AMERICAN GEOPHYSICAL UNION

The most common kind of lightning shoots down from clouds toward the ground. More elusive forms, dubbed sprites and blue jets, flit above the clouds. Now scientists have discovered another type of lightning--gigantic jets--that link the tops of clouds to the overlying charged atmosphere, known as the ionosphere.

Previous research had suggested that some blue jets could link clouds to the ionosphere, but they reached altitudes of only 40 kilometers. During a thunderstorm over the South China Sea last summer Han-Tzong Su of the National Cheng Kung University in Taiwan and his colleagues recorded five separate gigantic jets that attained altitudes of 90 kilometers. Depending on the shape their upper halves took on, the lightning streaks were classified as tree jets or carrot jets. The scientists report that the jets emerged from the core of the thunderclouds and lasted about half a second. All five jets, the authors write, "establish a direct link between a thundercloud and the ionosphere," and measurements showed that there was a significant flow of current moving charge upward from the clouds. The researchers also detected extremely-low-frequency radio waves that were detected thousands of kilometers away during four of the five events.

The study of so-called transient luminous events (TLEs), which now includes gigantic jets, blue jets, sprites and elves, is still in its infancy, Victor P. Pasko of Penn State University writes in an accompanying commentary. A worldwide survey using Earth-orbiting sensors is being planned, he notes, because "knowing how frequently these events occur will help us to understand their contribution to the global electrical circuit."