Aurora Cometalis: The Northern Lights of Comet 67P

Sept. 22, 2020: Imagine putting your thumb on a garden hose and sending a jet of water into the sky. At the apex of the stream, auroras form. It turns out, some comets can actually perform this trick.

In a paper published this week in Nature Astronomy, researchers described how comet 67P/Churyumov-Gerasimenko turns vaporous jets of water into auroras.

The European Space Agency’s Rosetta spacecraft observed the weird lights while it was orbiting Comet 67P in 2014-2016. At first researchers misunderstood what the glow was. It couldn’t be an aurora, could it? For one thing, the comet doesn’t even have a magnetic field–a key ingredient of geomagnetic storms. Also, the lights of Comet 67P are invisible to the human eye. They shine at far ultraviolet wavelengths, unlike the familiar red and green curtains that dance around Earth’s poles.

“Nevertheless, they are auroras,” says Marina Galand of Imperial College London, UK, lead author of the new study.

It took years for Galand and colleagues to figure out what was going on. Solving the mystery required data from five of Rosetta’s sensors plus a physics model to calculate how the solar wind interacts with the comet’s atmosphere.

They found that electric fields naturally occurring in the comet’s atmosphere can grab electrons from the solar wind and hurl them inward. Those electrons rush headlong into water molecules spewing out of the comet’s core. Debris from the collision–excited atoms of H and O–produce an ultraviolet glow: Aurora Cometalis.

Comet auroras respond to space weather much like Earth auroras do. Gusts of solar wind can rev them up, and a good coronal mass ejection (CME) can cause an outright auroral storm. Indeed, a CME that hit Comet 67P on Oct. 22, 2014, caused “a sharp intensification” of the UV brightness.

Galand says that “other comets should have these kind of auroras, too.” The basic physics is universal. 67P/Churyumov-Gerasimenko’s auroras are special only in the sense that the Rosetta spacecraft was there to observe them.

Thought experiment: Suppose you could see UV light. What would the auroras of Comet 67P look like from ground level?

“They would be diffuse, but not uniform,” speculates Galand. “Some parts of the sky would be brighter than others–especially if a jet of water crossed your field of view!”

And here’s the strangest part of all: The auroras would descend all the way down to the comet’s surface. “So you would be surrounded by the light,” she says.

Yet another reason to visit a comet….

Solar Cycle 25 Has Begun

Sept. 15, 2020: Solar Cycle 25 is officially underway. NASA and NOAA made the announcement during a media teleconference yesterday, Sept. 15th. According to an international panel of experts, the sunspot number hit rock bottom in Dec. 2019, bringing an end to old Solar Cycle 24. Since then, sunspot counts have been slowly increasing, heralding new Solar Cycle 25.

“How quickly solar activity rises is an indicator on how strong the next solar cycle will be,” says Doug Biesecker of NOAA’s Space Weather Prediction Center, co-chair of the Solar Cycle 25 Prediction Panel. “Although we’ve seen a steady increase in sunspot activity this year, it is slow.”

The panel believes that new Solar Cycle 25 will be a weak one, peaking in 2025 at levels similar to old Solar Cycle 24. If their prediction is correct, Solar Cycle 25 (like Solar Cycle 24 before it) will be one of the weakest since record-keeping began in 1755.

“While we are not predicting a particularly active Solar Cycle 25, violent eruptions from the sun can occur at any time,” warns Biesecker. Indeed, even Solar Minimum can produce a superstorm, so Solar Cycle 25 should not be taken lightly despite the panel’s low expectations. Radio blackouts, power outages and severe geomagnetic storms are possible in the years ahead.

Above: Solar Cycle 25 auroras photographed on Sept. 14, 2020, by Jani Ylinampa in Finland.

For now, solar activity should remain generally low. Sunspot counts still have a long way to go before they reach levels typical of Solar Maximum. For the rest of 2020, periods of quiet will be occasionally interrupted by minor solar storms, with only a slight chance of big events.

On the bright side, the first Northern Lights of Solar Cycle 25 are dancing around the Arctic Circle right now, and the coming season for aurora watching promises to be the best in years. Stay tuned! Aurora alerts: SMS Text.

A Warning from History: The Carrington Event Was Not Unique

Sept. 1, 2020: On Sept. 1st, 1859, the most ferocious solar storm in recorded history engulfed our planet. It was “the Carrington Event,” named after British scientist Richard Carrington, who witnessed the flare that started it. The storm rocked Earth’s magnetic field, sparked auroras over Cuba, the Bahamas and Hawaii, set fire to telegraph stations, and wrote itself into history books as the Biggest. Solar. Storm. Ever.

But, sometimes, what you read in history books is wrong.

“The Carrington Event was not unique,” says Hisashi Hayakawa of Japan’s Nagoya University, whose recent study of solar storms has uncovered other events of comparable intensity. “While the Carrington Event has long been considered a once‐in‐a‐century catastrophe, historical observations warn us that this may be something that occurs much more frequently.”


Drawings of the Carrington sunspot by Richard Carrington on Sept. 1, 1859, and (inset) Heinrich Schwabe on Aug. 27, 1859. [Ref]

To generations of space weather forecasters who learned in school that the Carrington Event was one of a kind, these are unsettling thoughts. Modern technology is far more vulnerable to solar storms than 19th-century telegraphs. Think about GPS, the internet, and transcontinental power grids that can carry geomagnetic storm surges from coast to coast in a matter of minutes. A modern-day Carrington Event could cause widespread power outages along with disruptions to navigation, air travel, banking, and all forms of digital communication.

Many previous studies of solar superstorms leaned heavily on Western Hemisphere accounts, omitting data from the Eastern Hemisphere. This skewed perceptions of the Carrington Event, highlighting its importance while causing other superstorms to be overlooked.

A good example is the great storm of mid-September 1770, when extremely bright red auroras blanketed Japan and parts of China. Captain Cook himself saw the display from near Timor Island, south of Indonesia. Hayakawa and colleagues recently found drawings of the instigating sunspot, and it is twice the size of the Carrington sunspot group. Paintings, dairy entries, and other newfound records, especially from China, depict some of the lowest-latitude auroras ever, spread over a period of 9 days.


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

“We conclude that the 1770 magnetic storm was comparable to the Carrington Event, at least in terms of auroral visibility,” wrote Hayakawa and colleagues in a 2017 Astrophysical Journal Letter. Moreover, “the duration of the storm activity was much longer than usual.”

Hayakawa’s team has delved into the history of other storms as well, examining Japanese diaries, Chinese and Korean government records, archives of the Russian Central Observatory, and log-books from ships at sea–all helping to form a more complete picture of events.

They found that superstorms in February 1872 and May 1921 were also comparable to the Carrington Event, with similar magnetic amplitudes and widespread auroras. Two more storms are nipping at Carrington’s heels: The Quebec Blackout of March 13, 1989, and an unnamed storm on Sept. 25, 1909, were only a factor of ~2 less intense. (Check Table 1 of Hayakawa et al‘s 2019 paper for details.)


Oriental reports of a giant naked-eye sunspot group (left) and auroras (right) in Feb. 1872. [Ref]

Contextualizing the Carrington Event has become a busy niche in space weather research. One team led by Jeff Love of the USGS recently confirmed the near equality of Carrington to the May 1921 superstorm. And Hayakawa’s team has just  unearthed new records of extreme auroras in South America.

Are we overdue for another Carrington Event? Maybe. In fact, we might have just missed one.

In July 2012, NASA and European spacecraft watched an extreme solar storm erupt from the sun and narrowly miss Earth. “If it had hit, we would still be picking up the pieces,” announced Daniel Baker of the University of Colorado at a NOAA Space Weather Workshop 2 years later. “It might have been stronger than the Carrington Event itself.”

History books, let the re-write begin.

Spiral Lights on Mars

August 25, 2020: NASA’s MAVEN spacecraft has discovered something unexpected on Mars–and researchers are struggling to explain it.

“There is a vast spiral of ultraviolet light over Mars’ South Pole,” says Nick Schneider of the University of Colorado’s Laboratory for Atmospheric and Space Physics. “We understand the origin of the light, but its shape is a mystery.”


Shown in false-color (green), a spiral of UV light is swirling around the Martian South Pole.

The light is “nightglow.” We have it here on Earth, too, where it’s called “airglow.” During the day, ultraviolet radiation from the sun breaks apart compounds in the upper atmosphere. At night, the atoms reassemble, glowing as they put themselves back together again. On Earth, airglow looks like the aurora borealis; people can actually see it. On Mars, the emission is ultraviolet, invisible to the human eye.

MAVEN has been monitoring Martian nightglow for years, yet the spiral pattern was only recently recognized. Schneider recalls the ‘Eureka moment’: “We were preparing a demo on our lab’s internal projection sphere (like Science on a Sphere), which turned out to be the first time we had plotted the UV glow in polar coordinates. The spiral ‘popped’ and we were all quite giddy.”


The chemistry of Martian nightglow. Nitric oxide (NO) also produces airglow on Earth.

The spiral is just the tip of the iceberg; the whole planet is surrounded by pulsating patterns of nightglow. High above the North Pole of Mars there is a luminous blob that pulses exactly twice a day. And around the equator there are three more blobs, evenly spaced, pulsing three times a day. Only the South Pole has a spiral, and it pulses once a day.

“It’s pretty complicated,” says Schneider.

To make sense of it all, Schneider and colleagues “spun up” a general circulation model (GCM) of the Martian atmosphere. (This part of the work was led by Francisco González-Galindo at the Instituto Astrofísica Andalucía.) GCMs are computer programs that model planetary atmospheres. They’re used all the time by weather forecasters on Earth, and planetary scientists have Martian versions as well, complete with accurate values for solar heating, winds, chemistry, cloud formation, and so on.

According to the GCM, nightglow on Mars is shaped by atmospheric tides. The gravity of the sun pulls on the Martian atmosphere, and the atmosphere has a natural response: It essentially rings like a bell three times a day. This accounts for the pulsating blobs around the equator.


Figure 6 of Schneider et al’s recent paper compares MAVEN data (panels a and b) to a GCM model of Martian nightglow (panel c).

“We were really impressed that the Mars General Circulation Model reproduced the equatorial patterns so well,” says Schneider. “The brightest spot hovers above the Martian prime meridian for months at a time, with two fainter spots placed on either side. Pulsating downdrafts at those locations boost the chemical reactions which give rise to nightglow.”

The model did not, however, account for the spiral. “We’ve really struggled to explain it,” he admits. “And however we might explain why a spiral should exist at the South Pole, we also need to explain why it should not exist at the North Pole!”

More data from MAVEN and updates to the Mars General Circulation Model might eventually provide an answer. For now, it remains a dizzying mystery.

Schneider et al‘s original research was just published in the August 6, 2020, edition of JGR Space Physics. Read it here.

Approaching Mars

Aug. 21, 2020: By the time you finish reading this sentence, you’ll be 40 km closer to Mars.

Earth and Mars are converging for a close encounter this Fall, one of the best since 2003, and their separation is rapidly shrinking–negative 8 km/s as of Aug. 21st. You are literally approaching the Red Planet. Just this week the brightness of Mars surpassed that of Sirius, the brightest star in the sky. Suddenly, Mars is almost bright enough to see in daylight and an easy target for backyard telescopes.

“Here is one hour of Mars rotating,” says amateur astronomer Maximilian Teodorescu of Magurele, Romania, who animated a sequence of photos taken Aug 18th using his 14-inch Newtonian:

“Lots of features are visible,” he says. “Note how the southern polar ice cap has contracted and broken–a result of it being late springtime on that side of Mars.”

The best is yet to come. By the night of closest approach on Oct. 6th (0.4149 AU), Mars will more than double in brightness again, outshining everything in the night sky except Venus and the Moon. Throughout this apparition, the south pole will remain tilted toward Earth, giving observers a good view of the breakup of the ice cap as Martian spring turns into summer.

Can’t wait? Right now, a good time to look is just before daybreak when Mars is high in the southern sky. Keep watching as twilight creeps up the sky. The planet’s burnt orange hue looks beautiful when surrounded by the day’s first hint of morning blue: sky map.

Congratulations, you’re now almost 500 km closer to the planet Mars.

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A Slow-Motion Solar Flare and CME

August 17, 2020: You know an explosion is powerful when it lasts for two hours. Yesterday, Aug. 16th (1726 UT), a B1-class solar flare took even longer to unfold. The 2.5 hr blast sent a powerful shock wave rippling through the sun’s atmosphere, shown here in a time-lapse movie from NASA’s Solar Dynamics Observatory:

No sunspot was involved. The explosion occured in a spotless region of the sun’s southern hemisphere. A magnetic filament snapped, hurling debris far and wide. Some of that debris formed the core of a coronal mass ejection (CME), which has escaped the sun and is now billowing into the Solar System.

Coronagraphs onboard the Solar and Heliospheric Observatory (SOHO) are tracking the CME:

Clearly, the storm cloud is not heading directly for Earth. However, NOAA models of the CME’s trajectory suggest it could deliver a glancing blow to Earth’s magnetic field on August 20th. Minor geomagnetic storms and high-latitude auroras are possible when the CME arrives. Stay tuned. Aurora alerts: SMS Text.

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Cosmic Rays and the Weakening Solar Cycle

August 18, 2020: Cosmic rays are bad–and they’re going to get worse. That’s the conclusion of a new study entitled “Galactic Cosmic Radiation in Interplanetary Space Through a Modern Secular Minimum” just published in the journal Space Weather.

“During the next solar cycle, we could see cosmic ray dose rates increase by as much as 75%,” says lead author Fatemeh Rahmanifard of the University of New Hampshire’s Space Science Center. “This will limit the amount of time astronauts can work safely in interplanetary space.”


Cosmic rays are the bane of astronauts. They come from deep space, energetic particles hurled in all directions by supernova explosions and other violent events. No amount of spacecraft shielding can stop the most energetic cosmic rays, leaving astronauts exposed whenever they leave the Earth-Moon system.

Back in the 1990s, astronauts could travel through space for as much as 1000 days before they hit NASA safety limits on radiation exposure. Not anymore. According to the new research, cosmic rays could limit trips to as little as 290 days for 45-year old male astronauts, and 204 days for females. (Men and women have different limits because of unequal dangers to reproductive organs.)

Why are cosmic rays growing stronger? Blame the sun. The sun’s magnetic field wraps the entire solar system in a protective bubble, normally shielding us from cosmic rays. In recent decades, however, that shield has been growing weaker–a result of the sputtering solar cycle.


The sunspot cycle has been trending weaker since the 1950s. The red curve is a prediction for upcoming Solar Cycle 25. [More]

Solar activity isn’t what it used to be. In the 1950s through 1990s, the sun routinely produced intense Solar Maxima with lots of sunspots and strong solar magnetic fields. Now look at the plot, above. Since the heyday of the late 20th century, the 11-year solar cycle has weakened, and the sun’s magnetic field has weakened with it.

Rahmanifard and colleagues believe we could be entering a Grand Minimum–a long, slow dampening of the 11-year solar cycle, which can suppress sunspot counts for decades. The most famous example of a Grand Minimum is the Maunder Minimum of the 17th century when sunspots practically vanished for 70 years.

“We are not in a Maunder Minimum,” stresses Rahmanifard. “The current situation more closely resembles the Dalton minimum of 1790-1830 or the Gleissberg minimum of 1890-1920.” During those lesser Grand Minima, the solar cycle became weak, but didn’t completely go away.


In these plots, Rahmanifard et al compare the Dalton and Gleissberg minima (top panels) to recent solar cycles (bottom panels).

For years, researchers have been monitoring cosmic rays using CRaTER, a sensor orbiting the Moon on board NASA’s Lunar Reconnaissance Orbiter (LRO). Recent data show that cosmic rays are at very high levels–the highest since LRO was launched in 2009. (See Figure 1 in their paper.)

“We took the latest readings from CRaTER and extrapolated them forward into Solar Cycle 25 (the next solar cycle),” says Rahmanifard. “We found that radiation doses will probably exceed already-high values by 34% for a Gleissberg-like minimum to 75% for a Dalton-like minimum.”

Study co-author Nathan Schwadron, also of the University of New Hampshire, wonders if NASA should rethink its safety limits to allow longer voyages. “Or,” he suggests, “maybe we should wait, and only conduct long-duration missions during Solar Maximum when galactic cosmic radiation falls to lower levels.”

For astronauts, it begs the question — How much can you get done in 200 days? Barring improvements in shielding technology, future space missions may be limited to only 6 or 7 months, probably too short for a Mars voyage.  Lunar exploration could be safer because the body of the Moon itself blocks radiation. But in interplanetary space, the researchers caution, “the next decade or two may be more dangerous than previously realized.”

Stay tuned for updates as Solar Cycle 25 unfolds.

Solar Cycle 25 is Coming to Life

August 3, 2020: There’s no longer any doubt. New Solar Cycle 25 is coming to life. The latest sign came today with the emergence of a new sunspot group, AR2770, inset in this magnetic map of the sun’s surface from NASA’s Solar Dynamics Observatory (SDO):


In this false-color image of the sun, intense magnetic fields are denoted by yellow (- polarity) and green (+ polarity).

AR2770 has two dark cores (each about the size of Mars) and is crackling with minor B-class solar flares. Its potential for even stronger flares will become clear in the days ahead as the sunspot turns toward Earth, more fully revealing its magnetic complexity.

Active regions from Solar Cycle 25 are now strewn across the sun’s northern hemisphere. These are places where magnetic fields are intensifying, creating islands of magnetism on the sun’s surface.


The -/+ magnetic polarities of these northern active regions mark them as members of Solar Cycle 25, per Hale’s Law.

In the cases of AR2769 and AR2770, the fields have intensified enough to form dark cores–that is, sunspots. A few days ago, AR2768 also had visible sunspots. It’s a targetrich environment for amateur astronomers with safe solar telescopes.

The appearance of so many active regions at once is a clear sign that Solar Cycle 25 is gaining steam. However, that doesn’t mean Solar Minimum is finished. These are just “starter sunspots,” pipsqueaks compared to the behemoths expected when Solar Cycle 25 reaches its peak a few years from now. Solar activity should remain generally low despite this uptick in sunspot counts.

On the other hand, even a starter sunspot can occasionally cause a very big storm–so stay tuned. Solar flare alerts: SMS Text.

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.”


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.


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.”


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.”


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.


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!