Dust devils produce magnetic fields. (Earth Science).

Scientists who chase dust devils report that the tiny twisters can produce a small magnetic field that changes magnitude between 3 and 30 times per second.

When grains of sand and clay collide inside a dust devil, they generate electric charges, says William M. Farrell, a geophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md. Negative charges typically transfer to the smaller, lighter particles, which are lofted higher than the heavier grains. As these charged particles swirl, they generate magnetic fields just the way electrons moving in an electromagnet's coiled wire do. Because a dust devil's charged particles move in circular paths at ever-changing speeds, they create a varying magnetic field.

On a typical summer day, several dozen dust devils spin across the dry lake bed in Nevada's Eldorado Valley. Farrell and his colleagues made their measurements by driving their instrument-laden pickup truck directly through or near dust devils. For one 10-meter-wide, 200-m-tall dust devil, the researchers could detect the magnetic field from several hundred meters away. The team reports its measurements in the Jan. 15 Geophysical Research Letters.

Similar instruments on a future Mars lander could measure the strength of Martian dust devils, which have shown up on images taken by the Mars Pathfinder lander and by probes orbiting the Red Planet.--S.P.