Magnetic Helicity Content in Solar Wind Flux Ropes

Sergio Dasso

Instituto de Astronomía y Física del Espacio (IAFE, UBA/CONICET) and Departamento de Física, Universidad de Buenos Aires, Buenos Aires, Argentina

Magnetic helicity (H) is an ideal MHD invariant that quantifies the twist and linkage of magnetic field lines. In magnetofluids of low resistivity H decays less than the energy, and it is almost conserved during times lower than the global diffusion timescale. The extended solar corona (i.e., the heliosphere) is one of the physical scenarios where H is expected to be conserved. The amount of H injected from the photospheric level can be reorganized at the corona, and finally ejected as flux ropes to the interplanetary medium. Thus, coronal mass ejections can be manifested as magnetic clouds (MCs), which are huge twisted flux tubes that transport large amounts of H through the solar wind. Quantification of H in MCs is one of the keys for linking them to their solar sources and tracking them along the heliosphere. Due to the content of H depends on the global configuration of the structure, one of the main difficulties to estimate it from single spacecraft (one point - multiple times) in situ observations is that a single 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 evolution, produced during the time of observation. Different methods to quantify H and magnetic fluxes from the analysis of in situ observations in MCs will be presented and revised in this talk. Some of these methods consider a MC in expansion and possible magnetic reconnection with its environment. From these methods it is possible to improve the quantification of H and fluxes in MCs; under the analyzed aspects, H in MCs seems to be a robust MHD quantity, in the sense that variations of H for a given MC when changing the method, are typically lower than changes of H when the cloud is changed. Quantification of H and magnetic fluxes let us to put constraints to models of coronal formation and ejection of flux ropes to the interplanetary medium, as well as for the dynamical evolution of MCs in the SW.