Causes and Consequences of Magnetic Cloud Expansion

DEMOULIN, P.; DASSO S.

ASTRONOMY AND ASTROPHYSICS

Año: 2008

0004-6361

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 of their size with solar distance demonstrate that MCs are strongly expanding structures. The aim of this work is to find the principal causes of this expansion and to derive a model toexplain 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 fluxropes in order to reduce the problem to the integration of ordinary differential equations. This allows us to explore a large variety of magnetic fields at a broad range of distances to the Sun.Results. We demonstrate that the rapid decrease of the total SW pressure with solar distance is the main driver of the flux-rope radial expansion. Other effects, such as internal over-pressure, the amount of twist, and its radial distribution within the flux rope, have muchweaker influence on the expansion. We demonstrate that any force-free flux rope will have a self similar expansion if its boundary total 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 be self similar with a nearly linear radial velocity profile across the flux rope, as observed. More over, we show that the expansion rate is proportional to the radius and to the global velocity away from the Sun, and that the model can explain the linear profile of velocities typically observed in MCs.Conclusions. The simple and universal law found for the radial expansion of flux ropes in the SWprovides a prediction of the typical size, magnetic structure, and radial velocity of MCs at various solar distances.