November 17-18, 1975, event: A clue to an internal structure of magnetic clouds?

During November 17-18, 1975, a magnetic cloud was observed by the IMP 8 sat ellite. The cloud was analyzed in several papers. It draws attention becaus e it is the most clear example where the magnetic field components behave d ifferently from the current single flux rope model. Various models and fits have been presented to explain the magnetic field measurements for this pa rticular event: single-polarity cylindrical flux rope, spheromak, toroidal flux rope, and two subsequent flux ropes (flux rope twins). We critically e xamine these models and fits and stress that not only magnetic field data b ut also plasma data must be taken into account. There is a remarkably sharp drop in the density inside the magnetic cloud. The most consistent explana tion of the behavior of magnetic field and plasma data for this event is th at the magnetic cloud consists of a dual-polarity flux rope with a low dens ity and strong magnetic field core surrounded by an annular region of the s ame chirality but opposite polarity. An implication of this possibility to explain other magnetic cloud observations is discussed.