The heliosphere and the Ulysses mission

Abstract

The interplanetary medium consists primarily of the supersonic solar wind, carrying the frozen-in magnetic field extending from the solar corona. The properties of this medium are controlled by the state of the corona and by dynamic processes occurring in the medium itself. As a result, there are significant variations in those properties as a function of heliolatitude. In situ observations over the past three decades have been largely confined to the neighbourhood of the solar equatorial plane. While many of the important processes have been identified and studied extensively, observations are required as a function of heliolatitude to define large-scale structures and their dependence on processes in the solar corona. The Ulysses mission, launched in October 1990, is the first space probe dedicated to the exploration of the heliosphere out of the ecliptic plane. By January 1994, the spacecraft had reached a heliolatitude of 50° south. The first results of the mission are summarized here, including the evolution and disappearance of the interplanetary magnetic sector structure; the onset of the dominance of the high-speed solar wind stream originating in the expanding southern coronal hole; observations of the signatures of complex coronal mass ejections; the high-latitude structure of the heliospheric magnetic field, and the evolution of corotating interaction regions as a function of heliolatitude. In particular, the abrupt change in the rotation rate of the sector structure in mid-1992, followed by the equatorward extension of the southern polar coronal hole, represent new observations related to the evolution of large-scale coronal structures and solar magnetic fields and to processes controlling the solar activity cycle.