Studying the Evolution of ICMEs in the Heliosphere Through Multipoint Observations

Interplanetary coronal mass ejections (ICMEs) are the main cause of severe space weather in near-Earth space. To better understand the evolution of ICMEs in interplanetary propagation, we identified 14 multipoint observed ICME events via various lists of ICMEs, which confirmed the multipoint associations previously made by other researchers for these events. We use the in situ measurements as well as model results of these 14 ICMEs to obtain the evolution of the ICMEs in interplanetary space. We found that the dependence of the average total magnetic field strength on the radial distance is $B_t\propto D^{-1.68\pm 0.17}$. The radial dependence of the axial magnetic field strength is $B_0\propto D^{-1.54\pm 0.15}$. These dependencies indicate that the decay rates of the magnetic field are slightly slower than expected from self-similar expansion. The relationship between radius of the flux rope and radial distance is $R\propto D^{0.84\pm 0.22}$, which is closed to the expectation of self-similar expansion but still slightly lower. However, for each group of ICME, their radial dependence varies significantly. Only 36% of events are basically self-similar expansions. This may be due to the effect of the magnetic pressure difference and velocity difference between an ICME and the background solar wind on the ICME expansion. In the process of ICME propagation, the axial magnetic flux and helicity can be considered essentially conserved. We also analyzed the changes in the flux rope axis orientation and found that the majority (71%) of the events changed the axis direction, and the flux rope axis tends to be horizontal and solar west-east or east-west.