Generation of Flows in the Solar Chromosphere Due to Magnetofluid Coupling (original) (raw)
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Generation of Flows in the Solar Atmosphere Due to Magnetofluid Coupling
The Astrophysical Journal, 2002
It is shown that a generalized magneto-Bernoulli mechanism can effectively generate high velocity flows in the Solar chromosphere by transforming the plasma pressure energy into kinetic energy. It is found that at reasonable heights and for realistic plasma parameters, there is a precipitous pressure fall accompanied by a sharp amplification of the flow speed.
2005
Within the framework of a two-fluid description possible pathways for the generation of fast flows (dynamical as well as steady) in the lower solar atmosphere is established. It is shown that a primary plasma flow (locally sub-Alfvénic) is accelerated when interacting with emerging/ambient arcade--like closed field structures. The acceleration implies a conversion of thermal and field energies to kinetic energy of the flow. The time-scale for creating reasonably fast flows ($\gtrsim 100$ km/s) is dictated by the initial ion skin depth while the amplification of the flow depends on local beta\beta beta. It is shown, for the first time, that distances over which the flows become "fast" are sim0.01Rs\sim 0.01 R_ssim0.01Rs from the interaction surface; later the fast flow localizes (with dimensions lesssim0.05RS\lesssim 0.05 R_Slesssim0.05RS) in the upper central region of the original arcade. For fixed initial temperature the final speed ($\gtrsim 500 km/s$) of the accelerated flow, and the modification of the field structure are independent of the time-duration (life-time) of the initial flow. In the presence of dissipation, these flows are likely to play a fundamental role in the heating of the finely structured Solar atmosphere.
2005
Within the framework of a two–fluid description possible pathways for the generation of fast flows (dynamical as well as steady) in the lower solar atmosphere is established. It is shown that a primary plasma flow (locally sub– Alfvénic) is accelerated when interacting with emerging/ambient arcade–like closed field structures. The acceleration implies a conversion of thermal and field energies to kinetic energy of the flow. The time–scale for creating reasonably fast flows (& 100 km/s) is dictated by the initial ion skin depth while the amplification of the flow depends on local β. It is shown, for the first time, that distances over which the flows become ”fast” are ∼ 0.01 Rs from the interaction surface; later the fast flow localizes (with dimensions . 0.05 RS) in the upper central region of the original arcade. For fixed initial temperature the final speed (& 500 km/s) of the accelerated flow, and the modification of the field structure are independent of the time-duration (life–...
DYNAMICS OF MAGNETIZED VORTEX TUBES IN THE SOLAR CHROMOSPHERE
The Astrophysical Journal, 2012
We use 3D radiative MHD simulations to investigate the formation and dynamics of small-scale (less than 0.5 Mm in diameter) vortex tubes spontaneously generated by turbulent convection in quiet-Sun regions with initially weak mean magnetic fields. The results show that the vortex tubes penetrate into the chromosphere and substantially affect the structure and dynamics of the solar atmosphere. The vortex tubes are mostly concentrated in intergranular lanes and are characterized by strong (near sonic) downflows and swirling motions that capture and twist magnetic field lines, forming magnetic flux tubes that expand with height and which attain magnetic field strengths ranging from 200 G in the chromosphere to more than 1 kG in the photosphere. We investigate in detail the physical properties of these vortex tubes, including thermodynamic properties, flow dynamics, and kinetic and current helicities, and conclude that magnetized vortex tubes provide an important path for energy and momentum transfer from the convection zone into the chromosphere.
The Astrophysical Journal, 2002
ABSTRACT By modeling the closed field structures in the solar atmosphere by "slowly" evolving double Beltrami two-fluid equilibria (created by the interaction of the magnetic and velocity fields), the conditions for catastrophic transformations of the original state are derived. The analysis for predicting sudden changes is carried out through a set of algebraic equations obtained by relating the four characteristic parameters (two eigenvalues and two amplitudes) of the equilibrium to the macroscopic constants of motion (helicities and energy). It is shown that a catastrophic loss of equilibrium occurs when the macro scale of a closed structure, interacting with its local surroundings, decreases below a critical value; the catastrophe is possible only if the total energy of the structure (for given helicities) also exceeds a well-defined threshold. It is further shown that at the transition much of the magnetic energy of the original state is converted to the flow kinetic energy.
The Astrophysical Journal, 2007
Identifying the two physical mechanisms behind the production and sustenance of the quiescent solar corona and solar wind poses two of the outstanding problems in solar physics today. We present analysis of spectroscopic observations from the Solar and Heliospheric Observatory that are consistent with a single physical mechanism being responsible for a significant portion of the heat supplied to the lower solar corona and the initial acceleration of the solar wind; the ubiquitous action of magnetoconvection-driven reprocessing and exchange reconnection of the Sun's magnetic field on the supergranular scale. We deduce that while the net magnetic flux on the scale of a supergranule controls the injection rate of mass and energy into the transition region plasma it is the global magnetic topology of the plasma that dictates whether the released ejecta provides thermal input to the quiet solar corona or becomes a tributary that feeds the solar wind.
The magnetohydrodynamics of solar activity
Plasma Physics
The Sun represents a cosmic laboratory in which we may observe the exotic behaviour of a magnetized plasma at high magnetic Reynolds number. The basic properties of this plasma will be described together with some recent theories for several features of solar activity. The crucial role of the magnetic field will be stressed. The upper atmosphere of the Sun (the corona) is very much hotter (lo6 K) than the lower layers. It may possibly be heated by the absorption of magnetoacoustic waves at cusp resonances or by phase-mixed Alfvtn waves with Kelvin Helmoltz and tearing mode instabilities creating small-scale structure. Prominences are huge sheets of cool plasma up in the corona that last for months. They are formed by a thermal instability and are supported by the magnetic field. They exhibit flows and fine structure that are not understood.
Time-Dependent Two-Fluid Magnetohydrodynamic Model and Simulation of the Chromosphere
Solar Physics
The Sun's chromosphere is a highly dynamic, partially ionized region where spicules (hot jets of plasma) form. Here we present a two-fluid MHD model to study the chromosphere, which includes ion-neutral interaction and frictionalheating. Our simulation recovers a magnetic-canopy shape that forms quickly, but it is also quickly disrupted by the formation of a jet. Our simulation produces a shock self-consistently, where the jet is driven by the frictional-heating, which is much greater than the ohmic-heating. Thus, our simulation demonstrates that the jet could be driven purely by thermal effects due to ion-neutral collisions and not by magnetic reconnection. We plan to improve the model to include photo-chemical effects and radiation.
Fast Magnetic Reconnection in the Solar Chromosphere Mediated by the Plasmoid Instability
The Astrophysical Journal, 2015
Magnetic reconnection in the partially ionized solar chromosphere is studied in 2.5 dimensional magnetohydrodynamic simulations including radiative cooling and ambipolar diffusion. A Harris current sheet with and without a guide field is considered. Characteristic values of the parameters in the middle chromosphere imply a high magnetic Reynolds number of ∼10 6 -10 7 in the present simulations. Fast magnetic reconnection then develops as a consequence of the plasmoid instability without the need to invoke anomalous resistivity enhancements. Multiple levels of the instability are followed as it cascades to smaller scales, which approach the ion inertial length. The reconnection rate, normalized to the asymptotic values of magnetic field and Alfvén velocity in the inflow region, reaches values in the range ∼0.01-0.03 throughout the cascading plasmoid formation and for zero as well as for strong guide field. The outflow velocity reaches ≈40 km s −1 . Slow-mode shocks extend from the X-points, heating the plasmoids up to ∼8 × 10 4 K. In the case of zero guide field, the inclusion of both ambipolar diffusion and radiative cooling causes a rapid thinning of the current sheet (down to ∼30 m) and early formation of secondary islands. Both of these processes have very little effect on the plasmoid instability for a strong guide field. The reconnection rates, temperature enhancements, and upward outflow velocities from the vertical current sheet correspond well to their characteristic values in chromospheric jets.