Causes and Consequences of Magnetic Cloud Expansion

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 magnetohydrodynamics (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.Fil: Démoulin, Pascal. Centre National de la Recherche Scientifique. Observatoire de Paris; FranciaFil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin

Forecast of solar ejecta arrival at 1 AU from radial speed

Las eyecciones transitorias de masa solar (EMS) pueden producir cambios en el campo geomagnético. Cuando la polaridad magnética de la EMS es adecuada, puede disparar intensas tormentas geomagnéticas. La predicción de la llegada de EMS desde el Sol al geoespacio tiene una importancia crucial para poder predecir el clima espacial. En este trabajo implementamos un modelo simple, desarrollado por Gopalswamy et al., 2000 para estimar el tiempo de llegada de EMS a una Unidad Astronómica. Este modelo requiere sólo un parámetro de entrada: la velocidad radial de la EMS en el momento de su expulsión desde el Sol. Cuando la velocidad de la EMS es medida desde una posición dentro de la línea Sol-Tierra, sólo la componente de la velocidad en el plano del cielo puede ser obtenida. Debido a que la predicción del modelo depende de la velocidad inicial de la EMS observada remotamente, es muy importante obtener esta velocidad lo más exactamente posible. Una de las mayores incertezas cuando se mide la velocidad inicial de la EMS es el efecto de proyección. El objetivo de este trabajo es corregir efectos de proyección a partir de la localización en la superficie solar de la erupción y del tamaño de apertura de la EMS. Encontramos que la corrección desarrollada acuerda con un modelo obtenido en observaciones estereoscópicas en el pasado.Solar ejecta produce changes in the interplanetary magnetic field of the terrestrial environment. When the magnetic polarity of the ejecta is suitable, it may trigger intense geomagnetic storms. Therefore, prediction of the arrival of solar ejecta in the geospace is of crucial importance for space weather applications. We implement a simple model, developed by Gopalswamy et al., (2000) to estimate the time of arrival for solar ejecta at 1AU. This model requires just one input parameter: the radial speed of the associated coronal mass ejection (CME) at the moment of its expulsion from the Sun. When the speed of the CME is measured from a location on the Sun-Earth line, only the plane of the sky speed can be obtained. Since the prediction model depends on the initial speed of the CMEs observed remotely, it is important to obtain this speed as accurately as possible. One of the major uncertainties in the measured initial speed is the extent of projection effects. We attempt to correct for projection effects using the solar surface location of the eruption and assuming a width to the CME. We found that the correction is in agreement with a model obtained from stereoscopic observations from the past.Fil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Gopalswamy, N.. National Aeronautics And Space Administration. Goddart Institute For Space Studies; Estados UnidosFil: Lara, A.. Universidad Nacional Autónoma de México; Méxic

Analysis of large scale MHD quantities in expanding magnetic clouds

Magnetic clouds (MCs) transport the magnetic flux and helicity released by the Sun. They are generally modeled as a static flux rope traveling in the solar wind, though they can present signatures of expansion. We analyze three expanding MCs using a self-similar free radial expansion model with a cylindrical linear force-free field (i.e., Lundquist solution) as the initial condition. We derive expressions for the magnetic fluxes, the magnetic helicity and the magnetic energy per unit length along the flux tube. We find that these quantities do not differ more than 25% when using the static or expansion model.Fil: Nakwacki, Maria Soledad. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Mandrini, Cristina Hemilse. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Démoulin, Pascal. Centre National de la Recherche Scientifique. Observatoire de Paris; Franci

Comparing generic models for interplanetary shocks and magnetic clouds axis configurations at 1 AU

Interplanetary coronal mass ejections (ICMEs) are the manifestation of solar transient eruptions, which can significantly modify the plasma and magnetic conditions in the heliosphere. They are often preceded by a shock, and a magnetic flux rope is detected in situ in a third to half of them. The main aim of this study is to obtain the best quantitative shape for the flux rope axis and for the shock surface from in situ data obtained during spacecraft crossings of these structures. We first compare the orientation of the flux rope axes and shock normals obtained from independent data analyses of the same events, observed in situ at 1 AU from the Sun. Then we carry out an original statistical analysis of axes/shock normals by deriving the statistical distributions of their orientations. We fit the observed distributions using the distributions derived from several synthetic models describing these shapes. We show that the distributions of axis/shock orientations are very sensitive to their respective shape. One classical model, used to analyze interplanetary imager data, is incompatible with the in situ data. Two other models are introduced, for which the results for axis and shock normals lead to very similar shapes; the fact that the data for MCs and shocks are independent strengthens this result. The model which best fits all the data sets has an ellipsoidal shape with similar aspect ratio values for all the data sets. These derived shapes for the flux rope axis and shock surface have several potential applications. First, these shapes can be used to construct a consistent ICME model. Second, these generic shapes can be used to develop a quantitative model to analyze imager data, as well as constraining the output of numerical simulations of ICMEs. Finally, they will have implications for space weather forecasting, in particular, for forecasting the time arrival of ICMEs at the Earth.Fil: Janvier, Miho. University of Dundee; Reino UnidoFil: Dasso, Sergio Ricardo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Démoulin, Pascal. Centre National de la Recherche Scientifique. Observatoire de Paris; FranciaFil: Masías Meza, Jimmy Joel. 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; ArgentinaFil: Lugaz, Noé. University Of New Hampshire; Estados Unido

Von Karman correlation similarity in solar wind magnetohydrodynamic turbulence

A major development underlying hydrodynamic turbulence theory is the similarity decay hypothesis due to von Karman and Howarth, here extended empirically to plasma turbulence in the solar wind. In similarity decay the second-order correlation experiences a continuous transformation based on a universal functional form and a rescaling of energy and characteristic length. Solar wind turbulence follows many principles adapted from classical fluid turbulence, but previously this similarity property has not been examined explicitly. Here, we analyze an ensemble of Elsässer autocorrelation functions computed from Advanced Composition Explorer data at 1 astronomical unit (AU), and demonstrate explicitly that the two-point correlation functions undergo a collapse to a similarity form of the type anticipated from von Karman's hypothesis applied to weakly compressive magnetohydrodynamic turbulence. This provides a firm empirical basis for employing the similarity decay hypothesis to the Elsässer correlations that represent the incompressive turbulence cascade. This approach is of substantial utility in space turbulence data analysis, and for adopting von Karman-type heating rates in global and subgrid-scale dynamical modeling.Fil: Roy, Sohom. University Of Delaware; Estados UnidosFil: Chhiber, R.. University Of Delaware; Estados UnidosFil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Ruiz, Maria Emilia. Ministerio de Defensa. Secretaria de Planeamiento. Servicio Meteorológico Nacional; Argentina. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Matthaeus, W .H.. University Of Delaware; Estados Unido

Taylor Scale and Effective Magnetic Reynolds Number Determination from Plasma Sheet and Solar Wind Magnetic Field Fluctuations

[1] Cluster data from many different intervals in the magnetospheric plasmas sheet and the solar wind are employed to determine the magnetic Taylor microscale from simultaneous multiple point measurements. For this study we define the Taylor scale as the square root of the ratio of the mean square magnetic field (or velocity) fluctuations to the mean square spatial derivatives of their fluctuations. The Taylor scale may be used, in the assumption of a classical Ohmic dissipation function, to estimate effective magnetic Reynolds numbers, as well as other properties of the small scale turbulence. Using solar wind magnetic field data, we have determined a Taylor scale value of 2400 ± 100 km, which is used to obtain an effective magnetic Reynolds number of about 260,000 ± 20,000, and in the plasma sheet we calculated a Taylor scale of 1900 ± 100 km, which allowed us to obtain effective magnetic Reynolds numbers in the range of about 7 to 110. The present determination makes use of a novel extrapolation technique to derive a statistically stable estimate from a range of small scale measurements. These results may be useful in magnetohydrodynamic modeling of the solar wind and the magnetosphere and may provide constraints on kinetic theories of dissipation in space plasmas.Fil: Weygand, James M.. University of California; Estados UnidosFil: Matthaeus, W. H.. University of Delaware; Estados UnidosFil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Kivelson, M. G.. University of California; Estados UnidosFil: Walker, R. J.. University of California; Estados Unido

Anisotropy of the Taylor scale and the correlation scale in plasma sheet magnetic field fluctuations as a function of auroral electrojet activity

Magnetic field data from the Cluster spacecraft in the magnetospheric plasma sheet are employed to determine the correlation scale and the magnetic Taylor microscale from simultaneous multiple‐point measurements for multiple intervals over a range of mean magnetic field directions for three different levels of geomagnetic activity. We have determined that in the plasma sheet the correlation scale along the mean magnetic field direction decreases from 19,500 ± 2200 to 13,100 ± 700 km as the auroral electrojet activity increases from quiet (200 nT). The reverse occurs for the correlation scale perpendicular to the magnetic field, which increases from 8200 ± 600 km to 13,000 ± 2100 km as the auroral electrojet activity increases from quiet to active conditions. This variation of the correlation scale with geomagnetic activity may mean either a change in the scale size of the turbulence driver or may mean a change in the predominance of one over another type of turbulence driving mechanism. Unlike the correlation scale, the Taylor scale does not show any clear variation with geomagnetic activity. We find that the Taylor scale is longer parallel to the magnetic field than perpendicular to it for all levels of geomagnetic activity. The correlation and Taylor scales may be used to estimate the effective magnetic Reynolds numbers separately for each angular channel. Reynolds numbers were found to be approximately independent of the angle relative to the mean magnetic field. These results may be useful in magnetohydrodynamic modeling of the magnetosphere and can contribute to our understanding of energetic particle diffusion in the magnetosphere.Fil: Weygand, James M.. University of California; Estados UnidosFil: Matthaeus, W. H.. University of Delaware; Estados UnidosFil: El Alaoui, M.. University of California; Estados UnidosFil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Kivelson, M. G.. University of California; Estados Unido

Eulerian decorrelation of fluctuations in the interplanetary magnetic field

A method is devised for estimating the two-time correlation function and the associated Eulerian decorrelation timescale in turbulence. With the assumptions of a single decorrelation time and a frozen-in flow approximation for the single-point analysis, the method compares two-point correlation measurements with single-point correlation measurements at the corresponding spatial lag. This method is applied to interplanetary magnetic field measurements from the Advanced Composition Explorer and Wind spacecraft. An average Eulerian decorrelation time of 2.9 hr is found. This measures the total rate of distortion of turbulent fluid elements—including sweeping, nonlinear distortion, and wave propagation.Fil: Matthaeus, W. H.. University of Delaware; Estados UnidosFil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Weygand, J. M.. University of California; Estados UnidosFil: Kivelson, M. G.. University of California; Estados UnidosFil: Osman, K. T.. University of Delaware; Estados Unido

Anisotropy of the Taylor Scale and the Correlation Scale in Plasma Sheet and Solar Wind Magnetic Field Fluctuations

Magnetic field data from nine spacecraft in the magnetospheric plasma sheet and the solar wind are employed to determine the correlation scale and the magnetic Taylor microscale from simultaneous multiple-point measurements for multiple intervals with a range of mean magnetic field directions. We have determined that in the solar wind the Taylor scale is independent of direction relative to the mean magnetic field, but the correlation scale along the mean magnetic field (2.7 106 ± 0.2 106 km) is longer than along the perpendicular direction (1.5 106 ± 0.1 106 km). Within the plasma sheet we found that the correlation scale varies from 16,400 ± 1000 km along the mean magnetic field direction to 9200 ± 600 km in the perpendicular direction. The Taylor scale is also longer parallel to the magnetic field (2900 ± 100 km) than perpendicular to it (1100 ± 100 km). In the solar wind the ratio of the parallel correlation scale to the perpendicular correlation scale is 2.62 ± 0.79; in the plasma sheet the ratio is 1.78 ± 0.16, which indicates that the turbulence in both regions is anisotropic. The correlation and Taylor scales may be used to estimate effective magnetic Reynolds numbers separately for each angular channel. Reynolds numbers were found to be approximately independent of the angle relative to the mean magnetic field. These results may be useful in magnetohydrodynamic modeling of the solar wind and the magnetosphere and can contribute to our understanding of solar and galactic cosmic ray diffusion in the heliosphere.Fil: Weygand, James M.. University of California; Estados UnidosFil: Matthaeus, W. H.. University of Delaware; Estados UnidosFil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Kivelson, M.G.. University of California; Estados UnidosFil: Kistler, L. M.. University of New Hampshire; Estados UnidosFil: Mouikis, C.. University of New Hampshire; Estados Unido

Technique for measuring and correcting the Taylor microscale

We discuss and develop methods to estimate and refine measurements of the Taylor microscale from discrete data sets. To study how well a method works, we construct a time series of discrete data with a known power spectrum and Taylor scale, but with various truncations of the resolution that eliminate higher frequencies in a controlled fashion. We compute the second-order structure function and correlation function, assuming that the unresolved dissipation range spectrum has various values of spectral index. A series of Taylor scale estimates are obtained from parabolic fits to subsets of the correlation function data, and these are extrapolated to the limit of zero separation. The error in this procedure, for finite time resolution sampling, depends on the spectral index in the dissipation range. When the spectral form is known, we can compute a correction factor that improves the estimate of the Taylor microscale value determined from the extrapolation method and band-limited data. Application of this technique to spacecraft observations of solar wind fluctuations is illustrated.Fil: Chuychai, P.. Mae Fah Luang University; Estados Unidos. Ministry of Education; TailandiaFil: Weygand, J. M.. University of California; Estados UnidosFil: Matthaeus, W. H.. University of Delaware; Estados UnidosFil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomia y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomia y Física del Espacio; ArgentinaFil: Smith, C. W.. University of New Hampshire; Estados UnidosFil: Kivelson, M. G.. University of California; Estados Unido