New insights into the Sun’s corona emerge from the first successful detailed measurements of its magnetic field – crucial for understanding solar eruptions
The Sun’s corona, the outermost atmospheric layer, holds the key to understanding solar activity, including phenomena such as solar flares and space weather events. For decades, scientists have struggled with the challenge of measuring the Sun’s coronal magnetic field, because this field powers much of the energy that leads to solar eruptions.
Now, in a groundbreaking achievement, Professor Tian Hui’s research team from Beijing University, in collaboration with international experts, has made the first conventional measurements of the global coronal magnetic field. Their findings, published in the journal Science (Vol. 386, No. 6717), provide new insights into the Sun’s magnetic activity over an eight-month period.
The Sun’s magnetic field is responsible for storing and releasing energy, which heats the plasma in the corona and causes solar flares. These outbursts, in turn, could have significant impacts on space weather, potentially impacting satellite operations, GPS systems, and even human spaceflight. However, due to the relatively weak nature of the coronal magnetic field compared to the magnetic field on the Sun’s surface (the photosphere), measuring this field has proven to be a significant challenge.
The importance of coronal magnetic field measurements
As the Sun rotates, there are variations in its magnetic fields, and the ability to regularly monitor the Sun’s coronal magnetic field will improve our understanding of solar eruptions and help protect high-tech systems on Earth and in space.
Routine measurements of the photospheric magnetic field have been made over the years, but the coronal field has remained elusive. This limitation has hindered scientists’ ability to fully understand the three-dimensional magnetic field structure and the dynamical processes taking place in the Sun’s atmosphere.
In 2020, Tian Hui’s team developed a method called “two-dimensional coronal shocks”, which allowed the first measurements of the global distribution of the coronal magnetic field. This was an important milestone and marked a crucial step toward the goal of routine coronal magnetic field measurements.
More recently, Tian’s team further refined this method, allowing them to track magnetohydrodynamic shear waves in the corona with greater precision. This made it possible to diagnose the coronal density distribution and, as a result, determine both the strength and direction of the magnetic field.
Using the Upgraded Coronal Multi-Channel Polarimeter (UCoMP), the research team conducted detailed observations of the Sun’s corona from February to October 2022. During this eight-month period, they collected 114 magnetograms, or magnetic field images, that allowed them to observe the evolution of the coronal magnetic field at different altitudes and latitudes over multiple solar rotations. The magnetic field strength measured between 1.05 and 1.60 solar radii and ranged from less than 1 gauss to about 20 gauss.
These measurements allowed them to create a global map of the magnetic field intensity in the Sun’s corona. This map showed how the magnetic field evolves over time and in different parts of the sun.
Compared to the predictions of the most advanced global coronal models – such as those developed by Predictive Science, a US-based company – the team found that their observational data closely matched the model’s predictions in mid- and low latitudes. However, they noticed greater differences in areas at high latitudes and active areas of the sun.
These findings are critical for improving current models of the Sun’s magnetic activity and understanding the dynamics of solar eruptions. As lead author Yang Zihao explains, the team’s observations provide an important basis for refining and optimizing coronal models, which could ultimately lead to more accurate predictions of solar eruptions and their potential impact on Earth’s space environment.
This study marks a shift in solar physics as the field enters a new era of routine coronal magnetic field measurements.
According to Tian Hui, this achievement is just the beginning. While their current methods allow measuring the magnetic field at the edge of the solar disk, the next goal is to develop techniques that allow a complete measurement of the entire coronal magnetic field, including the solar disk itself. This will require the integration of other measurement methods and instruments, but it represents a crucial objective for the solar physics community in the coming decades.
Via Science