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At least 1,027 records · Page 57
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NASA NTRS1980

Coronal heating by stochastic magnetic pumping

Recent observational data cast serious doubt on the widely held view that the Sun's corona is heated by traveling waves (acoustic or magnetohydrodynamic). It is proposed that the energy responsible for heating the corona is derived from the free energy of the coronal magnetic field derived from motion of the 'feet' of magnetic field lines in the photosphere. Stochastic motion of the feet of magnetic field lines leads, on the average, to a linear increase of magnetic free energy with time. This rate of energy input is calculated for a simple model of a single thin flux tube. The model appears to agree well with observational data if the magnetic flux originates in small regions of high magnetic field strength. On combining this energy input with estimates of energy loss by radiation and of energy redistribution by thermal conduction, we obtain scaling laws for density and temperature in terms of length and coronal magnetic field strength.

Sturrock, P. A.↗
NASA NTRS1980

Acousto-optic spectrometer for radio astronomy

A prototype acousto-optic spectrometer which uses a discrete bulk acoustic wave Itek Bragg cell, 5 mW Helium Neon laser, and a 1024 element Reticon charge coupled photodiode array is described. The analog signals from the photodiode array are digitized, added, and stored in a very high speed custom built multiplexer board which allows synchronous detection of weak signals to be performed. The experiment is controlled and the data are displayed and stored with an LSI-2 microcomputer system with dual floppy discs. The performance of the prototype acousto-optic spectrometer obtained from initial tests is reported.

Chin, G.↗
NASA NTRS1979

Effects of material inhomogeneities on ultrasonic measurements - The problem and a solution

Most ultrasonic measurements of materials involve the generation of an acoustic wave and the propagation of that wave from a transducer through a coupling medium to a specimen under test. After interacting with the specimen, the wave propagates through the coupling medium to a receiving transducer and is converted to an electrical signal. The information presented to the observer by the electrical signal depends on each element of the system. In this paper, the role that the receiving transducer plays in ultrasonic measurements is examined. The phase-sensitive nature of conventional receiving transducers has, for the most part, been neglected in nondestructive evaluations. This is shown to lead to significant data misinterpretation. A new acoustoelectric transducer (AET) has been developed which is phase insensitive. Comparative data obtained with both conventional and AET transducers are presented and discussed. The AET is shown to produce more accurate measurements for the cases investigated.

Heyman, J. S.↗
NASA NTRS1980

Stellar chromospheres

The interpretation of stellar chromospheric line intensities and widths is discussed, and semiempirical chromospheric models of single stars are reviewed. Theoretical chromospheric models based on the short period acoustic wave theory are shown to have promise for explaining the heating of the lower chromospheres of quiet chromosphere stars, but are inadequate to explain the heating of transition regions and coronae of active chromosphere stars and solar plages. Attention is given to the Wilson-Bappu (1957) relation between the widths of the Ca II H and K line emission cores and stellar absolute luminosity, while an examination of the calculations of Basri (1979) reveals that it is essential to solve the transfer equation properly before studying the physical basis of width-luminosity relations. Also considered are theoretical calculations of profiles of optically thick chromospheric lines in the presence of systematic flow patterns.

Linsky, J. L.↗
NASA NTRS1980

Noise transmission through stiffened panels

An analytical study is presented to predict low frequency noise transmission through finite stiffened panels into rectangular enclosures. Noise transmission is determined by solving the acoustic wave equation for the interior noise field and stiffened panel equations for vibrations of panels and stringers. The solution to this system of equations is obtained by a Galerkin-like procedure where the modes and frequencies for stiffened panels are determined by the transfer matrix method. Results include a comparison between theory and experiment and noise transmission through the sidewall of an aircraft.

Vaicaitis, R.↗
NASA NTRS1980

Acoustic loading in straight pipes

Based on linear one-dimensional acoustics, a geometrically perfect elastic waveguide would respond to an oscillatory internal pressure only in the presence of path deflectors (elbows and branches). In practice, a significant elasto-acoustic interaction results even in straight conduits as a result of manufacturing tolerances. A theoretical model of the linear acoustic loading in straight pipes is developed that considers the acoustic wave distortion due to perimeter, axial, and wall thickness nonuniformities.

El-Raheb, M.↗
NASA NTRS1980

Measurements of interactions between acoustic fields and nonuniform mean flow

Two problems crucial to the stability of longitudinal acoustic waves in solid rocket motors were examined experimentally. The first is the dissipation of energy associated with an average flow inward at the lateral boundary. Measurements reported here, though subject to considerable experimental error, show that the actual losses are much larger than predicted by the approximate one dimensional analysis. The second problem is the attenuation of waves accompanying reflection by the nonuniform flow in a choked exhaust nozzle. Emphasis on technique, to provide data relatively easily and inexpensively. It appears that good results can be obtained in a routine manner using small supersonic wind tunnel operated as an open cycle. At least for Mach numbers up to 0.04 at the nozzle entrance, difficulties with signal/noise are satisfactorily overcome with a tracking filter.

Magiawala, K.↗
NASA NTRS1981

Fast plasma heating by anomalous and inertial resistivity effects

Fast plasma heating by anomalous and inertial resistivity effects is described. A small fraction of the plasma contains strong currents that run parallel to the magnetic field and are driven by an exponentiating electric field. The anomalous character of the current dissipation is caused by the excitation of electrostatic ion cyclotron and/or ion acoustic waves. The role of resistivity due to geometrical effects is considered. Through the use of a marginal stability analysis, equations for the average electron and ion temperatures are derived and numerically solved. The evolution of the plasma is described as a path in the drift velocity diagram, in which the drift velocity is plotted as a function of the electron to ion temperature ratio.

Duijveman, A.↗
NASA NTRS1980

A suggestion concerning the generation of the physical state of stellar mantles

The physical state of the winds of B supergiants varies from one of a high level of ionization and rapid outflow to one of relatively low ionization and a moderate rate of outflow as one goes from type B0 to type A0. It is proposed here that this is the result of magnetodynamic energy being released in the mantles of these stars, much as occurs in the sun. Data are presented to show that this idea is plausible. It is noted that the release of magnetodynamic energy as the result of interaction between magnetic fields and differential motion in an inhomogeneous structure is probably the cause of the heating in all outer stellar atmospheres (mantles). In the late-type stars, the material in the mantle must first be ionized by the deposition of energy from acoustic waves; in the early-type stars, the material is already ionized at least once in the photosphere, as a result of the normal transfer of radiative energy, so no priming process is necessary for these stars.

Underhill, A. B.↗
NASA NTRS1980

Does H/-/ truly cool the solar chromosphere

The controversial problem of H(-) radiative cooling in the solar chromosphere is examined. It is found, in agreement with Praderie and Thomas (1972, 1976), that H(-) is a substantial source of radiative heating in the outer atmosphere, especially when departures from LTE are important. The role of H(-) as a chromospheric heating agent must be considered carefully before net radiative cooling rates can be assessed from empirical chromospheric models, or calculations of nonradiative heating, for example by acoustic waves, can be pursued meaningfully.

Ayres, T. R.↗
NASA NTRS1980

Momentum and energy deposition in late-type stellar atmospheres and winds

The present study calculates the response of the outer atmospheres of cool low-gravity stars to the passage of the mechanical energy fluxes of solar magnitude in the form of acoustic waves and Alfven waves. It is shown that Alfven waves are efficient in generating outflow, and can account for the order of magnitude of observed mass loss in late-type luminous stars. However, unless these magnetic waves undergo some dissipation within several stellar radii of the surface, the predicted terminal velocities of the resulting stellar winds are far too high. Alfven wave dissipation should give rise to extended warm chromospheres in low-gravity late-type stars, a prediction which can be observationally tested.

Hartmann, L.↗
NASA NTRS1980

Limits on the streaming and escape of electrons in thermal models for solar hard X-ray emission

Upper limits on the number of fast electrons streaming through and escaping from a plasma whose electrons have been heated to approximately 100 billion K and confined by a collisionless ion-acoustic thermal conduction front are determined. It is shown that such a front is fairly transparent to fast electrons with velocities much larger than the thermal velocity because the anisotropic ion-acoustic waves cannot scatter them, making them collisionless on a scale much larger than the thickness of the front. The collisionless analog of the collisional thermoelectric field is derived self-consistently and shown to offer a significant impediment to fast electrons because they must climb over a larger potential barrier than in the collisional case. The only factors limiting the escape of electrons able to surmount this barrier are their rate of production and the requirement that they carry less heat flux than the maximum heat flux allowable. The rate of production is determined for the case of a Maxwellian whose tail is being filled collisionally.

Smith, D. F.↗
NASA NTRS1980

Small-scale dissipative processes in stellar atmospheres

The outer atmospheres of stars must be heated by some non-thermal energy flux to produce chromospheres and coronae. Processes are discussed which convert the non-thermal energy flux of organized, macroscopic motions into random, microscopic (thermal) motions. Recent advances in the description of the chromosphere velocity field suggest that the acoustic waves observed there transmit very little energy, and hence are probably incapable of heating the upper chromosphere and corona. The apparent failure of this long held mechanism and the growing appreciation of the importance of strong magnetic fields in the chromosphere and corona have led to hypotheses of heating by the dissipation of currents (both oscillatory and quasi-steady). This follows discoveries in laboratory and ionospheric plasmas and work on solar flares, that instabilities can concentrate currents into thin high current density filaments where they dissipate rapidly.

Leibacher, J. W.↗
NASA NTRS1981

The coupling between flow instabilities and incident disturbances at a leading edge

It is now generally agreed that an external disturbance field, such as an incident acoustic wave, can effectively couple to instabilities of a flow past a trailing edge. One purpose of the present paper is to show that there are situations where a similar coupling can occur at a leading edge. The process is analyzed and the effects of experimentally controllable parameters are assessed. It is important to account for such phenomena when evaluating the effect of external disturbances on transition.

Goldstein, M. E.↗
NASA NTRS1981

Measurements of He D3 profiles in solar plages

Measurements of the profiles of the He D3 lines in solar plages are presented as a probe of the plage atmosphere. Observations were made at selected positions in quiescent plages during the period October 1978-April 1979 by the 25-cm coronagraph/3-m coude spectrograph at the Mees Solar Observatory on Mount Haleakala. Individual D3 plage profiles are fit well with a two-component, differentially moving atmosphere with constant source function and total Doppler width. The principal absorption is attributed to nearly stationary material, with a kinetic temperature less than 17,200 K, and velocity distribution is found to represent a net downflow in the plage atmosphere extending to around 12 km/sec. The present results for individual plage profiles are also noted to be consistent with those of Namba (1963). Time series of D3 profiles at fixed plage positions and the velocity spectra for nearby photospheric lines are observed to show general enhancements at periods of around 5 min, with redshift leading variations in line width by about 49 deg. Interpretations of the width variation as temperature variations are found not to be consistent with the properties of acoustic wave modes in a stratified atmosphere, presumably indicating magnetic field effects.

Landman, D. A.↗
NASA NTRS1981

Fast plasma heating by anomalous and inertial resistivity effects in the solar atmosphere

A simple model is presented to describe fast plasma heating by anomalous and inertial resistivity effects. It is noted that a small fraction of the plasma contains strong currents that run parallel to the magnetic field and are driven by an exponentiating electric field. The anomalous character of the current dissipation derives from the excitation of electrostatic ion-cyclotron and/or ion-acoustic waves. The possible role of resistivity deriving from geometrical effects ('inertial resistivity') is also considered. Using a marginal stability analysis, equations for the average electron and ion temperatures are derived and numerically solved. No loss mechanisms are taken into account. The evolution of the plasma is described as a path in the drift velocity diagram, where the drift velocity is plotted as a function of the electron to ion temperature ratio.

Duijveman, A.↗
NASA NTRS1981

Coronal heating by stochastic magnetic pumping

Recent observational data cast serious doubt on the widely held view that the sun's corona is heated by traveling waves (acoustic or magnetohydrodynamic). It is here proposed that the energy responsible for heating the corona is derived from the free energy of the coronal magnetic field derived from motion of the 'feet' of magnetic field lines in the photosphere. Stochastic motion of the feet of magnetic field lines leads, on the average, to a linear increase of magnetic free energy with time. This rate of energy input is calculated for a simple model of a single thin flux tube. The model appears to agree well with observational data if the magnetic flux originates in small regions of high magnetic field strength as proposed by Tarbell et al. (1979). On combining this energy input with estimates of energy loss by radiation and of energy redistribution by thermal conduction, scaling laws are obtained for density and temperature in terms of length and coronal magnetic field strength.

Sturrock, P. A.↗
NASA NTRS1979

Heating of chromospheres and coronae in cool stars

Recent evidence is summarized that suggests that neither of two traditional views of chromospheric heating (that the flux of energy required to heat the solar chromosphere) is only a small fraction of the total radiative energy emerging from the solar interior and that chromospheres are heated by acoustic waves generated in the convection zone (which lies close to the surface of the star) is valid. Particular attention is given to cool stars, both dwarfs and giants. The directions in which research is currently heading in attempting to understand heating of chromospheres and coronae in these stars are indicated. Hot stars are excluded from consideration because radiation pressure plays an important role in their atmospheric heating. It is concluded that the role of magnetic fields in giants is different from the role in dwarfs.

Mullan, D. J.↗