Interplanetary Coronal Mass Ejections Observed by Ulysses Through Its Three Solar Orbits

Abstract

An extended Ulysses interplanetary coronal mass ejections (ICMEs) list is used to statistically study the occurrence rate of ICMEs, the interaction of ICMEs with solar wind, and the magnetic field properties in ICMEs. About 43% of the ICMEs are identified as magnetic clouds (MCs). It is found that the occurrence rate of ICMEs approximately follows the solar activity level, except for the second solar orbit; the rate is higher in the southern heliolatitude than in the northern heliolatitude; and it roughly decreases with the increase of ICME speeds. Our results show that the speed difference between the ICME and the solar wind in front of it shows a slight decrease with increasing heliocentric distance for ICMEs preceded by a shock, whereas no such dependence is found for the ICMEs without shock association. It is also found that approximately 23% of the ICMEs are associated with radial field events, in which the interplanetary magnetic field with near-radial direction lasts for many hours, in the Ulysses detected range, and these associated events are not necessarily confined to fast ICMEs or the trailing portions of ICMEs. Nearly all these associated events occur during the period of declining solar wind speed and most of them occur at low heliolatitudes.