Bubbling, frothing and sloshing: Long-hypothesized plasma instabilities finally observed

Whether between galaxies or within doughnut-shaped fusion devices known as tokamaks, the electrically charged fourth state of matter known as plasma regularly encounters powerful magnetic fields, changing shape and sloshing in space. Now, a new measurement technique using protons, subatomic particles that form the nuclei of atoms, has captured details of this sloshing for the first time, potentially providing insight into the formation of enormous plasma jets that stretch between the stars.

Scientists at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) created detailed pictures of a magnetic field bending outward because of the pressure created by expanding plasma. As the plasma pushed on the magnetic field, bubbling and frothing known as magneto-Rayleigh Taylor instabilities arose at the boundaries, creating structures resembling columns and mushrooms.

Then, as the plasma's energy diminished, the magnetic field lines snapped back into their original positions. As a result, the plasma was compressed into a straight structure resembling the jets of plasma that can stream from ultra-dense dead stars known as black holes and extend for distances many times the size of a galaxy. The results suggest that those jets, whose causes remain a mystery, could be formed by the same compressing magnetic fields observed in this research.

"When we did the experiment and analyzed the data, we discovered we had something big," said Sophia Malko, a PPPL staff research physicist and lead scientist on the paper. "Observing magneto-Rayleigh Taylor instabilities arising from the interaction of plasma and magnetic fields had long been thought to occur but had never been directly observed until now. This observation helps confirm that this instability occurs when expanding plasma meets magnetic fields. We didn't know that our diagnostics would have that kind of precision. Our whole team is thrilled!"

"These experiments show that magnetic fields are very important for the formation of plasma jets," said Will Fox, a PPPL research physicist and principal investigator of the research reported in Physical Review Research. "Now that we might have insight into what generates these jets, we could, in theory, study giant astrophysical jets and learn something about black holes."

PPPL has world-renowned expertise in developing and building diagnostics, sensors that measure properties like density and temperature in plasma in a range of conditions. This achievement is one of several in recent years that illustrates how the Lab is advancing measurement innovation in plasma physics.

Using a new technique to produce unprecedented detail

The team improved a measurement technique known as proton radiography by creating a new variation for this experiment that would allow for extremely precise measurements. To create the plasma, the team shone a powerful laser at a small disk of plastic. To produce protons, they shone 20 lasers at a capsule containing fuel made of varieties of hydrogen and helium atoms. As the fuel heated up, fusion reactions occurred and produced a burst of both protons and intense light known as X-rays.

The team also installed a sheet of mesh with tiny holes near the capsule. As the protons flowed through the mesh, the outpouring was separated into small, separate beams that were bent because of the surrounding magnetic fields. By comparing the distorted mesh image to an undistorted image produced by X-rays, the team could understand how the magnetic fields were pushed around by the expanding plasma, leading to whirl-like instabilities at the edges.

"Our experiment was unique because we could directly see the magnetic field changing over time," Fox said. "We could directly observe how the field gets pushed out and responds to the plasma in a type of tug of war."

Diversifying a research portfolio

The findings exemplify how PPPL is expanding its focus to include research focused on high energy density (HED) plasma. Such plasmas, like the one created in this experiment's fuel capsule, are hotter and denser than those used in fusion experiments. "HED plasma is an exciting area of growth for plasma physics," Fox said. "This work is part of PPPL's efforts to advance this field. The results show how the Laboratory can create advanced diagnostics to give us new insights into this type of plasma, which can be used in laser fusion devices, as well as in techniques that use HED plasma to create radiation for microelectronics manufacturing."

"PPPL has an enormous amount of knowledge and experience in magnetized plasmas that can contribute to the field of laser-produced HED plasmas and help make significant contributions," Fox said.

"HED science is complex, fascinating and key to understanding a wide range of phenomena," said Laura Berzak Hopkins, PPPL's associate laboratory director for strategy and partnerships and deputy chief research officer. "It's incredibly challenging to both generate these conditions in a controlled manner and develop advanced diagnostics for precision measurements. These exciting results demonstrate the impact of integrating PPPL's breadth of technical expertise with innovative approaches."

More experiments and better simulations

The researchers plan to work on future experiments that will help improve models of expanding plasma. "Scientists have assumed that in these situations, density and magnetism vary directly, but it turns out that that's not true," Malko said.

"Now that we have measured these instabilities very accurately, we have the information we need to improve our models and potentially simulate and understand astrophysical jets to a higher degree than before," Malko said. "It's interesting that humans can make something in a laboratory that usually exists in space."

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Did a magnetic field collapse trigger the emergence of animals?

The Ediacaran Period, spanning from about 635 to 541 million years ago, was a pivotal time in Earth's history. It marked a transformative era during which complex, multicellular organisms emerged, setting the stage for the explosion of life.

But how did this surge of life unfold and what factors on Earth may have contributed to it?

Researchers from the University of Rochester have uncovered compelling evidence that Earth's magnetic field was in a highly unusual state when the macroscopic animals of the Ediacaran Period diversified and thrived. Their study, published in Nature Communications Earth & Environment, raises the question of whether these fluctuations in Earth's ancient magnetic field led to shifts in oxygen levels that may have been crucial to the proliferation of life forms millions of years ago.

According to John Tarduno, the William Kenan, Jr. Professor in the Department of Earth and Environmental Sciences, one of the most remarkable life forms during the Ediacaran Period was the Ediacaran fauna. They were notable for their resemblance to early animals -- some even reached more than a meter (three feet) in size and were mobile, indicating they probably needed more oxygen compared to earlier life forms.

"Previous ideas for the appearance of the spectacular Ediacaran fauna have included genetic or ecologic driving factors, but the close timing with the ultra-low geomagnetic field motivated us to revisit environmental issues, and, in particular, atmospheric and ocean oxygenation," says Tarduno, who is also the Dean of Research in the School of Arts & Sciences and the School of Engineering and Applied Sciences.

Earth's magnetic mysteries

About 1,800 miles below us, liquid iron churns in Earth's outer core, creating the planet's protective magnetic field. Though invisible, the magnetic field is essential for life on Earth because it shields the planet from solar wind -- streams of radiation from the sun. But Earth's magnetic field wasn't always as strong as it is today.

Researchers have proposed that an unusually low magnetic field might have contributed to the rise of animal life. However, it has been challenging to examine the link because of limited data about the strength of the magnetic field during this time.

Tarduno and his team used innovative strategies and techniques to examine the strength of the magnetic field by studying magnetism locked in ancient feldspar and pyroxene crystals from the rock anorthosite. The crystals contain magnetic particles that preserve magnetization from the time the minerals were formed. By dating the rocks, researchers can construct a timeline of the development of Earth's magnetic field.

Leveraging cutting-edge tools, including a CO2 laser and the lab's superconducting quantum interference device (SQUID) magnetometer, the team analyzed with precision the crystals and the magnetism locked within.

A weak magnetic field

Their data indicates that Earth's magnetic field at times during the Ediacaran Period was the weakest field known to date -- up to 30 times weaker than the magnetic field today -- and that the ultra-low field strength lasted for at least 26 million years.

A weak magnetic field makes it easier for charged particles from the sun to strip away lightweight atoms such as hydrogen from the atmosphere, causing them to escape into space. If hydrogen loss is significant, more oxygen may remain in the atmosphere instead of reacting with hydrogen to form water vapor. These reactions can lead to a buildup of oxygen over time.

The research conducted by Tarduno and his team suggests that during the Ediacaran Period, the ultraweak magnetic field caused a loss of hydrogen over at least tens of millions of years. This loss may have led to increased oxygenation of the atmosphere and surface ocean, enabling more advanced life forms to emerge.

Tarduno and his research team previously discovered that the geomagnetic field recovered in strength during the subsequent Cambrian Period, when most animal groups begin to appear in the fossil record, and the protective magnetic field was reestablished, allowing life to thrive.

"If the extraordinarily weak field had remained after the Ediacaran, Earth might look very different from the water-rich planet it is today: water loss might have gradually dried Earth," Tarduno says.

Core dynamics and evolution

The work suggests that understanding planetary interiors is crucial in contemplating the potential of life beyond Earth.

"It's fascinating to think that processes in Earth's core could be linked ultimately to evolution," Tarduno says. "As we think about the possibility of life elsewhere, we also need to consider how the interiors of planets form and develop."

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Researchers find oldest undisputed evidence of Earth's magnetic field

A new study, led by the University of Oxford and MIT, has recovered a 3.7-billion-year-old record of Earth's magnetic field, and found that it appears remarkably similar to the field surrounding Earth today. The findings have been published today in the Journal of Geophysical Research.

Without its magnetic field, life on Earth would not be possible since this shields us from harmful cosmic radiation and charged particles emitted by the Sun (the 'solar wind'). But up to now, there has been no reliable date for when the modern magnetic field was first established.

In the new study, the researchers examined an ancient sequence of iron-containing rocks from Isua, Greenland. Iron particles effectively act as tiny magnets that can record both magnetic field strength and direction when the process of crystallization locks them in place. The researchers found that rocks dating from 3.7 billion years ago captured a magnetic field strength of at least 15 microtesla comparable to the modern magnetic field (30 microtesla).

These results provide the oldest estimate of the strength of Earth's magnetic field derived from whole rock samples, which provide a more accurate and reliable assessment than previous studies which used individual crystals.

Lead researcher Professor Claire Nichols (Department of Earth Sciences, University of Oxford) said: 'Extracting reliable records from rocks this old is extremely challenging, and it was really exciting to see primary magnetic signals begin to emerge when we analysed these samples in the lab. This is a really important step forward as we try and determine the role of the ancient magnetic field when life on Earth was first emerging.'

Whilst the magnetic field strength appears to have remained relatively constant, the solar wind is known to have been significantly stronger in the past. This suggests that the protection of Earth's surface from the solar wind has increased over time, which may have allowed life to move onto the continents and leave the protection of the oceans.

Earth's magnetic field is generated by mixing of the molten iron in the fluid outer core, driven by buoyancy forces as the inner core solidifies, which create a dynamo. During Earth's early formation, the solid inner core had not yet formed, leaving open questions about how the early magnetic field was sustained. These new results suggest the mechanism driving Earth's early dynamo was similarly efficient to the solidification process that generates Earth's magnetic field today.

Understanding how Earth's magnetic field strength has varied over time is also key for determining when Earth's inner, solid core began to form. This will help us to understand how rapidly heat is escaping from Earth's deep interior, which is key for understanding processes such as plate tectonics.

A significant challenge in reconstructing Earth's magnetic field so far back in time is that any event which heats the rock can alter preserved signals. Rocks in the Earth's crust often have long and complex geological histories which erase previous magnetic field information. However, the Isua Supracrustal Belt has a unique geology, sitting on top of thick continental crust which protects it from extensive tectonic activity and deformation. This allowed the researchers to build a clear body of evidence supporting the existence of the magnetic field 3.7 billion years ago.

The results may also provide new insights into the role of our magnetic field in shaping the development of Earth's atmosphere as we know it, particularly regarding atmospheric escape of gases. A currently unexplained phenomenon is the loss of the unreactive gas xenon from our atmosphere more than 2.5 billion years ago. Xenon is relatively heavy and therefore unlikely to have simply drifted out of our atmosphere. Recently, scientists have begun to investigate the possibility that charged xenon particles were removed from the atmosphere by the magnetic field.

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Exoplanets true to size

A star's magnetic field must be considered in order to correctly determine the characteristics of their exoplanets from observations by space telescopes such as Kepler, James Webb, or PLATO. This is demonstrated by new model calculations presented today in the journal Nature Astronomy by a research group led by the Max Planck Institute for Solar System Research (MPS) in Germany. The researchers show that the distribution of the star's brightness over its disk depends on the star's level of magnetic activity. This, in turn, affects the signature of an exoplanet in observational data. The new model must be used in order to properly interpret the data from the latest generation of space telescopes pointed at distant worlds outside our Solar System.

700 light years away from Earth in the constellation Virgo, the planet WASP-39b orbits the star WASP-39. The gas giant, which takes little more than four days to complete one orbit, is one of the best-studied exoplanets: Shortly after its commissioning in July 2022, NASA's James Webb Space Telescope turned its high-precision gaze on the distant planet. The data revealed evidence of large quantities of water vapor, of methane and even, for the first time, of carbon dioxide in the atmosphere of WASP-39b. A minor sensation! But there is still one fly in the ointment: researchers have not yet succeeded in reproducing all the crucial details of the observations in model calculations. This stands in the way of an even more precise analyses of the data. In the new study led by the MPS, the authors, including researchers from the Massachusetts Institute of Technology (USA), the Space Telescope Science Institute (USA), Keele University (United Kingdom), and the University of Heidelberg (Germany), show a way to overcome this obstacle.

"The problems arising when interpreting the data from WASP-39b are well known from many other exoplanets -- regardless whether they are observed with Kepler, TESS, James Webb, or the future PLATO spacecraft," explains MPS scientist Dr. Nadiia Kostogryz, first author of the new study. "As with other stars orbited by exoplanets, the observed light curve of WASP-39 is flatter than previous models can explain," she adds.

Researchers define a light curve as a measurement of the brightness of a star over a longer period of time. The brightness of a star fluctuates constantly, for example because its luminosity is subject to natural fluctuations. Exoplanets can also leave traces in the light curve. If an exoplanet passes in front of its star as seen by an observer, it dims the starlight. This is reflected in the light curve as a regularly recurring drop in brightness. Precise evaluations of such curves provide information about the size and orbital period of the planet. Researchers can also obtain information about the composition of the planet's atmosphere, if the light from the star is split into its different wavelengths or colours.

A close look at a star's brightness distribution

The limb of a star, the edge of the stellar disk, plays a decisive role in the interpretation of its light curve. Just as in the case of the Sun, the limb appears darker to the observer than the inner area. However, the star does not actually shine less brightly further out. "As the star is a sphere and its surface curved, we look into higher and therefore cooler layers at the limb than in the center," explains coauthor and MPS-Director Prof. Dr. Laurent Gizon. "This area therefore appears darker to us," he adds.

It is known that the limb darkening affects the exact shape of the exoplanet signal in the light curve: The dimming determines how steeply the brightness of a star falls during a planetary transit and then rises again. However, it has not been possible to reproduce observational data accurately using conventional models of the stellar atmosphere. The decrease of brightness was always less abrupt than the model calculations suggested. "It was clear that we were missing a crucial piece of the puzzle to precisely understand the exoplanets' signal," says MPS-Director Prof. Dr. Sami Solanki, coauthor of the current study.

Magnetic field is the missing piece of the puzzle

As the calculations published today show, the missing piece of the puzzle is the stellar magnetic field. Like the Sun, many stars generate a magnetic field deep in their interior through enormous flows of hot plasma. For the first time, the researchers were now able to include the magnetic field in their models of limb darkening. They could show that the strength of the magnetic field has an important effect: The limb darkening is pronounced in stars with a weak magnetic field, while it is weaker in those with a strong magnetic field.

The researchers were also able to prove that the discrepancy between observational data and model calculations disappears if the star's magnetic field is included in the computations. To this end, the team turned to selected data from NASA's Kepler Space Telescope, which captured the light of thousands and thousands of stars from 2009 to 2018. In a first step, the scientists modeled the atmosphere of typical Kepler stars in the presence of a magnetic field. In a second step, they then generated "artificial" observational data from these calculations. As a comparison with the real data showed, by including the magnetic field, the Kepler data is successfully reproduced.

The team also extended its considerations to data from the James Webb Space Telescope. The telescope is able to split the light of distant stars into its various wavelengths and thus search for the characteristic signs of certain molecules in the atmosphere of the discovered planets. As it turns out, the magnetic field of the parent star influences the stellar limb darkening differently at different wavelengths -- and should therefore be taken into account in future evaluations in order to achieve even more precise results.

From telescopes to models

"In the past decades and years, the way to move forward in exoplanet research was to improve the hardware, the space telescopes designed to search for and characterize new worlds. The James Webb Space Telescope has pushed this development to new limits," says Dr. Alexander Shapiro, coauthor of the current study and head of an ERC-funded research group at the MPS. "The next step is now to improve and refine the models to interpret this excellent data," he adds.

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Twinkle twinkle baby star, 'sneezes' tell us how you are

Kyushu University researchers have shed new light into a critical question on how baby stars develop. Using the ALMA radio telescope in Chile, the team found that in its infancy, the protostellar disk that surrounds a baby star discharges plumes of dust, gas, and electromagnetic energy. These 'sneezes,' as the researchers describe them, release the magnetic flux within the protostellar disk, and may be a vital part of star formation. Their findings were published in The Astrophysical Journal.

Stars, including our Sun, all develop from what are called stellar nurseries, large concentrations of gas and dust that eventually condense to form a stellar core, a baby star.

During this process, gas and dust form a ring around the baby star called the protostellar disk.

"These structures are perpetually penetrated by magnetic fields, which brings with it magnetic flux. However, if all this magnetic flux were retained as the star developed, it would generate magnetic fields many orders of magnitude stronger than those observed in any known protostar," explains Kazuki Tokuda of Kyushu University's Faculty of Sciences and first author of the study.

For this reason, researchers have hypothesized that there is a mechanism during star development that would remove that magnetic flux.

The prevailing view was that the magnetic field gradually weakened over time as the cloud is pulled into the stellar core.

To get to the bottom of this mysterious phenomenon, the team set their sights on MC 27, a stellar nursery located approximately 450 light-years from earth.

Observations were collected using the ALMA array, a collection of 66 high-precision radio telescope constructed 5,000 meters above seas level in northern Chile.

"As we analyzed our data, we found something quite unexpected. There were these 'spike-like' structures extending a few astronomical units from the protostellar disk. As we dug in deeper, we found that these were spikes of expelled magnetic flux, dust, and gas," continues Tokuda.

"This is a phenomenon called 'interchange instability' where instabilities in the magnetic field react with the different densities of the gases in the protostellar disk, resulting in an outward expelling of magnetic flux. We dubbed this a baby star's 'sneeze' as it reminded us of when we expel dust and air at high speeds."

Additionally, other spikes were observed several thousands of astronomical units away from the protostellar disk.

The team hypothesized that these were indications of other 'sneezes' in the past.

The team expects their findings will improve our understanding of the intricate processes that shape the universe that continue to captivate the interest of both the astronomical community and the public.

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Telescope detects unprecedented behavior from nearby magnetar

Researchers using Murriyang, CSIRO's Parkes radio telescope, have detected unusual radio pulses from a previously dormant star with a powerful magnetic field.

​New results published today in Nature Astronomy describe radio signals from magnetar XTE J1810-197 behaving in complex ways.

​Magnetars are a type of neutron star and the strongest magnets in the Universe.

At roughly 8,000 light years away, this magnetar is also the closest known to Earth.

​Most are known to emit polarised light, though the light this magnetar is emitting is circularly polarised, where the light appears to spiral as it moves through space.

​Dr Marcus Lower, a postdoctoral fellow at Australia's national science agency -- CSIRO, led the latest research and said the results are unexpected and totally unprecedented.

​"Unlike the radio signals we've seen from other magnetars, this one is emitting enormous amounts of rapidly changing circular polarisation. We had never seen anything like this before," Dr Lower said.

​Dr Manisha Caleb from the University of Sydney and co-author on the study said studying magnetars offers insights into the physics of intense magnetic fields and the environments these create.

​"The signals emitted from this magnetar imply that interactions at the surface of the star are more complex than previous theoretical explanations."

​Detecting radio pulses from magnetars is already extremely rare: XTE J1810-197 is one of only a handful known to produce them.

​While it's not certain why this magnetar is behaving so differently, the team has an idea.

​"Our results suggest there is a superheated plasma above the magnetar's magnetic pole, which is acting like a polarising filter," Dr Lower said.

​"How exactly the plasma is doing this is still to be determined."

​XTE J1810-197 was first observed to emit radio signals in 2003.

Then it went silent for well over a decade. The signals were again detected by the University of Manchester's 76-m Lovell telescope at the Jodrell Bank Observatory in 2018 and quickly followed up by Murriyang, which has been crucial to observing the magnetar's radio emissions ever since.

​The 64-m diameter telescope on Wiradjuri Country is equipped with a cutting edge ultra-wide bandwidth receiver.

The receiver was designed by CSIRO engineers who are world leaders in developing technologies for radio astronomy applications.

​The receiver allows for more precise measurements of celestial objects, especially magnetars, as it is highly sensitive to changes in brightness and polarisation across a broad range of radio frequencies.

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Magnetic fields in the cosmos: Dark matter could help us discover their origin

The mini-halos of dark matter scattered throughout the Cosmos could function as highly sensitive probes of primordial magnetic fields. This is what emerges from a theoretical study conducted by SISSA and published in Physical Review Letters. Present on immense scales, magnetic fields are found everywhere in the Universe. However, their origin are still subjects of debate among scholars. An intriguing possibility is that magnetic fields originated near the birth of the universe itself, that is they are primordial magnetic fields.

In the study, researchers showed that if magnetic fields are indeed primordial then it could cause an increase in dark matter density perturbations on small scales.

The ultimate effect of this process would be the formation of mini-halos of dark matter, which, if detected would hint towards a primordial nature of magnetic fields.

Thus, in an apparent paradox, the invisible part of our Universe could be useful in resolving the nature of a component of the visible one.

Shedding light on the formation of Magnetic Fields

"Magnetic fields are ubiquitous in the Cosmos," explains Pranjal Ralegankar of SISSA, the author of the research.

"A possible theory regarding their formation suggests that those observed so far could be produced in the early stages of our Universe. However, this proposition lacks explanation in the standard model of physics. To shed light on this aspect and find a way to detect "primordial" magnetic fields, with this work we propose a method that we could define as 'indirect.' Our approach is based on a question: What is the influence of magnetic fields on dark matter?" It is known that there is no direct interaction.

Still, as Ralegankar explains, "there is an indirect one that occurs through gravity."

Right from the primordial Universe

Primordial magnetic fields can enhance density perturbations of electrons and protons in the primordial Universe.

When these become too large, they influence the magnetic fields themselves.

The consequence is the suppression of fluctuations on a small scale.

Ralegankar explains: "In the study, we show something unexpected. The growth in baryon density gravitationally induces the growth of dark matter perturbations without the possibility of subsequent cancellation. This would result in their collapse on small scales, producing mini-halos of dark matter." The consequence, continues the author, is that although fluctuations in the density of baryonic matter are cancelled, they would leave traces through the mini-halos, all solely through gravitational interactions.

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Researchers rely on Earth's magnetic field to verify an event mentioned in the Old Testament

A breakthrough achieved by researchers from four Israeli universities -- Tel Aviv University, The Hebrew University of Jerusalem, Bar-Ilan University and Ariel University- will enable archaeologists to identify burnt materials discovered in excavations and estimate their firing temperatures. Applying their method to findings from ancient Gath (Tell es-Safi in central Israel), the researchers validated the Biblical account: "About this time Hazael King of Aram went up and attacked Gath and captured it. Then he turned to attack Jerusalem" (2 Kings 12, 18). They explain that unlike previous methods, the new technique can determine whether a certain item (such as a mud brick) underwent a firing event even at relatively low temperatures, from 200°C and up. This information can be crucial for correctly interpreting the findings.

The multidisciplinary study was led by Dr. Yoav Vaknin from the Sonia & Marco Nadler Institute of Archaeology, Entin Faculty of Humanities, at Tel Aviv University, and the Palaeomagnetic Laboratory at The Hebrew University. Other contributors included: Prof. Ron Shaar from the Institute of Earth Sciences at The Hebrew University, Prof. Erez Ben-Yosef and Prof. Oded Lipschits from the Sonia & Marco Nadler Institute of Archaeology at Tel Aviv University, Prof. Aren Maeir from the Martin (Szusz) Department of Land of Israel Studies and Archaeology at Bar-Ilan University and Dr. Adi Eliyahu Beharfrom the Department of Land of Israel Studies and Archaeology and the Department of Chemical Sciences at Ariel University. The paper has been published in the scientific journal PLOS ONE.

Prof. Lipschits: "Throughout the Bronze and Iron Ages the main building material in most parts of the Land of Israel was mud bricks. This cheap and readily available material was used to build walls in most buildings, sometimes on top of stone foundations. That's why it's so important to understand the technology used in making these bricks."

Dr. Vaknin adds: "During the same era dwellers of other lands, such as Mesopotamia where stone was hard to come by, would fire mud bricks in kilns to increase their strength and durability. This technique is mentioned in the story of the Tower of Babel in the Book of Genesis: "They said one to another, Come, let us make bricks and fire them thoroughly. So they used brick for stone"(Genesis 11, 3). Most researchers, however, believe that this technology did not reach the Land of Israel until much later, with the Roman conquest. Until that time the inhabitants used sun-dried mud bricks. Thus, when bricks are found in an archaeological excavation, several questions must be asked: First, have the bricks been fired, and if so, were they fired in a kiln prior to construction or in situ, in a destructive conflagration event? Our method can provide conclusive answers."

The new method relies on measuring the magnetic field recorded and 'locked' in the brick as it burned and cooled down. Dr. Vaknin: "The clay from which the bricks were made contains millions of ferromagnetic particles -- minerals with magnetic properties that behave like so many tiny 'compasses' or magnets. In a sun-dried mud brick the orientation of these magnets is almost random, so that they cancel out one another. Therefore, the overall magnetic signal of the brick is weak and not uniform. Heating to 200°C or more, as happens in a fire, releases the magnetic signals of these magnetic particles and, statistically, they tend to align with the earth's magnetic field at that specific time and place. When the brick cools down, these magnetic signals remain locked in their new position and the brick attains a strong and uniformly oriented magnetic field, which can be measured with a magnetometer. This is a clear indication that the brick has, in fact, been fired.

In the second stage of the procedure, the researchers gradually 'erase' the brick's magnetic field, using a process called thermal demagnetization. This involves heating the brick in a special oven in a palaeomagnetic laboratory that neutralizes the earth's magnetic field. The heat releases the magnetic signals, which once again arrange themselves randomly, canceling each other out, and the total magnetic signal becomes weak and loses its orientation.

Dr. Vaknin: "We conduct the process gradually. At first, we heat the sample to a temperature of 100°C, which releases the signals of only a small percentage of the magnetic minerals. We then cool it down and measure the remaining magnetic signal. We then repeat the procedure at temperatures of 150°C, 200°C, and so on, proceeding in small steps, up to 700°C. In this way the brick's magnetic field is gradually erased. The temperature at which the signal of each mineral is 'unlocked' is approximately the same as the temperature at which it was initially 'locked', and ultimately, the temperature at which the magnetic field is fully erased was reached during the original fire."

The researchers tested the technique in the laboratory: they fired mud bricks under controlled conditions of temperature and magnetic field, measured each brick's acquired magnetic field, then gradually erased it. They found that the bricks were completely demagnetized at the temperature at which they had been burned -- proving that the method works.

Dr. Vaknin: "Our approach enables identifying burning which occurred at much lower temperatures than any other method. Most techniques used for identifying burnt bricks are based on actual changes in the minerals, which usually occur at temperatures higher than 500°C -- when some minerals are converted into others."

Dr. Eliyahu Behar: "One of the common methods for identifying mineralogical changes in clay (the main component of mud bricks) due to exposure to high temperatures is based on changes in the absorption of infrared radiation by the various minerals. In this study we used this method as an additional tool to verify the results of the magnetic method." Dr. Vaknin: "Our method is much more sensitive than others because it targets changes in the intensity and orientation of the magnetic signal, which occur at much lower temperatures. We can begin to detect changes in the magnetic signal at temperatures as low as 100°C, and from 200°C and up the findings are conclusive."

In addition, the method can determine the orientation in which the bricks cooled down. Dr. Vaknin: "When a brick is fired in a kiln before construction, it records the direction of the earth's magnetic field at that specific time and place. In Israel this means north and downward. But when builders take bricks from a kiln and build a wall, they lay them in random orientations, thus randomizing the recorded signals. On the other hand, when a wall is burned in-situ, as might happen when it is destroyed by an enemy, the magnetic fields of all bricks are locked in the same orientation."

After proving the method's validity, the researchers applied it to a specific archaeological dispute: was a specific brick structure discovered at Tell es-Safi -- identified as the Philistine city of Gath, home of Goliath -- built of pre-fired bricks or burned on location? The prevalent hypothesis, based on the Old Testament, historical sources, and Carbon-14 dating attributes the destruction of the structure to the devastation of Gath by Hazael, King of Aram Damascus, around 830 BCE. However, a previous paper by researchers including Prof. Maeir, head of the Tell es-Safi excavations, proposed that the building had not burned down, but rather collapsed over decades, and that the fired bricks found in the structure had been fired in a kiln prior to construction. If this hypothesis were correct, this would be the earliest instance of brick-firing technology discovered in the Land of Israel.

To settle the dispute, the current research team applied the new method to samples from the wall at Tell es-Safi and the collapsed debris found beside it. The findings were conclusive: the magnetic fields of all bricks and collapsed debris displayed the same orientation -- north and downwards. Dr. Vaknin: "Our findings signify that the bricks burned and cooled down in-situ, right where they were found, namely in a conflagration in the structure itself, which collapsed within a few hours. Had the bricks been fired in a kiln and then laid in the wall, their magnetic orientations would have been random. Moreover, had the structure collapsed over time, not in a single fire event, the collapsed debris would have displayed random magnetic orientations. We believe that the main reason for our colleagues' mistaken interpretation was their inability to identify burning at temperatures below 500°C. Since heat rises, materials at the bottom of the building burned at relatively low temperatures, below 400°C, and consequently the former study did not identify them as burnt -- leading to the conclusion that the building had not been destroyed by fire. At the same time, bricks in upper parts of the wall, where temperatures were much higher, underwent mineralogical changes and were therefore identified as burnt -- leading the researchers to conclude that they had been fired in a kiln prior to construction. Our method allowed us to determine that all bricks in both the wall and debris had burned during the conflagration: those at the bottom burned at relatively low temperatures, and those that were found in higher layers or had fallen from the top -at temperatures higher than 600°C."

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Furthest ever detection of a galaxy's magnetic field

Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have detected the magnetic field of a galaxy so far away that its light has taken more than 11 billion years to reach us: we see it as it was when the Universe was just 2.5 billion years old. The result provides astronomers with vital clues about how the magnetic fields of galaxies like our own Milky Way came to be.

Lots of astronomical bodies in the Universe have magnetic fields, whether it be planets, stars or galaxies. "Many people might not be aware that our entire galaxy and other galaxies are laced with magnetic fields, spanning tens of thousands of light-years," says James Geach, a professor of astrophysics at the University of Hertfordshire, UK, and lead author of the study published today in Nature.

"We actually know very little about how these fields form, despite their being quite fundamental to how galaxies evolve," adds Enrique Lopez Rodriguez, a researcher at Stanford University, USA, who also participated in the study. It is not clear how early in the lifetime of the Universe, and how quickly, magnetic fields in galaxies form because so far astronomers have only mapped magnetic fields in galaxies close to us.

Now, using ALMA, in which the European Southern Observatory (ESO) is a partner, Geach and his team have discovered a fully formed magnetic field in a distant galaxy, similar in structure to what is observed in nearby galaxies. The field is about 1000 times weaker than the Earth's magnetic field, but extends over more than 16,000 light-years.

"This discovery gives us new clues as to how galactic-scale magnetic fields are formed," explains Geach. Observing a fully developed magnetic field this early in the history of the Universe indicates that magnetic fields spanning entire galaxies can form rapidly while young galaxies are still growing.

The team believes that intense star formation in the early Universe could have played a role in accelerating the development of the fields. Moreover, these fields can in turn influence how later generations of stars will form. Co-author and ESO astronomer Rob Ivison says that the discovery opens up "a new window onto the inner workings of galaxies, because the magnetic fields are linked to the material that is forming new stars."

To make this detection, the team searched for light emitted by dust grains in a distant galaxy, 9io9. Galaxies are packed full of dust grains and when a magnetic field is present, the grains tend to align and the light they emit becomes polarised. This means that the light waves oscillate along a preferred direction rather than randomly. When ALMA detected and mapped a polarised signal coming from 9io9, the presence of a magnetic field in a very distant galaxy was confirmed for the first time.

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Neptune's disappearing clouds linked to the solar cycle

Astronomers have uncovered a link between Neptune's shifting cloud abundance and the 11-year solar cycle, in which the waxing and waning of the Sun's entangled magnetic fields drives solar activity.

This discovery is based on three decades of Neptune observations captured by NASA's Hubble Space Telescope and the W. M. Keck Observatory in Hawaii, as well as data from the Lick Observatory in California.

The link between Neptune and solar activity is surprising to planetary scientists because Neptune is our solar system's farthest major planet and receives sunlight with about 0.1% of the intensity Earth receives. Yet Neptune's global cloudy weather seems to be driven by solar activity, and not the planet's four seasons, which each last approximately 40 years.

At present, the cloud coverage seen on Neptune is extremely low, with the exception of some clouds hovering over the giant planet's south pole. A University of California (UC) Berkeley-led team of astronomers discovered that the abundance of clouds normally seen at the icy giant's mid-latitudes started to fade in 2019.

"I was surprised by how quickly clouds disappeared on Neptune," said Imke de Pater, emeritus professor of astronomy at UC Berkeley and senior author of the study. "We essentially saw cloud activity drop within a few months," she said.

"Even now, four years later, the most recent images we took this past June still show the clouds haven't returned to their former levels," said Erandi Chavez, a graduate student at the Center for Astrophysics | Harvard-Smithsonian (CfA) in Cambridge, Massachusetts, who led the study when she was an undergraduate astronomy student at UC Berkeley. "This is extremely exciting and unexpected, especially since Neptune's previous period of low cloud activity was not nearly as dramatic and prolonged."

To monitor the evolution of Neptune's appearance, Chavez and her team analyzed Keck Observatory images taken from 2002 to 2022, the Hubble Space Telescope archival observations beginning in 1994, and data from the Lick Observatory in California from 2018 to 2019.

In recent years, the Keck observations have been complemented by images taken as part of the Twilight Zone program and by Hubble's Outer Planet Atmospheres Legacy (OPAL) program.

The images reveal an intriguing pattern between seasonal changes in Neptune's cloud cover and the solar cycle -- the period when the Sun's magnetic field flips every 11 years as it becomes more tangled like a ball of yarn. This is evident in the increasing number of sunspots and increasing solar flare activity. As the cycle progresses, the Sun's tempestuous behavior builds to a maximum, until the magnetic field beaks down and reverses polarity. Then the Sun settles back down to a minimum, only to start another cycle.

When it's stormy weather on the Sun, more intense ultraviolet (UV) radiation floods the solar system. The team found that two years after the solar cycle's peak, an increasing number of clouds appear on Neptune. The team further found a positive correlation between the number of clouds and the ice giant's brightness from the sunlight reflecting off it.

"These remarkable data give us the strongest evidence yet that Neptune's cloud cover correlates with the Sun's cycle," said de Pater. "Our findings support the theory that the Sun's UV rays, when strong enough, may be triggering a photochemical reaction that produces Neptune's clouds."

Scientists discovered the connection between the solar cycle and Neptune's cloudy weather pattern by looking at 2.5 cycles of cloud activity recorded over the 29-year span of Neptunian observations. During this time, the planet's reflectivity increased in 2002 then dimmed in 2007. Neptune became bright again in 2015, then darkened in 2020 to the lowest level ever observed, which is when most of the clouds went away.

The changes in Neptune's brightness caused by the Sun appear to go up and down relatively in sync with the coming and going of clouds on the planet. However there is a two-year time lag between the peak of the solar cycle and the abundance of clouds seen on Neptune. The chemical changes are caused by photochemistry, which happens high in Neptune's upper atmosphere and takes time to form clouds.

"It's fascinating to be able to use telescopes on Earth to study the climate of a world more than 2.5 billion miles away from us," said Carlos Alvarez, staff astronomer at Keck Observatory and co-author of the study. "Advances in technology and observations have enabled us to constrain Neptune's atmospheric models, which are key to understanding the correlation between the ice giant's climate and the solar cycle."

However, more work is necessary. For example, while an increase in UV sunlight could produce more clouds and haze, it could also darken them, thereby reducing Neptune's overall brightness. Storms on Neptune rising up from the deep atmosphere affect the cloud cover, but are not related to photochemically produced clouds, and hence may complicate correlation studies with the solar cycle. Continued observations of Neptune are also needed to see how long the current near-absence of clouds will last.

The research team continues to track Neptune's cloud activity. "We have seen more clouds in the most recent Keck images that were taken during the same time NASA's James Webb Space Telescope observed the planet; these clouds were in particular seen at northern latitudes and at high altitudes, as expected from the observed increase in the solar UV flux over the past approximately 2 years," said de Pater.

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New clues on the source of the universe's magnetic fields

It isn't just your refrigerator that has magnets on it. The earth, the stars, galaxies, and the space between galaxies are all magnetized, too. The more places scientists have looked for magnetic fields across the universe, the more they've found them. But the question of why that is the case and where those magnetic fields originate from has remained a mystery and a subject of ongoing scientific inquiry.

A new paper by Columbia researchers offers insight into the source of these fields. The team used models to show that magnetic fields may spontaneously arise in turbulent plasma. Plasma is a kind of matter often found in ultra-hot environments like that near the surface of the sun, but plasma is also scattered across the universe in low-density environments, like the expansive space between galaxies; the team's research focused on those low-density environments. Their simulations showed that, in addition to generating new magnetic fields, the turbulence of those plasmas can also amplify magnetic fields once they've been generated, which helps explain how magnetic fields that originate on small scales can sometimes eventually reach to stretch across vast distances.

The paper was written by astronomy professor Lorenzo Sironi, astronomy research scientist Luca Comisso, and astronomy doctoral candidate Ryan Golant.

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Astronomers discover striking evidence of 'unusual' stellar evolution

Astronomers have found evidence that some stars boast unexpectedly strong surface magnetic fields, a discovery that challenges current models of how they evolve.

In stars like our sun, surface magnetism is linked to stellar spin, a process similar to the inner workings of a hand-cranked flashlight. Strong magnetic fields are seen in the hearts of magnetic sunspot regions, and cause a variety of space weather phenomena. Until now, low-mass stars -- celestial bodies of lower mass than our sun that can rotate either very rapidly or relatively slowly -- were thought to exhibit very low levels of magnetic activity, an assumption which has primed them as ideal host stars for potentially habitable planets.

In a new study, published today in The Astrophysical Journal Letters, researchers from The Ohio State University argue that a new internal mechanism called core-envelope decoupling -- when the surface and core of the star start out spinning at the same rate, then drift apart -- might be responsible for enhancing magnetic fields on cool stars, a process which could intensify their radiation for billions of years and impact the habitability of their nearby exoplanets.

The research was made possible due to a technique that Lyra Cao, lead author of the study and a graduate student in astronomy at Ohio State, and co-author Marc Pinsonneault, a professor of astronomy at Ohio State, developed earlier this year to make and characterize starspot and magnetic field measurements.

Although low-mass stars are the most common stars in the Milky Way and are often hosts to exoplanets, scientists know comparatively little about them, said Cao.

For decades, it was assumed that the physical processes of lower mass stars followed those of solar-type stars. Because stars gradually lose their angular momentum as they spin down, astronomers can use stellar spins as a device to understand the nature of a star's physical processes, and how they interact with their companions and their surroundings. However, there are times where the stellar rotation clock appears to stop in place, Cao said.

Using public data from the Sloan Digital Sky Survey to study a sample of 136 stars in M44, a star crib also known as Praesepe, or the Beehive cluster, the team found that the magnetic fields of the low-mass stars in the region appeared much stronger than current models could explain.

While previous research revealed that the Beehive cluster is home to many stars that defy current theories of rotational evolution, one of Cao's team's most exciting discoveries was determining that these stars' magnetic fields may be just as unusual -- far stronger than predicted by current models.

"To see a link between the magnetic enhancement and rotational anomalies was incredibly exciting," said Cao. "It indicates that there might be some interesting physics at play here." The team also hypothesized that the process of syncing up a star's core and the envelope might induce a magnetism found in these stars that would have a starkly different origin from the kind seen on the sun.

"We're finding evidence that there's a different kind of dynamo mechanism driving the magnetism of these stars," said Cao. "This work shows that stellar physics can have surprising implications for other fields."

According to the study, these findings have important implications for our understanding of astrophysics, particularly on the hunt for life on other planets. "Stars experiencing this enhanced magnetism are likely going to be battering their planets with high-energy radiation," Cao said. "This effect is predicted to last for billions of years on some stars, so it's important to understand what it might do to our ideas of habitability."

But these findings shouldn't put a damper on the search for extraplanetary existence. With further research, the team's discovery could help provide more insight into where to look for planetary systems capable of hosting life. But here on Earth, Cao believes her team's discoveries might lead to better simulations and theoretical models of stellar evolution.

"The next thing to do is verify that enhanced magnetism happens on a much larger scale," said Cao. "If we can understand what's going on in the interiors of these stars as they experience shear-enhanced magnetism, it's going to lead the science in a new direction."

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Astronomers observe the first radiation belt seen outside of our solar system

Astronomers have described the first radiation belt observed outside our solar system, using a coordinated array of 39 radio dishes from Hawaii to Germany to obtain high-resolution images. The images of persistent, intense radio emissions from an ultracool dwarf reveal the presence of a cloud of high-energy electrons trapped in the object's powerful magnetic field, forming a double-lobed structure analogous to radio images of Jupiter's radiation belts.

"We are actually imaging the magnetosphere of our target by observing the radio-emitting plasma -- its radiation belt -- in the magnetosphere. That has never been done before for something the size of a gas giant planet outside of our solar system," said Melodie Kao, a postdoctoral fellow at UC Santa Cruz and first author of a paper on the new findings published May 15in Nature.

Strong magnetic fields form a "magnetic bubble" around a planet called a magnetosphere, which can trap and accelerate particles to near the speed of light. All the planets in our solar system that have such magnetic fields, including Earth, as well as Jupiter and the other giant planets, have radiation belts consisting of these high-energy charged particles trapped by the planet's magnetic field.

Earth's radiation belts, known as the Van Allen belts, are large donut-shaped zones of high-energy particles captured from solar winds by the magnetic field. Most of the particles in Jupiter's belts are from volcanoes on its moon Io. If you could put them side by side, the radiation belt that Kao and her team have imaged would be 10 million times brighter than Jupiter's.

Particles deflected by the magnetic field toward the poles generate auroras ("northern lights") when they interact with the atmosphere, and Kao's team also obtained the first image capable of differentiating between the location of an object's aurora and its radiation belts outside our solar system.

The ultracool dwarf imaged in this study straddles the boundary between low-mass stars and massive brown dwarfs. "While the formation of stars and planets can be different, the physics inside of them can be very similar in that mushy part of the mass continuum connecting low-mass stars to brown dwarfs and gas giant planets," Kao explained.

Characterizing the strength and shape of the magnetic fields of this class of objects is largely uncharted terrain, she said. Using their theoretical understanding of these systems and numerical models, planetary scientists can predict the strength and shape of a planet's magnetic field, but they haven't had a good way to easily test those predictions.

"Auroras can be used to measure the strength of the magnetic field, but not the shape. We designed this experiment to showcase a method for assessing the shapes of magnetic fields on brown dwarfs and eventually exoplanets," Kao said.

The strength and shape of the magnetic field can be an important factor in determining a planet's habitability. "When we're thinking about the habitability of exoplanets, the role of their magnetic fields in maintaining a stable environment is something to consider in addition to things like the atmosphere and climate," Kao said.

To generate a magnetic field, a planet's interior must be hot enough to have electrically conducting fluids, which in the case of Earth is the molten iron in its core. In Jupiter, the conducting fluid is hydrogen under so much pressure it becomes metallic. Metallic hydrogen probably also generates magnetic fields in brown dwarfs, Kao said, while in the interiors of stars the conducting fluid is ionized hydrogen.

The ultracool dwarf known as LSR J1835+3259 was the only object Kao felt confident would yield the high-quality data needed to resolve its radiation belts.

"Now that we've established that this particular kind of steady-state, low-level radio emission traces radiation belts in the large-scale magnetic fields of these objects, when we see that kind of emission from brown dwarfs -- and eventually from gas giant exoplanets -- we can more confidently say they probably have a big magnetic field, even if our telescope isn't big enough to see the shape of it," Kao said, adding that she is looking forward to when the Next Generation Very Large Array, currently being planned by the National Radio Astronomy Observatory (NRAO), can image many more extrasolar radiation belts.

"This is a critical first step in finding many more such objects and honing our skills to search for smaller and smaller magnetospheres, eventually enabling us to study those of potentially habitable, Earth-size planets," said coauthor Evgenya Shkolnik at Arizona State University, who has been studying the magnetic fields and habitability of planets for many years.

The team used the High Sensitivity Array, consisting of 39 radio dishes coordinated by the NRAO in the United States and the Effelsberg radio telescope operated by the Max Planck Institute for Radio Astronomy in Germany.

"By combining radio dishes from across the world, we can make incredibly high-resolution images to see things no one has ever seen before. Our image is comparable to reading the top row of an eye chart in California while standing in Washington, D.C.," said coauthor Jackie Villadsen at Bucknell University.

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Neutron star's X-rays reveal 'photon metamorphosis'

A "beautiful effect" predicted by quantum electrodynamics (QED) can explain the puzzling first observations of polarized X-rays emitted by a magnetar -- a neutron star featuring a powerful magnetic field, according to a Cornell astrophysicist.

The extremely dense and hot remnant of a massive star, boasting a magnetic field 100 trillion times stronger than Earth's, was expected to generate highly polarized X-rays, meaning that the radiation's electromagnetic field did not vibrate randomly but had a preferred direction.

But scientists were surprised when NASA's Imaging X-ray Polarimetry Explorer (IXPE) satellite last year detected that lower- and higher-energy X-rays were polarized differently, with electromagnetic fields oriented at right angles to each other.

The phenomenon can be naturally explained as a result of "photon metamorphosis" -- a transformation of X-ray photons that has been theorized but never directly observed, said Dong Lai, Ph.D. '94, the Benson Jay Simon '59, MBA '62, and Mary Ellen Simon, M.A. '63, Professor of Astrophysics in the College of Arts and Sciences.

"In this observation of radiation from a faraway celestial object, we see a beautiful effect that is a manifestation of intricate, fundamental physics," Lai said. "QED is one of the most successful physics theories, but it had not been tested in such strong magnetic field conditions."

Lai is the author of "IXPE Detection of Polarized X-rays from Magnetars and Photon Mode Conversion at QED Vacuum Resonance," published April 18 in Proceedings of the National Academy of Sciences.

The research builds on calculations Lai and Wynn Ho, Ph.D. '03, published 20 years ago, incorporating observations NASA reported last November of the magnetar 4U 0142+61, located 13,000 light-years away in the Cassiopeia constellation.

Quantum electrodynamics, which describes microscopic interactions between electrons and photons, predicts that as X-ray photons exit the neutron star's thin atmosphere of hot, magnetized gas, or plasma, they pass through a phase called vacuum resonance.

There, Lai said, photons, which have no charge, can temporarily convert into pairs of "virtual" electrons and positrons that are influenced by the magnetar's super-strong magnetic field even in vacuum, a process called "vacuum birefringence." Combined with a related process, plasma birefringence, conditions are created for the polarity of high-energy X-rays to swing 90 degrees relative to low-energy X-rays, according to Lai's analysis.

"You can think about the polarization as two flavors of photons," he said. "A photon suddenly converting from one flavor to another -- you don't usually see this kind of thing. But it's a natural consequence of the physics if you apply the theory under these extreme conditions."

The IXPE mission did not see the polarization swing in observations of another magnetar, called 1RXS J170849.0-400910, with an even stronger magnetic field. Lai said that's consistent with his calculations, which suggest vacuum resonance and photon metamorphosis would occur very deep inside such a neutron star.

Lai said his interpretation of IXPE's observations of the magnetar 4U 0142+61 helped constrain its magnetic field and rotation, and suggested that its atmosphere was likely composed of partially ionized heavy elements.

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Tracking how magnetism affects animal behavior

For over 50 years, scientists have observed that the behaviour of a wide variety of animals can be influenced by the Earth's magnetic field. However, despite decades of research, the exact nature of this 'magnetic sense' remains elusive. Will Schneider and Richard Holland from Bangor University in Wales and their co-worker Oliver Lindecke from the Institute for Biology, Oldenburg, Germany have now written a comprehensive overview of this cross-disciplinary field, with an emphasis on the methodology involved. This work is now published in the journal EPJ Special Topics.

This magnetic sense, or 'magnetoreception', was first noticed in birds, and particularly in migratory songbirds. It has now been observed in many other species including mammals, fish and insects. However, the exact relationship between the magnetic field and the behaviour is difficult to pin down because it can be masked by other environmental factors. Experiments must be very carefully designed if their results are to be statistically sound.

"We aim to provide a balanced overview for researchers who wish to enter this exciting area of sensory biology," explains Schneider. He and his co-authors outlined a range of methods that are used to deduce whether an animal's behaviour is affected by a magnetic field. These include using GPS to mark animals' alignment with the Earth's field during normal activities, such as cows grazing; observing behaviour after tissues thought to be responsible for magnetoreception have been removed, or genes knocked out; and attaching small magnets on or near the animals' bodies to disrupt the mechanism. Further work by animal physiologists, neuroscientists, geneticists and others will also be necessary to truly understand this phenomenon.

And this research is not only of academic interest. "Understanding animal magnetoreception will help us to protect animals released into unknown environments in the wild," adds Lindecke.

From Science Daily

The mechanism of cosmic magnetic fields explored in the laboratory

Recent research shows that magnetic fields can spontaneously emerge in a plasma if the plasma has a temperature anisotropy. This mechanism is known as the Weibel instability. This new research is the first to unambiguously observe the Weibel instability in the laboratory. It offers a possible solution to the problem of the origin of the microgauss-level magnetic fields that permeate the galaxies.

Plasma is matter that is so hot that the electrons are separated from atoms. The electrons float freely and the atoms become ions. This creates an ionized gas -- plasma -- that makes up nearly all of the visible universe. Recent research shows that magnetic fields can spontaneously emerge in a plasma. This can happen if the plasma has a temperature anisotropy -- temperature that is different along different spatial directions. This mechanism is known as the Weibel instability. It was predicted by plasma theorist Eric Weibel more than six decades ago but only now has been unambiguously observed in the laboratory. The new research finds that this process can convert a significant fraction of the energy stored in the temperature anisotropy into magnetic field energy. It also finds that the Weibel instability could be a source of magnetic fields that permeate throughout the cosmos.

The Impact

The matter in our observable universe is plasma state and it is magnetized. Magnetic fields at the micro-gauss level (about a millionth of the Earth's magnetic fields) permeate the galaxies. These magnetic fields are thought to be amplified from weak seed fields by the spiral motion of the galaxies, known as the galactic dynamo. How the seed magnetic fields are created is a longstanding question in astrophysics. This new work offers a possible solution to this vexing problem of the origin of the microgauss level seed magnetic fields. The research used a novel platform that has great potential for studying the ultrafast dynamics of magnetic fields in the laboratory plasmas that are relevant to astro- and high-energy density physics.

Summary

First theorized six decades ago, the Weibel instability driven by temperature anisotropy is thought to be an important mechanism for self-magnetization of many laboratory and astrophysical plasmas. However, scientists have faced two challenges in unambiguously demonstrating the Weibel instability. First, until recently, researchers were not able to generate a plasma with a known temperature anisotropy as initially envisioned by Weibel. Second, researchers had no suitable technique to measure the complex and rapidly evolving topology of the magnetic fields subsequently generated in the plasma.

This work, enabled by the unique capability of the Accelerator Test Facility, a Department of Energy (DOE) user facility at Brookhaven National Laboratory, employed a novel experimental platform that allowed the researchers to create a hydrogen plasma with a known highly anisotropic electron velocity distributions on a tens of trillionth of a second timescale by using an ultrashort but intense carbon dioxide laser pulse. The subsequent thermalization of the plasma occurs via self-organization of plasma currents that produces magnetic fields driven by Weibel instability. These fields are large enough to deflect relativistic electrons to reveal an image of the magnetic fields a certain distance from the plasma. The researchers obtained a movie of the evolution of these magnetic fields with exquisite spatiotemporal resolution by using an one picosecond relativistic electron beam to probe these fields.

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How magnetism could help explain Earth's formation

There are several theories about how the Earth and the Moon were formed, most involving a giant impact. They vary from a model where the impacting object strikes the newly formed Earth a glancing blow and then escapes, through to one where the collision is so energetic that both the impactor and the Earth are vaporized.

Now scientists at the University of Leeds and the University of Chicago have analysed the dynamics of fluids and electrically conducting fluids and concluded that the Earth must have been magnetized either before the impact or as a result of it.

They claim this could help to narrow down the theories of the Earth-Moon formation and inform future research into what really happened.

Professor David Hughes, an applied mathematician in the School of Mathematics at the University of Leeds, said: "Our new idea is to point out that our theoretical understanding of the Earth's magnetic field today can actually tell us something about the very formation of the Earth-Moon system.

"At first glance, this seems somewhat surprising, and previous theories had not recognized this potentially important connection."

This new assessment is based on the resilience of Earth's magnetic field, which is maintained by a rotating and electrically conducting fluid in the outer core, known as a geodynamo.

Professor Fausto Cattaneo, an astrophysicist at the University of Chicago, said: "A peculiar property of the Earth's dynamo is that it can maintain a strong magnetic field but not amplify a weak one.

The scientists therefore concluded that if the Earth's field were to get switched off, or even reduced to a very small level, it would not have the capability to kick in again.

"It is this remarkable feature that allows us to make deductions about the history of the early Earth; including, possibly, how the Moon was formed," added Professor Cattaneo.

Professor Hughes added: "And if that is true, then you have to think, where did the Earth's magnetic field come from in the first place?

"Our hypothesis is that it got to this peculiar state way back at the beginning, either pre-impact or as an immediate result of the impact.

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Unraveling a perplexing explosive process that occurs throughout the universe

Mysterious fast radio bursts release as much energy in one second as the Sun pours out in a year and are among the most puzzling phenomena in the universe. Now researchers at Princeton University, the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) and the SLAC National Accelerator Laboratory have simulated and proposed a cost-effective experiment to produce and observe the early stages of this process in a way once thought to be impossible with existing technology.

Producing the extraordinary bursts in space are celestial bodies such as neutron, or collapsed, stars called magnetars (magnet + star) enclosed in extreme magnetic fields. These fields are so strong that they turn the vacuum in space into an exotic plasma composed of matter and anti-matter in the form of pairs of negatively charged electrons and positively charged positrons, according to quantum electrodynamic (QED) theory. Emissions from these pairs are believed to be responsible for the powerful fast radio bursts.

Pair plasma

The matter-antimatter plasma, called "pair plasma," stands in contrast to the usual plasma that fuels fusion reactions and makes up 99% of the visible universe. This plasma consists of matter only in the form of electrons and vastly higher-mass atomic nuclei, or ions. The electron-positron plasmas are composed of equal mass but oppositely charged particles that are subject to annihilation and creation. Such plasmas can exhibit quite different collective behavior.

"Our laboratory simulation is a small-scale analog of a magnetar environment," said physicist Kenan Qu of the Princeton Department of Astrophysical Sciences. "This allows us to analyze QED pair plasmas," said Qu, first author of a study showcased in Physics of Plasmas as a Scilight, or science highlight, and also first author of a paper in Physical Review Letters that the present paper expands on.

"Rather than simulating a strong magnetic field, we use a strong laser," Qu said. "It converts energy into pair plasma through what are called QED cascades. The pair plasma then shifts the laser pulse to a higher frequency," he said. "The exciting result demonstrates the prospects for creating and observing QED pair plasma in laboratories and enabling experiments to verify theories about fast radio bursts."

Laboratory-produced pair plasmas have previously been created, noted physicist Nat Fisch, a professor of astrophysical sciences at Princeton University and associate director for academic affairs at PPPL who serves as principle investigator for this research. "And we think we know what laws govern their collective behavior," Fisch said. "But until we actually produce a pair plasma in the laboratory that exhibits collective phenomena that we can probe, we cannot be absolutely sure of that.

Collective behavior

"The problem is that collective behavior in pair plasmas is notoriously hard to observe," he added. "Thus, a major step for us was to think of this as a joint production-observation problem, recognizing that a great method of observation relaxes the conditions on what must be produced and in turn leads us to a more practicable user facility."

The unique simulation the paper proposes creates high-density QED pair plasma by colliding the laser with a dense electron beam travelling near the speed of light. This approach is cost-efficient when compared with the commonly proposed method of colliding ultra-strong lasers to produce the QED cascades. The approach also slows the movement of plasma particles, thereby allowing stronger collective effects.

"No lasers are strong enough to achieve this today and building them could cost billions of dollars," Qu said. "Our approach strongly supports using an electron beam accelerator and a moderately strong laser to achieve QED pair plasma. The implication of our study is that supporting this approach could save a lot of money."

Currently underway are preparations for testing the simulation with a new round of laser and electron experiments at SLAC. "In a sense what we are doing here is the starting point of the cascade that produces radio bursts," said Sebastian Meuren, a SLAC researcher and former postdoctoral visiting fellow at Princeton University who coauthored the two papers with Qu and Fisch.

Evolving experiment

"If we could observe something like a radio burst in the laboratory that would be extremely exciting," Meuren said. "But the first part is just to observe the scattering of the electron beams and once we do that we'll improve the laser intensity to get to higher densities to actually see the electron-positron pairs. The idea is that our experiment will evolve over the next two years or so."

The overall goal of this research is understanding how bodies like magnetars create pair plasma and what new physics associated with fast radio bursts are brought about, Qu said. "These are the central questions we are interested in."

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Physicists explain how type of aurora on Mars is formed

Physicists led by the University of Iowa have learned how a type of aurora on Mars is formed.

In a new study, the physicists studied discrete aurora, a light-in-the-sky display that occurs mostly during the night in the red planet's southern hemisphere. While scientists have known about discrete aurora on Mars-which also occur on Earth -- they did not know how they formed. That's because Mars does not have a global magnetic field like Earth, which is a main trigger for aurora, also called the northern and southern lights on our planet.

Instead, the physicists report, discrete aurora on Mars are governed by the interaction between the solar wind -- the constant jet of charged particles from the sun -- and magnetic fields generated by the crust at southern latitudes on Mars. It's the nature of this localized interaction between the solar wind and the crustal magnetic fields that lead to discrete aurora, the scientists find.

"We have the first detailed study looking at how solar wind conditions affect aurora on Mars," says Zachary Girazian, associate research scientist in the Department of Physics and Astronomy and the study's corresponding author. "Our main finding is that inside the strong crustal field region, the aurora occurrence rate depends mostly on the orientation of the solar wind magnetic field, while outside the strong crustal field region, the occurrence rate depends mostly on the solar wind dynamic pressure."

The findings come from more than 200 observations of discrete aurora on Mars by the NASA-led Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. One of the instruments used to make the observations, the Solar Wind Ion Analyzer, is led by Jasper Halekas, associate professor in the Department of Physics and Astronomy and a co-author on the study.

"Now is a very fruitful and exciting time for researching aurora at Mars. The database of discrete aurora observations we have from MAVEN is the first of its kind, allowing us to understand basic features of the aurora for the first time," Girazian says.

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Scientists connect the dots between Galilean moon, auroral emissions on Jupiter

On November 8, 2020, NASA's Juno spacecraft flew through an intense beam of electrons traveling from Ganymede, Jupiter's largest moon, to its auroral footprint on the gas giant. Southwest Research Institute scientists used data from Juno's payload to study the particle population traveling along the magnetic field line connecting Ganymede to Jupiter while, at the same time, remotely sensing the associated auroral emissions to unveil the mysterious processes creating the shimmering lights.

"Jupiter's most massive moons each create their own auroras on Jupiter's north and south poles," said Dr. Vincent Hue, lead author of a paper outlining the results of this research. "Each auroral footprint, as we call them, is magnetically connected to their respective moon, kind of like a magnetic leash connected to the moon glowing on Jupiter itself."

Like the Earth, Jupiter experiences auroral light around the polar regions as particles from its massive magnetosphere interact with molecules in the Jovian atmosphere. However, Jupiter's auroras are significantly more intense than Earth's, and unlike Earth, Jupiter's largest moons also create auroral spots. The Juno mission, led by SwRI's Dr. Scott Bolton, is circling Jupiter in a polar orbit and flew through the electron "thread" connecting Ganymede with its associated auroral footprint.

"Prior to Juno, we knew that these emissions can be quite complex, ranging from a single auroral spot to multiple spots, which sometimes trail an auroral curtain that we called the footprint tail," said Dr. Jamey Szalay, a co-author from Princeton University. "Juno, flying extremely close to Jupiter, revealed these auroral spots to be even more complex than previously thought."

Ganymede is the only moon in our solar system that has its own magnetic field. Its mini-magnetosphere interacts with Jupiter's massive magnetosphere, creating waves that accelerate electrons along the gas giant's magnetic field lines, which can be directly measured by Juno.

Two SwRI-led instruments on Juno, the Jovian Auroral Distributions Experiment (JADE) and the Ultraviolet Spectrometer (UVS) provided key data for this study, which was also supported by Juno's magnetic field sensor built at NASA's Goddard Space Flight Center.

"JADE measured the electrons traveling along the magnetic field lines, while UVS imaged the related auroral footprint spot," said SwRI's Dr. Thomas Greathouse, a co-author on this study.

In this way, Juno is both able to measure the electron "rain" and immediately observe the UV light it creates when it crashes into Jupiter. Previous Juno measurements showed that large magnetic perturbations accompanied the electron beams causing the auroral footprint. However, this time, Juno did not observe similar perturbations with the electron beam.

"If our interpretation is correct, this a confirmation of a decade-old theory that we put together to explain the morphology of the auroral footprints," said Dr. Bertrand Bonfond, a co-author of the study from the Liège University in Belgium. The theory suggests that electrons accelerated in both directions create the multi-spot dance of auroral footprints.

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