**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="4119">Gulisano, A.M.</div>; Démoulin, P.; <div class="autor_fcen" id="2288">Dasso, S.</div>; <div class="autor_fcen" id="7602">Ruiz, M.E.</div>; Marsch, E.

**Publication Date:** 2010.

**Language:** English.

**Abstract:**

Context: Observations of magnetic clouds (MCs) are consistent with the presence of flux ropes detected in the solar wind (SW) a few days after their expulsion from the Sun as coronal mass ejections (CMEs). Aims: Both the in situ observations of plasma velocity profiles and the increase of their size with solar distance show that MCs are typically expanding structures. The aim of this work is to derive the expansion properties of MCs in the inner heliosphere from 0.3 to 1 AU. Methods: We analyze MCs observed by the two Helios spacecraft using in situ magnetic field and velocity measurements. We split the sample in two subsets: those MCs with a velocity profile that is significantly perturbed from the expected linear profile and those that are not. From the slope of the in situ measured bulk velocity along the Sun-Earth direction, we compute an expansion speed with respect to the cloud center for each of the analyzed MCs. Results: We analyze how the expansion speed depends on the MC size, the translation velocity, and the heliocentric distance, finding that allMCs in the subset of non-perturbed MCs expand with almost the same non-dimensional expansion rate (ζ).We find departures from this general rule for ζ only for perturbed MCs, and we interpret the departures as the consequence of a local and strong SW perturbation by SW fast streams, affecting the MC even inside its interior, in addition to the direct interaction region between the SW and the MC. We also compute the dependence of the mean total SW pressure on the solar distance and we confirm that the decrease of the total SW pressure with distance is the main origin of the observed MC expansion rate. We found that ζ was 0.91 ± 0.23 for non-perturbed MCs while ζ was 0.48 ± 0.79 for perturbed MCs, the larger spread in the last ones being due to the influence of the solar wind local environment conditions on the expansion. © ESO 2010.

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

**Keywords:**
Coronal mass ejections (CMEs) - Solar wind - Interplanetary medium; Magnetic fields - Magnetohydrodynamics (MHD) - Sun; Coronal mass ejection; Coronal mass ejections (CMEs) - Solar wind - Interplanetary medium; Interplanetary medium; Magnetohydrodynamics suns; Astrophysics; Boundary layer flow; Clouds; Interplanetary spacecraft; Magnetic fields; Magnetohydrodynamics; Semiconductor counters; Solar wind; Sun; Velocity; Velocity measurement; Expansion.

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

**Publication Date:** 2009.

**Language:** English.

**Abstract:**

Context. Magnetic clouds (MCs) are formed by magnetic flux ropes that are ejected from the Sun as coronal mass ejections. These structures generally have low plasma beta and travel through the interplanetary medium interacting with the surrounding solar wind. Thus, the dynamical evolution of the internal magnetic structure of a MC is a consequence of both the conditions of its environment and of its own dynamical laws, which are mainly dominated by magnetic forces.Aims. With in-situ observations the magnetic field is only measured along the trajectory of the spacecraft across the MC. Therefore, a magnetic model is needed to reconstruct the magnetic configuration of the encountered MC. The main aim of the present work is to extend the widely used cylindrical model to arbitrary cross-section shapes.Methods. The flux rope boundary is parametrized to account for a broad range of shapes. Then, the internal structure of the flux rope is computed by expressing the magnetic field as a series of modes of a linear force-free field.Results. We analyze the magnetic field profile along straight cuts through the flux rope, in order to simulate the spacecraft crossing through a MC. We find that the magnetic field orientation is only weakly affected by the shape of the MC boundary. Therefore, the MC axis can approximately be found by the typical methods previously used (e.g., minimum variance). The boundary shape affects the magnetic field strength most. The measurement of how much the field strength peaks along the crossing provides an estimation of the aspect ratio of the flux-rope cross-section. The asymmetry of the field strength between the front and the back of the MC, after correcting for the time evolution (i.e., its aging during the observation of the MC), provides an estimation of the cross-section global bending. A flat or/and bent cross-section requires a large anisotropy of the total pressure imposed at the MC boundary by the surrounding medium.Conclusions. The new theoretical model developed here relaxes the cylindrical symmetry hypothesis. It is designed to estimate the cross-section shape of the flux rope using the in-situ data of one spacecraft. This allows a more accurate determination of the global quantities, such as magnetic fluxes and helicity. These quantities are especially important for both linking an observed MC to its solar source and for understanding the corresponding evolution. © 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; Arbitrary cross section; Boundary shapes; Coronal mass ejection; Cylindrical models; Cylindrical symmetry; Dynamical evolution; Field strengths; Flux ropes; Global quantities; Helicities; In-situ data; In-situ observations; Internal structure; Interplanetary medium; Large anisotropy; Magnetic clouds; Magnetic configuration; Magnetic field orientations; Magnetic field profile; Magnetic field strengths; Magnetic flux ropes; Magnetic models; Minimum variance; Solar source; Sun: coronal mass ejection; Sun: magnetic field; Theoretical models; Time evolutions; Total pressure; Aspect ratio; Astrophysics; Boundary layer flow; Interplanetary spacecraft; Magnetic fields; Magnetic flux; Magnetic structure; Planetary surface analysis; Solar wind; Sun; Semiconductor counters.

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

**Authors**:
Poitevin, Augusto; Natalini, Bruno; Godoy, Luis Augusto

**Publication Date:** 2013.

**Language:** English.

**Abstract:**

Pressure coefficients due to wind in open canopy structures with parapets, such as those commonly used in gas stations, are presented. Two independent methodologies were used to obtain pressure coefficients: wind tunnel testing and Computational Fluid Dynamic (CFD) modeling. The experiments were performed in a low velocity atmospheric boundary layer wind tunnel. The atmospheric boundary layer flow was simulated in the tunnel and a small scale model of a canopy made with aluminum plates was tested considering wind incidence at 0 (normal-to-the ridge direction) and 30. Pressures were measured at a number of locations on the plate and the parapets. CFD models were investigated under steady state conditions using commercial software. The wind tunnel results for a canopy without parapets were found to be in agreement with results obtained by other authors. The CFD simulations were in good agreement with the wind tunnel results for canopies with parapets. The values of the net pressure coefficients on the main surface show downward pressures close to the windward zone, followed by suction at the center, and downward pressure on the leeward zone of the canopy. Differences with current recommendations for the design of canopies are highlighted.

**Author affiliation**: Poitevin, Augusto. Chm Structural Engineers; Estados Unidos

**Author affiliation**: Natalini, Bruno. Universidad Nacional del Nordeste. Facultad de Ingenieria; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

**Author affiliation**: Godoy, Luis Augusto. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

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