Whistlers may sing Neptune's lightning call.

Whistlers may sing Neptune's lightning call

Both Voyager spacecraft photographed bright "superbolts" of lightning in Jupiter's atmosphere in 1979 and recorded radio emissions called "whistlers" -- because of their declining frequencies--which lightning often triggers on Earth. At Saturn, the Voyagers neither saw lightning nor heard whistlers, but they did record a kind of high-frequency static associated with terrestrial lightning and audible on AM radios. And one group of scientists has proposed that Voyager 2 detected such static at Uranus.

Now Neptune gets nominated to the lightning club.

Voyager 2 neither saw lightning bolts nor encountered the high-frequency static as it flew past Neptune last August, but one of the spacecraft's instruments apparently detected whistlers. The instrument's chief scientist, Donald A. Gurnett of the University of Iowa in Iowa City, was reluctant at the time to call the signals that, even though he says "they behaved exactly like whistlers."

Now, he says, "I am confident that these are whistlers produced by lightning in the Neptune atmosphere."

Gurnett's initial uncertainty stemmed from measurements by another Voyager instrument, which he felt raised the possibility that the planet's ionosphere contained far too few electrons to carry whistlers to the spacecraft from the lightning bolts that would have spawned them.

The apparent Neptune whistlers took far longer to sweep down through their range of frequencies than those at Jupiter. According to Ralph L. McNutt Jr. of the Massachusetts Institute of Technology in Cambridge, that would happen if the whistlers followed either a long path with few electrons -- a low-density ionosphere -- or a short path through a dense ionosphere. Yet Voyager 2 was barely 5,000 kilometers from Neptune when it detected the whistler-like signals, too close for a long path to explain their sound -- which implies a concentration of electrons, called a plasma, much more dense than the plasma measured by the spacecraft.

After studying the problem, however, Gurnett reported last week at the American Geophysical Union meeting in Baltimore that the presence of a dense but relatively cold plasma at a temperature of only about 950 kelvins could resolve the discrepancy. This would be too cold for Voyager 2's plasma instrument to detect, but would contain far more electrons than the hotter plasma (at about 55,000 K) that the craft did measure. Confirming the answer will require more study, says McNutt, but he acknowledges that hot and cold plasmas could coexist at Neptune.