EXITING THE SOLAR WIND STREAM: After almost a week inside, Earth is exiting a stream of solar wind that has sparked bright auroras around the Arctic Circle. Another stream is coming. NOAA forecasters estimate a 40% chance of G1-class geomagnetic storms on Jan. 12th when the solar wind arrives. Free: Aurora Alerts
PILEUS CLOUDS: On Jan. 6th, Peter Lowenstein observed a rainbow-colored saucer over Mutare, Zimbabwe--but it wasn't a UFO. "This is a classic example of a pileus cloud," he says.
Pileus clouds form on sunny afternoons when the heat of the summer sun causes cumulus clouds to boil upwards. Roiling toward the sky, cumulus clouds push layers of moist air above them where they cool and condense to form droplet-rich cloud caps or 'pileus' (Latin for cap).
Sometimes, as in Mutare on Jan. 6th, pileus clouds form very quickly. In such cases their water droplets tend to be all the same size--the perfect condition for iridescent colors.
Lowenstein took four pictures over a period of just three minutes. "They show the cloud appearing, then changing shape and color," he says. "One minute later it had disappeared behind the summit of the growing cumulonimbus cloud."
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CORONAL HOLE: A relatively small hole has opened in the sun's atmosphere. On the sun, "small" means 25 times wider than Earth. NASA's Solar Dynamics Observatory took this picture of the coronal hole (CH) on Jan. 10th:
Coronal holes are places in the sun's atmosphere where the magnetic field opens up and allows solar wind to escape. A stream of solar wind flowing from this hole should reach Earth on Jan. 12th. Because the hole is small, the emerging solar wind stream is narrow. Earth will be inside it for only a day or so.
Coincidentally, Earth is expected to cross a fold in the heliospheric current sheet at about the same time. This will boost the chances of a polar geomagnetic storm to 40%, according to NOAA forecasters. Arctic sky watchers should be alert for auroras on Thursday and Friday nights. Free: Aurora Alerts
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