Interplanetary evolution of eleven magnetic clouds (MCs) recorded by at least
two radially aligned spacecraft is studied. The in situ magnetic field
measurements are fitted to a cylindrically symmetric Gold-Hoyle force-free
uniform-twist flux-rope configuration. The analysis reveals that in a
statistical sense the expansion of studied MCs is compatible with self-similar
behavior. However, individual events expose a large scatter of expansion rates,
ranging from very weak to very strong expansion. Individually, only four events
show an expansion rate compatible with the isotropic self-similar expansion.
The results indicate that the expansion has to be much stronger when MCs are
still close to the Sun than in the studied 0.47 - 4.8 AU distance range. The
evolution of the magnetic field strength shows a large deviation from the
behavior expected for the case of an isotropic self-similar expansion. In the
statistical sense, as well as in most of the individual events, the inferred
magnetic field decreases much slower than expected. Only three events show a
behavior compatible with a self-similar expansion. There is also a discrepancy
between the magnetic field decrease and the increase of the MC size, indicating
that magnetic reconnection and geometrical deformations play a significant role
in the MC evolution. About half of the events show a decay of the electric
current as expected for the self-similar expansion. Statistically, the inferred
axial magnetic flux is broadly consistent with it remaining constant. However,
events characterized by large magnetic flux show a clear tendency of decreasing
flux.
