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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A new solar cycle model including meridional circulation

A kinematic model is presented for the solar cycle which includes not only the transport of magnetic flux by supergranular diffusion and a poleward bulk flow at the sun's surface, but also the effects of turbulent diffusion and an equatorward 'return flow' beneath the surface. As in the earlier models of Babcock and Leighton, the rotational shearing of a subsurface poloidal field generates toroidal flux that erupts at the surface in the form of bipolar magnetic regions. However, such eruptions do not result in any net loss of toroidal flux from the sun (as assumed by Babcock and Leighton); instead, the large-scale toroidal field is destroyed both by 'unwinding' as the local poloidal field reverses its polarity, and by diffusion as the toroidal flux is transported equatorward by the subsurface flow and merged with its opposite hemisphere counterpart. The inclusion of meridional circulation allows stable oscillations of the magnetic field, accompanied by the equatorward progression of flux eruptions, to be achieved even in the absence of a radial gradient in the angular velocity. An illustrative case in which a subsurface flow speed of order 1 m/s and subsurface diffusion rate of order 10 sq km/s yield 22-yr oscillations in qualitative agreement with observations.

Wang, Y.-M.↗
NASA NTRS1993

On the generation of 'strong' magnetic fields

We rediscuss the nature of magnetic field generation in astrophysical systems. We show that as a result of ineffective three-dimensional turbulent diffusion in the presence of strong azimuthal magnetic fields, the standard dynamo equations are not likely to provide a reasonable description of magnetic dynamos in systems such as late-type stars and galaxies. Instead, we propose a new set of dynamo equations, which take into account the modifications of turbulent diffusion by strong magnetic fields.

Vainshtein, S. I.↗
NASA NTRS1985

The polar ionosphere as a source of the storm time ring current

The transport of ions from the polar ionosphere to the inner magnetosphere during storm time conditions has been computed using a Monte Carlo diffusion code. The effect of the electrostatic turbulence assumed to be present during the substorm expansion phase was simulated by a process that accelerated the ions stochastically perpendicular to the magnetic field with a diffusion coefficient proportional to the rate of energization of the ions by the induced electric field. This diffusion process was continued as the ions were convected from the plasma sheet boundary layer to the double-spiral injection boundary. Inward of the injection boundary the ions were convected adiabatically. By using as input an O(+) flux of 2.8 x 10 to the 8th per sq cm per s (w greater than 10 eV) and an H(+) flux of 5.5 x 10 to the 8th per sq cm per s (w greater than .63 eV) the computed distribution functions of the ions in the ring current were found to be in good agreement, over a wide range in L (4-8), with measurements made with the ISEE 1 satellite during a storm. This O(+) flux and a large part of the H(+) flux appear to be consistent with the DE-1 and DE-2 satellite measurements of the polar ionospheric outflow during disturbed times.

Cladis, J. B.↗
NASA NTRS1985

Transport of ions in presence of induced electric field and electrostatic turbulence - Source of ions injected into ring current

The transport of ions from the polar ionosphere to the inner magnetosphere during stormtime conditions has been computed using a Monte Carlo diffusion code. The effect of the electrostatic turbulence assumed to be present during the substorm expansion phase was simulated by a process that accelerated the ions stochastically perpendicular to the magnetic field with a diffusion coefficient proportional to the energization rate of the ions by the induced electric field. This diffusion process was continued as the ions were convected from the plasma sheet boundary layer to the double-spiral injection boundary. Inward of the injection boundary, the ions were convected adiabatically. By using as input an O(+) flux of 2.8 x 10 to the 8th per sq cm per s (w greater than 10 eV) and an H(+) flux of 5.5 x 10 to the 8th per sq cm per s (w greater than 0.63 eV), the computed distribution functions of the ions in the ring current were found to be in good agreement, over a wide range in L (4 to 8), with measurements made with the ISEE-1 satellite during a storm. This O(+) flux and a large part of the H(+) flux are consistent with the DE satellite measurements of the polar ionospheric outflow during disturbed times.

Cladis, J. B.↗
NASA NTRS1985

Simulations of magnetic-flux transport in solar active regions

The evolution of several observed solar active regions is simulated by solving a transport equation for magnetic flux at the photosphere. The rates of rotation, meridional flow, and diffusion of the flux are determined self-consistently in the calculations. The findings are in good quantitative agreement with previous measures of the rotation rate and diffusion constant associated with photospheric magnetic fields. Although the meridional velocities are consistent in direction and magnitude with recently reported poleward flows, relatively large uncertainties in the velocity determinations make this result inconclusive.

Devore, C. R.↗
NASA NTRS1979

Rates of mass, momentum, and energy transfer at the magnetopause

Empirical estimates of the global rates of transfer of solar wind mass, tangential momentum, and energy at the Earth's magnetopause are presented for comparison against model estimates based on the four principal mechanisms that have been proposed to explain such transfer. The comparisons, although not quite conclusive, strongly favor a model that incorporates some combination of direct magnetic connection and anomalous cross field diffusion. An additional global constraint, the rate at which magnetic flux is cycled through the magnetospheric convection system, strongly suggests that direct magnetic connection plays a significant if not dominant role in the solar wind/magnetosphere interaction.

Hill, T. W.↗
NASA NTRS1995

On the upstream boundary of electron foreshocks in the solar wind

The upstream boundary of electron foreshocks is defined as the path of the fastest electrons reflected by collisionless shocks and moving along the magnetic field in the solar wind. Considerable levels of magnetic fluctuations are found in these regions of the solar wind, and their effect is to create both a broadening and a fine structure of the electron foreshock boundary. The magnetic structure is studied by means of a 3-D numerical simulation of a turbulent magnetic field. Enhanced, anomalous diffusion is found, (Delta x(exp 2)) varies as s(sup alpha), where alpha is greater than 1 for typical values of the parameters (here, Delta x(exp 2) is the mean square width of the tangent magnetic surface and s is the field line length). This corresponds to a Levy flight regime for the magnetic field line random walk, and allows very efficient electron propagation perpendicular to the magnetic field. Implications on the observations of planetary foreshocks and of the termination shock foreshock are considered.

Zimbardo, G.↗
NASA NTRS1992

Venus internal magnetic field and its interaction with the interplanetary magnetic field

In a previous study, Knudsen et al. suggested that Venus has a weak internal magnetic dipole field of the order of 7 x 10 + 20 G cm(exp -3) that is manifested in the form of magnetic flux tubes threading the ionospheric holes in the Venus nightside ionosphere. They pointed out that any internal field of Venus, dipole or multipole, would be weakened in the subsolar region and concentrated in the antisolar region of the planet by the supersonic transterminator convection of the dayside ionosphere into the nightside hemisphere. The inferred magnitude of the dipole field does not violate the upper limit for an internal magnetic field established by the Pioneer Venus magnetometer experiment. The most compelling objection to the model suggested by Knudsen et al. has been the fact that it does not explain the observed interplanetary magnetic field (IMF) control of the polarity of the ionospheric hole flux tubes. In this presentation I suggest that a magnetic reconnection process analogous to that occurring at earth is occurring at Venus between the IMF and a weak internal dipole field. At Venus in the subsolar region, the reconnection occurs within the ionosphere. At Earth it occurs at the magnetopause. Reconnection will occur only when the IMF has an appropriate orientation relative to that of the weak internal field. Thus, reconnection provides a process for the IMF to control the flux tube polarity. The reconnection in the subsolar region takes place in the ionosphere as the barrier magnetic field is transported downward into the lower ionosphere by downward convection of ionospheric plasma and approaches the oppositely directed internal magnetic field that is diffusing upward. The reconnected flux tubes are then transported anti-Sunward by the anti-Sunward convecting ionospheric plasma as well as by the anti-Sunward-flowing solar wind. Reconnection will also occur in the Venus magnetic tail region, somewhat analogously to the reconnection that occurs in the magnetotail of the Earth. The possibility that reconnection is occurring between the IMF and an internal dipole field may be tested by measuring the orientation of the IMF projected into a plane perpendicular to the solar wind velocity during time intervals for which ionospheric holes are observed. The orientations of the IMV components should fall within a 180 deg angle.

Knudsen, W. C.↗
NASA NTRS1972

A model for Jupiter's proton radiation belt

A model for electron and proton energy and density in Jupiter's radiation belt is proposed. It is assumed that electrons diffuse inward from the solar wind. The Davis and Chang diffusion model, in which there is a very steep L-shell dependence in density and energy, is scaled outwards from the peak of the belts at about 1.5 to 2 Jupiter radii to the magnetopause as defined by the magnetic moment. The energetic electron density is calculated to be one billionth of the density in the solar wind, and that is assumed to be the trapping ratio of Jupiter's synchrotron belts. It is further assumed that the same figures apply to protons. The flux density at 80 MHz was measured by the Culgoora ring to be identical to that at 3000 MHz, implying a constant flux density of the synchrotron source of over 20 or 30 to 1 in frequency. It is suggested that the synchrotron spectrum is flat over more than an order of magnitude of frequency. Jupiter is an enormous synchrotron source and has a strong magnetic field, causing long diffusion times for electrons in the magnetosphere.

Warwick, J.↗
NASA NTRS2001

Measurement of persistence in 1D diffusion

Using a novel NMR scheme we observed persistence in 1D gas diffusion. Analytical approximations and numerical simulations have indicated that for an initially random array of spins undergoing diffusion, the probability p(t) that the average spin magnetization in a given region has not changed sign (i.e., "persists") up to time t follows a power law t(-straight theta), where straight theta depends on the dimensionality of the system. Using laser-polarized 129Xe gas, we prepared an initial "quasirandom" 1D array of spin magnetization and then monitored the ensemble's evolution due to diffusion using real-time NMR imaging. Our measurements are consistent with analytical and numerical predictions of straight theta approximately 0.12.

NASA Discipline Life Sciences Technologies↗
NASA NTRS1988

Test particle acceleration in turbulent reconnecting magnetic fields

The effect of turbulence on particle acceleration in a MHD field was investigated by computing test particle trajectories in turbulent MHD reconnecting fields, including reconnection simulations at different magnetic Reynolds numbers. The dynamics of individual particles were investigated making it possible to examine the acceleration mechanism in great detail. It was found that turbulence influences the acceleration in two ways. It enhances the reconnection electric field while producing a stochastic electric field that gives rise to momentum diffusion; and it produces magnetic 'bubbles' and other irregularities that can temporarily trap test particles in the strong reconnection electric field for times comparable to the magnetofluid characteristic time.

Ambrosiano, John↗
NASA NTRS2018

In Situ Observation of Magnetic Reconnection Between an Earthward Propagating Flux Rope and the Geomagnetic Field

It has been proposed that, in the near-Earth magnetotail, earthward propagating flux ropes can merge with the Earth's dipole magnetic field and dissipate its magnetic energy. However, the reconnection diffusion region related to this process has not been identified. Here we report the first in situ observation of magnetic reconnection between an earthward propagating flux rope and the closed magnetic field lines connecting to Earth. Magnetospheric Multiscale (MMS) spacecraft crossed a vertical current sheet between the leading edge of the flux rope (negative B(sub z)) and the geomagnetic field (positive B(sub z)). The subion-scale current sheet, super-Alfvénic electron outflow, Hall magnetic and electric field, conversion of magnetic energy to plasma energy (J·E > 0), and magnetic null were observed during the crossing. All the above signatures indicate that MMS detected the reconnection diffusion region. This result is also relevant to other planets with intrinsic magnetosphere.

Magnetospheric Multiscale↗
NASA NTRS1993

Magnetic reconnection with large separatrix angles

The magnetic reconnection process is studied here using incompressible MHD simulations with different inflow boundary conditions and different magnetic Reynolds numbers R(m). The angle between the magnetic separatrices is in steady state reconnection depends mainly on the normal magnetic field on the inflow boundary. In steady state nonuniform reconnection with large separatrix angles, field-aligned plasma jets appear slightly downstream of the magnetic separatrices. The field-aligned plasma jet are stronger when R(m) is larger. Each field-aligned plasma jet consists of two parts: a slow shock and a fast-mode compressional wave. The slow shock converts the magnetic energy into plasma kinetic energy by acceleration and heating. The fast-mode compressional wave decelerates the plasma to a smaller outflow speed and heats it further. Nearly all the magnetic energy flowing into the diffusion region is converted into other forms. The length and width of the diffusion region depend on the values of the reconnection rate, R(m), and the normal magnetic field on the inflow boundary.

Yan, M.↗
NASA NTRS1990

Magnetic field-aligned coupling effects on ionospheric plasma structure

This paper presents a mathematical description of the electrical coupling and dynamics of plasma structure in the E and F regions. The scale size dependence of the electric field coupling along the magnetic field is examined for a realistic background ionosphere and atmosphere. It is shown that, while normalized potentials map reciprocally between two altitudes, the potential disturbance caused by a fixed amplitude plasma density perturbation does not. The magnitude of electrostatic potential created by structured ionization is also shown to be strongly dependent on the altitude of the structure. The role of diffusion parallel to the magnetic field in the redistribution and decay of plasma structure is illustrated.

Heelis, R. A.↗
NASA NTRS1986

Collisional tearing instability in the current sheet with a low magnetic Lundquist number

An MHD model is used to calculate the growth rate of the collisional tearing mode instability in a current layer with a Lundquist number in the range 3-100,000. The maximum growth rate is found to be a function of the Alfven transit time in the current sheet and the Lundquist number. When the Lundquist number is below 50, the growth rate asymptotically approaches a value of about -0.075/transit time. The discovery of the dependence of the asymptotic value is considered to be of use in studies of the current sheet in the diffusion region of a magnetic reconnection configuration, where the Lundquist number will be in the range 2-100.

Lee, L. C.↗
NASA NTRS1991

Magnetic flux transport and the sun's dipole moment - New twists to the Babcock-Leighton model

The mechanisms that give rise to the sun's large-scale poloidal magnetic field are explored in the framework of the Babcock-Leighton (BL) model. It is shown that there are in general two quite distinct contributions to the generation of the 'alpha effect': the first is associated with the axial tilts of the bipolar magnetic regions as they erupt at the surface, while the second arises through the interaction between diffusion and flow as the magnetic flux is dispersed over the surface. The general relationship between flux transport and the BL dynamo is discussed.

Wang, Y.-M.↗