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December 2000

Quick as a flash?

Clockwise from left: Diana Bartels, Jim Dye, Tom Matejka, Jessica Hagen, and Eric Defer (seated). (Photos by Carlye Calvin.)

The phrase "lightning fast" may take on new meaning through a recently discovered class of flashes now being explored by scientists in MMM and their colleagues. A cloud-to-ground lightning flash with multiple strokes can last more than a second, and intracloud flashes average about a quarter of a second. In contrast, instruments deployed in Colorado in 1996 have documented a set of intracloud flashes that can play out as quickly as 23 millionths of a second, and perhaps even faster than that.

MMM's Eric Defer presented the group's recent findings at the International Lightning Detection Conference in Tucson last month; he'll also speak at this month's American Geophysical Union meeting in San Francisco. Eric's been studying a Colorado storm from the deep convection field project of STERAO (Stratosphere-Troposphere Experiment: Radiation, Aerosols and Ozone). Most lightning sensors report only location and polarity, but a French VHF interferometer deployed at STERAO can profile single flashes in three dimensions and judge duration. Out of about 5,400 flashes observed in a storm on 10 July 1996, only 83 were cloud-to-ground. More than 800 intracloud flashes had durations of less than a millisecond. Many of these lasted no more than 23 microseconds, which was the instrument's sharpest resolution.

Several other observing systems have spotted these brief flashes in the past few years, but Eric has produced the first analysis relating such flashes to radar output. For 10 July 1996, as well as for several other cases from STERAO, the short-duration lightning tends to occur at heights of 6 to 10 kilometers (4–7 miles) within the storm, in close proximity to the strongest updrafts and the most intense reflectivities (radar returns from water and ice) found at those heights. Thus, says Eric, the ultraquick flashes might someday serve as a real-time tool for judging storm severity.

What makes the flashes so brief? "Locally strong electric field may explain the ignition of the flashes, but we don't understand why this population of flashes does not last longer," says Eric. He's working with MMM colleagues Jim Dye and Jessica Hagen, as well as Diana Bartels and Tom Matejka (National Severe Storms Laboratory), to analyze more STERAO storms and sort out possible causes of the short-duration lightning. "We have an incredible set of data, but it takes time to analyze."

HAO scientist honored for astrophysics thesis

Hector Socas-Navarro.

A new technique for examining the solar chromosphere has gained Hector Socas-Navarro a prestigious award. The HAO associate scientist was recently recognized by the Spanish Astronomical Society (SEA) for producing the nation's best thesis in astrophysics during 1998–99. Hector accepted the honor, including a cash award of 1,500 euros, at the fourth meeting of the SEA, held in Santiago de Compostela 11–15 September.

For his thesis, Hector developed a way to infer the height distribution in the chromosphere of such variables as temperature, velocity, density, and magnetic field. In the low-density realm of the chromosphere, radiative processes become as important as collisions between atoms. These nonequilibrium conditions are still poorly understood, says Hector, and "it is very difficult to interpret the spectropolarimetric observations provided by the instruments."

Hector focused on umbrae, the dark regions at the heart of sunspots. He used the Stokes parameters from chromospheric lines measured with the oldest solar telescope at the Teide Observatory, housed at the Astrophysics Institute of the Canary Islands. Since the instrumentation was "rather modest," Hector says, the data were intended "merely as a first test of our technique. We didn't expect to find interesting results, but soon we realized that it was going to be very interesting work."

Hector's technique has shed light on the behavior of sunspots during oscillations that show up in the chromospheric data at three-minute intervals, regardless of sunspot size. "People have been thinking that the whole sunspot is moving up and down" during these oscillations, says Hector. His study revealed that "actually, most of the sunspot is downflowing in the chromosphere, then every three minutes there's an ejection concentrated in small jets too narrow to be resolved. That's why it hasn't been detected so far."

Over the past year in HAO, Hector has developed new codes for the solar community based on his thesis work. "Now most of this work is done, and I'm planning to take some time to continue the research on chromospheric dynamics and magnetism, which I find so fascinating and challenging." He adds, "I had friends competing for this award with longer publications lists than mine, so I was a little skeptical about my chances. But now, knowing that there was such a strong competition makes me feel even more proud."


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Edited by Bob Henson, bhenson@ucar.edu
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Last revised: Wed Dec 13 17:30:40 MST 2000