**Publication Date:** 2009.

**Language:** English.

**Abstract:**

Context. A magnetic cloud (MC) is a magnetic flux rope in the solar wind (SW), which, at 1 AU, is observed ∼2-5 days after its expulsion from the Sun. The associated solar eruption is observed as a coronal mass ejection (CME).Aims. Both the in situ observations of plasma velocity distribution and the increase in their size with solar distance demonstrate that MCs are strongly expanding structures. The aim of this work is to find the main causes of this expansion and to derive a model to explain the plasma velocity profiles typically observed inside MCs.Methods. We model the flux rope evolution as a series of force-free field states with two extreme limits: (a) ideal magneto-hydrodynamics (MHD) and (b) minimization of the magnetic energy with conserved magnetic helicity. We consider cylindrical flux ropes to reduce the problem to the integration of ordinary differential equations. This allows us to explore a wide variety of magnetic fields at a broad range of distances to the Sun.Results. We demonstrate that the rapid decrease in the total SW pressure with solar distance is the main driver of the flux-rope radial expansion. Other effects, such as the internal over-pressure, the radial distribution, and the amount of twist within the flux rope have a much weaker influence on the expansion. We demonstrate that any force-free flux rope will have a self-similar expansion if its total boundary pressure evolves as the inverse of its length to the fourth power. With the total pressure gradient observed in the SW, the radial expansion of flux ropes is close to self-similar with a nearly linear radial velocity profile across the flux rope, as observed. Moreover, we show that the expansion rate is proportional to the radius and to the global velocity away from the Sun.Conclusions. The simple and universal law found for the radial expansion of flux ropes in the SW predicts the typical size, magnetic structure, and radial velocity of MCs at various solar distances. © 2009 ESO.

**Author affiliation**: Dasso, S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

**Keywords:**
interplanetary medium; Sun: coronal mass ejections (CMEs); Sun: magnetic fields; Boundary pressure; Coronal mass ejection; Cylindrical flux ropes; Expansion rate; Flux ropes; Force free fields; In-situ observations; interplanetary medium; Magnetic clouds; Magnetic energies; Magnetic flux ropes; Magnetic helicity; Plasma velocity; Radial distributions; Radial expansions; Radial velocity; Self-similar; Solar eruption; Sun: coronal mass ejections (CMEs); Sun: magnetic fields; Astrophysics; Boundary layer flow; Energy conservation; Expansion; Fluid dynamics; Magnetic fields; Magnetic flux; Magnetic structure; Magnetohydrodynamics; Ordinary differential equations; Pressure gradient; Solar wind; Sun; Velocity; Velocity distribution; Solar energy.

**Repository:** Biblioteca Digital (UBA-FCEN). Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales

**Authors**:
<div class="autor_fcen" id="6067">Nakwacki, M.S.</div>; <div class="autor_fcen" id="2288">Dasso, S.</div>; Démoulin, P.; <div class="autor_fcen" id="5317">Mandrini, C.H.</div>; <div class="autor_fcen" id="4119">Gulisano, A.M.</div>

**Publication Date:** 2011.

**Language:** English.

**Abstract:**

Context. Significant quantities of magnetized plasma are transported from the Sun to the interstellar medium via interplanetary coronal mass ejections (ICMEs). Magnetic clouds (MCs) are a particular subset of ICMEs, forming large-scale magnetic flux ropes. Their evolution in the solar wind is complex and mainly determined by their own magnetic forces and the interaction with the surrounding solar wind. Aims. Magnetic clouds are strongly affected by the surrounding environment as they evolve in the solar wind. We study expansion of MCs, its consequent decrease in magnetic field intensity and mass density, and the possible evolution of the so-called global ideal-MHD invariants. Methods. In this work we analyze the evolution of a particular MC (observed in March 1998) using in situ observations made by two spacecraft approximately aligned with the Sun, the first one at 1 AU from the Sun and the second one at 5.4 AU. We describe the magnetic configuration of the MC using different models and compute relevant global quantities (magnetic fluxes, helicity, and energy) at both heliodistances. We also tracked this structure back to the Sun, to find out its solar source. Results. We find that the flux rope is significantly distorted at 5.4 AU. From the observed decay of magnetic field and mass density, we quantify how anisotropic is the expansion and the consequent deformation of the flux rope in favor of a cross section with an aspect ratio at 5.4 AU of ≈ 1.6 (larger in the direction perpendicular to the radial direction from the Sun). We quantify the ideal-MHD invariants and magnetic energy at both locations, and find that invariants are almost conserved, while the magnetic energy decays as expected with the expansion rate found. Conclusions. The use of MHD invariants to link structures at the Sun and the interplanetary medium is supported by the results of this multi-spacecraft study. We also conclude that the local dimensionless expansion rate, which is computed from the velocity profile observed by a single-spacecraft, is very accurate for predicting the evolution of flux ropes in the solar wind. © 2011 ESO.

**Author affiliation**: Nakwacki, M.S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

**Author affiliation**: Dasso, S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

**Author affiliation**: Mandrini, C.H. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

**Author affiliation**: Gulisano, A.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

**Keywords:**
Magnetic fields; Magnetohydrodynamics (MHD); Solar wind; Sun: coronal mass ejections (CMEs); Sun: heliosphere; Sun: magnetic topology; Cross section; Dynamical evolution; Expansion rate; Flux ropes; Global quantities; Helicities; In-situ observations; Interplanetary coronal mass ejections; Interplanetary medium; Interstellar mediums; Link structure; Magnetic clouds; Magnetic configuration; Magnetic energies; Magnetic flux ropes; Magnetic force; Magnetic-field intensity; Magnetized plasmas; Magnetohydrodynamics (MHD); Mass densities; Radial direction; Solar source; Sun: coronal mass ejections (CMEs); Surrounding environment; Velocity profiles; Aspect ratio; Clouds; Expansion; Interplanetary spacecraft; Magnetic fields; Magnetic flux; Magnetoplasma; Planetary surface analysis; Solar system; Solar wind; Wind; Magnetohydrodynamics.

**Repository:** Biblioteca Digital (UBA-FCEN). Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales

**Abstract:**

We present a numerical analysis of an incompressible decaying magnetohydrodynamic turbulence run on a grid of 15363 points. The Taylor Reynolds number at the maximum of dissipation is 1100, and the initial condition is a superposition of large-scale Arn'old-Beltrami-Childress flows and random noise at small scales, with no uniform magnetic field. The initial kinetic and magnetic energies are equal, with negligible correlation. The resulting energy spectrum is a combination of two components, each moderately resolved. Isotropy obtains in the large scales, with a spectral law compatible with the Iroshnikov-Kraichnan theory stemming from the weakening of nonlinear interactions due to Alfvén waves; scaling of structure functions confirms the non-Kolmogorovian nature of the flow in this range. At small scales, weak turbulence emerges with a k-2 spectrum, the perpendicular direction referring to the local quasiuniform magnetic field. © 2007 The American Physical Society.

**Author affiliation**: Mininni, Pablo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina

**Author affiliation**: Pouquet, A.. National Center for Atmospheric Research; Estados Unidos

**Keywords:**
Astronomía; Ciencias Físicas; CIENCIAS NATURALES Y EXACTAS.

**Repository:** CONICET Digital (CONICET). Consejo Nacional de Investigaciones Científicas y Técnicas

**Publication Date:** 2008.

**Language:** English.

**Abstract:**

Aims. We study the stability and modes of non - isothermal coronal loop models with different intensity values of equilibrium twisted magnetic field.Methods. We use an energy principle obtained by means of non - equilibrium thermodynamic arguments. The principle is expressed in terms of Hermitian operators and enables us to consider the coupled system of equations, the balance of energy equation and the equation of motion, to obtain modes and eigenmodes in a spectrum ranging from short to long-wavelength disturbances without using weak varying approximations of the equilibrium parameters. Long-wavelength perturbations introduce additional difficulties because the inhomogeneous nature of the medium produce disturbances corresponding to continuous intervals of eigenfrequencies, which cannot be considered as purely sinusoidal.Results. We analyze the modification of periods, modes structure, and stability when the helicity, the magnetic field strength, and the radius of the fluxtube are varied. The efficiency of the damping due to the resonant absorption mechanism is analyzed in terms of modes that can either impulsively release or store magnetic energy.Conclusions. We find that the onset of the instability is associated with a critical value of the helicity and the magnetic energy content has a determinant role on the instability of the system with respect to the stabilizing effect of the resonant absorption mechanism. © 2008 ESO.

**Author affiliation**: González, R. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

**Keywords:**
Sun: corona; Sun: magnetic fields; Sun: oscillations; Absorption; Energy efficiency; Equations of motion; Magnetic field measurement; Magnetic fields; Magnetic materials; Mathematical operators; Mechanisms; Solar energy; Thermodynamic stability; Thermodynamics; Coronal loops; Coupled systems; Critical values; Eigen-frequencies; Eigen-modes; Energy equation; Energy principles; Equation of motion; Equilibrium parameters; Helicity; Hermitian operators; Intensity values; Irreversible energy; Long wavelengths; Magnetic energies; Magnetic field configurations; Magnetic field strengths; Mode analysis; Non-equilibrium; Non-isothermal; Resonant absorption; Solar coronal loops; Stabilizing effects; Sun: corona; Sun: magnetic fields; Sun: oscillations; System stability.

**Repository:** Biblioteca Digital (UBA-FCEN). Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales

**Publication Date:** 2010.

**Language:** English.

**Abstract:**

We study decaying magnetohydrodynamics (MHD) turbulence stemming from the evolution of the Taylor-Green flow generalized recently to MHD, with equal viscosity and magnetic resistivity and up to equivalent grid resolutions of 20483 points. A pseudo-spectral code is used in which the symmetries of the velocity and magnetic fields have been implemented, allowing for sizable savings in both computer time and usage of memory at a given Reynolds number. The flow is non-helical, and at initial time the kinetic and magnetic energies are taken to be equal and concentrated in the large scales. After testing the validity of the method on grids of 5123 points, we analyze the data on the large grids up to Taylor Reynolds numbers of 2200. We find that the global temporal evolution is accelerated in MHD, compared to the corresponding neutral fluid case. We also observe an interval of time when such configurations have quasi-constant total dissipation, time during which statistical properties are determined after averaging over of the order of two turn-over times. A weak turbulence spectrum is obtained which is also given in terms of its anisotropic components. Finally, we contrast the development of small-scale eddies with two other initial conditions for the magnetic field and briefly discuss the structures that develop, and which display a complex array of current and vorticity sheets with clear rolling-up and folding. © 2010 Taylor & Francis.

**Author affiliation**: Pouquet, A.. National Center for Atmospheric Research; Estados Unidos

**Author affiliation**: Lee, E.. National Center for Atmospheric Research; Estados Unidos

**Author affiliation**: Brachet, M. E.. National Center for Atmospheric Research; Estados Unidos. Ecole Normale Supérieure; Francia

**Author affiliation**: Mininni, Pablo Daniel. National Center for Atmospheric Research; Estados Unidos. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina

**Author affiliation**: Rosenberg, Duane. National Center for Atmospheric Research; Estados Unidos

**Repository:** CONICET Digital (CONICET). Consejo Nacional de Investigaciones Científicas y Técnicas