"We believe we have found a new process that triggers the formation of these bubbles. Our discovery could lead to improved forecasts of this phenomenon because we now know what to look for," said David Hysell of Cornell University in Ithaca, N.Y., the experiment principal investigator.

The researchers traveled to Kwajalein Atoll in the Marshall Islands near the equator in the Pacific Ocean to conduct their experiment during Aug. 3-5, 2004. A radar facility operated by the U.S. Department of Defense provided the big picture with large-scale observations of the electrified upper atmosphere, also known as the ionosphere.

The team, supported by NASA engineering staff, launched two salvos of instrumented sounding rockets (three each on two different nights) into this region, and obtained detailed measurements of the wind speed and direction, the density, and the electric fields in a cross-section of the ionosphere.

When the team fed the radar and rocket observations into a computer simulation, the results supported a theory the team had developed to explain how the bubbles form.

Wind shears are common and well-known in the lower atmosphere, where they pose hazards to airplanes. A similar but more complicated phenomenon appears to operate in the upper atmosphere leading to the production of these giant bubbles that mushroom to great heights above the equator, analogous to the large cumulo-nimbus clouds of everyday experience in the lower atmosphere.

The rocket scientists discovered a shear flow just below the region in the ionosphere where the bubbles form. The ionosphere above the equator can flow in a westward direction at one altitude while just above, the ionosphere flows in an eastward direction due to forcing by prevailing neutral winds.

The shear set up by these oppositely directed flows is unstable and the flows begin to ripple, producing waves in the ionosphere that are the seeds that grow into the enormous Equatorial Spread-F bubbles. This scenario is fully supported by the Kwajalein experiments.

"Our (hypothesis) predicts that the shear flow will most often form waves with wavelengths (distance between wave crests or troughs) of either 30 kilometers (18.6 miles) or 200 kilometers (124.3 miles) in the ionosphere above the shear region.

This is in fact what we saw; one night, 30-kilometer waves were present, and another night, 200 kilometer waves formed," Hysell said.

The idea that bubbles in the ionosphere could be responsible for radio communication disruptions was developed in the mid-1970s, but until now, none of the theories proposed to explain the bubble formation have been conclusively demonstrated to work.

On any given day, there is about a 50-percent chance the bubbles will form. They appear about 400 kilometers (almost 250 miles) above Earth where the plasma in the ionosphere is densest. Within tens of minutes, they form towering plumes reaching altitudes of up to 2,000 kilometers (nearly 1,250 miles), with widths from tens to hundreds of kilometers.

They usually appear at night, when the ionosphere becomes unstable because a layer of dense plasma forms on top of low-density plasma. The layers can't mix easily because they are pinned in place by the Earth's magnetic field, which is horizontal and directed northward over the equator.

However, when conditions are right, bubbles of low-density plasma penetrate the dense plasma layer and rise high above Earth. The bubbles allow the two layers to mix, making the ionosphere stable again, but some trigger is necessary for the bubbles to form. The team's experiment supports the idea that the shear flow is the trigger.

The researchers will test their hypothesis further with the Communications and Navigation Outage Forecast System, a U.S. Air Force satellite projected to be launched in 2008.

"Sounding rocket experiments are a good way to do this kind of science, because they are modest experiments that give an initial test of your theory, allowing you to refine your ideas and focus your efforts before the much more expensive satellite is launched," Hysell said.

The results are described in a series of papers; one published in the December 2005 issue of Geophysical Research Letters, another in the May 2006 issue of Annales Geophysicae, with a third paper submitted to the Journal of Geophysical Research.

Related Links
CNOFS