Solar cycle–dependent helicity transport by magnetic clouds

Lynch, B. J.; Gruesbeck, J.; Zurbuchen, T. H.; Antiochos, S. K.

doi:10.1029/2005ja011137

Cited by 133 publications

(173 citation statements)

References 66 publications

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“…Understanding their origin and propagation is therefore key, not only for the Earth but also for understanding how the Sun loses both magnetic flux and magnetic helicity (Low 1996;Lynch et al 2005). Over the years, a wide variety of models have been put forward to explain the origin of CMEs, and a discussion of these can be seen in the reviews of Forbes et al (2006) and Chen (2011).…”

Section: Introductionmentioning

confidence: 99%

Magnetohydrodynamic simulations of the ejection of a magnetic flux rope

Pagano

Mackay

Poedts

2013

A&A

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Context. Coronal mass ejections (CME's) are one of the most violent phenomena found on the Sun. One model to explain their occurrence is the flux rope ejection model. In this model, magnetic flux ropes form slowly over time periods of days to weeks. They then lose equilibrium and are ejected from the solar corona over a few hours. The contrasting time scales of formation and ejection pose a serious problem for numerical simulations. Aims. We simulate the whole life span of a flux rope from slow formation to rapid ejection and investigate whether magnetic flux ropes formed from a continuous magnetic field distribution, during a quasi-static evolution, can erupt to produce a CME. Methods. To model the full life span of magnetic flux ropes we couple two models. The global non-linear force-free field (GNLFFF) evolution model is used to follow the quasi-static formation of a flux rope. The MHD code ARMVAC is used to simulate the production of a CME through the loss of equilibrium and ejection of this flux rope. Results. We show that the two distinct models may be successfully coupled and that the flux rope is ejected out of our simulation box, where the outer boundary is placed at 2.5 R . The plasma expelled during the flux rope ejection travels outward at a speed of 100 km s −1 , which is consistent with the observed speed of CMEs in the low corona. Conclusions. Our work shows that flux ropes formed in the GNLFFF can lead to the ejection of a mass loaded magnetic flux rope in full MHD simulations. Coupling the two distinct models opens up a new avenue of research to investigate phenomena where different phases of their evolution occur on drastically different time scales.

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Section: Introductionmentioning

confidence: 99%

Magnetohydrodynamic simulations of the ejection of a magnetic flux rope

Pagano

Mackay

Poedts

2013

A&A

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“…The magnetic field in MCs can be relatively well modeled by the so-called Lundquist model (Lundquist 1950), which considers a static and axially-symmetric linear force-free magnetic configuration (e.g., Burlaga 1988;Lepping et al 1990;Lynch et al 2003). Alternately, Farrugia et al (1999) considered a cylindrical shape for the cloud cross-section and a non-linear force-free field, while Mulligan et al (1999), Hidalgo et al (2002), and Cid et al (2002) have considered a cylindrical cloud with a finite plasma pressure.…”

Section: Introductionmentioning

confidence: 99%

Causes and consequences of magnetic cloud expansion

Démoulin¹,

Dasso

2009

A&A

143

191

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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.

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“…The simplest solution is obtained with a cylindrical boundary; this is the so-called Lundquist model (Lundquist 1950). It was, and is still, widely used to fit the magnetic field observed in MCs and to derive global quantities such as the magnetic flux and helicity (e.g., Burlaga 1988;Lepping et al 1990;Dasso et al 2003;Lynch et al 2003;Dasso et al 2005b;Mandrini et al 2005;Dasso et al 2006;Leitner et al 2007). An extension of this model to an elliptical boundary was realized by Vandas & Romashets (2003).…”

Section: Introductionmentioning

confidence: 99%

Magnetic cloud models with bent and oblate cross-section boundaries

Démoulin¹,

Dasso

2009

A&A

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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.

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Solar cycle–dependent helicity transport by magnetic clouds

Cited by 133 publications

References 66 publications

Magnetohydrodynamic simulations of the ejection of a magnetic flux rope

Magnetohydrodynamic simulations of the ejection of a magnetic flux rope

Causes and consequences of magnetic cloud expansion

Magnetic cloud models with bent and oblate cross-section boundaries

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