Direct observations of a complex coronal lattice reveal important clues about the mechanism that drives the solar wind

Dynamic coronal lattice catching

The Sun’s atmosphere: A computer simulation of the structure of the magnetic field in the central corona on August 17, 2018. The ray-like features in this snapshot are the underlying magnetic structure of the observed coronal lattice. In the central corona, the mostly closed magnetic field lines close to the Sun give way to the mostly open field lines from the outer corona. Credit: Nature Astronomy, Chitta et al.

Using observational data from the US GOES weather satellite, a team of researchers led by the Max Planck Institute for Solar System Research (MPS) in Germany has taken an important step toward revealing one of the Sun’s most enduring mysteries: How does our star release the particles that make up the solar wind into space? The data provides a unique view of a key region in the solar corona that researchers have had no access to until now.

For the first time, the team has captured a dynamic, web-like network of elongated, entangled plasma structures. Combined with data from other space probes and extensive computer simulations, a clear picture emerges: As elongated coronal lattice structures interact, magnetic energy is dumped — and particles escape into space.

NOAA’s Geostationary Operational Environmental Satellites (GOES) have traditionally been interested in things other than the sun. Since 1974, the system has been circling our planet at an altitude of about 36,000 km and is constantly providing data related to the Earth for example to forecast weather and storms.

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

An observational expedition to image the elongated solar corona took place in August and September 2018. For over a month, GOES’ Solar Ultraviolet Imager (SUVI) not only looked directly at the Sun as it usually does, but also took pictures on both sides of it.

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

By combining images from different angles of view, the instrument’s field of view can be greatly expanded, and thus, for the first time, the entire middle corona, a layer of the solar atmosphere 350,000 kilometers above the sun’s visible surface, can be imaged in infrared light. Ultraviolet.

Other spacecraft that study the Sun and collect data from the corona, such as NASA’s Solar Dynamics Observatory (SDO) and NASA’s Solar and Heliospheric Observatory and the European Space Agency’s (SOHO), look in the deeper, or higher, layers. “In the central corona region, solar research had a blind spot. GOES data now provides a significant improvement,” said Dr. Pradeep Chetta of MPS, lead author of the new study. In the central halo, researchers suspect the processes that drive and modify the solar wind.

Traveling through space at supersonic speeds

The solar wind is one of the most extensive features of our star. The stream of charged particles ejected into space by the sun travels all the way to the edge of our solar system, creating the heliosphere, the rarefied bubble of plasma that defines the sun’s field of influence. Depending on its speed, the solar wind is divided into fast and slow components.

The so-called fast solar winds, which reach speeds of more than 500 kilometers per second, originate from the interiors of coronal holes, which are regions that appear dark in coronal ultraviolet light. However, the source regions of the slow solar wind are less certain. But even the slow solar wind particles 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 greater than a million degrees need to escape from the sun to create the slow solar wind. What is the working mechanism here? Furthermore, the slow solar wind is not homogeneous, but reveals, at least in part, a ray-like structure of clearly distinguishable bands. Where and how does it arise? These are the questions the new study addresses.

Dynamic coronal lattice catching

Origin of the solar wind: This is a mosaic of images taken by the GOES SUVI instrument and SOHO coragraph LASCO on August 17, 2018. Outside the white-marked circle, LASCO’s field of view shows the currents of the slow solar wind. These connect seamlessly to the coronal web structures in the middle of the corona, which can be seen within the white-marked circle. Where the long filaments of the coronal lattice interact, the slow solar wind begins its journey out into space. Credit: Nature Astronomy, Chitta et al. / GOES / SUVI / SOHO / LASCO

In the GOES data, an area can be seen near the equator that particularly interested the researchers: two coronal holes, where the solar wind flows away from the sun unhindered, close to a region with a high-strength magnetic field. Interactions between systems like these are potential starting points for the slow solar wind.

Above this region, GOES data show elongated plasma structures in the central halo pointing diagonally outward. The team of authors refer to this phenomenon, which has now been directly imaged for the first time, as a coronal lattice. The web is in constant motion: its structures interact and reassemble.

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

The influence of the solar magnetic field

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

Using state-of-the-art computational techniques that incorporate remote-sensing observations of the sun, researchers can use supercomputers to build realistic 3D models of the elusive magnetic field in the solar corona. In this study, the team used an advanced magneto-hydrodynamic (MHD) model to simulate the magnetic field and plasma state of the corona over this time period.

Dr. Cooper Downes of Predictive Science Inc. said: “This has helped us connect the fascinating dynamics we have observed in the central halo to the prevailing theories of solar wind formation.”

As the calculations show, the coronal lattice structures follow magnetic field lines. “Our analysis indicates that the structure of the magnetic field in the central corona is imprinted on the slow solar wind and plays an important role in accelerating particles out into space,” Sheeta said. According to the team’s new findings, the hot solar plasma in the central corona flows along the open magnetic field lines of the coronal lattice. When field lines cross and interact, energy is released.

There is much to suggest that researchers are heading towards a fundamental phenomenon. “During periods of high solar activity, coronal holes often occur near the equator in very close proximity to regions of high magnetic field strength,” Sheeta said. “It is unlikely that the coronal lattice we observed is an isolated case,” he adds.

The team hopes to get more detailed insights from future solar missions. Some, like the European Space Agency’s Proba-3 mission planned for 2024, are equipped with instruments that specifically target the central corona. MPS is involved in processing and analyzing the data for this task. Combined with observational data from currently operating probes such as NASA’s Parker Solar Probe and ESA’s Solar Orbiter, which are leaving the Earth-Sun line, this will enable a better understanding of the 3D structure of the coronal lattice.

Research published in natural astronomy.

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

Provided by the Max Planck Society

the quote: Direct observations of a complex coronal web reveal important clue about the mechanism that drives the solar wind (2022, November 25) Retrieved November 25, 2022 from https://phys.org/news/2022-11-complex-coronal-web-uncover -important. html

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