Determining the Alpha to Proton Density Ratio for the New Horizons Solar Wind Observations

To help identify interplanetary coronal mass ejections (ICMEs) in New Horizons (NH) Solar Wind Around Pluto observations, we developed a method for determining the alpha to proton density ratio (n _α/n _p). Many common ICME signatures are derived from plasma and field parameters with values inside transient ICMEs distinct from values in the background solar wind. As the solar wind propagates, the plasma parameters evolve with increasing heliocentric distance, and ICMEs interact with the background solar wind. Some ICME signatures are based on composition such as the alpha (He⁺⁺) to proton (H⁺) number density ratio, which is frequently enhanced in ICMEs. Intervals with enhanced n _α/n _p ratios persist into the outer heliosphere even though individual solar wind parameters evolve as solar wind propagates farther from the Sun. Overall, the solar wind expands as it propagates, but parcels of differing speeds dynamically interact, forming compressions and rarefactions, and altering the solar wind parameters. Both n _α and n _p change in lock step during such dynamic interactions, keeping the n _α/n _p ratio fixed. Our n _α/n _p results are consistent with prior missions, and we find that enhanced levels of n _α/n _p often occur within intervals of low proton temperature, which is the only other reliable ICME signature that NH can measure. Eventually, enhanced n _α/n _p values will likely become the most reliable ICME indicator for NH if the ICME temperatures become indistinguishable from background levels. NH is heading toward the Energetic Neutral Atom ribbon, and should have enough power to reach the termination shock.