Iwao Aoyama

Three-Dimensional Structure of the Heliosphere as Inferred from Observations with a Japanese Halley Spacecraft

A sinusoidal neutral line with a pair of giant regions appeared on the sun about one year before the launch of “Sakigake”, the first of two Japanese Comet Halley spacecraft. The Sakigake magnetometer data during the early part of the mission (February – March, 1985) are well interpreted by an eastward shift of the tilting neutral sheet. The shift is further explained by an effect of a new giant region appearing at ∼10° heliolatitude and ∼50° Carrington longitude in August 1984. The toward polarity ratio of IMF observed by Sakigake changed from ∼22% in February to ∼62% in early June and then increased rapidly up to 98%. This ratio is interpreted as a decrease of the tilt angle of the sheet down to only ∼4°. It is the first spacecraft observation of “the disappearing sector structure” with such small tilt angle.