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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Erratum: Evidence That a Deep Meridional Flow Sets the Sunspot Cycle Period

An error was made in entering the data. This changes the results concerning the length of the time lag between the variations in the meridional flow speed and those in the cycle amplitude. The final paragraph on page 667 should read: Finally, we study the relationship between the drift velocities and the amplitudes of the hemisphere/cycles. We compare the drift velocity at the maximum of the cycle to the amplitude of that cycle for that hemisphere. There is a positive (0.5) and significant (95%) correlation between the two. However, an even stronger relationship is found between the drift velocity and the amplitude of the N + 2 cycle. The correlation is stronger (0.7) and more significant (99%), as shown. This relationship is suggestive of a "memory" in the solar cycle, again a property of dynamo models that use meridional circulation. Indeed, the two-cycle lag is precisely the relationship found by Charbonneau & Dikpati. This behavior is, however, more difficult to interpret, and we elaborate on this in the next section. In either case, these correlations only explain part of the variance in cycle amplitude (25% for the current cycle and 50% for the N + 2 cycle). Obviously, other mechanisms, such as variations in the gradient in the rotation rate, also contribute to the cycle amplitude variations. Our investigation of possible connections between drift rates and the amplitudes of the N + 1 and N + 3 cycles gives no significant correlations at these alternative time lags.

Hathaway, David H.↗
NASA NTRS1988

The possible role of meridional flows in suppressing magnetic buoyancy

The equation of motion for a toroidal flux ring in a stellar convective envelope is derived, and the equilibrium of such a ring is considered. Necessary conditions for the stability of toroidal flux rings are derived, and results of stability calculations for a particular model of the meridional flow are presented. The motions of the flux rings when the rings are far from their equilibrium position or when equilibrium does not exist are considered. The results confirm the linear stability analysis, and show that in the absence of stable equilibrium, the rings move toward the solar surface along a trajectory which is parallel to the rotation axis. It is expected that viscosity will tend to reduce the rotational velocity difference between the flux ring and its surroundings, thus reducing the Coriolis force and altering the equilibrium. The storage time of toroidal flux rings is estimated, and some implications for the sun are discussed.

Van Ballegooijen, A. A.↗
NASA NTRS1982

Evidence for a poleward meridional flow on the sun

Two subsets of all polar zone filaments, designated polemost filament and polar filament bands, are defined for observational study of their behavior, which is compared with the evolution of the polar magnetic field over Howard and LaBonte's (1981) activity cycle. The magnetic data show that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions originating in active region latitudes and subsequently drifting to the poles. F-polarity regions are carried poleward by a meridional flow, rather than by diffusion. It is noted that the mean latitude of the polemost filaments tracks the boundary of the polar field cap, and undergoes an equatorward dip during each arrival of additional polar field, and that the polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m/sec.

Topka, K.↗
NASA NTRS1999

Control of Meridional Flow in Circular Cylinders by a Travelling Axial Magnetic Field

Convective flow in a Bridgman or float zone configuration significantly affects the interface shape and segregation phenomena. While the primary causative factor for this flow is buoyancy induced convection in an enclosed Bridgman melt, the presence of a free surface gives rise to surface tension driven flows in the floating zone processing of melts. It is of interest to curtail these flows in order to realize near quiescent growth conditions that have shown to result in crystals with good longitudinal and radial homogeneity and thereby of better overall quality. While buoyancy effects can be reduced by careful processing in a low gravity (space) environment, the reduction of Marangoni flows due to surface tension variations is not that straight forward. Attempts have been made with some limited success with the use of external fields to affect the melt thermo-fluid behavior. The use of a static magnetic field that reduces convective contamination through the effects of a non-intrusively induced, dissipative Lorentz force in an electrically conducting melt is one such approach. Experiments have shown that axial fields of the order of 5 Tesla can significantly eliminate convection and yield close to diffusion limited crystal growth conditions. The generation and use of such high magnetic fields require substantial hardware and incur significant costs for its operation. Lately, the use of rotating magnetic fields has been tested in semiconductor crystal growth. The method is fairly well known and commonly used in metal processing but its adaptation to crystal growth of semiconductors is fairly recent. The elegance of the technique rests in its low power requirement (typically 10-20 milli-Tesla at 50-400 Hz) and its efficacy in curtailing deleterious temperature fluctuations in the melt. A rotating magnetic field imposes a rotational force and thereby induces a circulation within the melt that tends to dominate other sporadic convective effects. Thus a known low level of convective flow is introduced into the system. A new novel variation of the Lorentz force mechanism is proposed and investigated in this study. Since one of the desired process conditions in melt crystal growth is the minimization of convective effects, this investigation examines the use of an external field of magnetic origin to counteract existing convective flow within the melt. This is accomplished by utilizing a running or traveling axial magnetic wave in the system. The concept is similar to the use of vibrational means in order to induce streaming flows that oppose buoyant or surface tension driven convection in the system. The rotation direction as well as the magnitude (strength) of this circulation can be easily controlled by external inputs thus affording a direct means of controlling the developing shape of the crystallizing front (interface). The theoretical model of this technique is fully developed and presented in this paper. Results from the solution of the developed governing equations and boundary conditions are also presented. An experimental demonstration of the concept is presented through the suppression of natural convective flow in a mercury column. Implications to crystal growth systems will be fully explored in the final manuscript.

Mazuruk, K.↗
NASA NTRS2002

A New Component of Solar Dynamics: North-South Diverging Flows Migrating toward the Equator with an 11 Year Period

Time-distance helioseismology analysis of dopplergrams provides maps of torsional oscillations and meridional flows. Meridional flow maps show a time-varying component that has a banded structure which matches the torsional oscillations with an equatorward migration over the solar cycle. The time-varying component of meridional flow consists of a flow diverging from the dominant latitude of magnetic activity. These maps are compared with other torsional oscillation maps and with magnetic flux maps, showing a strong correlation with active latitudes. These results demonstrate a strong link between the time-varying component of the meridional flow and the torsional oscillations.

Beck, J. G.↗
NASA NTRS1972

Computer program for quasi-three-dimensional calculation of surface velocities and choking flow for turbomachine blade rows

Computer program, CHANEL, can obtain quasi-three-dimensional solutions in any well-guided channel. Conditions that can be handled by program that could not be handled previously are nonuniform inlet temperature, pressure, prewhirl, nonaxial flow where meridional flow angle, meridional stream-line curvature, and radius can vary as desired from hub to tip.

Katsanis, T.↗
NASA NTRS1984

The concentration of the large-scale solar magnetic field by a meridional surface flow

Analytical and numerical solutions to the magnetic flux transport equation in the absence of new bipolar sources of flux are calculated for several meridional flow profiles and a range of peak flow speeds. It is found that a poleward flow with a broad profile and a nominal 10 m/s maximum speed concentrates the large-scale field into very small caps of less than 15 deg half-angle, with average field strengths of several tens of gauss, contrary to observations. A flow which reaches its peak speed at a relatively low latitude and then decreases rapidly to zero at higher latitudes leads to a large-scale field pattern which is consistent with observations. For such a flow, only lower latitude sunspot groups can contribute to interhemispheric flux annihilation and the resulting decay and reversal of the polar magnetic fields.

Devore, C. R.↗
DOE OSTI2021

Generation of mean flows in rotating anisotropic turbulence: The case of solar near-surface shear layer

Results from helioseismology indicate that the radial gradient of the rotation rate in the near-surface shear layer (NSSL) of the Sun is independent of latitude and radius. Theoretical models using the mean-field approach have been successful in explaining this property of the NSSL, while global direct or large-eddy magnetoconvection models have so far been unable to reproduce this. We investigate the reason for this discrepancy by measuring the mean flows, Reynolds stress, and turbulent transport coefficients under conditions mimicking those in the solar NSSL. Simulations with as few ingredients as possible to generate mean flows were studied. These ingredients are inhomogeneity due to boundaries, anisotropic turbulence, and rotation. The parameters of the simulations were chosen such that they matched the weakly rotationally constrained NSSL. The simulations probe locally Cartesian patches of the star at a given depth and latitude. The depth of the patch was varied by changing the rotation rate such that the resulting Coriolis numbers covered the same range as in the NSSL. We measured the turbulent transport coefficient relevant for the nondiffusive (Λ-effect) and diffusive (turbulent viscosity) parts of the Reynolds stress and compared them with predictions of current mean-field theories. A negative radial gradient of the mean flow is generated only at the equator where meridional flows are absent. At other latitudes, the meridional flow is comparable to the mean flow corresponding to differential rotation. We also find that the meridional components of the Reynolds stress cannot be ignored. Additionally, we find that the turbulent viscosity is quenched by rotation by about 50% from the surface to the bottom of the NSSL. Our local simulations do not validate the explanation for the generation of the NSSL from mean-field theory where meridional flows and stresses are neglected. However, the rotational dependence of the turbulent viscosity in our simulations agrees well with theoretical predictions. Moreover, our results agree qualitatively with global convection simulations in that an NSSL can only be obtained near the equator.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2011

What Supergranule Flow Models Tell Us About the Sun's Surface Shear Layer and Magnetic Flux Transport

Models of the photospheric flows due to supergranulation are generated using an evolving spectrum of vector spherical harmonics up to spherical harmonic wavenumber l~1500. Doppler velocity data generated from these models are compared to direct Doppler observations from SOHO/MDI and SDO/HMI. The models are adjusted to match the observed spatial power spectrum as well as the wavenumber dependence of the cell lifetimes, differential rotation velocities, meridional flow velocities, and relative strength of radial vs. horizontal flows. The equatorial rotation rate as a function of wavelength matches the rotation rate as a function of depth as determined by global helioseismology. This leads to the conclusions that the cellular structures are anchored at depths equal to their widths, that the surface shear layer extends to at least 70 degrees latitude, and that the poleward meridional flow decreases in amplitude and reverses direction at the base of the surface shear layer (approx.35 Mm below the surface). Using the modeled flows to passively transport magnetic flux indicates that the observed differential rotation and meridional flow of the magnetic elements are directly related to the differential rotation and meridional flow of the convective pattern itself. The magnetic elements are transported by the evolving boundaries of the supergranule pattern (where the convective flows converge) and are unaffected by the weaker flows associated with the differential rotation or meridional flow of the photospheric plasma.

Hathaway, David↗
NASA NTRS2012

Photospheric Magnetic Flux Transport - Supergranules Rule

Observations of the transport of magnetic flux in the Sun's photosphere show that active region magnetic flux is carried far from its origin by a combination of flows. These flows have previously been identified and modeled as separate axisymmetric processes: differential rotation, meridional flow, and supergranule diffusion. Experiments with a surface convective flow model reveal that the true nature of this transport is advection by the non-axisymmetric cellular flows themselves - supergranules. Magnetic elements are transported to the boundaries of the cells and then follow the evolving boundaries. The convective flows in supergranules have peak velocities near 500 m/s. These flows completely overpower the superimposed 20 m/s meridional flow and 100 m/s differential rotation. The magnetic elements remain pinned at the supergranule boundaries. Experiments with and without the superimposed axisymmetric photospheric flows show that the axisymmetric transport of magnetic flux is controlled by the advection of the cellular pattern by underlying flows representative of deeper layers. The magnetic elements follow the differential rotation and meridional flow associated with the convection cells themselves -- supergranules rule!

Hathaway, David H.↗
NASA NTRS1988

Meridional plasma flow in the outer heliosphere

Voyager 2 observations made in the outer heliosphere near 25 AU and within 2 deg of the heliographic equatorial plane show periodic variations in the meridional (North/South) flow velocities that are much more prominent than the East/West variations. An autocorrelation analysis shows that the flow variation has a period of about 25.5 days in the latter half of 1986, in approximate agreement with the solar rotation period. The results suggest that increased pressure in interaction regions remains the best candidate for the driver of the nonradial flows.

Lazarus, A. J.↗
NASA NTRS2002

Large Scale Flows through the Solar Cycle

Large scale flows within the solar convection zone are the primary drivers of the Sun's magnetic activity cycle. Differential rotation amplifies the magnetic field and converts poloidal fields into toroidal fields. Poleward meridional flow near the surface carries magnetic flux that reverses the magnetic poles and converts toroidal fields into poloidal fields. The deeper, equatorward meridional flow carries magnetic flux toward the equator where it must reconnect with oppositely directed fields in the other hemisphere. These axisymmetric flows are themselves driven by large scale convective motions. Given these intimate connections between the large scale flows and solar activity, it would be surprising if there weren't solar cycle variations in the flow characteristics. Some variations, namely the torsional oscillations, are well established. Other variations, namely changes in the meridional flow and in the convective motions themselves, are more controversial. In this presentation I will describe the observed characteristics of the large scale flows and discuss possible variations associated with the solar cycle.

Hathaway, D. H.↗