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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Global circulation and the solar dynamo

The Sun is apparently rather typical of stars in its spectral class, with a convection zone of substantial depth and a modest rotation rate. These two factors are apparently enough to generate a substantial global circulation, seen so far principally as a differental rotation, as well as a nearly cyclic magnetohydrodynamic dynamo, seen principally as the 22 year "solar cycle". It would be expected therefore that many stars would have such dynamical characteristics. Recent observations of very large scale velocity fields of small velocity amplitude were reviewed.

Gilman, P. A.↗
NASA NTRS1983

Determining the alpha dynamo parameter in incompressible homogeneous magnetohydrodynamic turbulence

Alpha, an important parameter in dynamo theory, is proportional to either the kinetic, current, magnetic, or velocity helicity of the fluctuating magnetic field and fluctuating velocity field. The particular helicity to which alpha is proportional depends on the assumptions used in deriving the first order smoothed equations that describe the alpha effect. In two cases, when alpha is proportional to either the magnetic helicity or velocity helicity, alpha is determined experimentally from two point measurements of the fluctuating fields in incompressible, homogeneous turbulence having arbitrary symmetry. For the other two possibilities, alpha is determined if the turbulence is isotropic.

Matthaeus, W. H.↗
NASA NTRS1983

On the coronae of rapidly rotating stars. IV - Coronal activity in F dwarfs and implications for the onset of the dynamo

X-ray observations of 14 early F dwarfs are reported and these stars are used, together with a complete sample from the literature, to examine how the characteristics of coronal X-ray emission vary from dwarfs of spectral type A through G. Evidence for a rotation-activity relation in stars redder than B - V = 0.45 is found. Stellar duplicity and age, except insofar as they influence the rotation rate, do not appear to be important in determining the coronal X-ray flux level in F dwarfs. It is suggested that the appearance of a relation between rotation and activity at B - V = 0.45 indicates the turn-on of a solar-like dynamo. The high X-ray surface fluxes and small variance thereof for dwarfs with B - V = 0.3-0.45 are also discussed.

Walter, F. M.↗
NASA NTRS1984

The effect of closed boundary conditions on a stationary dynamo

One of two boundary conditions generally assumed in solutions of the dynamo equation is related to the disappearance of the azimuthal field at the boundary. Parker (1984) points out that for the realization of this condition the field must escape freely through the surface. Escape requires that the field be detached from the gas in which it is embedded. In the case of the sun, this can be accomplished only through reconnection in the tenuous gas above the visible surface. Parker concludes that the observed magnetic activity on the solar surface permits at most three percent of the emerging flux to escape. He arrives at the conclusion that, instead of B(phi) = 0, the partial derivative of B(phi) to r is equal to zero. The present investigation is concerned with the effect of changing the boundary condition according to Parker's conclusion. Implications for the solar convection zone are discussed.

Choudhuri, A. R.↗
NASA NTRS1984

A dynamo theory prediction for solar cycle 22 - Sunspot number, radio flux, exospheric temperature, and total density at 400 km

Using the 'dynamo theory' method to predict solar activity, a value for the smoothed sunspot number of 109 + or - 20 is obtained for solar cycle 22. The predicted cycle is expected to peak near December, 1990 + or - 1 year. Concommitantly, F(10.7) radio flux is expected to reach a smoothed value of 158 + or - 18 flux units. Global mean exospheric temperature is expected to reach 1060 + or - 50 K and global total average total thermospheric density at 400 km is expected to reach 4.3 x 10 to the -15th gm/cu cm + or - 25 percent.

Schatten, K. H.↗
NASA NTRS1984

What Would a Dynamo Theorist like to Know About the Dynamics of the Solar Convection Zone?

Observations about the current status of solar dynamo theory are given. The induction equation for magnetic field is solved using assumed velocities and parametric representations of the inductive or diffusive effects of velocities. The equations of motion governing these flow are not solved in parallel. Results from global compressible convection models are discussed. Differential rotation and convection are also investigated.

Gilman, P. A.↗
NASA NTRS1985

Planetary Dynamos

The MAGSAT-program has added significantly to our knowledge of planetary magnetism. The accuracy of observations has been improved such that a reliable extrapolation of the magnetic field to the core surface is now much more feasible than it has been before, and the prospect of further MAGSAT missions raises the expectation that the time dependence of the geomagnetic field will be known with similar accuracy in the future. In the research support it has been attempted to develop dynamo theory with these applications in mind.

Busse, F. H.↗
NASA NTRS1986

A dynamo theory prediction for solar cycle 22: Sunspot number, radio flux, exospheric temperature, and total density at 400 km

Using the dynamo theory method to predict solar activity, a value for the smoothed sunspot number of 109 + or - 20 is obtained for solar cycle 22. The predicted cycle is expected to peak near December, 1990 + or - 1 year. Concommitantly, F(10.7) radio flux is expected to reach a smoothed value of 158 + or - 18 flux units. Global mean exospheric temperature is expected to reach 1060 + or - 50 K and global total average total thermospheric density at 400 km is expected to reach 4.3 x 10 to the -15th gm/cu cm + or - 25 percent.

Schatten, K. H.↗
NASA NTRS1985

On the dynamo generation of flux ropes in the Venus ionosphere

Small scale magnetic field structures or 'flux ropes' observed in the ionosphere of Venus can be interpreted as the result of a kinematic dynamo process acting on weak seed fields. The seed fields result from the prevailing downward convection of magnetic flux from the vicinity of the ionopause, while small scale fluctuations in the velocity of the ionospheric plasma, which can be caused by collisional coupling to gravity waves in the neutral atmosphere, provide the mechanism by which the field is twisted and redistributed into features of similar scale. This mechanism naturally explains some of the average properties of flux ropes such as the variation of their characteristics with altitude and solar zenith angle. It also elucidates the relationship between the large scale and small scale ionospheric magnetic fields.

Luhmann, J. G.↗
NASA NTRS1986

The alpha dynamo parameter and measurability of helicities in magnetohydrodynamic turbulence

Alpha, an important parameter in dynamo theory, is shown to be proportional to either the kinetic, current, magnetic, or velocity helicities of the fluctuating magnetic field and fluctuating velocity field. The particular helicity to which alpha is proportional depends on the assumptions used in deriving the first-order smoothed equations that describe the alpha effect. In two cases, viz., when alpha is proportional to either the magnetic helicity or velocity helicity, alpha can be determined experimentally from two-point measurements of the fluctuating fields in incompressible, homogeneous turbulence with arbitrary rotational symmetry. For the other two possibilities, alpha can be determined if the turbulence is isotropic.

Matthaeus, W. H.↗
NASA NTRS1986

Magnetostrophic balance in planetary dynamos - Predictions for Neptune's magnetosphere

With the purpose of estimating Neptune's magnetic field and its implications for nonthermal Neptune radio emissions, a new scaling law for planetary magnetic fields was developed in terms of externally observable parameters (the planet's mean density, radius, mass, rotation rate, and internal heat source luminosity). From a comparison of theory and observations by Voyager it was concluded that planetary dynamos are two-state systems with either zero intrinsic magnetic field (for planets with low internal heat source) or (for planets with the internal heat source sufficiently strong to drive convection) a magnetic field near the upper bound determined from magnetostrophic balance. It is noted that mass loading of the Neptune magnetosphere by Triton may play an important role in the generation of nonthermal radio emissions.

Curtis, S. A.↗
NASA NTRS1989

Magnetic dynamo activity in mechanically driven compressible magnetohydrodynamic turbulence

Magnetic dynamo activity in a homogeneous, dissipative, polytropic, two-dimensional, turbulent magneto-fluid is simulated numerically. The magneto-fluid is simulated numerically. The magneto-fluid is, in a number of cases, mechanically forced so that energy input balances dissipation, thereby maintaining constant energy. In the presence of a mean magnetic field, a magneto-fluid whose initial turbulent magnetic energy is zero quickly arrives at a state of non-zero turbulent magnetic energy. If the mean magnetic field energy density is small, the turbulent magnetic field can achieve a local energy density more than four hundred times larger; if the mean magnetic field energy density is large, then equipartition between the turbulent magnetic and kinetic energy is achieved. Compared to the presence of a mean magnetic field, compressibility appears to have only a marginal effect in mediating the transfer of turbulent kinetic energy into magnetic energy.

Shebalin, John V.↗
NASA NTRS1989

Ionospheric dynamo theory for production of far ultraviolet emissions on Uranus

A model is presented to explain diffuse FUV emissions from the outer planets, specifically Uranus, in excess of those diffuse emissions that are currently explainable by scattering of sunlight and/or excitation by photoelectrons. These electroglow emissions in H Ly-alpha and H2 bands, which occur in the sunlit hemisphere slightly above the homopause, appear to require particle excitation in the 10- to 50-eV range. An in situ mechanism for accelerating photoelectrons (and ions is proposed, involving neutral wind dynamo generation of field-aligned currents analogous to what occurs in the earth's equatorial E and F regions. Sufficiently strong field-aligned currents are found in the model calculation for Uranus to produce a potential drop of about 100 eV or greater between the F peak and homopause, concentrated at lower altitudes, and capable in principle of accelerating photoelectrons (and ions) to the 10- to 50-eV energies required to explain the observed emissions. The fact that the excitation and ionization cross sections are larger than elastic scattering cross sections in an H2 atmosphere at these energies makes in situ acceleration feasible for the production of UV on the outer planets.

Hudson, M. K.↗
NASA NTRS1990

Evidence for fast dynamo action in a chaotic web

The evolution of a magnetic field in a chaotic web is studied. The model flow possessing the web is closely related to the nearly integrable ABC flow with A = B and C much less than 1. The magnetic diffusivity is taken to be zero and the field is followed using the Cauchy solution. It is found that the flow folds the magnetic field constructively, in the sense that the average magnetic field in a chaotic region grows exponentially in time. This is suggestive of fast dynamo action, although the effect of diffusion of the strong streamwise magnetic field remains to be assessed.

Gilbert, A. D.↗
NASA NTRS1990

A two-dimensional kinematic dynamo model of the ionospheric magnetic field at Venus

The results of a high-resolution, two-dimensional, time dependent, kinematic dynamo model of the ionospheric magnetic field of Venus are presented. Various one-dimensional models are considered and the two-dimensional model is then detailed. In this model, the two-dimensional magnetic induction equation, the magnetic diffusion-convection equation, is numerically solved using specified plasma velocities. Origins of the vertical velocity profile and of the horizontal velocities are discussed. It is argued that the basic features of the vertical magnetic field profile remain unaltered by horizontal flow effects and also that horizontal plasma flow can strongly affect the magnetic field for altitudes above 300 km.

Cravens, T. E.↗
NASA NTRS1991

Dynamo-based scheme for forecasting the magnitude of solar activity cycles

This paper presents a general framework for forecasting the smoothed maximum level of solar activity in a given cycle, based on a simple understanding of the solar dynamo. This type of forecasting requires knowledge of the sun's polar magnetic field strength at the preceding activity minimum. Because direct measurements of this quantity are difficult to obtain, the quality of a number of proxy indicators already used by other authors is evaluated, which are physically related to the sun's polar field. These indicators are subjected to a rigorous statistical analysis, and the analysis technique for each indicator is specified in detail in order to simplify and systematize reanalysis for future use. It is found that several of these proxies are in fact poorly correlated or uncorrelated with solar activity, and thus are of little value for predicting activity maxima. Also presented is a scheme in which the predictions of the individual proxies are combined via an appropriately weighted mean to produce a compound prediction. The scheme is then applied to the current cycle 22, and a maximum smoothed international sunspot number of 171 + or - 26 is estimated.

Layden, A. C.↗
NASA NTRS1991

Manifestations of dynamo driven large-scale magnetic field in accretion disks of compact objects

A turbulent dynamo nonlinear theory of turbulence was developed that shows that in the compact objects of accretion disks, the generated large-scale magnetic field (when the generation takes place) has a practically toroidal configuration. Its energy density can be much higher than turbulent pulsations energy density, and it becomes comparable with the thermal energy density of the medium. On this basis, the manifestations to which the large-scale magnetic field can lead at the accretion onto black holes and gravimagnetic rotators, respectively, are presented.

Chagelishvili, G. D.↗
NASA NTRS1992

Local magnetohydrodynamic instabilities and the wave-driven dynamo in accretion disks

We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the strength and organization of the magnetic field in a thin accretion disk. We discuss a model in which the wave-driven dynamo growth rate is balanced by the dissipative effects of the MSI. As in earlier work, the net helicity is due to small advective motions driven by nonlinear interactions between internal waves. Assuming a simple model of the internal wave spectrum generated from the primary m = 1 internal waves, we find that the magnetic energy density saturates at about (H/r) exp 4/3 times the local pressure (where H is the disk thickness and r is its radius). On very small scales the shearing instability will produce an isotropic fluctuating field. For a stationary disk this is equivalent to a dimensionless 'viscosity' of about (H/r) exp 4/3. The vertical and radial diffusion coefficients will be comparable to each other. Magnetic buoyancy will be largely suppressed by the turbulence due to the MSI. We present a rough estimate of its effects and find that it removes magnetic flux from the disk at a rate comparable to that caused by turbulent diffusion.

Vishniac, Ethan T.↗