New Horizons confirms: the solar wind slows as you travel outward
Figure 6 from the paper. Click for full resolution.
The science team for the New Horizons spacecraft, presently zipping outward towards the edge of the solar system, has confirmed earlier data from the Voyager spacecraft that the solar winds speed slows gradually as you move outward from the Sun.
You can read the paper here [pdf]. The distances are measured using astronomical units (AU), each equivalent to the distance from the Sun to the Earth, about 93 million miles.
Previously, New Horizons and Voyager 2 measurements between 30 and 43 AU indicated the solar wind was 5 to 10% slower than at 1 AU near Earth. Now, New Horizons researchers found at 58 AU that the solar wind is 13 to 15% slower than the wind at 1 AU. This gradual slowdown aligns with previous models of how interstellar material enters the heliosphere and affects the solar wind. It also demonstrates how the Sun’s influence decreases over long distances.
The scientists postulate that the slowing is caused by interaction with “interstellar neutral gas particles” that have managed to slip into the solar system. Their interaction with solar wind particles acts to slow the particles down.
New Horizons has not yet reached the termination shock that delineates the edge of the solar system. When it does, entering interstellar space, the wind speed will plummet significantly more, based on data from both Voyagers.
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