Why Are Sunsets Turning Purple?

Aug. 28, 2019: Every year, on average, about 60 volcanoes erupt somewhere on Earth, shooting ashy plumes of sulfurous gas thousands of feet into the air. Rarely do those plumes make it all the way up to the stratosphere. This summer, however, two volcanoes have done it. The Raikoke volcano in the Kirul Islands (June 22nd) and the Ulawun volcano in New Guinea (Aug. 3rd) both punched through to the stratosphere, sending material as high as 60,000 ft.

The action of these two volcanoes may explain why many sky watchers are starting to notice purple sunsets. Juli Fowler of Albuquerque, New Mexico, photographed this example on Aug. 24th:

“Spectacular sunsets are a regular thing here in the Land of Enchantment, but imagine my surprise when I saw these violet beams caused by volcanic aerosols in the stratosphere,” says Fowler. “Wow!”

Why purple? Fine volcanic aerosols in the stratosphere scatter blue light which, when mixed with ordinary sunset red, produces a violet hue. The purple color is often preceded by a yellow arch hugging the horizon. As the sun sets, violet beams emerge from the yellow, overlapping to fill the western sky with a soft purple glow. High-quality pictures of the phenomenon often show horizontal bands cross-crossing the yellow arch. These bands are the volcanic gas.

Ray Majoran of London, Ontario, Canada, captured this dramatic image using a drone:

“Sunsets lately have a beautiful purple/pink glow to them, so I have been launching my Mavic 2 Pro drone to photograph them,” says Majoran. “This picture was taken just a few days after the Ulawun volcano eruption in New Guinea.”

Sky watchers shouldn’t expect to see purple every night. The volcanic gas appears to be patchily distributed, bringing strange sunsets on some nights, not all. Clear air, lack of clouds, and country settings improves their visibility. If you photograph one, send us your pictures!

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A New Source of Space Radiation

August 9, 2019: Astronauts are surrounded by danger: hard vacuum, solar flares, cosmic rays. Researchers from UCLA have just added a new item to the list. Earth itself.

“A natural particle accelerator only 40,000 miles above Earth’s surface is producing ‘killer electrons’ moving close to the speed of light,” says Terry Liu, a newly-minted PhD who studied the phenomenon as part of his thesis with UCLA Prof. Vassilis Angelopoulos.

This means that astronauts leaving Earth for Mars could be peppered by radiation coming at them from behind–from the direction of their own home planet.


NASA’s THEMIS spacecraft ran across the particles in 2008 not far from the place where the solar wind slams into Earth’s magnetic field. Researchers have long known that shock waves at that location could accelerate particles to high energies–but not this high. The particles coming out of the Earth-solar wind interface have energies up to 100,000 electron volts, ten times greater than previously expected.

How is this possible? Liu found the answer using THEMIS data and computer simulations of the sun-Earth interface. When the solar wind meets Earth, it forms a shock wave around Earth’s magnetic field, shaped like the bow waves that form ahead of a boat moving through water. Within this “bow shock” immense stores of energy can be abruptly released akin to the sonic boom of an airplane.

Liu found that some electrons are shocked not just once, but twice or more, undergoing mirror-like reflections within the bow shock that build energy to unexpected levels. Most of the boosted particles shoot back into space away from Earth.


Above: Terry Liu created this diagram showing the location of the natural particle accelerator and how it spews radiation into space.

“Similar particles have been detected near Saturn, suggesting that the process is at work there as well,” says Liu.

“Indeed,” adds Angelopoulos, “this type of particle acceleration could be happening throughout the cosmos–from supernovas to solar storms–wherever a supersonic wind hits a barrier such as Earth’s magnetosphere.”

Meanwhile, back home, Earth-orbiting satellites and departing astronauts have a new source of radiation to contend with. It’s right over their shoulder.

Read the original research at Science Advances.

Raikoke Sunsets

Aug. 4, 2019: Over the weekend in DeSoto, Kansas, something strange happened to the sunset: It turned purple. “On Saturday night, I photographed a large dome of pinkish-purple light,” reports Doug Zubenel. “Strong crepuscular rays were also visible.”

Photo credit: Doug Zubenel of DeSoto, Kansas. August 3, 2019.

Purple sunsets are a sign of volcanic activity. Fine volcanic aerosols in the stratosphere scatter blue light which, when mixed with ordinary sunset red, produces a violet hue.

But which volcano? The answer is probably Raikoke, a volcano in the Kuril islands which erupted with such force on June 22, 2019, that it was seen from the International Space Station. NASA satellites confirm that aerosols from Raikoke reached the stratosphere and they have been circulating around the Northern Hemisphere ever since.

A similar eruption occurred 11 years ago, in Aug. 2008, when Alaska’s Kasatochi volcano spewed sulfurous gases into the stratosphere. For months sky watchers witnessed strange sunsets whenever a plume of Kasatochi’s emissions drifted overhead. The same thing, apparently, is happening now thanks to Raikoke.

Photo credit: Heiko Ulbricht in the Zittau Mountains of southeastern Germany. July 26, 2019.

Volcanic sunsets have also been seen in Halifax, Nova Scotia; in the Zittau Mountains of Germany; in Joshua Tree, California; in Orange, California.

Purple isn’t the only thing to look for, says atmospheric optics expert Les Cowley. In addition, he advises, sky watchers should “be alert for a very bright yellow twilight arch, fine cloud structure in the arch seen through binoculars, and long diffuse rays and shadows.”

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