Jean Heyvaerts

A FLUX-TUBE TECTONICS MODEL FOR SOLAR CORONAL HEATING DRIVEN BY THE MAGNETIC CARPET

We explore some of the consequences of the magnetic carpet for coronal heating. Observations show that most of the magnetic flux in the quiet Sun emerges as ephemeral regions and then quickly migrates to supergranule boundaries. The original ephemeral concentrations fragment, merge, and cancel over a time period of 10-40 hr. Since the network photospheric flux is likely to be concentrated in units of 1017 Mx or smaller, there will be myriads of coronal separatrix surfaces caused by the highly fragmented photospheric magnetic configuration in the quiet network. We suggest that the formation and dissipation of current sheets along these separatrices are an important contribution to coronal heating. The dissipation of energy along sharp boundaries we call, by analogy with geophysical plate tectonics, the tectonics model of coronal heating. Similar to the case on Earth, the relative motions of the photospheric sources will drive the formation and dissipation of current sheets along a hierarchy of such separatrix surfaces at internal dislocations in the corona. In our preliminary assessment of such dissipation we find that the heating is fairly uniform along the separatrices, so that each elementary coronal flux tube is heated uniformly. However, 95% of the photospheric flux closes low down in the magnetic carpet and the remaining 5% forms large-scale connections, so the magnetic carpet will be heated more effectively than the large-scale corona. This suggests that unresolved observations of coronal loops should exhibit enhanced heating near their feet in the carpet, while the upper parts of large-scale loops should be heated rather uniformly but less strongly.

The collimation of magnetized winds

It is established that any stationary axisymmetric magnetized wind will collimate along the symmetry axis at large distances from the source. This result is proved by consideration of the asymoptotic properties of the transfield equation, keeping the exact conserved quantities along field lines. The only consistent nonsingular solution with a nonvanishing poloidal current approaches a cylindrical structure. For singular solutions or those with a vanishing poloidal current, the asymptotic solutions can be paraboloidal. This result only applies to pure wind boundary conditions on the surface of the source. It is shown how the boundary conditions and the critical point analysis are related in our asymptotic analysis. This result demonstrates that axisymmetric magnetized flows tend generally to collimate, and it is hypothesized that this is the natural reason why there are so many collimated flows and jets. 15 refs.

Nature of the heating mechanism for the diffuse solar corona

The temperature of the Suns outer atmosphere (the corona) exceeds that of the solar surface by about two orders of magnitude, but the nature of the coronal heating mechanisms has long been a mystery. The corona is a magnetically dominated environment, consisting of a variety of plasma structures including X-ray bright points, coronal holes and coronal loops. The latter are closed magnetic structures that occur over a range of scales and are anchored at each end in the solar surface. Large-scale regions of diffuse emission are made up of many long coronal loops. Here we present X-ray observations of the diffuse corona from which we deduce its likely heating mechanism. We find that the observed variation in temperature along a loop is highly sensitive to the spatial distribution of the heating. From a comparison of the observations and models we conclude that uniform heating gives the best fit to the loop temperature distribution, enabling us to eliminate previously suggested mechanisms of low-lying heating near the footpoints of a loop. Our findings favour turbulent breaking and reconnection of magnetic field lines as the heating mechanism of the diffuse solar corona.

A self-consistent turbulent model for solar coronal heating

The rate of solar coronal heating induced by the slow random motions of the dense photosphere is calculated in the framework of an essentially parameter-free model. This model assumes that these motions maintain the corona in a state of small-scale MHD turbulence. The associated dissipative effects then allow a large-scale stationary state to be established. The solution for the macroscopic coronal flow and the heating flux is first obtained assuming the effective (turbulent) dissipation coefficients to be known. In a second step these coefficients are calculated by the sefl-consistency argument that they should result from the level of turbulence associated with this very heating flux

MAGNETIC FIELD DIFFUSION IN SELF-CONSISTENTLY TURBULENT ACCRETION DISKS

We show how the level of turbulence in accretion disks can be derived from a self-consistency requirement that the associated effective viscosity should match the instantaneous accretion rate. This method is applicable when turbulence has a direct energy cascade. Only limited information on the origin and properties of the turbulence, such as its injection scale and anisotropy, is needed. The method is illustrated by considering the case of turbulence originating from the magnetic shearing instability. The corresponding effective kinematic viscosity coefficient is shown to scale as the 1/3 power of surface mass density at a given radius in optically thick disks, and to be describable by a Shakura-Sunyaev law with α≈ 0.04. Mass flow in disks fed at a localized hot spot is calculated for accretion regimes driven by such turbulence, as well as passive magnetic field diffusion and dragging. An important result of this analysis is that thin disks supported by turbulence driven by the magnetic shearing instability, and more generally any turbulence with injection scale of order of the disk thickness, are very low magnetic Reynolds number systems. Turbulent viscosity-driven solutions with negligible field dragging and no emission of cold winds or jets are natural consequences of such regimes. Disks of accreting objects that are magnetized enough to be shielded by a magnetopause, however, may not operate in their innermost regions in the magnetic shearing instability regime. The possibility therefore remains to be explored of centrifugally driven winds emanating from such regions.

Global Asymptotic Solutions for Relativistic Magnetohydrodynamic Jets and Winds

We consider relativistic, stationary, axisymmetric, polytropic, unconfined, perfect MHD winds, assuming their five Lagrangian first integrals to be known. The asymptotic structure consists of field regions bordered by boundary layers along the polar axis and at null surfaces, such as the equatorial plane, which have the structure of charged column or sheet pinches supported by plasma or magnetic poloidal pressure. In each field-region cell, the proper current (defined here as the ratio of the asymptotic poloidal current to the asymptotic Lorentz factor) remains constant. Our solution is given in the form of matched asymptotic solutions separately valid outside and inside the boundary layers. A Hamilton-Jacobi equation, or equivalently a Grad-Shafranov equation, gives the asymptotic structure in the field regions of winds that carry Poynting flux to infinity. An important consistency relation is found to exist between axial pressure, axial current, and asymptotic Lorentz factor. We similarly derive WKB-type analytic solutions for winds that are kinetic energy-dominated at infinity and whose magnetic surfaces focus to paraboloids. The density on the axis in the polar boundary column is shown to slowly fall off as a negative power of the logarithm of the distance to the wind source. The geometry of magnetic surfaces in all parts of the asymptotic domain, including boundary layers, is explicitly deduced in terms of the first integrals.

A magnetic loop model for structure and activity in the Galactic center

A simple model for the production and propagation of magnetized loops in the Galactic center is developed. It is found that a magnetized model can reproduce the filamentary, asymmetric structures on all scales from 100 pc to subparsec scales in the Galactic center. The radio and molecular gas morphology on all scales inside the Galactic center lobe may be explained in terms of the expansion of strongly magnetized loops and their interaction with the 2-5 pc torus and 30-50 pc portion of the center lobe. The magnetic loops have a toroidal field which explains the braided appearance of the radio continuum emission on the 2 pc scale. The barlike feature on the 2 pc radio continuum maps can be viewed as a piece of a magnetic loop with field strength about 0.01 G which is colliding and reconnecting with the ionized inner edge of the 2 pc molecular torus. The loop-torus interaction feeds energetic particles and shear Alfven waves into the torus, and deposits mass and energy. 83 references.

Structuring and support by Alfvén waves around prestellar cores

Observations of molecular clouds show the existence of starless, dense cores, threaded by magnetic fields. Observed line widths indicate these dense condensates to be embedded in a supersonically turbulent environment. Under these conditions, the generation of magnetic waves is inevitable. In this paper, we study the structure and support of a 1D plane-parallel, self-gravitating slab, as a monochromatic, circularly polarized Alfven wave is injected in its central plane. Dimensional analysis shows that the solution must depend on three dimensionless parameters. To study the nonlinear, turbulent evolution of such a slab, we use 1D high resolution numerical simulations. For a parameter range inspired by molecular cloud observations, we find the following. 1) A single source of energy injection is sufficient to force persistent supersonic turbulence over several hydrostatic scale heights. 2) The time averaged spatial extension of the slab is comparable to the extension of the stationary, analytical WKB solution. Deviations, as well as the density substructure of the slab, depend on the wave-length of the injected wave. 3) Energy losses are dominated by loss of Poynting-flux and increase with increasing plasma beta. 4) Good spatial resolution is mandatory, making similar simulations in 3D currently prohibitively expensive.

THE MAGNETIC COUPLING OF PLANETS AND SMALL BODIES WITH A PULSAR'S WIND

We investigate the electromagnetic interaction of a relativistic stellar wind with a planet or a smaller body in orbit around a pulsar. This may be relevant to objects such as PSR B1257+12 and PSR B1620-26 that are expected to hold a planetary system, or to pulsars with suspected asteroids or comets. Most models of pulsar winds predict that, albeit highly relativistic, they are slower than Alfven waves. In that case, a pair of stationary Alfven waves, called Alfven wings (AW), is expected to form on the sides of the planet. The wings expand far into the pulsars wind and they could be strong sources of radio emissions. The Alfven wings would cause a significant drift over small bodies such as asteroids and comets.

A magnetic thrust action on small bodies orbiting a pulsar

Aims. We investigate the electromagnetic interaction of a relativistic stellar wind with small bodies in orbit around the star. Methods. Based on our work on the theory of Alfven wings to relativistic winds presented in a companion paper, we estimate the force exerted by the associated current system on orbiting bodies and evaluate the resulting orbital drift. Results. This Alfvenic structure is found to have no significant influence on planets or smaller bodies orbiting a millisecond pulsar. On the time scale of millions of years, it can however affect the orbit of bodies with a diameter of 100 km around standard pulsars with a period P ∼ 1 s and a magnetic field B ∼ 10 8 T. Kilometer-sized bodies experience drastic orbital changes on a time scale of 10 4 years.