  We analyze in situ measurements of solar wind velocity obtained by the
Advanced Composition Explorer (ACE) spacecraft during the solar activity cycle
23. We calculated a robust complexity measure, the permutation entropy (S) of
solar wind time series at different phases of a solar activity cycle. The
permutation entropy measure is first tested on the known dynamical data before
its application to solar wind time series. It is observed that complexity of
solar wind velocity fluctuations at 1 AU shows hysteresis phenomenon while
following the ascending and descending phases of the activity cycle. This
indicates the presence of multistability in the dynamics governing the solar
wind velocity over a solar activity cycle.
