First-Ever Sighting of Rare ‘Polar Rain’ Aurora from Earth

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A rare and mysterious aurora, unlike any seen before from the ground, was observed over the Arctic on Christmas Day 2022.

This unique “polar rain” aurora created a smooth, eerie green glow in the sky, captured by a camera in Longyearbyen, Norway.

What is a Polar Rain Aurora?

Unlike typical auroras that display vibrant, moving patterns, the polar rain aurora appeared as a faint, featureless glow spanning 2,485 miles (4,000 kilometers).

This kind of aurora has only been seen from space until now.

It was characterized by its smooth, steady light without the usual dancing or pulsing shapes.

How Auroras Usually Form

Auroras typically occur when electrons from the solar wind interact with Earth’s magnetic field, becoming trapped in a region called the magnetotail.

When these electrons collide with molecules in Earth’s atmosphere, they create colorful displays of light.

The colors vary based on the type of molecule and altitude:

  • Nitrogen: Produces blue emissions at lower altitudes.
  • Oxygen: Produces green emissions at higher altitudes and red emissions at even higher altitudes.

What Made the Christmas 2022 Aurora Unique?

The Christmas 2022 aurora was different due to a rare event where the solar wind, usually traveling at 250 miles (400 km) per second, almost completely stopped.

This calm allowed high-energy electrons from the sun, streaming at up to 500 miles (800 km) per second, to reach Earth unhindered.

These electrons traveled directly along open magnetic field lines from the sun’s surface to Earth’s north pole.

The Discovery Process

Researchers led by Keisuke Hosokawa from the University of Electro-Communications in Tokyo studied this event using satellite data.

They compared ground observations with images from the Special Sensor Ultraviolet Scanning Imager (SSUSI) on polar-orbiting satellites.

The satellites confirmed the aurora’s characteristics matched those of a polar rain aurora.

The data showed:

  • Satellite Imagery: Revealed a smooth, uniform aurora over the north polar cap.
  • Energy Measurements: Indicated high-energy electrons (>1 keV) were responsible for the aurora.

Why It Happened

The smooth, green aurora was due to the lack of regular solar wind, allowing high-energy electrons to flow directly onto Earth’s poles.

These electrons caused the green glow by ionizing oxygen molecules deep in the atmosphere.

Normally, the solar wind scatters these electrons, but the absence of the solar wind allowed them to travel directly along open magnetic field lines from the sun to Earth.

Detailed Explanation

The sun’s corona, especially at higher solar latitudes, has areas called coronal holes.

These holes allow high-energy solar wind particles to escape.

During this event:

  1. Coronal Hole Formation: A coronal hole formed at a lower latitude, sending a fast solar wind towards Earth.
  2. Solar Wind Cessation: The regular solar wind pressure dropped, creating a calm region around Earth.
  3. Direct Electron Travel: High-energy electrons from the coronal hole traveled directly along magnetic field lines connected to Earth’s north pole.
  4. Aurora Formation: These electrons rained down on the polar region, creating the smooth, green aurora.

The diameter of the magnetic funnel created by the coronal hole was about 4,600 miles (7,500 km), larger than Earth’s north polar cap, explaining the wide extent of the aurora.

Conclusion

The polar rain aurora, seen for the first time from Earth, was a remarkable event linked to a rare solar wind condition.

This discovery adds a new chapter to our understanding of auroras and space weather.

For more details, the full study can be found in the June 21st edition of Science Advances.


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