Rare Red Noctilucent Clouds

July 28, 2020: Noctilucent clouds (NLCs) are supposed to be electric blue. This past weekend in Sweden, photographer P-M Hedén saw a different color: Dark Red. “My 17 year-old son was out with friends and he texted me the message ‘Noctilucent!’ I looked out and didn’t really understand what I saw. The tops of the clouds were red.”

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Above: Red NLCs over Vallentuna, Sweden. July 25, 2020. Credit: P-M Hedén

Hedén hopped in his car and drove to a clear site for a better look. The movie he made, above, shows the dynamics of the clouds and the development of their amber crown. “This all happened around local midnight,” he says.

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space 83 km above the ground. Hedén’s video shows ordinary clouds scudding dark and low across the Swedish landscape. NLCs float high overhead, catching the rays of the sun, which is still “up” at their extremely high altitude.

This isn’t the first time people have seen red noctilucent clouds. There was a significant outbreak of red NLCs over Europe on June 21, 2019. However, they are rare and not fully understood.

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Above: Red NLCs over Piwnice, Poland. June 21, 2019. Credit: Piotr Majewski

To understand what makes NLCs red, first we have to ask What makes them blue? The answer is ozone. Research in the 1970s revealed that much of the sunlight hitting noctilucent clouds first passes through Earth’s ozone layer. Ozone absorbs red light, while allowing blue to pass. This filtered light gives NLCs an azure hue.

The origin of red is less certain. One idea, probably the best, comes from a 1988 paper in the Journal of Atmospheric and Terrestrial Physics entitled “The coloured edge of noctilucent clouds.” The authors note that “Noctilucent clouds are illuminated by sunlight which passes obliquely through the atmosphere. The lowest rays may pass only a few kilometres above sea level.” These low rays are strongly reddened (like sunsets) and bent by refraction; some of them may be redirected to the tops of NLCs.

Is that right? Even many specialists in NLC research aren’t sure, which means every sighting is a bit of a mystery. Northern sky watchers, if you’re seeing red, submit your photos here.

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The Solar Minimum Superstorm of 1903

July 29, 2020: Don’t let Solar Minimum fool you. The sun can throw a major tantrum even during the quiet phase of the 11-year solar cycle. That’s the conclusion of a new study published in the July 1st edition of the Astrophysical Journal Letters.

“In late October 1903, one of the strongest solar storms in modern history hit Earth,” say the lead authors of the study,  Hisashi Hayakawa (Nagoya University, Japan) and Paulo Ribeiro (Coimbra University, Portugal). “The timing of the storm interestingly parallels where we are now–just after the minimum of a weak solar cycle.”

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Above: The red line marks the 1903 solar superstorm in a plot of the 11-year solar cycle. [ref]

The 1903 event wasn’t always recognized as a great storm. Hayakawa and colleagues took an interest in it because of what happened when the storm hit. In magnetic observatories around the world, pens scrabbling across paper chart recorders literally flew offscale, overwhelmed by the disturbance. That’s the kind of thing superstorms do.

So, the researchers began to scour historical records for clues, and they found four magnetic observatories in Portugal, India, Mexico and China where the readings were whole. Using those data they calculated the size of the storm.

“It was enormous,” says Hayakawa. “The 1903 storm ranks 6th in the list of known geomagnetic storms since 1850, just below the extreme storm of March 1989, which blacked out the province of Quebec.”

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Above: A photo of the sun on Oct. 31, 1903, from the Royal Observatory in Greenwich. [ref]

In their paper, Hayakawa et al detail what happened. During the last week of October 1903, a moderately large new-cycle sunspot appeared. It was directly facing Earth on Oct. 30th when it unleashed a solar flare. The flare cannot be ranked using modern scales, because there were no Earth-orbiting satellites to measure its X-ray intensity. However, it must have been very strong; minutes after the explosion, Earth’s magnetic field lurched (a “magnetic crochet”) as radiation from the crackling sunspot caused strong electrical currents to flow in our planet’s upper atmosphere.

The real action began 27.5 hours later when the CME (coronal mass ejection) arrived. A massive plasma cloud slammed into Earth’s magnetic field–and pens flying off chart papers were the least of the effects. Surging ground currents disrupted communications around the world. In Chicago, voltages in telephone lines spiked to 675 volts–“enough to kill a man” according to headlines in the Chicago Sunday Tribune. Telegraph operators in London found they could not send clear messages to Latin America, France, Italy, Spain, Portugal, and Algeria.

Meanwhile, auroras spread across both hemispheres. Southern Lights were seen directly overhead in New South Wales, Australia, while Northern Lights descended past Colorado in the United States. “Shafts of cold gorgeous light [rose] almost to the zenith and gave the impression that a frightful conflagration was raging somewhere to the north of the city [of Leadville],” eyewitnesses reported in Colorado’s Herald Democrat newspaper.

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Above: Red dots mark aurora sightings during the Oct-Nov 1903 superstorm. [ref]

How big was it? Space weather researchers rank storms using Dst” (disturbance storm time index), a measure of geomagnetic activity that can be estimated from old magnetogram chart recordings. For the 1903 storm. Hayakawa and colleagues found Dst = -531 nT.  For comparison, the Carrington Event of 1859 and the Great Railroad Storm of May 1921 are both in the ballpark of Dst = -900 nT.  Arguably, this puts 1903 within spitting distance of the greatest storms in recorded history.

1903 isn’t the only time strong storms have interrupted Solar Minimum. “Similar storms (but less extreme) occurred around Solar Minimum in Feb 1986 (Garcia and Dryer, 1987; Dst=-307 nT) and Sept. 1998 (Daglis et al., 2007; Dst ~-200 nT),” notes Hayakawa.

As 2020 unfolds, the sun is experiencing, and perhaps just beginning to emerge from, a century-class Solar Minimum. Also, a new-cycle sunspot (AR2767) is directly facing Earth. Sound familiar?

Stay tuned!

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:

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“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.

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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?”

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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:

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“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.

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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:

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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.

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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:

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“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:

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“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.

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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.”

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“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:

Bum-Suk-Yeom-20200622_0627_c3_Comet_C2020F3_NEOWISE_bsyeom_1593387415

“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.

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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.

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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:

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“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.

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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:

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“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!

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