Claudio Chiuderi

Nonlinear kinetic regime of the Weibel instability in an electron–ion plasma

Kinetic numerical simulations of the evolution of the Weibel instability during the full nonlinear regime including ions dynamics are presented. The formation of strong density inhomogeneity and its influence on the resulting electrostatic and electromagnetic wave modes are shown.

Nonlinear wave coupling in type IV solar radio bursts

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.

NONRESONANT RESISTIVE DISSIPATION OF INCOMPRESSIBLE MAGNETOHYDRODYNAMIC WAVES

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. 17 refs.

Incompressible disturbances in nonuniform media : formation of small scales

The rate of energy deposition by propagating disturbances in a weakly dissipative (high Reynolds number) medium is strictly related to the formation of small scales. The basic mechanisms for producing them are either the nonlinear interactions among the propagating modes (turbulent cascade) or the linear interaction between the disturbance and the nonuniformities of the supporting medium. The efficiency of the first mechanism depends on the amplitude of the propagating disturbances, whereas the second one is active for all kinds of perturbation as long as some nonuniformity is present

A model for a stable coronal loop

We present here a new plasma-physics model of a stable active-region arch 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. We take advantage of the data and of the approximate cylindrical symmetry to develop a modified form of the commonly assumed sheared-spiral structure. The dynamic MHD behavior of this new pressure/field model is then evaluated by the Newcomb criterion, taken from controlled-fusion physics, and the results show short-wavelength stability in a specific parameter range. Thus we demonstrate the possibility, for pressure profiles with widths of the order of the magnetic-field scale, that such arches can persist for reasonable periods. Finally, the spatial proportions and magnetic fields of a characteristic stable coronal loop are described.

The structure of coronal magnetic loops

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.

Energy balance in the prominence-corona transition region

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.

Axisymmetric magnetohydrodynamic equations: Exact solutions for stationary incompressible flows

The paper describes a new set of exact analytical solutions of the axisymmetric magnetohydrodynamic (MHD) equations for stationary and incompressible flows. The magnetic and flow surfaces are assumed to form a regularly nested set to stress the physical character of the solutions. As a result, these show a self‐similar radial behavior. A suitable choice of the reference system reduces the problem to a single differential equation equivalent to the generalized Grad–Shafranov equation. The proposed method leads to the explicit expressions of the magnetic and flow surfaces. The analytical form of all the physical quantities for each possible shape of the surfaces can then be obtained in terms of two arbitrary functions of the self‐similar radial variable, one being the density. By comparison, standard treatments of the generalized Grad–Shafranov equation require the specification of up to five arbitrary functions.

CORONAL ABUNDANCE OF ELEMENTS AND A MODEL OF THE QUIET SUN FROM RADIO OBSERVATIONS.

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.

Induced Deposition of Magnetic Energy in the Solar Corona

In this paper a numerical study of the propagation and dissipation properties of magnetohydro-dynamic 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.