The Gradual Slowing of the Solar Wind in the Outer Heliosphere

The solar wind gradually slows with increasing distance from the Sun as interstellar material becomes ionized and picked up by the solar wind. When observing the solar wind in the outer heliosphere, it is difficult to differentiate temporal decreases in the speed originating at the Sun from the slowing of the solar wind with increase distance as interstellar material is picked up. In order to resolve this issue, we examine the solar wind speed in the outer heliosphere relative to the corresponding solar wind speed observed at 1 au. Additionally, we use a simulation to help discern the temporal and spatial changes in speed at New Horizons (NH) by driving the inner boundary of a heliospheric simulation with 1 au speed measurements. We conclude that the long interval of slow solar wind from 2018–2023 found at NH was caused by a combination of slow wind being directed toward NH owing to a low current sheet tilt angle keeping the streamers in the ecliptic, and the gradual slowing with increasing distance caused by additional pickup of interstellar material as the solar wind travels farther way from the Sun. When NH was between 50 and 58.5 au, the measured solar wind speed was reduced by ∼13%–15% relative to 1 au owing to pickup of interstellar material. This reduction is a further decrease from the previously reported slowing of 5%–7% in the 30–43 au region.