The Solar Wind Takes a Breather

I've always had a soft spot for an interplanetary pioneer called Ulysses. Built by the European Space Agency, it was launched in 1990 toward Jupiter, where the planet's powerful gravity yanked the craft out of the ecliptic plane and onto a looping path that carries it over and under the Sun every six years.

The initial mission concept, known as the International Solar Polar Mission, called for two identical craft — one European and one American — to study high-latitude regions of the Sun that can't be studied from Earth. But NASA reneged on its end of the deal, so Ulysses has soldiered on alone.

Recently it completed its third and final pass over the Sun's poles. That kind of longevity, far exceeding the planned 5-year-long mission, has really paid off. Ulysses's observations show that the solar wind is particularly feeble right now, with just 75% the strength it had a decade ago. In fact it's never been this weak since monitoring began a half century ago.

Space physicists had expected the flow to tail off, because the Sun's 11-year activity cycle is now at a minimum. But it's got far less punch than that seen during the last minimum. "The wind speed is almost the same, but the density and pressure are significantly lower," notes investigator David McComas (Southwest Research Institute), whose SWOOPS instrument aboard Ulysses has been key to the new finding.

The solar wind consists of plasma (ionized matter) and entrained solar magnetic field lines that pushed outward from the Sun's atmosphere into interplanetary space. Ulysses had previously shown that the wind comes off the Sun's poles faster and with less turbulence than it does from its midsection. But both the polar and equatorial flows have throttled back to historic lows.

There'd been earlier hints, in deep-space observations from IMP 8 and Voyager 2, that the solar wind variously ebbed and flowed during a solar cycle. Still, McComas and his colleagues, who detail their results in the September 18th issue of Geophysical Research Letters, don't know why the solar wind is taking a breather. One suspicion: perhaps the outflow is somehow being energized higher up in the Sun's corona, where there's less mass available to push outward into space.

In any case, the low flow means that the gigantic electromagnetic bubble that surrounds the Sun and planets must be shrinking inward and, with it, the solar system's boundary with interstellar space (called the heliopause). Both Voyager spacecraft are nearing this threshhold; they've aleady encountered a shock front inside the heliopause, and if this weak solar wind keeps up, Voyager 1 may find itself popping outside the heliosphere years sooner than expected.

Meanwhile, Ulysses itself is nearing the end of its historic mission. FLight controllers have been keeping a death watch all year, because the craft's source of heat and power (radioactive plutonium) has dwindled so much that the fuel lines are in imminent danger of freezing.

I contacted ESA project manager Richard Marsden for an update on the craft's health. "True to its name, Ulysses refuses to give up without a fight," he replied. "We're still getting science data, albeit only a few hours per day." The team has kept the fuel from freezing by firing thrusters every two hours. But the fuel is running low, and the team expects Ulysses to run dry sometime between the end of September and December. "With a bit of luck," Marsden adds, "we'll encounter the slow solar wind once again before then."

Hang in there, Ulysses!