Switchbacks Science: Explaining Parker Solar Probe’s Magnetic Puzzle

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. 

animated illustration of solar switchbacks
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. 

infographic explaining five theories explaining switchbacks
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


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.

Animation of Parker passing through switchback
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.

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NASA’s Goddard Space Flight Center, Greenbelt, Md.

Researchers Comb Atoms into a Novel Swirl

When you brush your hair in the morning, there’s a pretty good chance you’re not thinking about quantum physics. But the way your hair swirls as you brush is actually related to some features of the quantum world.

Important properties of quantum particles are described by topology—a field of mathematics that classifies objects according to how many holes they have. For instance, a coconut is topologically the same as a pizza (no holes), but different from a donut (one hole). Importantly, topology also dictates the kinds of hairdos you can style on these shapes. On a fuzzy coconut, no matter how much you brush, you’ll end up with at least one whorl—a spot that all the nearby hairs swirl around. On the other hand, a hairy donut can be brushed down flat without incurring any unsightly tufts or whorls. It can also have pairs of whorls, swirling in opposite directions, but never a single whorl.

This is not merely a question of fashion. In physical materials, topology can determine many interesting properties, like whether a material conducts electricity or not—particularly at its edges. Additionally, these edge properties are extremely precise and stable—so stable that they now serve as the standard for measuring electrical resistance.

Physicists don’t usually re-shape the materials they study into donuts or attach hairs to them. Instead, they often look at the topology of the quantum states of a particle moving around in the material. The coconut or donut shapes are found in abstract landscapes, inhabited by the possible speeds and directions the particle is moving—in technical language, each point in this space corresponds to a particular momentum. At each momentum, a “hair” sticks out, representing the internal quantum state of the particle. For solid materials, whose atoms line up into repeating crystal structures, the landscape is usually donut-shaped, and quantum whorls either don’t form at all or appear in pairs.Scientist created a swirl around a central point (white dot) in the quantum state of atoms (left), akin to a whorl of hair on a baby’s head (right). (Credit: Ana Valdes/JQI)Scientist created a swirl around a central point (white dot) in the quantum state of atoms (left), akin to a whorl of hair on a baby’s head (right). (Credit: Ana Valdes/JQI)

Now, a team of JQI researchers has engineered a new kind of topological matter—one with a single whorl—by breaking free from the constraints of crystalline solids. They managed to do this by grooming their atomic states into a whorl situated in an abstract, infinite plane, rather than a coconut or donut shape. The team was led by former UMD graduate student Ana Valdes-Curiel (currently a postdoc at the University of California, Los Angeles) in the group of Adjunct Professor and JQI Fellow Ian Spielman, a fellow at the National Institute of Standards and Technology. They reported their findings in a recent paper published in Nature Communications.(link is external)

In most solids, be they metals, insulators or superconductors, atoms arrange themselves in a repeating grid. Some electrons in the atoms can travel around in this grid, moving up and down or left and right. But no matter where an electron travels, it will end up experiencing deja vu: The grid of atoms repeats, so any given spot in the crystal looks exactly like many others.

This repeating structure has an effect on an electron’s momentum, too. In fact, topologically speaking, it’s as if the electron’s momentum is constrained to the surface of a donut. And since the donut can be brushed smooth, most materials have quantum state “hairs” that are swirl free. (They can also form two whorls, with hairs spiraling around them in opposite directions.)

To create their quantum whorl without the benefit of a neatly arranged crystal, the JQI team took a cloud of rubidium atoms and cooled it down to extremely low temperatures—so cold that quantum effects take over. At these temperatures, the atoms can inhabit only a few distinct quantum states. But, crucially, without the regular order of a crystal, there was no restriction on their momenta—they could move at any speed, up and down as well as left and right, forming a topological landscape of possible momenta akin to an infinite sheet.

Once the atoms were cooled, the team used three laser beams with precisely chosen colors and orientations to shuffle the atoms between three of the possible quantum states in sequence: first to second to third and back to first again. With each hop between states, the lasers imparted a small kick to the atoms, causing atoms in different states to move with slightly different speeds (and thus different momenta). This ensured that the quantum “hairs” pointed in different directions at different momenta and created a swirl around a point in the momentum landscape—the quantum whorl.

“And it turns out with this kind of atom, implementing that idea in a robust way is just super hard,” Spielman says.

Not all states can be manipulated to hop in a circle, and the ones that can are pretty unstable, quickly decaying and heating the cold atomic cloud. They are also easily disturbed by the tiniest changes in the lab’s magnetic field—something as small as a person walking by with a metal keychain can throw the whole thing off.

To mitigate these issues, the researchers employed a technique that they recently pioneered(link is external). They bathed the atoms in a strong, carefully chosen radio-frequency signal before the lasers were turned on. In this field, three stable states are transformed into quantum superpositions of each other, with parts of the other states mixed in. It’s normally impossible to make an atom hop in a circle through these stable states, but once they are jumbled together this restriction is lifted. On top of that, the strong magnetic field of the radio waves inoculated the atoms against small magnetic field disturbances.

They confirmed the novel topology of their cloud of atoms by directly measuring the quantum states they created, at a host of different atom momenta. Traversing their momentum landscape in a loop, they measured what mixture of the constituent states (the first, second, and third states in the circle) the atoms lived in, analogous to looking which direction the hairs were pointing. They confirmed that there was a single swirl at the loop’s center.

One of the most important features of topological materials is the edge effects that occur at an interface with a material that has a different topology. For example, if an insulator happens to have two swirls on its momentum donut, there will be a sharp change whenever it’s bordered by swirl-free air or a vacuum. At the place where materials with different swirl counts meet, something discontinuous has to happen. In this case, the boundary becomes a conductor sandwiched between two insulators. The number of these conducting edge channels exactly corresponds to the topology in the bulk, and they cannot be easily disrupted or destroyed. The discovery and topological explanation of these edge effects was the subject of the 2016 Nobel Prize in Physics(link is external), and has inspired much recent research and development of new types of devices.

“The whole big thing about topology is you get these edge states that are very robust,” says Valdes-Curiel “And now you have this system that is topological, but it doesn’t have the usual topological features. So what happens with the usual bulk-edge correspondence? What kind of edge states do you have? And what kind of topological devices can you build?”

Story by Dina Genkina

In addition to Valdes-Curiel and Spielman, co-authors of this paper included Dimitios Trypogeorgos, a former JQI postdoc now at the at the Institute of Nanotechnology in Lecce, Italy; Qiyu Liang, a postdoc at the JQI; and Russel Anderson, a former visiting researcher at the JQI now at La Trobe University and Q-CTRL.

Research Contact:  Ian Spielman, This email address is being protected from spambots. You need JavaScript enabled to view it.
(link sends e-mail)

Buonanno Receives Galileo Galilei Medal

Alessandra Buonanno has been awarded the Galileo Galilei Medal by the National Institute for Nuclear Physics (INFN). Buonanno was cited with Thibault Damour of the Institut des Hautes Études Scientifiques in Paris and Frans Pretorius of Princeton University “for the fundamental understanding of sources of gravitational radiation by complementary analytic and numerical techniques, enabling predictions that have been confirmed by gravitational wave observations and are now key tools in this new branch of astronomy”.  

Stefania De Curtis, director of the Galileo Galilei Institute, wrote that "Professors Buonanno and Damour, and professor Pretorius proposed two complementary approaches, analytical and numerical, to describe the behavior of two black holes spiraling around each other until they collide. Their description was used for the analysis of experimental data that, in 2015, led the LIGO and VIRGO scientific collaborations to the observation of the first gravitational waves emitted by the collision of two black holes". 2021 Galileo Galilei medal2021 Galileo Galilei medal

Buonanno is the director of the Astrophysical and Cosmological Relativity Department at the Max Planck Institute for Gravitational Physics in Potsdam and a Research Professor at the University of Maryland. She joined the UMD Physics in 2005, and received an Alfred P. Sloan Foundation Fellowship and the Richard A. Ferrell Distinguished Faculty Fellowship. She is a Fellow of the American Physical Society and the International Society of General Relativity and Gravitation. In 2018, she received the Leibniz Prize, Germany's prestigious research award. 

In discussing the work that led to the Galilei Medal, Buonanno explained that "To identify the source that generated the gravitational waves we observe on Earth, we need hundred thousand of waveform models. To achieve this goal about 20 years ago we introduced a novel approach to solve analytically the two-body problem in general relativity. This approach paved the way to develop the highly precise waveform models that today are routinely used by LIGO and VIRGO to detect binary systems composed of black holes and neutron stars and infer unique information about astrophysics, cosmology and gravity”. She offers futher discussion in this video.  

Buonanno and others detailed UMD's contributions to gravitational studies in a 2016 forum, A Celebration of Gravitational Waves


This story was adapted from the INFN website; for further information on the award, see https://home.infn.it/en/media-outreach/press-releases/4303-the-2021-galileo-galilei-medal-goes-to-alessandra-buonanno-thibault-damour-and-frans-pretorius

CU²MiP: Online and Expanded

In January 2021, the University of Maryland’s Department of Physics and the National Institute of Standards and Technology (NIST) hosted the third Conference for Undergraduate Underrepresented Minorities in Physics (CU²MiP). The conference launched in 2016 to address the historically low representation of minorities in the physics community.

This year, UMD President Darryll J. Pines gave a welcoming and encouraging address. UMD College of Computer, Mathematical, and Natural Sciences Dean Amitabh Varshney, NIST director Walter Copan. Physics Chair Steve Rolston, Rowan University’s Tabbatha Dobbins and Howard University Thomas A. Searles were among many speakers, workshop leaders and panelists.

Though COVID-19 required an online gathering this year, organizers adapted and expanded the program in significant ways, offering a research panel on quantum science, helpful videos and an entire slate for high school students.

“The quantum panel and quantum speakers for both undergrad and high school were very well received,” said Donna Hammer, director of education for the Department of Physics. Among the speakers at the quantum panel was alumna Ana Maria Rey (Ph.D. ’04), recipient of a MacArthur “genius” grant.

CU²MiP videos included several lab tours, as well as interviews with UMD students explaining their choice and enjoyment of physics.CU2MiP Collage

Other CU2MiP highlights included a fireside chat where College Park Professor Sylvester James Gates Jr. was interviewed by his daughter, Delilah Gates (B.S. ’15), who is now a physics Ph.D. candidate at Harvard University. The elder Gates mentioned events in his life that helped him succeed as a physicist and contribute to society. He also addressed “imposter syndrome,” which is a sense of not belonging or being good enough, and discussed ways that students might overcome it.

Jorge Ramirez Ortiz and Daniel Serrano of UMD gave a presentation on Rostros Físicos, a new multimedia celebration of the successes of Latinx/Latin American physicists across all stages of the scientific career path.

Fostering collegiality has always been a primary CU²MiP goal, and this year’s virtual gathering continued this emphasis.

“Adding mentoring chats throughout the conference fostered meaningful networking beyond the breakout rooms associated with the panels and workshops,” Hammer said. “Drop-in mentoring provided shared stories, guidance and collaboration in real time.”

Undergraduates responded positively.

“It was great to meet people and I found all of the speakers inspiring and engaging!” wrote one participant. Another expressed gratitude for the conference, noting, “I spoke with a lot of supportive people on the prospects of research.” 

The high school conference featured a plenary talk by Professor Willie Rockward, the physics department chair at Morgan State University, on “Your Pathway in Physics using Passion, Purpose, and Problem-solving.” High school student Anisha Musti discussed founding Q-munity, a group of high school students working together in quantum computing. College Park Professor and Nobel Laureate Bill Phillips, along with NIST’s Angie Hight Walker, held a Quantum Science Showcase. 

Erin Lukomska-Schlauch, chair of the science department at Charles Herbert Flowers High School in Prince George’s County, helped to organize the conference, and found the experience memorable.

"As an educator, I will be taking a lot of what I learned back to my students, especially from the diversity workshops,” she said. "All the sessions that I attended were all really engaging, well planned and well executed."

Cindy Hollies, a teacher who has led many UMD physics summer programs, wrote, “I logged out of the conference on Sunday evening feeling proud and impressed with the young people leading the future of physics and amazed at the inspiring opportunities this conference presented for high school students. May there be many more such conferences.”  

Hammer observed that high school students learned about the many career opportunities opened by degrees in physics. As one student wrote, “…although I have taken a physics class, I didn't know much about its applications. I am very excited to take more related classes in college.”

Rolston, the department chair, was pleased with the undergraduate program and the extended efforts for high school students.

“We are grateful to everyone who contributed to CU²MiP,” he said. “Studying physics is a great path, not only to research and teaching careers, but to an extremely wide range of interesting professions. And the discipline itself helps develop a discerning way of seeing the world.”

"CU²MiP is a catalyst for change,” summarized Hammer. “The outcomes of each conference inspire me to keep moving forward and to know, not just believe, that real, positive change is possible and happening right now. As one student said to me, ‘This conference showed me that with each day I study physics, I'm part of the solution.’”