Magnetohydrodynamic turbulent evolution of a magnetic cloud in the outer heliosphere

Abstract

This Letter exploits joint observations of the same interplanetary coronal mass ejection by widely separated spacecraft to study, for the first time, the turbulent evolution of the magnetohydrodynamic (MHD) properties of the embedded magnetic cloud, during its propagation throughout interplanetary space. Specifically, the event was observed by Wind at 1 au on 1998 March 4-6 and tracked to the location of Ulysses at 5.4 au 18 days later, when the two spacecraft were radially aligned with the Sun. The analysis of the MHD invariants within the magnetic cloud, along with its energy budget, provides compelling evidence of magnetic erosion of the structure thanks to its interaction with a trailing magnetic cloud. The helical configuration is thus largely deformed and degraded, and the initial dominance of magnetic over kinetic energy is observed to evolve toward a less imbalanced condition. This is consistent with the expected conversion of magnetic energy into kinetic energy due to magnetic reconnection processes. Local interaction of the magnetic cloud's (MC) outer layers with the solar wind acts to generate larger amplitude Alfvénic fluctuations in the downstream region, leading the MC to turbulently evolve toward a more complex cross-helicity configuration in the outer heliosphere. Finally, evidence of a flux rope locally generated by magnetic reconnection events at 1 au that likely decays by the time it reaches Ulysses is also reported.