MULTI-ION COLLISIONLESS PLASMAS ARE UBIQUITOUS IN ASTROPHYSICAL AND SPACE SYSTEMS. COLLISIONLESS PLASMAS ARE OFTEN NOT IN THERMODYNAMIC EQUILIBRIUM SO THAT A COLD THERMAL ION POPULATION CAN COEXIST WITH A HOT SUPRATHERMAL POPULATION THAT IS CREATED BY CHARGE EXCHANGE ELECTRON IMPACT IONIZATION AND PHOTOIONIZATION OF AN AMBIENT NEUTRAL GAS COMPONENT. THIS PROPOSAL FOCUSES ON NONEQUILIBRIUM PLASMAS WITH A COLD THERMAL ION POPULATION AND A HOT SUPRATHERMAL ONE SUCH AS THE SOLAR WIND PLASMA IN THE HELIOSHEATH THE REGION WHERE THE SOLAR WIND IS SUBSONIC. IT HAS REMAINED LARGELY UNEXPLORED HOW MAGNETOSONIC WAVES PROPAGATE AND FORM SHOCKS IN SUCH PLASMAS. RECENTLY THE PROPOSERS DEVELOPED A NEW MULTI-FLUID MHD MODEL OF THE SOLAR WIND THAT TREATS SUPRATHERMAL IONS THERMAL SOLAR WIND IONS AND ELECTRONS AS THREE SEPARATE FLUIDS. THIS MODEL WAS APPLIED TO THE TERMINATION SHOCK WHERE THE SOLAR WIND GOES SUBSONIC [ZIEGER ET AL. 2015]. THE ADDITION OF HOT SUPRATHERMAL IONS TO THE MIXTURE OF THERMAL SOLAR WIND PROTONS AND ELECTRONS RESULTS IN A HIGH-FREQUENCY FAST MODE (OR SUPRATHERMAL MODE) AND A LOW-FREQUENCY FAST MODE (OR THERMAL PROTON MODE). IN SUCH PLASMAS QUASISTATIONARY NONLINEAR MAGNETOSONIC STRUCTURES EXIST KNOWN AS SOLITONS AND OSCILLITONS [SAUER ET AL. 2001; 2003; DUBININ ET AL. 2006]. IT WAS SHOWN THAT THE CROSSING OF THE TERMINATION SHOCK BY VOYAGER 2 HAS THE CHARACTERISTICS OF A MULTI-ION SHOCK WITH A TRAILING WAVE TRAIN WHICH IS A QUASI-STATIONARY NONLINEAR FAST MAGNETOSONIC OSCILLITON [ZIEGER ET AL. 2015]. THE PREVIOUSLY UNKNOWN DENSITY AND TEMPERATURE OF THE SUPRATHERMAL COMPONENT HAVE BEEN CONSTRAINED THROUGH THE COMPARISON OF NUMERICAL MULTI-FLUID MHD SIMULATIONS WITH VOYAGER 2 DATA.