanical energy flux decreases to about 0.3 of its photospheric value. The consequent value for the (outward directed) turbulent acceleration is 24 cm/sec-squared, more than one half the observationally determined effective acceleration of gravity.

Spaan, F. H. P.↗
NASA NTRS1995

Anisotropic microturbulence near the Sun

Interplanetary scintillation observations which sample the spatial spectrum of electron density at scales between 10 and 100 km show power-law spectra which are flatter than the Kolmogorov spectra observed at larger scales by spacecraft and also, indirectly, by phase scintillation of coherent radio signals. Furthermore, angular broadening observations of compact radio sources have shown that the microscale density fluctuations are field-aligned and become more anisotropic as R decreases inside 10 solar radius. We present angular broadening observations taken in October of 1992 which were recorded nearly simultaneously on the VLA and VLBA arrays. The VLA samples structure at scales between 3 and 30 km, whereas the VLBA samples scales between 200 and 4000 km. The small scale VLA measurements of the south polar source 1246-075 showed lower turbulence than those of the equatorial source 1256-057, consistent with previous work showing that the density delta N(exp 2)(sub e) is a factor of 10-15 lower in coronal holes. The VLA observations inside of 10 solar radius were anisotropic, as expected. We were not able to measure the equatorial source with the VLBA inside of 10 solar radius because the scattering was too strong, however we did observe the polar source just inside this distance with both the VLA and the VLBA. Significant anisotropy was seen on the smaller scales, but the larger scales were essentially isotropic. This suggests that the process responsible for the anisotropic microturbulence is distinct from the larger scale isotropic turbulence.

Coles, William A.↗
NASA NTRS1974

The nature of microturbulence in the solar photosphere

It is shown that the increase in apparent Doppler width as the limb is approached is a natural consequence of the fact that the solar atmosphere is not homogeneous. The work on the sodium D line cores which assumes the solar atmosphere to be homogeneous is reviewed. It appears that the cores of the sodium D lines have been distorted by fluctuations in the solar atmosphere in a systematic manner. It is concluded that the increase in apparent Doppler width observed in the sodium D line is a natural consequence of the temperature fluctuations in the solar atmosphere. The presence of a velocity field is not required to produce this effect.

Wilson, A. M.↗
NASA NTRS1988

The geomagnetic field - An explanation for the microturbulence in coaxial gun plasmas

The complexity introduced by the geomagnetic field in several regions of a coaxial gun plasma device is described. It is shown that the annihilation of the swept-up geomagnetic flux, trapped within the highly compressed turbulent plasma, provides an explanation for varied performance and experimental results. The results indicate that the device should be aligned along the direction of the local geomagnetic field or enclosed in a mu-metal shield.

Mather, J. W.↗
NASA NTRS1975

Hydrodynamic models of a Cepheid atmosphere. III - Line spectrum and radius determinations

Line profiles are computed on the basis of the moving atmospheres from the hydrodynamic models investigated by Karp (1975). It is found that the velocity gradients in the atmosphere can be used to explain the apparent, slightly supersonic microturbulence. The total observed microturbulence is seen to be consistent with the linear sum of the classical microturbulence and that caused by the velocity gradients.

Karp, A. H.↗
NASA NTRS1993

CO(J = 1-0) line emission from giant molecular clouds

Numerical models of the (C-12)O (J = 1-0) line emission from giant molecular clouds (GMCs) have been constructed. Line profiles are presented for both microturbulent and macroturbulent clouds. The observed width of the CO (J = 1-0) line cannot be physically interpreted as a microturbulent gas velocity dispersion. Microturbulent models produce emission with a wide range of profile shapes and brightness temperatures at line center over a cloud mass range of 100 to 10 exp 6 solar masses. Macroturbulent models produce smooth, centrally peaked profiles. Clumpy GMC models can produce peak brightness temperatures over about 6 K provided that clump densities are larger than 1000/cu cm. If there are no significant velocity gradients, photoelectric heating dominates in the region where the CO line arises. The observed relation between cloud mass and CO luminosity can be constructed from theoretical models.

Wolfire, Mark G.↗