Focus: Ringing above the Aurora
Behind the shimmering curtains of light in an aurora tremendously energetic events take place. 12 years ago researchers studying the electrons and ions whizzing through and above auroras came across puzzling electric field patterns that have taken years to explain. One current theory is a new phenomenon in plasma physics: an electric field oscillation that stays put, rather than propagating away, as a normal electromagnetic wave would. In the 29 June PRL a team claims to provide the most convincing evidence to date of these localized “knots” of resonating fields.
The sun constantly spews energetic charged particles that are funneled in by the Earth’s magnetic field, and their energy is eventually released in the beautiful light shows of auroral displays. But many links in the chain of events that leads to auroras remain uncertain, including the dynamics of the upper ionosphere–the plasma of atmospheric electrons and ions. After instruments on several rockets launched through auroras showed unexpected electric field structures, some theorists proposed an explanation: regions (perhaps 100 m across) with slightly lower or higher densities of particles could “trap” electromagnetic waves in a type of “bound state” that oscillates at certain resonant frequencies. “It’s like taking a hammer and ringing a bell,” says Greg DeLory, of the University of California at Berkeley, because traveling waves hit the regions and excite the resonances, which are called LHSS, for “lower hybrid solitary structures.”
Data from several rockets have seemed to confirm the theory, but John Bonnell, of Los Alamos National Laboratory in New Mexico, says there were still some uncertainties in the data. The theory predicts electric field structures that in one configuration, or “mode,” look like the magnetic field surrounding a bar magnet–except that in the atmosphere above an aurora the whole pattern should rotate at about 2 kHz in the direction perpendicular to the Earth’s magnetic field. In the same region, a similar field pattern should rotate in the opposite sense at about 4 kHz, according to the theory. Bonnell was a member of a team from Cornell University that measured electric fields from a rocket launched in early 1997, and the team now reports in PRL that it captured both of the counter-rotating fields in at least 22 different density-depleted regions above an aurora. The authors say the new data constitute “definitive evidence” of these electric field structures.
“It’s a neat, interesting result found in space about how plasmas work in general,” says DeLory, since any plasma with slight density variations could support such resonances. He points out that 99.9% of the universe is plasma, so understanding these phenomena is important far beyond Earth’s dazzling light shows.