Interstellar atoms flow past Earth as the solar system passes through the surrounding interstellar cloud at 23 kilometers per second (50,000 mph). The latest IBEX measurements of the interstellar wind direction were discovered to differ from those made by the Ulysses spacecraft in the 1990s. That difference led the IBEX team to compare the IBEX measurements to data gathered by 11 spacecraft between 1972 and 2011. Statistical testing of the Earth-orbiting and interplanetary spacecraft data showed that, over the past 40 years, the longitude of the interstellar helium wind has changed by 6.8 ± 2.4 degrees.
“We concluded it’s highly likely that the direction of the interstellar wind has changed over the past 40 years. It’s also highly unlikely that the direction of the interstellar helium wind has remained constant,” said Priscilla Frisch, lead author of the study and a senior scientist in the Department of Astronomy and Astrophysics at the University of Chicago.
“We think the change in wind direction could be explained by turbulence in the interstellar cloud around the Sun,” she said.
The spacecraft data used for this study were gathered using three methods to measure the neutral interstellar helium wind direction: IBEX and Ulysses provided direct in situ measurements of the neutral wind; the earliest measurements from the 1970s used fluorescence of solar extreme ultraviolet radiation of the helium atoms near the Sun; and measurements also were included of the helium flow direction from “pickup ions,” neutral particles in the solar system that become ionized near the Sun and join the solar wind.
“This result is really stunning,” said Dave McComas, IBEX principal investigator, assistant vice president of the Space Science and Engineering Division at Southwest Research Institute, and an author on the paper. “Previously, we thought the very local interstellar medium was very constant, but these results show just how dynamic the solar system’s interaction is.”