The dazzling green, purple, and red lights of the aurora borealis, also known as the Northern Lights, are one of the most spectacular natural phenomena on Earth.
It is an unraveled fact that electrons that bounce around the magnetosphere of the planet cause the colorful aurorae. Scientists, however, have not been able to understand in depth the process that produces them, until now.
An international research team has recently directly observed what causes one kind of Aurora, the pulsating aurora. They have published their findings in the journal Nature on Feb. 15.
Pulsating Aurorae
Most common at sunrise, the pulsating aurorae are named after their characteristic of shifting and brightening in well-defined patches, instead of the long arcs that can be seen for other aurorae in the sky. They stretch in the sky for more than hundreds of kilometers and can be seen at a height of around 100 kilometers.
The random electrons release into the upper atmosphere generates the pulsating feature, however, the physical mechanism that causes the process had never been seen directly before.
The research team used data collected from the ERG satellite, also referred to as the Arase spacecraft, to observe that the pulsating aurorae are generated by interactions in the planet's magnetosphere between plasma and electrons. Plasma is the energized gas where electrons have broken free from their parent molecules. It is one of the four elementary states of matter in addition to gases, liquids, and solids.
Electromagnetic fluctuations in the magnetosphere scatter the electrons, which cause them to fall back into the upper atmosphere, simultaneously creating colorful light.
“By analyzing 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 the lead author of the paper Satoshi Kasahara, from the University of Tokyo in Japan.
Auroral Substorms
Kasahara added that auroral substorms are the result of global reconfiguration in the magnetosphere. The phenomena lead to stored solar wind energy being released. Auroral substorms lead to auroral brightening from sunrise to midnight, leading to subsequent violent motions of distinct auroral arcs, which finally get broken and result in pulsating, diffuse patches at dawn.
The chorus waves, which are a certain type of plasma wave, is driven by the global reconfiguration to rain electrons into the upper atmosphere. The process stabilizes the system and releases a colorful light as the electrons start to fall.
Researchers had, however, questioned if the chorus waves were strong enough to stimulate the electrons enough to create auroras. Now, they witnessed the phenomena directly for the first time.
