Magnetic Helicity Content in Solar Wind Flux Ropes

DASSO, S.

Universal Heliophysical Processes

Cambridge University Press

Año: 2009; p. 379 - 389

Magnetic helicity (H) is an ideal magnetohydrodynamical (MHD) invariant thatquantifies the twist and linkage of magnetic field lines. In magnetofluids with low resistivity, Hdecays much less than the energy, and it is almost conserved during times shorter than the globaldiffusion timescale. The extended solar corona (i.e., the heliosphere) is one of the physical scenarioswhere H is expected to be conserved. The amount of H injected through the photosphericlevel can be reorganized in the corona, and finally ejected in flux ropes to the interplanetarymedium. Thus, coronal mass ejections can appear as magnetic clouds (MCs), which are hugetwisted flux tubes that transport large amounts of H through the solar wind. The content ofH depends on the global configuration of the structure, then, one of the main difficulties toestimate it from single spacecraft in situ observations (one point - multiple times) is that asingle spacecraft can only observe a linear (one dimensional) cut of the MC global structure.Another serious difficulty is the intrinsic mixing between its spatial shape and its time evolutionthat occurs during the observation period. However, using some simple assumptions supportedby observations, the global shape of some MCs can be unveiled, and the associated H and magneticfluxes (F) can be estimated. Different methods to quantify H and F from the analysisof in situ observations in MCs are presented in this review. Some of these methods consider aMC in expansion and going through possible magnetic reconnections with its environment. Weconclude that H seems to be a ‘robust’ MHD quantity in MCs, in the sense that variations ofH for a given MC deduced using different methods, are typically lower than changes of H whena different cloud is considered. Quantification of H and F lets us constrain models of coronalformation and ejection of flux ropes to the interplanetary medium, as well as of the dynamicalevolution of MCs in the solar wind.