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 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 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.

2021 May Be A Good Year for Noctilucent Clouds

May 27, 2021: Something unusual is happening at the top of Earth’s atmosphere. Noctilucent clouds (NLCs) are forming, and people are seeing them from the ground even though it is only May. Andy Stables sends this photo from the Isle of Skye, Scotland, taken May 26th:

The electric-blue ripples “were clearly visible to the unaided eye,” says Stables. “This is the earliest I have ever seen them here in Scotland.”

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space about 83 km above the ground. The clouds form when summertime wisps of water vapor rise up to the mesosphere, allowing water to crystallize around specks of meteor smoke. The season for bright naked-eye NLCs typically stretches from June through August.

This year NLCs are getting an early start. We’ve already received multiple reports of sightings in Europe from latitudes as low as 50 degrees, and according to NASA’s AIM spacecraft the clouds are rapidly intensifying. In only 4 days since the clouds were first spotted, their coverage of the Arctic has multiplied 10-fold:

The reason may be extra water in the mesosphere: NASA satellite data show that 2021 is one of the wettest years since 2007. NLCs have more H2O to work with–hence the early start and rapid growth.

In recent years, summertime noctilucent clouds have spilled as far south as Los Angeles and Las Vegas, setting records for low-latitude sightings. 2021 is shaping up to be such a year. Pro tip for northern sky watchers: Look west 30+ minutes after sunset. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud

Noctilucent Clouds are Back

May 22, 2021: The 2021 season for noctilucent clouds (NLCs) is underway. NASA’s AIM spacecraft detected the first electric-blue NLCs over Ellesmere Island in northern Canada on May 20th:

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space about 83 km above the ground. The clouds form around Earth’s poles when summertime wisps of water vapor rise up to the mesosphere, allowing water to crystallize around specks of meteor smoke. In recent years they have spread as far south as Los Angeles and Las Vegas, setting records for low-latitude sightings.

This year the mesosphere is unusually wet. “2021 is one of the wettest years in the AIM record,” says Lynn Harvey of the University of Colorado’s Laboratory for Atmospheric and Space Physics, who processed data from NASA’s Microwave Limb Sounder (MLS) to check conditions in the noctilucent zone. Click here to see her results. Only once or twice in the past 14 years have NLCs had more water to work with.

The extra water bodes well for a strong season. In fact, there have already been ground sightings. “We saw the first NLC from Germany on May 21st,” reports Gerd Baumgarten of the Leibniz-Institute of Atmospheric Physics. “The clouds appeared in the feed from one of our high resolution cameras located in Collm (51.3N, 13.0E).”

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“The display was quite faint,” he notes. They may not remain faint for long, however. Previous data from AIM show that NLCs are like a great “geophysical light bulb.” They turn on every year in late spring, reaching almost full intensity over a period of no more than 10 days.

Observing tips: Look west 30 to 60 minutes after sunset. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud. Early season NLCs are typically confined to high latitudes, spreading south as northern summer unfolds.

100 Years Later: The Great Geomagnetic Storm of May 1921

May 15, 2021: You know a solar storm is serious when buildings burst into flame. Sounds crazy? It really happened 100 years ago today.

On May 15, 1921, the biggest solar storm of the 20th century hit Earth. Around 02:00 GMT that Sunday morning a telegraph exchange in Sweden burst into flames. Across the Atlantic, the same thing was going on in New York. Flames engulfed the switch-board at the Brewster station of the Central New England Railroad and quickly spread to destroy the whole building. During the conflagration, long distance telephone lines burned out in New Brunswick; voltages on telegraph lines in the USA spiked as high as 1000 V; and auroras were sighted by ships at sea crossing the equator. It was a Big. Solar. Storm.

The outburst happened during the lazy tail end of Solar Cycle 15, an unremarkable cycle that was almost over in 1921. Sunspot numbers were low–but it only took one. Giant sunspot AR1842 appeared in mid-May and started flaring, hurling multiple coronal mass ejections (CMEs) toward Earth. In those days scientists had never heard of “CMEs,” so they were completely surprised when the clouds of plasma struck Earth. Around the world, magnetometers suddenly went offscale, pens in strip chart recorders pegged uselessly to the tops of their papers.

In response to the pummeling, Earth’s magnetic field swayed back and forth, rippling with energy. Fires were a direct result. Physics 101: When a magnetic field changes rapidly, electricity flows through conductors in the area. It’s called “magnetic induction.” Early 20th century telegraph lines suddenly found themselves buzzing with induced currents. In Sweden and New York, wires grew so hot they ignited telegraph papers and other combustibles.

What would happen if the same storm struck today? A 2013 Royal Academy of Engineering report summarizes the possibilities. Suffice it to say, fire would be the least of our worries. Modern technology is far more sensitive to solar activity than the simple copper wires of 1921. The same solar storm today could black out regional power grids, expose air travelers to radiation, knock out satellites, and disable radio-based technologies such as GPS.

Loss of electricity is often cited as the worst likely side-effect of a solar superstorm, but power systems are more resilient than they used to be. Thanks to improvements made after the Great Quebec Blackout of 1989, many modern grids would bounce back quickly. A more worrisome loss might be GPS. We think of GPS as our main way of finding things: ambulances finding accidents, pilots finding runways, and so on. But there’s more to it than that. GPS tells us what time it is, a service of atomic clocks onboard the satellites. In fact, GPS time is woven into the fabric of modern society.

Consider the following paragraph from a report in the Atlantic entitled “What Happens if GPS Fails?

“Telecom networks rely on GPS clocks to keep cell towers synchronized so calls can be passed between them. Many electrical power grids use the clocks in equipment that fine-tunes current flow in overloaded networks. The finance sector uses GPS-derived timing systems to timestamp ATM, credit card, and high-speed market transactions. Computer network synchronization, digital television and radio, Doppler radar weather reporting, seismic monitoring, even multi-camera sequencing for film production—GPS clocks have a hand in all.”

“What if all these flying clock radios were wiped out, and everything on the ground started blinking 12:00?” asks the author, Dan Glass. Answer: “Nobody knows.”

Space weather scholars routinely call the May 1921 event a “100 year storm.” However, recent research (both historical and statistical) suggests that such storms come along more often–every 40 to 60 years. Either way, we’re overdue.

Happy 100th anniversary, May 1921!

Additional Reading:

The Great Storm of May 1921: An Exemplar of a Dangerous Space Weather Event” by Mike Hapgood (Rutherford Appleton Laboratory, UK)

Intensity and Impact of the New York Railroad Superstorm of May 1921” by Jeffrey Love (US Geological Survey) and colleagues.

The Sodium Tail of Mercury

May 9, 2021: The biggest comet in the Solar System is actually a planet. It’s Mercury. Researchers have known for years that Mercury has an enormous tail. Last week, Andrea Alessandrini photographed it from the balcony of his house in Veroli, Italy:

“I took the picture on May 5th using a 66 mm (2.5 inch) refracting telescope and a Pentax K3-II camera,” says Alessandrini, an amateur astronomer who works by day as an aerospace engineer. “This is a 7 minute exposure @ ISO 1000.”

First predicted in the 1980s, Mercury’s tail was discovered in 2001. Its source is Mercury’s super-thin atmosphere. Mercury is so close to the sun, pressure from sunlight itself can push atoms out of the atmosphere and into space. The escaping gas forms a tail more than 24 million km long.

The key to detecting Mercury’s tail is sodium. There are many elements in Mercury’s tail; sodium is only one. But because sodium is so good at scattering yellow light, it is the best element for tracing the long plume of gas. “I use a special 589 nm filter tuned to the yellow glow of sodium,” says Alessandrini. “Without that filter, Mercury’s tail would be invisible.”

NASA’s MESSENGER spacecraft spent years observing Mercury’s tail often from close range. This movie shows how the brightness of the tail varies as Mercury orbits the sun:

For reasons having to do with the Doppler shift of sodium absorption lines in the solar spectrum, Mercury’s tail is brightest when the planet is ±16 days from perihelion (closest approach to the sun). Read the research here.

That special date is this week: On May 13th, Mercury will be 16 days past perihelion and the tail could be as much as 10 times brighter than Alessandrini saw last week. Coincidentally, on that same day the crescent Moon will pass by Mercury in the evening sky. Can you say “photo-op”? Just don’t forget your sodium filter…. [sky map]

China’s New Space Station is Visible from Earth

May 6, 2021: China’s new space station is just getting started, but it’s already putting on a good show. “I’ve seen it twice this week,” reports Mark A. Brown of Marion, Iowa. “The space station’s Tianhe-1 module has been making brilliant passes across Middle America.” Here it is rivaling the star Vega on May 4th:

“In the photo, I also caught the tumbling Long March 5B booster, which helped loft the module into orbit,” says Brown. “The booster flares brilliantly (visual magnitude -1) in reflected sunlight as it tumbles along Tianhe-1’s path. Both objects are easily seen with the naked eye.”

China launched the Tianhe-1 module on April 29th. It is the first of three modules that will eventually join to create the Chinese Space Station (CSS). When the CSS is finished, it will be about as big as Russia’s old Mir space station, roughly a quarter of the mass of the ISS. Three astronauts will live onboard.

On May 5th, Tianhe-1 flew over Manorville, New York, where Philip Smith photographed it through a 14-inch telescope:

“I was lucky to get this image,” says Smith. “The module was already 56 degrees above the horizon when it popped out of Earth’s shadow–so I didn’t have much time. Less than a minute later it was at its maximum altitude of 76 degrees, and that’s when I caught it.”

Readers, you can see the new space station with your own eyes. Visit and select flyby predictions for Tianhe-1. Many towns and cities in the USA have a good view this week. Pro tip: Go outside at least 10 minutes early; you might see the tumbling booster rocket, too.

EXTRA: The Long March 5B booster rocket is out of control. Forecasters expect it to re-enter Earth’s atmosphere on May 8th or 9th, with pieces landing … no one knows where. There is a 70% chance of an ocean splashdown, and experts say human injuries are unlikely.

A Timeline of Great Aurora Storms

April 30, 2021: Imagine living in Florida. You’ll never see the Northern Lights … right? Actually, the odds may be better than you think. A new historical study just published in the Journal of Space Climate and Space Weather shows that great aurora storms occur every 40 to 60 years.

“They’re happening more often than we thought,” says Delores Knipp of the University of Colorado, the paper’s lead author. “Surveying the past 500 years, we found many extreme storms producing auroras in places like Florida, Cuba and Samoa.”

This kind of historical research is not easy. Hundreds of years ago, most people had never even heard of the aurora borealis. When the lights appeared, they were described as “fog,” “vapors”, “spirits”–almost anything other than “auroras.” Making a timeline 500 years long requires digging through unconventional records such as personal diaries, ship’s logs, local weather reports–often in languages that are foreign to the researchers.

“We defined a ‘Great Storm’ simply as one in which auroras were visible to the unaided eye at or below 30 degrees magnetic latitude,” says Knipp. 

Visual sightings were key. The human eye is a sensor we’ve had in common with observers since the beginning of recorded history. Pre-modern scientists didn’t have satellites or magnetometers to measure solar storms, but they could look up at the night sky. In all, Knipp’s team tallied 14 examples of storms where many people saw auroras within 30 degrees of the equator.

“There may be more,” she notes. “For example, I am aware of a low latitude event that occurred between February and April 1648. It’s not on the timeline, though, because we haven’t yet been able to pin down the date.”

Look at the timeline again; there’s a whole cluster of sightings in Sept. 1770. “The Great Storm of 1770 appears to be a 500-year event,” says Knipp. “There were low-latitude auroras for 9 nights in a row.”

Above: An eyewitness sketch of red auroras over Japan in mid-September 1770. [ref]

During the 1770 storm, extremely bright red auroras blanketed Japan and parts of China. Captain James Cook himself saw the display from the HMS Endeavour near Timor Island, south of Indonesia. Knipp’s colleague Hisashi Hayakawa (Nagoya University) has found drawings of the instigating sunspot; it is twice the size of the sunspot that caused the infamous Carrington Event of 1859. Knipp’s timeline suggests that this was not “just another Great Storm”; something exceptional happened in 1770 that researchers still don’t fully understand.

Today’s senior space weather researchers were taught in school that Great Storms are rare. The Carrington Event was long thought to be a singular event, alone in the historical record. Recent studies are finding otherwise. Just last month Jeffrey Love of the US Geological Survey published a paper in the research journal Space Weather showing that extreme geomagnetic storms recur every ~45 years or so–a result in accord with Knipp’s. He used completely different techniques (extreme value statistics and magnetometer records) to reach a similar conclusion. 

The last Great Storm in Knipp’s timeline occurred 32 years ago. Soon, it will be time for another. Stay tuned.