The Northern Lights - with their dazzling display of green, red and purples - are one of the most beautiful and alluring natural phenomena on Earth.
But while scientists know that the colourful aurorae are the result of electrons bouncing around the Earth's magnetosphere – the region of space surrounding our planet that is controlled by its magnetic field – they have not been able to fully explain the processes that produce them.
Now, an international team of researchers have directly observed the cause of one type of aurora – known as pulsating aurorae - for the first time. Their findings are published in the journal Nature.
Pulsating aurorae are common at dawn and are so-called because their features shift and brighten in distinct patches, rather than elongated arcs across the sky like other aurorae. They stretch over tens to hundreds of kilometres of sky and appear at an altitude of approximately 100 kilometres.
The pulsing is generated by the intermittent release of electrons into the upper atmosphere, but the physical mechanisms driving this process has never been observed until now.
Using data taken from the Exploration of energization and Radiation in Geospace (ERG) satellite - also known as the Arase spacecraft – the scientists observed that pulsating aurorae are caused by interactions in the Earth's magnetosphere between electrons and plasma – a gas that has been energised to the point that electrons break free from their host molecules. (Plasma is one of the four fundamental states of matter, along with solids, liquids and gases).
Electrons in the magnetosphere are scattered by electromagnetic fluctuations, causing them to rain back down into the upper atmosphere while producing colourful light.
"By analysing data collected by the ERG spacecraft more comprehensively, we will reveal the variability and further details of plasma physics and resulting atmospheric phenomena, such as auroras," said Satoshi Kasahara, an associate professor in the Department of Earth and Planetary Science at the Graduate School of Science of the University of Tokyo in Japan, and lead author of the new paper.