Heliospheric Evolution of Magnetic Clouds

The interplanetary evolution of 11 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 the 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 isotropic self-similar expansion. The results indicate that the expansion has to be much stronger when the 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 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 behavior compatible with 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 self-similar expansion. Statistically, the inferred axial magnetic flux is broadly consistent with remaining constant. However, events characterized by a large magnetic flux show a clear tendency toward decreasing flux.