Author name code: chiuderi ADS astronomy entries on 2022-09-14 author:"Chiuderi, Claudio" ------------------------------------------------------------------------ Title: Basics of Plasma Astrophysics Authors: Chiuderi, C.; Velli, M. Bibcode: 2015bps..book.....C Altcode: No abstract at ADS Title: A new fluid scheme for weakly collisional plasmas: 2. The case of solar wind Authors: Pietrini, P.; Torricelli-Ciamponi, G.; Chiuderi, C. Bibcode: 2012JGRA..11712105P Altcode: Chiuderi et al. (2011) (paper 1) investigated the possibility of extending to weakly collisional plasmas the new fluid scheme recently developed by Chen et al. (2000) for neutral gases. Paper 1 established the conditions under which such an extension is feasible and identified the solar wind as a good candidate to test the new theory. Such a test is performed in the present paper, where we actually build a model for the fast solar wind and compare it with the available data. We define the basic assumptions adopted: steadiness, spherical symmetry, and neglect of magnetic field and of minor ions. We limit our treatment to the range 2-215 R⊙to cover the details of the transition from collisional to collisionless regimes, excluding the low corona and the chromosphere that appear to be far too complex to be described by our simple model. We present a detailed discussion concerning the choice of the parameters and boundary conditions entering the model. We then proceed to the actual construction of the model, whose main merit lies in its simplicity: it is a two-fluid model capable of accounting for both viscosity and thermal conduction, in spite of the very limited number of equations to be solved. We then critically compare our results with the observations and with other recent solar wind models. The level of agreement with the data is quite satisfactory and is comparable with that obtained by more sophisticated models. Title: A new fluid scheme for weakly collisional plasmas: 1. General theory Authors: Chiuderi, C.; Pietrini, P.; Torricelli-Ciamponi, G. Bibcode: 2011JGRA..116.4107C Altcode: We present a new approach to the study of two-fluid hydrodynamics of weakly collisional plasma systems, such as those frequently encountered in the astrophysical context. Our starting point is the novel procedure developed in past years by Chen, Rao, and Spiegel (CRS) for the fluid description of semicollisional neutral monoatomic gases. The new system of fluid equations that include viscous and conductive effects has been successfully tested. However, the extension of such a procedure to plasmas is not a straightforward one. We have to deal with (at least) two components, with vastly different values of the masses, and the nature of collisions between charged particles is substantially different from the neutral particle case. We perform a preliminary careful examination of the basic requirements of the CRS method and identify the conditions under which an extension of such method is indeed possible. We then derive the system of fluid equations appropriate to the description of a weakly collisional two-component plasma in the new scheme and discuss the differences with respect to the more familiar Navier-Stokes approach. This paper is therefore of a general theoretical nature. However, we also point out that solar wind is a good testing ground for the newly derived system of fluid equations. This application will be the subject of a subsequent paper. Title: Solar MHD: An Introduction Authors: Chiuderi, C.; Velli, M. Bibcode: 2009LNP...778...45C Altcode: The Universe is filled by plasmas. In fact, it can be reasonably estimated that more than 95% of (standard) cosmic matter is found in the plasma state. Given this basic fact, it is apparent that plasma physics is the basic tool to understand the mechanisms that are at work in the astrophysical context and to interpret the observations. The Earth represents a notable exception as far as the presence of natural plasmas is concerned, a lucky circumstance for living beings. In practice, terrestrial plasmas are almost exclusively produced during electrical discharges, such as lightning. If plasmas are almost absent on Earth and in the low-altitude atmosphere, they start to be the dominant state of matter immediately beyond the ionosphere, the magnetosphere, and the whole heliosphere that includes the entire solar system made up of plasmas. The Sun, like the other stars, is made up of ionized gas almost everywhere. Title: Effect of suprathermal particles on the quiet Sun radio emission Authors: Chiuderi, C.; Chiuderi Drago, F. Bibcode: 2004A&A...422..331C Altcode: The bremsstrahlung emissivity and absorption coefficient in the radiofrequency range are derived under the assumption that the electron population is not purely thermal, but presents a tail of high energy particles. This population is approximated by a two-component Maxwellian distribution and by the kappa-functions of different (integer) index. It is shown that, if the temperature ratio of the two Maxwellians is larger than 10, the absorption coefficient and the effective temperature (the quantities entering the radio transfer equation) depend only on the fraction R of particles in the highest temperature Maxwellian. In the case of kappa-functions the above quantities depend on the index n of the functions.

The microwave radio spectrum is computed for different values of R and for 3 ≤ n ≤ 6, finding, in all cases, brightness temperatures lower than those computed with a pure thermal distribution. This could explain some inconsistencies found between radio and EUV observations.

Appendix is only available in electronic form at http://www.edpsciences.org Title: Effect of suprathermal particles on the quiet sun radio emission Authors: Chiuderi, C.; Chiuderi Drago, F. Bibcode: 2004cosp...35..289C Altcode: 2004cosp.meet..289C The bremsstrahlung emissivity and absorption coefficient in the radiofrequencies range are derived in the assumption that the electron population is not purely thermal, but presents a tail of high energy particles. This population is approximated by a bi-Maxwellian distribution and by kappa-functions of different (integer) index. It is shown that, if the temperature ratio of the two Maxwellians is larger than 10, the absorption coefficient and the effective temperature (the quantities entering the radio transfer equation) depend only on the fraction R of particles in the highest temperature maxwellian. In case of kappa-functions the above qunatities depend on the index n of the functions. The microwave radio spectrum is computed for different values of R and for 3 ≤ n ≤ 6, finding, in all cases, brightness temperatures lower than those computed with a pure thermal distribution. This fact could explain some inconsistencies found between radio and EUV observations. Title: Effect of Superthermal Particles in the Quiet Sun Radio Emission Authors: Chiuderi, C.; Chiuderi Drago, F. Bibcode: 2003MmSAI..74..695C Altcode: The bremsstrahlung emissivity and absorption coefficient, in the radiofrequencies range, are derived in the assumption that the electron population is not purely thermal, but presents a tail of high energy particles. This population is approximated by a bi-Maxwellian distribution. It is shown that, if the temperature ratio of the two Maxwellians is larger than 10, the absorption coefficient and the effective temperature depend only on the fraction R of particles in the highest temperature Maxwellian. The microwave radio spectrum is computed for some values of R, finding brightness temperatures lower than those computed with a pure thermal distribution. This fact could explain some inconsistencies found between radio and EUV observations. Title: The World Space Observatory Project WSO/UV Authors: Pagano, I.; Rodonò, M.; Bonanno, G.; Buson, L.; Cassatella, A.; De Martino, D.; Wamsteker, W.; Shustov, B.; Barstow, M.; Brosch, N.; Fu-Zhen, Cheng; Dennefeld, M.; Gomez de Castro, A. I.; Kappelmann, N.; Sahade, J.; Van der Hucht, K.; Solheim, J. -E.; Haubold, H.; Altamore, A.; Andretta, V.; Badiali, M.; Becciani, U.; Busà, I.; Cappellaro, E.; Cardini, D.; Catalano, S.; Castellani, V.; Chiaberge, M.; Chieffi, A; Chiuderi, C.; Cosentino, R.; Cremonese, G.; Cutispoto, G.; Falomo, R.; Ferrini, F.; Franchini, M.; Frasca, A.; Giovannelli, F.; Gori, L.; Gomez, M.; Hack, M.; Lanza, A. F.; Lanzafame, A.; Malagnini, M. L.; Marilli, E.; Marziani, P.; Matteucci, F.; Morossi, C.; Munari, U.; Pace, E.; Panagia, N.; Pasinetti, L.; Piotto, G.; Polcaro, F.; Radovich, M.; Ragaini, S.; Rifatto, A.; Rossi, C.; Scuderi, S.; Selvelli, P.; Silvotti, R. L. Terranegra5, M. Turatto3, M. Uslenghi22, R. Viotti4 Bibcode: 2003MSAIS...3..327P Altcode: 2003astro.ph..6554P The World Space Observatory Project is a new space mission concept, grown out the needs of the Astronomical community to have access to the part of the electromagnetic spectrum where all known physics can be studied on all possible time scales: the Ultraviolet range. The physical diagnostics in this domain supply a richness of new experimental data unmatched by any other wavelength range, for the studies of the Universe. As WSO/UV has been driven by the needs of scientists from many different countries, a new implementation model was needed to bring the World Space Observatory to reality. The WSO/UV consists of a single Ultraviolet Telescope in orbit, incorporating a primary mirror of 1.7 m diameter feeding a UV spectrograph and UV Imagers. Title: Nonlinear processes in heliospheric plasma: models and observations Authors: Velli, M.; Einaudi, G.; Chiuderi, C.; Veltri, P. L.; MM02242342 project Team; Betta, R.; Londrillo, P.; Rappazzo, F.; Del Zanna, L.; Landi, S.; Malara, F.; Carbone, V.; Zimbardo, G.; Primavera, L.; Greco, A.; Sorriso-Valvo, L.; Pommois, P.; Lepreti, F. Bibcode: 2003MmSAI..74..425V Altcode: We present the scientific objectives of the research project `Nonlinear processes in heliospheric plasma: models and observations', co-financed by the Italian Ministry for Universities and Scientific Research, as well as a summary of some of the results obtained. The objective of the proposal was a detailed study of the nonlinear and dissipation-scale dynamics of heliospheric plasmas. The project focused on the study of wave propagation and properties of turbulence at the various scales, from the macroscopic scales of the solar wind, down to the microscopic scales of magnetic reconnection and turbulence dissipation, in its two aspects of evolutionary internal dynamics, and its effects on the transport of energetic particles of both heliospheric and extra-solar origins (cosmic rays, interstellar neutrals ionized in the solar wind as pickup ions). Title: Resistivity Independent Alfven Waves Dissipation Authors: Chiuderi, C.; Califano, F. Bibcode: 1999ESASP.446..235C Altcode: 1999soho....8..235C The heating of high temperature plasmas by MHD waves is one of the most interesting and challenging problems of plasma physics especially when the energy is injected into the system at length scales much larger than the dissipative ones. It has been conjectured that in 2-D MHD systems the possibility exist of establishing a state in which energy is dissipated at a rate that is independent of the ohmic resistivity and that the time needed to reach such a state is finite and independent of resistivity as well. We study the nonlinear interaction of long-wavelength, small amplitude perturbations with an inhomogeneous magnetic field showing that energy is transferred to the fluctuations from the background equilibrium. In the case of an isolated system, the free energy initially stored in the equilibrium would decrease and the field would tend to become homogeneous. To model the process of energy injection into the system, we force the large scale field to maintain its initial profile. In these circumstances, we prove that it is actually possible after a finite time, independent of the dissipative mechanism, to generate a well developed 2-D MHD turbulent state where the dissipation rate does not depend on the Lundquist number, at least for values of the order of a few thousands. To our knowledge, this is the first time in which the existence of the conjectured resistivity independent dissipation has been explicitly demonstrated. Title: Resistivity-independent dissipation of magnetohydrodynamic waves in an inhomogeneous plasma Authors: Califano, F.; Chiuderi, C. Bibcode: 1999PhRvE..60.4701C Altcode: The heating of high temperature plasmas by magnetohydrodynamic (MHD) waves is one of the most interesting and challenging problems of plasma physics especially when the energy is injected into the system at length scales much larger than the dissipative ones. It has been conjectured that in two-dimensional MHD systems the possibility exists of establishing a state in which energy is dissipated at a rate that is independent of the Ohmic resistivity and that the time needed to reach such a state is finite and independent of resistivity as well. In this paper we prove that this is actually possible as a result of the nonlinear interaction of long-wavelength, ``small'' amplitude perturbations with a constant, inhomogeneous magnetic field, at least in the relatively moderate Lundquist number (magnetic Reynolds) range 100<=S<=3200. Title: A New Model for Gas Excitation in the Beam of Stellar Jets Authors: Bacciotti, F.; Pouquet, A.; Chiuderi, C. Bibcode: 1998ajop.conf..239B Altcode: No abstract at ADS Title: Optical Emission in the Beam of Stellar Jets: A Possible Mechanism Authors: Bacciotti, F.; Chiuderi, C.; Pouquet, A. Bibcode: 1997ApJ...478..594B Altcode: We propose a mechanism for the optical emission observed in the beam section of stellar jets based on the stability properties of circularly polarized Alfvén waves propagating in the partially ionized medium filling the jet's channel. We first derive the relevant magnetohydrodynamic equations, which include the Hall term for such a partially ionized medium, a term which give rise to dispersive effects. Quasi-equipartition of thermal and magnetic energies is assumed, and the model is developed in the one-dimensional approximation but keeping the three components of the vectorial fields. Mild compressions or very weak shocks occur in the flow when instabilities develop, depending on the relative sign of the angular momentum of the underlying accretion disk and the external magnetic field. Simulations performed under quite general conditions show that the temperature/density perturbations associated with the instability are consistent with the average luminosity contrast observed in the jet's beam between bright knots and interknot regions. Title: Concluding remarks: solar physics and the advancement of physics Authors: Chiuderi, C. Bibcode: 1997MmSAI..68..449C Altcode: No abstract at ADS Title: A mechanism for the optical emission of stellar jets Authors: Chiuderi, Claudio; Bacciotti, Francesca; Pouquet, Annick Bibcode: 1997IAUS..182P..79C Altcode: A mechanism for the optical emission of stellar jets is proposed, based on the stability properties of circularly polarized Alfven waves propagating in the partially ionized medium filling the jet channel. We adopt a MHD description including the Hall term, as appropriate for such a medium. Quasi-equipartition of thermal and magnetic energies is assumed. Shock-like features appear in the flow when instabilities develop, depending on the alignment of the angular momentum vector of the accretion disk along or against the background magnetic field. We show that the proposed mechanism is capable of reproducing the observed luminosity contrast between bright knots and interknot regions. Title: Induced Deposition of Magnetic Energy in the Solar Corona Authors: Einaudi, Giorgio; Califano, Francesco; Chiuderi, Claudio Bibcode: 1996ApJ...472..853E Altcode: In this paper a numerical study of the propagation and dissipation properties of magnetohydrodynamic waves in an incompressible magnetized plasma is presented. The magnetic field is assumed to be unidirectional, but its magnitude varies in a direction perpendicular to the field. The analysis concerns both linear and nonlinear waves. The main findings are the following.

Among the waves whose amplitude never exceeds the linear limit, short-wavelength waves dissipate more efficiently than those of long wavelength. The dissipated energy is at most the energy initially injected into the system in the form of waves, the background magnetic field remaining unaltered. When the initial amplitude is significantly increased from very small values, but remains still substantially lower than the background field, a nonlinear cascade is excited and dissipation is greatly enhanced in the long- wavelength limit. The dissipated energy in this case exceeds that contained in the waves initially injected into the system, which shows that also part of the unperturbed field is actually dissipated.

A second important point concerns the formation of localized current sheets in a finite time as a result of the propagation of the waves. Such current sheets are formed in a nonlinear process triggered by the same mechanism responsible for the formation of linear resonant normal modes. The dissipation rate of such modes is known to be independent of the Reynolds number. By analogy, it is conjectured that the time of formation of current sheets might not depend on the magnetic Reynolds number. Title: Exact solutions for symmetric magnetohydrodynamic equilibria with mass flow. Authors: Del Zanna, L.; Chiuderi, C. Bibcode: 1996A&A...310..341D Altcode: In this paper the problem of ideal magnetohydrodynamic equilibria with mass flow is treated. Under the assumption of general symmetry (i.e. one ignorable spatial coordinate) we derive a generalized Grad-Shafranov equation in an unspecified curvilinear coordinate system. If incompressibility is assumed an elliptical equation is derived and a new, totally analytical method of solution is proposed. This is based on a particular self-similar separation of the variables in the unknown flux function and leads to an ordinary, non-linear differential equation for the profile of the magnetic and flow surfaces. Three novel classes of solutions are derived in different geometries, all being flexible (they contain a minimum of three free functions) and regular, which makes them suitable for astrophysical applications. These are flows in magnetic flux tubes with non-circular section, flows in magnetic arcades above the solar surface and collimated, axisymmetric outflows. Title: Theories of Coronal Heating (Invited) Authors: Chiuderi, Claudio Bibcode: 1996ASPC..111...69C Altcode: 1997ASPC..111...69C This paper reviews the most recent trends in the theory of coronal heating. It is argued that the corona does most likely find itself in a fully developed turbulent state. The turbulence is the direct consequence of the random photospheric motions that stress the footpoints of magnetic field lines. Although the details of the propagation of energy from the photosphere to the corona could depend on the frequency of the driving motions, in all cases the key feature to explain coronal heating seems to be the formation and disruption of localized current sheets. This suggests a statistical approach to the theory of coronal heating as well as a possible link between heating and flares. Title: Stellar Jets: Spectral Diagnostics and Fluidodynamic Models Authors: Bacciotti, F.; Chiuderi, C.; Hirth, G.; Natta, A.; Oliva, E. Bibcode: 1996LNP...465..329B Altcode: 1996doay.conf..329B We present a non-standard, straightforward procedure for a model-independent determination of crucial physical parameters of the linear section of stellar jets, such as the hydrogen ionization fraction x and the temperature T e of the emitting gas. The method can be easily applied to every Herbig-Haro jet for which the brightest red lines have been measured, even if the lines are not calibrated and the ratios are not corrected for reddening. In the cases of HH 34 and HH 111 we find x∼0.1 and T e∼6000. The momentum rates evaluated with the derived total number densities (N∼104 cm-3) give strong support to the picture in which the luminous jet is only the fastest and most ionized component of a wider neutral flow that, in turn, has the capability of accelerating a molecular outflow. The ionization fraction cannot be explained in terms of equilibrium processes reflecting the local physical conditions of the medium, however the recombination length scales turn out to be comparable to the linear extension of the most luminous part of the jets: the observed degree of ionization is most likely the remnant of the heating and the excitation occurred in the initial accelerating region. This picture is supported by our diagnostics of RW Aur's optical jet, for which spatially resolved spectra have been taken: in this case the ionization fraction (around 10% again) is found to decrease slowly with distance from the star. Title: Plasma Astrophysics Authors: Chiuderi, Claudio; Einaudi, Giorgio Bibcode: 1996LNP...468.....C Altcode: 1996plas.conf.....C The book contains the notes of the lectures presented by outstanding experts at the 7th EADN School on plasma astrophysics. It is an up-to-date review of a number of basic topics in the physics of cosmic plasmas. The subject is treated both from a theoretical point of view and from that of the observational and diagnostic tools that provide us with the physically relevant data. The reader will have at hands a comprehensive and rather complete presentation of the subject, thanks also to the parallel development of the theoretical and experimental aspects. The book addresses graduate students and researchers in different areas who want to have a rapid and up-to-date introduction to this subject. Title: Stellar Jets: Spectral Diagnostics and Fluidodynamic Models Authors: Bacciotti, F.; Chiuderi, C.; Hirth, G.; Natta, A.; Oliva, E. Bibcode: 1996LNP...465E.781B Altcode: No abstract at ADS Title: The structure of optical stellar jets: a phenomenological analysis. Authors: Bacciotti, F.; Chiuderi, C.; Oliva, E. Bibcode: 1995A&A...296..185B Altcode: In this paper we discuss the possible mechanisms of formation ot the nodular structure of Herbig-Haro jets. The available spectral observations of the linear jet section in the HH 34 and the HH 111 complexes are reviewed, in order to diagnostic the temperature and ionization state of the emitting material in a way as model-independent as possible. The values we find by means of an original although straightforward diagnostic procedure (T~6000K, x=n_H^+^_/n_H_~0.1, in both cases), are similar to those empirically determined by Bruegel et al. (1981) and Boehm & Solf (1990) for the compact object HH 7. Our results lead to a value of the momentum rate in these flows comparable to the rates observed in high-speed neutral winds (Lizano et al. 1988), giving support to the view according to which the bright optical jet actually identifies the axial portion of the neutral flow, and marks the location where the matter attains its maximum velocity and an observable amount of ionization. The ionization derived from the observations is likely to be a remnant of the heating and the excitation occurred in the accelerating region, where the wind is probably focused by the passage through a nozzle. The recombination time scale appropriate for the rather low electron densities in the jet is found to be close to the crossing time of the visible portion of the jet. Combining kinematical and energetic considerations we show that physical conditions consistent with the observed jet's emission can be obtained through "soft" compressions of the central portion of the flow, caused by 'damped' Kelvin-Helmoltz instabilities generated at the jet-ambient contact discontinuity. These compressions concentrate in smaller volumes the already available internal energy that is eventually radiated in optical and IR lines, unlike the weak shocks that actually increase the internal energy content by transforming the bulk kinetic energy into thermal random motions. A picture of this kind seems capable of resolving the long-standing problem of conciling the high supersonic velocity of the ionized material and the low excitation nature of the emission, and, at the same time, provides a very natural explanation for a number of observational constraints, among which the increase of the [SII]/Hα ratio along the jet and the disappearance of the optical emission after a characteristic scale length. Title: Normal modes of a resistive nonuniform plasma Authors: Chiuderi, C.; Califano, F. Bibcode: 1994R&QE...37..355C Altcode: The propagation and dissipation properties of magnetohydrodynamic waves in a nonuniform, highlyconducting plasma, is investigated with a normal mode approach. The interaction between the perturbation and the non-uniform supporting medium is analyzed as the main mechanism able to produce the small scale spatial structure necessary to dissipate efficiently the wave energy. Two fundamental classes of modes are found, characterized by their resistive or ideal asymptotic behavior; the damping rates are shown to be orders of magnitude larger than those obtained when the plasma is perfectly homogeneous, and an application to the problem of solar coronal heating is discussed. Title: Coronal heating and solar activity: The role of waves Authors: Einaudi, G.; Chiuderi, C.; Califano, F. Bibcode: 1993AdSpR..13i..85E Altcode: 1993AdSpR..13...85E Coronal heating and solar activity are manifestations of the complex interaction between the magnetic field and the photospheric plasma motions. As a result of such an interaction magnetohydrodynamic waves are generated: their dissipation may provide the basic physical mechanism to heat the corona. This paper describes the present status of knowledge of the physics underlying the dissipation of MHD waves. After having stressed the importance of the formation of small spatial scales for achieving an efficient dissipation, we review the theory of linear MHD waves in nonuniform media based on a normal mode approach. We then present the results of some recent numerical experiments and interpret them in the framework of the theory of MHD turbulence. A plan of future work will also be presented. Title: Keys to understanding the corona. Authors: Chiuderi, C. Bibcode: 1993ESASP1157...25C Altcode: 1993srfs.book...25C The theoretical understanding of the solar corona involves such different phenomena as the coronal heating and the occurrence of flares. It is argued that all of these phenomena could be seen as a manifestation of the same basic physics, the differences being a question of coherence. The theoretical work needed to support this view and its possible impact on future observational strategies are outlined. Title: LY-A-CO-PO (LY α coronograph/polarimeter): an instrument to measure coronal magnetic fields Authors: Fineschi, S.; Chiuderi, C.; Poletto, G.; Hoover, R. B.; Walker, A. B. C., Jr. Bibcode: 1993MmSAI..64..441F Altcode: No abstract at ADS Title: New Exact Solutions for Magneto-Hydrodynamic Flows and Their Application to Stellar Jets Authors: Bacciotti, F.; Chiuderi, C. Bibcode: 1993ASSL..186..357B Altcode: 1993sjbo.conf..357B No abstract at ADS Title: Incompressible Disturbances in Nonuniform Media: Formation of Small Scales Authors: Malara, Francesco; Veltri, Pierluigi; Chiuderi, Claudio; Einaudi, Giorgio Bibcode: 1992ApJ...396..297M Altcode: The study proposes a phenomenological interpretation of the mechanism of the formation of small scales during the propagation of an MHD wave where only z(+) fluctuations are excited. The nonlinear effects are shown to be negligible when the initial disturbance travels in the direction of the background magnetic field and its wavelength is comparable to the nonuniformity scale length. It is shown that the interaction between the perturbation and the inhomogeneous background has two effects: the oscillations of neighboring field lines become rapidly out of phase, producing small scales in the direction of nonuniformity, and the pressure gradients caused by the perturbation have the tendency to concentrate energy in the neighborhood of the point where the wave frequency resonates with the local Alfven frequency, with a subsequent local creation of small scales. Title: Nonresonant Resistive Dissipation of Compressible Magnetohydrodynamic Waves Authors: Califano, Francesco; Chiuderi, Claudio; Einaudi, Giorgio Bibcode: 1992ApJ...390..560C Altcode: The effect of compressibility on the propagation and resistive dissipation properties of magnetohydrodynamic waves in nonuniform media is studied by using a normal mode analysis. The paper concentrates on wave models for which the resistivity acts over the entire system, thus excluding localized resonant absorption. The wave modes in nonhomogeneous situations are identified in term of their familiar homogeneous counterparts and it is shown that only the shear Alfven and slow magnetosonic modes survive in the resistive regime. The most promising heating agents in the astrophysical context are shown to be the shear Alfven waves that behave almost incompressibly. Title: Energy Balance in the Prominence / Corona Transition Region Authors: Chiuderi, C.; Chiuderi Drago, F. Bibcode: 1991SoPh..132...81C Altcode: The prominence-corona transition region can be observed both at UV and radio wavelengths. The physical parameters needed to explain one set of observations are, however, in disagreement with those consistent with the other one. A solution of the problem is proposed, based on the proper consideration of the dependence of the thermal conduction on the angle between the magnetic field and the direction of the local temperature gradient. Title: Nonresonant Resistive Dissipation of Incompressible Magnetohydrodynamic Waves Authors: Califano, Francesco; Chiuderi, Claudio; Einaudi, Giorgio Bibcode: 1990ApJ...365..757C Altcode: The influence of departures from homogeneity on the dissipative properties of incompressible MHD waves are examined within the framework of an analytically solvable model in plane geometry. Only resistive dissipation is taken into account. The existence of a new class of rapidly oscillating solutions is proved for which the role of resistivity is not restricted to a single narrow layer, as in the well-known resonating case, but extends to the whole system. Title: Energy Balance in Prominence-Corona Transition Regions Authors: Chiuderi, C.; Chiuderi-Drago, F. Bibcode: 1990IAUS..142..325C Altcode: No abstract at ADS Title: A Case for Alfven Wave Heating Authors: Califano, F.; Chiuderi, C.; Einaudi, G. Bibcode: 1990IAUS..142..223C Altcode: The present study examines the resistive dissipation of Alfven waves in magnetically structured media within the framework of an analytically solvable model in plane geometry. It is shown that consideration of nonideal asymptotic boundary conditions gives rise to a new type of solutions for the resistively damped Alfven waves that are characterized by the appearance of very small scales in the x-direction. Since the normal modes can be thought of as the asymptotic state of the temporal evolution of an initial perturbation of arbitrary scale, the appearance of the small scales is interpreted as the result of the bilinear interaction of the initial perturbation and the nonuniform equilibrium structure. During the transient the spectrum of modes in the z-direction remains essentially unchanged, showing that the formation of small scales across the field takes place well before the possible development of the nonlinear mode-mode coupling along the field. Title: MHD instabilities Authors: Chiuderi, Claudio Bibcode: 1990MmSAI..61..359C Altcode: Recent theoretical studies of MHD instabilities in plasmas are reviewed, with an emphasis on those relevant to the solar corona. Consideration is given to the time evolution equations for a heated magnetized plasma losing energy via optically thin radiation, with dissipative effects described in terms of a finite constant isotropic resisitivity and shear viscosity and finite thermal conductivity channeled along the magnetic-field lines. Rayleigh-Taylor, Kruskal-Schwarzschild, and Kelvin-Helmholtz instabilities are characterized for various combinations of plasma parameters, and the refinements introduced to provide a greater degree of realism in solar-corona modeling are discussed. It is concluded that, although numerical simulations can now reproduce many features of solar plasmas, the study of analytical solutions must be pursued as a check on modeling accuracy. Title: MHD of Solar Activity Authors: Chiuderi, C. Bibcode: 1990PDHO....7..146G Altcode: 1990ESPM....6..146G; 1990dysu.conf..146G No abstract at ADS Title: MHD of solar activity. Authors: Chiuderi, C. Bibcode: 1990PDHO....7..121C Altcode: 1990dysu.conf..121C After a short overview of the subject, the general trends of the present efforts of modelisation of solar activity by means of MHD theory are discussed. Such trends are illustrated with the help of a few examples. Title: Extragalactic Jets as Current-carrying Systems. I. Equilibrium and Stability Authors: Chiuderi, Claudio; Pietrini, Paola; Ciamponi, Guidetta Torricelli Bibcode: 1989ApJ...339...70C Altcode: We present a model for extragalactic jets considered as magnetized current-carrying structures. Unlike previous models the return currents are assumed to flow within the jet itself. Standard MHD equations in cylindrical geometry are used to investigate the equilibrium and stability properties of such a model. Incompressible thermal matter flows are included in the equilibrium, but not in the stability analysis. Absolute stability regions are found and their boundaries are determined. Linear growth rates for the unstable m = 0 and m = 1 modes are computed. The paper is intended as a basis for a subsequent one where the behavior of distribution of relativistic electrons in the given field is studied and predictions of observable polarization characteristics are made. Title: MHD instabilities of a cylindrical plasma with a realistic energy equation Authors: Torricelli-Ciamponi, Guidetta; Ciampolini, Vittorio; Chiuderi, Claudio Bibcode: 1987JPlPh..37..175T Altcode: The influence of a realistic energy equation on the stability of a cylindrical magnetized plasma in a force-free magnetic field is discussed. Thermal conduction, heating and line radiation are included in the treatment. Explicit growth rates for the m = 0 and m = 1 modes are derived and compared with the standard adiabatic or incompressible time-scales. Finally, the relevance of these results for laboratory and solar plasmas is discussed. Title: Resistive Instabilities in Coronal Conditions Authors: Batistoni, P.; Einaudi, G.; Chiuderi, C. Bibcode: 1985SoPh...97..309B Altcode: Resistive instabilities in a context referring to the solar corona are rigorously investigated. Various equilibrium configurations are considered, differing, among other things, by their behaviour with respect to fast, ideal instabilities. The computations presented cover in a unified scheme all known regimes of resistive modes and allow one to determine the fastest timescale over which resistivity can play a role. Comparisons with previous work as well as possible extensions are presented. Title: Heating in the solar mantle. Authors: Chiuderi, C. Bibcode: 1985NASCP2358..101C Altcode: 1985onhm.rept..101C In the case of the solar chromosphere and corona (the solar mantle) the primary energy source is the mechanical energy from photospheric motions. Plenty of energy is available; the problem is to transfer the needed amount of energy to the proper place to account for the observations. The global problem is reviewed from the point of view of the generation and transmission of energy, the intermediate storage of energy, and the release of energy in such a way that the observed features are generated. Title: Resistive instabilities in astrophysical conditions - A critical discussion Authors: Batistoni, P.; Einaudi, G.; Rubini, F.; Chiuderi, C.; Torricelli, G. Bibcode: 1985IAUS..107..277B Altcode: The linear development of resistive instabilities in astrophysical conditions is investigated numerically with allowance for the effects of gradients of the current density at the singular points that produce asymmetries in the F-profiles. An analysis is also made of the resistive behavior of a system with cylindrical symmetry under modes possessing an m = 1 azimuthal symmetry. The computational method used in the study is essentially a numerical boundary layer, with the internal solution computed exactly; a shooting technique is used to find the eigenvalues. This method is sufficiently fast and accurate for magnetic Reynolds numbers up to 10 to the 9th. The spatial structure and the maximum growth rate of the resistive modes are examined. Title: Magnetic Fields and Thermal Structure of Solar Plasmas Authors: Einaudi, G.; Torricelli-Ciamponi, G.; Chiuderi, C. Bibcode: 1984SoPh...92...99E Altcode: The connection between the magnetic field geometry and the thermal properties of solar coronal structures is investigated. Gravitational effects, that can modify the spatial dependence of the thermodynamical quantities, but have no influence on the plasma-field interaction, are omitted to simplify the problem. A series of two-dimensional models is constructed, in which a strong coupling between the magnetic field shear and the thermal structure of the loop is clearly pointed out. Title: The role of magnetic fields in the heating of stellar atmospheres - Theory Authors: Chiuderi, C. Bibcode: 1983IAUS..102..375C Altcode: The last ten years of observations have shown beyond doubt the fundamental role played by the magnetic field in the heating of stellar atmospheres. After the recognition of the extreme inhomogeneity of the solar corona, two new basic trends have appeared in the theoretical literature on the coronal-heating problem. One is the adoption of a global point of view that stresses the connection of the properties of the upper layers to those of the underlying ones. In this way a general framework is provided, capable of accommodating many possible heating mechanisms. The second trend is the explicit inclusion in the theory of the inhomogeneous nature of the stellar envelopes as a result of the presence of magnetic fields. The present status of knowledge on the subject, as determined by the above evolution of the theoretical approach, is reviewed. Title: Equilibrium and Stability of Magnetized Plasmas Authors: Chiuderi, C. Bibcode: 1982MmSAI..53..863C Altcode: No abstract at ADS Title: Plasmi: il quarto stato della materia. Authors: Chiuderi, C. Bibcode: 1982GAst....8..361C Altcode: No abstract at ADS Title: Differential rotation, magnetic activity and X-ray emission of late type giants Authors: Belvedere, G.; Chiuderi, C.; Paterno, L. Bibcode: 1982A&A...105..133B Altcode: The observations of X-ray emission from late type main sequence and giant stars show that a coronal heating due to acoustic waves is unlikely.

As suggested by Vaiana and Rosner, the conversion of magnetic into thermal energy may be responsible for the X-ray emission from such a kind of stars.

The proposed ingredients in our analysis are the differential rotation and dynamo action which are able to generate a magnetic activity at the star's surface which in turn gives rise to the observed X-ray emission. We assume that the interaction of rotation with convection, in stars possessing outer convective envelopes, is the dominant mechanism for generating differential rotation and dynamo action, and that the stressing of the coronal magnetic flux tubes by the surface turbulence, converting magnetic into thermal energy, determine the level of X-ray emission.

Therefore we compute a series of models of luminosity class III giant stars, and determine the surface X-ray flux.

Comparing these results with those concerning the late type main sequence star models, previously computed by the same authors, and with observations, it appears that the proposed mechanism is plausible. Title: Diagnostic of coronal heating processes based on the emission measure of UV lines Authors: Torricelli-Ciamponi, G.; Einaudi, G.; Chiuderi, C. Bibcode: 1982A&A...105L...1T Altcode: It is shown how data on the emission measure (EM) of solar UV lines can be used to gather information on the structure and heating processes of solar coronal loops, where the most important feature that must be incorporated is the presence of a minimum of the EM as a function of temperature. By doing this, and observing the requirement that the values of the observable parameters remain in the proper range, it is possible to strictly limit the value of the temperature exponent in the heating function. The loop models proposed present strong variations of the conductive fluxes, and exhibit a marked increase in the region where heating and radiation balance. Title: The Formation of Solar Prominences Authors: Chiuderi, C. Bibcode: 1982ASSL...96..137C Altcode: 1982spls.meet..137C A discussion of the physical conditions and numerical models describing the formation of solar prominences is presented. Prominences are noted to occur when thermal conduction is inhibited, with constraints by gravity and the strength of the magnetic field, which supports the filament against gravity. Linear growth has been calculated to take place where the perturbation energy is perpendicular to the field lines. The fastest linear stage has served as a starting point for nonlinear computations, which continue on to a new equilibrium. Location of the equilibrium has been accomplished with three different sets of boundary conditions, i.e., a fixed slab with no mass or heat flux from the boundaries, a fixed slab with no mass flux from the boundaries, and a fixed mass of plasma free to expand or contract, with boundaries at constant temperature and pressure. It is concluded that a homogeneous system linearly unstable to thermal instability evolves toward a stable inhomogeneous state where thin, cool condensations have formed. Title: Loops, fields and all that Authors: Chiuderi, C. Bibcode: 1981SSRv...29..349C Altcode: A greater understanding of solar coronal loops may explain the more elusive processes that heat the solar corona. Loops are strong emitters in the EUV, XUV, and X-ray ranges and have been recognized as the seat of coronal flares. They connect regions of opposite magnetic polarity and therefore delineate the gross topology of the coronal magnetic field. Since they result from the interplay of pressure, gravitational and magnetic forces, a quantitative model of the loops must be devised which will properly account for these forces. The mathematical problem to be solved is summarized, and the basic equations are presented. The solutions are simplified, however, since information is missing for some terms in the equations. It is concluded that high resolution instruments are needed to probe the solar atmosphere above the chromosphere for more information if these models are to be completed. Title: Current Confinement in Solar Coronal Loops Authors: Chiuderi, C.; Einaudi, G. Bibcode: 1981SoPh...73...89C Altcode: In this paper solar coronal loops are regarded as regions of localized current flows. The main purpose is to investigate the consequences of current confinement rather than to produce a model. The physical and observational basis for this assumption are presented as well as the connection with previous studies on loop structure. A proper choice of the current profile allows us to treat quantitatively the equilibrium structure of the loops and their MHD and resistive stability properties. Regions of absolute stability against ideal kink modes are found. Explicit growth rates for the tearing-mode instability are computed. The possible relevance of other resistive effects is also discussed and the crucial importance of the small-scale geometry of the magnetic field outlined. Title: What can we learn from static models of coronal loops Authors: Chiuderi, C.; Einaudi, G.; Torricelli-Ciamponi, G. Bibcode: 1981A&A....97...27C Altcode: The possible use of static coronal loop models to derive information on coronal heating mechanisms is considered. The conditions for the thermal stability of coronal loops with different sets of boundary conditions are investigated, and it is shown that stable configurations exist, including those with zero heat flux at the base. The use of generalized scaling laws permits the conclusion that no meaningful inferences on heating processes can be obtained from static models. A comparison with the results of other authors is also included. Title: Stellar X-ray emission as a consequence of magnetic activity Authors: Belvedere, G.; Chiuderi, C.; Paterno, L. Bibcode: 1981A&A....96..369B Altcode: It is noted that the discovery of stellar coronae for main sequence stars later than FO through K and M is one of the most important results of Einstein satellite observations. A theoretical estimate is given of the surface X-ray fluxes for these stars. It is noted that at coronal temperatures greater than 2,000,000 K X-ray luminosity is given by the radiative losses of systems of magnetic loops forming the active regions on the star's surface, namely by the summation of the single loop emission over the total number of loops in the active regions. Passing from luminosities to fluxes, it is shown that the surface X-ray flux is independent of the dimensions of the active regions present on the star's surface and depends only on the surface turbulence and the typical strength of the magnetic field in the active regions. The field twisted by turbulent motions is identified with the emerging mean toroidal field as computed by stellar dynamo models. Title: Magnetic heating in the sun. Authors: Chiuderi, C. Bibcode: 1981ASIC...68..269C Altcode: 1981spss.conf..269C The observational evidence for magnetic heating in the solar corona is presented. The possible ways of investigating theoretically the nature of the heating processes are critically discussed. Merits and disadvantages of the basic mechanisms so far proposed are reviewed. Finally, a preliminary application of the magnetic heating concept to stellar coronae is presented. Title: High energy phenomena in the sun Authors: Chiuderi, C. Bibcode: 1981IAUS...94..367C Altcode: Solar flares are considered as a model of a widespread astrophysical phenomenon: the violent conversion of some form of energy into heat and the kinetic energy of accelerated particles. Following a brief description of the observational characteristics of solar flares in the chromosphere and corona, attention is given to mechanisms for the accumulation of magnetic energy in the solar atmosphere and its subsequent release by processes of resistive dissipation and dynamical reconnection giving rise to a flare. It is noted that measurements of the coronal magnetic field made by EUV polarimeters of the type planned for the Solar Maximum Mission and Spacelab are necessary for the definitive solution of the problem of energy release in flares. Title: The stability properties of cylindrical force-free fields: effect of an external potential field Authors: Chiuderi, C.; Einaudi, G.; Ma, S. S.; van Hoven, G. Bibcode: 1980JPlPh..24...39C Altcode: A large-scale potential field with an embedded smaller-scale force-free structure ( × B = αB) is studied in cylindrical geometry. We consider cases in which α goes continuously from a constant value α0 on the axis to zero at large r. Such a choice of α(r) produces fields which are realistic (few field reversals) but not completely stable. The MHD-unstable wavenumber regime is found. Since the considered equilibrium field exhibits a certain amount of magnetic shear, resistive instabilities can arise. The growth rates of the tearing mode in the limited MHD-stable region of º space are calculated, showing time-scales much shorter than the resistive decay time. Title: The solar corona. Authors: Chiuderi, C. Bibcode: 1980HiA.....5..335C Altcode: Recent observations from space have shown that the solar corona is spatially a very structured medium and temporally a very dynamic one. The consequent changes in the current theoretical ideas about coronal physics are reviewed. Particular emphasis is placed on the role of magnetic fields in shaping and heating the coronal structures. Title: The filament instability in a sheared field Authors: Chiuderi, C.; van Hoven, G. Bibcode: 1980IAUS...91..295C Altcode: A self-consistent calculation of the thermal instability in a non-uniform field is performed. It is shown how the dynamic response of density and temperature to the competing effects of optically-thin radiation and field-collimated thermal conduction leads to the formation of characteristic knife-blade filaments. The dynamic calculation reflects the sheared magnetic field and the finite geometry, as well as the intimate relation of the sheared magnetic fields to flares. Title: Recent development in coronal structure thermodynamics and stability. Authors: Chiuderi, C. Bibcode: 1980AnPh....5..425C Altcode: 1980mhda.conf..425C No abstract at ADS Title: Wave propagation in a non-isothermal atmosphere and the solar five-minute oscillations Authors: Chiuderi, C.; Giovanardi, C. Bibcode: 1979SoPh...64...27C Altcode: This paper presents a detailed discussion of the properties of linear, periodic acoustic waves that propagate vertically in a non-isothermal atmosphere. In order to retain the basic feature of the solar atmosphere we have chosen a temperature profile presenting a minimum. An analytical solution of the problem is possible if T/μ, μ being the mean molecular weight, varies parabolically with height. The purpose of this study is to point out the qualitative differences existing between the case treated here and the customary analysis based on a locally isothermal treatment. The computed velocity amplitude and the temperature-perturbation as functions of the wave period exhibit a sharp peak in the region between 180 and 300 s, thus showing the possibility of interpreting the five-minute oscillations as a resonant phenomenon. The propagating or stationary nature of the waves is investigated by a study of the phase of the proposed analytical solution. Title: The dynamics of filament formation: the thermal instability in a sheared magnetic field. Authors: Chiuderi, C.; van Hoven, G. Bibcode: 1979ApJ...232L..69C Altcode: The dynamics of the thermal plasma instability in a nonuniform magnetic field, involved in the formation of solar filaments or prominences in sheared coronal magnetic fields, are examined. An expression is derived for a perturbation of equilibrium conditions corresponding to the physical configuration to observed filaments, which allows the dynamic response of plasma pressure, density and temperature to optically thin radiation and field-collimated thermal conduction to be determined. Numerical solutions to the equations demonstrate the formation of characteristic knife blade condensations and can predict the initial temporal and spatial scales of coronal filament appearance. Title: High-Energy Phenomena on the Solar Surface Authors: Chiuderi, C. Bibcode: 1979ASSL...75...33C Altcode: 1979sss..meet...33C This paper reviews observations of events in the solar atmosphere that involve the presence of suprathermal particles and the emission of radiation whose brightness temperature exceeds the kinetic temperature. Observed characteristics of solar flares are discussed, emphasizing the buildup of energy prior to a flare and the mechanism of primary energy release. Magnetic-field reconnection is taken to be the best candidate for the mechanism of energy conversion required to fuel a flare. Several phenomenological flare models are considered, along with shortcomings of these models. Title: Does the structure of the chromosphere-corona arise entirely from small-scale MHD effects? Authors: Chiuderi, C. Bibcode: 1979ssms.conf..105C Altcode: No abstract at ADS Title: Polytropic models of radio stars. Authors: Chiuderi, C.; Torricelli Ciamponi, G. Bibcode: 1978A&A....69..333C Altcode: Summary. This paper extends our previous work on models for radio emitting stars undergoing mass loss. The emission is assumed to be of thermal nature and the dynamics of the envelope is treated in the polytropic approximation. No limitations on the value of the polytropic index are introduced. Analytic results are derived whenever possible. For the other cases, the results of numerical integration of the model are fitted in a simple mathematical form that allows an easy application of the model to the observations. Detailed predictions, in particular for the mass-loss rate, are made for a few selected stars. A very good agreement is found with the estimates of the same quantities found by different authors. The possibility of reproducing the infrared spectrum is also discussed. Key words: radiostars stellar winds - mass loss Title: The structure of coronal magnetic loops. II: MHD stability theory. Authors: Giachetti, R.; van Hoven, G.; Chiuderi, C. Bibcode: 1977SoPh...55..371G Altcode: We present the second part of a complete theory for the plasma and field structure of a cool coronal arch, corresponding to those observed in the EUV from Skylab. The global magneto-hydrodynamic (MHD) stability of a previously described equilibrium-loop model is evaluated, and compared with that of an unmodified ambient force-free field. The influence of the photospheric boundary condition is also evaluated, producing a specification of stability limits which depend on the relative field and plasma pressures and scale widths. The resulting restrictions on the allowable field configuration of a coronal loop are then compared with observed values. The implications of this general method for deducing small-scale coronal magnetic-field structure from the measured plasma profile of an emissive feature are also described. Title: The structure of coronal magnetic loops. I: Equilibrium theory. Authors: Chiuderi, C.; Giachetti, R.; van Hoven, G. Bibcode: 1977SoPh...54..107C Altcode: We present here a model, based on observations, for the magnetic-field equilibrium of a cool coronal loop. The pressure structure, taken from the Harvard/Skylab EUV data, is used to modify the usual force-free-field form in quasi-cylindrical symmetry. The resulting field, which has the same direction but different strength, is calculated and its variation displayed. Finally, localized interchange stability is evaluated and discussed, as the first step in a subsequent complete magnetohydrodynamic-stability analysis. Title: The solar corona and the origin of the solar wind Authors: Chiuderi, C. Bibcode: 1977MmSAI..48..561C Altcode: The paper deals with the properties of the solar corona and the techniques by which they have been observed. The corona is treated, in general, as a spherically symmetric stationary gas. This model, although admittedly unrealistic, has a certain right to exist if it is considered as an average model (corresponding to a lower resolution or to a greater distance to the sun). Particular attention is given to the problem of coronal heating and to the solar wind. Title: Radio emission from mass-outflow stars. Authors: Chiuderi, C.; Torricelli Ciamponi, G. Bibcode: 1977A&A....59..395C Altcode: The problem of the radio emission from the extended envelopes surrounding early-type stars undergoing mass loss is considered. The emission is interpreted as due to thermal bremsstrahlung processes. The dynamics of the expanding atmosphere is treated in the polytropic approximation without prescribing the density and temperature profiles, as done in previous models. For a particular value of the polytropic index falling in the physically interesting range, analytical expressions are deduced for the spectral index, the radio flux, and the angular size of the emitting object, as well as a relationship between these quantities and the mass-loss rate. The model applied to P Cyg and MWC 349 gives good agreement with the existing data. Title: Cyclotron instabilities of a magnetized electron plasma with anisotropic temperatures Authors: Chiuderi, C.; Einaudi, G.; Giachetti, R. Bibcode: 1977JPlPh..17..453C Altcode: The dispersion relation for an electron plasma in a magnetic field is investigated for a bi-Maxwellian distribution function. A new set of solutions for non-perpendicular propagation is found. The linear growth rates are computed and the instability regions in the (k, cos θ) plane are determined. An approximate analytical treatment of the problem is also given for certain ranges of the parameters. Title: Flare mechanisms Authors: Chiuderi, C. Bibcode: 1977MmSAI..48..321C Altcode: The paper considers a 'standard' model of a solar flare as defined by morphology, energetics (thermal, gravitational and magnetic), and temporal evolution; the quantitative data considered are characteristic of a large flare (3 plus magnitude). Detailed consideration is given to the three stages of flare evolution: preparatory, peak, and decay. Flare evolution is discussed in reference to three determining factors: (1) the macroscopic instability which leads to intensification of local currents, (2) plasma turbulence, and (3) particle acceleration mechanisms. Title: A model for a stable coronal loop. Authors: van Hoven, G.; Chiuderi, C.; Giachetti, R. Bibcode: 1977ApJ...213..869V Altcode: A plasma-physics model of a stable active-region arch is presented which corresponds to the structure observed in the EUV. Pressure gradients are seen, so that the equilibrium magnetic field must depart from the force-free form valid in the surrounding corona. The data and the approximate cylindrical symmetry are exploited to develop a modified form of the commonly assumed sheared-spiral structure. The dynamic MHD behavior of this pressure/field model is then evaluated by the Newcomb criterion (1960) from controlled-fusion physics, and the results show short-wavelength stability in a specific parameter range. For pressure profiles with widths of the order of the magnetic-field scale, the possibility is demonstrated that such arches can persist for reasonable periods. Finally, the spatial proportions and magnetic fields of a characteristic stable coronal loop are described. 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: Circular polarization and magnetic fields of white dwarfs. Authors: Chiuderi, C.; Silvi, M. Bibcode: 1976MmSAI..47...65C Altcode: A simple method to estimate the strength of the surface magnetic field of a white dwarf from the degree of circular polarization in its continuum spectrum is proposed. The method is applied to the observed spectra of G99-37 and G227-35 giving a fairly good fit in the optical range. Title: Solar five-minute oscillations as a resonance phenomenon. Authors: Chiuderi, C.; Giovanardi, C. Bibcode: 1975ApJ...200L.165C Altcode: The response of a nonisothermal gravitational atmosphere to an oscillatory perturbation is studied in the case of a parabolic temperature profile. The exact analytical solution is then applied to a model solar atmosphere. An interpretation of the 5-minute oscillations as free oscillations of the atmosphere as a whole is proposed. Title: The LOFER (Landau Orbital Ferromagnetism) Mechanism for the Generation of Intense Magnetic Fields Authors: Chiuderi, C.; Lee, H. J. Bibcode: 1975NYASA.257...82C Altcode: No abstract at ADS Title: Acoustic Waves in the Lower Solar Atmosphere Authors: Chiuderi, C.; Giovanardi, C. Bibcode: 1975SoPh...41...35C Altcode: The propagation and dissipation of acoustic waves in the lower solar atmosphere is studied. The level of shock formation is computed for various initial conditions. It is shown that shocks form rather low in the atmosphere and that this result does not depend critically on the assumed initial conditions. Title: Proceedings of the First European Solar Meeting. Florence, February 25 - 27, 1975. Authors: Chiuderi, C.; Landini, M.; Righini, A.; Zwaan, C. Bibcode: 1975MmArc.105....1C Altcode: No abstract at ADS Title: CESRA-5 Authors: Chiuderi, C.; Landini, M.; Righini, A. Bibcode: 1975cesra...5.....C Altcode: No abstract at ADS Title: European solar meeting : 1 : 1975 Authors: Chiuderi, C. Bibcode: 1975esm..meet.....C Altcode: The papers in this collection report on current European research in solar physics, recently developed instruments for solar observations, and future developments in this field. Specific topics include a theoretical review of dynamic processes in the solar atmosphere, magnetic structures in the chromosphere and photosphere, magnetic structures observed in the corona, and the solar brightness temperature in the far infrared. Attention is given to future solar-physics programs in space, current research being conducted in a number of Western and Eastern European countries, the design of a high-sensitivity radioheliograph, a proposal for a collecting array of 144 antennas, and a film of the solar spectrum during an eclipse. Individual items are announced in this issue. Title: Neutrino Reactions in Strong Magnetic Field Authors: Canuto, V.; Chiuderi, C.; Chou, C. K.; Fassio-Canuto, L. Bibcode: 1974Ap&SS..28..145C Altcode: We present the energy losses due to several neutrinos processes: (1) synchrotron neutrinos, (2) pair annihilation neutrinos, (3) plasmon neutrinos, and (4) photoneutrinos in the presence of a superstrong magnetic field. Numerical results are tabulated and illustrated for several values of densities and temperatures. In the low density regime (ϱ≲107 g cm-3) the presence of a magnetic field decreases the luminosity, whereas the opposite is true at higher densities. This last effect is however almost entirely due to the existence of a new process the synchrotron neutrinos that disappear whenH→0. Even though the overall effect can only be quantitatively ascertain after a complete cooling computation is performed, one should however expect a much lower temperature for neutron star surface than the one computed in theH=0 case. Title: Spectral and Polarization Properties of Radiation Generated by Plasma Turbu- lence Authors: Chiuderi, C.; Veltri, P. Bibcode: 1974A&A....30..265C Altcode: Summary. We have studied a system which consists of a cold plasma containing an ultrarelativistic distribution of electrons. The response of such a system to a periodic electromagnetic disturbance is first discussed. The problem is then specialized to the case of a longitudinal stationary plasma wave. The emission and propagation of radiation in such a system is analysed in detail by a method which is analogous to the one used in synchro tron radiation theory. Explicit results are given for a power-law distribution of relativistic electrons, and they are compared to synchrotron emission under the same conditions. It is shown that the frequency dependence and polarization are quite similar in the two cases. Key words: plasma radiation mechanism - transfer equation - polarized radiation Title: High Resolution Studies of Type III Solar Radio Bursts (presented by H. Rosenberg) Authors: Chiuderi, C.; Giachetti, R.; Mercier, C.; Rosenberg, H.; Slottje, C. Bibcode: 1974IAUS...57..225C Altcode: No abstract at ADS Title: Fine Structure in Type IV Solar Radio Bursts (presented by H. Rosenberg) Authors: Caroubalos, C.; Pick, M.; Chiuderi, C.; Giachetti, R.; Rosenberg, H.; Slottje, C. Bibcode: 1974IAUS...57..291C Altcode: No abstract at ADS Title: A Dynamical Model for the Chromosphere-Corona Transition Region Authors: Chiuderi, Claudio; Riani, Iacopo Bibcode: 1974SoPh...34..113C Altcode: A dynamical, homogeneous model of the chromosphere-corona transition region and of the lower corona is presented, based on the hydrodynamical equations and on a semi-empirical relation deduced from radio observations. The model is shown to be in agreement with radio and UV observations and with the particle flux given by solar wind measurements. A comparison with the analogous static model shows that dynamical effects are very small. Title: Excitation of Longitudinal Waves in a Magnetized Plasma under Coronal Conditions Authors: Chiuderi, C.; Giachetti, R.; Rosenberg, H. Bibcode: 1974cesra...4..155C Altcode: No abstract at ADS Title: NonLinear Wave Coupling in Type IV Solar Radio Bursts Authors: Chiuderi, Claudio; Giachetti, Riccardo; Rosenberg, Hans Bibcode: 1973SoPh...33..225C Altcode: In order to explain a fine structure of parallel ridges in stationary type IV continua, the emission due to the coupling of electrostatic upper hybrid waves and Bernstein waves at the sum frequency of the upper hybrid and harmonics of the gyro frequency has been calculated. If the energy density of these electrostatic waves is of the order of 10-3 times the thermal energy density, then the observed zebra pattern can be emitted by a region with a diameter of ∼ 103 km. Title: Nonlinear wave coupling in type IV solar radio bursts. Authors: Chiuderi, C.; Giachetti, R.; Rosenberg, H. Bibcode: 1973UtrOv.237.....C Altcode: No abstract at ADS Title: Coronal Abundance of Elements and a Model of the Quiet Sun from Radio Observations Authors: Chiuderi, Claudio; Chiuderi Drago, Franca; Noci, Giancarlo Bibcode: 1972SoPh...26..343C Altcode: It is shown that the combined use of radio observations of the quiet Sun and UV line intensities allows to compute the absolute coronal abundance of the elements. The abundances found by this method agree very well with the most recent determinations. A model of the transition region and corona in hydrostatic equilibrium is also presented. Similarities and differences with models based on UV observations are discussed. Title: Coronal Abundances and a Model of the Quiet Sun from Radio Observations Authors: Chiuderi, C.; Chiuderi Drago, F.; Noci, G. Bibcode: 1972BAAS....4T.379C Altcode: No abstract at ADS Title: Evidence for non linear effects in solar metric radiation Authors: Chiuderi, C. Bibcode: 1972ppsr.conf..288C Altcode: No abstract at ADS Title: Radio Model of the Transition Layer in Solar Active Regions Authors: Chiuderi, Claudio; Drago, Franca Chiuderi; Noci, Giancarlo Bibcode: 1971SoPh...17..369C Altcode: A model of the transition layer of an active region of the Sun is presented based on radio observations. The model is deduced by using the Laplace transform of the brightness temperature and the hydrostatic equilibrium equation. A rational function, well-behaved in the coronal region, has been used to represent the observed brightness temperature. The model indicates the existence of a very steep temperature gradient and suggests the presence of a constant conductive flux from the corona into the chromosphere. Both these conclusions are quantitatively in a very good agreement with those deduced from the UV emission lines observations, thus removing a previous discrepancy between radio and optical based models. It is also shown that the presence of a weak magnetic field does not alter the above conclusions. Title: Model of the quiet sun and coronal abundance of elements from radio observations Authors: Chiuderi, C.; Chiuderi-Drago, F.; Noci, G. Bibcode: 1971cesra...2...55C Altcode: No abstract at ADS Title: Plasmon Neutrinos Emission in a Strong Magnetic Field. II: Longitudinal Plasmons Authors: Canuto, V.; Chiuderi, C.; Chou, C. K. Bibcode: 1970Ap&SS...9..453C Altcode: The decay of a longitudinal plasmon into two neutrinos is studied in the presence of a strong magnetic field. Contrary to the transverse case, for longitudinal plasmons the existence of a new mode, entirely dependent on the magnetic field, greatly enhances the energy loss at high densities. Denoting byQ HandQ 0the neutrino energy losses with and without magnetic field respectively, the situation is as follows: atH≃1011 G andT≥109K,Q 0≫105 Q Hfor ϱ<1011g cm-3, WhileQ H≫1010 Q 0for ϱ>1011g cm-3. A second physically interesting feature is the anisotropic character of the energy loss which is highly peaked along the field lines, giving rise to a shorter cooling time in that direction than in any other. Title: Plasmon Neutrinos Emission in a Strong Magnetic Field. I: Transverse Plasmons Authors: Canuto, V.; Chiuderi, C.; Chou, C. K. Bibcode: 1970Ap&SS...7..407C Altcode: In this paper we generalize the Adams, Ruderman, Woo and Zaidi plasmon decay process to include the presence of a strong magnetic field. Two cases are studied; propagation parallel and perpendicular to the magnetic field. In either case we found that relevant changes only show forH≌1012 1013G. Title: Shape of the Crab Pulsar and its Period Fluctuations Authors: Chiuderi, Claudio; Occhionero, Franco Bibcode: 1970Natur.226..337C Altcode: RICHARDS et al.1 have recently recorded sinusoidal variations in the arrival times of radio pulses from NP 0532 with a period of about three months and an amplitude Δτ= 6 × 10-4 s. This effect can be interpreted in terms of genuine sinusoidal variations of the circular frequency of the Crab pulsar where Ω ~= 200 s-1, ω ~= 10-6 s-1, A ~= 10-10. Equation (1) can be explained by the widely accepted oblique rotator model2-4 simply by investigating an interesting consequence of the possible ellipticity of the neutron star. To account for the history and energy balance of the Crab Nebula3, an upper limit must be placed4,5 to this ellipticity, such that the gravitational quadrupole radiation dominates the magnetic dipole radiation only in the early life of the pulsar. It turns out that the explanation given here for the anomalies in the pulse arrival times provides in the end an even stronger limit to the deviations from spherical symmetry allowed in a neutron star. Title: Transverse Electrical Conductivity of a Relativistic Gas in an Intense Magnetic Field Authors: Canuto, V.; Chiuderi, C. Bibcode: 1970PhRvD...1.2219C Altcode: The transverse electrical conductivity is computed for a system of degenerate relativistic electrons in a strong magnetic field H~1010-1013 G. As suggested by pulsar models, such fields exist in nature in collapsed bodies, like neutron stars. The present computation is valid in the outer regions of the star where the scatterers are not degenerate, while the electrons are taken to be at zero temperature. The scattering mechanism is assumed to be the electron-ion Coulomb scattering. Numerical values of the transverse conductivity are given in the range 109<=H<=1013 G, and a comparison is made with the longitudinal and zero-field conductivities. It is found that for densities 107<=ρ<=5×107, σII~20σ0 and σ0~3σ⊥. As the density increases, both σ⊥ and σII tend to σ0. Title: Physical Sciences: New Source of Intense Magnetic Fields in Neutron Stars Authors: Canuto, V.; Chiu, H. Y.; Chiuderi, C. Bibcode: 1970Natur.225...47C Altcode: RECENTLY there has been interest in the problem of intense magnetic fields (IMF) in gravitationally collapsed bodies1-6. In particular, the radio emission from neutron stars (pulsars) suggests strong magnetic fields of the order of 1012 G (refs. 5 and 7). According to the law of flux conservation, in a collapse process the magnetic fields strength increases as the square of the contracting factor a (> 1). If the initial magnetic field is of the order of 103 G (the field in sunspots) before collapse, then assuming contraction from a star of one solar radius ~1011 cm to that of a neutron star of 106 cm, a field strength of 1013 G may be achieved. Magnetic fields in ordinary stars are usually attributed to non-equilibrium processes such as the existence of currents in the form of drifting charges, but we have recently shown that a new kind of quasi -equilibrium state exists, which possesses a uniform self-consistent magnetization8. Title: New State of Ferromagnetism in Degenerate Electron Gas and Magnetic Fields in Collapsed Bodies Authors: Lee, Hyung Joon; Canuto, Vittorio; Chiu, Hong-Yee; Chiuderi, Claudio Bibcode: 1969PhRvL..23..390L Altcode: A new state of "ferro" magnetism in a degenerate electron gas is found and shown to be stable. This magnetism is the sum of all microscopic magnetic moments associated with electrons in their respective Landau levels while the Landau levels of the system are in turn maintained by this macroscopic magnetization. The maximum field in the Landau orbital ferromagnetism state is 107 G for white-dwarf densities and 1012 G for neutron-star densities. Title: Solution of the Dirac equation in orthogonal electric and magnetic fields. Authors: Canuto, V.; Chiuderi, C. Bibcode: 1969NCimL...2..223C Altcode: No abstract at ADS