Amazing Airglow above Easter Island

by Dr. Tony Phillips (

Oct. 4, 2016: Not every colorful light in the night sky is an aurora.  Especially not in the South Pacific. Yuri Beletsky was on a beach in Easter Island, Chile, two nights ago when the starry canopy turned red:

“There was no fire,” says Beletsky. “This is an amazing display of airglow.”

Airglow is aurora-like phenomenon caused by chemical reactions in the upper atmosphere. Human eyes seldom notice the faint glow, because it is usually very faint, but it can be photographed on almost any clear dark night, anywhere in the world.

Beletsky is a veteran photographer of airglow, having captured it dozens of times from sites in Chile and the South Pacific. “The intensity of airglow varies, and sometimes it can be more prominent, as it was on Oct. 2nd,” he says.

The curious thing about Beletsky’s photo is not the intensity of the airglow, but rather its color–red. Airglow is usually green, the color of light from oxygen atoms some 90 km to 100 km above Earth’s surface. Where does the red come from? Instead of oxygen, OH can produce the ruddy hue. These neutral molecules (not to be confused with the OH- ion found in aqueous solutions) exist in a thin layer 85 km high where gravity waves often impress the red glow with a dramatic rippling structure.

Realtime Airglow Photo Gallery

Sprites above Hurricane Matthew

by Dr. Tony Phillips (

Oct. 2, 2016: On Oct. 1st, Earth weather met space weather above Hurricane Matthew.  As the giant storm system was approaching the Greater Antilles, Frankie Lucena of Puerto Rico photographed red sprites shooting up from the thunderclouds:

Sprites are a strange and beautiful form of lightning that shoot up from the tops of electrical storms. They reach all the way up to the edge of space alongside meteors, auroras, and noctilucent clouds. Some researchers believe cosmic rays help trigger sprites, making them a  true space weather phenomenon.

Seeing sprites above a hurricane is rare. Many hurricanes don’t even have regular lightning because the storms lack a key ingredient for electrical activity: vertical winds. (For more information read the Science@NASA article “Electric Hurricanes.”) But Matthew is not a typical hurricane.  It’s one of the most powerful in recent years, briefly reaching Category 5 at about the time Lucena photographed the sprites.  Perhaps extra-strong winds in the vicinity of the storm set the stage for upward-reaching bolts.

Sprite photographers across the Caribbean and the southeastern USA should be alert for more as the storm system approaches the mainland: observing tips.

Realtime Sprite Photo Gallery

New Maps of the South Atlantic Anomaly

by Dr. Tony Phillips (

Sept. 30, 2016: Researchers have long known that one of the van Allen Radiation Belts dips down toward Earth over South America, creating a zone of high radiation called “The South Atlantic Anomaly” (SAA). Since its discovery in 1958, the SAA has been shape-shifting, growing larger and intensifying.  A map published just last week in the American Geophysical Union’s journal Space Weather Quarterly outlines the anomaly with new precision:

When a spacecraft in low-Earth orbit passes through the anomaly, “the radiation causes faults in spacecraft electronics and can induce false instrument readings,” explains Bob Schaefer of the Johns Hopkins University Applied Physics Lab, lead author of the paper reporting the results. “We actually used these spurious signals to map out the radiation environment at an altitude of 850 km.”

Specifically, they looked at pulses of noise in an ultraviolet photometer carried aboard many polar orbiting Defense Meteorological Satellite Program (DMSP) satellites. When high-energy protons in the SAA pass through these sensors, they  produce spurious signals–or, in the case of this study, valuable data. By monitoring the rate of spurious UV pulses, the researchers were able to trace the outlines of the anomaly and monitor its evolution over a period of years.

They found that the anomaly is slowly drifting north and west at rates of 0.16 deg/yr and 0.36 deg/yr, respectively. Currently, it is most intense over a broad region centered on Sao Paulo, Brazil, including much of Paraguay, Uruguay, and northern Argentina. They also detected a seasonal variation: On average, the SAA is most intense in February and again in September-October. In this plot, yearly average counts have been subtracted to reveal the double-peaked pattern:

One maximum coincides with an equinox, but the other does not. The authors were not able to explain the origin of this unexpected pattern.

The solar cycle matters, too, as the data revealed a yin-yang anti-correlation with sunspots. “During years of high solar activity, the radiation intensity is lower, while during solar quiet years the radiation intensity is higher,” writes Schaefer.

According to orthodox thinking, the SAA reaches down from space to within about 200 km of Earth’s surface. Below that altitude, its effects should be mitigated by the shielding of Earth’s atmosphere and geomagnetic field. To test this idea, and Earth to Sky Calculus have undertaken a program to map the SAA from below using weather balloons equipped with radiation sensors.  Next week we will share the results of our first flight from a launch site in Chile.  Stay tuned!

Autumn is Aurora Season

by Dr.Tony Phillips (
Sept. 4, 2016

Summer is ending in the northern hemisphere.  That’s good news for sky watchers because autumn is “aurora season.” Autumn is special in part because lengthening nights and crisp pleasant evenings tempt stargazers outside; they see things they ordinarily wouldn’t. But there’s more to it than that: autumn really does produce a surplus of geomagnetic storms–almost twice the annual average.

see captionIn fact, both spring and autumn are good aurora seasons. Winter and summer are poor. This is a puzzle for researchers because auroras are triggered by solar activity. The Sun doesn’t know what season it is on Earth–so how could one season yield more auroras than another?

Left: Geomagnetic activity from 1875 to 1927, from “Semiannual Variation of Geomagnetic Activity” by C.T. Russell and R.L. McPherron, JGR, 78(1), 92, 1973. See also this analysis by NASA solar physicist David Hathaway.

To understand the answer, we must first understand what causes auroras themselves.

Auroras appear during geomagnetic storms–that is, when Earth’s magnetic field is vibrating in response to a solar wind gust. Such gusts pose no danger to people on the ground because our magnetic field forms a bubble around Earth called the magnetosphere, which protects us. The magnetosphere is filled with electrons and protons. “When a solar wind gust hits the magnetosphere, the impact knocks loose some of those trapped particles,” explains space physicist Tony Lui of Johns Hopkins University. “They rain down on Earth’s atmosphere and cause the air to glow where they hit–like the picture tube of a color TV.”

Below: Still frames from a digital movie show how solar wind gusts rattle Earth’s magnetosphere and trigger auroras. Click to view the 750 kb Quicktime animation created by Digital Radiance, Inc.

see caption
Some solar wind gusts (“coronal mass ejections”) are caused by explosions near sunspots, others are caused by holes in the Sun’s atmosphere (“coronal holes”) that spew solar wind streams into interplanetary space. These gusts sweep past Earth year-round, which returns us to the original question: why do auroras appear more often during spring and autumn?

The answer probably involves the Sun’s magnetic field near Earth. The Sun is a huge magnet, and all the planets in the solar system orbit within the Sun’s cavernous magnetosphere. Earth’s magnetosphere, which spans about 50,000 km from side to side, is tiny compared to the Sun’s.

The outer boundary of Earth’s magnetosphere is called the magnetopause–that’s where Earth’s magnetic field bumps into the Sun’s and fends off the solar wind. Earth’s magnetic field points north at the magnetopause. If the Sun’s magnetic field tilts south near the magnetopause, it can partially cancel Earth’s magnetic field at the point of contact.

see caption“At such times the two fields (Earth’s and the Sun’s) link up,” says Christopher Russell, a Professor of Geophysics and Space Physics at UCLA. “You can then follow a magnetic field line from Earth directly into the solar wind.” Researchers call the north-south component of the Sun’s nearby magnetic field “Bz” (pronounced “Bee-sub-Zee”). Negative (south-pointing) Bz‘s open a door through which energy from the solar wind can reach Earth’s inner magnetosphere. Positive (north-pointing) Bz‘s close the door.

Above: Coronal holes spewing solar windappear as dark areas in ultraviolet and x-ray images of the Sun.

In the early 1970’s Russell and colleague R. L. McPherron recognized a connection between Bz and Earth’s changing seasons. “It’s a matter of geometry,” explains Russell. Bz is the component of the Sun’s magnetic field near Earth which is parallel to Earth’s magnetic axis. As viewed from the Sun, Earth’s tilted axis seem to wobble slowly back and forth with a one-year period. The wobbling motion is what makes Bz wax and wane in synch with the seasons.

In fact, Bz is always fluttering back and forth between north and south as tangled knots of solar magnetic field drift by Earth. What Russell and McPherron realized is that the average size of the flutter is greatest in spring and fall. When Bz turns south during one of those two seasons, it really turns south and “opens the door wide” for the solar wind.

see captionLeft: A solar wind gust triggered these bright auroras in Finland on Sept. 7, 2002. Photo credit: Martti Tenhunen. [more]

Mystery solved? Not yet. In a Geophysical Research Letter (28, 2353-2356, June15, 2001), Lyatsky et al argued that Bz and other known effects account for less than one-third of the seasonal ups-and-downs of geomagnetic storms. “This is an area of active research,” remarks Lui. “We still don’t have all the answers because it’s a complicated problem.”

But not too complicated to enjoy. Dark nights, bright stars, an occasional meteor–and the promise of Northern Lights. Perhaps scientists haven’t figured out why auroras prefer autumn, but it’s easy to understand why sky watchers do….

Cosmic Rays are Intensifying

by Dr. Tony Phillips (

Aug. 30, 2016: Researchers have long known that solar activity and cosmic rays have a yin-yang relationship. As solar activity declines, cosmic rays intensify. Lately, solar activity has been very low indeed. Are cosmic rays responding? The answer is “yes.” and the students of Earth to Sky Calculus have been using helium balloons to monitor cosmic rays in the stratosphere over California. Their latest data show an increase of almost 13% since 2015.

Cosmic rays, which are accelerated toward Earth by distant supernova explosions and other violent events, are an important form of space weather. They can seed clouds, trigger lightning, and penetrate commercial airplanes. Furthermore, there are studies ( #1, #2, #3, #4) linking cosmic rays with cardiac arrhythmias and sudden cardiac death in the general population.

Why are cosmic rays intensifying? The main reason is the sun. Solar storm clouds such as coronal mass ejections (CMEs) sweep aside cosmic rays when they pass by Earth. During Solar Maximum, CMEs are abundant and cosmic rays are held at bay. Now, however, the solar cycle is swinging toward Solar Minimum, allowing cosmic rays to return. Another reason could be the weakening of Earth’s magnetic field, which helps protect us from deep-space radiation.

The radiation sensors onboard our helium balloons detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV. These energies span the range of medical X-ray machines and airport security scanners.

The data points in the graph above correspond to the peak of the Reneger-Pfotzer maximum, which lies about 67,000 feet above central California. When cosmic rays crash into Earth’s atmosphere, they produce a spray of secondary particles that is most intense at the entrance to the stratosphere. Physicists Eric Reneger and Georg Pfotzer discovered this maximum using balloons in the 1930s and it is what we are measuring today.

Cosmic Rays vs. Clouds

The connection between cosmic rays and clouds has long been controversial.  Some researchers hold that cosmic rays hitting Earth’s atmosphere create aerosols which, in turn, seed clouds.  This could make cosmic rays an important player in weather and climate.  Other researchers are less convinced.  Although some laboratory experiments support the idea that cosmic rays help seed clouds, skeptics say the effect is too small to substantially affect the cloudiness of our planet or to avert the course of climate change.

A new study just published in the Aug. 19th issue of Journal of Geophysical Research: Space Physics comes down in favor of cosmic rays. A team of scientists from the Technical University of Denmark (DTU) and the Hebrew University of Jerusalem has linked sudden decreases in cosmic rays (called “Forbush Decreases”) to changes in Earth’s cloud cover.

Forbush Decreases occur when solar storms called “coronal mass ejections (CMEs)” sweep past Earth.  Magnetic fields in CMEs deflect cosmic rays and, essentially, sweep some of the cosmic rays away from our planet.  The research team led by Jacob Svensmark of DTU identified the strongest 26 Forbush Decreases between 1987 and 2007, and looked at ground-based+satellite records of cloud cover to see what happened.  In a press release, their conclusions were summarized as follows: “[Strong Forbush Decreases] cause a reduction in cloud fraction of about 2 percent corresponding to roughly a billion tonnes of liquid water disappearing from the atmosphere.”

If true, that’s amazing.  It would also underscore the importance of measuring cosmic rays in the atmosphere.  Recent balloon flights by and Earth to Sky Calculus show that cosmic rays are intensifying. Cloudy days, anyone?

Space Lightning Over China

On Aug. 13th in China, photographer Phebe Pan was photographing the night sky, hoping to catch a Perseid meteor. Instead, he witnessed a spectacular bolt of “space lightning.” Working atop Shi Keng Kong, the highest mountain peak in the Guangdong province, “I was using a fisheye lens to capture as much of the sky as possible,” says Pan. “Suddenly we saw a flash of blue and purple ejected from the top of a nearby thundercloud. It just looked like a tree with branches, and grew up very fast. So awesome!”

“It just looked like a tree with branches, and grew up very fast,” says Pan. “It lasted just less than one second. So awesome!”

Oscar van der Velde, a member of the Lightning Research Group at the Universitat Politècnica de Catalunya, explains what Pan saw: “This is a very lucky capture of a gigantic jet. It’s the first time I’ve seen one captured using a fisheye lens!”

Think of them as sprites on steroids: Gigantic jets are lightning-like discharges that spring from the tops of thunderstorms, reaching all the way to the ionosphere more than 50 miles overhead. They’re enormous and powerful.

“Gigantic jets are much more rare than sprites,” says van der Velde. “While sprites were discovered in 1989 and have since been photographed by the thousands, it was not until 2001-2002 that gigantic jets were first recorded from Puerto Rico and Taiwan.” Only a few dozen gigantic jets have ever been seen.

Like their cousins the sprites, gigantic jets reach all the way up to the edge of space alongside meteors, noctilucent clouds, and some auroras. This means they are a true space weather phenomenon. Indeed, some researchers believe cosmic rays help trigger these exotic forms of lightning, but the link is controversial.

Realtime Sprite Photo Gallery

Perseid Meteor Outburst

Every year in August, Earth passes through a stream of debris from Comet Swift-Tuttle, source of the annual Perseid meteor shower. The shower is beloved by sky watchers. It is rich in fireballs and plays out over a two-week period of warm, starry summer nights.

This year’s display is going to be even better than usual. “Our models predict an outburst on Aug. 11-12 with peak rates greater than 200 meteors/hour under ideally dark skies,” explains Bill Cooke of NASA’s Meteoroid Environment Office. “That’s about twice as many Perseids as usual.”

Perseids in Aug. 2015, a composite image by Petr Horalek of Kolonica, Slovakia [more]

In ordinary years, Earth grazes the edge of Swift-Tuttle’s debris zone. Occasionally, though, Jupiter’s gravity tugs the huge network of dust trails closer, and Earth plows through closer to the middle. This appears to be one of those years. Experts at NASA and elsewhere agree that three or more streams are on a collision course with Earth–hence the outburst.

Observing tips: Go outside between midnight and dawn on the morning of Aug. 12th. Allow about 45 minutes for your eyes to adjust to the dark. Lie on your back and look straight up. Perseids can appear anywhere in the sky, but their tails will point back to a single point in the constellation Perseus: sky map. Increased activity may also be seen on the morning of Aug. 13th.

Got clouds? NASA is planning a live broadcast of the Perseid meteor shower overnight on Aug. 11-12 and Aug. 12-13, beginning at 10 p.m. EDT. You can also listen to radar echoes from the Perseids on Space Weather Radio. More webcasts: from Israel, from Alabama.

Realtime Perseid Photo Gallery

A Mysterious Form of Aurora

Humans have been watching the aurora borealis for thousands of years, with scientific studies of the phenomenon underway for centuries.  Despite all that watching and studying, however, there are still some auroral forms that remain a mystery–namely, the “proton arc.” This one appeared over the Grande Cache area of Alberta, Canada, on July 29th:

“As I was driving to the Kakwa river, I saw a purple ‘proton arc’ crossing the sky from east to west, pulsing and dancing with the Northern lights,” says photographer Catalin Tapardel. “Quite a show….”

Aurora photographers see these structures from time to time–tight ribbons of light, sometimes red, sometimes green, writhing across the night sky.  They are commonly called “proton arcs.”

Yet aurora scientists say they probably have nothing to do with protons.

“My opinion, and I believe the consensus of most aurora scientists, is that these arcs are not proton related, ” says Jason Ahrns, a researcher at the University of Alaska Fairbanks, “but I don’t know what does cause them.”

“Ordinary auroras we see from the ground and space are caused by electrons precipitating down into the atmosphere,” says Dennis Gallagher of the NASA Marshall Space Flight Center. “Protons can cause auroras, too, but they are different. For one thing, proton auroras are brightest in the UV part of the spectrum, invisible to the human eye.”

There is some visible light from proton auroras, but the structures they make are not tight and filamentary, but rather broad and diffuse–“in part because the gyroradius of protons is large,” says Ahrns. In other words, massive protons circle around magnetic fields in broad lazy arcs unlike lightweight electrons, which can tightly circle magnetic fields to form narrow structures.

Ahrns photographed an authentic proton aurora in February 2014: photo. “It appearance matched the description of proton arcs in the scientific literature – ‘a dim and diffuse glow’ with ‘very little structure in the observed brightness’ with a total brightness of only a few kiloRayleighs, which is just on the verge of visual threshold (Lummerzheim 2001).”

So what are the “proton arcs” often photographed by amateur aurora chasers? “I don’t know,” says Ahrns, “but it is something many of us would like to get to the bottom of!”  For more examples of this mystery in the sky, browse the Proton Arc Photo Gallery.

Realtime Proton Arc Photo Gallery

Realtime Aurora Photo Gallery

What lies inside Jupiter?

July 5, 2016: Jupiter’s swirling clouds can be seen through any department store telescope. With no more effort than it takes to bend over an eyepiece, you can witness storm systems bigger than Earth navigating ruddy belts that stretch hundreds of thousands of kilometers around Jupiter’s vast equator. It’s fascinating.

It’s also vexing. According to many researchers, the really interesting things–from the roots of monster storms to stores of exotic matter–are located at depth. The clouds themselves hide the greatest mysteries from view.

NASA’s Juno probe, which went into orbit on July 4,2016, could change all that. The goal of the mission is to answer the question, What lies inside Jupiter?

juno crop for ICYMI 160701

“Our knowledge of Jupiter is truly skin deep,” says Juno’s principal investigator, Scott Bolton of the SouthWest Research Institute in San Antonio, TX. “Even the Galileo probe, which dived into the clouds in 1995, penetrated no more than about 0.2% of Jupiter’s radius.”

There are many basic things researchers would like to know—like how far down does the Great Red Spot go? How much water does Jupiter hold? And what is the exotic material near the planet’s core?

Juno will lift the veil without actually diving through the clouds. Bolton explains how: “Swooping as low as 5000 km above the cloudtops, Juno will spend a full year orbiting nearer to Jupiter than any previous spacecraft. The probe’s flight path will cover all latitudes and longitudes, allowing us to fully map Jupiter’s gravitational field and thus figure out how the interior is layered.”

Jupiter is made primarily of hydrogen, but only the outer layers may be in gaseous form. Deep inside Jupiter, researchers believe, high temperatures and crushing pressures transform the gas into an exotic form of matter known as liquid metallic hydrogen–a liquid form of hydrogen akin to the slippery mercury in an old-fashioned thermometer. Jupiter’s powerful magnetic field almost certainly springs from dynamo action inside this vast realm of electrically conducting fluid.

“Juno’s magnetometers will precisely map Jupiter’s magnetic field,” says Bolton. “This will tell us a great deal about the planet’s inner magnetic dynamo and the role liquid metallic hydrogen plays in it.”

diagram of Jupiter's interior.

Juno will also probe Jupiter’s atmosphere using a set of microwave radiometers.

“Our sensors can measure the temperature and water content at depths where the pressure is 50 times greater than what the Galileo probe experienced,” says Bolton.

Jupiter’s water content is of particular interest. There are two leading theories of Jupiter’s origin: One holds that Jupiter formed more or less where it is today, while the other suggests Jupiter formed at greater distances from the sun, later migrating to its current location. (Imagine the havoc a giant planet migrating through the solar system could cause.) The two theories predict different amounts of water in Jupiter’s interior, so Juno should be able to distinguish between them—or rule out both.

Finally, Juno will get a grand view of the most powerful Northern Lights in the Solar System.

“Juno’s polar orbit is ideal for studying Jupiter’s auroras,” explains Bolton. “They are really strong, and we don’t fully understand how they are created.”

Auroras on JupiterUnlike Earth, which lights up in response to solar activity, Jupiter makes its own auroras. The power source is the giant planet’s own rotation. Although Jupiter is ten times wider than Earth, it manages to spin around 2.5 times as fast as our little planet. As any freshman engineering student knows, if you spin a magnet—and Jupiter is a very big magnet—you’ve got an electric generator. Induced electric fields accelerate particles toward Jupiter’s poles where the aurora action takes place. Remarkably, many of the particles that rain down on Jupiter’s poles appear to be ejecta from volcanoes on Io. How this complicated system actually works is a puzzle.

It’s a puzzle that members of the public will witness at close range thanks to JunoCam—a public outreach instrument modeled on the descent camera for Mars rover Curiosity. When Juno swoops low over the cloudtops, JunoCam will go to work, snapping pictures better than the best Hubble images of Jupiter.

“JunoCam will show us what you would see if you were an astronaut orbiting Jupiter,” says Bolton. “I am looking forward to that.”