Direct observation of large scale magnetohydrodynamic quantities in magnetic clouds

DASSO, S.; GULISANO, A.M.; MANDRINI, C.H.; DÉMOULIN, P.

París

Congreso; 35th COSPAR Scientific Assembly; 2004

COSPAR

Magnetic Clouds (MCs) are the interplanetary manifestation of Coronal Mass Ejections (CMEs). Despite these astrophysical objects have been observed for more than 20 years, several details of their magnetic configuration at 1AU, as consequence of their dynamical evolution through the inner heliosphere, are not well known. The magnetic flux (F) and helicity (MH) are very important magnetohydrodynamic (MHD) quantities characterizing the magnetic configuration in these objects, and there are very few studies quantifying their typical values. MH measures several aspects of a given magnetic structure, and it is practically conserved in the solar atmosphere and the heliosphere. MCs travel from the Sun toward the external heliosphere carrying an important amount of MH. In situ observations of MCs at 1 AU agree with a local helical magnetic structure. However, since spacecrafts only register data along a unique direction, several aspects of the global configuration are not observed. We select a set of MCs observed by the spacecraft Wind and we analyze them, obtaining values for F and MH directly from the observations. We apply first the Minimum Variance method to obtain the components of the magnetic field in the coordinates of the cloud, then we apply a method to calculate the MH and the F from these components, assuming only a cylindrical symmetry for the magnetic field configuration in the cloud. We compare the results obtained from our direct method with that obtained under the assumption of a helical linear force-free field (i.e., the Lundquist's solution). Our direct method is useful to compare the MH content in a given MC, with the MH variation in the solar corona before and after the associated CME.