Noctilucent Clouds over the South Pacific

March 26, 2020: You don’t see this everyday–or even in 30 years. “Noctilucent clouds have appeared over the South Pacific,” reports  meteorologist Ashleigh Wilson of the Australian Antarctic Division. “I photographed them from Macquarie Island on Jan. 4th.”


Noctilucent clouds over Macquarie Island on Jan. 4, 2020. Photo credit: Ashleigh Wilson of the Macquarie Island Bureau of Meteorology team, Australian Antarctic Division. [more]

Wilson is just wrapping up a year-long stint on the remote island where the Australian government maintains a research station to study climate change and wildlife. She’s been there, off and on, since the year 2000, and this is the first time she’s ever seen NLCs.

“According to John French of the Australian Antarctic Division Atmospheric Science Program, the only other photographic recording of NLCs on Macquarie Island was in the mid-1990s,” Wilson says.

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space more than 80 km above the planet’s surface. NLCs form when summertime wisps of water vapor rise up to the edge of space and crystalize around specks of ‘meteor smoke.’ Prime time for sightings is June-July in the north, December-January in the south.

“It was a breathtaking moment when I first realized what I was seeing,” recalls Wilson. “I had just finished my shift in the Met office at 11:30 pm and I was walking back to our dorms. Luminous filaments were crisscrossing the sky, blue in color with some wave formations. I spent two hours down by the ocean with a camera and tripod, adjusting ISO and lenses, trying to capture the extraordinary event. The clouds were so bright, I could see their reflection in the water.”

Wilson’s sighting caps a remarkable year for noctilucent clouds. Once confined to Earth’s polar regions, NLCs have recently spread to lower and lower latitudes. In 2019, the clouds appeared over Rome, Italy; Las Vegas, Nevada; Albuquerque, New Mexico; Paris, France; and outside Los Angeles, California. Many of the sightings smashed old records for low-latitude visibility.


The recent spread of noctilucent clouds. The shaded area shows the traditional domain of NLCs. Black circles denote sightings during the northern summers of 2003-2011. Red circles are for June 2019. All data come from The map originally appeared in JGR Atmospheres: article.

In the southern hemisphere, reports of *any* noctilucent clouds are quite rare–in part because there are fewer people on that side of the world; in part because southern NLCs are usually weaker than their northern counterparts. Nevertheless, NLCs were sighted over New Zealand in Dec. 2019 and Macquarie Island in Jan. 2020.

Why are NLCs so revved up? No one knows for sure. It could be a result of climate change and/or the solar cycle. Satellites have recently detected extra moisture in the mesosphere–a key ingredient of NLCs. That moisture may be surviving thanks to low solar activity. Right now a deep Solar Minimum is underway. Ultraviolet radiation that would normally destroy water in the mesosphere is at low ebb.

Get ready for more. The northern hemisphere season for NLCs is about to begin as summertime wisps of water vapor drift up toward the edge of space. The first electric-blue filaments are typically sighted in mid- to late-May. If recent events are any guide, 2020 could be very special indeed.

Stay tuned!


Comet ATLAS is Half as Wide as the Sun

March 24, 2020: No one knows how big the icy core of Comet ATLAS (C/2019 Y4) might be–possibly no wider than a few kilometers. One thing’s for sure, though, the comet’s atmosphere is huge. New images from amateur astronomers around the world show that ATLAS’s gaseous envelope has ballooned in diameter to ~720,000 km–about half as wide as the sun.


“Comet ATLAS’s coma (atmosphere) is approximately 15 arcminutes in diameter,” reports Michael Jäger of Weißenkirchen, Austria, who took the picture, above, on March 18th. “Its newly-formed tail is about the same size.”

Other astronomers are getting similar results. 15 arcminutes = a quarter of a degree. Given Comet ATLAS’s distance of 1.1 AU on March 18th, that angle corresponds to a physical size of 720,000 km.

On the scale of big things in the solar system, Comet ATLAS falls somewhere between the sun (1,392,000 km  diameter) and Jupiter (139,820 km). It’s not unusual for comets to grow so large. While their icy solid cores are typically mere kilometers in diameter, they can spew prodigious amounts of gas and dust into space, filling enormous volumes. In the fall of 2007, Comet 17P/Holmes partially exploded and, for a while, had an atmosphere even larger than the sun. The Great Comet of 1811 also had a sun-sized coma. Whether Comet ATLAS will eventually rival those behemoths of the past remains to be seen.

Right now, Comet ATLAS is certainly the biggest green thing in the Solar System. Its verdant hue comes from diatomic carbon, C2, a molecule commonly found in comets.  Gaseous C2 emits a beautiful green glow in the near-vacuum of space.


Currently, Comet ATLAS is shining like an 8th magnitude star–invisible to the unaided eye but an easy target for backyard telescopes. The comet is brightening rapidly as it comes closer to Earth and the sun. By late May it could rival Venus in the evening twilight sky. Stay tuned!

Comet ATLAS resources: sky map; 3D orbit; ephemeris, light curve.

Comet ATLAS is Brightening Faster than Expected

March 17, 2020: Get ready for a wild ride. Comet ATLAS (C2019 Y4) is plunging toward the sun, and if it doesn’t fly apart it could soon become one of the brightest comets in years.

“Comet ATLAS continues to brighten much faster than expected,” says Karl Battams of the Naval Research Lab in Washington DC. “Some predictions for its peak brightness now border on the absurd.”


Above: Comet ATLAS (C/2019 Y4) photographed on March 6, 2020, by Austrian astrophotographer Michael Jäger. The comet’s diffuse green atmosphere is about twice as wide as the planet Jupiter.

The comet was discovered in December 2019 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii. Astronomers quickly realized it might be special. On May 31, 2020, Comet ATLAS will pass deep inside the orbit of Mercury only 0.25 AU from the sun. If it can survive the blast furnace of solar heating, it could put on a good show.

However, no one expected the show to start this soon. More than 2 months before perihelion (closest approach to the sun), Comet ATLAS is already “heating up.” The worldwide Comet Observation Database shows it jumping from magnitude +17 in early February to +8 in mid-March–a 4000-fold increase in brightness. It could become visible to the naked eye in early April.

“Right now the comet is releasing huge amounts of its frozen volatiles (gases),” says Battams. “That’s why it’s brightening so fast.”


Can ATLAS sustain this crazy pace? If it has a big nucleus with large stores of frozen gas, then yes; we could get a very bright comet. Otherwise, Comet ATLAS might “run out of gas”, crumbling and fading as it approaches the sun.

Current best estimates of the comet’s peak brightness in May range from magnitude +1 to -5. If Comet ATLAS hits the high end of that range, a bit brighter than Venus, it could become visible in broad daylight.

Comet McNaught (C/2006 P1) performed that very trick 13 years ago. On Jan. 13, 2007, it swooped past the sun shining at magnitude -5. The absurdly-bright comet was visible at high noon with its tail jutting across blue sky.


Above: Comet McNaught in broad daylight on Jan. 13, 2007. Photo credit: Peter Rosen
 of Stockholm, Sweden. [More]

Battams is not optimistic, though: “My personal intuition is that Comet ATLAS is over-achieving, and I wouldn’t be surprised to see it start to fade rapidly and possibly even disintegrate before reaching the sun,” he says.

Come to think of it, that would be a good show, too. Solar glare may challenge ground-based observers, but NASA has spacecraft with cameras that specialize in seeing things close to the sun.

“The Heliospheric Imager on NASA’s STEREO spacecraft will get a great view of ATLAS from mid-May through early June,” says Battams. “The camera is very sensitive, so we might be able to observe ATLAS’s tail interacting with the solar wind and outflows–as well as any potential breakup events.”

Stay tuned!

Cosmic Rays are Increasing at Aviation Altitudes

March 13, 2020: We’re back from the Arctic, and we have some new results to share. In January 2020, the students of Earth to Sky Calculus and traveled to Abisko, Sweden, to launch a pair of cosmic ray balloons. We’d been there before, launching three identical balloons in March 2017. Putting all the data together, 2017+2020, we find that radiation has increased +12% in the past 3 years:


The graph shows radiation dose rate (uGy/hr) vs. altitude (feet) all the way from ground level to the stratosphere. Radiation appears to be increasing at nearly all altitudes–even in the range 25,000 ft to 40,000 ft where airplanes fly. Polar flight crews and passengers are therefore absorbing ~12% more cosmic radiation than they did only a few years ago.

What’s causing the increase? Solar Minimum. At the moment, the sun is near the bottom of the 11-year solar cycle. During Solar Minimum, the sun’s magnetic field weakens, allowing extra cosmic rays from deep space to penetrate the solar system. These cosmic rays are hitting Earth’s atmosphere, creating a spray of secondary cosmic rays that shower toward the ground below.

Secondary cosmic rays are what we measure. Radiation sensors onboard our helium balloons detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV, similar to what you get from medical X-ray machines and airport security scanners.

Above: Schematic diagram of a cosmic ray air shower. Learn more from CERN.

We’ve been launching radiation sensors almost weekly for 5 years–mainly from California, the “home base” of Cosmic rays in the stratosphere have been increasing the entire time, a sign of deepening Solar Minimum.

The new data from Abisko, Sweden, show the increase is not limited to the stratosphere. It is also happening at aviation altitudes with a 3-year increase of ~12% even below 40,000 ft. We’re planning another ballooning trip to Sweden in August 2020 to confirm these results. Stay tuned.

A “Radio-Active” Sunspot from the Next Solar Cycle

March 8, 2020: A new sunspot is emerging in the sun’s southern hemisphere, and it looks like a member of new Solar Cycle 25. Numbered “AR2758,” the sunspot is inset in this magnetic map of the sun’s surface from NASA’s Solar Dynamics Observatory:

How do we know this sunspot belongs to Solar Cycle 25? Its magnetic polarity tells us so. Southern sunspots from old Solar Cycle 24 have a -/+ polarity. This sunspot is the opposite: +/-. According to Hale’s Law, sunspots switch polarities from one solar cycle to the next. AR2744 is therefore a member of Solar Cycle 25.

AR2758 continues a trend of increasing Solar Cycle 25 activity. So far this year, there have been 4 numbered sunspots. Three of them (75%) have been from Solar Cycle 25. This compares to only 17% in 2019 and 0% in 2018. Solar Cycle 25 is still weak, but it is coming.

Sunspot AR2758 has produced one of the first solar radio bursts of Solar Cycle 25. “I was surprised to see distinct solar radio bursts on my spectrograph this morning,” reports Thomas Ashcraft. Click to play the slow-rolling roar of static that emerged from the loudspeakers of his amateur radio telescope in New Mexico:

These radio sounds are caused by beams of electrons–in this case, accelerated by unrest in the sunspot’s fast-changing magnetic canopy. As the electrons slice through the sun’s atmosphere, they generate a ripple of plasma waves and radio emissions detectable on Earth 93 million miles away. Astronomers classify solar radio bursts into five types; Ashcraft’s recording captured a mixture of Type III and Type V.

“Even in the height of solar maximum, this would be considered a strong solar burst,” says Ashcraft, who has been listening to solar radio many years. “I am hoping for more!”

Extra: There is growing evidence that solar radio bursts can disorient the navigation of grey whales, causing them to strand on beaches. According to the radical-pair hypothesis of magnetoreception, shortwave bursts such as the one Ashcraft recorded may be especially effective at sending them off course.

Get Ready for Sprite Season

March 7, 2020: Sprite season is coming. Spring thunderstorms often produce the year’s first big bursts of upward-directed lightning. To get ready, Puerto Rican sprite chaser Frankie Lucena has prepared a chart to identify the different forms, including a newly-discovered type of sprite called “the Ghost.”


“This chart provides just a glimpse of what can be seen and photographed above very strong thunderstorms,” says Lucena. “I used actual images, enhanced and slightly modified to better show what they actually look like.”

“This is the first chart to show the Ghost and a Negative Sprite event,” he continues. “The Ghost is a green colored shadow that appears above some sprites. The green color is caused by electrons exciting oxygen molecules in the mesosphere, approximately 80 km high. A Negative Sprite is triggered by a -CG lightning discharge as opposed to a regular sprite which is triggered by a +CG lightning discharge.” (Note: CG means “cloud to ground.”)


HALO = Frankie Lucena
SPRITE = Jason Ahrns
GHOST = Paul Smith and Hank Schyma
TROLL = Nova Documentary
ELVE = Frankie Lucena
BLUE JET = Jon Heppell’s Blog
GIGANTIC JET = Gemini Observatory/AURA
NEGATIVE SPRITE = Frankie Lucena
PIXIES = Paul Smith


SPRITES: They initiate between 65 and 85 km in altitude and are typically triggered by a positive cloud-to-ground lightning strike. The streamers first grow downward and then upwards before disappearing. They are mainly red in color but as they grow downward they will transition to purple and then to blue.

HALOS: They are typically triggered by a positive cloud-to-ground lightning strike like sprites and will sometimes appear together with sprites. They initiate at about 80-85 KM and will typically look like a red oval shaped cloud.

TROLLS: They occur during long-lasting sprite events with ongoing lightning activity in the cloud and with long lasting +CG currents to ground. Its hard to link them to a particular cloud-to-ground lightning strike. They are mainly purple in color like the tendrils of a sprite and will transition to blue towards the bottom.

ELVES: They are an electromagnetic pulse that originates at the same time as the cloud-to-ground lightning strike. They project a ring of red light as electrons at the base of the ionosphere excite nitrogen molecules. They typically appear at about 90 Km in altitude and are considered the most common type of Transient Luminous Event.

BLUE STARTERS: A blue starter is an electric streamer discharge that initiates under the screening layer at the cloud top. They start out as a white channel rising from the cloud top that quickly transitions to a blue fan shaped plume as it travels upwards to about 26 Km in altitude. The white channel is rarely seen because they are typically obscured by the cloud.

BLUE JETS: They are often linked to charge removal by negative cloud-to-ground lightning. They initiate under the screening layer at the cloud top and they start out as a bright white channel rising from the top of the storm cloud that quickly transitions to a blue coned shaped plume as it travels upwards to about 40 Km in altitude. The white channel is rarely seen because they are often obscured by the cloud.

GIGANTIC JETS: Unlike Blue Jets and Starters, a Gigantic Jet initiates in the middle of the storm cloud. They start out as a bright white channel rising from the cloud top that quickly transitions to blue and then to red as it climbs as high as 80-90 Km in altitude. They are usually preceded by multiple negative cloud-to-ground lightning and will typically appear during a null in the lightning activity.

NEGATIVE SPRITE: These sprites are similar to regular sprites but are triggered by a negative polarity cloud to ground lightning discharge (-CG). A negative sprite requires a higher Charge Moment Change than a regular sprite and is usually located above the convective core, or the surrounding area, because this is where -CG lightning usually occurs.

GHOST: Newly discovered so little is known. They sometimes appear at the top of a Sprite event and are green in color. The green color is caused by electrons exciting Oxygen molecules high up in the Mesosphere at around ~80 Km.

PIXIES: They are blue emissions of light that are very small in size and usually located either on top or on the outer wall of the convective core of a storm cell.

SECONDARY GIGANTIC JET: They originate from the cloud top, under the shielding area of the preceding sprites, and develop upward to reach the lower ionosphere at ∼90 km. The streamers branch outward towards the top, which is similar to what occurs in a Gigantic Jet event.



Comet ATLAS is Plunging Toward the Sun

March 4, 2020: Comet ATLAS (C/2019 Y4) is coming … and the sun is waiting. In late May, the dirty snowball will dip inside the orbit of Mercury, passing only 0.25 AU from the sun. No one knows what will happen when the comet’s icy core is exposed to solar heat at point blank range.


Above: Comet ATLAS photographed on Feb. 29, 2020, by Hisayoshi Kato of Makioka, Japan

“ATLAS is a bit of a wildcard, and there’s a spectrum of possibilities as it nears the sun,” says Karl Battams of the Naval Research Lab in Washington DC. “At one extreme, it could simply crumble away in the coming weeks, and at the other extreme it could brighten up tremendously. It has an unusually small perihelion distance inside of Mercury’s orbit, which bodes well for getting those frozen gases fizzing furiously.”

At the moment Comet ATLAS is beyond the orbit of Mars. Even at that great distance, it is already brightening, shooting up 100-fold since the beginning of February. Currently, it looks like a fuzzy star of 11th magnitude in the Big Dipper–an easy target for backyard telescopes.

By the end of May, the comet could be as bright as a 1st magnitude star. Astronomers will be struggling to see it, however, through the glare of the approaching sun.

“All is not lost!” says Battams. “We have a number of space telescopes designed to view objects very close to the sun. For instance, the Heliospheric Imager on NASA’s STEREO spacecraft will get a great view of ATLAS from mid-May through early June. The camera is very sensitive, so we might be able to observe ATLAS’s tail interacting with the solar wind and outflows.”

Above: STEREO’s Heliospheric Imager recorded Comet PanSTARRS in 2013.

“There’s also the WISPR camera on NASA’s Parker Solar Probe,” he continues. “We will potentially be able view ATLAS from WISPR, concurrently with the STEREO observations, but it would require a non-standard pointing configuration for us, so the operation teams are currently assessing our options.”

Can’t wait for May? Amateur astronomers with mid-sized backyard telescopes can observe Comet ATLAS now. Monitoring is encouraged. Outbursts are possible in the weeks ahead as new veins of volatile material are exposed by intensifying sunlight. Point your optics using this ephemeris from JPL and submit your images here.

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