NASA's SDO approaches a sundog - the rainbow-colored splash - before destroying it. Click the link below to watch what happens.
Credit: NASA
MARYLAND: One year ago, NASA's Solar Dynamics Observatory (SDO) amazed observers when it destroyed a sundog en route to orbit. A new analysis of the event is shedding light on the surprising way rocket shock-waves interact with clouds.
Sundogs are rainbow-colored splashes formed by plate-shaped ice crystals drifting down from the sky that are often seen to the left or right of the morning Sun. As the SDO rocket pierced the ice crystal alignment in 2010, a mysterious ‘rocket halo’ was produced.
"It's a new form of ice halo," said Les Cowley from the British Atmospheric Optics site of the phenomenon that causes rings and arcs of light to appear when sunlight shines through ice crystals in the air. "We saw it for the first time at the launch of SDO - and it is teaching us new things about how shock waves interact with clouds."
The mystery of the ‘rocket halo’
SDO lifted off from Cape Canaveral on 11 February 2010. As the countdown timer ticked to zero, a sundog formed over the launch pad. "When the rocket penetrated the cirrus, shock waves rippled through the cloud and destroyed the alignment of the ice crystals," said Cowley. "This extinguished the sundog."
The sundog's destruction was understood. The events that followed, however, were not. "A luminous column of white light appeared next to the Atlas V - the latest in a family of U.S. space launch vehicles - and followed the rocket up into the sky," said Cowley. "We'd never seen anything like it."
Cowley and colleague Robert Greenler from the University of Wisconsin-Milwaukee set to work figuring out what the mystery-column was. Somehow, shock waves from the rocket must have scrambled the ice crystals to produce the 'rocket halo.' But how? Computer models of sunlight shining through ice crystals tilted in every possible direction failed to explain the SDO event.
New research field of halo dynamics
Then came the epiphany: The crystals weren't randomly scrambled, Cowley and Greenler realised. On the contrary, the plate-shaped hexagons were organised by the shock waves as a dancing army of microscopic spinning tops.
Cowley explained their successful model: "The crystals are tilted between 8 and 12 degrees. Then they gyrate so that the main crystal axis describes a conical motion. Toy tops and gyroscopes do it. The earth does it once every 26,000 years. The motion is ordered and precise."
Blasting a rocket through a cirrus cloud can produce a surprising degree of order. "This could be the start of a new research field - halo dynamics," he added.
Forget about the rocket
The simulations show that the white column beside SDO was only a fraction of a larger oval that would have appeared if the crystals and shock waves had been more wide-ranging.
"We'd love to see it again and more completely," said Cowley.
"If you ever get a once-in-a-lifetime opportunity to be at a rocket launch," he said with a laugh, "forget about the rocket! Look out instead for halos."
