Scientists have finally solved the mystery of northern lights or auroras by directly observing for the first time the scattering of electrons bouncing across the Earth’s magnetosphere.

Auroras are the colorful lights that appear over both the Arctic (northern lights) and Antarctic (southern lights). This spectacular light show is considered one of the greatest natural wonders, and people living regions near the poles also associate it with good luck (as well as evil spirits). These lights can take different forms, but the most amazing are the “pulsating auroras” that present themselves as flickering displays across the dark sky at dawn. They are seen about 100 km above the surface of Earth, and cover tens to hundreds of kilometres.

While scientists have theoretically understood the underlying mechanisms behind this phenomenon, they had no experimental evidence so far to prove that what they believe is correct. Now scientists have solved this mystery by directly observing how scattering of electrons bouncing across the Earth’s magnetosphere generate these colorful lights.

“We, for the first time, directly observed scattering of electrons by chorus waves generating particle precipitation into the Earth’s atmosphere,” said Professor Satoshi Kasahara, the lead researcher and a planetary scientist at the University of Tokyo.

“The precipitating electron flux was sufficiently intense to generate pulsating aurora.”

Scientists have long believed that pulsating auroras are created due to interaction of electrons (in the magnetosphere) and chorus waves (the natural oscillations in the Earth’s outer magnetosphere). Scientists believed that intermittent bursts of chorus waves cause the pulsations of “electron precipitation” which eventually result in auroras. Results of theoretical models and computer simulations have also suggested that chorus waves are capable of scattering the electrons bouncing along the Earth’s geomagnetic field. This scattering causes electrons to rain down into the upper atmosphere, thereby stabilizing the system and producing colorful auroras.

While scientists had theoretically understood the underlying mechanism behind auroras, it was unclear whether these theories actually work in the real world, and whether chorus waves can really excite electrons and drive them into the atmosphere. The sensors that scientists earlier used were unable to differentiate between electrons entering the Earth’s atmosphere from other electrons whizzing around. To solve this problem, Professor Kasahara and his team created a new electron sensor that was capable of observing the precise movement of electrons participating in auroras. This new sensor was installed on Japan’s Exploration of energization and Radiation in Geospace (ERG) satellite, and it enabled scientists to observe electron scattering by chorus waves occurring across the Earth’s magnetosphere, thereby generating pulsating auroras.

“Auroral substorms are caused by global reconfiguration in the magnetosphere, which releases stored solar wind energy,” writes Satoshi Kasahara.

“They are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that eventually break up, and emerge as diffuse, pulsating auroral patches at dawn.”

Scientists now believe this phenomenon probably takes place on other planets, including Jupiter and Saturn, which exhibit chorus waves.

The detailed findings of the stud were published in Nature.