40 years later, solar wind problem solved

U. WARWICK (UK) — Physicists have explained the unusual observations of turbulence in solar wind made by the probe Mariner 5 in 1971.

Current understanding tells us turbulence in the solar wind should not be affected by the speed and direction of travel of that solar wind. However when the first space probes attempted to measure that turbulence they found their observations didn’t quite match that physical law.

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The first such data to be analyzed from Mariner 5 in 1971 found a small but nonetheless irritatingly clear pattern in the turbulence perpendicular to both the direction of the travel and the magnetic field through which the solar wind was traveling. While it was an irritating aberration, the affect was relatively small and has been essentially ignored by physicists until now.

The most recent space missions to look at the solar wind, such as the Cluster mission, are examining it with such sensitive and highly accurate modern instrumentation that what was once a small aberration was threatening to become a significant stumbling block to deeper understanding of solar wind—which is effectively the solar system’s largest and most interesting natural turbulence lab.

Research led by scientists at the University of Warwick has found a solution to this 40-year-old problem. The research team looked at data from the Cluster mission and they also created a virtual model of how magnetohydrodynamic (MHD) turbulence builds up in the solar wind.

They then flew a virtual space probe through that virtual model in a range of directions unlike the single direction of travel open to a probe such as Mariner 5. The findings are reported in the journal Physical Review Letters.

“The analysis clearly showed that when all these results were considered together any correlation between changes in the turbulence in the solar wind and the direction of travel simply disappeared,” says lead researcher Andrew Turner. “The observed non-axisymmetric anisotropy may simply arise as a sampling effect of using just one probe taking a single particular path through the solar wind.”

Collaborators include researchers at the Institute of Theoretical Physics, Ilia State University, and the Max-Planck-Institut für Plasmaphysik.

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