Rare Auroras - 'Proton Arcs'

A surprisingly strong G3-class geomagnetic storm erupted on Aug. 15th when a CME hit Earth's magnetic field. Two nights later, as the storm was subsiding, midnight sky watchers in North America witnessed a rare and beautiful form of aurora--a "proton arc." Paul Zizka photographed the phenomenon on Aug. 17th from Banff, Alberta (see below, left).  "It was incredible," says Zizka. "The whitish pillar remained nearly stationary for over 30 minutes--enough time for a self-portrait."

In Val Marie, Saskatchewan, photographer Sherri Grant saw a purple proton arc cutting across the Milky Way. And in Oroville, Washington, at the Table Mountain Star Party, campers witnessed at least two more arcs (see below, centre and right).

Ordinary auroras are caused by electrons, which rain down on Earth's atmosphere from above. Atoms of oxygen and nitrogen, excited by the pitter-patter of electrons, form dynamic curtains of light. Protons have a different effect. For reasons not fully understood, protons normally trapped in our planet's ring current sometimes rain down on Earth's atmosphere during geomagnetic storms. En route, they excite a type of plasma wave called "EMIC"--short for electromagnetic ion cyclotron waves. The result is not a curtain, but rather a tight arc of light as shown above.

Many of the photographers who witnessed proton arcs on Aug. 17th have been observing auroras for years, yet they had never seen this phenomenon before. Geomagnetic storms still have the capacity to surprise!
www.spaceweather.com