The particles streaming into the Solar System from interstellar space have changed direction over the last forty years, says a group of astrophysicists led by Dr Priscilla Frisch from the University of Chicago.
Their finding, reported in the journal Science, is based on data from 11 different spacecraft, including NASA’s Interstellar Boundary Explorer (IBEX).
“It was very surprising to find that changes in the interstellar flow show up on such short time scales because interstellar clouds are astronomically large,” said study co-author Dr Eberhard Moebius from the University of New Hampshire.
“However, this finding may teach us about the dynamics at the edges of these clouds—while clouds in the sky may drift along slowly, the edges often are quite fuzzy and dynamic. What we see could be the expression of such behavior.”
The IBEX data show that neutral interstellar atoms are flowing into the Solar System from a different direction than previously observed. Interstellar atoms flow past the Earth as the interstellar cloud surrounding the Solar System passes the Sun at 50,000 miles per hour.
The latest measurements of the interstellar wind direction differed from those made by the Ulysses spacecraft in the 1990s. That difference led the scientists to compare the new measurements to data gathered by 11 spacecraft between 1972 and 2011. They wanted to gather as much evidence from as many sources as possible to determine whether the newer instruments simply provided more accurate results, or whether the wind direction itself changed over the years.
The various sets of observations relied on three different methods to measure the incoming interstellar wind. IBEX and Ulysses directly measured neutral helium atoms as they coursed through the inner Solar System. IBEX’s measurements are close to Earth, while Ulysses’ measurements were taken between 1.3 and 2 times further from the Sun.
In the final analysis, the direction of the wind obtained most recently by IBEX data differs from the direction obtained from the earlier measurements, which strongly suggests the wind itself has changed over time.
Bibliographic information: P. C. Frisch et al. 2013. Decades-Long Changes of the Interstellar Wind Through Our Solar System. Science, vol. 341, no. 6150, pp. 1080-1082; doi: 10.1126/science.1239925