For the first time, scientists have taken near-daily measurements of the Sun's global coronal magnetic field, a region of the Sun that has only been observed irregularly in the past. The resulting observations are providing valuable insights into the processes that drive the intense solar storms that impact fundamental technologies, and thus lives and livelihoods, here on Earth.
An analysis of the data, collected over eight months by an instrument called the Upgraded Coronal Multi-channel Polarimeter (UCoMP), is published today in Science.
The solar magnetic field is the primary driver of solar storms, which can pose threats to power grids, communication systems, and in-space technologies like GPS. However, our ability to understand how the magnetic field builds up energy and erupts has been limited by the challenge of observing it in the solar corona, the Sun's upper atmosphere.
Measuring the magnetism of the region through standard polarimetric methods typically requires large, expensive equipment that to date has only been able to study small segments of the corona. However, the combined use of coronal seismology and UCoMP observations makes it possible for researchers to produce consistent and comprehensive views of the magnetic field of the global corona -- the whole-Sun view one sees 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 who pursued this research as a PhD graduate at Peking University, China, and is 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.?"
The international team is made of researchers from Northumbria University, UK; NSF NCAR; Peking University, China; and University of Michigan. The research was funded by a grant from the National Natural Science Foundation of China and the National Key R&D Program of China and supported by the Newkirk graduate student fellowship awarded to Yang by NSF NCAR. The UCoMP instrument was developed with support from the U.S. National Science Foundation (NSF) and is operated by NSF NCAR at the Mauna Loa Solar Observatory.
Upgraded instrument
Although scientists have been able to routinely measure the magnetic field on the Sun's surface, known as the photosphere, it has long been difficult to measure the much dimmer coronal magnetic field. This has limited a deeper understanding of the three-dimensional structure and evolution of the magnetic field of the corona, where solar storms brew.
To measure the three-dimensional coronal magnetic fields in depth, big telescopes like NSF's Daniel K. Inouye Solar Telescope (DKIST) are needed. With a 4-meter-diameter aperture, DKIST is the world's largest solar telescope, and recently demonstrated its groundbreaking ability for making detailed observations of the coronal magnetic field. However, DKIST is not able to map the Sun all at once. The smaller UCoMP instrument is actually better-suited to give scientists global pictures of the coronal magnetic field, albeit at lower resolution and in a two-dimensional projection. The observations from both sources are thus highly-complementary to a holistic view of the coronal magnetic field.
UCoMP is primarily a coronagraph, an instrument that uses a disc to block out light from the Sun, similar to an eclipse, making it easier to observe the corona. It also combines a Stokes polarimeter, which images other spectral information such as coronal line intensity and Doppler velocity. Even though UCoMP has a much smaller aperture (20 cm), it is able to take a wider view which makes it possible to study the entire Sun on most days.
The researchers applied a method called coronal seismology to track magnetohydrodynamic (MHD) transverse waves in the UCoMP data. The MHD waves gave them information that made it possible to create a two-dimensional map of the strength and direction of the coronal magnetic field.
In 2020, a previous study used UCoMP's predecessor and the coronal seismology method to produce the first map of the global coronal magnetic field. This was a crucial step toward routine coronal magnetic field measurements. UCoMP has expanded capabilities that makes it possible to make more detailed, routine measurements. During the UCoMP study, the research team produced 114 magnetic field maps between February and October 2022, or one almost every other day.
"We are entering a new era of solar physics research where we can routinely measure the coronal magnetic field," said Yang.
Completing the picture
The observations also produced the first measurements of the coronal magnetic field in the polar regions. The Sun's poles have never been directly observed because the curve of the Sun near the poles keeps it just beyond our view from Earth. Though the researchers didn't directly view the poles, for the first time they were able to take measurements of the magnetism emitting from them. This was due in part to the improved data quality provided by UCoMP and because the Sun was near solar maximum. The typically weak emissions from the polar region have been much stronger, making it easier to obtain coronal magnetic field results in the polar regions.
As a postdoctoral fellow at NSF NCAR, Yang will continue his research of the Sun's magnetic field; he hopes to improve existing coronal models that are based on measurements of the photosphere. Since the current method used with UCoMP is limited to two dimensions, it still doesn't capture the full three-dimensional magnetic field. Yang and his colleagues hope to combine their research with other techniques to get a deeper understanding of the full vector of the magnetic field in the corona.
The third dimension of the magnetic field, oriented along a viewer's line of sight, is of particular importance for understanding how the corona is energized leading up to a solar eruption. Ultimately, a combination of a large telescope and a global field of view is needed to measure all the three-dimensional twists and tangles behind phenomena like solar eruptions; this is the motivation behind the proposed Coronal Solar Magnetism Observatory (COSMO), a 1.5-meter-diameter solar refracting telescope undergoing its final design study.
"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 co-author on 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."
