Researchers discover source of super-fast electron 'rain'

UCLA scientists have discovered a new source of super-fast, energetic electrons raining down on Earth, a phenomenon that contributes to the colorful aurora borealis but also poses hazards to satellites, spacecraft and astronauts.

The researchers observed unexpected, rapid "electron precipitation" from low-Earth orbit using the ELFIN mission, a pair of tiny satellites built and operated on the UCLA campus by undergraduate and graduate students guided by a small team of staff mentors.

By combining the ELFIN data with more distant observations from NASA's THEMIS spacecraft, the scientists determined that the sudden downpour was caused by whistler waves, a type of electromagnetic wave that ripples through plasma in space and affects electrons in the Earth's magnetosphere, causing them to "spill over" into the atmosphere.

Their findings, published March 25 in the journal Nature Communications, demonstrate that whistler waves are responsible for far more electron rain than current theories and space weather models predict.

"ELFIN is the first satellite to measure these super-fast electrons," said Xiaojia Zhang, lead author and a researcher in UCLA's department of Earth, planetary and space sciences. "The mission is yielding new insights due to its unique vantage point in the chain of events that produces them."

Central to that chain of events is the near-Earth space environment, which is filled with charged particles orbiting in giant rings around the planet, called Van Allen radiation belts. Electrons in these belts travel in Slinky-like spirals that literally bounce between the Earth's north and south poles. Under certain conditions, whistler waves are generated within the radiation belts, energizing and speeding up the electrons. This effectively stretches out the electrons' travel path so much that they fall out of the belts and precipitate into the atmosphere, creating the electron rain.

One can imagine the Van Allen belts as a large reservoir filled with water -- or, in this case, electrons, said Vassilis Angelopolous, a UCLA professor of space physics and ELFIN's principal investigator. As the reservoir fills, water periodically spirals down into a relief drain to keep the basin from overflowing. But when large waves occur in the reservoir, the sloshing water spills over the edge, faster and in greater volume than the relief drainage. ELFIN, which is downstream of both flows, is able to properly measure the contributions from each.

The low-altitude electron rain measurements by ELFIN, combined with the THEMIS observations of whistler waves in space and sophisticated computer modeling, allowed the team to understand in detail the process by which the waves cause rapid torrents of electrons to flow into the atmosphere.

The findings are particularly important because current theories and space weather models, while accounting for other sources of electrons entering the atmosphere, do not predict this extra whistler wave-induced electron flow, which can affect Earth's atmospheric chemistry, pose risks to spacecraft and damage low-orbiting satellites.

The researchers further showed that this type of radiation-belt electron loss to the atmosphere can increase significantly during geomagnetic storms, disturbances caused by enhanced solar activity that can affect near-Earth space and Earth's magnetic environment.

"Although space is commonly thought to be separate from our upper atmosphere, the two are inextricably linked," Angelopoulos said. "Understanding how they're linked can benefit satellites and astronauts passing through the region, which are increasingly important for commerce, telecommunications and space tourism."

Read more at Science Daily

Physicists report definitive evidence how auroras are created

The aurora borealis, or northern lights, that fill the sky in high-latitude regions have fascinated people for thousands of years. But how they're created, while theorized, had not been conclusively proven.

In a new study, a team of physicists led by University of Iowa reports definitive evidence that the most brilliant auroras are produced by powerful electromagnetic waves during geomagnetic storms. The phenomena, known as Alfven waves, accelerate electrons toward Earth, causing the particles to produce the familiar atmospheric light show.

The study, published online June 7 in the journal Nature Communications, concludes a decades-long quest to demonstrate experimentally the physical mechanisms for the acceleration of electrons by Alfven waves under conditions corresponding to Earth's auroral magnetosphere.

"Measurements revealed this small population of electrons undergoes 'resonant acceleration' by the Alfven wave's electric field, similar to a surfer catching a wave and being continually accelerated as the surfer moves along with the wave," says Greg Howes, associate professor in the Department of Physics and Astronomy at Iowa and study co-author.

Scientists have known that energized particles that emanate from the sun -- such as electrons racing at approximately 45 million miles per hour -- precipitate along the Earth's magnetic field lines into the upper atmosphere, where they collide with oxygen and nitrogen molecules, kicking them into an excited state. These excited molecules relax by emitting light, producing the colorful hues of the aurora.

The theory was supported by spacecraft missions that frequently found Alfven waves traveling Earthward above auroras, presumably accelerating electrons along the way. Although space-based measurements had supported the theory, limitations inherent to spacecraft and rocket measurements had prevented a definitive test.

The physicists were able to find confirmatory evidence in a series of experiments conducted at the Large Plasma Device (LPD) in UCLA's Basic Plasma Science Facility, a national collaborative research facility supported jointly by the U.S. Department of Energy and National Science Foundation.

"The idea that these waves can energize the electrons that create the aurora goes back more than four decades, but this is the first time we've been able to confirm definitively that it works," says Craig Kletzing, professor in the Department of Physics and Astronomy at Iowa and a study co-author. "These experiments let us make the key measurements that show that the space measurements and theory do, indeed, explain a major way in which the aurora are created."

The phenomenon of electrons "surfing" on the electric field of a wave is a theoretical process known as Landau damping, first proposed by Russian physicist Lev Landau in 1946. Through numerical simulations and mathematical modeling, the researchers demonstrated that the results of their experiment agreed with the predicted signature for Landau damping.

The agreement of experiment, simulation, and modeling provides the first direct evidence that Alfven waves can produce accelerated electrons, causing the aurora, says Troy Carter, professor of physics at UCLA and director of the UCLA Plasma Science and Technology Institute.

Read more at Science Daily

To catch a wave, rocket launches from top of world

Earth's magnetosphere, showing the northern and southern polar cusps (illustration).

On Jan. 4, 2019, at 4:37 a.m. EST the CAPER-2 mission launched from the Andøya Space Center in Andenes, Norway, on a 4-stage Black Brant XII sounding rocket. Reaching an apogee of 480 miles high before splashing down in the Arctic Sea, the rocket flew through active aurora borealis, or northern lights, to study the waves that accelerate electrons into our atmosphere.

CAPER-2, short for Cusp Alfvén and Plasma Electrodynamics Rocket-2, is a sounding rocket mission -- a type of spacecraft that carries scientific instruments on short, targeted trips to space before falling back to Earth. In addition to their relatively low price tags and quick development time, sounding rockets are ideally suited for launching into transient events -- like the sudden formation of the aurora borealis, or northern lights.

For CAPER-2 scientists, flying through an aurora provides a peek into a process as fundamental as it is complex: How do particles get accelerated throughout space? NASA studies this phenomenon in an effort to better understand not only the space environment surrounding Earth -- and thus protect our technology in space from radiation -- but also to help understand the very nature of stars and atmospheres throughout the solar system and beyond.

"Throughout the universe you have charged particles getting accelerated -- in the Sun's atmosphere, in the solar wind, in the atmospheres of other planets, and in astrophysical objects," said Jim LaBelle, space physicist at Dartmouth College in Hanover, New Hampshire, and principal investigator for the CAPER-2 mission. "An aurora presents us with a local laboratory where we can observe these acceleration processes close at hand."

Technically, the CAPER-2 team is interested in what happens just before an aurora starts glowing. Electrons, pouring into our atmosphere from space, collide with atmospheric gases and trigger the aurora's glow. Somehow, they pick up speed along the way.

"By the time they crash into our atmosphere, these electrons are traveling over 10 times faster than they were before," said Doug Rowland, space physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who also studies particle acceleration. "We still don't understand the fundamental physics of how that happens."

The CAPER-2 team focused on a special kind of aurora that forms during the day. Unlike the nighttime aurora, the daytime aurora is triggered by electrons that stream in directly from the Sun -- and we know far less about them.

"There's been a huge amount of research done on the regular nighttime aurora, but the daytime aurora is much less studied," said Craig Kletzing, space physicist at the University of Iowa in Iowa City and coinvestigator for the mission. "There are good indications that there are some similarities and there are also some differences."

The team is focusing on how the electrons that create daytime auroras are jostled around by waves, in ways that may or may not differ from nighttime auroras. Two kinds of waves are of special interest, and have opposite effects. Alfvén waves, named after Swedish Nobel laureate Hannes Alfvén who first predicted their existence in 1942, are thought to accelerate the electrons. These huge waves -- measuring tens to hundreds of miles long from peak to peak -- propagate along Earth's magnetic field lines, whipping electrons to and fro.

On the other side are Langmuir waves, which are generated by the electrons themselves -- a process that steals some of the electrons' energy and slows them down. CAPER-2 will carry a high-resolution wave-particle correlator to measure them, the first sounding rocket mission to do so for the daytime aurora.

"This is very data-intensive," said LaBelle. "It's unique to sounding rockets to be able to look at this mechanism in this level of detail."

For the launch, the CAPER-2 team traveled to northern Norway, one of the few places that can put a rocket within range of the daytime aurora. Every day, northern Norway rotates under an opening in Earth's magnetic field known as the northern polar cusp, where particles from the Sun can funnel into our upper atmosphere.

Meeting the aurora right where they form is the best way to understand physical processes that are far too large to replicate in a lab.

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