Author name code: kuperus ADS astronomy entries on 2022-09-14 author:"Kuperus, Max" ------------------------------------------------------------------------ Title: The effect of delays on filament oscillations and stability Authors: van den Oord, G. H. J.; Schutgens, N. A. J.; Kuperus, M. Bibcode: 1998A&A...339..225V Altcode: We discuss the linear response of a filament to perturbations, taking the finite communication time between the filament and the photosphere into account. The finite communication time introduces delays in the system. Recently Schutgens (1997ab) investigated the solutions of the delay equation for vertical perturbations. In this paper we expand his analysis by considering also horizontal and coupled oscillations. The latter occur in asymmetric coronal fields. We also discuss the effect of Alfven wave emission on filament oscillations and show that wave emission is important for stabilizing filaments. We introduce a fairly straightforward method to study the solutions of delay equations as a function of the filament-photosphere communication time. A solution can be described by a linear combination of damped harmonic oscillations each characterized by a frequency, a damping/growth time and, accordingly, a quality factor. As a secondary result of our analysis we show that, within the context of line current models, Kippenhahn/Schlüter-type filament equilibria can never be stable in the horizontal and the vertical direction at the same time but we also demonstrate that Kuperus/Raadu-type equilibria can account for both an inverse or a normal polarity signature. The diagnostic value of our analysis for determining, e.g., the filament current from observations of oscillating filaments is discussed. Title: Prominence Oscillations and the Influence of the Distant Photosphere Authors: Schutgens, N. A. J.; Kuperus, M.; van den Oord, G. H. J. Bibcode: 1998ASPC..150..147S Altcode: 1998IAUCo.167..147S; 1998npsp.conf..147S No abstract at ADS Title: The Double Inverse Polarity Paradigm|The Sign of Magnetic Fields in Quiescent Prominences Authors: Kuperus, Max Bibcode: 1996SoPh..169..349K Altcode: A model is presented for the origin of inverse polarity magnetic fields in the perpendicular as well as in the axial direction of quiescent prominences. The model is based on the presence of a discrete coronal arcade structure where magnetic separating surfaces can be identified. On the crossing of these separating surfaces magnetic reconnection driven by photospheric shear and converging motions can create the observed field direction in quiescent prominences. Title: A magnetic explanation for the rapid burster. Authors: Kuijpers, J.; Kuperus, M. Bibcode: 1994A&A...286..491K Altcode: The main observations of Type II bursts in the low-mass X-ray binary MXB 1730-335 (the Rapid Burster) can be understood if the neutron star's magnetic field is strong (7 x 10^11^ - 2 x 10^12^ gauss at the surface) and rotationally symmetric around the axis of a steadily accreting disk. We show that a Keplerian disk of matter develops inside the pressure balance radius where it is compressed into a very thin disk by the external magnetic field. At the inner edge of the disk a ring of matter is suspended in the stellar magnetic field and corotates with the star. During quiescence the disk penetrates more and more into the magnetosphere, the ring accumulates gas and shifts to lower altitudes. The stellar field sets a maximum to the amount of supported gas. As soon as the threshold is passed an ideal mhd instability occurs, the matter ring falls onto the surface and triggers a violent burst of accretion. As the stellar field relaxes outward accretion onto the star continues to be enhanced until a new magnetic equilibrium is attained, the burst ends and a smaller ring is left at higher altitudes. As the (steady) external accretion continues the ring load increases again and moves inward until the threshold is reached and the sequence repeats. Such a bursting behaviour only occurs if the star is an aligned rotator. Title: A magnetic explanation for the rapid burster Authors: Kuijpers, J.; Kuperus, M. Bibcode: 1994SSRv...68..333K Altcode: The observations of X-ray Type II bursts from the low-mass X-ray binary MXB 1730-335 can be explained by a particular form of magnetic gating in the presence of steady external accretion. The requirements are a strong magnetic field of the neutron star (7×1011 2×1012 gauss at the surface), rotational symmetry and alignment of the field axis with the axis of a steadily accreting disk to within 6°. Title: Foreword Authors: van den Oord, Bert; Kuijpers, Jan; Kuperus, Max; Benz, A. O.; Brown, J. C.; Einaudi, G.; Kuperus, M.; Raadu, M. A.; Trottet, G.; van den Oord, G. H. J.; Vlahos, L.; Zheleznyakov, V. V.; Wijburg, Marion; Fletcher, Lyndsay; Volwerk, Martin Bibcode: 1994SSRv...68D..17V Altcode: No abstract at ADS Title: Accretion disk flares in energetic radiation fields. A model for hard X-rays from black hole candidates. Authors: van Oss, R. F.; van den Oord, G. H. J.; Kuperus, M. Bibcode: 1993A&A...270..275V Altcode: An investigation is conducted of the magnetic flares in the energetic radiation field of an accretion-disk corona, whose X-ray emission may be the source of the hard power-law component in the X-ray spectra of galactic black-hole candidates in their 'high' spectral state. The concept of the low plasma-beta accretion-disk corona (ADC), which produces the hard X-rays via inverse Compton scattering of lower-energy photons from the disk on enegetic electrons in the corona, is elaborated. A spectrum is derived for the up-scattered radiation from current sheets in the ADC; this spectrum mimics a power-law above a critical photon energy. Title: Accretion Disk Flares in Energetic Radiation Fields Authors: van Oss, R. F.; van den Oord, G. H. J.; Kuperus, M. Bibcode: 1993IAUS..157..217V Altcode: No abstract at ADS Title: The Effect of Retardation on the Stability of Current Filaments Authors: van den Oord, G. H. J.; Kuperus, M. Bibcode: 1992SoPh..142..113V Altcode: We investigate the influence of the finite Alfvén velocity on the evolution of an active region filament. In general, variations of a current result in variations of the magnetic fields which spread around with the Alfvén velocity. As a consequence of the fact that a magnetic field can only change with the Alfvén velocity, a filament will experience the photospheric boundary conditions as these were at an Alfvén travel time back in time. The inclusion of this retardation effect in the momentum equation of a filament leads effectively to an extra force term. This force contribution acts in the direction in which the filament moves and has therefore a destabilizing effect on the filament. Because a moving filament acts as an antenna of Alfvén waves, the filament loses energy by the emission process. This leads to a radiative damping term in the equation of motion of the filament. In general, the radiative damping will be sufficiently strong to counteract the retardation instability. Numerical simulations show that during the energy build-up phase a filament follows the van Tend-Kuperus equilibrium curve. After the van Tend-Kuperus equilibrium has disappeared the filament goes through a transient phase moving with a sub-Alfvénic velocity upward. At greater heights the repulsive Lorentz force of the photospheric surface current magnetic field is balanced by the radiative damping, resulting in a decreasing filament velocity. Title: Retardation effects and the origin of filament oscillations. Authors: Kuperus, M.; van den Oord, G. H. J. Bibcode: 1992ESASP.346...39K Altcode: 1992ssts.rept...39K A filament current subject to a perturbation experiences the photospheric boundary an Alfvén bounce time later. This retardation effect gives rise to an extra force which causes the filament to oscillate around an equilibrium position, emitting Alfvén waves which damp the filament oscillations. The onset of filament oscillations is a natural consequence of the presence of the inert photospheric boundary and the finite Alfvén speed. Numerical simulations are presented that show the evolution of a filament in a bipolar magnetic field as the filament current slowly increases. The emission of waves during this evolution is an important energy source for the coronal heating. Title: The Effect of Coupling of a Corona to the Turbulence in an Accretion Disk Authors: Geertsema, G. T.; Kuperus, M. Bibcode: 1992A&A...254..426G Altcode: Assuming a preferred length scale at which energy flows from magnetohydrodynamic turbulence in a thin accretion disk to a magnetically coupled corona, we investigate the differences between the energy dissipated in the accretion disk and the energy flowing into the corona. The study shows that the turbulence is very stable even against a strong coronal energy sink. A weak coronal energy sink reduces the amplitude of the fluctuations in the turbulent energy.

The temporal spectra of the disk turbulence i.e. the dissipated energy and of the energy flowing into the corona are compared. The slopes of the power spectra of the energy which flows into the corona are steeper than those for the dissipated energy. A case of strong coronal coupling is further examined. Remarkable peaks occur in the Fourier spectra, which we identify with energy bursts in the total turbulent energy. Title: Book reviews Authors: Linssen, P. F. J.; Valníček, B.; Pecker, Jean-Claude; Marov, M. Ya; Abalakin, V. K.; Achterberg, A.; Kuperus, M.; Sterzik, M. Bibcode: 1991SSRv...56..419L Altcode: No abstract at ADS Title: Book Review: Accretion disks and magnetic fields in astrophysics / Kluwer, 1989 Authors: Kuperus, M. Bibcode: 1991SSRv...56..422K Altcode: No abstract at ADS Title: Book Review: An introduction to Alfven waves / Adam Hilger, 1988 Authors: Kuperus, M. Bibcode: 1990SSRv...53..347K Altcode: 1990SSRv...53..347C No abstract at ADS Title: Magnetohydrodynamics of accretion disks. Authors: Kuperus, M. Bibcode: 1990CoPhR..12..275K Altcode: Accretion disks in close binary systems originate when mass overflow occurs from the primary star onto the compact star. When the compact star is a neutron star or a black hole the inner parts of the thin disk extend to the Alfvén radius respectively a few times the Schwarzschild radius. In the Keplerian rotating highly turbulent inner parts of the accretion disk magnetic fields are strongly amplified and expelled from the disk thus leading to the formation of a magnetically structured accretion disk corona. The magnetic energy supplied to the corona is radiated by inverse Compton scattering of soft X-ray photons produced in the disk by the viscous heating of the accreting matter. The interaction of the inner parts of an accretion disk with the magnetosphere around a neutron star leads to channeled accretion onto the magnetic poles, resulting in the phenomenon of X-ray pulsars. The interaction of disk coronal structures with the relatively weak magnetic fields of old fast spinning neutron stars leads to a new form of interaction around the so called beat frequency that can be used as a model for quasiperiodic oscillations in low-mass X-ray binaries. Title: Accretion disk coronae Authors: Kuperus, M. Bibcode: 1989HiA.....8..535K Altcode: No abstract at ADS Title: Coupling of an accretion disk corona with a weakly magnetic neutron star and a possible relation with quasi-periodic oscillations. Authors: Stollman, G. M.; Kuperus, M. Bibcode: 1988A&A...203..104S Altcode: We propose a mechanism whereby rotating, weakly magnetized neutron stars, which are thought to be present in some low mass X-ray binaries, may lose energy to the accretion disk via an electrodynamical coupling between the magnetic field of the star and magnetic loops in a disk corona. This mechanism may solve some of the problems posed by the observations of these systems. In particular, the properties of the power spectra of some of the sources, in which quasi-periodic oscillations are observed, can be explained, and especially it can be understood why the lowfrequency noise in some sources may be much reduced in power with respect to the power in the oscillations. Title: Equivalent circuit representation of Alfven waves Authors: Scheurwater, R.; Kuperus, M. Bibcode: 1988A&A...194..213S Altcode: By applying basic electric circuit theory to the linearized MHD equations the authors determine the voltage and current distribution in the magnetoplasma associated with the Alfvén waves and derive an expression for the input impedance. The input impedance, which is a function of both the plasma parameters and the frequency of the Alfvén waves, in general, may be represented by an equivalent electric circuit. The authors construct equivalent circuit representations for different ranges of the plasma parameters and frequency and calculate the quality factors for the circuits in the vicinity of the natural frequencies. Title: Accretion disk coronae and the contribution to angular-momentum transport in the disk Authors: Burn, H.; Kuperus, M. Bibcode: 1988A&A...192..165B Altcode: The differential rotation of an accretion disk amplifies the magnetic field in a standard thin accretion disk and drives large flux cells out of the disk which form an accretion disk corona. These cells, which are treated as force-free arcades in this paper are sheared by the same differential rotation. The reaction force of these cells that acts on the disk plasma is described in the frame work of an effective shear stress and can give a significant contribution to the total angular momentum transport in the corona-disk system. Title: Report of IAU Commission 12: Radiation and structure of the solar atmosphere (Radiation et structure de l'atmosphère solaire). Authors: Kuperus, M. Bibcode: 1988IAUTA..20...91K Altcode: No abstract at ADS Title: Varying self-inductance and energy storage in a sheared force-free arcade Authors: Zuccarello, F.; Burm, H.; Kuperus, M.; Raadu, M.; Spicer, D. S. Bibcode: 1987A&A...180..218Z Altcode: The authors utilize an electric circuit analogy to model the build-up and storage of magnetic energy in the coronal loops known to exist in the atmosphere of the Sun. The parameterization of magnetic energy storage in an electric circuit analogue uses a bulk current I flowing in the circuit and a self-inductance L. Because the self-inductance is determined by the geometry of the magnetic configuration any change in its dimensions will change L. If L is increased, the amount of magnetic energy stored and the rate at which magnetic energy is stored are both increased. One way of increasing L is to shear the magnetic field lines and increase their effective geometrical length. Using the force-free field approximation for a magnetic arcade whose field lines are sheared by photospheric motions, the authors demonstrate that the increase of magnetic energy is initially due to the increase of the current intensity I and later mainly due to the increase of the self-inductance. Title: Accretion disk electrodynamics. Authors: Kuperus, M. Bibcode: 1987gac..conf..195K Altcode: Contents: 1. The standard thin disk. 2. Turbulent dynamo in accretion disks. 3. Electrodynamic coupling of accretion-disk coronae. 4. The interaction of a neutron star with an accretion disk: Spin up of neutron stars. Quasiperiodic oscillations. Title: Black hole accretion disks. Electrodynamic coupling of accretion-diskcoronae and the partitioning of soft and hard X-ray emission. Authors: Kuperus, M.; Ionson, J. A. Bibcode: 1985A&A...148..309K Altcode: It is demonstrated that the observed large ratio of hard to soft X-ray emission and the bimodel behavior of black hole accreting X-ray sources such as Cygnus X-1 can be described in terms of a magnetically structured accretion disk corona which is electrodynamically coupled to the disk turbulent motions while the disk is thermodynamically coupled to the corona as described by a feedback parameter delta. The observed ratio of hard to soft X-ray emission is independent of the disk thickness, and weakly dependent of the disk parameter alpha relating the disk viscous stresses to the total pressure. Observed values of the luminosity ratio point towards strong differences of the feedback of the low state compared to the high state, in the sense that low state means small feedback (delta less than 0.2) and high state means strong feedback delta of about 0.5. Title: Solar physics: "progress in solar hydromagnetics". Authors: Kuperus, M. Bibcode: 1985plph.conf..997K Altcode: No abstract at ADS Title: The role of differential rotation in magnetic energy storage. Authors: Zappala, R. A.; Zuccarello, F.; Kuperus, M. Bibcode: 1984ESASP.220..159Z Altcode: 1984ESPM....4..159Z The authors investigate a possible mechanism for magnetic energy storage in coronal loops without making assumptions about dissipative mechanisms. Title: Black hole accretion disks - Coronal stabilization of the Lightman-Eardley instability Authors: Ionson, J. A.; Kuperus, M. Bibcode: 1984ApJ...284..389I Altcode: Physical processes by which the presence of a corona around a black hole can raise the threshold of onset of the Lightman-Eardley (L-E, 1976) instability are explored analytically. The L-E model predicts that an optically thick disk becomes unstable when the disk radiation pressure exceeds the disk gas pressure. The model has important implications for the validity of either the coronal disk or two-temperature disk models for accretion zones around black holes. It is shown that a corona can dissipate accreting gravitational energy through radiative cooling. Specific ratios of hard/soft X-rays are quantified for stable and unstable conditions. X-ray spectra from Cyg X-1 are cited as residing below the instability threshold value and thus are supportive of the coronal disk model. Title: Book-Review - Solar and Stellar Magnetic Fields - Origins and Coronal Effects Authors: Stenflo, J. O.; Kuperus, M. Bibcode: 1984SSRv...38..387S Altcode: No abstract at ADS Title: Book reviews Authors: Kuijpers, Jan; Kleczek, Josip; Lamers, H. J. G. L. M.; Kruit, P. C. V. D.; Kuperus, M.; Knott, K. Bibcode: 1984SSRv...38..385K Altcode: No abstract at ADS Title: Book-Review - Solar Magnetohydrodynamics Authors: Priest, E. R.; Kuperus, M. Bibcode: 1983SSRv...36..424P Altcode: No abstract at ADS Title: Book reviews Authors: Kleczek, J.; Nussbaumer, H.; van der Hucht, K. A.; De Greve, J. P.; Ooms, G.; Rutten, R. J.; van der Laan, H.; Jäger, F. W.; Reijnen, G. C. M.; Bijleveld, W.; Kistemaker, J.; de Jager, C.; Mustel, E. R.; Ne'Eman, Y.; Priest, E. R.; Stiller, H.; Seifert, W.; Namba, O.; Kuperus, M.; Hoekstra, Roel; Stumpers, F. L. H. M.; Frank, S.; Zimmerman, J. T. F.; De Loore, C.; Gendrin, R.; Schrijver, J.; Mulder, P. S.; Pounds, K. A.; Young, R. S.; Houziaux, L.; Engvold, O.; Bok, B. J.; de Graaff, W. Bibcode: 1983SSRv...36..415K Altcode: No abstract at ADS Title: Spatially coherent oscillations in microwave bursts. Authors: Kattenberg, A.; Kuperus, M. Bibcode: 1983SoPh...85..185K Altcode: Solar microwave burst observations (made with the WSRT) with high time and high spatial resolution show large-scale (> 8000 km) short-period (1.5 s) modulations of the source. It is argued that an interpretation in terms of Alfvén oscillations in the microwave source is ruled out by this observation. Instead it must be the source of the fast electrons, that produce the microwaves, that is oscillating. The fluctuating acceleration region is identified with a volume where a sheared field is compressed against a flux tube by an unstable current. MHD oscillations in the overlying fluxtube are caused by the pushing force. The rapidly expanding current plays a major role in the flare theory of van Tend and Kuperus (1978). Title: Electrodynamics of the Outer Solar Atmosphere Authors: Kuperus, M. Bibcode: 1983SSRv...34...47K Altcode: The structure of the outer solar atmosphere and its magnetic coupling to the photospheric motions indicate the existence of large-scale current systems. The heating and the dynamics of coronal structures is therefore governed by electrodynamic coupling of these structures to the underlying photosphere. In a structured corona, the heating is enhanced because of several processes such as resonance absorption of Alfvénic surface waves, anomalous Joule heating, reconnection and the related topological dissipation. The global thermal and dynamic behaviour of coronal structures can be fruitfully described in terms of equivalent electrodynamic circuits, taking into account the paramount role of the photospheric boundaries. Coronal current systems may be stable, as in the case of coronal loops, but occassionally they show catastrophic behaviour if the current intensity surpasses a critical threshold. Title: A thermal catastrophe in a resonantly heated coronal loop Authors: Martens, P. C. H.; Kuperus, M. Bibcode: 1983IAUS..102..397M Altcode: A theory for the thermal stability of hot coronal loops is presented, which is based on the resonant electrodynamic heating theory of Ionson (1982) and the evaporation/condensation scenario of Krall and Antiochos (1980). The theory predicts that gradual changes in the length of a loops or in its magnetic field strength can trigger catastrophic changes in the X-ray visibility of the loop, without the need for a change in the magnetic field topology. A natural explanation is thereby given for the observations of X-ray brightenings in loops and loop evacuations with coronal rain. Title: Resonant electrodynamic heating and the thermal stability of coronal loops Authors: Martens, P. C. H.; Kuperus, M. Bibcode: 1982A&A...113..324M Altcode: The resonant electrodynamic heating theory of Ionson (1982) and the evaporation/condensation scenario of Krall and Antiochos (1980) are invoked by a theory of hot coronal loop thermal stability. The theory predicts that gradual changes in loop length or magnetic field strength can trigger catastrophic X-ray visibility changes in the loop without accompanying changes in magnetic field topology. This is judged to constitute a natural explanation for observed X-ray brightening in loops, as well as loop evacuations with coronal rain. Title: Heating processes of the solar corona Authors: Kuperus, M. Bibcode: 1981ESASP.161..113K Altcode: 1981plas.work..113K No abstract at ADS Title: Heating processes of the solar corona Authors: Kuperus, M. Bibcode: 1981plap.rept..113K Altcode: Coronal heating processes are discussed in the context of the magnetic structure of the solar corona. Existing theories are divided into acoustic theories, where the magnetic field plays a passive role, and electrodynamic theories where the magnetic field plays an active role in the sense that transversal waves are possible. The dissipation mechanisms are shock wave dissipation in the acoustic theory and viscous and Joule dissipation of Alfven waves in the electrodynamic theory. The dissipation of Alfven waves appears to be much more effective in a structured medium than in a homogeneous medium. Anomalous Joule heating is likely to be important as a steady heating mechanism. Title: The Eruption of Active Region Filaments and its Relation to the Triggering of a Solar Flare Authors: Kuperus, M.; van Tend, W. Bibcode: 1981SoPh...71..125K Altcode: The formation and eruption of active region filaments is supposed to be caused by the increase of a concentrated current embedded in the active region background magnetic field of an active region according to the theory of Van Tend and Kuperus (1978). Title: On the theory of coronal heating mechanisms Authors: Kuperus, M.; Ionson, J. A.; Spicer, D. S. Bibcode: 1981ARA&A..19....7K Altcode: The present state-of-the-art of two classes of theories of coronal heating is examined: (1) heating by acoustic processes in the 'nonmagnetic' parts of the atmosphere (the shock-wave theory is an example); and (2) heating by electrodynamic processes in the magnetic regions of the corona (beta much less than 1) either by MHD waves or current heating in regions with high electric current densities (flare-type heating). It is concluded that the mechanism of the heating of the solar chromosphere and corona remains an open question, especially in explaining detailed atmospheric structures. The acoustic theory might be correct with little modification for most of the chromosphere, but as soon as the atmosphere shows a high degree of structure as in the corona and transition layer the magnetic field must play a dominant role. It appears that the current heating theories have a small range of applicability, while the MHD-wave theories are the most promising. Title: On the theory of coronal heating mechanisms Authors: Kuperus, M.; Ionson, J. A.; Spicer, D. S. Bibcode: 1980STIN...8115928K Altcode: Theoretical models describing solar coronal heating mechanisms are reviewed in some detail. The requirements of chromospheric and coronal heating are discussed in the context of the fundamental constraints encountered in modelling the outer solar atmosphere. Heating by acoustic processes in the 'nonmagnetic' parts of the atmosphere is examined with particular emphasis on the shock wave theory. Also discussed are theories of heating by electrodynamic processes in the magnetic regions of the corona, either magnetohydrodynamic waves or current heating in the regions with large electric current densities (flare type heating). Problems associated with each of the models are addressed. Title: Brief Report of Meeting - IAU / Solar Maximum Year Authors: Švestka, Zdenk; Van Hoven, Gerard; Hoyng, Peter; Kuperus, Max Bibcode: 1980SoPh...67..379S Altcode: No abstract at ADS Title: Solar and Interplanetary Dynamics Authors: Kuperus, M. Bibcode: 1980IAUS...91..547K Altcode: 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: The development of coronal electric current systems in active regions and their relation to filaments and flares. Authors: van Tend, W.; Kuperus, M. Bibcode: 1978SoPh...59..115V Altcode: The equilibrium positions of coronal currents are determined. It is shown that the fact that the photosphere has a very large inertia as compared to the corona, poses an important boundary condition at the surface. Electric currents flowing in a coronal active region show a tendency to concentrate above a neutral line. Only here equilibria are possible, determined at low heights by the background field and at large heights by gravity. An instability may occur when the current at low heights exceeds a certain value. The model given is compared with observations of the corona, of prominences, and of fibril motions. Also, the relation with solar flares is discussed in general terms. 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: The Coronal Active Region and the Preflare Environment Authors: Kuperus, M.; Svetska, Z. Bibcode: 1978pfsl.conf..169K Altcode: 1978ESPM....2..169K No abstract at ADS Title: Mechanisms of Heating and Heat Transfer in the Outer Solar Atmosphere Authors: Chiuderi, C.; Kuperus, M. Bibcode: 1977ebhs.coll..223C Altcode: 1977IAUCo..36..223C No abstract at ADS Title: Solar Flares Authors: de Jager, C.; Kuperus, M.; Rosenberg, H. Bibcode: 1976RSPTA.281..507D Altcode: 1976RSLPT.281..507D A summary is given of some recent observational data on solar flares. Particularly we discuss the flare build-up process and the time scales involved. We suggest as a possible magnetic field configuration a multiply kinked or supertwisted flux tube. The role of plasma turbulence and the Fermi mechanism in particle acceleration is discussed. Title: Physics of the Solar Atmosphere Authors: de Jager, C.; Kuperus, M.; Rosenberg, H. Bibcode: 1976RSPTA.281..415D Altcode: 1976RSLPT.281..415D A summary is given on recent results on the physics of the quiet solar atmosphere, and active regions. This includes: solar rotation, velocity fields and waves, magnetic field concentration, the transition region, coronal magnetic field structure, and prominences. Title: Solar Flares and Magnetospheric Substorms (Invited summary) Authors: Kuperus, M. Bibcode: 1976SoPh...47..361K Altcode: No abstract at ADS Title: The role of plasma turbulence in the development of solar flares. Authors: Kuperus, M. Bibcode: 1976SoPh...47...79K Altcode: The amount of energy released during a solar flare and the relatively short timescale in which all the flare associated events occur lead to the conclusion that a solar flare is a magnetohydrodynamic instability taking place in a strongly turbulent plasma. Title: Book reviews Authors: de Jager, C.; van Rensbergen, Walter; Kuperus, M.; Falthammar, Carl-Gunne; Mewe, R.; Reunen, G. C. M.; Bruzek, A.; Swanenburg, B. N.; Kleczek, J.; Millman, Peter M.; Vesseur, H. J. A.; Pacini, F.; Monfils, A. Bibcode: 1976SSRv...18..541D Altcode: No abstract at ADS Title: The Support of Prominences Formed in Neutral Sheets Authors: Kuperus, M.; Raadu, M. A. Bibcode: 1974A&A....31..189K Altcode: Summary. It is shown that prominence filaments formed in a magnetically neutral sheet in the corona can be supported against gravity by magnetic forces due to induced currents in the photospheric boundary. The supporting force is inversely proportional to the height above the photo sphere and the filaments seem stable against small perturbations. The adopted magnetic field configuration allows for a prominence eruption. However, it is suggested that the so called disparition brusque may only occur if either the prominence loses weight by mass loss or through the influence of large scale changes in the photospheric magnetic field. Key words: solar prominences - neutral sheets - magnetohydrodynamics Title: The evolution of magnetic fields and unstable solar plasmas. Authors: Kuperus, M. Bibcode: 1974UtrOv.295.....K Altcode: No abstract at ADS Title: Onderzoek naar magnetohydrodynamische en plasma instabiliteiten in de zonnekorona. Authors: Kuperus, M.; Rosenberg, H. Bibcode: 1974NTNA...39..130K Altcode: No abstract at ADS Title: The Evolution of Magnetic Fields and Unstable Solar Plasmas Authors: Kuperus, M. Bibcode: 1974cesra...4...17K Altcode: No abstract at ADS Title: The next decade of theoretical solar physics. Authors: Kuperus, M. Bibcode: 1974HiA.....3..133K Altcode: The next decade in theoretical solar physics will probably be characterized by the understanding that the sun is the nearest cosmical laboratory in which a large class of astrophysical problems can be studied in detail. The current state of knowledge regarding solar physics is briefly reviewed and the most important developments expected for the coming years are considered. Attention is given to the neutrino problem, questions of solar oblateness, solar rotation and convection, the structure of the photosphere and the chromosphere, the corona, and solar magnetic fields. Title: Characteristics of non-thermally heated stellar atmospheres. Authors: Lamers, H. J. G. L. M.; Kuperus, M. Bibcode: 1974ssse.conf..169L Altcode: No abstract at ADS Title: Thermal Instability of Coronal Neutral Sheets and the Formation of Quiescent Prominences Authors: Raadu, M. A.; Kuperus, M. Bibcode: 1973SoPh...28...77R Altcode: It is argued that the quiscent prominences are a natural consequence of the formation and thermal instability of current sheets in the corona. Thus observation and theory of prominences can give vital information on the presence of currents and the topology of magnetic fields in the corona. Conversely by developing the theory of the structure and evolution of current sheets under coronal conditions we can attempt to gain a comprehensive understanding of the structure, evolution, and mass and energy balance of quiescent prominences. A stability analysis for coronal material permeated by a vertical magnetic field rooted in the photosphere, indicates that a condensation will take the form of a thin vertical wedge of cool matter. The development of a finite condensation is followed and it is shown that photospheric line tying is only important in the initial stages. A perturbation analysis of vertical motions at the neutral sheet shows that thermal instability can lead to overstable oscillations. Cooling of coronal material can lead to both upward and downward mass motions, and gravitational energy release is important to the thermal balance of prominences. Relevant optical and radio observations are discussed. Synoptic observations of the development of active regions and magnetic fields are needed to test the basic hypothesis of the formation of prominences from neutral sheets. Title: Apollo Telescope Mount (ATM). Authors: Kuperus, M. Bibcode: 1973HemD...71..167K Altcode: No abstract at ADS Title: Physics of Solar Prominences. Report on an International Colloquium Held at the German Solar Observatory, Anacapri, September 29 to October 1, 1971 Authors: Bruzek, Anton; Kuperus, Max Bibcode: 1972SoPh...24....3B Altcode: This colloquium on solar prominences - the first ever held - has shown that a major part of activity in prominence research in recent years concentrated on both observation and computation of the magnetic conditions which were found to play a crucial role for the development and the maintainance of prominences. Remarkable progress was made in fine-scale measurements of photospheric magnetic fields around filaments and in internal field measurements in prominences. In addition, important information on the structure of the magnetic fields in the chromosphere adjacent to the filaments may be derived from high resolution photographs of the Hα fine structure around filaments which have become available recently; unfortunately, an unambiguous determination of the vector field in the chromosphere is not yet possible.

It is quite clear, now, that stable filaments extend along `neutral' lines which divide regions of opposite longitudinal magnetic fields. Different types of neutral lines are possible, depending on the history and relationship of the opposite field regions. There is convincing evidence that the magnetic field in the neighbouring chromosphere may run nearly parallel to the filament axis and that there are two field components in stable prominences: an axial field dominant in the lower parts and a transverse field dominant in the higher parts.

Methods for the computation of possible prominence field configurations from measured longitudinal photospheric fields were developed in recent years. In a number of cases (e.g. for loop prominences) the observed configuration could be perfectly represented by a force-free or even a potential field; poor agreement was found between computed and measured field strengths in quiescent prominences. In order to reconcile both of them it is necessary to assume electric currents. Unambiguous solutions will not be found until measurements of the vector field in the photosphere and in the prominences are available.

The two-dimensional Kippenhahn-Schlüter model is still considered a useful tool for the study of prominence support and stability. However, a more refined model taking into account both field components and considering also thermal stability conditions is available now. It was proposed that quiescent prominences may form in magnetically neutral sheets in the corona where fields of opposite directions meet.

As for the problem of the origin of the dense prominence material there are still two opposite processes under discussion. The injection of material from below, which was mainly applied to loop prominences, has recently been considered also a possible mechanism for the formation of quiescent prominences. On the other hand, the main objections against the condensation mechanism could be removed: it was shown that (1) sufficient material is available in the surrounding corona, and that (2) coronal matter can be condensed to prominence densities and cooled to prominence temperatures in a sufficiently short time.

The energy balance in prominences is largely dependent on their fine structure. It seems that a much better radiative loss function for optically thin matter is now available. The problem of the heat conduction can only be treated properly if the field configuration is known. Very little is known on the heating of the corona and the prominence in a complicated field configuration. For the optically thick prominences the energy balance becomes a complicated radiative transfer problem.

Still little is known on the first days of prominence development and on the mechanism of first formation which, both, are crucial for the unterstanding of the prominence phenomenon. As a first important step, it was shown in high resolution Hα photographs that the chromospheric fine structure becomes aligned along the direction of the `neutral' line already before first filament appearance. More Hα studies and magnetic field measurements are badly needed.

Recent studies have shown that even in stable prominences strong small-scale internal rotational or helical motions exist; they are not yet understood. On the other hand, no generally agreed interpretation of large-scale motions of prominences seems to exist. A first attempt to explain the ascendance of prominences, the `Disparitions Brusques', as the result of a kink instability was made recently.

New opportunities in prominence research are offered by the study of invisible radiations: X-rays and meterwaves provide important information, not available otherwise, on physical conditions in the coronal surroundings of prominences; EUV observations will provide data on the thin transition layer between the cool prominence and the hot coronal plasma. Title: On the Directional Dependence of the Emission of Acoustic Noise by Convective Turbulence in a Gravitational Atmosphere Authors: Kuperus, M. Bibcode: 1972SoPh...22..257K Altcode: The directional dependence of the emission of sound waves in the solar atmosphere is studied. It is shown that quadrupole acoustic radiation generated in convective turbulence is strongly enhanced in the direction of the mean convective flow. The intensity in a direction θ with the convective velocity is proportional to (1 − Mc cosθ)−17/2, where Mc is the Mach number of the convective motion. The influence of the atmospheric cut off frequency on the transmitted acoustic spectrum is taken into account. It is suggested that low frequency atmospheric oscillations may modulate the flux of high frequency sound waves. Title: Structure and Dynamics of the Solar Corona Authors: Kuperus, M. Bibcode: 1972ASSL...29....9K Altcode: 1972sun..conf....9K No abstract at ADS Title: Physical characteristics of the solar atmosphere Authors: Kuperus, M. Bibcode: 1972ppsr.conf...89K Altcode: No abstract at ADS Title: Structure and dynamics of the solar corona. Authors: Kuperus, M. Bibcode: 1972sstp.conf....9K Altcode: No abstract at ADS Title: On the Analogy of Sound Waves and Shallow Water Waves Authors: Kuperus, M. Bibcode: 1971A&A....12...85K Altcode: The analogy between the propagation of shallow water waves and sound waves is discussed. It is suggested that laboratory experiments with shallow water in a vessel with varying depth and width can be used to study the propagation of sound waves in a gravitational atmosphere. It is shown that in some cases the depth and the width of the vessel serve as the analogon for the temperature and the density in the atmosphere. Key words: sound waves - stellar atmospheres - analogy experiment Title: Solar radioastronomy and corona magnetohydrodynamics Authors: Kuperus, M. Bibcode: 1971cesra...1....4K Altcode: No abstract at ADS Title: On the Determination of the Velocity of the Exciters of Type II Solar Radiobursts Authors: Van Bueren, H. G.; Kuperus, M. Bibcode: 1970SoPh...14..208V Altcode: Under the assumption that type II bursts are excited by strong shock waves it is shown that the velocity determined from dynamic spectrograms should be corrected by a term proportional to the gradient of the shock strength. The magnitude of the correction strongly depends on the properties of the pre-shock atmosphere and the geometry of the shock propagation. Title: Thermally Driven Motions in a Gravitational Atmosphere Authors: Bessey, R. J.; Kuperus, M. Bibcode: 1970SoPh...12..216B Altcode: Numerical solutions of the non-linear equations of fluid dynamics for a compressible inviscid initially isothermal atmosphere are given using Lax' method for the integration of the equations when discontinuities occur in the flow. The motion of the atmosphere is studied following the heating of a thin layer in the atmosphere. It is found that for a sufficiently large heat input the atmosphere strongly expands towards the regions of lower densities. In most cases a shock wave is formed which precedes the expanding region. The possible occurrence of thermally generated motions in the solar chromosphere is discussed. Title: Thermally Driven Motions in a Gravitational Atmosphere Authors: Bessey, R. J.; Kuperus, M. Bibcode: 1969cctr.conf..191B Altcode: No abstract at ADS Title: The Structure of the Transition Region between the Chromosphere and the Corona Authors: Kuperus, M. Bibcode: 1969cctr.conf....1K Altcode: No abstract at ADS Title: The Heating of the Solar Corona Authors: Kuperus, M. Bibcode: 1969SSRv....9..713K Altcode: In this paper a discussion is given of the present state of the theory of the heating of the solar corona by shock waves. Arguments are presented why the main contribution to the mechanical energy flux is of acoustic origin, while estimates for the amount of acoustic energy generated in the convection zone as well as the deviations from isotropy are given. During propagation through the atmosphere acoustic waves develop into shock waves after a distance of a few scale heights in the chromosphere. The heating of the outer layers by dissipation of shock waves is found to be sufficient to account for the observed radiative and corpuscular energy losses. Much emphasis is laid on the competitive role played by the four fundamental processes of energy transfer: mechanical heating, radiation, heat conduction and convection of energy in establishing the equilibrium structure of the corona. The atmosphere may be divided in several regions according to the predominance of one of the energy processes mentioned above. The physical properties of the chromosphere and the solar wind are discussed only where they are intimately connected with the problem of the heating of the corona. The most important aspects of the influence of a magnetic field on the structure and the heating of the corona in magnetically active regions are briefly mentioned. Special attention is paid to the strong channelling of heat flow along the field lines and its consequences for the structure and dynamics of the chromosphere-corona transition layer. Title: Oscillatory Phenomena in Quiescent Prominences Authors: Kleczek, J.; Kuperus, M. Bibcode: 1969SoPh....6...72K Altcode: A model for horizontal oscillations of prominences is presented. It is found that the model of a freely oscillating prominence surrounded by coronal matter explains satisfactorily the observed periods and damping times, as well as the changes in the prominence shape. Title: Magnetic Fields and the Temperature Structure of the Chromosphere-Corona Interface Authors: Kopp, Roger A.; Kuperus, Max Bibcode: 1968SoPh....4..212K Altcode: The temperature structure of the transition region between the chromosphere and corona is discussed in the context of current ideas about magnetic fields in these layers. Magnetic channeling of the downward conductive heat flow from the corona into the regions of enhanced field at the supergranulation boundaries is proposed as a mechanism for explaining the measured intensities of solar ultraviolet emission lines which originate in layers with temperatures below 105 °K. It is shown that nearly all of the observed ultraviolet line emission originates in interspicule regions, and that this emission plays an important part in the energy balance of the cooler layers of the transition region. It is suggested that certain motions observed in the upper chromosphere may represent the earliest visual evidence for conversion of inflowing conduction energy into kinetic motions. Title: Magnetic Fields and the Temperature Structure of the Chromosphere-Corona Interface. Authors: Kopp, Roger A.; Kuperus, Max Bibcode: 1968AJS....73Q..67K Altcode: The temperature structure of the transition region between the chromosphere and corona is discussed in the context of current ideas about magnetic fields in these layers. Magnetic channeling of the downward conductive heat flow from the corona into the regions of enhanced field at the supergranulation boundaries is proposed as a mechanism for explaining the measured intensities of solar ultraviolet lines which are emitted by layers with temperatures below 10~ 0K. It is suggested that nearly all of the observed ultraviolet line emission originates in interspicule regions, and that this emission plays an important part in the energy balance of the cooler layers of the transition region. Title: The Nature of Quiescent Solar Prominences Authors: Kuperus, Max; Tandberg-Hanssen, Einar Bibcode: 1967SoPh....2...39K Altcode: Quiescent prominences occur as long-lasting cool sheets of matter in the surrounding hot corona at the base of coronal streamers. Seen on the disk they appear as dark filaments dividing regions of opposite magnetic polarity. Title: On the Origin of Spicules in the Chromosphere-Corona Transition Region Authors: Kuperus, Max; Athay, R. Grant Bibcode: 1967SoPh....1..361K Altcode: Qualitative arguments are presented to show that in the chromosphere-corona transition region the energy which is transported downwards by heat conduction cannot be disposed of by radiative losses only. The region therefore is unstable and forced to be in motion, which may give rise to spicule-like phenomena. Title: Book reviews Authors: Kuperus, M.; Nijboer, B. R. A. Bibcode: 1967SSRv....6..892K Altcode: No abstract at ADS Title: The Heating of the Solar Corona by Photospheric Waves Authors: Kuperus, M. Bibcode: 1966IAUTB..12..564K Altcode: No abstract at ADS Title: On the thermal generation of magnetic fields in the ionization zone of rotating stars Authors: Kuperus, M. Bibcode: 1966BAN....18..353K Altcode: No abstract at ADS Title: The transfer of mechanical energy in the sun and the heating of the corona Authors: Kuperus, Max Bibcode: 1965RAOU...17.....K Altcode: No abstract at ADS Title: The Transfer of Mechanical Energy in the Sun and the Heating of the Corona Authors: Kuperus, M. Bibcode: 1965RAOU...17....1K Altcode: No abstract at ADS Title: The transfer of mechanical energy in the sun and the heating of the corona Authors: Kuperus, Max Bibcode: 1965PhDT........55K Altcode: No abstract at ADS Title: On the selection concentration in a prominence observed during the total solar eclipse of oktober 2nd 1959. Authors: Koelbloed, D.; Kuperus, M. Bibcode: 1963PRNAA..66Q..36K Altcode: 1963RNAAS..66Q..36K No abstract at ADS Title: On the electron concentration in a prominence observed during the total solar eclipse of october 2nd. 1959. Authors: Kuperus, M.; Koelbloed, D. Bibcode: 1963PRNAA..66R..36K Altcode: 1963RNAAS..66R..36K No abstract at ADS Title: The acoustic energy flux of the Sun and the formation of the corona Authors: de Jager, C.; Kuperus, M. Bibcode: 1961BAN....16...71D Altcode: No abstract at ADS