Scientists monitoring a joint European Space Agency (ESA)–NASA solar probe in the waning months of its 18-year mission have collected a fortuitous but startling set of data. The solar wind--a steady stream of charged particles flowing outward from the sun in all directions at about 1.5 million kilometers per hour--is currently blowing at the lowest intensity ever recorded. If the as-yet-unexplained phenomenon continues, it could provide clues about the role of cosmic rays in climate change.
By counting sunspots, dark zones of violent activity on the star's surface, scientists have known for centuries that the sun undergoes an 11-year energy cycle. In the middle of the cycle, called the Solar Maximum, sunspots are most plentiful, the solar wind is most intense, and the sun's strengthened magnetic field sometimes wreaks havoc on satellite communications and electric-power grids on the ground. But during the Solar Minimum, which comes at the end and beginning of each energy cycle, sunspots are conspicuously absent, and the solar wind and magnetic field are relatively tepid. Right now, for example, except for one lone sunspot detected last week, our local star is quiet.
To learn more about the solar cycle and the structure and dynamics of the solar wind, NASA and ESA in 1990 launched the Ulysses spacecraft. The probe first headed to Jupiter, where it traveled high above the sun's north and south poles in circuits that last more than 6 years. The idea was to coordinate Ulysses's observations with those of satellites in more conventional orbits and detectors on Earth. Now, near the end of its third orbit, Ulysses has transmitted data showing that the solar wind is about 20% weaker than it was at the last solar minimum, making it the weakest wind in the 50 years scientists have been able to measure it.
"We don't know what's causing this," says space physicist David McComas of the Southwest Research Institute in San Antonio, Texas. McComas and colleagues report the data collected by Ulysses in this week's Geophysical Research Letters. "We don't know where this [event] is headed,” he says. “Will it turn around, or will it continue to degrade?"
Unfortunately, Ulysses won't be able to help, McComas explains. The spacecraft's fuel supply is slowly freezing, and it will soon become derelict. But a new NASA mission, called the Interstellar Boundary Explorer (IBEX), will launch next month, and about a year from now, IBEX will be able to see the effect of the weakened solar wind on the heliopause, the zone where the solar wind meets incoming cosmic rays. "It's a great bit of timing," McComas says.
The Ulysses data show "just how important long-term monitoring missions are to science," says physicist Larry Paxton of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. If the spacecraft hadn't been monitoring the solar cycle for so long, we would not have been given such a complete picture of the diminished solar wind, he says. Paxton says the data could help illuminate a long-standing mystery about whether cosmic rays increase cloud formation on Earth, perhaps leading to prolonged cold spells such as the Little Ice Age, which ran from the 15th to the 19th centuries. "We'll have to wait to see if this solar minimum shows a link between our climate and the behavior of the solar wind," he says.