Scientists solve solar secret

The further we move away from a heat source, the cooler the air gets. Bizarrely, the same can't be said for the Sun, but University of Otago scientists may have just explained a key part of why.

Study lead Dr Jonathan Squire, of the Department of Physics, says the surface of the Sun starts at 6000 degree C, but over a short distance of only a few hundred kilometers, it suddenly heats up to more than a million degrees, becoming its atmosphere, or corona.

"This is so hot that the gas escapes the Sun's gravity as 'solar wind', and flies into space, smashing into Earth and other planets.

"We know from measurements and theory that the sudden temperature jump is related to magnetic fields which thread out of the Sun's surface. But, exactly how these work to heat the gas is not well understood -- this is known as the Coronal Heating Problem.

"Astrophysicists have several different ideas about how the magnetic-field energy could be converted into heat to explain the heating, but most have difficulty explaining some aspect of observations," he says.

Dr Squire and co-author Dr Romain Meyrand have been working with scientists at Princeton University and the University of Oxford and found two previous theories can be merged into one to solve a key piece of the 'problem'. The group's findings have just been published in Nature Astronomy.

The popular theories are based on heating caused by turbulence, and heating caused by a type of magnetic wave called ion cyclotron waves.

"Both, however, have some problem -- turbulence struggles to explain why Hydrogen, Helium and Oxygen in the gas become as hot as they do, while electrons remain surprisingly cold; while the magnetic waves theory could explain this feature, there doesn't seem to be enough of the waves coming off the Sun's surface to heat up the gas," Dr Meyrand says.

The group used six-dimensional supercomputer simulations of the coronal gas to show how these two theories are actually part of the same process, linked together by a bizarre effect called the 'helicity barrier'.

This intriguing occurrence was discovered in an earlier Otago study, led by Dr Meyrand.

"If we imagine plasma heating as occurring a bit like water flowing down a hill, with electrons heated right at the bottom, then the helicity barrier acts like a dam, stopping the flow and diverting its energy into ion cyclotron waves. In this way, the helicity barrier links the two theories and resolves each of their individual problems," he explains.

For this latest study, the group stirred the magnetic field lines in simulations and found the turbulence created the waves, which then caused the heating.

"As this happens, the structures and eddies that form end up looking extremely similar to cutting-edge measurements from NASA's Parker Solar Probe spacecraft, which has recently become the first human-made object to actually fly into the corona.

"This gives us confidence that we are accurately capturing key physics in the corona, which -- coupled with the theoretical findings about the heating mechanisms -- is a promising path to understanding the coronal heating problem," Dr Meyrand says.

Understanding more about the Sun's atmosphere and the subsequent solar wind is important because of the profound impacts they have on Earth, Dr Squire explains.

Effects which result from solar wind's interaction with the Earth's magnetic field is called 'space weather', which causes everything from Aurora to satellite-destroying radiation and geomagnetic currents which damage the power grid.

"All of this is sourced, fundamentally, by the corona and its heating by magnetic fields, so as well as being interesting for our general understanding of the solar system, the solar-corona's dynamics can have profound impacts on Earth.

Read more at Science Daily

A solar illusion: Coronal loops may not be what they seem

Many coronal loops -- ropey strands of plasma that scientists have long thought existed in the Sun's atmosphere -- may actually be optical illusions, according to a new paper that challenges prevailing assumptions of what we know, and don't know, about the Sun.

The research, led by the National Center for Atmospheric Research (NCAR) and published in The Astrophysical Journal, relied on a cutting-edge, realistic 3D simulation of the solar corona. The simulation, carried out at NCAR several years ago, allowed the scientists to slice the corona in distinct sections in an effort to isolate individual coronal loops.

What they found is that many of the loops weren't loops at all.

While the research team was able to pinpoint some of the coronal loops they were looking for, they also found that in many cases what appear to be loops in images taken of the Sun may actually be wrinkles of bright plasma in the solar atmosphere. As sheets of bright plasma fold over themselves, the folds look like bright thin lines, mimicking the look of distinct and self-contained strands of plasma.

The findings, which the research team is calling the "coronal veil" hypothesis, have significant implications for our understanding of the Sun, since the presumed coronal loops have been used for decades as a way to infer information about density, temperature, and other physical characteristics of the solar atmosphere.

"I have spent my entire career studying coronal loops," said NCAR scientist Anna Malanushenko, who led the study. "I was excited that this simulation would give me the opportunity to study them in more detail. I never expected this. When I saw the results, my mind exploded. This is an entirely new paradigm of understanding the Sun's atmosphere."

The research was funded by NASA and included collaborators from NCAR's High Altitude Observatory, Lockheed Martin Solar and Astrophysics Laboratory, the Southwest Research Institute, and NASA Goddard. NCAR is sponsored by the National Science Foundation.

Challenging intuition

What appears to be coronal loops can be seen in images taken of the Sun in extreme ultraviolet light. The assumption that they exist is a natural one for scientists because it fits our most basic understanding of magnetism.

Most schoolchildren have at some point seen what happens when iron filings are sprinkled near a bar magnet. The filings orient themselves along magnetic field lines that loop from one pole of the bar magnet to the other. These curving lines spread out, becoming weaker and less dense, the further they are from the magnet.

The apparent coronal loops in images of the Sun look strikingly similar, and since there is a significant magnetic field in the Sun, the existence of magnetic field lines that could trap a rope of plasma between them and create loops seems like an obvious explanation. And in fact, the new study confirms that such loops likely exist.

However, the coronal loops seen on the Sun have never behaved exactly as they should, based on our understanding of magnets. For example, scientists would expect the magnetic field lines on the Sun to spread apart, just as in the iron filings experiment, as you move higher in the corona. If this happened, the plasma trapped between the field lines would also spread out between the boundaries, creating thicker, less bright loops. But images of the Sun do not show this phenomenon. Instead, the loops further out still appear thin and bright.

The possibility that these loops are instead wrinkles in a coronal veil helps explain this and other discrepancies with our expectations of the loops -- but it also asks new questions. For example, what determines the shape and thickness of the folds? And how many of the apparent loops in images of the Sun are actually real strands, and how many are optical illusions?

"This study reminds us as scientists that we must always question our assumptions and that sometimes our intuition can work against us," Malanushenko said.

Innovative model offers new view of the Sun

The discovery that coronal loops may be illusions was made possible thanks to an extremely detailed simulation of the solar corona produced by MURaM, a radiative magnetohydrodynamic model that was extended to model the solar corona in an effort led by NCAR.

The simulation was groundbreaking when it was first produced because it was able to simultaneously model what was happening in multiple regions of the Sun, from the upper part of the convective zone -- about 10,000 kilometers below the Sun's surface -- through the solar surface and beyond, up to nearly 40,000 kilometers into the solar corona. These varied regions of the Sun cover a vast range of physical conditions, including differences in density and pressure, and so scientists had not previously figured out a way to mathematically represent these regions in a unified simulation.

Among other results, the new simulation was able to capture the entire life cycle of a solar flare for the first time, from the build up of energy below the solar surface to the emergence of flare at the surface, and finally to the explosive release of energy.

The model also produced 3-dimensional data sets that contain the structure of the magnetic field and plasma, which can be used to generate "synthetic" observations. Because the solar corona is optically thin -- meaning it's relatively easy to see through it -- structures in the corona overlap one another in images of the Sun. This makes it difficult to tell whether a "loop" that is overlapping other loops is in front or behind. It's also difficult to tell whether the loop itself has a compact cross section, like a garden hose, or resembles a long ribbon viewed edge on. It's also possible that what appears to be a thin strand may be an optical artifact caused by a fold in a sheet of bright plasma.

The cubes of data produced by MURaM provide scientists the opportunity to dissect the solar atmosphere and study the overlapping structures separately, something that is not possible with the observatories and instruments we currently have.

While the MURaM simulation is one of the most realistic ever created of the solar corona, it's still just a model. Understanding how many coronal loops are actually optical illusions will require carefully designed observational methods that probe the corona and new data analysis techniques.

Read more at Science Daily

Bringing the sun into the lab

Why the Sun's corona reaches temperatures of several million degrees Celsius is one of the great mysteries of solar physics. A "hot" trail to explain this effect leads to a region of the solar atmosphere just below the corona, where sound waves and certain plasma waves travel at the same speed. In an experiment using the molten alkali metal rubidium and pulsed high magnetic fields, a team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a German national lab, has developed a laboratory model and for the first time experimentally confirmed the theoretically predicted behavior of these plasma waves -- so-called Alfvén waves -- as the researchers report in the journal Physical Review Letters.

At 15 million degrees Celsius, the center of our Sun is unimaginably hot. At its surface, it emits its light at a comparatively moderate 6000 degrees Celsius. "It is all the more astonishing that temperatures of several million degrees suddenly prevail again in the overlying Sun's corona," says Dr. Frank Stefani. His team conducts research at the HZDR Institute of Fluid Dynamics on the physics of celestial bodies -- including our central star. For Stefani, the phenomenon of corona heating remains one of the great mysteries of solar physics, one that keeps running through his mind in the form of a very simple question: "Why is the pot warmer than the stove?"

That magnetic fields play a dominant role in heating the Sun's corona is now widely accepted in solar physics. However, it remains controversial whether this effect is mainly due to a sudden change in magnetic field structures in the solar plasma or to the dampening of different types of waves. The new work of the Dresden team focuses on the so-called Alfvén waves that occur below the corona in the hot plasma of the solar atmosphere, which is permeated by magnetic fields. The magnetic fields acting on the ionized particles of the plasma resemble a guitar string, whose playing triggers a wave motion. Just as the pitch of a strummed string increases with its tension, the frequency and propagation speed of the Alfvén wave increases with the strength of the magnetic field.

"Just below the Sun's corona lies the so-called magnetic canopy, a layer in which magnetic fields are aligned largely parallel to the solar surface. Here, sound and Alfvén waves have roughly the same speed and can therefore easily morph into each other. We wanted to get to exactly this magic point -- where the shock-like transformation of the magnetic energy of the plasma into heat begins," says Stefani, outlining his team's goal.

A dangerous experiment?

Soon after their prediction in 1942, the Alfvén waves had been detected in first liquid-metal experiments and later studied in detail in elaborate plasma physics facilities. Only the conditions of the magnetic canopy, considered crucial for corona heating, remained inaccessible to experimenters until now. On the one hand, in large plasma experiments the Alfvén speed is typically much higher than the speed of sound. On the other hand, in all liquid-metal experiments to date, it has been significantly lower. The reason for this: the relatively low magnetic field strength of common superconducting coils with constant field of about 20 tesla.

But what about pulsed magnetic fields, such as those that can be generated at the HZDR's Dresden High Magnetic Field Laboratory (HLD) with maximum values of almost 100 tesla? This corresponds to about two million times the strength of the Earth's magnetic field: Would these extremely high fields allow Alfvén waves to break through the sound barrier? By looking at the properties of liquid metals, it was known in advance that the alkali metal rubidium actually reaches this magic point already at 54 tesla.

But rubidium ignites spontaneously in air and reacts violently with water. The team therefore initially had doubts as to whether such a dangerous experiment was advisable at all. The doubts were quickly dispelled, recalls Dr. Thomas Herrmannsdörfer of the HLD: "Our energy supply system for operating the pulse magnets converts 50 megajoules in a fraction of a second -- with that, we could theoretically get a commercial airliner to take off in a fraction of a second. When I explained to my colleagues that a thousandth of this amount of chemical energy of the liquid rubidium does not worry me very much, their facial expressions visibly brightened."

Pulsed through the magnetic sound barrier

Nevertheless, it was still a rocky road to the successful experiment. Because of the pressures of up to fifty times the atmospheric air pressure generated in the pulsed magnetic field, the rubidium melt had to be enclosed in a sturdy stainless steel container, which an experienced chemist, brought out of retirement, was to fill. By injecting alternating current at the bottom of the container while simultaneously exposing it to the magnetic field, it was finally possible to generate Alfvén waves in the melt, whose upward motion was measured at the expected speed.

The novelty: while up to the magic field strength of 54 tesla all measurements were dominated by the frequency of the alternating current signal, exactly at this point a new signal with halved frequency appeared. This sudden period doubling was in perfect agreement with the theoretical predictions. The Alfvén waves of Stefani's team had broken through the sound barrier for the first time. Although not all observed effects can yet be explained so easily, the work contributes an important detail to solving the puzzle of the Sun's corona heating. For the future, the researchers are planning detailed numerical analyses and further experiments.

Read more at Science Daily

A spacecraft has 'touched' the sun for the first time

NASA's Parker Solar Probe reached the sun's extended solar atmosphere, known as the corona, and spent five hours there. The spacecraft is the first to enter the outer boundaries of our sun.

"This marks the achievement of the primary objective of the Parker mission and a new era for understanding the physics of the corona," said Justin C. Kasper, the first author, Deputy Chief Technology Officer at BWX Technologies, and a professor at the University of Michigan. The mission is led by the Johns Hopkins University Applied Physics Laboratory (JHU/APL).

The probe made the first direct observations of what lies within the sun's atmosphere, measuring phenomena previously only estimated.

The sun's outer edge begins at the Alfvén critical surface: the point below which the sun and its gravitational and magnetic forces directly control the solar wind. Many scientists think that sudden reverses in the sun's magnetic field, called switchbacks, emerge from this area.

"The concept of sending spacecraft into the magnetized atmosphere of the sun -- sufficiently close that the magnetic energy is greater than both ion and electron kinetic and thermal energy -- predated NASA itself," said Kasper.

In 2018, NASA launched Parker Solar Probe with the goal of finally reaching the sun's corona and making humanity's first visit to a star.

This past April, the probe spent five hours below the Alfvén critical surface in direct contact with the sun's plasma. Below that surface, the pressure and energy of the sun's magnetic field was stronger than the pressure and energy of the particles. The spacecraft passed above and below the surface three separate times during its encounter. This is the first time a spacecraft has entered the solar corona and touched the atmosphere of the sun.

Surprisingly, the researchers discovered that the Alfvén critical surface is wrinkled. The data suggest that the largest and most distant wrinkle of the surface was produced by a pseudostreamer -- a large magnetic structure more than 40 degrees across, found back on the innermost visible face of the sun. It is not currently known why a pseudostreamer would push the Alfvén critical surface away from the sun.

Researchers noticed far fewer switchbacks below the Alfvén critical surface than above it. The finding could mean that switchbacks do not form within the corona. Alternatively, low rates of magnetic reconnection on the sun's surface could have pumped less mass into the observed wind stream, resulting in fewer switchbacks.

The probe also recorded some evidence of a potential power boost just inside the corona, which may point to unknown physics affecting heating and dissipation.

"We have been observing the sun and its corona for decades, and we know there is interesting physics going on there to heat and accelerate the solar wind plasma. Still, we cannot tell precisely what that physics is," said Nour E. Raouafi, the Parker Solar Probe Project Scientist at JHU/APL. "With Parker Solar Probe now flying into the magnetically-dominated corona, we will get the long-awaited insights into the inner workings of this mysterious region."

The observations took place during Parker Solar Probe's eighth encounter with the sun. All data is publicly available in the NASA PSP archive. Several previous studies predicted the probe would first pass within the sun's boundaries in 2021.

The fastest known object built by humans, Parker Solar Probe has made many new discoveries since its launch, including on explosions that create space weather and the dangers of super-speedy dust.

The new findings suggest that direct observations by spacecraft have much to illuminate about the physics of coronal heating and solar wind formation. Having achieved its goal of touching the sun, Parker Solar Probe will now descend even deeper into the sun's atmosphere and linger for longer periods of time.

Read more at Science Daily