Noctilucent clouds, behaving strangely

The southern season for noctilucent clouds (NLCs) has come to an end. NASA's AIM spacecraft observed the last wisps of electric-blue over Antarctica on Feb. 20, 2015. The end of the season was no surprise: The polar clouds always subside in late summer. Looking back over the entire season, however, reveals something unexpected. In an 8-year plot of Antarctic noctilucent cloud frequencies, the 2014-2015 season is clearly different from the rest:

© SpaceWeather

These data come from the AIM spacecraft, which was launched in 2007 to monitor NLCs from Earth orbit. The curves show the abundance ("frequency") of the clouds vs. time for 120 days around every southern summer solstice for the past 8 years.

"This past season was not like the others," notes Cora Randall, a member of the AIM science team and the chair of the Department of Atmospheric and Oceanic Sciences at the University of Colorado. "The clouds were much more variable, and there was an enormous decrease in cloud frequency 15 to 25 days after the summer solstice. That's when the clouds are usually most abundant."

What does this mean? Previous research shows that NLCs are a sensitive indicator of long-range teleconnections in Earth's atmosphere, which link weather and climate across hemispheres. The strange behavior of noctilucent clouds in 2014-2015 could be a sign of previously unknown linkages. "Preliminary indications are that it is indeed due to inter-hemispheric teleconnections," says Randall. "We're still analyzing the data, so stay tuned."

Now attention turns to the northern hemisphere, where the season for NLCs typically begins in May. Will the northern season ahead be as strangely variable as the southern season, just concluded? Says Randall, "I can't wait to find out."

Boosting the chances of auroras even more is a high-speed solar wind stream waiting on the other side of the heliospheric current sheet. The wind is flowing from a coronal hole on the sun, shown in this extreme UV image from NASA's Solar Dynamics Observatory:

© SpaceWeather

Coronal holes are places in the sun's atmosphere where the sun's magnetic field opens up, allowing hot gas to escape. In the image, above, the magnetic field is traced by curved white lines. Arrows show where the solar wind is escaping along open field lines. Forecasters expect the gassy stream to reach Earth on Feb. 28-March 1. Keep an eye on the realtime photo gallery for aurora sightings this weekend.