O. Steiner

Magnetic tornadoes as energy channels into the solar corona

Heating the outer layers of the magnetically quiet solar atmosphere to more than one million kelvin and accelerating the solar wind requires an energy flux of approximately 100 to 300 watts per square metre, but how this energy is transferred and dissipated there is a puzzle and several alternative solutions have been proposed. Braiding and twisting of magnetic field structures, which is caused by the convective flows at the solar surface, was suggested as an efficient mechanism for atmospheric heating. Convectively driven vortex flows that harbour magnetic fields are observed to be abundant in the photosphere (the visible surface of the Sun). Recently, corresponding swirling motions have been discovered in the chromosphere, the atmospheric layer sandwiched between the photosphere and the corona. Here we report the imprints of these chromospheric swirls in the transition region and low corona, and identify them as observational signatures of rapidly rotating magnetic structures. These ubiquitous structures, which resemble super-tornadoes under solar conditions, reach from the convection zone into the upper solar atmosphere and provide an alternative mechanism for channelling energy from the lower into the upper solar atmosphere.

Dynamical Interaction of Solar Magnetic Elements and Granular Convection: Results of a Numerical Simulation

Nonstationary convection in the solar photosphere and its interaction with photospheric magnetic structures (flux sheets in intergranular lanes) have been simulated using a numerical code for two-dimensional MHD with radiative energy transfer. Dynamical phenomena are identified in the simulations, which may contribute to chromospheric and coronal heating. Among these are the bending and horizontal displacement of a flux sheet by convective flows and the excitation and propagation of shock waves both within and outside the magnetic structure. Observational signatures of these phenomena are derived from calculated Stokes profiles of Zeeman-sensitive spectral lines. We suggest that the extended red wings of the observed Stokes V profiles are due to downward coacceleration of magnetized material in a turbulent boundary layer between the flux sheet and the strong external downflow. Upward-propagating shocks in magnetic structures should be detectable if a time resolution of about 10 s is achieved, together with a spatial resolution that allows one to isolate individual magnetic structures. Determination of the complicated internal dynamics of magnetic elements requires observations with a spatial resolution better than 100 km in the solar photosphere.

Dynamics of the Solar Magnetic Network: Two-dimensional MHD Simulations

The aim of this work is to identify the physical processes that occur in the network and contribute to its dynamics and heating. We model the network as consisting of individual flux tubes, each with a nonpotential field structure, that are located in intergranular lanes. With a typical horizontal size of about 150 km at the base of the photosphere, they expand upward and merge with their neighbors at a height of about 600 km. Above a height of approximately 1000 km the magnetic field starts to become uniform. Waves are excited in this medium by means of motions at the lower boundary. We focus on transverse driving, which generates both fast and slow waves within a flux tube and acoustic waves at the interface of the tube and the ambient medium. The acoustic waves at the interface are due to compression of the gas on one side of the flux tube and expansion on the other. These longitudinal waves are guided upward along field lines at the two sides of the flux tube, and their amplitude increases with height due to the density stratification. Being acoustic in nature, they produce a compression and significant shock heating of the plasma in the chromospheric part of the flux tube. For impulsive excitation with a time constant of 120 s, we find that a dominant feature of our simulations is the creation of vortical motions that propagate upward. We have identified an efficient mechanism for the generation of acoustic waves at the tube edge, which is a consequence of the sharp interface of the flux concentration. We examine some broad implications of our results.

DETECTION OF VORTEX TUBES IN SOLAR GRANULATION FROM OBSERVATIONS WITH SUNRISE

We have investigated a time series of continuum intensity maps and corresponding Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory SUNRISE. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. From cross sections through the computational domain of the simulation, we conclude that these granular lanes are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. We propose that the observed vortex tubes may represent only the large-scale end of a hierarchy of vortex tubes existing near the solar surface.

THE HORIZONTAL INTERNETWORK MAGNETIC FIELD: NUMERICAL SIMULATIONS IN COMPARISON TO OBSERVATIONS WITH HINODE

Observations with the Hinode space observatory led to the discovery of predominantly horizontal magnetic fields in the photosphere of the quiet internetwork region. Here we investigate realistic numerical simulations of the surface layers of the Sun with respect to horizontal magnetic fields and compute the corresponding polarimetric response in the Fe i 630 nm line pair. We find a local maximum in the mean strength of the horizontal field component at a height of around 500 km in the photosphere, where, depending on the initial state or the boundary condition, it surpasses the vertical component by a factor of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean horizontal field component that is 1.6 or 4.3 times stronger than the vertical component, depending on the initial state or the boundary condition. This is a consequence of both the intrinsically stronger flux density of and the larger area occupied by the horizontal fields. We find that convective overshooting expels horizontal fields to the upper photosphere, making the Poynting flux positive in the photosphere, whereas the Poynting flux is negative in the convectively unstable layer below it.

Spectral signature of magnetic flux tubes in sunspot penumbrae

We study the polarization of spectral lines in the penumbra by integrating the radiative transfer equation of polarized light for a three-dimensional model atmosphere of a sunspot. In this model, the Evershed flow is confined to magnetic flux tubes which are embedded in a static background atmosphere, in accordance with the moving tube model of Schlichenmaier et al. (1998a,b). The gradients and/or discontinuities in the fluid velocity and the magnetic field at the flux tube boundaries give rise to asymmetric Stokes profiles. We concentrate on the Stokes-V profiles and study the net circular polarization (NCP) of two photospheric spectral lines of neutral iron, Fe I 630.25 nm and Fe I 1564.8 nm. The dierent behavior of these two lines, which are exemplary for atomic spectral lines with a large Lande factor and significantly dierent wavelength, is based on the dierence in excitation potential of the corresponding atomic transitions and the fact that the wavelength dependence of the Doppler shift is linear, while that of the Zeeman splitting is quadratic. We find that the azimuthal variation of the NCP,N( ), is a predominantly antisymmetric function of with respect to the line connecting disk center and spot center (line-of-symmetry) for the infrared line of Fe I 1564.8 nm, while the variation is predominantly symmetric for Fe I 630.25 nm. We show that the antisymmetric variation is caused by anomalous dispersion (Faraday pulsation) and the discontinuity in the azimuthal angle of the magnetic field, which is due to the relative inclination between flux tube and background field. We then compute synthetic NCP maps of a sunspot and compare them with observational results. Finally, the center-to-limb variation of the NCP,N(), of these spectral lines is investigated. We show that the location of the zero-crossing point ofN() on the center side of the line-of-symmetry represents a diagnostic tool to determine the inclination angle of the Evershed flow: A vanishing NCP on the center-side of the line-of-symmetry is an indirect evidence of downflows in the penumbra.

Dynamic Interaction of Convection with Magnetic Flux Sheets: First Results of a New MHD Code

Methods, tests, and first results of a new code for the simulation of solar magnetoconvection are presented. Special emphasis is laid on a reliable and economic treatment of small-scale structures like magnetic flux concentrations, current layers and shocks in 3-D, time-dependent simulations. The numerical grid is adjusted to the local resolution requirements using the method of Adaptive Mesh Refinement (AMR). We describe the implementation of the AMR technique in an MHD code and show a number of test cases. First results on the interaction of magnetic flux sheets with radiative, non-stationary convection in the solar atmosphere have been obtained with a 2-D version of the code. Besides recovering a number of basic features of previous models for such structures, we find a spectacular new phenomenon: strong bending of a flux sheet by asymmetric convective flow followed by rapid sweeping back due to buoyancy and magnetic tension. Such events may lead to the excitation of transversal MHD waves and therefore possibly contribute to heating the upper solar atmosphere.

Wave propagation and energy transport in the magnetic network of the Sun

Aims. We investigate wave propagation and energy transport in magnetic elements, which are representatives of small scale magnetic flux concentrations in the magnetic network on the Sun. This is a continuation of earlier work by Hasan et al. (2005, ApJ, 631, 1270). The new features in the present investigation include a quantitative evaluation of the energy transport in the various modes and for different field strengths, as well as the effect of the boundary-layer thickness on wave propagation. Methods. We carry out 2D MHD numerical simulations of magnetic flux concentrations for strong and moderate magnetic fields for which β (the ratio of gas to magnetic pressure) on the tube axis at the photospheric base is 0.4 and 1.7, respectively. Waves are excited in the tube and ambient medium by a transverse impulsive motion of the lower boundary. Results. The nature of the modes excited depends on the value of β. Mode conversion occurs in the moderate field case when the fast mode crosses the β = 1 contour. In the strong field case the fast mode undergoes conversion from predominantly magnetic to predominantly acoustic when waves are leaking from the interior of the flux concentration to the ambient medium. We also estimate the energy fluxes in the acoustic and magnetic modes and find that in the strong field case, the vertically directed acoustic wave fluxes reach spatially averaged, temporal maximum values of a few times 10 6 erg cm −2 s −1 at chromospheric height levels. Conclusions. The main conclusions of our work are twofold: firstly, for transverse, impulsive excitation, flux tubes/sheets with strong fields are more efficient than those with weak fields in providing acoustic flux to the chromosphere. However, there is insufficient energy in the acoustic flux to balance the chromospheric radiative losses in the network, even for the strong field case. Secondly, the acoustic emission from the interface between the flux concentration and the ambient medium decreases with the width of the boundary layer.

The formation of asymmetric Stokes V profiles in the presence of a magnetopause

A magnetopause that separates two regimes of different flow, additional to the separation of a magnetic field from a field-free plasma, gives rise to the formation of asymmetric Stokes profiles. Using a simple two-layer model atmosphere, where one layer comprises a magnetic field, the other being field-free, it is shown by analytical derivation that a wide variety of Stokes V profiles can be produced, having amplitude asymmetries δa in the range −∞≤δa≤∞. These include two-humped V profiles, which have two lobes of equal sign. For the most simple models, the asymmetry depends on the ratio of continuum intensity to the Planck radiation intensity of the magnetic layer at the wavelength of the spectral line under consideration, and on the line depth. Two-humped profiles (|δa|>1) require the temperature of the magnetic layer to surpass the temperature of the line-core forming region, implying a temperature inversion, so that the V profile is partially in emission. The confrontation of this formation scenario with properties of observed one-lobe profiles of quiet-Sun network regions is inconclusive due to insufficient spatial resolution and lack of a sufficient sample of simultaneously recorded Stokes spectral lines of varying line depths. It seems, however, to be in good agreement with the observed frequent occurrence of abnormal V profiles of the very strong Na i D2 and D2 spectral line. A possible observational verification for the present formation scenario of abnormal Stokes V profiles and a novel method of Stokes inversion are discussed.

Excitation of Slow Modes in Network Magnetic Elements Through Magnetic Pumping

From radiation magnetohydrodynamic simulations of the solar atmosphere, we find a new mechanism for the excitation of longitudinal slow modes within magnetic flux concentrations. We find that the convective downdrafts in the immediate surroundings of magnetic elements are responsible for the excitation of slow modes. The coupling between the external downdraft and the plasma motion internal to the flux concentration is mediated by the inertial forces of the downdraft that act on the magnetic flux concentration. These forces, in conjunction with the downward movement, pump the internal atmosphere in the downward direction, which entails a fast downdraft in the photospheric and chromospheric layers of the magnetic element. Subsequent to the transient pumping phase, the atmosphere rebounds, causing a slow mode traveling along the magnetic flux concentration in the upward direction. It develops into a shock wave in chromospheric heights, possibly capable of producing some kind of dynamic fibril. We propose an observational detection of this process.