Explainer: How auroras light up the sky

The northern and southern lights are considered natural wonders of the world. Here’s how these and related splendid sky glows form.

Explainer: How auroras light up the sky

Some of the most awesome sights in the night sky are the auroras, or northern and southern lights. These dazzling light shows appear as rippling curtains, bright arcs and diffuse glows. Most glimmer green, but some shine red, blue or purple. Such breathtaking displays are considered one of the natural wonders of the world. But few people ever see them. Why? Auroras largely appear in the skies of Earth’s remote polar regions.

Understanding light and other forms of energy on the move

They trace back to a stream of charged particles that continually flows out from the sun. This plasma is known as the solar wind. Earth’s magnetic field deflects most of the solar wind away from the planet. But that magnetic field snags some particles in the plasma gale. These high-energy charged particles travel along the magnetic field lines to Earth’s poles. There, the particles plunge into the atmosphere, smashing into oxygen and nitrogen atoms.

Those collisions excite the oxygen and nitrogen. That is, they give the atoms a little extra energy. But excited atoms are not stable. They quickly relax into a non-excited — or ground — state. In the process, the atoms release energy in the form of light particles, or photons. These photons make up the auroras.

These lights dance across the sky above Earth’s poles in two oval-shaped zones. The northern lights, or aurora borealis, are most reliably visible to sky watchers in Alaska and Canada. The aurora borealis also shimmers above Greenland, Iceland and Norway. The southern lights, or aurora australis, can be seen over Tasmania, New Zealand and Antarctica.

Auroras are rarely seen closer to the equator than at latitudes of about 70 degrees North or South. But sometimes the sun belches out an enormous plume of plasma. Such an intense burst is called a coronal mass ejection. These plumes can severely distort Earth’s magnetic field. And that can trigger intense auroras that extend much closer to the equator than normal.

Sometimes the sun launches a giant plume of plasma known as a coronal mass ejection. This can trigger intense northern lights that are visible in skies much farther south than usual. A coronal mass ejection in June 2015 set auroras aglow across the United States, as seen in this photo from West Virginia. NASA, courtesy of Michael Charnick

A variety of lights

Auroras paint the sky hundreds of kilometers above the ground in a layer of the atmosphere called the thermosphere. An aurora’s color depends on the energy of the incoming charged particles. Low-energy particles can’t dive very deep into the atmosphere. They excite oxygen atoms at high altitudes — above about 240 kilometers (150 miles). Those oxygen atoms glow red.

Explainer: Our atmosphere — layer by layer

More energetic particles slam into oxygen at lower altitudes. These collisions happen around 100 kilometers (60 miles) to 240 kilometers above Earth. Due to the incoming particles’ higher energy, they cause oxygen atoms to emit higher frequency light. It appears green.

The most energetic particles plunge below 100 kilometers. At such low altitudes, nitrogen atoms outnumber oxygen. Cascading particles cause the nitrogen to give off blue or purple light. That light will have an even higher wavelength than oxygen’s green glow.

Auroras come in many shapes and sizes. Their features will depend on conditions of both the atmosphere and Earth’s magnetic field. One common auroral form is a tall curtain of light. These towering structures trace particles raining in from space. The particles are drawn down by disturbances in Earth’s magnetic field called Alfvén waves.

A recently discovered type of aurora, named the dunes, includes a row of green stripes that run parallel to the ground (here, pointing to the left).
Ripples in the magnetic field around Earth can kick pulses of charged particles into the atmosphere. A series of those pulses creates an aurora that flickers on and off. This fast-blinking aurora was spotted over Iceland in March 2015.

A rarer aurora is known as the dunes. This type appears as a series of green bands parallel to the ground. Scientists think the green streaks come from ripples of gas in the atmosphere, which create a row of bands where oxygen is especially dense. Those bands glow bright green when incoming particles excite the tightly packed oxygen.

Pulsating auroras, meanwhile, are glowing swaths of light. These patches of sky can span hundreds of miles. They repeatedly brighten and dim. This aurora is caused by undulations in Earth’s magnetic field called chorus waves. The waves periodically dump bunches of electrons into the atmosphere. That creates rhythmic flashes of light.

STEVE sky glow
The sky glow known as STEVE is a cousin to the auroras. Its bright smear of purple is formed by a torrent of charged particles rushing across the atmosphere. That plasma heats surrounding air particles to make them glow purple. NASA Goddard Space Flight Center, courtesy of Krista Trinder

Not all colorful lights in the night sky are auroras. Other sky glows include a ribbon of light called STEVE. That’s short for Strong Thermal Emission Velocity Enhancement. STEVE shimmers closer to the equator than auroras do. Its main feature is a purple smear that stretches east to west. Unlike auroras, that light is not produced by particles arriving from space. Instead, a river of plasma rushing through the atmosphere heats surrounding particles through friction. The hot, glowing particles drape the sky in purple.

telescope images of Jupiter show ultraviolet auroras in blue over the planet's north pole
Observations from NASA’s Hubble Space Telescope reveal the auroral lights that crown Jupiter. Hubble observed the auroras in ultraviolet light. That image is overlaid on a Hubble image of Jupiter taken in visible light. NASA, ESA, and J. Nichols (University of Leicester)

Auroras beyond Earth

Auroras aren’t unique to Earth. These stunning phenomena can also develop on other planets with atmospheres and magnetic fields. Saturn and Jupiter’s auroras are so powerful that they heat up the gas giants’ atmospheres. This may explain why the planets are so warm, despite being so far from the sun.

Jupiter’s moons Europa and Ganymede also sport auroras. Even Comet 67P/Churyumov-Gerasimenko has one. This space rock was visited by the European Space Agency’s Rosetta spacecraft. Comet 67P’s aurora forms when solar wind strikes molecules in the comet’s shroud of gas.

It turns out that auroras are not just one of the natural wonders of our world — but a marvel on many other worlds, too.

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Chemists win Nobel Prize for faster, cleaner way of making molecules

Both scientists independently came up with new process — asymmetric organocatalysis. That name may be a mouthful, but it’s not that hard to understand.

Chemists win Nobel Prize for faster, cleaner way of making molecules

Making molecules is hard work. Atoms must be bonded together in specific arrangements through a series of chemical reactions. Those reactions often are slow and far from straightforward. They also can waste resources. The 2021 Nobel Prize in chemistry goes to two scientists who developed a tool some 20 years ago that revolutionized how chemists create new molecules. Their process is not only faster but also friendlier to the environment.

Explainer: The Nobel Prize

“This is a fitting recognition of very important work,” says H.N. Cheng. He’s president of the American Chemical Society, based in Washington, D.C. “We can think of chemists as magicians having magic wands in the lab,” Cheng says. “We wave the wand and a reaction goes on.” These Nobel laureates gave chemists “a new wand,” that’s drastically more efficient and less wasteful, he says.

That wand is a new way to speed the reactions that build specific molecules. It’s a process known as asymmetric organocatalysis (AY-sih-MEH-trik Or-gan-oh-kah-TAL-ih-sis). This year’s winners came up with the idea for it independently. One of the chemists, Benjamin List, works at the Max Planck Institute for Coal Research. It’s in Mülheim an der Ruhr, Germany. The other is David MacMillan. He works at Princeton University in New Jersey.

Making new drugs or designing novel materials often requires assembling simple chemical building blocks to form new molecules. But these chemical building blocks can’t just be thrown together. Instead, they must be carefully combined through a step-by-step series of processes. Many of these procedures create two versions of a molecule, ones that that are mirror images of each another. And often those left- and right-handed versions can have very different effects.

For example, thalidomide (Thah-LID-uh-myde) is a drug prescribed in the 1950s and ‘60s to prevent morning sickness in early pregnancy. But it caused birth defects in more than 10,000 babies. The problem: This drug supplied both mirror-image forms of the molecule, and one of them was toxic.

It was a painful lesson for chemists. Today, building such asymmetric molecules and controlling which version gets produced is extremely important. That’s especially true for medicines.

The role of catalysts

Chemical reactions can be coaxed to take place using catalysts. These molecular workhorses speed up those reactions without being transformed by them.

Chemists have long known about two kinds of catalysts: enzymes and metal complexes. Enzymes are big, clunky proteins. Through evolution, organisms have developed enzymes that perform very specific chemical actions in the body. Many can be reproduced in the lab. But making them on a large scale can be hard. Metals, such as platinum or cobalt, can kick-start some reactions too. But many only work in airless, waterfree conditions. And they can be hard to achieve in manufacturing plants. What’s more, many metal catalysts are toxic and costly.

Explainer: What is a catalyst?

For much of history, these were the only tools chemists had to make new molecules. “But in the year 2000, everything changed,” says Pernilla Wittung-Stafshede. She’s a chemist at Chalmers University of Technology in Gothenburg, Sweden. She’s also a member of the Nobel Committee for Chemistry, which selected this year’s winners. 

Back in 2000, Benjamin List worked at the Scripps Research Institute. It’s in La Jolla, Calif. He was studying a reaction used to link two organic molecules together through bonds in their carbon atoms. In organisms, such reactions are key to converting food into energy. And they depend on a large and complex enzyme called aldolase A.

Benjamin List, pictured here, discovered that proline, an amino acid, could do the work of a big, unwieldy enzyme to trigger chemical reactions. This work led to a new route to catalytic reactions and the 2021 Nobel Prize for chemistry.© Frank Vinken for MPG

Only a small part of this enzyme actually catalyzes the reaction, however. List discovered that one amino acid — proline — could do the work of this big clunky protein. And by using proline, chemists could make far more of one of the mirror-image final products.

“When I did this experiment, I didn’t know what would happen,” List said at an October 6 news conference. “I thought maybe it’s a stupid idea.” But when it worked, he now recalls, he realized “it could be something big.”

At about the same time, MacMillan was working at the University of California, Berkeley. He was focusing on another chemical reaction — one that forms rings of carbon atoms. It’s an important reaction. Chemists use it all the time to make products as different as rubber and medicines. While it works, it tends to be very slow. And it relies on finicky metal catalysts that won’t work when wet.

So MacMillan designed small carbon-based molecules — organic molecules — that mimicked the metals’ catalytic action. They also worked more simply. And these, too, favored the production of one of two possible mirror-image forms of the final product. MacMillan coined a term for this process: asymmetric organocatalysis.

David MacMillan sitting in his office
David MacMillan won a Nobel Prize for his design of certain small organic molecules. These mimicked the catalytic action of metals, but in a simple way.Denise Applewhite/Office of Communications/Princeton University (2012)

The value of this research

List’s and MacMillan’s work prompted others to seek out more organic catalysts and to study how they might be used. These catalysts tend to be small carbon-and-hydrogen molecules which might also include oxygen, nitrogen, sulfur and/or phosphorus.

Catalysis is a big deal. Roughly one-third of the world’s collective income depends on it, notes Peter Somfai. He’s a chemist at Lund University in Sweden and another member of the Nobel Committee for Chemistry. At an October 6 news conference announcing the new winners, he noted “We now have a new powerful tool available for making organic molecules.” He said it’s one that can be drastically more efficient and “greener” than previous methods.

Explainer: In chemistry, what does it mean to be organic?

To highlight how much more efficient this process is, Somfai pointed to strychnine (STRIK-nyne). The toxic molecule is made by certain plants and is sometimes sold as a rat poison. Its complex structure makes it a good benchmark for comparing how efficiently chemicals can be made using different processes in the lab or by a company. Chemists once had to use an extremely wasteful process to make the compound. There were 29 different chemical steps. And in the end, just 0.0009 percent of the initial raw materials ended up as strychnine. But through organocatalysis, strychnine can now be made in just 12 steps. In fact, Somfai said, this process is 7,000 times more efficient than the old one.

And because this process eliminates use of toxic chemicals, it’s also a far more environmentally friendly process.

If building new molecules is like playing chess, asymmetric organocatalysis has “completely changed the game,” Somfai said. “It’s like adding a new chess piece that can move in different ways.”

For their achievements, List and MacMillan will each get a medal and share 10 million Swedish kroner (more than $1.1 million).

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