Engineering topics

Strauss, H. R.

Publications and source records attributed to Strauss, H. R..

10 records

NASA NTRS1991

Kelvin-Helmholtz instability in an Alfven resonant layer of a solar coronal loop

A Kelvin-Helmholtz instability has been identified numerically on an azimuthally symmetric Alfven resonant layer in an axially bounded, straight cylindrical coronal loop. The set of equations is solved numerically as an initial value problem. The linear growth rate of this instability is shown to be approximately proportional to the Alfven driving amplitude and inversely proportional to the width of the Alfven resonant layer. It is also shown that the linear growth rate increases linearly with m - 1 up to a certain m, reaches its maximum value for the mode whose half wavelength is comparable to the Alfven resonant layer width, and decreases at higher azimuthal mode number.

Uchimoto, E.↗

NASA NTRS1991

Nonresonant absorption of shear Alfven waves

Resonant absorption of shear Alfven waves is thought to be a likely candidate to explain heating of the solar corona and acceleration of the solar wind. A difficulty with the theory is that the absorption process is slow. Moreover, heating occurs in a very thin layer. A faster absorption mechanism is nonresonant absorption by compressional viscosity, in a curved magnetic field. Heating is nonresonant and is not localized to a narrow layer. The effect could be quite important where the solar coronal magnetic field is strongly curved, in the chromosphere. It could also be important on open field lines in the upper corona, where the compressional viscosity is large. It might imply that a significant part of outgoing Alfven waves are absorbed in the corona.

Strauss, H. R.↗

NASA NTRS1990

Three-dimensional driven reconnection in magnetic loops

A numerical study of reconnection in a three-dimensional axially bounded magnetic flux tube is presented. An axial magnetic field is embedded in a conducting fluid, between conducting end plates. An applied flow in the end plates causes distortion and stretching of the magnetic field in the flux tube, and the creation of current sheets. After the current sheets form, the reconnection rate is fast; with a merging rate M of about 1. The reconnection mechanism resembles flux pileup in two dimensions. It is also similar to the nonlinear behavior of a subcritically excited current-driven MHD mode. The numerical model is relevant to a mechanism thought to be important for heating the solar corona, as well as producing solar flares.

Strauss, H. R.↗

NASA NTRS1989

Computer simulation of Alfven resonance in a cylindrical, axially bounded flux tube

The resonant absorption of Alfven waves in an axially bounded cylindrical flux tube is investigated in a dissipative MHD simulation. It is found that in an axially bounded flux tube, in contrast to an infinite periodic model, the resonant frequency is nearly independent of the poloidal component of the magnetic field. This is a consequence of the 'ballooning' structure of the resonant Alfven waves. The scaling with resistivity and viscosity of the width of the resonance layer, the dissipation rate, and the time for steady state absorption to occur, are all in agreement with theory.

Strauss, H. R.↗

NASA NTRS1989

The effect of ballooning modes on thermal transport and magnetic field diffusion in the solar corona

Presently favored mechanisms of coronal heating (current sheet dissipation and Alfven wave resonant heating) deposit heat in thin layers. Classical thermal conduction cannot explain how heat is transported across the magnetic field. If heating occurs in thin layers, large pressure gradients can be created which can give rise to ballooning modes. These instabilities are caused by the pressure gradient and the curvature of the magnetic field, and are stabilized by magnetic tension. The modes are broad band in wavelength and should produce turbulence. A mixing length expression for the turbulent heat transport shows that it is more than adequate to rapidly convect heat into much broader layers. Furthermore, the turbulent resistivity implies that heating occurs over most of the width of these broadened layers. The broadening also implies that much shorter time scales are required for heating. The beta values in the corona suggest that 1-10 turbulent layers are formed in typical loop or arch structures.

Strauss, H. R.↗

NASA NTRS1988

Turbulent reconnection

The reconnection rate in the presence of tearing mode turbulence is calculated. Tearing modes can produce turbulence in three-dimensional sheared magnetic fields. They produce a turbulent anomalous electron viscosity or hyperresistivity. Using quasi-linear and direct interaction approximations, the scaling of the hyperresistivity with magnetic perturbation strength and other parameters is estimated. This yields a reconnection rate, or inflow Mach number M, scaling as delta B, the ratio of rms magnetic fluctuations to mean magnetic field. This can greatly exceed the Sweet-Parker rate. The rate of heating of the current sheet is also large and can be many orders of magnitude larger than in a Sweet-Parker current sheet. Hyperresistivity produced by tearing mode turbulence may explain both fast reconnection and the heating of the solar corona.

Strauss, H. R.↗

NASA NTRS1988

Current sheets in the solar corona

Coronal magnetic fields are twisted up by motion of their footpoints in the photosphere. When the twist exceeds a critical amount, kink-ballooning instabilities occur. These instabilities are studied numerically, in long, thin, axially bounded magnetic fields. Nonlinearly, the three-dimensional kinking motion compresses magnetic flux, forming a current sheet. Magnetic energy can be dissipated at a rate orders of magnitude greater than without the current sheets. The energy of footpoint motion can then go into coronal heating, via Ohmic dissipation in the current sheets.

Strauss, H. R.↗

NASA NTRS1988

Current-driven resistive ballooning modes in axially bounded solar flare plasmas

The most unstable current-driven resistive modes of an axially bounded coronal loop are found in computer simulations to exhibit the spatial structure of ballooning modes. The observed modes are not confined to mode rational surfaces, but instead have broad radial extent. A theory assuming ballooning mode spatial structure predicts that a minimum current should be required for linear instability, and that, when the mode is unstable, the linear growth rate scales linearly with the resistivity eta below a critical resistivity, and scales as cu root of eta for larger resistivities. Both predictions are borne out by simulation results. Both theory and simulation analyses of the mode suggest that the strong radial structure of the mode near the ends of the system is the primary contributing factor to the instability of the mode. A helical current sheet is formed in the nonlinear evolution of the mode near the edge of the current channel and is accompanied by a strong radial gradient in the current and partial current reversal.

Otani, Niels F.↗

NASA NTRS1986

Hyper-resistivity produced by tearing mode turbulence

Tearing mode turbulence produces a hyperresistivity or effective anomalous electron viscosity. The hyperresistivity is calculated for the mean magnetic field quasi-linearly, and for long-wavelength modes using the direct interaction approximation. The hyperresistivity accounts for current relaxation in reversed-field pinch experiments, and gives a magnetic fluctuation scaling of S to the -1/3. It causes enhanced tearing mode growth rates in the turbulent phase of tokamak disruptions. In astrophysics, it limits magnetic energy growth because of the dynamo effect, and may explain rapid reconnection phenomena such as solar flares.

Strauss, H. R.↗

NASA NTRS1986

Resonant fast dynamo

A resonant fast dynamo is found in chaotic shear flows. The dynamo effect is produced by resonant perturbations of the velocity field, similar to resonant diffusion in plasma physics. The dynamo is called fast because the flow produces an electric field independent of the fluid resistivity.

Strauss, H. R.↗