Scientists have determined that, contrary to expectations, the forerunners of electrical sparks can split, according to a new study. The findings, published in the April 29 issue of Physical Review Letters, could help explain a recently observed lightning flash that reached an altitude of 70 kilometers.
As electrons move through an electric field, they liberate electrons from surrounding gas molecules and create an ionized gas, which in turn generates its own electric field. Previous research had suggested that such a situation would create streamers (precursors to visible sparks) having a tube-like channel with a fixed diameter. But when Manuel Arrays and colleagues at the Center for Mathematics and Computer Science in Amsterdam simulated streamers moving through nitrogen gas at 50,000 volts--twice the electric field strength utilized in previous studies--they observed spontaneous splitting. As the streamers grew, their leading edges expanded and attenuated, the researchers found. Once a streamer thinned, bulges in its leading front caused the electric field to focus around them, which in turn increased the speed of ionization and caused the bumps to grow faster than the rest of the front. Branching followed.
This mechanism mirrors a process that can occur in coral reefs: if one section of the organism moves ahead of the rest, it has access to more food and grows ever faster. And because recently observed blue jets tens of kilometers above the earth appear to branch in much the same way that sparks closer to the ground do, the new work could shed light on the forces underlying those elusive flashes.