NLCs, Stuck Inside the Arctic Circle

June 14, 2021: The season for noctilucent clouds (NLCs) is now 3 weeks old. This animation from NASA’s AIM spacecraft shows a thickening spiral of frosted meteor smoke around the North Pole:


Above: Noctilucent clouds from May 21st through June 11th. Credit: NASA/AIM

Noctilucent clouds form every year, approximately now, when summertime wisps of water vapor rise to the edge of space and crystallize around disintegrated meteoroids. The icy clouds float 83 km above Earth’s surface, making them (by far) our planet’s highest clouds.

Mid-June to July is typically when NLCs are most widespread. At the moment, though, they’re mostly confined inside the Arctic Circle. Some spillage into mid-latitudes has been reported from Canada, Scotland, Poland, the Netherlands, and England. This is a far cry from the low latitude excursions of recent years. In June 2019, for instance, NLCs were seen as far south as Los Angeles and Las Vegas.

What’s holding the clouds back? To answer this question, Lynn Harvey of the University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP) took a look at current conditions in the mesosphere where NLCs form. “The time series below shows that water vapor in the mesosphere is relatively high,” she notes.

This is actually good news for noctilucent clouds, which form only when the mesosphere is both cold and wet. In the plot, red traces 2021. This year has been among the wettest since AIM was launched in 2007.

Now for the not-so-good news: Harvey has also looked at temperature in the mesosphere and “it is just ‘middle of the road,'” she says. Unremarkable cold above the North Pole appears to be slowing the clouds’ growth.

There is plenty of water in the mesosphere. The temperature just needs to drop so more H2O molecules can latch on to specks of meteor smoke. If that happens, 2021 could still shape up to be a good year for noctilucent clouds.

Stay tuned.

The Termination Event

June 10, 2021: Something big may be about to happen on the sun. “We call it the Termination Event,” says Scott McIntosh, a solar physicist at the National Center for Atmospheric Research (NCAR), “and it’s very, very close to happening.”

If you’ve never heard of the Termination Event, you’re not alone.  Many researchers have never heard of it either. It’s a relatively new idea in solar physics championed by McIntosh and colleague Bob Leamon of the University of Maryland – Baltimore County. According to the two scientists, vast bands of magnetism are drifting across the surface of the sun. When oppositely-charged bands collide at the equator, they annihilate (or “terminate”). There’s no explosion; this is magnetism, not anti-matter. Nevertheless, the Termination Event is a big deal. It can kickstart the next solar cycle into a higher gear.

Above: Oppositely charged magnetic bands (red and blue) march toward the sun’s equator where they annihilate one another, kickstarting the next solar cycle. [full caption]

“If the Terminator Event happens soon, as we expect, new Solar Cycle 25 could have a magnitude that rivals the top few since record-keeping began,” says McIntosh.

This is, to say the least, controversial. Most solar physicists believe that Solar Cycle 25 will be weak, akin to the anemic Solar Cycle 24 which barely peaked back in 2012-2013. Orthodox models of the sun’s inner magnetic dynamo favor a weak cycle and do not even include the concept of “terminators.”

“What can I say?” laughs McIntosh. “We’re heretics!”

The researchers outlined their reasoning in a December 2020 paper in the research journal Solar Physics. Looking back over 270 years of sunspot data, they found that Terminator Events divide one solar cycle from the next, happening approximately every 11 years. Emphasis on approximately. The interval between terminators ranges from 10 to 15 years, and this is key to predicting the solar cycle.

Above: The official forecast for Solar Cycle 25 (red) is weak; McIntosh and Leamon believe it will be more like the strongest solar cycles of the past.

“We found that the longer the time between terminators, the weaker the next cycle would be,” explains Leamon. “Conversely, the shorter the time between terminators, the stronger the next solar cycle would be.”

Example: Sunspot Cycle 4 began with a terminator in 1786 and ended with a terminator in 1801, an unprecedented 15 years later. The following cycle, 5, was incredibly weak with a peak amplitude of just 82 sunspots. That cycle would become known as the beginning of the “Dalton” Grand Minimum.

Solar Cycle 25 is shaping up to be the opposite. Instead of a long interval, it appears to be coming on the heels of a very short one, only 10 years since the Terminator Event that began Solar Cycle 24. Previous solar cycles with such short intervals have been among the strongest in recorded history.

These ideas may be controversial, but they have a virtue that all scientists can appreciate: They’re testable. If the Termination Event happens soon and Solar Cycle 25 skyrockets, the “heretics” may be on to something. Stay tuned for updates.

Sunrise Solar Eclipse

June 7, 2021: Sunrise has never been so beautiful … or weird. On Thursday, June 10th, dawn will break over the northeastern USA and Canada with a solar eclipse in progress. This map from GreatAmericanEclipse.com shows who can see it:

Beach communities up and down the Atlantic Coast will have a great view of the sun rising over ocean waves. If you’re in New York City, find a tall building with an unobstructed view of the eastern horizon; an eclipse over the cityscape is a great photo-op. Later, after the eclipsed sun climbs into the morning sky, iconic landmarks such as the Statue of Liberty can be framed next to the fiery crescent.

This is not a total eclipse. It’s annular; the Moon is a fraction too small to cover the entire solar disk. Even when the Moon is dead-center in front of the sun, a little bit of sun will stick out around the Moon’s circumference, forming the fabled “ring of fire.” Only a few people in the northern reaches of Canada, Greenland and Russia will see it: visibility map.

For most people, the sun will look like a crescent. Dennis Put of Maasvlakte, the Netherlands, photographed a similar eclipse on Jan. 4, 2011:

“The eclipse was absolutely stunning!” recalls Put. “At first some clouds threatened to hide the event–an eclipse of the eclipse! I was very pleased to see the two peaks of the crescent sun finally emerging from the morning clouds.”

Similar scenes will play out on June 10th over urban population centers such as New York City, Philadelphia, Washington DC, Montreal and Toronto. Much of the action occurs at the crack of dawn, so plan to wake up early. Larry Koehn of ShadowandSubstance.com has posted a good discussion of viewing times.

Warning: Even during an eclipse, the sun can damage your eyes. Always use safe solar filters and ISO-approved eclipse viewing glasses.

Noctilucent Cloud Season is Getting Longer

June 3, 2021: No it’s not your imagination. Noctilucent cloud (NLC) season really is getting longer. New data from NASA’s AIM spacecraft show the first NLCs of summer have been trending earlier since the spacecraft was launched in 2007. This plot prepared by Cora Randall of the University of Colorado’s Laboratory for Atmospheric and Space Physics shows the change:

Each little blue box shows the day of year when AIM’s CIPS sensor detected the first NLC of northern summer. “The season appears to be starting earlier, which is making it longer by about 5 days,” says Randall.

Interestingly, the season is not also ending later; it still stops in August. Nevertheless, the early start is giving sky watchers an extra 5 days a year of noctilucent clouds.

The first NLCs of the season typically appear inside the Arctic Circle. Then, they spin outward to lower latitudes–a process which is underway now:

Above: 12 days of NLCs (May 21-June 1, 2021). Credit: AIM/CIPS

In recent years, NLCs have spilled as far south as Los Angeles and Las Vegas, setting records for low-latitude sightings. Mid-June to July is when the clouds are most widespread.

Observing tip: Look west 30+ minutes after sunset. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud.