You are no doubt familiar with active aurorae, like the Northern Lights. When particles from the Sun impact Earth’s magnetic field, they can send electrons shooting towards the poles, which can create glowing lights coloured green, purple and other colors across the sky.
But did you know there is another type of aurora, known as a pulsating aurora? These do not rely on the solar wind to form, and can come into being at any moment. During a pulsating aurora, it will shift and brighten in certain patches, instead of forming giant arcs like an active aurora. A previous study suggested that electromagnetic waves may be driving them, but the cause of their shifting intensity was a bit of a mystery. However, a new study in the Journal of Geophysical Research provides an answer.
Pulsating aurorae are caused by wave motions of high-energy electrons in the magnetosphere. The electrons travel along Earth’s magnetic field lines, which magnetically connect the north and south hemispheres, so if there is a pulsating aurora near the North Pole, there will also be one near the South Pole. "Electrons are constantly pinging back and forth along this magnetic field line during an aurora event," said Robert Michell, a space physicist at NASA Goddard and one of the study’s authors, in a statement.
In this study, the scientists found that the intensity of the pulsating aurorae depended on the quantity of low-energy secondary electrons, which were sent traveling between the hemispheres by the higher-energy electrons in the magnetosphere. The biggest changes in the structure and shape of the pulsating aurorae were found to be occurring when fewer low-energy particles were traveling along the magnetic field lines.
"It turns out that secondary electrons could very well be a big piece of the puzzle to how, why, and when the energy that creates auroras is transferred to the upper atmosphere," said Marilia Samara, a space physicist at NASA Goddard and lead author of the study, in the statement.
The exact mechanics behind the change in quantity of low-energy electrons does not seem to be well understood, but it appears they may have a bigger role to play than thought.
Gif in text: Time-lapse of a pulsating aurora on January 3, 2012, in Poker Flat, Alaska. NASA.
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