New era of solar observation begins with routine coronal magnetic field data

The findings, collected over an eight-month period using the Upgraded Coronal Multi-channel Polarimeter (UCoMP), are detailed in a study published today in 'Science'.

Solar storms, which can disrupt major technologies, are driven by the Sun's magnetic field. However, the challenge of observing the magnetic field in the Sun's upper atmosphere, or corona, has limited scientists' understanding of how energy builds and erupts there. Traditional methods for measuring the magnetic field in the corona require large, expensive equipment and only cover small areas.

Thanks to UCoMP and coronal seismology, researchers now have a consistent, full-Sun view of the magnetic field, similar to what can be seen during a solar eclipse. "Global mapping of the coronal magnetic field has been a big missing part in the study of the Sun," said Zihao Yang, lead author and now a postdoctoral fellow at the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR). "This research is helping us fill a crucial gap in our understanding of coronal magnetic fields, which are the source of the energy for storms that can impact Earth."

This international research effort includes scientists from Northumbria University, UK; NSF NCAR; Peking University, China; and the University of Michigan. The research was funded by the National Natural Science Foundation of China and the National Key R and D Program of China, with additional support from NSF NCAR's Newkirk graduate student fellowship awarded to Yang. UCoMP, developed with NSF support, operates at the Mauna Loa Solar Observatory.

Advancing coronal magnetic field measurement
While measurements of the Sun's surface magnetic field (the photosphere) have been routine, measuring the coronal magnetic field has been more difficult due to its dimmer nature. Instruments like the NSF's Daniel K. Inouye Solar Telescope (DKIST) have improved our understanding, but UCoMP, though smaller, provides a global view of the Sun's magnetic field that complements DKIST's higher-resolution, localized observations.

UCoMP, a coronagraph equipped with a Stokes polarimeter, blocks the Sun's light, similar to an eclipse, allowing for clearer views of the corona. Despite its smaller aperture (20 cm), UCoMP enables near-daily, whole-Sun observations. Using coronal seismology, the team tracked magnetohydrodynamic (MHD) transverse waves to map the coronal magnetic field's strength and direction in two dimensions.

In 2020, a predecessor to UCoMP produced the first map of the global coronal magnetic field. UCoMP has since expanded on that work, allowing for more detailed and regular observations. Between February and October 2022, the researchers created 114 magnetic field maps, covering almost every other day. "We are entering a new era of solar physics research where we can routinely measure the coronal magnetic field," Yang said.

Expanding solar knowledge
For the first time, scientists also took measurements of the coronal magnetic field in the Sun's polar regions, areas typically hidden from Earth's view due to the Sun's curvature. The Sun's proximity to solar maximum and improved data quality from UCoMP helped capture the magnetic fields from these regions, which are usually weak and hard to observe.

Yang will continue his research at NSF NCAR, focusing on improving coronal models based on surface measurements. Current techniques, limited to two-dimensional views, don't capture the full three-dimensional nature of the magnetic field. Yang and colleagues aim to use new methods to enhance their understanding of the corona's magnetic field, particularly its role in solar eruptions.

A more complete understanding of the magnetic field's third dimension, which aligns with the viewer's line of sight, is vital for understanding how the corona is energized. This has important implications for predicting solar eruptions. To achieve full 3D observations, scientists are planning the Coronal Solar Magnetism Observatory (COSMO), a 1.5-meter solar telescope currently in its design phase.

"Since coronal magnetism is the force that sends mass from the Sun flying across the solar system, we have to observe it in 3D - and everywhere all at once, throughout the global corona," said Sarah Gibson, COSMO Development Lead and an NSF NCAR scientist and co-author of the paper. "Yang's work represents a huge step forward in our ability to understand how the Sun's global coronal magnetic field changes from day to day. This is critical to our ability to better predict and prepare for solar storms, which are an ever-increasing danger to our ever-more technologically dependent lives here on Earth."

Research Report:Observing the evolution of the Sun's global coronal magnetic field over eight months