Parallel lines of cumulus clouds often appear when frigid, dry winds rush over comparatively warm bodies of water. NASA satellites have observed the striking cloud formations – which atmospheric scientists call “cloud streets” -- over the Hudson Bay, Greenland Sea, Bering Sea, and the Amery Ice Shelf a number of times in the past. Recently, University of Wisconsin scientist Steve Ackerman was combing through data from NASA’s MODIS instrument as part of an effort to catalog and classify different cloud types. Something about the street clouds in this image of Lake Superior (above) struck him as peculiar. We caught up with him during a poster session at an American Geophysical Union meeting in San Francisco to find out more. WoE: What are we looking at here?
Ackerman: These are cloud streets. They’re really quite interesting clouds. They occur when you get cold air blowing over warm water. You get them frequently over the Great Lakes and off the East Coast as well.
WoE: What was it about this particular cloud street set that you found notable?
Ackerman: We actually looked at a series of these, and what we found was that the clouds start small, grow in altitude, get thicker optically, and then do something quite strange and unexpected.
WoE: Strange and unexpected? Please explain…
Ackerman: Yes, often what happens is that the size of the cloud droplets grow as we’d expect at first, but then partway across the lake the size of the particles starts to decrease.
WoE: And that’s surprising?
Ackerman: Yes, we have no idea why they’d do that. They should be getting progressively bigger as they move across the lake and pick up moisture.
WoE: About how big are these cloud droplets, and how do they change over time?
Ackerman: They start off at about 5 microns. (For reference, human hair is about 100 microns.) They grow up to about 20 microns, and then they drop down to 10 microns.
WoE: How long does that process take?
Ackerman: About four hours. 
WoE: Why do think it’s happening?
Ackerman: We’re really not sure. Perhaps dry air is coming in from above.
WoE: Is this the only time you’ve observed this phenomenon?
Ackerman: It’s pretty rare. We found it in the MODIS imagery in the five years that we looked about 15 times.
WoE: What makes a peculiar phenomenon like this worth studying?
Ackerman: The next step is to work with cloud modelers and to see if they’re modeling things well enough to explain what’s going on. If the models can’t recreate unusual events like these cloud streets, we know they’re not getting things right. We need models to get the global climate right, and also the weather prediction right.
--Adam Voiland, NASA's Earth Science News Team
Combined with the movie of swirling lights you might even have guessed some kind of spacecraft launch or radio tower.
In fact, both the sound and image are of completely natural origin. The movie shows what's known as a pulsating aurora – a very common, but hard to see, weak aurora that blinks on and off up to 12 times per minute in the night sky.
The sound is of something no one knew was connected to these auroras until recently: a special kind of electromagnetic wave some 40,000 km higher in Earth's magnetosphere called a chorus wave, since it sounds like birds chirping when played through a speaker. 
How the pulsating auroras form has long been a mystery. Stable auroras form when electrons and ionized particles from the solar wind travel down magnetic field lines towards Earth. These collide with nitrogen and oxygen particles in the ionosphere, some100 km above Earth, and the collisions send out blue, green, and red photons to create the colorful light shows of the aurora.
But no one knew what could cause an aurora to turn into a strobe light until scientists at UCLA looked at data from NASA's THEMIS spacecraft. They discovered that the auroras pulsed in sync to chorus waves far above Earth's atmosphere. The chorus waves apparently drive the light-inducing solar wind particles down to Earth following its own unique beat. Linking the two phenomena does more than explain the origins of the pulsating aurora. Using the electromagnetic waves and the aurora to define end points of magnetic field lines gives scientists a new tool to physically map Earth's constantly changing magnetic field. Knowing the way that the magnetic field moves, in turn, is crucial for understanding space weather and phenomena that can threaten Earth-observing satellites.
Top Image: Pulsating aurora image taken on Oct 30, 2008 in Laukvik, Lofoten Islands, Norway. Courtesy of Jan Koeman. Middle Image: A snapshot of the pulsating aurora taken by a ground-based camera. The black square in the middle is the THEMIS spacecraft. Bottom Image: Schematic diagram showing aurora over North America and spacecraft in space (magenta) embedded in the energetic plasma source (blue cloud). These two regions are connected by the Earth's magnetic field line. Energetic plasma interacts with waves (red) and precipitate into the upper atmosphere (blue arrows) and generate aurora. The geometry of the plasma cloud determines the aurora shape. Courtesy of Toshi Nishimura
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