Switchbacks Science: Explaining Parker Solar Probe’s Magnetic Puzzle

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Published: Wednesday, March 10 2021 05:33

When NASA’s Parker Solar Probe sent back the first observations from its voyage to the Sun, scientists found signs of a wild ocean of currents and waves quite unlike the near-Earth space much closer to our planet. This ocean was spiked with what became known as switchbacks: rapid flips in the Sun’s magnetic field that reversed direction like a zig-zagging mountain road. 

Scientists think piecing together the story of switchbacks is an important part of understanding the solar wind, the constant stream of charged particles that flows from the Sun. The solar wind races through the solar system, shaping a vast space weather system, which we regularly study from various vantage points around the solar system – but we still have basic questions about how the Sun initially manages to shoot out this two-million-miles-per-hour gust. 

Parker Solar Probe observed switchbacks — traveling disturbances in the solar wind that caused the magnetic field to bend back on itself — an as-yet unexplained phenomenon that might help scientists uncover more information about how the solar wind is accelerated from the Sun.

Credits: NASA's Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez

 

Solar physicists have long known the solar wind comes in two flavors: the fast wind, which travels around 430 miles per second, and the slow wind, which travels closer to 220 miles per second. The fast wind tends to come from coronal holes, dark spots on the Sun full of open magnetic field. Slower wind emerges from parts of the Sun where open and closed magnetic fields mingle. But there is much we’ve still to learn about what drives the solar wind, and scientists suspect switchbacks – fast jets of solar material peppered throughout it – hold clues to its origins. 

Since their discovery, switchbacks have sparked a flurry of studies and scientific debate as researchers try to explain how the magnetic pulses form. 

“This is the scientific process in action,” said Kelly Korreck, Heliophysics program scientist at NASA Headquarters. “There are a variety of theories, and as we get more and more data to test those theories, we get closer to figuring out switchbacks and their role in the solar wind.”

Magnetic fireworks

On one side of the debate: a group of researchers who think switchbacks originate from a dramatic magnetic explosion that happens in the Sun’s atmosphere. 

Signs of what we now call switchbacks were first observed by the joint NASA-European Space Agency mission Ulysses, the first spacecraft to fly over the Sun’s poles. But decades later when the data streamed down from Parker Solar Probe to the Johns Hopkins Applied Physics Lab in Laurel, Maryland, which manages the mission, scientists were surprised to find so many. 

As the Sun rotates and its superheated gases churn, magnetic fields migrate around our star. Some magnetic field lines are open, like ribbons waving in the wind. Others are closed, with both ends or “footpoints” anchored in the Sun, forming loops that course with scorching hot solar material. One theory – initially proposed in 1996 based on Ulysses data – suggests switchbacks are the result of a clash between open and closed magnetic fields. An analysis published last year by scientists Justin Kasper and Len Fisk of the University of Michigan further explores the 20-year-old theory. 

When an open magnetic field line brushes against a closed magnetic loop, they can reconfigure in a process called interchange reconnection – an explosive rearrangement of the magnetic fields that leads to a switchback shape. “Magnetic reconnection is a little like scissors and a soldering gun combined into one,” said Gary Zank, a solar physicist at the University of Alabama Huntsville. The open line snaps onto the closed loop, cutting free a hot burst of plasma from the loop, while “gluing” the two fields into a new configuration. That sudden snap throws an S-shaped kink into the open magnetic field line before the loop reseals – a little like, for example, the way a quick jerk of the hand will send an S-shaped wave traveling down a rope. 

Other research papers have looked at how switchbacks take shape after the fireworks of reconnection. Often, this means building mathematical simulations, then comparing the computer-generated switchbacks to Parker Solar Probe data. If they’re a close match, the physics used to create the models may successfully help describe the real physics of switchbacks. 

Zank led the development of the first switchbacks model. His model suggests not one, but two magnetic whips are born during reconnection: One travels down to the solar surface and one zips out into the solar wind. Like an electric wire made from a bundle of smaller wires, each magnetic loop is made of many magnetic field lines. “What happens is, each of these individual wires reconnects, so you produce a whole slew of switchbacks in a short period of time,” Zank said. 

Zank and his team modelled the very first switchback Parker Solar Probe observed, on Nov. 6, 2018. This first model fit the observations well, encouraging the team to develop it further. The team’s results were published in The Astrophysical Journal on Oct. 26, 2020. 

Another group of scientists, led by University of Maryland physicist James Drake, agrees on the import of interchange reconnection. But they differ when it comes to the nature of switchbacks themselves. Where others say switchbacks are a kink in a magnetic field line, Drake and his team suggest what Parker is observing is the signature of a kind of magnetic structure, called a flux rope. 

In Drake’s simulations, the kink in the field didn’t travel very far before fizzling out. “Magnetic field lines are like rubber bands, they like to snap back to their original shape,” he explained. But the scientists knew the switchbacks had to be stable enough to travel out to where Parker Solar Probe could see them. On the other hand, flux ropes – which are thought to be core components of many solar eruptions – are sturdier. Picture a magnetic striped candy cane. That’s a flux rope: strips of magnetic field wrapped around a bundle of more magnetic field. 

Drake and his team think flux ropes could be an important part of explaining switchbacks, since they should be stable enough to travel out to where Parker Solar Probe observed them. Their study – published in Astronomy and Astrophysics on Oct. 8, 2020 – lays the groundwork for building a flux rope-based model to describe the origins of switchbacks. 

What these scientists have in common is they think magnetic reconnection can explain not only how switchbacks form, but also how the solar wind is heated and slings out from the Sun. In particular, switchbacks are linked to the slow solar wind. Each switchback shoots a gob of hot plasma into space. “So we’re asking, ‘If you add up all those bursts, can they contribute to the generation of the solar wind?’” Drake said. 

Illustration of five current theories explaining how switchbacks form. Image is not to scale.

Credits: NASA’s Goddard Space Flight Center/Miles Hatfield/Lina Tran/Mary-Pat Hrybyk Keith

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Going with the flow

On the other side of the debate are scientists who believe that switchbacks form in the solar wind, as a byproduct of turbulent forces stirring it up.

Jonathan Squire, space physicist at New Zealand’s University of Otago, is one of them. Using computer simulations, he studied how small fluctuations in the solar wind evolved over time. “What we do is try and follow a small parcel of plasma as it moves outwards,” Squire said. 

Each parcel of solar wind expands as it escapes the Sun, blowing up like a balloon. Waves that undulate across the Sun create tiny ripples in that plasma, ripples that gradually grow as the solar wind spreads out.

“They start out first as wiggles, but then what we see is as they grow even further, they turn into switchbacks,” Squire said. “That's why we feel it's quite a compelling idea – it just happened by itself in the model.”  The team didn’t have to incorporate any guesses about new physics into their models – the switchbacks appeared based on fairly agreed-upon solar science.

Squire’s model, published on Feb. 26, 2020, suggests switchbacks form naturally as the solar wind expands into space. Parts of the solar wind that expand more rapidly, he predicts, should also have more switchbacks – a prediction already testable with the latest Parker dataset.

Other researchers agree that switchbacks begin in the solar wind, but suspect they form when fast and slow streams of solar wind rub against one another. One October 2020 study, led by Dave Ruffolo at Mahidol University in Bangkok, Thailand, outlined this idea.

Bill Matthaeus, a co-author on the paper and space physicist at the University of Delaware in Newark, points to the shearing at the boundary between fast and slow streams. This shearing between fast and slow creates characteristic swirls seen all over in nature, like the eddies that form as river water flows around a rock. Their models suggest that these swirls ultimately become switchbacks, curling the magnetic field lines back on themselves.

Illustration of Parker Solar Probe flying through a switchback in the solar wind.

Credits: NASA's Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez

 

But the swirls don’t form immediately – the solar wind has to be moving pretty fast before it can bend its otherwise rigid magnetic field lines. The solar wind reaches this speed about 8.5 million miles from the Sun. Mattheaus’ key prediction is that when Parker gets significantly closer to the Sun than that – which should happen during its next close pass 6.5 million miles from the Sun, on April 29, 2021 – the switchbacks should disappear.

“If this is the origin, then as Parker moves into the lower corona this shearing can't happen,” Mattheaus said. “So, the switchbacks caused by the phenomenon we're describing should go away.”

One aspect of switchbacks that these solar wind models haven’t yet successfully simulated is the fact that they tend to be stronger when they twist in a particular direction – the same direction of the Sun’s rotation. However, both simulations were created with a Sun that was still, not rotating, which may make the difference. For these modelers, incorporating the actual rotation of the Sun is the next step. 

Twisting in the wind

Finally, some scientists think switchbacks stem from both processes, starting with reconnection or footpoint motion at the Sun but only growing into their final shape once they get out into the solar wind. A paper published today by Nathan Schwadron and David McComas, space physicists at the University of New Hampshire and Princeton University, respectively, adopts this approach, arguing that switchbacks form when streams of fast and slow solar wind realign at their roots.

After this realignment fast wind ends up “behind” slow wind, on the same magnetic field line. (Imagine a group of joggers on a race track, Olympic sprinters at their heels.) This could happen in any case where slow and fast wind meet, but most notably at the boundaries of coronal holes, where fast solar wind is born. As coronal holes migrate across the Sun, scooting underneath streams of slower solar wind, the footpoint from the slow solar wind plugs into a source of fast wind. Fast solar wind races after the slower stream ahead of it. Eventually the fast wind overtakes the slower wind, inverting the magnetic field line and forming a switchback. 

Schwadron thinks the motion of coronal holes and of solar wind sources across the Sun is also a key puzzle piece. Reconnection at the leading edge of coronal holes, he suggests, could explain why switchbacks tend to “zig” in a way that’s aligned with the Sun’s rotation.

“The fact that these are oriented in this particular way is telling us something very fundamental,” Schwadron said.

Though it starts with the Sun, Schwadron and McComas think those reconnecting streams only become switchbacks within the solar wind, where the Sun’s magnetic field lines are flexible enough to double-back on themselves. 

As Parker Solar Probe swoops closer and closer to the Sun, scientists will eagerly look for clues that will support – or debunk – their theories. “There are different ideas floating around,” Zank said. “Eventually something will pan out.” 

Parker Solar Probe is part of NASA’s Living With a Star program to explore aspects of the Sun–Earth system that directly affect life and society. The Living With a Star flight program is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington, D.C. The Johns Hopkins University Applied Physics Laboratory implements the mission for NASA. Scientific instrumentation is provided by teams led by the Naval Research Laboratory, Princeton University, the University of California, Berkeley, and the University of Michigan.

Researchers Comb Atoms into a Novel Swirl

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Published: Friday, March 05 2021 05:33

Pushing the Frontier of Extreme Light-Matter Interaction Research

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Published: Monday, February 08 2021 05:33

University of Maryland Physics Professor Howard Milchberg and the students and postdoctoral researchers in his lab explore the dramatic results of experiments that push light to extremes in the presence of matter. In Milchberg’s opinion, researching the intense interactions between light and matter—which are only possible thanks to the revolutionary technology of lasers—brings together the most interesting aspects of physics.

“Once you considered the effect that an intense laser beam has on matter, the number of basic physics and application areas exploded,” said Milchberg, who also holds appointments in the Department of Electrical and Computer Engineering and the Institute for Research in Electronics and Applied Physics. “And understanding the interaction between intense light and matter requires bringing in tools from all areas of physics. It flexes all your physics muscles, experimental and theoretical, and it overlaps with all of the major areas. You have to deal with atomic physics, plasma physics, condensed matter physics, high-pressure physics and quantum physics. It is intellectually challenging and, as a bonus, it has many practical applications.”

In explorinIntense Laser-Matter Interactions Lab, University of Marylandg this research topic, his lab has discovered new physics and technological opportunities. From cutting-edge, powerful laser pulses to vortices made of light, recent research from Milchberg’s lab keeps revealing new ways that light and matter can come together and deliver new results.

The richness of this line of research comes from the fact that light is much more than just illumination traveling in straight lines at a constant speed. It is energy that can dance intricately as it travels and can interact with matter in exotic ways—including tearing atoms into pieces.

Light is traveling electric and magnetic energy, and it is often convenient to visualize light as traveling waves in the electric and magnetic fields. The hills and troughs in the waves represent the shifting of the strength and direction of the fields that push and pull on charged particles, like the protons and electrons that make up atoms. Powerful lasers can even accelerate charged particles to near the speed of light where the unusual behaviors described by Einstein’s theory of relativity come into play.

Milchberg’s lab often investigates the dramatic results of dialing a laser up to extreme strengths where it exceeds the field that connects the cores of atoms to its electrons. As researchers like Milchberg push lasers to greater and greater power levels, they reach a roadblock: A laser tends to spread its power out over an increasing area as it travels, and a laser with enough power will vaporize any solid piece of equipment that researchers try to use to corral it.

So an optical fiber, like those that commonly carry internet signals, are useless when faced with a powerful research laser. The central core and outer cladding that make up the fiber get destroyed without any chance to keep the light concentrated as it blazes forward.

But Milchberg’s research has uncovered multiple ways to use the interactions of light with matter to keep powerful laser beams contained. In papers published in the journals Physical Review Letters and Physical Review Research over the past year, Milchberg and his colleagues described two new ways to keep intense laser beams contained in a way that can accelerate particles and produce advanced coherent light sources. In addition to improving our understanding of how light and matter interact, the techniques might be implemented as tools for research in other areas, like high-energy physics, and for use in industrial and medical settings.Researchers have generated vortices of light that they describe as “edge-first flying donuts” and developed a new technique for imaging them in mid-flight. (Credit: Scott Hancock/University of Maryland)

In these projects, the group used its expertise to give the technology of optical fibers an extreme indestructible makeover. The key lies in building the waveguides—devices such as optical fibers that transport waves down a confined path—out of a material that has already been vaporized. The team forms the material—an energetic state of matter called a plasma—by letting a laser rip electrons free from their atoms to form a cloud of charged particles.

“A plasma waveguide has all the structure of an optical fiber, the classic core, the classic cladding,” Milchberg said. “Although in this case, it's indestructible. The hydrogen plasma forming the waveguide is already ripped up into its protons and electrons, so there's not much more violence you can do to it.”

For the first project, the group used two laser pulses: a solid beam and then a hollow tube of light matching the desired form of the outer shell. These two lasers allowed the team to independently craft the low-density cores and outer shells more carefully than previous approaches. This refined control improved the amount of power the techniques could concentrate and the distance the powerful pulses could travel­­­­­—a key to achieving desired levels of acceleration with a compact system.

The second approach produced a similar plasma waveguide but sacrificed the ability to tailor the resulting waveguide in favor of using a simpler, more accessible technique. In this technique, the researchers created a tube of low-density gas and then relied on the front edge of the powerful pulse to rip the electrons free and create the waveguide structure on-the-fly.

“It's actually simpler than the first method,” Milchberg said. “But there's less control. And we did an analysis which shows that if you want to have big diameter waveguides, the first method is actually more efficient than the second method.”

Both methods have potential uses as laser accelerators that can generate bursts of electrons of energy 10 billion electron volts, and Milchberg’s group is already doing those experiments.

In addition to these two techniques, Milchberg’s group is developing a technique that uses a 1,000 times higher density hydrogen gas to accelerate electrons without constructing a waveguide, while using 1,000 times less laser energy. This new technique improves on established methods by avoiding negative effects from the light vibrating the electrons as they get accelerated behind the intense laser pulse.

To do this, they used pulses of circularly polarized light that aren’t even as long as two full lengths of the waves of the light that is used. The field of circularly polarized light rotates as the light travels and this variation cancels out the effect of the asymmetries of the vibration from the light waves, enabling electron beams of higher quality than previous attempts. The denser gas used in this approach isn’t compatible with the 10 billion electron volt energies of the other approaches, but the technique might have its own niche to fill.

“Our experiments have spanned the higher through lower energy ends of laser-based acceleration,” Milchberg said. “The plasma waveguide effort is aimed at 10 billion electron volts, which is of high energy physics interest, while the newer research using millijoule pulses and dense hydrogen generates 15 million electron volts.  Although a high energy, it isn’t enough for high energy physics. But the energy is more than sufficient to do time-resolved medical imaging and materials imaging.”

But accelerating particles is not the only aspect of light-matter interactions that Milchberg’s group investigates. For instance, they also discovered new intricate effects that can be created in light pulses.

In a paper published in the journal Optica in December 2019, they generated and observed a new kind of light structure called a spatiotemporal optical vortex (STOV). STOVs are whorls in the way the phase—the property of light and other waves that tells you are where you are on the wave—changes in space and time. The researchers had to first develop a method to create these vortices and then figure out how to observe them in flight. The observation required analysis of the interaction between the STOVs and another bit of light while traveling through a thin glass window.

Understanding STOVs provides insight into how light produces the high-intensity laser effect called self-guiding. Milchberg’s team had previously discovered a naturally occurring form of STOV that behave like “optical smoke rings” and are crucial for all self-guiding processes. These vortices may also have applications in transmitting information because the twisting structure tends to stabilize itself by filling any sections that get knocked out—say by water droplets in the air that the signal travels through.

All of these research results represent new techniques that may be useful tools for researchers and industry, and they deepen our understanding of the intricate back-and-forth that can be engineered between light and matter.

“One of the things that I would say my group is known for is that all of our papers include theory and simulation that accompanies the experiments,” Milchberg said. “And that has provided an important feedback loop to guide and refine the experiments.”

Milchberg credits his group’s steady generation of new discoveries to his graduate students.

“I don't think I could have done any of this in a non-university setting,” Milchberg said. “I think the sort of relationship one has with students and they have with each other where we're all batting ideas back-and-forth and having a continuous free-for-all discussion—with crazy thoughts encouraged—is not the same as in a place filled with longtime Ph.D.s and an established hierarchy. The freedom to ask naïve questions and argue a lot is essential.”

Written by Bailey Bedford

Researchers Create On-Demand Cold Spots to Generate Electromagnetic Cone of Silence

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Published: Monday, January 25 2021 05:33

In modern society, we are accustomed to having electronic systems that always work, regardless of the conditions. Protection of sensitive electronics to interference through unwanted coupling between components or intentional electromagnetic attack is important to ensure uninterrupted service. However, the environments in which we operate are growing increasingly complex and the electromagnetic spectrum is more congested. Additionally, certain environments such as a passenger cabin on an aircraft or train, can act as reverberant cavities, resulting in random fluctuations in signal levels. These effects are dynamic, so preventing significant performance degradation necessitates an approach that is capable of adapting to changing conditions.

An electromagnetic enclosure can be characterized by its scattering parameters, which are voltage to voltage transfer functions defining the behavior of transmission and reflection between inputs and outputs. One method of dynamically changing the scattering parameters is to install a programmable metasurface inside the cavity. A programmable metasurface consists of multiple unit cells, each of which can modify its reflection coefficient, allowing the direction of reflected rays to be adjusted on-the-fly. Conceptual overview of the metasurface-enabled cavity as a closed-loop system. The cavity S parameters (scattering parameters) are measured with a network analyzer and passed to a controller that updates the metasurface elements with a new set of commands. The controller can generate cold spots at port 2 at an arbitrary set of frequencies, or drive candidate S-matrix eigenvalues towards the origin, and includes a stochastic iterative optimization algorithm. The three ports allow additional angular and spatial diversity to be added at the inputs. The inset shows a closeup view of one of the metasurface unit cells.

Researchers in the Wave Chaos Group at the University of Maryland, College Park (UMD) have used this approach to create on-demand coldspots, or nulls in transmission, effectively generating an electromagnetic cone of silence. Their work, published on December 29 in Physical Review Research, used a binary tunable metasurface manufactured by the Johns Hopkins University Applied Physics Laboratory. The relationship between commands and cavity scattering parameters is extremely complex, so simple linear techniques fail to converge. The team, led by electrical and computer engineering Ph.D. student Benjamin Frazier, developed an efficient stochastic algorithm and experimentally demonstrated the ability to generate coldspots at arbitrary frequencies, with arbitrary bandwidths, and even when driving multiple inputs.

“Chaotic microwave cavities are extremely useful as surrogates to probe the behavior of electromagnetic waves in larger complicated enclosures and are used in many of the research projects being investigated both by our group and collaborators at facilities such as the Naval Research Lab,” said Frazier. “The ability to dynamically modify the cavity in a very detailed and controllable manner is a significant advancement towards harnessing waves as they propagate through these rich scattering environments.”

In addition, they showed the ability to induce coherent perfect absorption states inside the cavity. Coherent perfect absorption is a special condition inside the cavity where all incoming energy injected into the cavity is absorbed and has great promise as a method to enable long range wireless power transfer.

Other authors of the paper include UMD Electrical and Computer Engineering and Physics Professors Thomas M. Antonsen,  Edward Ott and  Steven M. Anlage.

DOI: https://doi.org/10.1103/PhysRevResearch.2.043422

Original story: https://ece.umd.edu/news/story/umd-researchers-create-ondemand-cold-spots-to-generate-electromagnetic-cone-of-silence 

A Frankenstein of Order and Chaos

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Published: Wednesday, January 06 2021 15:33