The Synchronic Bands of Comet NEOWISE

July 17, 2020: Comet NEOWISE (C/2020 F3) is doing something usually reserved for Great Comets. It has sprouted synchronic bands. Also known as “striae,” these bands divide the comet’s dust tail into linear regions of greater and lesser density. Chris Cook of Cape Cod, MA, captured the phenomenon on the evening of July 15th:


“Comet NEOWISE is now in its full glory for northern hemisphere observers,” says Cook. “This image is a stack of thirty 25s exposures at ISO1600. It clearly shows the formation of synchronic bands within the dust tail.”

Synchronic bands have been seen in comet tails for centuries, yet only recently have astronomers begun to understand what they are. The turning point came in 2007 when European and NASA spacecraft observed the formation of striae in Comet McNaught (C/2006 P1). The process starts when a chunk of comet detaches itself from the nucleus. Boulder-sized chunks fragment into smaller and smaller pieces, a cascade shaped into long streamers by solar radiation pressure.


Above: A close-up of the dusty striae photographed by Chris Cook

A few years ago, then-PhD student Ollie Price of University College London’s Mullard Space Science Laboratory was looking at old pictures of McNaught’s striae and noticed some “weird goings-on.” The bands were occasionally being bent and disrupted by some invisible force. “So I set out to investigate what might have happened to create this weird effect,” he recalls.

Price and colleagues ultimately found the answer. The disruptions occured when Comet McNaught crossed the heliospheric current sheet (HCS)–a vast wavy structure in interplanetary space separating regions of opposite magnetic polarity. “It appears the dust may be electrically charged, and gets rearranged as it crosses the HCS boundary,” says Karl Battams of the Naval Research Lab, a co-author of their 2018 paper.

Could the same thing happen to Comet NEOWISE? It’s possible. Photographers monitoring NEOWISE are encouraged to keep a sharp eye on the striae. Changes may be in the offing. Sky maps: July 18, 19, 20.

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A Surprise Visit from STEVE

July 16, 2020: Even STEVE wants to see Comet NEOWISE. On July 14th, the geomagnetic phenomenon appeared over Canada, streaking the sky with mauve ribbons of light. Harlan Thomas of Calgary, Alberta, reports: “I was out shooting the comet when I noticed a mauve-looking cloud. Wow!” I thought. “STEVE has come to visit NEOWISE. How cool is that?”


STEVE is a recent discovery. It looks like an aurora, but it is not. The purple glow is caused by hot (3000°C) ribbons of gas flowing through Earth’s magnetosphere at speeds exceeding 6 km/s (13,000 mph). It appears during some geomagnetic storms, often alongside a type of green aurora known as the “picket fence,” also shown in Thomas’s photo.

Statistics suggest that STEVE appears most often in spring and fall. What summoned STEVE in mid-summer? It may have been a CME that grazed Earth’s magnetic field on July 13th. As our planet passed through the CME’s magnetized wake on July 14th, hot currents and plasma waves rippled through Earth’s magnetosphere. STEVE was the result.

Christy Turner saw it too:


“I was shooting the comet outside Calgary when STEVE started to form,” she says. “It was a huge purple pillar–a total delight!”

Many observers across western Canada witnessed the display. During a normal summer, STEVE might have been overlooked, but with Comet NEOWISE drawing photographers outdoors, his visit was well documented. “Summertime STEVE” might be more common than previously thought.

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Solar Cycle Update

July 14, 2020: NOAA has released a new interactive tool to explore the solar cycle. It lets you scroll back through time, comparing sunspot counts now to peaks and valleys of the past. One thing is clear. Solar Minimum is here, and it’s one of the deepest in a century.


Solar Minimum is a natural part of the solar cycle. Every ~11 years, the sun transitions from high to low activity and back again. Solar Maximum. Solar Minimum. Repeat. The cycle was discovered in 1843 by Samuel Heinrich Schwabe, who noticed the pattern after counting sunspots for 17 years. We are now exiting Solar Cycle 24 and entering Solar Cycle 25.

During Solar Minimum, the sun is usually blank–that is, without sunspots. The solar disk often looks like a big orange billiard ball:


The spotless sun on July 13, 2020

In 2019, the sun went 281 days without sunspots, and 2020 is producing spotless suns at about the same rate. To find a year with fewer sunspots, you have to go all the way back to 1913, which had 311 spotless days. This makes 2019-2020 a century-class Solar Minimum; solar flares are rare, geomagnetic storms are almost non-existent, and Earth’s upper atmosphere is cooling.

Some people worry that the sun could “get stuck” in Solar Minimum, producing a mini-Ice Age caused by low solar activity. There is no evidence this is happening. On the contrary, the next solar cycle (Solar Cycle 25) is showing unmistakable signs of life.

On May 29th, the sun unleashed the strongest solar flare in years–an M1-class eruption that just missed Earth. The blast came from an active region belonging to Solar Cycle 25.


An M1-class solar flare on May 29, 2020–the strongest flare in three years.

Observers are also seeing a growing number of Solar Cycle 25 sunspots. So far in 2020, the sun has produced a dozen sunspots. Nine of them (75%) have the magnetic polarity of Solar Cycle 25.  This compares to only 17% in 2019 and 0% in 2018. The sun is clearly  tipping from one solar cycle to the next.

A NOAA-led panel of experts actually predicted this behaviour. Last year they said that Solar Minimum would hit rock bottom sometime in late 2019-early 2020. Activity would then quicken in 2021-22, ramping up to a new Solar Maximum in 2023-26.

So far, so good.

A Major Outbreak of Noctilucent Clouds

July 6, 2020: Last night, July 5-6, a major outbreak of noctilucent clouds (NLCs) blanketed Europe. Electric-blue tendrils of frosted meteor smoke rippled over almost every European capital from Scandinavia to the Adriatic. “It was the most phenomenal display of NLCs I’ve seen in my life,” says Viktor Veres, who photographed the outbreak from Budapest, Hungary:


“I was just getting ready for dinner when one of my friends, Alex, cried ‘NLC party time!’,” says Veres. “The electric-blue clouds were almost directly overhead. I sprinted to the car (partially dressing in the street) and drove up Gellért Hill for a view of the clouds over the most famous sights of Budapest–the Danube River, Chain Bridge, Buda Castle, and Parliament. And, yes, my dinner got cold.”

Paris was also “overcast” by noctilucent clouds. “They were very bright,” reports Bertrand Kulik, who shot them floating above the Eiffel Tower:


“The shapes of the noctilucent waves were out of this world!” he says.

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space 83 km above the ground. The clouds form during summer when wisps of water vapor rise up to the mesosphere, allowing water to crystallize around specks of meteor smoke. This summer, record cold temperatures in the mesosphere are boosting their production.

Last night’s mega-display in Europe comes on the heels of a 4th of July sighting in southern California at the same latitude as Los Angeles. It seems that everyone should be alert for noctilucent clouds. Dusk and dawn are the best times to look; here’s why.

Realtime Noctilucent Cloud Photo Gallery
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The Invisible Lunar Eclipse

July 4, 2020: The Moon is about to pass through the shadow of Earth, producing a penumbral lunar eclipse. Unfortunately, it might be invisible.

Eclipse expert Fred Espenak explains: “During past lunar eclipses, I have made a concerted effort to determine when I can first see the subtle shading of Earth’s penumbral shadow on the Moon (using naked eye and binoculars). I have consistently found the penumbral shading is only detectable when at least 2/3 of the Moon lies within the penumbral shadow.”


“Because the Moon will only pass 1/3 of the way into Earth’s penumbral shadow during the July 4/5 lunar eclipse, it will NOT BE VISIBLE to the naked eye,” he says.  “Digital photography can reveal the subtle shading if the contrast of the image is greatly increased.

Penumbral eclipses differ from total eclipses as follows: In a total lunar eclipse, the Moon passes directly through the darkest crimson-colored core of Earth’s shadow. It produces a “Blood Moon.” In a penumbral lunar eclipse, the Moon passes through the pale outskirts of Earth’s shadow. Penumbral eclipses are notoriously subtle–and in this case potentially invisible.

“I fear the general media is hyping this event when there’s really nothing more to see than a Full Moon–although that’s beautiful in its own right,” he says.

The Rapid Brightening of Comet NEOWISE

June 30, 2020: Comet NEOWISE (C/2020 F3) is passing by the sun this week–and it’s looking good. The comet just experienced a sharp increase in brightness recorded by coronagraphs onboard the Solar and Heliospheric Observatory (SOHO). Click to play a 4-day movie of the surge:


“During the transit, Comet NEOWISE increased in brightness from magnitude +4 to +1.8–an almost 8-fold jump,” says planetary scientist Qicheng Zhang of Caltech, who analyzed the images. “If the comet maintains this brightness, it will be visible to the naked eye when it emerges from the sun’s glare in July.”

Zhang is a bit concerned, however, that the rapid brightening might be too much of a good thing. “When a comet brightens this quickly (2.2 magnitudes in only ~4 days) it could be a sign that the nucleus is unstable. Comet NEOWISE might yet disintegrate,” he cautions.

Among astronomers, this is a bit of a sore point. Earlier this year two comets, ATLAS (C/2019 Y4) and SWAN (C/2020 F8), approached the sun with much fanfare, then disintegrated before they could become naked-eye objects. Disappointing.


The disintegration of Comet ATLAS when it passed by the sun at a similar distance in April 2020. Credit: Hubble Space Telescope. [More]

Will Comet NEOWISE end the same way?

“I don’t think so,” says Karl Battams of the Naval Research Lab in Washington DC. “Comet NEOWISE looks healthy to me. It showed a smooth-and-steady brightness increase, perhaps a little steeper than we might like, but nothing that makes me enormously concerned for its health, as long as it has a reasonably large nucleus.”

We’ll soon find out. “Later this week, around perihelion (closest approach to the sun) the comet may be observable by skilled astronomers low in the morning twilight,” says Zhang. “If it maintains anything close to its brightness as it left SOHO’s field of view, it will most likely be recovered from the ground by then.”

Any sightings this week could be very good news, indeed. As July unfolds, the comet will swing around the sun and pop up in the evening sky–perfectly placed for casual sky watchers if anything remains to be seen.  Stay tuned for updates.

Out of Nowhere, a Global Magnetic Anomaly

June 25, 2020: Lately, Earth’s magnetic field has been quiet. Very quiet. The sun is in the pits of what may turn out to be the deepest Solar Minimum in a century. Geomagnetic storms just aren’t happening.

“That’s why I was so surprised on June 23rd when my instruments picked up a magnetic anomaly,” reports Stuart Green, who operates a research-grade magnetometer in his backyard in Preston UK. “For more than 30 minutes, the local magnetic field oscillated like a sine wave.”

20200623_PC5 _Wave

This chart recording shows a magnetic wave rippling through Preston UK on June 23, 2020. Credit: Stuart Green.

Green quickly checked solar wind data from NOAA’s DSCOVR satellite. “There was nothing–no uptick in the solar wind speed or other factors that might explain the disturbance,” he says.

He wasn’t the only one who noticed. In the Lofoten islands of Norway, Rob Stammes detected a similar anomaly on his magnetometer. “It was remarkable,” he says. “Our magnetic field swung back and forth by about 1/3rd of a degree. I also detected ground currents with the same 10 minute period.”

What happened? Space physicists call this phenomenon a “pulsation continuous” or “Pc” for short. Imagine blowing across a piece of paper, making it flutter with your breath. Solar wind can have a similar effect on magnetic fields. Pc waves are essentially flutters propagating down the flanks of Earth’s magnetosphere excited by the breath of the sun. During more active phases of the solar cycle, these flutters are easily lost in the noise of rambunctious geomagnetic activity. But during the extreme quiet of Solar Minimum, such waves can make themselves “heard” like a pin dropping in an silent room.


Magnetic observatories around the world detected the wave on June 23, 2020. Credit: INTERMAGNET

Earth’s magnetic field was so quiet on June 23rd, the ripple was heard all around the world. INTERMAGNET‘s global network of magnetic observatories picked up wave activity at the same time from Hawaii to China to the Arctic Circle. There’s even a hint of it in Antarctica.

Pc waves are classified into 5 types depending on their period. The 10-minute wave on June 23rd falls into category Pc5. Slow Pc5 waves have been linked to a loss of particles from the van Allen radiation belts. Energetic electrons surf these waves down into Earth’s atmosphere, where they dissipate harmlessly.

With Solar Minimum in full swing, there’s never been a better time to study these waves. Keep quiet … and stay tuned for more.


Noctilucent Clouds over London

June 23, 2020: On June 21st, something rare and magical happened in London. The skies of the great city filled with noctilucent clouds (NLCs). Phil Halper noticed the display, grabbed a camera, and raced from one landmark to another, hurriedly recording pictures like this:


“Even the bright lights of the London Eye on the river Thames couldn’t drown out the display,” says Halper. “These were the most spectacular NLCs I’ve ever seen.”

If NLCs look alien–that’s because they are. The clouds are seeded by meteoroids. They form every year around this time when summertime wisps of water vapor rise up to the mesosphere, allowing water to crystallize around specks of meteor smoke.

Usually you have to be under a dark sky at high latitudes to see these rare clouds–but 2020 is not usual. Record-cold temperatures in the mesosphere are boosting NLCs, brightening them enough to see from places like London.


Halpert is a longtime observer of NLCs, and when he saw their electric-blue ripples forming over the city, he immediately realized it was a special occasion. “I hired a bike and cycled from landmark to landmark, stopping at the London Eye, the Tower of London, Blackfriar’s Bridge, St. Paul’s Cathedral and Temple,” he says. “I had to hurry because there is only about one hour of night when the NLCs are visible.’

“Alas, in the rush, I fell off my bike and I ended up in hospital,” he says. “But still it was well worth it. I was thrilled to see this magical display.” (Thank you, Phil!)

The outbreak over London is a sign that even more intense NLCs could be on the way. Last summer, bright noctilucent clouds over European capitals heralded a descent to even lower latitudes. Indeed, just this morning sky watchers in Oregon witnessed their own outbreak. James W. Young sends this picture from Seaside, OR:


“The NLCs were incredibly intense on the morning of June 23rd,” says Young. “I was able to take this picture using just a 1/4-second exposure (ISO 800).”

Observing tips: The best time to look for noctilucent clouds is during the hours after sunset (or before sunrise) when the sun is at least 6 degrees below the horizon: diagram. If you see electric-blue tendrils spreading across the sky … submit your photos here!

Realtime Noctilucent Cloud Photo Gallery
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Naked-Eye Comet NEOWISE?

June 15, 2020: Here we go again. A comet is falling toward the sun, and it could become a naked-eye object after it skims past the orbit of Mercury on July 3rd. Michael Mattiazzo photographed Comet NEOWISE (C/2020 F3) on June 10th from Swan Hill, Australia:


“Pushing the limits of comet observing, I had to leave home to find a clear horizon,” says Mattiazzo. “When I took the picture, Comet NEOWISE was very close to the sun and only 5 degrees above the local horizon. Its visual magnitude was near +7.0, below the threshold for naked-eye visibility.”

It might not look like much now, but this comet could blossom in the weeks after perihelion (closest approach to the sun). Forecasters say Comet NEOWISE could become as bright as a 2nd or 3rd magnitude star. Northern hemisphere observers would be able to easily see it in the evening sky in mid-July.

At this point, readers may be experiencing a feeling of déjà vu. Almost the exact same forecast was issued for Comet ATLAS (C/2019 Y4) in March and Comet SWAN (C/2020 F8) in May. Both comets dived toward the sun and … instead of blossoming, died. Intense solar heat can do that to a fragile ball of ice.


Planets, comets, and asteroids transiting SOHO’s C3 coronagraph in 2020. Comet NEOWISE is marked by a red arrow.

Mattiazzo, who is one of the world’s most experienced amateur comet observers, thinks Comet NEOWISE could turn out better. “I’d say there’s a 70% chance this comet will survive perihelion,” he says, basing his guess on the stability of the comet’s light curve, which sets it apart from Comets ATLAS and SWAN. “Comet NEOWISE could be a case of third time lucky.”

We’ll know soon enough. On June 22nd, the comet will enter the field of view of SOHO‘s C3 coronagraph–a space-based instrument that blocks the glare of the sun to reveal nearby stars, planets and comets. For a whole week, astronomers will be able to monitor Comet NEOWISE as it approaches the orbit of Mercury. If it falls apart, the event may be visible in the images. Ditto if it survives.

Stay tuned for updates.

Record Cold in the Mesosphere

June 9, 2020: It’s getting cold in the mesosphere. Very cold. “At polar latitudes (60N-80N) temperatures have been breaking 14-year records in the last few days,” reports Lynn Harvey of the University of Colorado Laboratory for Atmospheric and Space Physics.  This development is causing noctilucent clouds (NLCs) to spill out of the Arctic to middle latitudes.

“I’ve been waiting for years to see NLCs, and finally it happened!” reports Phil Halpert from London, England, on June 7th. He noticed their electric-blue ripples over local rooftops, then rushed out to photograph them in open sky over Clissold Park:


“This is the first time I have ever seen noctilucent clouds over London!” he says.

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space 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. Usually they are best seen after the summer solstice, but this year they are getting an early start.

What’s happening? To find out, Harvey has been looking at data from NASA’s Microwave Limb Sounder (MLS), which can sense conditions 83 km high where NLCs form. “These plots show that 2020 is shaping up to be a cold and wet year in the mesosphere,” she says.


“Temperatures, in particular, are very cold,” she says. “In fact, mid-latitude temperatures (35N-55N) in late May (DOY 142-148) were the coldest of the AIM record”–that is, since 2007 when NASA’s AIM spacecraft began monitoring noctilucent clouds.

Last summer, NLCs spread as far south as Los Angeles and Las Vegas, setting records for low-latitude sightings. The growing chill today suggests this summer could be just as good–or maybe even better. In fact, the first sightings in the continental USA (Washington and Minnesota) have already happened.

Observing tips: The best time to look is during the hours after sunset (or before sunrise) when the sun is more than 6 degrees below the horizon: diagram. If you see electric-blue tendrils spreading across the sky, you may have spotted a noctilucent cloud.

Realtime Noctilucent Cloud Photo Gallery
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