The solar wind, which creates beautiful auroras on Earth, is now less mysterious

NASA’s Parker Solar Probe, on its journey to the Sun, has revealed previously unseen structures of the solar wind, right where it is formed. The way the probe perceives it, the solar wind emanates from the Sun’s surface much like water jets from a showerhead.

A paper published in the journal Nature this week, led by scientists Stuart D. Bale from the University of California, Berkeley, and James Drake of the University of Maryland-College Park, illustrates how the Parker Solar Probe has detected streams of high-energy particles. These particles mirror the supergranulation flows within coronal holes, suggesting these regions as the birthplace of the “fast” solar wind.

Coronal holes are unique areas where magnetic field lines radiate from the Sun’s surface without looping back inward, consequently forming open field lines that encompass most of the space surrounding the Sun.

During quiet periods of the Sun, these holes are primarily located at the poles, preventing the fast solar wind they produce from reaching Earth. However, every 11 years, the Sun’s activity peaks, its magnetic field flips, and these holes can be seen all over the Sun’s surface, leading to bursts of solar wind aimed directly at Earth.

Origins of the solar wind

The process of how and where the solar wind originates is crucial for predicting solar storms. These storms, while responsible for the Earth’s breathtaking auroras, can also disrupt satellites and the electrical grid. As Drake puts it, “Winds carry lots of information from the Sun to Earth, so understanding the mechanism behind the Sun’s wind is important for practical reasons on Earth.”

The scientists describe the coronal holes like showerheads. From the bright spots where magnetic field lines dive into and out of the Sun’s surface, evenly spaced jets burst forth. These jets of charged particles are flung out of the Sun when the oppositely directed magnetic fields in these funnels collide and reconnect.

Bale explains, “The photosphere is covered by convection cells, like in a boiling pot of water, and the larger scale convection flow is called supergranulation.” He goes on to illustrate how these supergranulation cells pull the magnetic field down with them into a funnel, intensifying the magnetic field. “The spatial separation of those little drains, those funnels, is what we’re seeing now with solar probe data,” he said.

The Parker Solar Probe’s detection of some exceptionally high-energy particles – ones traveling at speeds 10 to 100 times faster than the solar wind average – led the researchers to believe that this process, known as magnetic reconnection, must be the source of the wind. The primary purpose of launching the probe in 2018 was to establish the origin of these high-energy particles: were they caused by magnetic reconnection or acceleration by plasma or Alfvén waves?

“The big conclusion is that it’s magnetic reconnection within these funnel structures that’s providing the energy source of the fast solar wind,” Bale concluded. The research strongly suggests that it’s reconnection that’s responsible for the high-energy particles detected.

The same funnel structures, as Nour Raouafi of the Applied Physics Laboratory at Johns Hopkins University suggests, probably align with the bright jetlets that can be seen from Earth within coronal holes. “Solving the mystery of the solar wind has been a six-decade dream of many generations of scientists. Now, we are grasping at the physical phenomenon that drives the solar wind at its source — the corona,” Raouafi said.

Sampling the solar wind

The solar wind evolves into a homogeneous, turbulent flow as it traverses the 93 million miles to Earth. The charged particles and entangled magnetic fields interact with Earth’s magnetic field, dumping electrical energy into the upper atmosphere, which excites atoms and produces vivid auroras. Understanding and predicting intense solar winds, or solar storms, and their effects near Earth are some of the key goals of NASA’s Living With a Star program, which funded the Parker Solar Probe.

The probe’s main task is to determine the structure and acceleration of the solar wind near its birthplace, the Sun’s surface or photosphere. The probe must approach the Sun closer than ever before – within 13 million miles, or about 25 to 30 solar radii.

According to Bale, once the probe dives below that altitude, the solar wind undergoes minimal evolution and displays more of its initial structure. This allows scientists to see the imprints of the Sun on the wind.

In 2021, the probe’s instruments identified magnetic field switchbacks in Alfvén waves, closely associated with the regions generating the solar wind. As the probe neared the Sun, it became clear that the probe was moving through structured jets of material, rather than mere turbulence. Bale, Drake, and their colleagues traced these jets back to the supergranulation cells in the photosphere, where the magnetic fields accumulate and funnel into the Sun.

Mysteries and questions remain

The question that loomed was, what accelerates the charged particles in these funnels? Is it magnetic reconnection, which would catapult the particles outward, or hot plasma waves streaming out of the Sun, which would allow particles to ride a wave?

The detection of extremely high-energy particles within these jets suggested to Bale that magnetic reconnection must be responsible for particle acceleration and the generation of Alfvén waves, which could give the particles an additional thrust.

Bale further explained, “Our interpretation is that these jets of reconnection outflow excite Alfvén waves as they propagate out. That’s an observation that’s well known from Earth’s magnetotail, as well, where you have similar kind of processes. I don’t understand how wave damping can produce these hot particles up to hundreds of keV, whereas it comes naturally out of the reconnection process.”

The Parker Solar Probe can approach the Sun no closer than 4 million miles, or about 8.8 solar radii above the surface, without risking damage to its instruments. Bale anticipates that data collected from this altitude will confirm their conclusions, despite the Sun’s heightened activity during its solar maximum. This increased activity might obscure the processes that scientists aim to observe.

“We’re lucky that we launched it in the solar minimum,” Bale commented, reflecting on the Parker Solar Probe’s journey. He implied that the quieter solar activity during the probe’s launch allowed them to make discoveries that would have been impossible during more chaotic solar conditions.

More about NASA’s Parker Solar Probe

NASA’s Parker Solar Probe is a pioneering mission that has set its sights on our very own star, the Sun. The probe’s main goal is to uncover the mysteries of the corona, the Sun’s outer atmosphere, and to learn more about the highly energetic particles present in the solar wind.

Named after astrophysicist Eugene Parker, the probe was launched on August 12, 2018. It’s the first spacecraft to be named after a living individual. Eugene Parker is the scientist who first theorized the existence of the solar wind back in 1958.

The Parker Solar Probe is unique because it ventures closer to the Sun than any previous spacecraft. It’s designed to withstand the extreme temperatures and conditions near the Sun. The probe is shielded by a 4.5-inch-thick carbon-composite shield, which can withstand temperatures up to about 2,500 degrees Fahrenheit.

The probe is equipped with four major instrument suites that are designed to measure magnetic fields, plasma and energetic particles, and capture images of the solar wind. These suites include the Fields Experiment (FIELDS), the Integrated Science Investigation of the Sun (ISIS), the Wide-Field Imager for Solar Probe (WISPR), and the Solar Wind Electrons Alphas and Protons (SWEAP) investigation.

Over the course of its seven-year mission, the probe will complete 24 orbits of the Sun, coming within 4 million miles of the Sun’s surface. This is about seven times closer than any previous mission.

The Parker Solar Probe has several primary science objectives. One is to trace the flow of energy that heats and accelerates the solar corona and solar wind. Another is to determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind. The probe also aims to explore the mechanisms that accelerate and transport energetic particles.

By achieving these objectives, scientists hope to improve their ability to forecast space weather events that impact life on Earth, as well as satellites and astronauts in space. The mission encapsulates the essence of exploration, as it journeys into a region of space never before explored by human-made objects.