Structure of interplanetary magnetic cloud on April 16, 1999 and its origin estimated by fitting the torus-shaped flux rope model

We examined the relationship between interplanetary magnetic cloud observed by the ACE satellite on April 16, 1999 and its solar origin. The solar and interplanetary background was relatively quiet for 10 clear days before April 16, enabling us to unambiguously assume the solar origin of the magnetic cloud. However, the results of fitting a constant-alpha force-free cylindrical flux rope model is inconsistent with findings from previous studies about magnetic clouds and their solar origins, although corresponding solar surface phenomena occurred near the central meridian. We therefore attempted to fit another model with the torus-shaped magnetic field structure, which is a simple extension of the conventional cylindrical model with more geometrical flexibility. Using the estimated planar structure of the flux rope, we were able to interpret reasonably the observed magnetic cloud as an expansion into interplanetary space of a filament eruption or halo CME (coronal mass ejection) on April 13, 1999.