Relation between current sheets and vortex sheets in stationary incompressible MHD

Magnetohydrodynamic configurations with strong localized current concentrations and vortices play an important role for the dissipation of energy in space and astrophysical plasma. Within this work we investigate the relation between current sheets and vortex sheets in incompressible, stationary equilibria. For this approach it is helpful that the similar mathematical structure of magnetohydrostatics and stationary incompressible hydrodynamics allows us to transform static equilibria into stationary ones. The main control function for such a transformation is the profile of the Alfven-Mach number M_A, which is always constant along magnetic field lines, but can change from one field line to another. In the case of a global constant M_A, vortices and electric current concentrations are parallel. More interesting is the nonlinear case, where M_A varies perpendicular to the field lines. This is a typical situation at boundary layers like the magnetopause, heliopause, the solar wind flowing around helmet streamers and at the boundary of solar coronal holes. The corresponding current and vortex sheets show in some cases also an alignment, but not in every case. For special density distributions in 2D it is possible to have current but no vortex sheets. In 2D vortex sheets of field aligned-flows can also exist without strong current sheets, taking the limit of small Alfven Mach numbers into account. The current sheet can vanish if the Alfven Mach number is (almost) constant and the density gradient is large across some boundary layer. It should be emphasized that the used theory is not only valid for small Alfven Mach numbers M_A<<1, but also for M_A~1. Connection to other theoretical approaches and observations and physical effects in space plasmas are presented. Differences in the various aspects of theoretical investigations of current sheets and vortex sheets are given.Comment: 10 pages, 2 figures, accepted for publication in Annales Geophysica

Are heliospheric flows magnetic line- or flux-conserving?

International audienceThis article discusses and tests the validity of the frozen in magnetic field paradigm (or 'ideal magnetohydrodynamics (MHD) constraint') which is usually adopted by many authors dealing with heliospheric physics. To show the problem of using ideal MHD in such a counterflow configuration like the heliosphere, we first recapitulate the basic concepts of freezing-in of magnetic fields, respectively magnetic topology conservation and its violation (= magnetic reconnection) in 3-D, already done by other authors with different methods with respect to derivations and interpretations. Then we analyse different heliospheric plasma environments. As a model of the stagnation region/stagnation point in front of the heliospheric nose, we present and discuss the general solution of the ideal MHD Ohm's law in the vicinity of a 2-D stagnation point, which was found by us. We show that ideal MHD either leads necessarily to a diverging magnetic field strength in the vicinity of such a stagnation point, or to a vanishing mass density on the heliopause boundaries. In the case that components of the electric field parallel to the magnetic field do not exist due to the chosen form of the non-ideal Ohm's law, it is always possible to formulate the transport equation of the magnetic field as a modified ideal Ohm's law. We find that the form of the Ohm's law which is often used in heliospheric physics (see e.g. Baranov and Fahr, 2003), is not able to change magnetic topology and thus cannot lead to magnetic reconnection, which necessarily has to occur at the stagnation point. The diverging magnetic field, for instance, implies the breakdown of the flux freezing paradigm for the heliosphere. Its application, especially at the heliospheric nose, is therefore rather doubtful. We conclude that it is necessary to search for an Ohm's law which is able to violate magnetic topology conservation

Self-consistent stationary MHD shear flows in the solar atmosphere as electric field generators

Magnetic fields and flows in coronal structures, for example, in gradual phases in flares, can be described by 2D and 3D magnetohydrostatic (MHS) and steady magnetohydrodynamic (MHD) equilibria. Within a physically simplified, but exact mathematical model, we study the electric currents and corresponding electric fields generated by shear flows. Starting from exact and analytically calculated magnetic potential fields, we solveid the nonlinear MHD equations self-consistently. By applying a magnetic shear flow and assuming a nonideal MHD environment, we calculated an electric field via Faraday's law. The formal solution for the electromagnetic field allowed us to compute an expression of an effective resistivity similar to the collisionless Speiser resistivity. We find that the electric field can be highly spatially structured, or in other words, filamented. The electric field component parallel to the magnetic field is the dominant component and is high where the resistivity has a maximum. The electric field is a potential field, therefore, the highest energy gain of the particles can be directly derived from the corresponding voltage. In our example of a coronal post-flare scenario we obtain electron energies of tens of keV, which are on the same order of magnitude as found observationally. This energy serves as a source for heating and acceleration of particles.Comment: 11 pages, 9 figures, accepted to Astronomy and Astrophysic

On the validity of ideal MHD in the vicinity of stagnation points in the heliosphere and other astrospheres

International audienceThe paradigm of ideal MHD is investigated in the vicinity of null points of flows and magnetic fields. These null points determine the location and geometrical shape of the heliopause (or other astropauses). We investigate the question whether regular and stable solutions of the ideal MHD equations in the vicinity of null points of flow and magnetic field exist. This is done to test the validity of ideal MHD in the vicinity of flow and magnetic field of the plasma boundaries of stellar winds and their local interstellar medium. We calculate the general solutions of ideal MHD in the vicinity of magnetic null points and use the standard form of stagnation point flows to analyse all possible time evolutions of these plasma environments. We show that the solution space in 2-D consists almost exclusively of either exponentially (in time) growing velocity or magnetic fields, or collapse solutions. Regular solutions must be three-dimensional and seem to be unstable with respect to small perturbations. This is an argument that reconnection has to take place in such regions and that therefore nonideal terms in Ohm's law are necessary, allowing for reconnection. We conclude that the use of ideal MHD in the vicinity of singular points of flow and magnetic field has to be analysed very carefully with respect to simulation results as those simulations show numerical dissipation (resistivity). These simulations can therefore produce unphysical reconnection regimes. Thus one has to search for a realistic Ohm's law, allowing for reconnection at the heliospheric boundaries

Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73

We aim to improve our knowledge on the structure and dynamics of the circumstellar disk of the LMC B[e] supergiant LHA 120-S 73. High-resolution optical and near-IR spectroscopic data were obtained over a period of 16 and 7 years, respectively. The spectra cover the diagnostic emission lines from [CaII] and [OI], as well as the CO bands. These features trace the disk at different distances from the star. We analyzed the kinematics of the individual emission regions by modeling their emission profiles. A low-resolution mid-infrared spectrum was obtained as well, which provides information on the composition of the dusty disk. All diagnostic emission features display double-peaked line profiles, which we interpret as due to Keplerian rotation. We find that LHA 120-S 73 is surrounded by at least four individual rings of material with alternating densities (or by a disk with strongly non-monotonic radial density distribution). Moreover, we find that the molecular ring must have gaps or at least strong density inhomogeneities, or in other words, a clumpy structure. The mid-infrared spectrum displays features of oxygen- and carbon-rich grain species, which indicates a long-lived, stable dusty disk. We cannot confirm the previously reported high value for the stellar rotation velocity. The line profile of HeI 5876 A is strongly variable in both width and shape and resembles of those seen in non-radially pulsating stars. A proper determination of the real underlying stellar rotation velocity is hence not possible. The existence of multiple stable and clumpy rings of alternating density recalls ring structures around planets. Although there is currently insufficient observational evidence, it is tempting to propose a scenario with one (or more) minor bodies or planets revolving around LHA 120-S 73 and stabilizing the ring system, in analogy to the shepherd moons in planetary systems.Comment: 14 pages, 13 figure, accepted for pulication in A&

Resolving the circumstellar environment of the Galactic B[e] supergiant star MWC 137, II : nebular kinematics and stellar variability

The Galactic B[e] supergiant MWC 137 is surrounded by a large-scale optical nebula. To shed light on the physical conditions and kinematics of the nebula, we analyze the optical forbidden emission lines [N II] λλ 6548,6583 and [S II] λλ 6716,6731 in long-slit spectra taken with ALFOSC at the Nordic Optical Telescope. The radial velocities display a complex behavior but, in general, the northern nebular features are predominantly approaching while the southern ones are mostly receding. The electron density shows strong variations across the nebula with values spreading from about zero to ~800 cm-3. Higher densities are found closer to MWC 137 and in regions of intense emission, whereas in regions with high radial velocities the density decreases significantly. We also observe the entire nebula in the two [S II] lines with the scanning Fabry-Perot interferometer attached to the 6 m telescope of the Special Astrophysical Observatory. These data reveal a new bow-shaped feature at P.A. = 225°-245° and a distance 80″ from MWC 137. A new Hα image has been taken with the Danish 1.54 m telescope on La Silla. No expansion or changes in the nebular morphology appear within 18.1 yr. We derive a mass of 37+9−5M⊙ and an age of 4.7 ± 0.8 Myr for MWC 137. Furthermore, we detect a period of 1.93 d in the time series photometry collected with the TESS satellite, which could suggest stellar pulsations. Other, low-frequency variability is seen as well. Whether these signals are caused by internal gravity waves in the early-type star or by variability in the wind and circumstellar matter currently cannot be distinguished. * Based on observations collected with (1) the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias, (2) the Danish 1.54 m telescope at La Silla, Chile, and (3) the 6 m telescope of the Special Astrophysical Observatory (SAO), Russia.Fil: Kraus, Michaela. Czech Academy of Sciences; República ChecaFil: Liimets, Tiina. Czech Academy of Sciences; República ChecaFil: Moiseev, Alexei. Special Astrophysical Observatory; RusiaFil: Sánchez Arias, Julieta Paz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Nickeler , Dieter H.. Astronomický Ústav, Rep. Checa; República ChecaFil: Cidale, Lydia Sonia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Jones, D.. Instituto de Astrofisica de Canarias; Españ

Flux and field line conservation in 3--D nonideal MHD flows: Remarks about criteria for 3--D reconnection without magnetic neutral points

We make some remarks on reconnection in plasmas and want to present some calculations related to the problem of finding velocity fields which conserve magnetic flux or at least magnetic field lines. Hereby we start from views and definitions of ideal and non-ideal flows on one hand, and of reconnective and non-reconnective plasma dynamics on the other hand. Our considerations give additional insights into the discussion on violations of the frozen--in field concept which started recently with the papers by Baranov & Fahr (2003a; 2003b). We find a correlation between the nonidealness which is given by a generalized form of the Ohm's law and a general transporting velocity, which is field line conserving.Comment: 9 pages, 2 figures, submitted to Solar Physic

Stationary field-aligned MHD flows at astropauses and in astrotails. Principles of a counterflow configuration between a stellar wind and its interstellar medium wind

A stellar wind passing through the reverse shock is deflected into the astrospheric tail and leaves the stellar system either as a sub-Alfvenic or as a super-Alfvenic tail flow. An example is our own heliosphere and its heliotail. We present an analytical method of calculating stationary, incompressible, and field-aligned plasma flows in the astrotail of a star. We present a recipe for constructing an astrosphere with the help of only a few parameters, like the inner Alfven Mach number and the outer Alfven Mach number, the magnetic field strength within and outside the stellar wind cavity, and the distribution of singular points of the magnetic field within these flows. Within the framework of a one-fluid approximation, it is possible to obtain solutions of the MHD equations for stationary flows from corresponding static MHD equilibria, by using noncanonical mappings of the canonical variables. The canonical variables are the Euler potentials of the magnetic field of magnetohydrostatic equilibria. Thus we start from static equilibria determined by the distribution of magnetic neutral points, and assume that the Alfven Mach number for the corresponding stationary equilibria is finite. The topological structure determines the geometrical structure of the interstellar gas - stellar wind interface. Additional boundary conditions like the outer magnetic field and the jump of the magnetic field across the astropause allow determination of the noncanonical transformations. This delivers the strength of the magnetic field at every point in the astrotail region beyond the reverse shock. The mathematical technique for describing such a scenario is applied to astrospheres in general, but is also relevant for the heliosphere. It shows the restrictions of the outer and the inner magnetic field strength in comparison with the corresponding Alfven Mach numbers in the case of subalfvenic flows.Comment: 19 pages, 17 figures, accepted for publication in A&

Molecular emission from GG Carinae's circumbinary disk

Context. The appearance of the B[e] phenomenon in evolved massive stars such as B[e] supergiants is still a mystery. While these stars are generally found to have disks that are cool and dense enough for efficient molecule and dust condensation, the origin of the disk material is still unclear. Aims. We aim at studying the kinematics and origin of the disk in the eccentric binary system GG Car, whose primary component is proposed to be a B[e] supergiant. Methods. Based on medium- and high-resolution near-infrared spectra we analyzed the CO-band emission detected from GG Car. The complete CO-band structure delivers information on the density and temperature of the emitting region, and the detectable 13CO bands allow us to constrain the evolutionary phase. In addition, the kinematics of the CO gas can be extracted from the shape of the first 12CO band head. Results. We find that the CO gas is located in a ring surrounding the eccentric binary system, and its kinematics agrees with Keplerian rotation with a velocity, projected to the line of sight, of 80±1-km-s-1. The CO ring has a column density of (5±3)×1021 cm-2 and a temperature of 3200±500-K. In addition, the material is chemically enriched in 13C, which agrees with the primary component being slightly evolved off the main sequence. We discuss two possible scenarios for the origin of the circumbinary disk: (i) non-conservative Roche lobe overflow; and (ii) the possibility that the progenitor of the primary component could have been a classical Be star. Neither can be firmly excluded, but for Roche lobe overflow to occur, a combination of stellar and orbital parameter extrema would be required.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Molecular emission from GG Carinae's circumbinary disk

The appearance of the B[e] phenomenon in evolved massive stars such as B[e] supergiants is still a mystery. While these stars are generally found to have disks that are cool and dense enough for efficient molecule and dust condensation, the origin of the disk material is still unclear. We aim at studying the kinematics and origin of the disk in the eccentric binary system GG Car, whose primary component is proposed to be a B[e] supergiant. Based on medium- and high-resolution near-infrared spectra we analyzed the CO-band emission detected from GG Car. The complete CO-band structure delivers information on the density and temperature of the emitting region, and the detectable 13CO bands allow us to constrain the evolutionary phase. In addition, the kinematics of the CO gas can be extracted from the shape of the first 12CO band head. We find that the CO gas is located in a ring surrounding the eccentric binary system, and its kinematics agrees with Keplerian rotation with a velocity, projected to the line of sight, of (80pm 1) km/s. The CO ring has a column density of (5pm 3)x10^21 cm^-2 and a temperature of 3200pm 500 K. In addition, the material is chemically enriched in 13CO, which agrees with the primary component being slightly evolved off the main sequence. We discuss two possible scenarios for the origin of the circumbinary disk: (i) non-conservative Roche lobe overflow, and (ii) the possibility that the progenitor of the primary component could have been a classical Be star. Neither can be firmly excluded, but for Roche lobe overflow to occur, a combination of stellar and orbital parameter extremawould be required.Fil: Kraus, M.. Astronomický ústav; República ChecaFil: Oksala, M. E.. Astronomický ústav; República ChecaFil: Nickeler, D. H.. Astronomický ústav; República ChecaFil: Muratore, M. F.. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Borges Fernandes, M.. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; BrasilFil: Aret, A.. Tartu Observatory; EstoniaFil: Cidale, Lydia Sonia. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: de Wit, W. J.. European Southern Observatory; Chil