Slowing of the solar wind in the outer solar system

An article published in “The Astrophysical Journal” reports a study on changes in the solar wind in the outer region of the heliosphere, the bubble in which the influence of the Sun is felt. A team of researchers from the Southwest Research Institute led by Dr. Heather Elliott used measurements collected by NASA’s New Horizons space probe’s Solar Wind Around Pluto (SWAP) instrument to compare the speed of the solar wind at distances from the Sun between 1 and 3 times that of the Earth from the Sun and at distances between 21 and 43 times that distance reveals a slowdown between 5% and 7% at distances between 30 and 43 times that of the Earth from the Sun.

Some NASA space probes explored the outer solar system and the two Voyagers are now in interstellar space, where their still working instruments are providing information on areas of space that represent a new frontier. The New Horizons space probe is well within the heliosphere since its distance is almost 46 times that of the Earth from the Sun while the Voyager 1 crossed the first heliosphere boundary, called in jargon termination shock, at a distance about 94 times that of the Earth from the Sun and Voyager 2 crossed it at a distance about 84 times that of the Earth from the Sun. New Horizons, however, has the advantage of having on board instruments much more advanced than those built about thirty years before enabling to obtain more information.

In particular, the New Horizons space probe’s SWAP instrument carries out detailed daily detections of the solar wind and other particles such as ions from interstellar space that enters the heliosphere as a neutral material and get ionized by sunlight and by interactions with solar wind ions. These interstellar materials increase with the distance from the Sun and, according to the theory, when they get ionized they’re picked up by the solar wind which as a consequence slows down and warms up.

To try to verify this theory, Dr. Heather Elliott’s team examined solar wind speed measurements made using the SWAP instrument at distances between 21 and 42 times that of the Earth from the Sun and those made at a distance from the Sun close to that of the Earth conducted by the Advanced Composition Explorer (ACE) and Solar TErrestrial RElations Observatory (STEREO) space probes. The data collected by the SWAP instrument indicate that there’s indeed a slowdown that becomes relevant at distances between 33 and 42 times that of the Earth from the Sun, even reaching 7% compared to the speed it has at a distance from the Sun close to that of the Earth. In short, there’s a confirmation of the theoretical predictions.

The heliosphere changes continuously following the solar cycles and in different areas the boundaries are at different distances from the Sun, the reason why the two Voyagers crossed the termination shock at very different distances from the Sun. Measurements such as those made by the New Horizons space probe’s SWAP instrument, along with the information available on solar wind temperature and density will help estimate when it will cross termination shock as well.

In this period solar activity is relatively mild and that leads to the termination shock getting closer and an estimate of its crossing as early as the mid-2020s. When solar activity increases again, the termination shock will be pushed to the previous distances, perhaps before New Horizons reaches it.

The image (Courtesy Southwest Research Institute; background artist rendering by NASA and Adler Planetarium. All rights reserved) shows the situation of the heliosphere and the solar wind resulting from the detections examined. The estimates will be improved over time thanks to the data that the New Horizons space probe keeps on sending. This means an improvement in our understanding of both the influence of solar activity through its cycles and the effects of the arrival of particles from interstellar space. Even from this point of view, the New Horizons mission continues to be extraordinary.

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