Solar Wind in the Outer Heliosphere

Different source regions on the Sun emit solar wind parcels with different speeds. The fastest wind originates in large coronal holes. Smaller coronal holes emit moderately fast wind. The slow wind has a variety of possible sources such as coronal streamers and edges of coronal holes. As these differing speed parcels leave the Sun, they interact with one another altering the solar wind properties with distance. Fast parcels catch up with (run away from) slower ones emitted at an earlier (later) time forming a compression (rarefaction). Eventually, the solar wind is so far from the Sun that it begins to interact with incoming interstellar material and slows and heats as interstellar material is ionized and picked up by the solar wind. We quantify how much the solar wind slows in the outer heliosphere owing to interaction with the interstellar medium as the solar wind moves away from the Sun and into an increasing amount of interstellar neutral material. Recently New Horizons (NH) observed a distinct decrease in the solar wind speed. By comparing NH solar wind observations to those at 1 au, we find that the recent slow and steady wind in the outer heliosphere is a result of three factors: 1) the solar wind emanating from the Sun was slow, 2) solar wind structures merged and were worn down as they propagated away from the Sun, and 3) the solar wind continued to slow as additional interstellar material was picked up as the solar wind moved farther away from the Sun. When NH was between 45 and 51 au, the solar wind was 12 to 13.5% slower than at 1 au, which as expected exceeds the 5 -7% amount of slowing previously found between 30 and 43 au. Such slow steady wind has implications for the overall size and shape of the heliosphere, how deeply Galactic Cosmic Rays (GCRs) penetrate into the heliosphere, and the suprathermal ion tail development.