Author name code: heyvaerts ADS astronomy entries on 2022-09-14 author:"Heyvaerts, Jean" ------------------------------------------------------------------------ Title: General solution for the vacuum electromagnetic field in the surroundings of a rotating star Authors: Bonazzola, S.; Mottez, F.; Heyvaerts, J. Bibcode: 2020arXiv200702539B Altcode: Aims. Many recent observations of pulsars and magnetars can be interpreted in terms of neutron stars (NS) with multipole electromagnetic fields. As a first approximation, we investigate the multipole magnetic and electric fields in the environment of a rotating star when this environment is deprived of plasma. Methods. We compute a multipole expansion of the electromagnetic field in vacuum for a given magnetic field on the conducting surface of the rotating star. Then, we consider a few consequences of multipole fields of pulsars. Results. We provide an explicit form of the solution. For each spherical harmonic of the magnetic field, the expansion contains a finite number of terms. A multipole magnetic field can provide an explanation for the stable sub-structures of pulses, and they offer a solution to the problem of current closure in pulsar magnetospheres. Conclusions. This computation generalises the widely used model of a rotating star in vacuum with a dipole field. It can be especially useful as a first approximation to the electromagnetic environment of a compact star, for instance a neutron star, with an arbitrarily magnetic field. Title: Magnetic coupling of planets and small bodies with a pulsar wind (Corrigendum) Authors: Mottez, F.; Heyvaerts, J. Bibcode: 2020A&A...639C...2M Altcode: No abstract at ADS Title: Dressed diffusion and friction coefficients in inhomogeneous multicomponent self-gravitating systems Authors: Heyvaerts, Jean; Fouvry, Jean-Baptiste; Chavanis, Pierre-Henri; Pichon, Christophe Bibcode: 2017MNRAS.469.4193H Altcode: 2017arXiv170606009H General self-consistent expressions for the coefficients of diffusion and dynamical friction in a stable, bound, multicomponent self-gravitating, and inhomogeneous system are derived. They account for the detailed dynamics of the colliding particles and their self-consistent dressing by collective gravitational interactions. The associated Fokker-Planck equation is shown to be fully consistent with the corresponding inhomogeneous Balescu-Lenard equation and, in the weak self-gravitating limit, to the inhomogeneous Landau equation. Hence, it provides an alternative derivation to both and demonstrates their equivalence. The corresponding stochastic Langevin equations are presented: they can be a practical alternative to numerically solving the inhomogeneous Fokker-Planck and Balescu-Lenard equations. The present formalism allows for a self-consistent description of the secular evolution of different populations covering a spectrum of masses, with a proper accounting of the induced secular mass segregation, which should be of interest to various astrophysical contexts, from galactic centres to protostellar discs. Title: Do asteroids evaporate near pulsars? Induction heating by pulsar waves revisited Authors: Kotera, Kumiko; Mottez, Fabrice; Voisin, Guillaume; Heyvaerts, Jean Bibcode: 2016A&A...592A..52K Altcode: 2016arXiv160505746K
Aims: We investigate the evaporation of close-by pulsar companions, such as planets, asteroids, and white dwarfs, by induction heating.
Methods: Assuming that the outflow energy is dominated by a Poynting flux (or pulsar wave) at the location of the companions, we calculate their evaporation timescales, by applying the Mie theory.
Results: Depending on the size of the companion compared to the incident electromagnetic wavelength, the heating regime varies and can lead to a total evaporation of the companion. In particular, we find that inductive heating is mostly inefficient for small pulsar companions, although it is generally considered the dominant process.
Conclusions: Small objects like asteroids can survive induction heating for 104 yr at distances as small as 1 R⊙ from the neutron star. For degenerate companions, induction heating cannot lead to evaporation and another source of heating (likely by kinetic energy of the pulsar wind) has to be considered. It was recently proposed that bodies orbiting pulsars are the cause of fast radio bursts; the present results explain how those bodies can survive in the pulsar's highly energetic environment. Title: General solution for the vacuum electromagnetic field in the surroundings of a rotating star Authors: Bonazzola, S.; Mottez, F.; Heyvaerts, J. Bibcode: 2015A&A...573A..51B Altcode:
Aims: Many recent observations of pulsars and magnetars can be interpreted in terms of neutron stars with multipole electromagnetic fields. As a first approximation, we investigate the multipole magnetic and electric fields in the environment of a rotating star when this environment is deprived of plasma.
Methods: We compute a multipole expansion of the electromagnetic field in vacuum for a given magnetic field on the conducting surface of the rotating star. Then, we consider a few consequences of multipole fields of pulsars.
Results: We provide an explicit form of the solution. For each spherical harmonic of the magnetic field, the expansion contains a finite number of terms. A multipole magnetic field can provide an explanation for the stable sub-structures of pulses, and they offer a solution to the problem of current closure in pulsar magnetospheres.
Conclusions: This computation generalises the widely used model of a rotating star in vacuum with a dipole field. It can be especially useful as a first approximation to the electromagnetic environment of a compact star, for instance a neutron star, with an arbitrarily magnetic field. Title: Towards a theory of extremely intermittent pulsars. II. Asteroids at a close distance Authors: Mottez, F.; Bonazzola, S.; Heyvaerts, J. Bibcode: 2013A&A...555A.126M Altcode: 2013arXiv1305.6726M
Aims: We investigate whether there may be one or many companions orbiting at close distance to the light cylinder around the extremely intermittent pulsars PSR B1931+24 and PSR J1841-0500. These pulsars, behaving in a standard way when they are active, also "switch off" for durations of several days, during which their magnetospheric activity is interrupted or reduced.
Methods: We constrained our analysis on eight fundamental properties of PSR B1931+24 that summarise the observations. We considered that the disruption/activation of the magnetospheric activity would be caused by the direct interaction of the star with the Alfvén wings emanating from the companions. We also considered the recurrence period of 70 days to be the period of precession of the periastron of the companions orbit. We analysed in which way the time scale of the "on/off" pseudo-cycle would be conditioned by the precession of the periastron and not by the orbital time scale, and we derived a set of orbital constraints that we solved. We then compared the model, based on PSR 1931+24, with the known properties of PSR 1841+0500.
Results: We conclude that PSR B1931+24 may be surrounded at a close distance to the star by a stream of small bodies of kilometric or sub-kilometric sizes that could originate from the tidal disruption of a body of moderate size that fell at a close distance to the neutron star on an initially very eccentric orbit. This scenario is also compatible with the properties of PSR J1841-0500, although the properties of PSR J1841-0500 are, as of now, less constrained.
Conclusions: These results raise new questions. Why are the asteroids not yet evaporated? What kind of interaction can explain the disruption of the magnetospheric activity? These questions are the object of two Papers in preparation that will complete the present analysis. Title: Towards a theory of extremely intermittent pulsars. I. Does something orbit PSR B1931+24? Authors: Mottez, F.; Bonazzola, S.; Heyvaerts, J. Bibcode: 2013A&A...555A.125M Altcode: 2013arXiv1305.6724M
Aims: We investigate whether one or many companions are orbiting the extremely intermittent pulsar PSR B1931+24.
Methods: We constrained our analysis on previous observations of eight fundamental properties of PSR B1931+24. The most puzzling properties are the intermittent nature of the pulsar's activity, with active and quiet phases that alternate quasi-periodically; the variation of the slowing-down rate of its period between active and quiet phases; and because there are no timing residuals, it is highly unlikely that the pulsar has a massive companion. Here, we examine the effects that one putative companion immersed in the magnetospheric plasma or the wind of the pulsar might have, as well as the associated electric current distribution. We analysed several possibilities for the distance and orbit of this hypothetical companion and the nature of its interaction with the neutron star.
Results: We show that if the quasi-periodic behaviour of PSR B1931+24 was caused by a companion orbiting the star with a period of 35 or 70 days, the radio emissions, usually considered to be those of the pulsar would in that specific case be emitted in the companion's environment. We analysed four possible configurations and conclude that none of them would explain the whole set of peculiar properties of PSR 1931+24. We furthermore considered a period of 70 days for the precession of the periastron associated to an orbit very close to the neutron star. This hypothesis is analysed in a companion paper. Title: Polarization transfer in relativistic magnetized plasmas Authors: Heyvaerts, Jean; Pichon, Christophe; Prunet, Simon; Thiébaut, Jérôme Bibcode: 2013MNRAS.430.3320H Altcode: 2013MNRAS.tmp..843H; 2012arXiv1211.7352H The polarization transfer coefficients of a relativistic magnetized plasma are derived. These results apply to any momentum distribution function of the particles, isotropic or anisotropic. Particles interact with the radiation either in a non-resonant mode when the frequency of the radiation exceeds their characteristic synchrotron emission frequency or quasi-resonantly otherwise. These two classes of particles contribute differently to the polarization transfer coefficients. For a given frequency, this dichotomy corresponds to a regime change in the dependence of the transfer coefficients on the parameters of the particle's population, since these parameters control the relative weight of the contribution of each class of particles. Our results apply to either regimes as well as the intermediate one. The derivation of the transfer coefficients involves an exact expression of the conductivity tensor of the relativistic magnetized plasma that has not been used hitherto in this context. Suitable expansions valid at frequencies much larger than the cyclotron frequency allow us to analytically perform the summation over all resonances at high harmonics of the relativistic gyrofrequency. Title: The magnetic coupling of planets and small bodies with a pulsar's wind Authors: Mottez, F.; Heyvaerts, J. Bibcode: 2012sf2a.conf..597M Altcode: 2012arXiv1210.0684M 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 Alfvén waves. In that case, a pair of stationary Alfvén waves, called Alfvén wings (AW), is expected to form on the sides of the planet. The wings expand far into the pulsar's wind and they could be strong sources of radio emissions. The Alfvén wings would cause a significant drift over small bodies such as asteroids and comets. Title: The magnetic wake of planets and small bodies in a pulsar's wind Authors: Mottez, F.; Heyvaerts, J. Bibcode: 2012epsc.conf..153M Altcode: 2012espc.conf..153M We investigate the electromagnetic interaction of a relativistic and magnetized 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 other pulsars with suspected asteroids or comets. Most models predict that, albeit highly relativistic, pulsar's winds are slower than Alfvén waves. In that case, a pair of stationary Alfvén waves, called Alfvén wings (AW), is expected to form on the sides of the planet. They are the magnetic wake of the body into the plasma flow, like the wake of a boat left into the sea (with a similar shape). The theory of Alfvén wings was initially developed in the context of the Io- Jupiter interaction. We have extended it to relativistic winds, and we have studied the possible consequences that could be relevant for observations : possible radio emissions from pulsar's planets, and a magnetic force configuration that can deeply modify the orbit of the smaller bodies (asteroids, comets). Title: Non-linear simple relativistic Alfvén waves in astrophysical plasmas Authors: Heyvaerts, J.; Lehner, T.; Mottez, F. Bibcode: 2012A&A...542A.128H Altcode:
Aims: Large amplitude MHD perturbations are generated in magnetized tenuous relativistically moving plasmas, such as winds emitted by compact stellar objects or galactic nuclei, when a rapid change occurs at their boundaries or when an obstacle is present in them. These perturbations may involve relativistic motions in the rest-frame of the unperturbed plasma. In this paper, we calculate the characteristics and the structure of relativistic non-linear Alfvén waves.
Methods: We establish these properties for special-relativistic perturbations occurring in a particular type of non-linear waves, the simple waves.
Results: We derive the conditions applicable to Alfvénic perturbations in a cold flow. We calculate the characteristics of these perturbations and the structure of wave trains of finite extent in the propagation direction of these characteristics, as observed in the unperturbed fluid proper frame. We determine the velocity of the characteristics with respect to any observer. This velocity is found to be a first integral, constant in time and space. This implies that relativistic Alfvénic perturbations are channeled, in the unperturbed fluid proper frame, by the unperturbed magnetic field and travel along this field neither steepening nor breaking into shocks. For finite wave trains, the Lorentz factor is found to be limited by some maximum value that we calculate and that depends on the ratio of proper magnetic energy density to material energy density in the unperturbed fluid. Title: The magnetic coupling of planets and small bodies with a pulsar's wind Authors: Mottez, F.; Heyvaerts, J. Bibcode: 2011epsc.conf...23M Altcode: 2011DPS....43...23M 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 Alfvén waves. In that case, a pair of stationary Alfvén waves, called Alfvén wings (AW), is expected to form on the sides of the planet. The wings expands far into the pulsar's wind. We have extended the theory of Alfvén wings, initially developed in the context of the Io-Jupiter interaction, to relativistic winds, and we have studied the possible consequences that could be relevant for observations. Title: Magnetic coupling of planets and small bodies with a pulsar wind Authors: Mottez, F.; Heyvaerts, J. Bibcode: 2011A&A...532A..21M Altcode: 2011arXiv1106.0657M
Aims: We investigate the electromagnetic interaction of a relativistic stellar wind with a planet or a smaller body in orbit around the star. This may be relevant to objects orbiting a pulsar that are expected to hold a planetary system, such as PSR B1257+12 and PSR B1620-26, or to pulsars with suspected asteroids or comets.
Methods: We extend the theory of Alfvén wings to relativistic winds.
Results: When the wind is relativistic but slower than the total Alfvén speed, a system of electric currents carried by a stationary Alfvénic structure is driven by the planet or by its surroundings.For an Earth-like planet around a "standard" second pulsar, the associated current can reach the same magnitude as the Goldreich-Julian current that powers the pulsar's magnetosphere. Title: A magnetic thrust action on small bodies orbiting a pulsar Authors: Mottez, F.; Heyvaerts, J. Bibcode: 2011A&A...532A..22M Altcode: 2011arXiv1106.0659M
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 Alfvén 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 Alfvénic 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 ~ 108 T. Kilometer-sized bodies experience drastic orbital changes on a time scale of 104 years. Title: A Balescu-Lenard-type kinetic equation for the collisional evolution of stable self-gravitating systems Authors: Heyvaerts, J. Bibcode: 2010MNRAS.407..355H Altcode: 2010MNRAS.tmp.1045H A kinetic equation for the collisional evolution of stable, bound, self-gravitating and slowly relaxing systems is established, which is valid when the number of constituents is very large. It accounts for the detailed dynamics and self-consistent dressing by collective gravitational interaction of the colliding particles, for the system's inhomogeneity and for different constituents' masses. It describes the coupled evolution of collisionally interacting populations, such as stars in a thick disc and the molecular clouds off which they scatter.

The kinetic equation derives from the BBGKY hierarchy in the limit of weak, but non-vanishing, binary correlations, an approximation which is well justified for large stellar systems. The evolution of the 1-body distribution function is described in action-angle space. The collective response is calculated using a biorthogonal basis of pairs of density-potential functions.

The collision operators are expressed in terms of the collective response function allowed by the existing distribution functions at any given time and involve particles in resonant motion. These equations are shown to satisfy an H theorem. Because of the inhomogeneous character of the system, the relaxation causes the potential as well as the orbits of the particles to secularly evolve. The changing orbits also cause the angle Fourier coefficients of the basis potentials to change with time. We derive the set of equations which describes this coupled evolution of distribution functions, potential and basis Fourier coefficients for spherically symmetric systems. In the homogeneous limit, which sacrifices the description of the evolution of the spatial structure of the system but retains the effect of collective gravitational dressing, the kinetic equation reduces to a form similar to the Balescu-Lenard equation of plasma physics. Title: A Balescu-Lenard type kinetic equation fot the collisional evolution of stable self-gravitating systems Authors: Heyvaerts, Jean Bibcode: 2010arXiv1005.0195H Altcode: A kinetic equation for the collisional evolution of stable, bound, self gravitating and slowly relaxing systems is established, which is valid when the number of constituents is very large. It accounts for the detailed dynamics and self consistent dressing by collective gravitational interaction of the colliding particles, for the system's inhomogeneity and for different constituent's masses. The evolution of the one-body distribution function is described in action angle space. The collision operators are expressed in terms of the collective response function allowed by the existing distribution functions at any given time and involve particles in resonant motions. The set of equations which describe the coupled evolution of the distribution functions and of the potential is derived for spherical systems. In the homogeneous limit, which sacrifices the description of the evolution of the spatial structure of the system, but retains the effects of collective gravitational dressing, the kinetic equation reduces to a form similar to the Balescu-Lenard equation of plasma physics. Title: Luminosity of a quark star undergoing torsional oscillations and the problem of γ-ray bursts Authors: Heyvaerts, J.; Bonazzola, S.; Bejger, M.; Haensel, P. Bibcode: 2009A&A...496..317H Altcode: 2008arXiv0811.1090H Aims: We discuss whether the winding-up of the magnetic field by differential rotation in a new-born quark star can produce a sufficiently-high, energy, emission rate of sufficiently long duration to explain long gamma-ray bursts.
Methods: In the context of magnetohydrodynamics, we study the torsional oscillations and energy extraction from a new-born, hot, differentially-rotating quark star.
Results: The new-born compact star is a rapid rotator that produces a relativistic, leptonic wind. The star's torsional oscillation modulates this wind emission considerably when it is odd and of sufficient amplitude, which is relatively easy to reach. Odd oscillations may occur just after the formation of a quark star. Other asymmetries can cause similar effects. The buoyancy of wound-up magnetic fields is inhibited, or its effects are limited, by a variety of different mechanisms. Direct electromagnetic emission by the torsional oscillation in either an outside vacuum or the leptonic wind surrounding the compact object is found to be insignificant. In contrast, the twist given to the outer magnetic field by an odd torsional oscillation is generally sufficient to open the star's magnetosphere. The Poynting emission of the star in its leptonic environment is then radiated from all of its surface and is enhanced considerably during these open episodes, tapping at the bulk rotational energy of the star. This results in intense energy shedding in the first tens of minutes after the collapse of magnetized quark stars with an initial poloidal field of order of 1014 Gauss, sufficient to explain long gamma-ray bursts. Title: Coronal Heating at Separators and Separatrices Authors: Priest, E. R.; Longcope, D. W.; Heyvaerts, J. Bibcode: 2005ApJ...624.1057P Altcode: Several ways have been proposed for heating the solar corona by magnetic reconnection in current sheets, depending on the nature of both the coronal magnetic field and the photospheric driving. Two ways that have recently been considered involve the formation of such current sheets either along separatrices (surfaces that separate topologically distinct regions) or along separators (intersections of separatrices linking one null point to another). The effect of slow photospheric motions on complex coronal magnetic configurations will in general be to generate three forms of electric current, namely, nonsingular distributed currents, singular currents on separatrices and singular currents on separators. These currents are not mutually exclusive but will in general coexist in the same configuration. The aim of this paper is to compare energy storage and heating that occurs at separatrices and separators. We use reduced MHD to model coronal loops that are much longer than they are wide, and we construct a series of examples for the formation of current sheets along separatrices and separators. We deduce that coronal heating is of comparable importance at separatrices and separators. Separatrices are twice as effective for observed small footpoint motions, while separators are twice as effective in the initial build-up of a new flux domain. Title: Effects of Complexity on the Flux-Tube Tectonics Model Authors: Close, R. M.; Heyvaerts, J. F.; Priest, E. R. Bibcode: 2004SoPh..225..267C Altcode: The quiet-Sun magnetic field emerges through the solar photosphere in a multitude of mixed-polarity magnetic concentrations and is subsequently tangled up into intricate regions of interconnecting flux. Moreover, since these discrete concentrations are likely to be extremely small in size, with fluxes of around only 1017 Mx, the number of such flux sources in, say, a supergranule, will be extremely large. The flux-tube tectonics model of Priest, Heyvaerts, and Title (2002) demonstrated how the formation and dissipation of current sheets along the separatrices that separate the regions of different connectivity are likely to make an important contribution to coronal heating. Since the full complexity of the magnetic field is below present observable scales, this study examines the effect of having the magnetic flux emerge through configurations structured on smaller and smaller scales. It is found that, by fixing the amount of flux emerging into a given 2D region, the main factors influencing the current build-up along the separatrices are the number of sources through which the flux emerges and the spatial distribution of the sources on the photosphere. The free energy (i.e., that above potential) is stored lower and lower in the atmosphere as the complexity of the system increases. A simple comparison is then made between coronal heating by separator currents and by separatrix currents. It is found that both result in comparable amounts of energy release, with separatrix heating being the more dominant. Title: Structuring and support by Alfvén waves around prestellar cores Authors: Folini, D.; Heyvaerts, J.; Walder, R. Bibcode: 2004A&A...414..559F Altcode: 2003astro.ph.10525F 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 Alfvén 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. Title: Course 1: Accretion and Ejection-Related MHD Authors: Heyvaerts, Jean Bibcode: 2004adjh.conf....3H Altcode: 2004LHSS...78....3H This lecture is an introduction to MHD. Relevant equations, both in the classical and special-relativistic regimes are derived. The magnetic field evolution is considered both in the perfect-MHD limit and when weak resistivity is present, giving rise to reconnection flows. A short section gives a flavour of dynamo theory. Examples of simple stationnary flows and equilibria are then presented. Stationnary, axisymmetric, rotating perfect-MHD winds and jets are discussed in some more detail. Their asymptotic structure is described. The last sections deal with small motions about an equilibrium and stability. These issues are illustrated by a few classical examples. The last section discusses linear aspects of the magneto-rotationnal instability. Title: Cross-field charge transport by the diocotron instability in pulsar magnetospheres with gaps Authors: Pétri, J.; Heyvaerts, J.; Bonazzola, S. Bibcode: 2003A&A...411..203P Altcode: In a previous work, we have shown by linear analysis that a thin charged disk in differential rotation in the magnetosphere of a neutron star with vacuum gaps is unstable to a collisionless instability induced in non-neutral plasmas by differential rotation, the diocotron instability. In this paper we study the long-time-scale evolution of this instability in the non-linear regime by means of both direct numerical simulations and a quasilinear model. We show that, when the disk is externally fed with charged particles produced by a moderate pair creation activity in the magnetosphere, the diocotron instability causes diffusion of the charged particles across the magnetic field lines outwards. An equatorial cross-field electric current is observed to form, carrying a net charge flux radially outwards. This constitutes a hitherto ignored charge transport mechanism in the pulsar magnetosphere. We briefly discuss how this turbulent charge transport mechanism could bear on the problem of electric current closure in pulsar's magnetospheres.

Appendices A and B are only available in electronic form at\ http://www.edpsciences.org Title: Global Asymptotic Solutions for Nonrelativistic Magnetohydrodynamic Jets and Winds Authors: Heyvaerts, Jean; Norman, Colin Bibcode: 2003ApJ...596.1270H Altcode: 2003astro.ph..9128H We present general and global analytical solutions, valid from pole to equator, for the asymptotic structure of nonrelativistic, rotating, stationary, axisymmetric, polytropic, unconfined, perfect MHD winds. The standard five Lagrangian first integrals along field lines are assumed known. The asymptotic structure of such winds consists of field regions virtually devoid of poloidal current. We show that a Hamilton-Jacobi equation, or equivalently a Grad-Shafranov equation, gives the asymptotic structure in the field regions. These field regions are bordered by current-carrying boundary layers around the polar axis and near null magnetic surfaces. Current closure is achieved in a number of separate cells bordered by null surfaces. The solution is given in the form of matched asymptotics separately valid outside and inside these boundary layers. The polar boundary layer is pressure supported against the pinching force exerted by the axial poloidal current and has the structure of a current pinch, while the null-surface boundary layers have the structure of current sheet pinches. We establish a consistency relation between the residual poloidal current at large distances and the axial pressure. We find a similar relation for the current sheets at null surfaces. We further consider the case where the polar boundary layer is force-free. The geometry of magnetic surfaces in all parts of the asymptotic domain is explicitly deduced in terms of the first integrals. The solutions have the following general properties:1. For winds that are kinetic energy-dominated at infinity we derive WKBJ analytic solutions whose magnetic surfaces focus into paraboloids. The current slowly weakens as the inverse of the logarithm of the distance to the wind source, while the axial plasma density falls off as a negative power of this logarithm.2. For winds carrying Poynting flux at large distances the solutions asymptotically approach to nested cylindrical and conical magnetic surfaces. Title: Kinetic Energy Flux versus Poynting Flux in Magnetohydrodynamic Winds and Jets: The Intermediate Regime Authors: Heyvaerts, Jean; Norman, Colin Bibcode: 2003ApJ...596.1256H Altcode: 2003astro.ph..9143H We show that the formal asymptotic limit for all rotating polytropic axisymmetric perfect MHD flows is a kinetic energy-dominated wind that collimates to paraboloids around the symmetry axis. We reach this result by showing that another a priori possible solution with finite Poynting flux can be excluded on the following physical grounds: (1) the wind velocity does not exceed the fast mode speed everywhere and (2) the circumpolar current increases with distance from the source. We show that asymptotic hoop stress collimation is mathematically robust and we give strong arguments why recent ``anticollimation'' claims are not correct. However, in practice, because of the very slow logarithmic decline of the circumpolar current with increasing distance from the source, there is a broad intermediate regime with significant Poynting flux. This intermediate regime, rather than the mathematically exact asymptotic regime, may well apply along the finite length of the jet. We briefly discuss peculiarities that would be associated with Poynting jets in the intermediate regime. Force-free initial conditions in the near field are most likely to produce such jets, in which most of the energy flux is electromagnetic. Title: Global Asymptotic Solutions for Relativistic Magnetohydrodynamic Jets and Winds Authors: Heyvaerts, Jean; Norman, Colin Bibcode: 2003ApJ...596.1240H Altcode: 2003astro.ph..9132H 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. Title: Magnetized accretion Authors: Heyvaerts, J. Bibcode: 2003EAS.....7..309H Altcode: This lecture reviews in simple terms the general subject of large scale magnetic field coupling to plasma flows in the vicinity of accreting compact stars. The relevant astrophysical phenomenology is summarized. Disk interaction with the magnetosphere of accreting stars is first discussed, in particular the structure of the magnetopause, its stability and plasma ejection in so-called propeller systems. The physics of accretion/ejection is then considered. Acceleration and focusing mechanisms of jets from accretion disks around compact stars or black holes and the question of the self-consistency of accretion and ejection are described. By contrast, small scale MHD turbulence in disks is not discussed, neither are accretion columns near the polar caps of neutron stars or white dwarfs. The reader is only assumed to have some basic knowledge of astrophysics and of fluid mechanics and electromagnetism. Title: A Flux-Tube Tectonics Model for Solar Coronal Heating Driven by the Magnetic Carpet Authors: Priest, Eric R.; Heyvaerts, Jean F.; Title, Alan M. Bibcode: 2002ApJ...576..533P Altcode: 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. Title: Diocotron instability in pulsar electrospheres. I. Linear analysis Authors: Pétri, J.; Heyvaerts, J.; Bonazzola, S. Bibcode: 2002A&A...387..520P Altcode: In a previous work, we have shown that the electrospheric plasma of a pulsar's magnetosphere with vacuum gaps is subject to differential rotation in the equatorial belt. Here we examine the stability of a simplified model of this belt. The model consists of a charged thin plasma disc in differential rotation, embedded in a dipolar magnetic field anchored in the neutron star. In the linearized electrostatic perturbation approximation, this disc appears to be unstable to the so-called diocotron instability. We present several eigenspectra and eigenfunctions for different disc models, which differ by the total charge of the disc-star system. Velocity and electric field perturbations are also shown in the disc plane for a few fast-growing modes. Increasing the total charge has a stabilizing effect, in the sense that the growth rate of the fastest-growing eigenvalue decreases with total charge and eventually vanishes. When the system is unstable, the growth rates are of the order of the azimuthal rotation rate. This means that the diocotron instability, which develops in a few pulsar's periods, should be very efficient in causing particles to migrate across magnetic surfaces. Title: Global static electrospheres of charged pulsars Authors: Pétri, J.; Heyvaerts, J.; Bonazzola, S. Bibcode: 2002A&A...384..414P Altcode: In this paper we present self-consistent models of the magnetosphere of inactive, charged, aligned rotator pulsars. We have devised an efficient semi-analytical and numerical algorithm to construct such models. The only free parameter is the total charge of the system. These charge-separated ``electrospheres" consist of an equatorial belt carrying charge of one sign, partially in differential rotation, and of two oppositely charged domes located over the poles which corotate with the neutron star. The dependence of the shape of these plasma-filled regions surrounding the star on the total charge of the system and of their differential rotation is investigated. It is shown that our solutions are stable to vacuum breakdown by electron-positron pair production in most of the light-cylinder volume, except perhaps in the case of millisecond pulsars. The small regions where vacuum breakdown occurs are shown to behave merely as an effective extension of the star's volume. We have also found that no permanent null-charged wind emanating from the polar caps can exist in a stationary state. Indeed, for a given total charge of the system determined by the net outgoing charged flux, the potential configuration becomes unfavorable to particles escaping to infinity. Finally, we have shown that the geometric and kinematic structure of the electrosphere is uniquely determined by the total charge of the system. Title: Asymptotic structure of MHD winds and jets Authors: Heyvaerts, J.; Norman, C. Bibcode: 2001A&AT...20..295H Altcode: We describe asymptotic solutions for stationary, axisymmetric, perfect MHD, polytropic winds, both classical and relativistic. They are expressed as field-region solutions and current-carrying boundary layer solutions smoothly joined by asymptotic matching. The vicinity of the polar axis is one of these boundary layers. In general, the boundary layers are null surfaces. It is argued that the boundary layer regions, in particular the axial one, should stand out observationally because of their larger density and activity. We associate the axial boundary layer with a jet. Current closure is self-consistently achieved in these solutions, which we obtain both in the case of vanishing or non-vanishing circumpolar asymptotic current. It is shown that the total current about the polar axis is simply related to the set of the five first integrals which characterize the flow and that non-vanishing values of this quantity are not available to all winds, but only to a restricted class which we present here. We show that winds of this class separate clearly into an axial jet and a circum-equatorial conical wind. Title: Magnetohydrodynamics of Astrophysical Winds Authors: Heyvaerts, J. Bibcode: 2000eaa..bookE2227H Altcode: Stellar mass loss became an issue when Parker (1958) first introduced the concept of the solar wind. Such a thermal wind results from coronal heating processes indirectly associated with the existence of a convection zone, also responsible, with rotation, for the star's dynamo action (see articles on SOLAR WIND: THEORY, CORONAL HEATING MECHANISMS, SOLAR INTERIOR: CONVECTION ZONE and MAGNETOHYDROD... Title: Coronal Heating Mechanisms Authors: Heyvaerts, J. Bibcode: 2000eaa..bookE2269H Altcode: Evidence for a hot coronal medium... Title: A Method to Determine the Heating Mechanisms of the Solar Corona Authors: Priest, E. R.; Foley, C. R.; Heyvaerts, J.; Arber, T. D.; Mackay, D.; Culhane, J. L.; Acton, L. W. Bibcode: 2000ApJ...539.1002P Altcode: One of the paradigms about coronal heating has been the belief that the mean or summit temperature of a coronal loop is completely insensitive to the nature of the heating mechanisms. However, we point out that the temperature profile along a coronal loop is highly sensitive to the form of the heating. For example, when a steady state heating is balanced by thermal conduction, a uniform heating function makes the heat flux a linear function of distance along the loop, while T7/2 increases quadratically from the coronal footpoints; when the heating is concentrated near the coronal base, the heat flux is small and the T7/2 profile is flat above the base; when the heat is focused near the summit of a loop, the heat flux is constant and T7/2 is a linear function of distance below the summit. It is therefore important to determine how the heat deposition from particular heating mechanisms varies spatially within coronal structures such as loops or arcades and to compare it to high-quality measurements of the temperature profiles. We propose a new two-part approach to try and solve the coronal heating problem, namely, first of all to use observed temperature profiles to deduce the form of the heating, and second to use that heating form to deduce the likely heating mechanism. In particular, we apply this philosophy to a preliminary analysis of Yohkoh observations of the large-scale solar corona. This gives strong evidence against heating concentrated near the loop base for such loops and suggests that heating uniformly distributed along the loop is slightly more likely than heating concentrated at the summit. The implication is that large-scale loops are heated in situ throughout their length, rather than being a steady response to low-lying heating near their feet or at their summits. Unless waves can be shown to produce a heating close enough to uniform, the evidence is therefore at present for these large loops more in favor of turbulent reconnection at many small randomly distributed current sheets, which is likely to be able to do so. In addition, we suggest that the decline in coronal intensity by a factor of 100 from solar maximum to solar minimum is a natural consequence of the observed ratio of magnetic field strength in active regions and the quiet Sun; the altitude of the maximum temperature in coronal holes may represent the dissipation height of Alfvén waves by turbulent phase mixing; and the difference in maximum temperature in closed and open regimes may be understood in terms of the roles of the conductive flux there. Title: Astrophysical MHD jets Authors: Heyvaerts, J. Bibcode: 2000NuPhS..80...51H Altcode: No abstract at ADS Title: Introduction to MHD Authors: Heyvaerts, Jean Bibcode: 2000LNP...553....1H Altcode: 2000tech.conf....1H LNP 553, p. 1 ff. Title: Outflows from magnetic rotators. II. Asymptotic structure and collimation Authors: Lery, T.; Heyvaerts, J.; Appl, S.; Norman, C. A. Bibcode: 1999A&A...347.1055L Altcode: 1999astro.ph..2365L The asymptotic structure of outflows from rotating magnetized objects confined by a uniform external pressure is calculated. The flow is assumed to be perfect MHD, polytropic, axisymmetric and stationary. The well known associated first integrals together with the confining external pressure, which is taken to be independent of the distance to the source, determine the asymptotic structure. The integrals are provided by solving the flow physics for the base within the framework of the model developed in Paper I (Lery et al. 1998), which assumes conical geometry below the fast mode surface, and ensures the Alfvén regularity condition. Far from the source, the outflow collimate cylindrically. Slow (i.e. with small rotation parameter omega ) rigid rotators give rise to diffuse electric current distribution in the asymptotic region. They are dominated by gas pressure. Fast rigid rotators have a core-envelope structure in which a current carrying core is surrounded by an essentially current free region where the azimuthal magnetic field dominates. The total asymptotic poloidal current carried away decreases steadily with the external pressure. A sizeable finite current remains present for fast rotators even at exceedingly small, but still finite, pressure. Title: Astrophysical Jets Authors: Heyvaerts, J. Bibcode: 1998tx19.confE.372H Altcode: No abstract at ADS Title: Influence of viscosity laws on the transition to the self-gravitating part of accretion disks Authors: Bardou, Anne; Heyvaerts, Jean; Duschl, Wolfgang J. Bibcode: 1998A&A...337..966B Altcode: In this paper we obtain a new solution for accretion disks influenced by self-gravity using the viscosity law proposed by Heyvaerts et al. (1996). We show how to solve analytically the problem taking into account self-gravity. Using this new viscosity law, we obtain a slightly different solution (compared with the standard one) for the transition between the non self-gravitating and the self-gravitating part of the disk but no solution in the self-gravitating part where additional viscosity mechanisms may play a role. Title: Outflows from magnetic rotators. I. Inner structure Authors: Lery, T.; Heyvaerts, J.; Appl, S.; Norman, C. A. Bibcode: 1998A&A...337..603L Altcode: 1999astro.ph..2364L A simplified model for the stationary, axisymmetric structure of magnetized winds with a polytropic equation of state is presented. The shape of the magnetic surfaces is assumed to be known (conical in this paper) within the fast magnetosonic surface. The model is non-self-similar. Rather than solving the equilibrium perpendicular to the flux surfaces everywhere, solutions are found at the Alfven surface where it takes the form of the Alfven regularity condition and at the base of the flow. This constrains the Transfield equilibrium in that the Alfven regularity condition is imposed and the regularity of the magnetic surfaces at the Alfven critical surface is ensured. The model imposes criticality conditions at the slow and fast magnetosonic critical points using the Bernoulli equation. These Alfven regularity and criticality conditions are used to evaluate three constants of motion, the total energy, angular momentum, and the ratio of mass to magnetic flux alpha , as well as the shape of the critical surfaces. The rotation rate Omega and the polytropic constant Q as a function of the magnetic surfaces, together with the mass-to-magnetic flux ratio on the axis alpha_0 entirely specify the model. Analytic results are given for limiting cases, and parameter studies are performed by numerical means. The model accepts any boundary conditions. Numerical calculations yield the value of the rotation parameter omega . Rotators can be defined as slow, intermediate or fast according to whether omega is much less or close to unity or near its maximum value for fast rotators, ((3)/(2))((3)/(2)) . Given the properties of astrophysical objects with outflows, the model allows their classification in terms of the rotation parameter. Critical surfaces are nearly spherical for slow rotators, but become strongly distorted for rapid rotators. The fast point remains at a finite distance for finite entropy flows, in contrast to cold flows.It is found that for a given mass loss rate, the rotation rate is limited. Title: Nature of the heating mechanism for the diffuse solar corona Authors: Priest, E. R.; Foley, C. R.; Heyvaerts, J.; Arber, T. D.; Culhane, J. L.; Acton, L. W. Bibcode: 1998Natur.393..545P Altcode: The temperature of the Sun's 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. Title: Alfven wave support of a dwarf molecular cloud. I. an isothermal model. Authors: Martin, C. E.; Heyvaerts, J.; Priest, E. R. Bibcode: 1997A&A...326.1176M Altcode: Dwarf (or dark) molecular clouds and molecular clumps have a lifetime which is greater than their dynamical time and must therefore be, in an average sense, in mechanical equilibrium. Equilibrium perpendicular to a global magnetic field is by magnetic forces and it is proposed that along the field the gas is supported by an Alfven wave pressure force. A self-consistent analytical model, utilising a WKB approximation, is developed for such support. It is found that Alfven waves are indeed a good candidate for this support, generating model cloud thicknesses consistent with observations. The effect of damping by the linear process of ion-neutral friction is considered. It is found that the damping of the waves is not a necessary condition for the support of the cloud although weak damping is an advantage. The possible sources of these waves are discussed. Title: Instabilities in Magnetized Jets: An Origin for Knots? Authors: Lery, T.; Appl, S.; Heyvaerts, J.; Norman, C. A. Bibcode: 1997AAS...190.5506L Altcode: 1997BAAS...29R1108L No abstract at ADS Title: Instabilities in MHD Jets Authors: Lery, T.; Heyvaerts, J.; Appl, S.; Norman, C. A. Bibcode: 1997AAS...190.4117L Altcode: 1997BAAS...29Q.835L No abstract at ADS Title: Interaction Of A Stellar Magnetic Field With A Turbulent Accretion Disk Authors: Bardou, A.; Heyvaerts, J. Bibcode: 1997ASPC..121..205B Altcode: 1997apro.conf..205B; 1997IAUCo.163..205B No abstract at ADS Title: Asymptotic Structure of Rotating MHD Winds and its Relation to Wind Boundary Conditions Authors: Heyvaerts, J.; Norman, C. A. Bibcode: 1997IAUS..182..275H Altcode: Approximate asymptotic solutions for rotating MHD winds are obtained analytically in terms of the first integrals of the motion. It is shown that the paraxial region of such winds is a line-shaped boundary layer which has, even at large distances, the structure of a pressure-supported current pinch. A necessary condition for cylindrically focused asymptotics to be possible is derived. A simplified model by which the asymptotic structure of such winds can be obtained in terms of general boundary conditions at the wind source is introduced. Results of semi numerical solutions of the model are reported. The model is analytically solved in the limit of very fast rotators, giving in this particular case an explicit and complete description of the wind outputs and asymptotic structure in terms of arbitrary boundary conditions at the wind source. Title: Outflows of magnetized rotators: structure and collimation Authors: Lery, Thibaut; Heyvaerts, Jean; Appl, Stefan; Norman, Colin A. Bibcode: 1997IAUS..182P.152L Altcode: How can outflows such as astrophysical winds and jets form and collimate ? What differences exhibit these two types of plasma flows ? This work makes an attempt to answer those questions thanks to a simple model that deals with the time-independent and axisymmetric MHD equations. The geometry of magnetic surfaces has been assumed to be conical up to the fast magnetosonic critical point. The transversal force balance is calculated along the Alfven surface, and the critically conditions are derived from the Bernoulli equation at the two other critical points. They are used to calculate the specific energy, the angular momentum and the mass loss rate, that are constant for each flux surface. This allows to deal with the asymptotic structure in pressure equilibrium with the surrounding medium. The choice of boundary conditions is unrestricted. It is found that rigid slow rotators, associated to stellar winds, carry a diffused poloidal current, and are gas pressure dominated. On the other hand, rigid fast rotators, corresponding to jets, carry a current concentrated around the polar axis, show strongly distorted critical surfaces and are magnetic pressure dominated at the border of the jet. Regardless the class of rotator, the angular velocity is bounded from above for a given mass loss rate, and regardless an external confining pressure, the collimation of magnetic rotators is asymptotically cylindrical. Title: Magnetic Field Diffusion in Self-consistently Turbulent Accretion Disks Authors: Heyvaerts, J.; Priest, E. R.; Bardou, A. Bibcode: 1996ApJ...473..403H Altcode: 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. Title: Outflows of magnetized rotators: structure and collimation. Authors: Lery, T.; Appl, S.; Heyvaerts, J. Bibcode: 1996AGAb...12..195L Altcode: No abstract at ADS Title: (Erratum) Stationary subalfvenic and low-β MHD flows in solar coronal loops and arcades. Authors: Surlantzis, G.; Demoulin, P.; Heyvaerts, J.; Sauty, C. Bibcode: 1996A&A...310..351S Altcode: Erratum to Astron. Astrophys. 284, 985-999 (1994) Title: Electric current concentration and kink instability in line-tied coronal loops. Authors: Baty, H.; Heyvaerts, J. Bibcode: 1996A&A...308..935B Altcode: A fully three dimensional non-linear magnetohydrodynamic (MHD) simulation of the evolution of ideal kink modes in line-tied cylindrical coronal loops is presented. Two distinct initially unstable equilibria are considered: the uniform- twist force-free Gold-Hoyle profile, and a more realistic non force free field with variable and localized twist profile. In this latter case the instability non-linearly develops at the axial midplane of the loop a current concentration radially localized at a resonant point r_s_, where the condition {vec}(k).{vec}(B)=0 is satisfied, {vec}(k) being the local wave vector of the mode and {vec}(B) the equilibrium magnetic field. No such fine scale current structure forms for the Gold-Hoyle profile since {vec}(k).{vec}(B) then never vanishes. This current concentration extends along all the loop length down to the photosphere, and it takes the form of an helical ribbon of intense current with a weakly variable helicity along the axial direction. A kinked bifurcated equilibrium is reached in which the current concentration generated is non singular in the sense that this has a non-zero thickness. Moreover, the non-linear perturbed current seems to develop a filamentary structure superposed on the current layer. We discuss the resistive dissipation mechanism of such fine-scale structures. Title: Interaction of a stellar magnetic field with a turbulent accretion disk. Authors: Bardou, A.; Heyvaerts, J. Bibcode: 1996A&A...307.1009B Altcode: This paper studies systems where a central magnetized object is surrounded by a turbulent accretion disk. In such systems, magnetic field lines are embedded in the disk. The purpose here is to find a stationary magnetic configuration in the corona. The magnetic field of the central star is considered to be dipolar in the absence of the accretion disk. In the presence of a thin, turbulent and keplerian accretion disk, the configuration is not dipolar anymore. We look for a stationary solution. We show that the interaction of the disk with the magnetic field stretches magnetic field lines along the disk and that most of the non-magnetospheric magnetic flux is expelled outside the disk. A few field lines might open. Title: Outflows of magnetized rotators: structure and collimation. Authors: Lery, T.; Appl, S.; Heyvaerts, J. Bibcode: 1996AGAb...12..237L Altcode: No abstract at ADS Title: Rotating MHD Winds Authors: Heyvaerts, J. Bibcode: 1996LNP...468...31H Altcode: 1996plas.conf...31H No abstract at ADS Title: Collimation of MHD outflows. Authors: Heyvaerts, J.; Norman, C. A. Bibcode: 1996ASIC..481..459H Altcode: This paper discusses the magnetic focusing of MHD winds, in particular the possibility that they asymptotically converge to a cylindrical geometry. Basics of magnetized rotating MHD winds are quickly reviewed. The condition for the solution to remain regular at the Alfvén surface is discussed and explicitly expressed in terms of surface functions which determine the flow solution. Theorems are established, which constrain the asymptotic shape of rotating polytropic MHD winds. In particular it is proved that winds which carry a non-vanishing Poynting flux and poloidal current to infinity must contain a cylindrically collimated core, whereas other winds focus parabolically. Title: Interaction of turbulent accretion disks with embedded magnetic fields. Authors: Heyvaerts, J. F.; Bardou, A.; Priest, E. R. Bibcode: 1996ASIC..481..659H Altcode: The authors show that 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. When turbulence originates in the magnetic shearing instability, the effective kinematic viscosity coefficient is shown to be describable by a Shakura-Sunyaev law with α ≍ 0.04. It is shown that thin disks suported by 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. Such disks are shown to expell the magnetic field of the accreting object from their Keplerian regions radially outwards, resulting in a flux distribution in the disk which differs very much from a dipolar one. Title: Outflows of magnetized rotators: structure and collimation Authors: Lery, T.; Appl, S.; Heyvaerts, J. Bibcode: 1996AGM....12.J237L Altcode: No abstract at ADS Title: Disques d'accrétion turbulents auto-cohérents. Authors: Bardou, A.; Heyvaerts, J.; Priest, E. Bibcode: 1995JAF....49...51B Altcode: No abstract at ADS Title: Erratum - Turbulent Coronal Heating Authors: Inverarity, G. W.; Priest, E. R.; Heyvaerts, J. Bibcode: 1995A&A...299..640I Altcode: No abstract at ADS Title: Turbulent coronal heating. I. Sheared arcade. Authors: Inverarity, G. W.; Priest, E. R.; Heyvaerts, J. Bibcode: 1995A&A...293..913I Altcode: The model of Heyvaerts & Priest for heating the solar corona by turbulent processes in a sheared arcade is examined in more detail by numerically solving their basic equations for the turbulent viscosity, magnetic diffusivity, velocity and energy flux density. A weighted spectrum for the mean square velocity in the photosphere and a scale-dependent turbulent viscosity and magnetic diffusivity for local eddy relaxation are introduced which improve the self-consistency of the model. Estimates are made of the turbulent variables listed above which are found to accord well with the requirements for heating a solar active region (=~10^7^erg/cm^2^/s). Title: Closing address Authors: Heyvaerts, J. Bibcode: 1994SSRv...68..371H Altcode: Impressions from the workshop are summarized. Difficulties within the present observational and theoretical context are outlined and directions for fruitful future work are indicated. The workshop subtitle, “the interface between MHD and plasma physics”, still appears to be a goal for the future, though some encouraging results have been reported at this workshop. Title: Stationary subalfvenic and low-β MHD flows in solar coronal loops and arcades. Authors: Surlantzis, G.; Demoulin, P.; Heyvaerts, J.; Sauty, C. Bibcode: 1994A&A...284..985S Altcode: We present general two-dimensional solutions for low-β and subalfvenic stationary MHD flow. Our method of solution applies to any type of boundary conditions. It solves for the pertubation of the magnetic configuration brought about by flows and by the development of shock waves in it. Solutions in cartesian and cylindrical geometries are presented to model flows in coronal loops and counter-Evershed flows above spots. In symmetrical magnetic configurations, when the distribution of pressure at the foot points is symmetrical, the flow is necessarily subsonic. Otherwise it can become supersonic at the summit of the magnetic field line and then passes through a shock. Such shocks can be very inclined to the magnetic field and the shocked material may form a dense hot sheet around a cooler core, a situation which seems to be observed in cool loops. For asymmetrical magnetic configurations, the flow accelerates towards the low gas pressure foot point and could be subsonic or trans-sonic depending on the pressure difference between the foot points. Loops can have a significant density contrast against their environment only if their energy flux differs markedly from the background one. In asymmetrical loops one leg can be much less dense than the other and poorly visible. Near spots, the sign of the difference of pressure between the two foot points is such as to drive a reverse Evershed flow towards the spot. Additional effects would be needed to drive a direct Evershed flow. Title: Magnetic Energy Dissipation and Coronal Heating by DC Currents Authors: Heyvaerts, J. Bibcode: 1994ASIC..422...25H Altcode: 1994coma.conf...25H No abstract at ADS Title: MHD Equilibria in Uniform Gravity Authors: Surlantzis, G.; Démoulin, P.; Heyvaerts, J.; Sauty, C. Bibcode: 1993ASSL..183..629S Altcode: 1993pssc.symp..629S No abstract at ADS Title: Jeans collapse of turbulent gas clouds - Tentative theory Authors: Bonazzola, S.; Perault, M.; Puget, J. L.; Heyvaerts, J.; Falgarone, E.; Panis, J. F. Bibcode: 1992JFM...245....1B Altcode: The conditions for gravitational instability in a statistically homogeneous turbulent fluid is investigated using a renormalization technique, with particular reference to the problem of stability of molecular clouds and the origin of protostellar density fluctuations. It is shown how turbulence at scales smaller than the potentially collapsing scale builds up a turbulent pressure force that effectively resists compression if the kinetic energy is sufficient to balance gravitational attraction. Title: Physics of coronal loops: heating and reconnection. Authors: Heyvaerts, J. Bibcode: 1992ESASP.344...69H Altcode: 1992spai.rept...69H The importance of small scale structures in the current distribution is outlined, and the smallest size that these current structures might have is discussed. It is shown that they must be dynamic, turbulent, structures. The physical processes that may be at the origin of the formation of such structures are discussed. The braiding of field lines, the phase mixing of oscillations in inhomogeneous magnetic configurations, and a number of secondary instabilities are likely to be the dominant ones. A selfconsistent calculation of a large scale coronal flow with associated turbulence is described. This parameter-free theory gives, as a function of boundary motions which cause it, the rate of coronal heating by fine scaled DC current structures. Title: A Self-consistent Turbulent Model for Solar Coronal Heating Authors: Heyvaerts, J.; Priest, E. R. Bibcode: 1992ApJ...390..297H Altcode: 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 self-consistency argument that they should result from the level of turbulence associated with this very heating flux. For the sake of tractability the derivation is restricted to a two-dimensional situation where boundary flows are translationally symmetric. The resulting value of the heating rate and the predicted level of microturbulent velocity compare satisfactorily with the observational data. Title: Design Rationale of the Solar Ultraviolet Network / Sun Authors: Dame, L.; Acton, L.; Bruner, M. E.; Connes, P.; Cornwell, T. J.; Curdt, W.; Foing, B. H.; Hammer, R.; Harrison, R.; Heyvaerts, J.; Karabin, M.; Marsch, E.; Martic, M.; Mattic, W.; Muller, R.; Patchett, B.; Roca-Cortes, T.; Rutten, R. J.; Schmidt, W.; Title, A. M.; Tondello, G.; Vial, J. C.; Visser, H. Bibcode: 1992ESOC...39..995D Altcode: 1992hrii.conf..995D No abstract at ADS Title: Magnetohydrodynamic Effects in Active Galactic Nuclei Accretion Disks Authors: Heyvaerts, J. Bibcode: 1992pagn.conf..445H Altcode: No abstract at ADS Title: Magnetohydrodynamic Forces in Astrophysical Disks and Jets Authors: Heyvaerts, J. Bibcode: 1991assm.conf..399H Altcode: No abstract at ADS Title: A solar interferometric mission for ultrahigh resolution imaging and spectroscopy: SIMURIS Authors: Damé, L.; Acton, L.; Bruner, M.; Connes, P.; Cornwell, T.; Foing, B. H.; Heyvaerts, J.; Lemaire, P.; Martić, M.; Muller, R.; Porteneuve, J.; Roca Cortés, T.; Riehl, J.; Rutten, R.; Séchaud, M.; Smith, P.; Thorne, A. P.; Title, A. M.; Vial, J. -C.; Visser, H.; Weigelt, G. Bibcode: 1991AdSpR..11a.383D Altcode: 1991AdSpR..11..383D SIMURIS is an interferometric investigation of the very fine structure of the solar atmosphere from the photosphere to the corona. It was proposed to ESA /1/, November 30 1989, for the Next Medium Size Mission - M2, and accepted in February 1990 for an Assessment Study in the context of the Space Station. The main scientific objectives will be outlined, and the ambitious model payload featuring the Solar Ultraviolet Network (SUN), a 2 m long monolithic array of 4 telescopes of Ø20 cm, and the Imaging Fourier Transform Spectrometer (IFTS), an UV and Visible Imaging Fourier Transform Spectrometer coupled to a Ø40 cm Gregory, described. Title: Solar physics at ultrahigh resolution from the space station with the Solar Ultraviolet Network (SUN) Authors: Damé, L.; Acton, L.; Bruner, M.; Connes, P.; Cornwell, T.; Foing, B.; Heyvaerts, J.; Lemaire, P.; Martić, M.; Muller, R.; Roca Cortés, T.; Riehl, J.; Rutten, R.; Title, A. M.; Vial, J. -C.; Visser, H.; Weigelt, G. Bibcode: 1991AdSpR..11e.267D Altcode: 1991AdSpR..11..267D The SUN experiment is a UV and visible Space Interferometer aimed at ultra-high resolution in the solar atmosphere. It has been proposed to ESA as part of the SIMURIS Mission Proposal which has recently been accepted for an Assessment Study in the framework of the Space Station. The 4 × 20 cm telescopes of the SUN linear array are non-redundantly placed to cover a 2 m baseline, and the instrument makes full use of stabilized interferometry potential, the 4 telescopes being co-aligned and co-phased on a reference field on the sun. After a brief outline of the scientific objectives, the concept of the instrument is described, and its image reconstruction potential is illustrated. Title: The Role of Magnetic Fields in Accretion Disks. Authors: Heyvaerts, J. Bibcode: 1991sepa.conf..109H Altcode: 1991IAUCo.129..109H No abstract at ADS Title: Basic Hydrodynamics Authors: Heyvaerts, Jean Bibcode: 1991LNP...373..313H Altcode: 1991lsse.conf..313H No abstract at ADS Title: MHD forces in astrophysical disks and jets. Authors: Heyvaerts, J. Bibcode: 1991gamp.conf..399H Altcode: Accretion disks and jets are found to be associated with several kinds of astrophysical objects which are briefly described. Observational evidence and theoretical arguments indicating that accretion disks are likely to be magnetized are presented, and the consequences for the structure and evolution of disks are discussed. It is shown that dynamo action in the disk results in a magnetized coronal halo around it. Being stressed by the disk's differential rotation, the halo is heated by a process similar to solar coronal heating. Conversely, Lorentz forces are exerted on the disk, which cause non-local interaction and redistribution of angular momentum. The disk, even at relatively low magnetization, may become non-keplerian, and eventually differential rotation may come to be frozen. A model incorporating these effects is outlined. If the disk is threaded by an open field configuration, the latter can support a rotating magnetized wind. It is shown that such a wind can remove angular momentum from the disk, thus allowing a secular contraction of it to take place. Finally, the shape of the wind in regions remote from its source is discussed. Recent results (Heyvaerts and Norman, 1989) show that the asymptotic form of the momentum equation perpendicular to flux surfaces implies that, for almost all such winds, the asymptotic structure consists of a family of nested cylindrical surfaces, potentially explaining the collimation of the observed "jets", in particular the famous jets from active galactic nuclei without resorting to any external confinement. Title: Possible scenarios of coronal loops reconnection/heating processes to be observed at high spatial resolution Authors: Dame, L.; Heyvaerts, J.; Foing, B. H. Bibcode: 1991AdSpR..11a.327D Altcode: 1991AdSpR..11..327D Recent improvements in interferometric techniques could allow to achieve 0.01 arcsec angular resolution on the Sun, i.e. 10 km. Such a high resolution is of direct interest to understand the coronal loop structure since current observations at low resolution cannot distinguish between major dissipation/heating theories which all involve very small scale dissipating processes. Three simplified scenarios of loop instabilities are investigated in this paper and the resulting fine structure and contrast that they might induce on observable quantities (temperature, density) are deduced. Title: High Resolution Solar Physics from the Space Station with Interferometric Techniques: The Solar Ultraviolet Network (SUN) - Instrument &Objectives Authors: Damé, L.; Acton, L.; Bruner, M.; Connes, P.; Cornwell, T.; Foing, B.; Heyvaerts, J.; Jalin, R.; Lemaire, Ph.; Martic, M.; Moreau, B.; Muller, R.; Roca Cortés, T.; Riehl, J.; Rutten, R.; Title, A. M.; Vial, J. -C.; Visser, H.; Weigelt, G. Bibcode: 1990PDHO....7..262D Altcode: 1990dysu.conf..262D; 1990ESPM....6..262D No abstract at ADS Title: The Collimation of Magnetohydrodynamic Winds - Bipolar Flows and Jets Authors: Norman, C. A.; Heyvaerts, J. Bibcode: 1990IAUS..140..353N Altcode: No abstract at ADS Title: Coronal Heating by DC Currents Authors: Heyvaerts, J. Bibcode: 1990IAUS..142..207H Altcode: Recent views on dc current coronal heating are presented. The relation to ac mechanisms, the importance of MHD turbulence in both processes, and the convergence of recently proposed ideas are outlined. Attention is given to the role of the field line stochasticity in coronal structures and tentative theories of the 'extrinsic' and 'intrinsic' type. Title: The Collimation of Magnetized Winds Authors: Heyvaerts, Jean; Norman, Colin Bibcode: 1989ApJ...347.1055H Altcode: 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. Title: Solar Ultraviolet Network: an interferometric investigation of the fundamental solar astrophysical scales Authors: Dame, Luc; Moreau, Bernard G.; Cornwell, Timothy J.; Visser, H.; Title, Alan M.; Acton, Loren W.; Aime, Claude; Braam, Bart M.; Bruner, Marilyn E.; Connes, Pierre; Faucherre, Michel; Foing, B. H.; Haisch, Bernhard M.; Hoekstra, Roel; Heyvaerts, Jean; Jalin, Rene; Lemaire, Philippe; Martic, Milena; Muller, R.; Noens, J. C.; Porteneuve, Jacques; Schulz-Luepertz, E.; von der Luehe, Oskar Bibcode: 1989SPIE.1130..126D Altcode: The Solar UV Network (SUN) presently proposed is an interferometric system, based on the principles of stabilized interferometry, which will be capable of solar observations with spatial resolutions better than 0.013 arcsec. SUN will consist of four 20-cm diameter telescopes aligned nonredundantly on a 2-m baseline. SUN is judged to be ideally deployable by the NASA Space Station, if implemented on a pointing platform whose performance is of the order of the Instrument Pointing System flown on Spacelab 2. The compact, nonredundant configuration of SUN's telescopes will allow high-resolution imaging of a 2 x 2 arcsec field on the solar disk. Title: A model for a non-Keplerian magnetic accretion disk with a magnetically heated corona Authors: Heyvaerts, J. F.; Priest, E. R. Bibcode: 1989A&A...216..230H Altcode: MHD stresses are used to model a thin disk dynamically interacting with a self-created magnetic corona. A simple model of the coronal magnetic structure produced by the turbulent MHD relaxation of the stresses exerted by the disk's differential rotation can explain the effect of coronal heating and the back reaction of the corona on the energy balance and angular momentum flow in the disk. The results show the disk to acquire a non-Keplerian rotation profile, with the trailing foot point of the magnetic loops being accelerated, and the leading one being decelerated, in comparison to a Keplerian distribution. It is found that the coronal heating and non-Keplerian effects are weak when the coronal scale becomes much larger than the disk size. Title: MHD Aspects of Galactic Center Physics (review) Authors: Heyvaerts, J. Bibcode: 1989IAUS..136..301H Altcode: No abstract at ADS Title: La formation doctorale à l'heure européenne. Authors: Heyvaerts, J. Bibcode: 1989JAF....34...26H Altcode: No abstract at ADS Title: Topological catastrophe in massive current sheets. Authors: Peterle, P.; Heyvaerts, J. Bibcode: 1989sasf.confP.285P Altcode: 1989IAUCo.104P.285P; 1988sasf.conf..285P A two-dimensional sheet model for solar filaments (Kippenhahn and Schlüter configuration) is considered. The authors investigate the quasi-static evolution of gravito-magnetohydrostatic equilibria in exploring the response of massive current sheets to a slow continuous variation of the mass/flux ratio with fixed boundary conditions. A catastrophic behavior of the field topology is found to occur in the sequence following the formation of a cusp point (bifurcation). Title: A Magnetic Loop Model for Structure and Activity in the Galactic Center Authors: Heyvaerts, Jean; Norman, Colin; Pudritz, Ralph E. Bibcode: 1988ApJ...330..718H Altcode: 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. Title: Magnetohydrodynamic Modes of a Periodic Magnetic Medium Authors: Berton, R.; Heyvaerts, J. Bibcode: 1987SoPh..109..201B Altcode: The oscillations of a magnetic medium periodic in the x-direction with B parallel to z, have been studied. The case with no gravity and a stepwise profile for B(x), allowing a normal mode analysis, has been examined and dispersion relations have been derived. The dispersion curves in the diagram kz − ω display two types of modes, kink and sausage, like in the isolated slab, but the profiles are different and depend on Bloch's number k0. Moreover, modes usually absent in the isolated slab (propagating and tunelling) appear here, connecting surface- or body-wave domains. The detectability of this characteristic structure of the diagnostic diagram on the observations is discussed, and prospects for a more realistic analysis including gravity are given. Title: About the interest of Solar Interferometric Observations Authors: Heyvaerts, J. Bibcode: 1987ESASP.273...11H Altcode: 1987ois..work...11H It is shown that the understanding of some basic solar physical processes calls for high spatial resolution observation on the visible and in EUV lines. These processes are of general astrophysical interest. The scientific return expected from UV line observations of coronal loops with an angular resolution of 0arcsec.01 across the loop is described in some more detail. Title: Solar interferometry with a 4-aperture non-redondant and stabilised network Authors: Dame, L.; Aime, C.; Faucherre, M.; Heyvaerts, J. Bibcode: 1987ESASP.273..189D Altcode: 1987ois..work..189D The design of a solar interferometer is intrinsically complex since many requirements, often found separately, and difficult by themselves, are brought together: UV spectral range, limb observations, resolved structures (low contrast) and time resolution. The stabilized interferometry technique, applied to a non-redundant array of 4 telescopes, provides an elegant solution to those complex problems. Title: A magnetic loop model for activity in the Galactic centre Authors: Heyvaerts, Jean; Pudritz, Ralph E.; Norman, Colin A. Bibcode: 1987AIPC..155..176H Altcode: 1987gace.conf..176H We propose that radio structures on 0.1-100 pc scales in the galactic centre radio lobe (GCL) are manifestations of magnetic activity in a central source object. The observations indicate that truly one dimensional structures occur and we hold that this is strong evidence for magnetic loops. They are generated on subpc scales and expand out to 10-100 pc. Their interaction with the 2 pc molecular torus as well as the GCL on 30-50 pc scales reproduces the observed irregular ionized and molecular emission on 2 pc scales, as well as the bridge, radio arc, and thread-like filaments on the larger ones. The Sgr A radio lobe itself (on 10 pc scales) is comprised of a system of lower energy magnetized loops which have been trapped in the molecular torus. The main thrust of this theory is that all of these exotic structures may be understood within the framework of one rather simple model. Title: Jeans collapse in a turbulent medium Authors: Bonazzola, S.; Heyvaerts, J.; Falgarone, E.; Perault, M.; Puget, J. L. Bibcode: 1987A&A...172..293B Altcode: According to the classical Jeans criterion, all the observed molecular clouds of mass M greater than 100 solar masses, radius R greater than 1 pc and gas kinetic temperature Tk less than 30 K are gravitationally unstable. However, millimetric and infrared observations show that low mass dense cores (M greater than 3 solar masses) can collapse and form stars within clouds for which there is no evidence for global collapse. It is shown that, if the power spectrum of the turbulent internal motions which support them against gravity is steep enough, the molecular clouds are stable as long as their gas density remains close to the observed low mean densities (a few 100 per cu cm). For larger densities, only a narrow range of scales become gravitationally unstable. In that context, unstable low mass dense cores eventually form only when the density fluctuations within the clouds happen to be large. Title: Formation à et par la recherche en astronomie: mythes et réalités. Authors: Heyvaerts, J. Bibcode: 1987JAF....30...10H Altcode: No abstract at ADS Title: Jeans criterion in a turbulent medium Authors: Bonazzola, S.; Falgarone, E.; Heyvaerts, J.; Perault, M.; Puget, J. L. Bibcode: 1986inpr.conf...41B Altcode: According to the classical Jeans analysis, all the molecular clouds of mass larger than a few 100 M(solar), size larger than about 1pc and kinetic temperature Tk less than 30K are gravitationally unstable. We have shown that in clouds supported by internal supersonic motions, local gravitational instabilities may appear within molecular clouds which are globally stable. The argument is threefold: (1) when the turbulent kinetic energy is included into the internal energy term, the virial equilibrium condition shows that molecular clouds such as those observed, which are gravitationally unstable according to the Jeans criterion, are indeed globally stable if supported by a turbulent velocity field of power spectrum steeper than 3; (2) 2D compressible hydrodynamical simulations show that a supersonic turbulent velocity field generates a turbulent pressure within clouds, the gradients of which stabilize the unstable scales (i.e., the largest scales and the cloud itself) against gravitational collapse; (3) an analysis similar to the Jeans approach but including the turbulent pressure gradient term, gives basically the same results as those given in (1). Clouds of mean density lower than a critical value are found to be stable even though more massive than their Jeans mass. In clouds of mean density larger than that critical value, the gravitational instability appears only over a range of scales smaller than the cloud size, the largest scales being stable. In practice, the observed mean densities are lower than this critical value: the observation of a small number of cores and stars of a few solar masses embedded in clouds of several hundred solar masses can only be understood in terms of small scale density fluctuations of large amplitude generated by the supersonic turbulence which would occasionally overtake the limit of gravitational stability. Title: Gamma-Ray Bursts Authors: Heyvaerts, J. Bibcode: 1986ppm..conf..180H Altcode: No abstract at ADS Title: Quasistatic Evolution of Magnetic Arcades in the Sun : Ideal and Dissipative Case (Invited paper) Authors: Heyvaerts, J. Bibcode: 1986mrt..conf..121H Altcode: No abstract at ADS Title: Solar Fine Structures - Their Importance in the Sun's Physics and Their Observation Authors: Heyvaerts, J. Bibcode: 1985ESASP.235..169H Altcode: 1985shpp.rept..169H; 1985fmsh.work..169H An overview of the structures in the sun's atmosphere is given. The essential role of these structures in the physics of the sun's convective zone and atmosphere is stressed, and it is emphasized that the same physics is crucial in a number of other systems of planetological or astrophysical interest. The problem of solar coronal heating is evocated with some more detail, and its relation to the solar wind acceleration problem is discussed. The diagnosing of non thermal heating phenomena is shown to involve elaborate observations with high spectral, spatial and temporal resolution. It is concluded that the SOHO project, presently considered by ESA, appears in this respect to be the best next step to take. Title: Spectra of gamma-ray bursts Authors: Hameury, J. M.; Lasota, J. P.; Bonazzola, S.; Heyvaerts, J. Bibcode: 1985ApJ...293...56H Altcode: It is shown that gamma-ray burst spectra are basically thermal synchrotron spectra emitted in a cold 'photosphere' by electrons excited to high Landau levels by high-energy photons that are beamed along the magnetic field lines. The high-energy radiation is produced in a corona by the interaction of soft, thermal photons and synchrotron photons with one-dimensional, relativistic electrons in a strong magnetic field. These coronal electrons are accelerated by short-scale magnetic reconnection. The physical parameters of the coronal layer are self-consistently determined. Monte Carlo simulations that include Compton and resonant scattering produce spectra in good agreement with the observations between 20 keV and 1 MeV. It is speculated that the recently discovered high-energy tail of the spectrum is formed in the outer corona and the wind zone. It is also shown that the emission is strongly anisotropic, and consequently, the most intense bursts need not be the closest. Title: Anomalous magnetic field diffusion during star formation Authors: Norman, C.; Heyvaerts, J. Bibcode: 1985A&A...147..247N Altcode: The authors discuss the physics of magnetic field dissipation and reconnection during protostellar formation. In their analysis of quasi-equilibrium collapse they have incorporated both ambipolar diffusion and magnetic dissipation processes. Both the final flux and angular momentum problems can be resolved in their model which incorporates some of the detailed physics that was previously lacking in such theories. The energy input during flux destruction is considerable and the authors have speculated that consequences may include the formation of a hot corona with an observable X-ray flux, and the pumping of OH masers. Radio emission may be observable in this phase. Title: Energy dissipation mechanisms in the solar corona Authors: Heyvaerts, J. Bibcode: 1985IAUS..107...95H Altcode: Present ideas concerning the electric heating of the solar corona are reviewed. More detailed consideration is given to the dissipation of MHD waves in strong horizontal gradients of the Alfven velocity. Then the evolution of dc currents in the solar corona is considered. Some theories aiming at the evaluation of the net rate of energy dissipation by such mechanisms are described. A short account is given of a recent analytical study based on a generalization of Taylor's hypothesis concerning the evolution of magnetic helicity in plasma with a large magnetic Reynolds number. Title: Mechanisms of magnetic heating of the solar corona. Authors: Heyvaerts, J. Bibcode: 1984ESASP.220..123H Altcode: 1984ESPM....4..123H Present ideas concerning the electric heating of the solar corona are briefly reviewed. The author considers mechanisms of MHD wave dissipation in strong horizontal gradients of the Alfvén velocity, as well as turbulence associated dissipation. Then he considers the evolution of D.C. coronal currents in the presence of active magnetic reconnection, and stresses the interest of considering the existence of approximate global invariants associated with such phenomena. Title: Coronal heating by reconnection in DC current systems - A theory based on Taylor's hypothesis Authors: Heyvaerts, J.; Priest, E. R. Bibcode: 1984A&A...137...63H Altcode: The rate of coronal heating expected from complex reconnection processes is analyzed by adapting Taylor's hypothesis to solar and stellar physical conditions. The fact that the magnetic helicity of a region which is not a closed flux tube is not a gauge-invariant quantity is addressed, and the time evolution of a magnetic arcade undergoing slow footpoint motions and infinitely rapid relaxation by reconnection is calculated. It is shown that, when the stresses are relaxed instantaneously the amount of heating vanishes exactly. A second order theory is developed which permits the heating effect due to a small but finite reconnection time to be calculated. It is concluded that DC current coronal heating is mainly due to motions comparable in size to or smaller than the characteristic size of the magnetic structure, and that these motions must tend to produce a nonconstant alpha force-free magnetic structure. The theory illuminates the connection between general coronal heating and solar flares. Title: The production of gamma-rays in gamma-ray burst sources Authors: Bonazzola, S.; Hameury, J. M.; Heyvaerts, J.; Lasota, J. P. Bibcode: 1984A&A...136...89B Altcode: The authors propose, as a model for gamma-ray bursts, that the energy released by the thermonuclear runaway at the surface of a magnetized neutron star is converted into gamma-ray photons by small scale magnetic field reconnection. Most of the energy flux is transported up to the atmosphere by Alfvén waves, driven by oscillatory magnetoconvection. The electric field created by reconnection accelerates electrons to very high energies. The excitation of Landau levels by collisions is followed by gamma-ray synchrotron emission. The time scales, energies, and luminosities involved agree with the observations. Title: Resonant reception in the Solar System of gravitational waves from external sources Authors: Bonazzola, S.; Carter, B.; Heyvaerts, J.; Lasota, J. P. Bibcode: 1984Natur.308..163B Altcode: Here we aim to point out the rather stringent limitations, particularly those due to frequency stability requirements, that can be placed on conceivable (for example, binary) sources of any gravitational waves that might be detected by resonant stimulation of an oscillation mode in the Solar System. This question has recently been raised by widespread informal discussions1,2 of the possibility that unexpectedly persistent low-frequency (160-min period) oscillations observed in the Sun3 might be due to such a mechanism4. We show here that this particular possibility can be excluded. Title: Radiative transfer in optically thick hot astrophysical plasmas. Authors: Heyvaerts, J. Bibcode: 1984PhST....7...94H Altcode: 1984PhyS....7...94H Inelastic radiation transfer in hot plasmas is reviewed. It is shown that a variety of situations can be described by combining a Kompaneets type of transfer equation with suitable photon sources. Some emphasis is put on the particular case of very magnetized plasmas (B ≅ 1012G). It is shown that the spectrum of magnetized X-ray sources is best understood as resonant-scattering cooling of very hot photons in a medium with negative temperature gradient. Title: Magnétohydrostatique. Authors: Heyvaerts, J. Bibcode: 1984cms..conf..179H Altcode: No abstract at ADS Title: Thermonuclear flashes on magnetized neutron stars as a model for gamma-ray bursts Authors: Hameury, J. M.; Bonazzola, S.; Heyvaerts, J.; Lasota, J. P. Bibcode: 1984AdSpR...3j.297H Altcode: 1984AdSpR...3..297H We propose, as a model for gamma ray bursts, that the energy released by a thermonuclear runaway at the surface of a slowly accreting, magnetized neutron star, is converted into gamma ray photons by small scale magnetic field reconnection. Most of the energy flux is transported up to the atmosphere by Alfvén waves, driven by oscillatory magnetoconvection. The electric field created by reconnection accelerates electrons to very high energies. The excitation of Landau levels by collisions is followed by gamma ray synchrotron emission. The time scales, energies, and luminosities involved agree with the observations. Title: Thermonuclear Flashes on Magnetized Neutron Stars as a Model for Gamma-Ray Bursts Authors: Hameury, J. M.; Bonazzola, S.; Heyvaerts, J.; Lasota, J. P. Bibcode: 1984heac.conf..297H Altcode: No abstract at ADS Title: Mechanisms for Heating the Solar Corona by the Dissipation of AC or DC Currents Authors: Heyvaerts, J. Bibcode: 1984apoa.conf..318H Altcode: No abstract at ADS Title: Gamma-ray Bursts: a Model Authors: Hameury, J. M.; Bonazzola, S.; Heyvaerts, J.; Lasota, J. P. Bibcode: 1984ASIC..134..345H Altcode: 1984pcnr.conf..345H The authors propose, as a model for gamma-ray bursters, that the energy released by a thermonuclear runaway under the surface of a slowly accreting, magnetized, neutron star, is transported up to the atmosphere by Alfvén waves, driven by overstable magnetoconvection. In the atmosphere, the short scale magnetic field lines reconnection generates an electric field. The electrons accelerated by this process produce gamma-rays by inverse Compton scatterings with blackbody photons of a few keV. Title: Magnetohydrostatics in the polar caps of the gamma-ray burst sources Authors: Hameury, J. M.; Bonazzola, S.; Heyvaerts, J.; Lasota, J. P. Bibcode: 1983A&A...128..369H Altcode: The ability of the magnetic field existing at the surface of a neutron star to confine a layer of hydrogen inside the polar caps has been studied. It is found that, except for low values of the magnetic field (less than 10 to the 11th G), diffusion across the field lines is negligible. A numerical solution of the magnetohydrostatic equations shows that the disruption of the field lines can occur only if the ratio beta of the gas pressure to the magnetic pressure exceeds L/h (L = polar cap radius, h = height of the accreted layer). When the amount of accreted matter reaches the critical value at which a thermonuclear flash is triggered, the value of the critical magnetic field is a few times 10 to the 11th G; it is much smaller than what is deduced from Mazets line features (Mazets et al., 1981), i.e. about 5 x 10 to the 12th G. It is therefore concluded that the magnetic field acts as a stiff container, and that it is not distorted on a large scale either before or during the burst. Title: Current sheet models for solar prominences. II - Energetics and condensation process Authors: Malherbe, J. M.; Priest, E. R.; Forbes, T. G.; Heyvaerts, J. Bibcode: 1983A&A...127..153M Altcode: A steady state dynamic model for solar prominences of the Kuperus and Raadu type was previously proposed by Malherbe and Priest (1983), but only the motion through series of quasi-static states was investigated there. The mechanisms for formation, condensation and cooling of plasma in this model are studied. As hot coronal material approaches the filament sheet, it is expected to cool and condense. Cold material is then carried up through the prominence by rising magnetic field lines due to converging photospheric motions below the filament. Two possible ways are suggested of triggering a thermal instability and so producing such a stationary condensation process: a larger pressure in the sheet, or a smaller wave heating in the reconnected field than in the surrounding corona. This paper presents a simple model to simulate the plasma condensation: the thermodynamics of the cooling process, as well as the dynamics of new material entering the prominence sheet, are described in detail. Title: The final state of a solar flare. Authors: Norman, C. A.; Heyvaerts, J. Bibcode: 1983A&A...124L...1N Altcode: It is shown that the final state of a solar flare is a force free field with constant α where α is determined by the boundary conditions. This result is independent of the details of magnetic energy build up and release. The crucial assumptions are that during the period of rapid magnetic energy release the effects of slow photospheric foot point motions can be neglected and that magnetic field reconnection occurs in regions whose total volume is small compared to that of the overall configuration. Title: Gravitational settling in layers accreted on neutron stars and its relations to gamma ray bursts Authors: Hameury, J. M.; Heyvaerts, J.; Bonazzola, S. Bibcode: 1983A&A...121..259H Altcode: The use of Hansen's (1978) diffusion coefficient has reduced metallicities by two to three orders of magnitude in the present study of the possibility of the occurrence of hydrogen-helium flashes at the surface of accreting neutron stars, which takes into account heavy element depletion due to sedimentation. The abundance profile is found to be Rayleigh-Taylor unstable under the influence of a mean molecular weight gradient when there is no magnetic field, in which case metallicity is reduced. The minimum hydrogen flash-associated accretion rate is of the order of 10 to the -15th solar masses/year per sq km, rendering these neutron stars only marginally detectable in the X-ray emission range by the Einstein satellite. Title: A mathematical model of solar flares. Authors: Heyvaerts, J.; Lasry, J. -M.; Schatzman, M.; Witomski, P. Bibcode: 1983QApMa..41....1H Altcode: The phenomenon of solar flares is modeled assuming that the magnetic field is force-free and that its evolution is quasi-static. This model is simplified so as to be tractable and yields a semi-linear elliptic equation in a half-plane depending on a parameter lambda which describes the time evolution. It is proved that there are (at least) two branches of solutions which have distinct asymptotic behaviors at infinity. The upper branch exists for all lambda greater than 0, but the lower branch exists only on a finite interval /0, lambda exp c/. As stable solutions must have the same asymptotic behavior as the lower branch of solutions, and as this is impossible after lambda exp c, it is contended that no stable solution exists after lambda exp c and that a solar flare is thus triggered. Title: Can γ-ray bursts originate from low-mass binaries? Authors: Ventura, J.; Bonazzola, S.; Hameury, J. M.; Heyvaerts, J. Bibcode: 1983Natur.301..491V Altcode: γ-Ray bursts have been attributed to binary systems1-3 with the burst resulting from an accretion instability1,2, or a thermonuclear explosion at the surface of an accreting magnetic neutron star4-6. Recent deep sky surveys in the X-ray7-9 and optical wavelengths (S. Illovaiski and C. Chevalier, personal communication), impose stringent new limitations on the theory. Assuming an average distance of ~300 pc, these observations limit the optical absolute magnitude of bursters in quiescence to Mv>13 and their X-ray luminosity to <1031 erg s-1 for polar cap accretion (K. Hurley, S. Illovaiski and G. Pizzichini, personal communication). If these are local binary systems in our Galaxy, as suggested by their isotropic distribution in the sky10,11, the binary companion would clearly have to be a very low mass object. Although very peculiar, the presence of such an `invisible' companion is possibly hinted at by the recently proposed identification of the 19 November 1978 γ-ray burst to a 1928 optical transient event lasting <=10 min discovered by Schaefer12. Motivated by these data, we examine here the possibility of obtaining such low luminosity systems as the evolutionary end products of galactic low-mass binary systems with a neutron star primary. Title: Coronal heating by phase-mixed shear Alfven waves. Authors: Heyvaerts, J.; Priest, E. R. Bibcode: 1983A&A...117..220H Altcode: The authors consider the physical processes which occur when a shear Alfvén wave propagates in a structure with a large gradient of the Alfvén velocity. Although these waves do not possess local resonances (unlike magneto acoustic modes) they nevertheless suffer intense phase mixing during which the oscillations of neighbouring field lines become rapidly out of phase. The authors study this effect and show that the resulting large growth of gradients dramatically enhances the viscous and ohmic dissipation. The cases of propagating and standing waves are considered, and a detailed calculation is given of the rate of dissipation achieved in a finite length structure like a loop, in the presence of a random excitation at its ends. The authors prove that, after a long enough time, phase mixing can actually ensure the dissipation of all the wave mechanical energy that a loop can pick up from the excitation, in agreement with a previous claim by Ionson. The general conclusion of the study is that phase mixing is the process most able to ensure the dissipation of shear Alfvén waves in loops and in open regions of strong reflectivity, and that loops, in particular, must be in a permanent state of Kelvin-Helmholtz and tearing turbulence. Title: Observational constraints on the thermonuclear flash model for gamma-ray bursts Authors: Ventura, J.; Bonazzola, S.; Hameury, J. M.; Heyvaerts, J. Bibcode: 1983MitAG..58..111V Altcode: No abstract at ADS Title: Thermonuclear runaways at the surface of slowly accreting neutron stars. Authors: Bonazzola, S.; Hameury, J. M.; Heyvaerts, J.; Ventura, J. Bibcode: 1982ans..conf..241B Altcode: 1982ans..conf..241H Thermonuclear runaways at the surface of neutron stars with mass accretion rates of the order of 10 to the -15th power solar mass/yr/sq km are discussed. This slow accretion results in longer recurrence time scales, of the order of a few years; the shell temperature between two bursts is also lower and consequently the accreted material is pushed to higher densities before the flash. A strong magnetic field causes non-thermal emission during the flash, and may explain the gamma-ray emission. Moreover, it focusses the accretion onto the polar caps. The accretion rate per unit surface may then be 1000 times larger than the rate which result from spherical accretion. The possibility of producing hydrogen-helium flashes in such conditions is investigated. In a previous paper it was found that a metallicity Z = .04 in the accreted layer, thermonuclear instabilities where obtained when the accretion rate was larger than a critical value. However depletion of the C, N, and O nuclei due to gravitational settling seriously affect these results. Title: Hydrogen-helium flashes on accreting neutron stars as a possible origin of gamma-ray bursts Authors: Hameury, J. M.; Bonazzola, S.; Heyvaerts, J.; Ventura, J. Bibcode: 1982A&A...111..242H Altcode: Accretion of interstellar matter on a galactic disc population of old neutron stars can lead to thermonuclear runaway offering a very plausible explanation for the phenomenon of gamma-ray bursts. We have studied numerically the possibility of having hydrogen and helium flashes in system with low accretion rates Mdot < 10-12 Msun yr-1. We find instabilities when the accretion rate exceeds 3 10-13 Msun (in the absence of magnetic channeling of the accretion). Each burst is then triggered by electron capture on protons, when the accreted envelope reaches a mass of 3 1023 g. The resulting hydrogen flash will lead to helium detonation after a short heating time of several minutes. Our conclusions are:

a) Only slowly moving neutron stars (3 km s-1 with respect to the interstellar medium) can give a burst.

b) The model is compatible with the data if the old neutron star density is ∼10-2 PC-3 and their velocity dispersion is ∼50km s-1, somewhat lower then that of radio pulsars.

c) The amount of fuel available for the detonation has a standard value of ∼10+23 g, but can also depend on the neutron star's previous history. Exceptionally intense events could, for instance, be explained by a cold neutron star entering a favorable environment of low relative velocity.

d) Persistent X-ray emission after the bursts and a gradual increase above the X-ray back ground starting a few minutes before the burst should be a signature of the mechanism proposed. Title: Blowing up of two-dimensional magnetohydrostatic equilibria by an increase of electric current or pressure Authors: Heyvaerts, J.; Lasry, J. M.; Schatzmann, M.; Witomsky, P. Bibcode: 1982A&A...111..104H Altcode: General properties are deduced for a series of two-dimensional magnetohydrostatic configurations. An existence theorem for the Dirichlet boundary conditions on the frontier of a semiinfinite domain is established, and the magnetic topology of the always-existing solution is shown to be of the open type. While the existence of a closed topology solution is also proved for the case of small currents, this solution disappears at a bifurcation point for some finite value of the stretching parameter lambda. The relevance of these results to the study of eruptive solar flares and other solar active phenomena is discussed. Title: Population III objects and the shape of the cosmological background radiation. Authors: Heyvaerts, J.; Puget, J. -L. Bibcode: 1982cp...proc..297H Altcode: No abstract at ADS Title: High energy particle acceleration in flares Authors: Heyvaerts, J. Bibcode: 1982AdSpR...2k.187H Altcode: 1982AdSpR...2..187H Not Available

The permanent participants in this team at Annecy SMY workshop (October 1981) have been, in alphabetical order : R. BOCCHIA, S. ENOME, J. HEYVAERTS, C. KOUVELIOTOU, B. IWERS, M. PESSSES, E. RIEGER, J.M. ROBILLOT, J. RYAN, G. TROTTET, N. VILMER, L. VLAHOS, G. WIBBERENZ Very useful contributions by many other participants are acknowledged, specially (but not only) by K. ANDERSON, A. BENZ, J. BROWN, M. KUNDU, M. PICK, D. RUST, R. STEWART. Title: Magnetic reconnection: A problem of general physical and astrophysical interest, with special implications in solar physics Authors: Heyvaerts, J. Bibcode: 1982AdSpR...2i..21H Altcode: 1982AdSpR...2...21H The astrophysical applications of magnetic reconnection are addressed. MHD flows involving reconnection are considered for various situations and different media, and the importance of thermal and gravitational effects in plasma MHD flows is argued. The nature of reconnection where the plasma is not collisional enough to be regarded as an MHD medium is addressed, and microturbulence in reconnection processes is discussed. Phenomena related to the nonlinear evolution of tearing modes in time-dependent reconnection are considered, including magnetic braiding, internal disruptions involving an apparently unique helical mode, and situations resulting when the primary periodicity of the tearing perturbation is inapplicable or where finite resistivity is included. Title: Are γ-ray bursters neutron stars accreting interstellar matter? Authors: Bonazzola, S.; Hameury, J. M.; Heyvaerts, J.; Ventura, J. Bibcode: 1981SSRv...30..471B Altcode: A model for the approximately 150 gamma-ray burst events observed in recent years is presented. The existence of population I, old neutron stars with a density of one/100 cu pc was considered, with an accretion of interstellar matter at a rate defined by Bondi's law. The thermal evolution of the subsequent thick shells was calculated, with an eye to the possibility of thermonuclear runaway. Account was made of the energy exchanges between the accreted shell and the star interior, and a range of parameters was derived which would result in a runaway. Higher accretion rates were found to lead to a runaway when the shell reaches 50 cm in thickness and electron capture was occurring. Various changes in burning rates, changing temperatures, and accretion rates which would disrupt steady-state stages into a runaway condition and subsequent gamma-ray burst are discussed, noting that a significant role may also be played by the magnetic field. Title: Expected characteristics of pulsar gamma-ray radiation and the problem of its location Authors: Heyvaerts, J.; Signore, M. Bibcode: 1981A&A....96...36H Altcode: The transfer of gamma rays in the near pulsar magnetosphere of a parallel rotator is studied, with emphasis placed on determining the parameters characterizing the emitted spectrum, namely high energy cutoff, spectral slopes, and the energy range of secondary emission. A systematic and analytical investigation is made of their relaxation to the electron acceleration. It is shown that there exists a maximum pulsar period above which pair creation in the vicinity of the crust is impossible, whatever the electron acceleration conditions may be Title: Population III objects and the shape of the cosmological background radiation Authors: Heyvaerts, J.; Puget, J. -L. Bibcode: 1981copa.conf..297H Altcode: 1981copa.conf..297P The spectrum of the cosmological radiation may keep track of nonthermal processes having followed the decoupling era, in the form of departures from a strictly Planckian spectrum. The consequences of energy and metals release by a population of pregalactic objects is examined. The latter are asumed to condense into dust. The formation of the universal spectrum under these conditions is described in a self-consistent manner. It is shown that a good enough agreement can be obtained with presently available data. It is concluded that the observed spectrum can be explained if the star burst occurred before the epoch z near 30 and after z near 300, with a release varying between 0.3 MeV/nucleon for z equals 30 and 2 MeV/nucleon at z equals 200, while the mass fraction in grains vary from 1/10,000-1/1,000,000. The results point to the possibility that the population III even might have occurred recently, at z near 30 to 50, this being still consistent with all the considered constraints. Title: Recent developments in solar flare models Authors: Heyvaerts, J. Bibcode: 1981soac.reptR....H Altcode: Theoretical models which explain why and how solar flares occur are surveyed. Processes that govern the dissipation of magnetic energy are considered fundamental. The storage of magnetic energy and what produces its violent conversion into thermal and suprathermal energy are considered. A magnetic model of a flare which is presented comprises three phases: a stable flare buildup phase evolves into an impulsive phase which leads to the principle phase of a flare phenomenon. The stability of the preeruptive state and current sheets is discussed as well as the thermal equilibrium of the preeruptive structure. Reconnection in the preeruptive state and the occurence of spontaneous reconnection are treated. Results show that reconnection can have different geometries and, hence, can provoke nonlinear developments of widely varying magnitudes in the context of the solar corona. Title: Energy Balance in Current Sheets - from Petschek to Gravity Driven Reconnection Authors: Mercier, C.; Heyvaerts, J. Bibcode: 1980SoPh...68..151M Altcode: It has been shown earlier that energy balance processes play a very important role in the determination of the reconnection regime in the central diffusive region of a steady Petschek flow (usually considered elsewhere as isothermal and incompressible): as a consequence of the plasma thermal properties, abrupt transitions in the reconnection regime may occur for special external conditions. The regime becomes then a dynamical one, and it was suggested that onset of plasma microturbulence may result and act as a primary triggering mechanism in solar flares. Title: Stability of accretion column flows. Authors: Hameury, J. M.; Bonazzola, S.; Heyvaerts, J. Bibcode: 1980A&A....90..359H Altcode: The stability of both a spherically symmetric optically thin accretion and an optically thick accretion column are examined. For optically thick conditions, it is shown that radiation-pressure dominated flows near the Eddington limit are, under the usual physical conditions, unstable to a Rayleigh-Taylor type of instability. Yet an optically thin flow is stable against radial modes, both in cases where a time lag between accretion fluctuations and the resulting changes in luminosity is incorporated or not. These studies indicate that realistic accretion models should take into account strong micro-inhomogeneity likely to be present in accretion columns. Title: Population III stars and the shape of the cosmological black body radiation Authors: Puget, J. L.; Heyvaerts, J. Bibcode: 1980A&A....83L..10P Altcode: The paper presents a theory of the distortion of the cosmological thermal spectrum by pregalactic dust which appears to fit the recent data. The basic idea is to consider the possibility that the opacity of the universe in the pregalactic phase (redshift of the order of 10 or more) may have been dominated by dust at a wavelength near the peak of intensity of the cosmological black body at that time. This hypothesis is quite natural within the framework of a model of the universe in which population III stars can produce a fraction of the observed abundances of helium and of the heavier elements, which might condense into dust. It is suggested that the formation of a small mass fraction of dust (about 0.001) near z equals 10, together with sufficient energy release, explains the shape of the observed distortion. Title: MHD stability of coronal structures. Authors: Heyvaerts, J. Bibcode: 1980AnPh....5..337H Altcode: 1980mhda.conf..337H No abstract at ADS Title: Resistive stability. Authors: Heyvaerts, J. Bibcode: 1980AnPh....5..379H Altcode: 1980mhda.conf..379H No abstract at ADS Title: Solar corona magnetohydrostatics. Authors: Heyvaerts, J. Bibcode: 1980AnPh....5..315H Altcode: 1980mhda.conf..315H No abstract at ADS Title: Kinetic effects on the propagation of surface waves and their relevance to the heating of the solar corona. Authors: Kuperus, M.; Heyvaerts, J. Bibcode: 1980AnPh....5..483K Altcode: 1980mhda.conf..483K No abstract at ADS Title: Electro Magnetic Heating of Coronae Authors: Heyvaerts, J.; Schatzman, E. Bibcode: 1980jfss.conf...77H Altcode: No abstract at ADS Title: Gyro-synchrotron modulation in the moving type IV bursts. Authors: Trottet, G.; Pick, M.; Heyvaerts, J. Bibcode: 1979A&A....79..164T Altcode: Three successive moving type IV bursts were observed in association with an eruptive prominence on April 12, 1977. The radiation which is attributed to the gyrosynchrotron emission from energetic electrons trapped in an expanding magnetic arch exhibits an important modulation on a time scale of about one minute. The modulation takes predominantly its origin in two components which are assumed to be the mirror points of the expanding arch where the electrons are trapped. The possibility that synchrotron radiation near the mirror points is modulated by a mere MHD oscillation of the arch is investigated. A second kind of explanation is proposed which involves a quasiperiodic downward precipitation of particles. Title: Cyclotron line formation by resonant Compton-cyclotron scattering in Hercules X-1. Authors: Bonazzola, S.; Heyvaerts, J.; Puget, J. L. Bibcode: 1979A&A....78...53B Altcode: Calculations are presented which show that a spectrum similar to the one observed for Hercules X-1 can be generated. It is shown that there is no straightforward way to explain how observed 'lines' are produced. The effect of the cyclotron process as resonance scattering and Compton diffusion on a thermal spectrum in a simple model (cold plasma, plane-parallel geometry, isotropic scattering cross section) is studied. It is shown that even in such a crude model, a good account is given of the formation of the observed spectrum, continuum as well as lines. The cold-gas approximation is shown not to be inconsistent with the thermal balance of the accreting column. Title: Plasma and solar physics. The solar flare phenomenon Authors: Heyvaerts, J. Bibcode: 1979JPhys..40...37H Altcode: No abstract at ADS Title: Discussion Authors: Gaizauskas, V.; Glencross, W. M.; Heyvaerts, J. Bibcode: 1979phsp.coll..182G Altcode: 1979IAUCo..44..182G No abstract at ADS Title: Discussion Authors: Acton, L. W.; Engvold, O.; Heasley, J. N.; Heyvaerts, J.; Hirayama, T.; Kundu, M. R.; Leroy, J. L.; Malville, J.; Rust, D. M.; Zirin, H. Bibcode: 1979phsp.coll...31A Altcode: 1979IAUCo..44...31A No abstract at ADS Title: Plasma and solar physics. The solar flare phenomenon. Authors: Heyvaerts, J. Bibcode: 1979JPhys..40C..37H Altcode: No abstract at ADS Title: Force-Free Equilibria, Solar Flares and Coronal Transients. Authors: Heyvaerts, J.; Lasry, J. M.; Schatzman, M.; Witomsky, P. Bibcode: 1979phsp.coll..174H Altcode: 1979IAUCo..44..174H; 1979phsp.conf..174H The properties of magnetic configurations able to exist in the solar corona are considered. The solar flare phenomenon is due to the sudden dissipation of magnetic energy in the solar corona, and it was shown that photospheric shearing motions are essential in triggering flares which occur in closed magnetic configurations. The flare problem is simplified by considering 2-dimensional structures and deriving expressions for the components of the field and the electric current density. The shear function is considered, and it is concluded that the current could be increased in a closed magnetic structure only up to a certain value which depends on the current distribution. If the current is pushed above this limit, no neighboring equilibrium can be found and the structure undergoes a dynamical evolution driven by unbalanceable Laplace forces towards the open configuration which is the only equilibrium solution left. The motion could be the 'coronal transient' which is observed to follow flares, and should be considered as part of it. Title: Discussion Authors: Acton, L. W.; Brueckner, J.; Heyvaerts, J.; Maltby, P.; Spicer, D. S. Bibcode: 1979phsp.coll..314A Altcode: 1979IAUCo..44..314A No abstract at ADS Title: Discussion Authors: Gaizauskas, V.; Heyvaerts, J.; Hirayama, T.; Pneuman, G. W.; Spicer, D. S.; Withbroe, G. L.; Zirin, H. Bibcode: 1979phsp.coll..301G Altcode: 1979IAUCo..44..301G No abstract at ADS Title: Discussion Authors: Heyvaerts, J.; Schmahl, E. J.; Spicer, D. S. Bibcode: 1979phsp.coll..179H Altcode: 1979IAUCo..44..179H No abstract at ADS Title: Discussion Authors: Heyvaerts, J.; Sheridan, K. V.; Zirin, H. Bibcode: 1979phsp.coll..320H Altcode: 1979IAUCo..44..320H No abstract at ADS Title: Polarization and location of metric radiobursts in relationship with the emergence of a new magnetic field. Authors: Heyvaerts, J.; Kerdraon, A.; Mangeney, A.; Pick, M.; Slottje, C. Bibcode: 1978A&A....66...81H Altcode: Spike bursts have been observed in association with some solar type III burst groups. In this event, the spikes appear according to bandwidth and duration to be mini type I bursts. Their circular polarization cannot be explained by emission of the ordinary mode in the magnetic field of the dominant photospheric polarity. This observation is interpreted by the expansion at 0.3 solar radii of magnetic loops related to the emergence of a satellite polarity. A coronal-field model is proposed for the whole active center. Application of the theory of Mangeney and Veltri (1976) for type I bursts leads to a consistent picture of the generation of these spikes. Title: The triggering of plasma turbulence during fast flux emergence in the solar corona. Authors: Heyvaerts, J.; Kuperus, M. Bibcode: 1978A&A....64..219H Altcode: The paper discusses the physics of current sheet formation in regions of contact between old and new flux; this formation occurs during flux emergence from the solar photosphere into the corona. Particular attention is paid to the case of fast emergence, in which, at first, no field-line reconnection can take place. It is shown to be almost impossible to trigger microturbulence in the one-dimensional phase of sheet development. An approximation based on the fact that the sheet is very long and very thin is used to study analytically the two-dimensional phase of flow induced by compression. It is shown that the flow evacuates the region of the sheet where the pinching is strongest, and that the conditions for microturbulence are easily fulfilled there after some time. The behavior of the current sheet after turbulence sets in is considered. Title: Formation of X-ray spectra by Compton and cyclotron resonant diffusion. Application to Hercules X-1. Authors: Bonazzola, S.; Heyvaerts, J.; Pujet, J. L. Bibcode: 1978sss..meet...B3B Altcode: No abstract at ADS Title: Problems related to Particle Acceleration and Plasma Turbulence during the Impulsive Phase of Solar Flares Authors: Heyvaerts, J. Bibcode: 1978pfsl.conf..193H Altcode: 1978ESPM....2..193H No abstract at ADS Title: The downward motions in quiescent prominences. Authors: Mercier, C.; Heyvaerts, J. Bibcode: 1977A&A....61..685M Altcode: Two possible causes of the observed downward mass loss in quiescent prominences are considered: Joule dissipation in subphotospheric regions and downward diffusion of neutral atoms due to gravity. A model of the complete electrical circuit in a quiescent prominence is examined in which current lines are closed in the convection zone. Arguments are presented suggesting that Joule dissipation occurs mainly in the convection zone, and it is shown that the resulting downward velocity of a prominence could be as high as several hundred meters per second. The possibility is discussed that material motions in the photosphere and the convection zone may act as a current generator in addition to the motion of the prominence itself. Downward diffusive motion of neutral atoms in a prominence as a result of gravity are studied quantitatively, taking into account magnetic-field-induced anisotropies in momentum transfer between particles. It is concluded that this process allows only slow downward differential motions and is therefore incapable of explaining the observed mass loss. Title: An emerging flux model for the solar phenomenon. Authors: Heyvaerts, J.; Priest, E. R.; Rust, D. M. Bibcode: 1977ApJ...216..123H Altcode: An outline is presented of the physical processes involved in the emerging flux model, which appears to explain naturally many solar flare observations. The separate physical phases of the basic model include a preflare heating phase as the new flux emerges, an impulsive phase as high-energy particles are accelerated, a flash (or explosive) phase when the H-alpha intensity increases, and a main phase while it decreases. The extent and morphology of the main phase emission depend on the structure of the magnetic field region in which the new flux finds itself imbedded. It is suggested that a (small) simple loop flare occurs if the new flux appears in a region where no great amount of magnetic energy in excess of potential is stored. A two-ribbon flare occurs if the flux emerges near the polarity inversion line of an active region that has begun to develop filaments. Title: An emerging flux model for solar flares. Authors: Heyvaerts, J.; Priest, E.; Rust, D. M. Bibcode: 1977SoPh...53..255H Altcode: No abstract at ADS Title: Broadening and anomalous shift of Pioneer 6 telemetry line. An effect of coronal inhomogeneities useful for diagnostics. Authors: Chastel, A. A.; Heyvaerts, J. Bibcode: 1976A&A....51..171C Altcode: The observational results of Goldsten (1969) on the perturbations of the frequency and bandwidth of the Pioneer VI 2.3 GHz telemetry signal during its occultation by the sun are reexamined from the viewpoint of whether the observed redshifts are of instrumental or solar origin. The observations are shown to be unreliable since the probe frequency was not controlled. Analysis of possible solar influences on the frequency arising from electron density inhomogeneities on the global scale, macroscopic scale, mesoscale, or microscopic scale shows that classical physics can explain the observed anomalies, and that a new model of photon-photon interaction is not needed. A likely explanation is that in the experiment the corona was almost always being observed when relaxing after the passage of a discontinuity, most probably a shock. Title: General-relativistic kinetic theory of waves in a massive particle medium Authors: Asseo, E.; Gerbal, D.; Heyvaerts, J.; Signore, M. Bibcode: 1976PhRvD..13.2724A Altcode: In this paper, we give a general-relativistic kinetic theory of waves propagating in a medium filled with massive particles. A major difficulty of this problem is to handle simultaneously dispersive and expansion effects. Matter itself is at the root of both phenomena, and in our treatment they are conveniently separated by using a two-time scale approximation. It turns out that the expansion modifies both the amplitude and the frequency of the waves. Dispersion effects give rise to proper modes, which are shown to be the 0, 1, and 2 helicity components of the total field. The dispersion equations for these different components are obtained in a general form. The propagation of gravitational modes is examined in more detail for the two extreme cases of cold and ultrarelativistic matter. A lower cutoff frequency appears, and no Landau damping is found in the case of a thermalized gas. Title: Thermal evolution of current sheets and flash phase of solar flares. Authors: Heyvaerts, J.; Priest, E. R. Bibcode: 1976SoPh...47..223H Altcode: The physical conditions in a stationary flow of the Petchek type, allowing reconnection between flux emerging from below the solar photosphere and a preexisting magnetic field, are discussed. It is shown that, when rising in the solar atmosphere, the reconnection region has at first a rather low temperature as compared with its environment. Above a certain critical height, however, this low temperature thermal equilibrium often ceases to be possible, and the sheet rapidly heats, seeking a new thermal equilibrium. During this dynamical process, current-driven microinstabilities may be triggered in the current sheet, giving rise to an enhanced resistivity. High energy particles might be produced by the induced electric field developed during the rapid readjustment of MHD flows that results from this change in the transport properties of the plasma. Title: The effect of electron density fluctuations on the fundamental radiation of type III bursts. Authors: Heyvaerts, J. Bibcode: 1975A&A....38...45H Altcode: Summary. The effect of longwavelength electron density inhomogeneities on the fundamental radiation ofType III bursts is discussed. It is shown that the effective radiation transfer coefficients in the source are mainly determined by these fluctuations if the relative electron density of fluctuations exceeds 0.1 %. In this case both the bandwidth in the vicinity of a given altitude and the overall size of the source reflect the properties of the fluctuations of the electronic density. Key words: solar radio burst - stochastic processes radiative transfer- coronal electron density fluctuations plasma turbulence Title: The thermal instability in a magnetohydrodynamic medium. Authors: Heyvaerts, J. Bibcode: 1974A&A....37...65H Altcode: The modes that can propagate in a resistive, current carrying and radiating plasma are studied. The energy equation includes wave heating, radiation cooling, anisotropic thermal conduction as well as Joule heating. It is shown that when the thermal instability occurs in such a medium, it tends to develop field aligned fine structures. The Joule effect, when it is sufficiently important, can be responsible for a number of particular instabilities: a thermal Joule instability, which can make unstable modes that would otherwise be stable when the current is less than some threshold value, and an 'antidiffusion' mode, the effect of which is to concentrate the electric currents in fine field-aligned fibers. The nature and stability of modes whose frequency is less than the characteristic thermal frequencies is also studied. Title: Coronal Electric Currents Produced by Photospheric Motions Authors: Heyvaerts, J. Bibcode: 1974SoPh...38..419H Altcode: Due to its high electrical conductivity along field lines, the corona provides an electric coupling between those remote points in the photosphere which are linked by coronal lines of force. This coupling essentially acts as a shunt for potential differences induced between such points by the electromotive fields created by photospheric motions. As a result, important coronal currents are driven. This paper studies the implications of this idea for a model which schematizes both the magnetic configuration and the photospheric motions observed in flare producting regions. In this particular geometry, coronal currents would be mainly driven along a sheet, whose trace in the plane of the photosphere would look as a two-ribbon structure on both sides of an inversion line of the photospheric polarity. We show that vertical currents of the order of 10−2, 10−1 A m−2 can be generated, and possibly interrupted if the photospheric flow velocity exceeds about 1 km s−1, then releasing the 1032 ergs of magnetic energy previous stored in the circuit. Title: Pairs of Non Fundamental-Harmonic Type III Bursts Authors: Caroubalos, C.; Heyvaerts, J.; Pick, M.; Trottet, G. Bibcode: 1974SoPh...37..205C Altcode: Different forms of pairs of type III bursts have been discussed in the literature. We report here a new aspect revealed by high time resolution radioheliography. In some groups of these bursts, each element appears to be split into two components. These pairs recur with a characteristic time, and in a given group the time splitting of the two components of each pair is the same (one second or less). The nature of these pairs is discussed: the fundamental-harmonic hypothesis is excluded. Alternative interpretations are reviewed. Title: A Clue to the Trigger for Both the Type III Solar Radioburst and the Solar Flare Authors: Priest, E. R.; Heyvaerts, J. Bibcode: 1974SoPh...36..433P Altcode: Recent observations of `neutral line absorbing features' in the solar atmosphere may give an important clue to the mechanism whereby both type III solar radiobursts and solar flares are triggered. It is suggested that as new satellite magnetic flux emerges at the edge of an active region in an area of opposite polarity a neutral sheet builds up between the new and old flux. When the sheet has a length of about a megametre its thermal insulation from the surrounding plasma is effective enough for a thermal instability to occur. The resulting compression and inflow of plasma is observed in Hα on the disc as a neutral line absorbing feature. Furthermore, the electric field of the accompanying collisionless tearing mode instability in a thin slab near the centre of the sheet exceeds the runaway field; it may therefore accelerate electrons to high enough energies to produce the type III burst which usually occurs at the same time as the absorbing feature. Perhaps the flare which sometimes ensues is triggered when the quasi-equilibrium state is destroyed by the development of turbulence in the neutral sheet. Title: Physical sciences: Perturbations of Pioneer 6 telemetry signal during solar occultation Authors: Chastel, Arnaud A.; Heyvaerts, Jean F. Bibcode: 1974Natur.249...21C Altcode: ATTENTION has been drawn recently to unexplained perturbations in the telemetry signal of Pioneer 6 (2,300 MHz) during solar occultation. The results1 shown in Fig. 1 present the following odd features: (1) An anomalous redshift is added to a normal linear redshift due to the spacecraft oscillator. This residual redshift which is symmetrical with respect to the centre of the Sun is of the order of z = 5.18-8 at four solar radii. (2) The bandwidth increases sharply when the telemetry signal grazes the Sun. (3) There are some extremely sharp pulses in the bandwidth. In Fig. 2 we show that these pulses are clearly associated with a sharp increase of the redshift and correspond to solar flares. Title: On Neutral Line Absorbing Features Authors: Heyvaerts, J.; Priest, E. Bibcode: 1974cesra...4..147H Altcode: No abstract at ADS Title: Effects of Turbulence Anisotropy on Propagation and Electromagnetic Radiation of Particle Streams in the Solar Corona Authors: Heyvaerts, J.; Verdier de Genouillac, G. Bibcode: 1974A&A....30..211H Altcode: Summary. This paper studies the evolution of a plasmabeam instability in the presence of non-linear effects. These consist of induced 11-scattering of Langmuir waves on the polarization clouds of ions; Langmuir waves are considered to have all possible angular- patterns. This problem is set in relation to Type III solar radio-bursts, hence the parameters used are those presumed to be characteristic of coronal conditions. Generally, the electromagnetic radiation of Type III bursts is attributed to the conversion of Langmuir waves. The non-linear effects have been introduced in the literature as a mechanism for sustaining the streams of fast electrons or ions in which such plasma waves may originate. Here we take explicitly into account Langmuir turbulence anisotropy as it may influence this stabilizing mechanism. Mathematical results show a saturation of the Langmuir turbulence due to non4inear effects, but this regime is not acceptable from a physical point of view. To complete the study, we look into the influence of the turbulence level and anisotropy on the electromagnetic radiation resulting from it by the two conversion processes usually invoked for Type III bursts. We confirm Smith's result that the fundamental radiation could be amplified in the source. Key words: solar corona - solar radio-burst - particle stream - plasma turbulence - plasma-beam instability Title: Plasma Physics and Solar Radioastronomy Authors: Heyvaerts, J.; Pick, M. Bibcode: 1972ppsr.conf.....H Altcode: No abstract at ADS Title: Some aspects of coronal structures Authors: Heyvaerts, J.; Pick, M. Bibcode: 1972ppsr.conf..118H Altcode: No abstract at ADS Title: Structures Coronales Authors: Heyvaerts, Jean; Pick, Monique Bibcode: 1972stco.book.....H Altcode: No abstract at ADS