  Ion acoustic (IA) solitons are accompanied by vortex-shaped nonlinear
structures (e.g. hollows, plateaus or humps) in the electron distribution
function, called electron holes, portraying trapped electrons. These structures
appear as charged flexible clouds (shielded by the background plasma) in the
phase space with their own inertia, depending on the number of trapped
electrons. According to simulation studies, electron holes tend to merge in
pairs until one accumulative hole remains in the simulation box. This tendency
has been analytically and qualitatively explained in the frame of the energy
conservation principle. However, electron holes accompanying IA solitons should
not merge due to stability of IA solitons against mutual collisions. In this
report based on a fully kinetic simulation approach, detailed study of the
collisions of IA solitons reveals the behavior of electron holes under these
two conflicting predictions, i.e. stability against mutual collisions and
merging tendency. Four main results are reported here. Firstly, we find that
among the three different types of collisions possible for electron holes, just
two of them happen for electron holes accompanying IA solitons. We present
different collisions, e.g. two large/small and large versus small holes, to
cover all the these three different types of collisions. Secondly, we show that
although electron holes merge during collisions of IA solitons, the stability
of IA solitons forces the merged hole to split and form new electron holes.
Thirdly, we reveal that holes share their trapped population during collisions.
Post-collision holes incorporate some parts of the oppositely propagating
before-collision holes. Finally, it is shown that the newly added population of
trapped electrons goes through a spiral path inside the after-collision holes
hole because of dissipative effects.
