Dynamical evolution of a magnetic cloud from the Sun to 5.4 AU

NAKWACKI M.S.; DASSO S.; DEMOULIN P.; MANDRINI C.H.; GULISANO A.M.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2011 vol. 535 p. 52 - 62

0004-6361

Context. Significant quantities of magnetized plasma are transported from the Sun to the interstellarmedium via interplanetary coronalmass ejections (ICMEs). Magnetic clouds (MCs) are a particular subset of ICMEs, forming large-scale magnetic flux ropes. Theirevolution in the solar wind is complex and mainly determined by their own magnetic forces and the interaction with the surroundingsolar wind.Aims. Magnetic clouds are strongly affected by the surrounding environment as they evolve in the solar wind. We study expansion ofMCs, its consequent decrease in magnetic field intensity and mass density, and the possible evolution of the so-called global ideal-MHD invariants.Methods. In this work we analyze the evolution of a particular MC (observed in March 1998) using in situ observations made bytwo spacecraft approximately aligned with the Sun, the first one at 1 AU from the Sun and the second one at 5.4 AU. We describethe magnetic configuration of the MC using different models and compute relevant global quantities (magnetic fluxes, helicity, andenergy) at both heliodistances. We also tracked this structure back to the Sun, to find out its solar source.Results. We find that the flux rope is significantly distorted at 5.4 AU. From the observed decay of magnetic field and mass density,we quantify how anisotropic is the expansion and the consequent deformation of the flux rope in favor of a cross section with anaspect ratio at 5.4 AU of ≈1.6 (larger in the direction perpendicular to the radial direction from the Sun). We quantify the ideal-MHDinvariants and magnetic energy at both locations, and find that invariants are almost conserved, while the magnetic energy decays asexpected with the expansion rate found.Conclusions. The use of MHD invariants to link structures at the Sun and the interplanetary medium is supported by the results ofthis multi-spacecraft study.We also conclude that the local dimensionless expansion rate, which is computed from the velocity profileobserved by a single-spacecraft, is very accurate for predicting the evolution of flux ropes in the solar wind.