May 21, 2019: Every summer since the late 1970s, radars probing Earth’s upper atmosphere have detected strong echoes from altitudes between 80 km and 90 km. What’s up there? Noctilucent clouds (NLCs). NASA’s AIM spacecraft is still waiting to spot the first NLCs of the 2019 season, but the echoes have already begun. Rob Stammes of the Polarlightcenter in Lofoten, Norway, detected them on May 19th and 20th:
“I detected these VHF signals coming from transmitters in Eastern Europe,” he explains. “Before they reached my receiver in Norway, they bounced off something in the mesosphere. The patterns were recognizable and very strong.”
Researchers call them “Polar Mesospheric Summer Echoes” or “PMSEs.” They occur over the Arctic during the months of May through August, and over the Antarctic during the months of November through February. These are the same months that NLCs appear.
The underlying physics of these echoes is still uncertain. A leading theory holds that the ice particles in noctilucent clouds are electrically charged, and this makes them good reflectors of radio waves. However, NLCs are not always visible when the radar echoes are observed and vice versa.
The echoes Stammes detected suggest that the season for NLCs is about to begin.
“It certainly should be starting soon!” says Cora Randall of the AIM science team at the University of Colorado. “We’ve been looking at MLS temperature and water vapor data. As of last week, the north polar mesopause was colder and wetter than in any other years of the AIM record at this time.” In other words, conditions are ripe for water vapor to crystallize around meteor smoke, forming icy tendrils of electric-blue at the edge of space.
High-latitude sky watchers should be alert for their return. Observing tips: Look west 30 to 60 minutes after sunset when the sun has dipped 6o to 16o below the horizon. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud.