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Expansion of magnetic clouds in the outer heliosphere View all abstracts by submitter

Pascal Demoulin   Submitted: 2012-04-17 09:55

A large amount of magnetized plasma are frequently ejected from the Sun as Coronal Mass Ejections (CMEs). A part of these ejections are detected in the solar wind as magnetic clouds (MCs) which have flux rope signatures. MCs are typically expanding structures in the inner heliosphere. The aim of this work is to derive the expansion properties of MCs in the outer heliosphere from 1 to 5 AU and to compare them to the ones in the inner heliosphere. We analyze MCs observed by the Ulysses spacecraft using in situ magnetic field and plasma measurements. The MC boundaries are defined in the MC frame after defining the MC axis with a minimum variance method applied only to the flux rope structure. As in the inner heliosphere, a large fraction of the velocity profile within MCs is close to a linear function of time. This implies a self-similar expansion and a MC size that locally follows a power-law of the solar distance with an exponent called zeta. We derive the value of zeta from the in situ velocity data. We analyze separately the non-perturbed MCs (cases presenting a linear velocity profile almost for the full event), and perturbed MCs (cases presenting a strongly distorted velocity profile). We find that non-perturbed MCs expand with a similar non-dimensional expansion rate (zeta = 1.05 ± 0.34), i.e. slightly faster than the solar distance and than in the inner heliosphere (zeta = 0.91± 0.23). The subset of perturbed MCs expands, as in the inner heliosphere, with a significant lower rate and with a larger dispersion (zeta = 0.28 ± 0.52) as expected from the temporal evolution found in numerical simulations. This local measure of the expansion is also in agreement with the distribution with distance of MC size, mean magnetic field and plasma parameters. The MCs in interaction with a strong field region, e.g. another MC, have the most variable expansion rate (ranging from compression to over-expansion).

Authors: A.M. Gulisano, P. Demoulin, S. Dasso, L. Rodriguez
Projects: None

Publication Status: in press, A&A
Last Modified: 2012-04-17 13:54
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