SDO Sundog Mystery   
       
   February 11, 2011: NASA's Solar Dynamics Observatory (SDO), best known for   
   cutting-edge images of the sun, has made a discovery right here on Earth.   
   [...]   
   How ice crystals make sundogs. [more] "It's a new form of ice halo," says   
   atmospheric optics expert Les Cowley of England. "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."   
       
   Ice halos are rings and arcs of light that appear in the sky when sunlight   
   shines through ice crystals in the air. A familiar example is the sundog-a   
   rainbow-colored splash often seen to the left or right of the morning sun.   
   Sundogs are formed by plate-shaped ice crystals drifting down from the sky   
   like leaves fluttering from trees.*   
       
   Last year, SDO destroyed a sundog-and that's how the new halo was discovered.   
   [...]   
   SDO lifted off from Cape Canaveral on Feb. 11, 2010-one year ago today. It was   
   a beautiful morning with only a handful of wispy cirrus clouds crisscrossing   
   the wintry-blue sky. As the countdown timer ticked to zero, a sundog formed   
   over the launch pad. Play the movie, below, to see what happened next-and   
   don't forget to turn up the volume to hear the reaction of the crowd:   
   [...]   
   SDO has a close encounter with a sundog. "The shock waves were amazing,   
   fantastical!" says high school student Amelia Phillips who watched the event   
   alongside friend and photographer Anna Herbst of Bishop, California. "We were   
   shouting and jumping up and down when SDO destroyed the sundog."  Movie   
   credit: Anna Herbst.   
       
   "When the rocket penetrated the cirrus, shock waves rippled through the cloud   
   and destroyed the alignment of the ice crystals," explains Cowley. "This   
   extinguished the sundog."   
   [...]   
   A luminous column of white light follows SDO into the sky. [more] 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 and followed   
   the rocket up into the sky," says Cowley. "We'd never seen anything like it."   
   [...]   
   Cowley and colleague Robert Greenler 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.   
       
   Then came the epiphany: The crystals weren't randomly scrambled, Cowley and   
   Greenler realized. On the contrary, the plate-shaped hexagons were organized   
   by the shock waves as a dancing army of microscopic spinning tops.   
       
   Cowley explains 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 26000   
   years. The motion is ordered and precise."   
   [...]   
   According to Cowley and Greenler, spinning and gyrating plate-shaped crystals   
   are responsible for the mystery halo. Credit: L. Cowley. Bottom line: 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 adds.   
       
   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. A picture of the hypothetical complete halo may be found   
   here.   
       
   "We'd love to see it again and more completely," says Cowley.   
       
   "If you ever get a once-in-a-lifetime opportunity to be at a rocket launch,"   
   he advises with a laugh, "forget about the rocket! Look out instead for halos."   
       
       
   Author: Dr. Tony Phillips | Credit: Science@NASA   
       
   More Information   
   Solar Dynamics Observatory -- home page   
       
   Cool Movie: SDO Destroys a Sundog -- Science@NASA   
       
   More images of the SDO-sundog encounter: from Romeo Durscher of Stanford,   
   California; from Barbara Tomlinson of Beachton, Georgia; from George C. Privon   
   of the University of Virginia.   
       
   Sundog Formation -- from Les Cowley's authoritative web site "Atmospheric   
   Optics"   
       
   * Regarding the statement "Sundogs are formed by plate-shaped ice crystals   
   drifting down from the sky....", Cowley notes that this can be an   
   oversimplification. "The crystals do not always drift down from the sky. They   
   drift slowly down relative to air currents in the cloud. Half of the time they   
   will be ascending relative to the ground. The drift velocity is only a few   
   mm/s."   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.59   
    * Origin: NCS BBS (1:3828/7)