CHANCE OF STORMS: A high-speed stream of solar wind is approaching Earth. NOAA forecasters estimate a 15% chance of polar geomagnetic storms when the stream arrives on April 16th increasing to 55% on April 17th as it fully envelops our planet's magnetic field. High-latitude sky watchers should be alert for auroras. Free: Aurora Alerts
AURORAS VS. TWILIGHT: Around the Arctic Circle, aurora season is coming to an end as waxing Spring twilight washes out the velvety black night sky. But it's not over yet. Last night in Salla, Finland, B.Art Braafhart photographed a green band pushing back against the midnight sun:
"Around midnight On April 14-15, while the sunset sky was still visible, a bright auroral arch appeared, forming a kind of gate with smaller activity areas inside," says Braafhart. "Meanwhile the temperature dropped to -13C. The season continues, so keep an eye on the sky!"
The display was caused by a coronal mass ejection (CME), which brushed against Earth's magnetic field on April 14th. The CME's weak impact did not spark a geomagnetic storm. However, near-storm conditions developed during the early hours of April 15th as Earth passed through the CME's magnetized wake. More auroras are in the offing as a solar wind stream approaches Earth ETA: April 16th and 17th. Free: Aurora Alerts.
DOUBLE RAINBOW: Whenever you see one rainbow, look for another, because rainbows always come in pairs. Consider this example, photographed by Kamila Mazurkiewicz Osiak of Trzcianki, Poland, on April 13th:
"This huge double rainbow appeared after an intense rainstorm passed through our area," says Osiak. "It was raining so hard, it looked like the clouds were walking on the ground."
The bright inner rainbow is the primary bow, caused by sunbeams reflecting once inside falling raindrops. It's the bow you usually see. The less-bright outer rainbow is the secondary bow, caused by sunbeams reflecting twice inside raindrops. Secondary bows often go unnoticed, because they are usually very faint, but they are always there.
In Osiak's case, "you couldn't miss it," she says. Bright sunlight lancing through the clearing clouds struck droplet-laden air behind the retreating storm, resulting in an unusually intense secondary bow.
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