Direct observations of a complex coronal web provide an important clue as to what mechanism drives the solar wind

Direct observations of a complex coronal web provide an important clue as to what mechanism drives the solar wind

The Sun’s atmosphere: computer simulation of the architecture of the magnetic field in the central corona on August 17, 2018. The radiating features in this snapshot are the underlying magnetic architecture of the observed coronal web. In the middle corona, the efficiently closed magnetic field lines near the Sun give way to the efficiently open field lines of the outer corona. Photo credit: Naturastronomy, Chitta et al.

With the help of observation data from the US weather satellite GOES, a research team led by the Max Planck Institute for Solar System Research (MPS) in Germany has taken an important step towards solving one of the sun’s most stubborn mysteries: How does our star start? particles that make up the solar wind into space? The data provide a unique insight into a key region in the solar corona that researchers previously had little access to.

The team has captured for the first time a dynamic web-like network of elongated, interwoven plasma structures. Together with data from other space probes and extensive computer simulations, a clear picture emerges: Where the elongated coronal network structures interact, magnetic energy is discharged – and particles escape into space.

The US National Oceanic and Atmospheric Administration (NOAA) Geostationary Operational Environmental Satellites (GOES) have traditionally been concerned with things other than the sun. The system has been orbiting our planet at an altitude of around 36,000 kilometers since 1974 and continuously provides earth-related data for weather and storm forecasts, for example.

Over the years, the original configuration has been expanded to include newer satellites. The three youngest currently in operation are more heavily equipped with instruments that look at the sun for space weather forecasts. They can image ultraviolet radiation from our star’s corona.

An exploratory observing campaign to image the extended solar corona took place in August and September 2018. For more than a month, GOES’ Solar Ultraviolet Imager (SUVI) not only looked directly at the sun as usual, but also took pictures from both sides of it.

“We had a rare opportunity to use an instrument in an unusual way to observe a region that hasn’t really been explored,” said Dr. SwRI’s Dan Seaton, who served as chief scientist for SUVI during the monitoring campaign. “We didn’t even know if it would work, but we knew that if it did work, we would make important discoveries.”

By combining the images from the different viewing angles, the field of view of the instrument was significantly enlarged and the entire middle corona, a layer of the sun’s atmosphere from 350,000 kilometers above the visible surface of the sun, was recorded for the first time in ultraviolet light.

Other spacecraft that study the Sun and collect data from the corona, such as NASA’s Solar Dynamics Observatory (SDO) and NASA-ESA’s Solar and Heliospheric Observatory (SOHO), are peering deeper or higher. “In the middle corona, solar research had something of a blind spot. The GOES data now provide a significant improvement,” said Dr. MPS’ Pradeep Chitta, lead author of the new study. In the middle corona, researchers suspect processes that drive and modulate the solar wind.

Travel through space at supersonic speeds

The solar wind is one of the most far-reaching features of our star. The stream of charged particles that the Sun hurls into space travels to the edge of our solar system, creating the heliosphere, a bubble of rarefied plasma that marks the Sun’s sphere of influence. Depending on its speed, the solar wind is divided into fast and slow components.

The so-called fast solar wind, reaching speeds of more than 500 kilometers per second, originates from the interior of coronal holes, regions that appear dark in coronal ultraviolet radiation. However, the source regions of the slow solar wind are less certain. But the particles of the slow solar wind also race through space at supersonic speeds of 300 to 500 kilometers per second.

This slower component of the solar wind still raises many questions. Hot coronal plasma of over a million degrees must escape the Sun to form the slow-moving solar wind. What mechanism is at work here? In addition, the slow solar wind is not homogeneous, but at least partially shows a ray-like structure of clearly distinguishable streamers. Where and how are they created? The new study addresses these questions.

Capture the dynamic coronal mesh

The origin of the solar wind: This is a mosaic of images acquired by the GOES instrument SUVI and the SOHO coronagraph LASCO on August 17, 2018. Outside the circle marked in white, LASCO’s field of view shows the streams of the slow solar wind. These connect seamlessly to the structures of the coronal mesh in the center of the corona, which can be seen within the circle marked in white. Where the long filaments of the coronal web converge, the slow solar wind begins its journey into space. Photo credit: Naturastronomy, Chitta et al. /GOES/SUVI/SOHO/LASCO

A region near the equator can be seen in the GOES data that sparked the researchers’ particular interest: two coronal holes where the solar wind freely flows away from the Sun, in close proximity to a region of high magnetic field strength. Interactions between such systems are considered possible origins of the slow solar wind.

Above this region, the GOES data show elongated plasma structures in the middle corona pointing radially outward. The team of authors calls this phenomenon, which is now directly imaged for the first time, the coronal network. The web is constantly in motion: its structures interact and regroup.

Researchers have long known that the solar plasma of the outer corona exhibits a similar architecture. For decades, the LASCO (Large Angle and Spectrometric Coronagraph) coronagraph on board the SOHO spacecraft, which celebrated its 25th anniversary last year, has been providing visible-light images of this region. Scientists interpret the jet-like currents in the outer corona as the structure of the slow solar wind, which begins its journey into space there. As the new study now impressively shows, this structure already prevails in the middle corona.

Influence of the solar magnetic field

To better understand the phenomenon, the researchers also analyzed data from other spacecraft: NASA’s Solar Dynamics Observatory (SDO) provided a simultaneous view of the Sun’s surface; The STEREO-A spacecraft, which has preceded Earth in its orbit around the Sun since 2006, provided a side perspective.

Using modern computational techniques that include remote sensing observations of the Sun, researchers can use supercomputers to create realistic 3D models of the elusive magnetic field in the Sun’s corona. In this study, the team used an advanced magnetohydrodynamic (MHD) model to simulate the corona’s magnetic field and plasma state for this time period.

“This helped us to relate the intriguing dynamics we observed in the middle corona to prevailing theories of solar wind formation,” said Dr. Cooper Downs of Predictive Science Inc. who ran the computer simulations.

As the calculations show, the structures of the coronal network follow the magnetic field lines. “Our analysis suggests that the architecture of the magnetic field in the middle corona is imprinted on the slow solar wind and plays an important role in accelerating the particles into space,” Chitta said. According to the team’s new results, the hot solar plasma in the central corona flows along the open magnetic field lines of the coronal grid. Where the field lines cross and interact, energy is released.

There is much to suggest that the researchers are on the trail of a fundamental phenomenon. “During times of high solar activity, coronal holes near the equator often appear in close proximity to areas of high magnetic field strength,” Chitta said. “The coronal network observed by us should therefore not be an isolated case,” he adds.

The team hopes to gain further and more detailed insights from future solar missions. Some of them, like ESA’s Proba-3 mission planned for 2024, are equipped with instruments specifically targeting the middle corona. The MPS is involved in the processing and analysis of the data from this mission. This, combined with observational data from currently operating probes such as NASA’s Parker Solar Probe and ESA’s Solar Orbiter, which are exiting the Earth-Sun line, will provide a better understanding of the three-dimensional structure of the coronal web.

The study was published in natural astronomy.

More information:
LP Chitta et al, Direct observations of a complex coronal web driving a highly structured slow-moving solar wind, natural astronomy (2022). DOI: 10.1038/s41550-022-01834-5

Provided by the Max Planck Society

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