New research shows unprecedented atmospheric changes during May's geomagnetic superstorm

On May 11, a gorgeous aurora surprised stargazers across the southern United States. That same weekend, a tractor guided by GPS missed its mark.

What do the visibility of the northern lights have in common with compromised farming equipment in the Midwest?

A uniquely powerful geomagnetic storm, according to two newly published papers co-authored by Virginia Tech's Scott England.

"The northern lights are caused by energetic, charged particles hitting our upper atmosphere, which are impacted by numerous factors in space, including the sun," said England, associate professor in the Kevin T. Crofton Department of Aerospace and Ocean Engineering. "During solar geomagnetic storms, there's a lot more of these energetic charged particles in the space around Earth, so we see a brightening of the northern lights and the region over which you can see them spreads out to include places like the lower 48 states that usually don't see this display."

England and a team of university and industry collaborators tracked the upper atmospheric event on May 11 using NASA's GOLD instrument. It turned out to be the strongest geomagnetic storm captured in the last 20 years. Their findings were recently published in Geophysical Research Letters in two studies, both co-authored by England. The first study, by first author Deepak Karan, from the University of Colorado, Boulder, showed unprecedented changes in location and spread of particles in the upper atmosphere. The second study, by first author and Virginia Tech alumnus J. Scott Evans '88, documented composition and temperature changes.

Among the collected data, England noted witnessing some "delightful swirly patterns" for the first time, and a dramatic motion of the air away from the aurora causing the formation of enormous vortices that moved air in a spiral larger than a hurricane. Specific observations included:

• Unpredictable movement of low energy charged particles from around the equator toward the aurora
• Charged particles that can be divided into two buckets: low energy and high energy, the latter of which can hurt humans working in space and damage electronics
• Changes in temperature and pressure that likely lead to the swirls and vortices seen
• Changes in locations and spread of low energy particles, which can negatively impact GPS, satellites, and even the electrical grid

"As the aurora intensifies, you see more lights, but along with that, there's more energy entering the atmosphere, so it makes the atmosphere near the poles very hot, which starts to push air away from the poles and towards the equator," England said. "This data poses a lot of questions like, did something really different happen during this geomagnetic storm than has happened previously, or do we just have better instruments to measure the changes?"

Furthermore, what could those changes mean for the human-made technology that orbits that region of the atmosphere?

More than a northern lights show

Earth's upper atmosphere, spanning from about 60 to 400 miles above us, borders space and is the hang-out zone for satellites and the International Space Station. The upper atmosphere is made up of some of the same particles as the lower atmosphere, where we live and breathe. But it also has another side, the ionosphere that can be thought of almost like an electric blanket -- highly charged and constantly fluctuating. These charged particles in the ionosphere are one thing that makes this region of space so dynamic. It's common for the temperature and composition of the upper atmosphere and ionosphere to change. In fact, it does so predictably during the day and night and even changes overtime with seasons.

England said the particles in earth's atmosphere are impacted by numerous factors in space, including the sun's activity. During a solar geomagnetic storm flare, an intense burst of radiation from the sun changes the composition and speed of the particles within the earth's atmosphere. So why in recent months all over the globe the northern lights have been visible in places where they've not been seen before now?

"The number of sunspots, flares, and storms changes with an 11-year cycle that we call the solar cycle," England said. "The number of flares we are seeing has been increasing gradually for the last couple of years as we move toward the peak of the solar cycle."

In addition to the visibility of the northern lights, geomagnetic storms have a range of impacts on our technology. Because radio and GPS signals travel through this constantly fluctuating "electric blanket," changes in this layer of the atmosphere can disrupt signals and impede navigation and communication systems such as GPS. Various factors from both earth's weather and space weather can impact this crucial layer, but there's much to be learned about why changes in the upper and lower atmosphere occur and how they might impact life as we know it.

"These storms can also increase electrical currents that flow around the Earth, which can impact technological devices that use very long wires. In recent years, there have been impacts to the power grid when too much current was flowing through the wires. During the largest geomagnetic storm ever recorded, the Carrington Event in 1859, these caused telegraph systems -- peak technology at that time -- to catch on fire," England said.

Scientists suspect that a storm similar to the 1859 Carrington Event, if it happened today, could cause an internet apocalypse, sending large numbers of people and businesses offline. While the May 11 storm did not cause drastic disruptions, with the peak of the solar cycle expected to reach in July 2025, we are still about a year away from knowing those potential effects.

Read more at Science Daily

Scientists solve 40-year mystery over Jupiter's X-ray aurora

A research team co-led by UCL (University College London) has solved a decades-old mystery as to how Jupiter produces a spectacular burst of X-rays every few minutes.

The X-rays are part of Jupiter's aurora -- bursts of visible and invisible light that occur when charged particles interact with the planet's atmosphere. A similar phenomenon occurs on Earth, creating the northern lights, but Jupiter's is much more powerful, releasing hundreds of gigawatts of energy, enough to briefly power all of human civilisation*.

In a new study, published in Science Advances, researchers combined close-up observations of Jupiter's environment by NASA's satellite Juno, which is currently orbiting the planet, with simultaneous X-ray measurements from the European Space Agency's XMM-Newton observatory (which is in Earth's own orbit).

The research team, led by UCL and the Chinese Academy of Sciences, discovered that X-ray flares were triggered by periodic vibrations of Jupiter's magnetic field lines. These vibrations create waves of plasma (ionised gas) that send heavy ion particles "surfing" along magnetic field lines until they smash into the planet's atmosphere, releasing energy in the form of X-rays.

Co-lead author Dr William Dunn (UCL Mullard Space Science Laboratory) said: "We have seen Jupiter producing X-ray aurora for four decades, but we didn't know how this happened. We only knew they were produced when ions crashed into the planet's atmosphere.

"Now we know these ions are transported by plasma waves -- an explanation that has not been proposed before, even though a similar process produces Earth's own aurora. It could, therefore, be a universal phenomenon, present across many different environments in space."

X-ray auroras occur at Jupiter's north and south poles, often with clockwork regularity -- during this observation Jupiter was producing bursts of X-rays every 27 minutes.

The charged ion particles that hit the atmosphere originate from volcanic gas pouring into space from giant volcanoes on Jupiter's moon, Io.

This gas becomes ionised (its atoms are stripped free of electrons) due to collisions in Jupiter's immediate environment, forming a donut of plasma that encircles the planet.

Co-lead author Dr Zhonghua Yao (Chinese Academy of Sciences, Beijing) said: "Now we have identified this fundamental process, there is a wealth of possibilities for where it could be studied next. Similar processes likely occur around Saturn, Uranus, Neptune and probably exoplanets as well, with different kinds of charged particles 'surfing' the waves."

Co-author Professor Graziella Branduardi-Raymont (UCL Mullard Space Science Laboratory) said: "X-rays are typically produced by extremely powerful and violent phenomena such as black holes and neutron stars, so it seems strange that mere planets produce them too.

"We can never visit black holes, as they are beyond space travel, but Jupiter is on our doorstep. With the arrival of the satellite Juno into Jupiter's orbit, astronomers now have a fantastic opportunity to study an environment that produces X-rays up close."

For the new study, researchers analysed observations of Jupiter and its surrounding environment carried out continuously over a 26-hour period by the Juno and XMM-Newton satellites.

They found a clear correlation between waves in the plasma detected by Juno and X-ray auroral flares at Jupiter's north pole recorded by X-MM Newton. They then used computer modelling to confirm that the waves would drive the heavy particles towards Jupiter's atmosphere.

Why the magnetic field lines vibrate periodically is unclear, but the vibration may result from interactions with the solar wind or from high-speed plasma flows within Jupiter's magnetosphere.

Jupiter's magnetic field is extremely strong -- about 20,000 times as strong as Earth's -- and therefore its magnetosphere, the area controlled by this magnetic field, is extremely large. If it was visible in the night sky, it would cover a region several times the size of our moon.

Read more at Science Daily

Physicists describe new type of aurora

For millennia, humans in the high latitudes have been enthralled by auroras -- the northern and southern lights. Yet even after all that time, it appears the ethereal, dancing ribbons of light above Earth still hold some secrets.

In a new study, physicists led by the University of Iowa report a new feature to Earth's atmospheric light show. Examining video taken nearly two decades ago, the researchers describe multiple instances where a section of the diffuse aurora -- the faint, background-like glow accompanying the more vivid light commonly associated with auroras -- goes dark, as if scrubbed by a giant blotter. Then, after a short period of time, the blacked-out section suddenly reappears.

The researchers say the behavior, which they call "diffuse auroral erasers," has never been mentioned in the scientific literature. The findings appear in the Journal of Geophysical Research Space Physics.

Auroras occur when charged particles flowing from the sun -- called the solar wind -- interact with Earth's protective magnetic bubble. Some of those particles escape and fall toward our planet, and the energy released during their collisions with gases in Earth's atmosphere generate the light associated with auroras.

"The biggest thing about these erasers that we didn't know before but know now is that they exist," says Allison Jaynes, assistant professor in the Department of Physics and Astronomy at Iowa and study co-author. "It raises the question: Are these a common phenomenon that has been overlooked, or are they rare?

"Knowing they exist means there is a process that is creating them," Jaynes continues, "and it may be a process that we haven't started to look at yet because we never knew they were happening until now."

It was on March 15, 2002, that David Knudsen, a physicist at the University of Calgary, set up a video camera in Churchill, a town along Hudson Bay in Canada, to film auroras. Knudsen's group was a little disheartened; the forecast called for clear, dark skies -- normally perfect conditions for viewing auroras -- but no dazzling illumination was happening. Still, the team was using a camera specially designed to capture low-level light, much like night-vision goggles.

Though the scientists saw only mostly darkness as they gazed upward with their own eyes, the camera was picking up all sorts of auroral activity, including an unusual sequence where areas of the diffuse aurora disappeared, then came back.

Knudsen, looking at the video as it was being recorded, scribbled in his notebook, "pulsating 'black out' diffuse glow, which then fills in over several seconds."

"What surprised me, and what made me write it in the notebook, is when a patch brightened and turned off, the background diffuse aurora was erased. It went away," says Knudsen, a Fort Dodge, Iowa, native who has studied aurora for more than 35 years and is a co-author on the study. "There was a hole in the diffuse aurora. And then that hole would fill back in after a half-minute or so. I had never seen something like that before."

The note lay dormant, and the video unstudied, until Iowa's Jaynes handed it to graduate student Riley Troyer to investigate. Jaynes learned about Knudsen's recording at a scientific meeting in 2010 and referenced the eraser note in her doctoral thesis on diffuse aurora a few years later. Now on the faculty at Iowa, she wanted to learn more about the phenomenon.

"I knew there was something there. I knew it was different and unique," says Jaynes, assistant professor in the Department of Physics and Astronomy. "l had some ideas how it could be analyzed, but I hadn't done that yet. I handed it to Riley, and he went much further with it by figuring out his own way to analyze the data and produce some significant conclusions."

Troyer, from Fairbanks, Alaska, took up the assignment with gusto.

"I've seen hundreds of auroras growing up," says Troyer, who is in his third year of doctoral studies at Iowa. "They're part of my heritage, something I can study while keeping ties to where I'm from."

Troyer created a software program to key in on frames in the video when the faint erasers were visible. In all, he cataloged 22 eraser events in the two-hour recording.

"The most valuable thing we found is showing the time that it takes for the aurora to go from an eraser event (when the diffuse aurora is blotted out) to be filled or colored again," says Troyer, who is the paper's corresponding author, "and how long it takes to go from that erased state back to being diffuse aurora. Having a value on that will help with future modeling of magnetic fields."

Jaynes says learning about diffuse auroral erasers is akin to studying DNA to understand the entire human body.

Read more at Science Daily

Comet discovered to have its own northern lights

This composite is a mosaic comprising four individual NAVCAM images taken from 19 miles (31 kilometers) from the center of comet 67P/Churyumov-Gerasimenko on Nov. 20, 2014. The image resolution is 10 feet (3 meters) per pixel.

Data from NASA instruments aboard the ESA (European Space Agency) Rosetta mission have helped reveal that comet 67P/Churyumov-Gerasimenko has its own far-ultraviolet aurora. It is the first time such electromagnetic emissions in the far-ultraviolet have been documented on a celestial object other than a planet or moon. A paper on the findings was released today in the journal Nature Astronomy.

On Earth, aurora (also known as the northern or southern lights) are generated when electrically charged particles speeding from the Sun hit the upper atmosphere to create colorful shimmers of green, white, and red. Elsewhere in the solar system, Jupiter and some of its moons -- as well as Saturn, Uranus, Neptune, and even Mars -- have all exhibited their own version of northern lights. But the phenomena had yet to be documented in comets.

Rosetta is space exploration's most traveled and accomplished comet hunter. Launched in 2004, it orbited comet 67P/Churyumov-Gerasimenko (67P/C-G) from Aug. 2014 until its dramatic end-of-mission comet landing in Sept. 2016. The data for this most recent study is on what mission scientists initially interpreted as "dayglow," a process caused by photons of light interacting with the envelope of gas -- known as the coma -- that radiates from, and surrounds, the comet's nucleus. But new analysis of the data paints a very different picture.

"The glow surrounding 67P/C-G is one of a kind," said Marina Galand of Imperial College London and lead author of the study. "By linking data from numerous Rosetta instruments, we were able to get a better picture of what was going on. This enabled us to unambiguously identify how 67P/C-G's ultraviolet atomic emissions form."

The data indicate 67P/C-G's emissions are actually auroral in nature. Electrons streaming out in the solar wind -- the stream of charged particles flowing out from the Sun -- interact with the gas in the comet's coma, breaking apart water and other molecules. The resulting atoms give off a distinctive far-ultraviolet light. Invisible to the naked eye, far-ultraviolet has the shortest wavelengths of radiation in the ultraviolet spectrum.

Exploring the emission of 67P/C-G will enable scientists to learn how the particles in the solar wind change over time, something that is crucial for understanding space weather throughout the solar system. By providing better information on how the Sun's radiation affects the space environment they must travel through, such information could ultimately can help protect satellites and spacecraft, as well as astronauts traveling to the Moon and Mars.

"Rosetta is the gift that keeps on giving," said Paul Feldman, an investigator on Alice at the Johns Hopkins University in Baltimore and a co-author of the paper. "The treasure trove of data it returned over its two-year visit to the comet have allowed us to rewrite the book on these most exotic inhabitants of our solar system -- and by all accounts there is much more to come."

NASA Instruments Aboard ESA's Rosetta

NASA-supplied instruments contributed to this investigation. The Ion and Electron Sensor (IES) instrument detected the amount and energy of electrons near the spacecraft, the Alice instrument measured the ultraviolet light emitted by the aurora, and the Microwave Instrument for the Rosetta Orbiter (MIRO) measured the amount of water molecules around the comet (the MIRO instrument includes contributions from France, Germany, and Taiwan). Other instruments aboard the spacecraft used in the research were the Italian Space Agency's Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS), the Langmuir Probe (LAP) provided by Sweden, and the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) provided by Switzerland.

Read more at Science Daily

Citizen science discovers a new form of the northern lights

Working together with space researchers, Finnish amateur photographers have discovered a new auroral form. Named 'dunes' by the hobbyists, the phenomenon is believed to be caused by waves of oxygen atoms glowing due to a stream of particles released from the Sun.

In the recently published study, the origins of the dunes were tracked to a wave guide formed within the mesosphere and its boundary, the mesopause. The study also posits that this new auroral form provides researchers with a novel way to investigate conditions in the upper atmosphere.

The study was published in the first issue of the journal AGU Advances.

An unknown fingerprint appears in the sky

Minna Palmroth, Professor of Computational Space Physics at the University of Helsinki, heads a research group developing the world's most accurate simulation of the near-Earth space and space weather that cause auroral emissions.

The sun releases a steady flow of charged particles, known as the solar wind. Reaching Earth's ionised upper atmosphere, the ionosphere, they create auroral emissions by exciting atmospheric oxygen and nitrogen atoms. The excitation state is released as auroral light.

In late 2018, Palmroth published a book entitled 'Revontulibongarin opas' ('A guide for aurora borealis watchers'). The book was born out of Palmroth's cooperation with Northern Lights enthusiasts and the answers she provided to questions about the physics of the phenomenon in the hobbyists' Facebook group.

Thousands of magnificent photographs of the Northern Lights taken by hobbyists were surveyed and categorised for the book. Each auroral form is like a fingerprint, typical only of a certain phenomenon in the auroral zone. During the classification, hobbyists pointed out that a certain auroral form did not fit into any of the pre-existing categories. Palmroth set aside these unusual forms for later consideration.

By an almost unbelievable coincidence, just days after the book was published, the hobbyists saw this unusual form again and immediately informed Palmroth. The form appeared as a green-tinged and even pattern of waves resembling a striped veil of clouds or dunes on a sandy beach.

"One of the most memorable moments of our research collaboration was when the phenomenon appeared at that specific time and we were able to examine it in real time," says Northern Lights and astronomy hobbyist Matti Helin.

Waves newly revealed by the aurora

Investigations into the phenomenon were launched, with hobbyist observations and scientific methods coming together to explain the waves.

"It was like piecing together a puzzle or conducting detective work," says Helin. "Every day we found new images and came up with new ideas. Eventually, we got to the bottom of it..."

The phenomenon was photographed at the same time in both Laitila and Ruovesi, southwest Finland, with the same detail observed in the auroral emission in both images. Maxime Grandin, a postdoctoral researcher in Palmroth's team, identified stars behind the emission and determined the azimuths and elevations of the stars with the help of the astronomy software program Stellarium. This made it possible to use the stars as points of reference when calculating the altitude and extent of the auroral phenomenon.

Grandin found that the auroral dunes occur at a relatively low altitude of 100 kilometres, in the upper parts of the mesosphere. The wavelength of the wave field was measured to be 45 kilometres.

A total of seven similar events -- where a camera had recorded the same even pattern of waves -- were further identified from the 'Taivaanvahti' ('Sky Watch') service maintained by the Finnish Amateur Astronomer Association, Ursa.

Unexplored region

The part of the auroral zone where Earth's electrically-neutral atmosphere meets the edge of space is an extremely challenging environment for satellites and other space-borne instruments. Palmroth says this is why it is one of the least studied places on our planet.

"Due to the difficulties in measuring the atmospheric phenomena occurring between 80 and 120 kilometres in altitude, we sometimes call this area 'the ignorosphere'," she says.

The dunes were observed precisely in that particular region of the auroral zone. The observed phenomenon guided the researchers towards a middle ground between atmospheric research and space research, as the usual methodology of space physics could not explain it alone.

"The differences in brightness within the dune waves could be due to either waves in the precipitating particles coming from space, or in the underlying atmospheric oxygen atoms," says Palmroth. "We ended up proposing that the dunes are a result of increased oxygen atom density."

Next, the team had to determine how the variability in the density of the oxygen atoms caused by gravity waves in the atmosphere results in such an even and widespread field of waves. Normally at the altitude of study there are many different kinds of gravity waves travelling in different directions at different wavelengths, which is why they do not easily form the even wavefields exhibited by the dunes.

The Northern Lights illuminate a tidal bore

The study suggests that the phenomenon in question is a mesospheric bore, a rare and little-studied phenomenon that takes place in the mesosphere. The tidal bore phenomenon is a wave common to many rivers, where the tide travels up the river channel.

Various types of gravity wave are born in the atmosphere and then rise. In very rare cases, gravity waves can get filtered as they rise between the mesopause and an inversion layer that is intermittently formed below the mesopause. The inversion layer makes the filtered waves bend and enables them to travel long distances through the channel without attenuation.

When the oxygen atoms in the bore collide with the electrons precipitating down upon the atmosphere, they become excited. When releasing this excitation, they create the auroral light. This is why mesospheric bores -- a phenomenon thus far considered a very challenging subject of research -- can occasionally be seen with the naked eye.

Space researchers focus on the atmosphere

Prior to this discovery, mesospheric bores were not observed in the auroral zone, nor have they been investigated via auroral emissions.

"The auroral zone as a whole is usually discounted in studies focused on the bore, as auroral emissions impair the technique used to identify mesospheric bores," says Palmroth.

Traditionally, researchers specialising in the atmosphere and space have largely investigated their topics of interest separate from each other. This is because there are only a handful of known mechanisms of interaction between the ionosphere bathing in the precipitating electrons, and the neutral atmosphere.

With the help of measuring devices operated by the Finnish Meteorological Institute, the dunes were found to occur simultaneously and in the same region where the electromagnetic energy originating in space is transferred to the ignorosphere.

Read more at Science Daily

Flares in the universe can now be studied on Earth

Solar flares are caused by magnetic reconnection in space and can interfere with our communications satellites, affecting power grids, air traffic and telephony. Now, researchers at Chalmers University of Technology, Sweden, have found a new way to imitate and study these spectacular space plasma phenomena in a laboratory environment.

Solar flares, cosmic radiation, and the northern lights are well known phenomena. But exactly how their enormous energy arises is not as well understood.

Now, physicists at Chalmers University of Technology, Sweden, have discovered a new way to study these spectacular space plasma phenomena in a laboratory environment. The results have been published in the journal Nature Communications.

"Scientists have been trying to bring these space phenomena down to earth for a decade. With our new method we can enter a new era, and investigate what was previously impossible to study. It will tell us more about how these events occur," says Longqing Yi, researcher at the Department of Physics at Chalmers.

The research concerns so-called 'magnetic reconnection' -- the process which gives rise to these phenomena. Magnetic reconnection causes sudden conversion of energy stored in the magnetic field into heat and kinetic energy. This happens when two plasmas with anti-parallel magnetic fields are pushed together, and the magnetic field lines converge and reconnect. This interaction leads to violently accelerated plasma particles that can sometimes be seen with the naked eye -- for example, during the northern lights.

Magnetic reconnection in space can also influence us on earth. The creation of solar flares can interfere with communications satellites, and thus affect power grids, air traffic and telephony.

In order to imitate and study these spectacular space plasma phenomena in the laboratory, you need a high-power laser, to create magnetic fields around a million times stronger than those found on the surface of the sun. In the new scientific article, Longqing Yi, along with Professor Tünde Fülöp from the Department of Physics, proposed an experiment in which magnetic reconnection can be studied in a new, more precise way. Through the use of grazing incidence of ultra-short laser pulses, the effect can be achieved without overheating the plasma. The process can thus be studied very cleanly, without the laser directly affecting the internal energy of the plasma.

The proposed experiment would therefore allow us to seek answers to some of the most fundamental questions in astrophysics.

"We hope that this can inspire many research groups to use our results. This is a great opportunity to look for knowledge that could be useful in a number of areas. For example, we need to better understand solar flares, which can interfere with important communication systems. We also need to be able to control the instabilities caused by magnetic reconnection in fusion devices," says Tünde Fülöp.

Read more at Science Daily

The Northern Lights Make Strange Sounds: Here's Why

For more than 15 years, a lone scientist in southern Finland has spent countless winter nights among the snowy fields and frozen lakes around his village, in pursuit of one of the most ephemeral mysteries of the heavens: the faint, almost phantasmagorical sounds heard during intense displays of the aurora borealis, or northern lights.

The epic study by acoustician Unto K. Laine includes the first audio recordings of the muffled crackling or popping sometimes heard overhead during spectacular aurora displays.

Over the years, the sounds of the northern lights have been explained as illusions, imagination, inebriation or even voices from the spiritual world.

But Laine has shown the sounds are real, and he thinks he has found what causes them: sparks of electricity discharging beneath the aurora in an inversion layer of the atmosphere that can form in clear and calm weather conditions.

Laine told Live Science that he became interested in the phenomenon of so-called auroral acoustics about 25 years ago, when he and his friends heard sounds from an aurora after a nighttime music gathering in Finnish Lapland, in the far north of the country.

"This experience never left me. We had to concentrate — we did not move or talk at all," he said. "A few of us did not hear it, because at that time, the aurora wasn't very strong, and it was a very low-intensity sound. I could never forget this experience; it was so strange."

In 2000, Laine started to keep a careful watch on space weather forecasts for reports of intense solar flares that could supercharge auroras over the following nights. Auroras are caused by charged particles from solar flares interacting with the Earth's magnetic field and raining into the upper atmosphere, where they excite the nitrogen and oxygen atoms of the air to create the dramatic and colorful light shows.

"During all these years, I have checked the space weather two to three times a day, so not to miss any aurora events — and always, when the weather conditions are good, I go," he said.

Sounds out of space

Laine's solitary hunt for the elusive sound of the northern lights demanded great patience. There have been few intense auroras over Finland in the past 16 years, thanks in part to the "solar minimum," a period of decreased activity in the sun's natural 11-year solar cycle. Between 2004 and 2007, the sun experienced a lull in activity, according to NASA, which meant fewer solar flares and fewer auroras. In 2011, as solar activity ramped up, Laine started using a VLF loop antenna to measure magnetic fields and a microphone array to triangulate the locations of crackles and pops in the ever-changing sounds from the aurora.

"The sounds are diverse and can vary quite a lot, and it is very possible that there are many different mechanisms creating the sounds," he said. "I have been concentrating more on the clapping, popping and crackling, because they are good for estimating the direction of the sound."

In September 2011, during an intense aurora overhead, Laine's microphone array was able to triangulate the locations of several distinct "clap" sounds from the celestial display. To his surprise, the results showed the sounds were originating just 230 feet (70 meters) above the ground — much lower than the auroras themselves, which occur at altitudes of up to 300 kilometers (185 miles).

Laine's unexpected discovery partly explained one of the mysteries of the acoustics of auroras: How can faint sounds from auroras so high in the atmosphere be heard at the surface of the Earth?

In a research paper published in 2012, which included the first recordings of auroral sounds ever made, Laine also ruled out a theory that the sounds could be made by trees, because his microphones had been set in open fields and beside frozen lakes.

The acoustics of auroras

Now, Laine thinks he may have discovered a mechanism in the atmosphere that explains at least some of the sounds caused by auroras.

During an intense display of the northern lights over southern Finland on March 17 and 18, 2013, when the temperature in the village of Fiskars was minus 4 degrees Fahrenheit (minus 20 degrees Celsius), Laine recorded hundreds of auroral sound events.

He also measured magnetic pulses that occurred immediately before each sound event, which corresponded in strength to the volume of the sounds.

Then, Laine matched his measurements to data from the Finnish Meteorological Institute for the same night, and found they had measured a thermal inversion layer in the atmosphere — a blanket of relatively warm air that traps cold air next to the ground in calm conditions — at the same altitude where the noises originated.

Read more at Discovery News