CHANCE OF STORMS THIS WEEK: NOAA forecasters estimate a 50% chance of polar geomagnetic storms on Dec. 30th when a co-rotating interaction region (CIR) is expected to hit Earth's magnetic field. CIRs are transition zones between fast- and slow-moving solar wind streams. Solar wind plasma piles up in these regions, producing density gradients and shock waves that do a good job of sparking auroras. Free: Aurora Alerts.
EVOLUTION OF A CORONAL HOLE: Christmas 2016 was special for sky watchers around the Arctic Circle. The skies filled with some of the best Northern Lights of the year, including rare outbursts of white and pink. The source of the display: A giant "coronal hole" in the sun's atmosphere sprayed our planet with solar wind. The hole opened up in July 2016 and it has been strobing Earth with solar wind every ~28 days ever since as the hole pirouettes with the slowly rotating sun.
Spaceweather.com reader Stuart Green has prepared a plot showing the evolution of the coronal hole and the effect it has had on the magnetic field at his private observatory in Preston, England. Click on the image to inspect the full 6 months:
Inset images come from NASA's Solar Dynamics Observatory. The coronal hole is the giant dark region, starting small in July, then growing and shape-shifting as the year unfolds.
The background strip chart recording shows the instability of the magnetic field around Green's private observatory. When the coronal hole is facing Earth, big changes are measured.
"I've been recording geomagnetic activity over the past three years using a home built/ home designed magnetometer," says Green. "The sensor is buried in my garden about 0.5m below the surface in an East/West orientation to allow very sensitive (sub nanotesla) measurements of magnetic declination during geomagnetic storms. The plots show the change in magnetic flux density in nanotesla occurring between readings every 2.5 minutes."
Green's presentation suggests that this yawning hole is a long-lived feature on the sun, and it will probably be back as potent as ever 28 days from now. Stay tuned for magnetic unrest--and more Arctic auroras--in January.IRIDESCENT CLOUDS OVER FRANCE: To see fantastic colors in the sky, it is not always necessary to travel to the Arctic Circle. Today, Dec. 27th, Antoine Gerhardt photographed this display over Serres, France:
"Such beautiful colors in the southern French sky," says Gerhardt.
These are iridescent clouds, colored by sunlight shining through tiny droplets of water. Many clouds contain water droplets, but only a few exhibit this kind of splendid iridescence. For such vivid pastel colors to appear, all of the droplets in the cloud must be of a uniform small size. Only then can the physics of diffraction work its color-creating magic.
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