NASA scientists have used a combination of satellite data and ground-based cameras to study pulsating auroras, which appear as flickering patches of bright light in the night sky. The research gave rise to an unexpected discovery, with the data revealing that secondary electrons may play a bigger part in the occurrences than was previously thought.
There are two kinds of aurora. First, there are active auroras, which appear as elongated arcs of light across the sky, and are caused by dense waves of solar material – such as solar winds or coronal mass ejections – hitting the Earth's magnetic field. The collision causes a release of electrons that race towards the poles, interacting with particles in the upper atmosphere as they go, creating the long patches of glowing sky.
Pulsating auroras are a little different. They're a more internal affair, in that they're brought about by complex wave motions in the magnetosphere, rather than being stimulated by external events. Because of this, they can happen at any moment, not only at times when solar material hits the magnetic field.
The magnetic connection between the north and south poles means that pulsating aurora events occur simultaneously at both poles, and electrons move constantly back and forth between hemispheres during events, shooting along magnetic field lines.
Those electrons that ping between hemispheres are lower-charged secondary electrons, kicked up by collisions with more highly-charged particles at the poles, themselves accelerated by the wave motions in the magnetosphere.
During the new study, the team used data from two satellites – one from the Japanese Aerospace Exploration Agency and another from the US Department of Defense's Defense Meteorological Satellite Program – examining the volume and energy of electrons during pulsating aurora events.
The data was compared with footage from all-sky cameras at the the European Incoherent Scatter Scientific Association Radar Facility in Norway, and the Poker Flat Research Range in Alaska.
Previous to the study, it was thought these secondary electrons, which possess far less energy than their primary counterparts, played little to no part in pulsating aurorae, but the combined observations tell a different story.
The researchers found that the most significant changes in the shape and structure of the pulsating auroras was observed when there were a low number of secondary electrons moving along magnetic field lines between the hemispheres. This strongly suggests that secondary electrons play a big part in pulsating aurora events, but further study will be required to clarify the findings.
"We need targeted observations to figure out exactly how to incorporate these low-energy secondary electrons into our models," says the NASA Goddard Space Flight Center's Marilia Samara. "But it seems clear that they may very well end up playing a more important role than previously thought."
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