Rare 'Green Polar Rain' Aurora Observed from Earth for the First Time

In a remarkable and unprecedented event, a rare and eerie green aurora was observed from the ground over the Arctic during Christmas 2022. 

This unique phenomenon, described as a 'polar rain' aurora, left scientists both puzzled and fascinated. 

Now, Japanese and US-based researchers have uncovered the mystery behind this extraordinary celestial display.

The green glow that appeared during the holiday season was not the typical aurora display most are familiar with. 

Unlike the usual auroras, which are characterized by dynamic shapes and pulsating lights, this aurora was a smooth, featureless glow that spanned an impressive 2,485 miles (4,000 kilometers). 

The tranquil aurora, observed by an All-Sky Electron Multiplying Charge-Coupled Device (EMCCD) camera in Longyearbyen, Norway, had never been seen from Earth before.

Auroras generally occur when electrons from the solar wind become trapped in the Earth’s magnetotail — an extension of the Earth's magnetic field. 

These electrons travel along magnetic field lines to the poles, where they collide with atmospheric molecules, causing the sky to glow in various colors. 

However, the smooth aurora observed on December 25-26, 2022, was different. 

The solar wind had almost completely ceased, creating a calm region around Earth and allowing for this rare event to occur.

The research team, led by Keisuke Hosokawa from the Center for Space Science and Radio Engineering at the University of Electro-Communications in Tokyo, compared ground observations with data from the Special Sensor Ultraviolet Scanning Imager (SSUSI) on the Defense Meteorological Satellite Program (DMSP) satellites. 

These satellites confirmed that the aurora bore all the hallmarks of a rare 'polar rain' aurora, previously only observed from space.

The sun’s corona, full of holes particularly at higher solar latitudes, sometimes allows exceptionally fast solar wind to stream out. 

These coronal holes can occasionally appear at lower latitudes, as happened in December 2022. 

During this period, the regular solar wind pressure was negligible, permitting high-energy electrons from the coronal holes to travel unhindered across space and reach Earth’s poles.

Normally, these high-energy electrons are scattered by regular solar wind particles, but the absence of such interference allowed the electrons to travel directly to Earth’s magnetic poles. 

The open magnetic field lines from the coronal holes connected with Earth's magnetic field, creating a magnetic funnel with a diameter of about 4,600 miles (7,500 kilometers) at Earth’s distance from the sun. 

This funnel allowed the electrons to rain down directly onto the poles, causing the smooth, green aurora.

The polar rain aurora was purely green due to the high energy of the electrons, which ionized oxygen deeper in the atmosphere, producing a green glow rather than the red typically seen at higher altitudes. 

The DMSP satellites observed this aurora only over the north magnetic pole, tilted towards the sun during the Northern Hemisphere winter.

“When the solar wind disappeared, an intense flux of electrons with an energy of >1keV was observed by the DMSP, which made the polar rain aurora visible even from the ground as bright greenish emissions,” explained Hosokawa’s team in their published research.

This groundbreaking observation marks the first time a polar rain aurora has been seen from Earth. 

The phenomenon, previously studied by particle detectors on satellites, had remained invisible to the naked eye on the ground until now. 

The full explanation of this rare event is detailed in the June 21 edition of the journal Science Advances.

The rare 'green polar rain' aurora observed from Earth during Christmas 2022 offers new insights into the complex interactions between solar wind particles and Earth’s magnetic field. 

This discovery not only enriches our understanding of auroras but also highlights the dynamic and ever-changing nature of space weather. 

As scientists continue to study these phenomena, we can expect to uncover even more mysteries about the cosmos and its impact on our planet.