Scattering of electrons responsible for northern lights observed for first time by scientists

The mystery behind the elusive phenomenon commonly known as the aurora, or “northern lights”, has been unravelled by scientists using measurements taken from space.

Auroras appear over both the Arctic and Antarctic – where the phenomenon is known as southern lights. They have inspired mythology and superstition for centuries.

In cultures traditionally based near the poles, the lights have been associated with everything from good luck to evil spirits.

Now, the origins of these light displays have been demystified after scientists directly observed showers of electrons thought to underpin some of these phenomena bouncing across the Earth’s magnetosphere for the first time.

Auroras take different forms, but perhaps the most stunning are the “pulsating auroras” that present not as sheets of green light but intense, flickering displays across the dark sky.

These lights cover tens to hundreds of kilometres, and appear around 100 kilometres above the Earth’s surface.

While the mechanisms underpinning auroras are roughly understood by scientists, the chain of events leading to pulsating auroras in particular have only ever been demonstrated in theory.

"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, a planetary scientist at the University of Tokyo who led the research.

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

Scientists have long thought pulsating auroras result from electrons in the magnetosphere – a region of space surrounding Earth in which the planet’s magnetic field exerts a strong effect – interacting with so-called “chorus waves”.

Intermittent bursts of chorus waves – natural oscillations in the Earth's outer magnetosphere – were thought to cause the pulsations of “electron precipitation” and associated auroral illumination.

Electrons usually bounce along the Earth’s geomagnetic field, but theories and computer simulations have suggested chorus waves were capable of scattering them.

According to these models, this scattering caused electrons to rain down into the upper atmosphere and result in the effects we can see from the Earth’s surface.

However, prior to the work by Professor Kasahara and his colleagues, it was unclear whether these theories played out in the real world. Specifically, scientists had no direct evidence that chorus waves were powerful enough to sufficiently excite electrons and drive them into the atmosphere.

This has been a difficult phenomenon to measure because traditional electron sensors lacked the capacity to distinguish electrons entering the Earth’s atmosphere from all the other electrons zooming around.

To overcome this, the research team led by Professor Kasahara designed a specialised electron sensor – mounted on Japan’s Exploration of energisation and Radiation in Geospace (ERG) satellite – which was capable of observing the exact movement of electrons participating in auroras.

Using this device, they could watch electron scattering by chorus waves taking place across the Earth’s magnetosphere.

The results of the study were published in the journal Nature.

According to the scientists, this same phenomenon is also likely to take place on other planets that exhibit chorus waves, including Jupiter and Saturn.

Making use of their novel sensor technology, the researchers now hope to develop their research further in order to understand the complex phenomena taking place high above our heads.

"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 Professor Kasahara.