Causes and consequences of magnetic cloud expansion

Démoulin, P.; <div class="autor_fcen" id="2288">Dasso, S.</div>
Publication Year
Published version
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.
Fil:Dasso, S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Astron. Astrophys. 2009;498(2):551-566
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
Solar eruption
Sun: coronal mass ejections (CMEs)
Sun: magnetic fields
Boundary layer flow
Energy conservation
Fluid dynamics
Magnetic fields
Magnetic flux
Magnetic structure
Ordinary differential equations
Pressure gradient
Solar wind
Velocity distribution
Solar energy
Access level
Open access
Biblioteca Digital (UBA-FCEN)
Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales
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