The Sun's corona, its outermost atmospheric layer, holds the key to understanding solar activity, including phenomena such as solar flares and space weather events. For decades, scientists have faced the challenge of measuring the Sun's coronal magnetic field, since this field drives much of the energy that drives solar flares.
Now, in a groundbreaking achievement, Professor Tian Hui's research team from Peking 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 (Volume 386, No. 6717), offer 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 triggers solar flares. These eruptions, in turn, can have significant impacts on space weather, potentially affecting 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 of 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 magnetic fields and the ability to regularly monitor the Sun's coronal magnetic field will improve our understanding of solar flares and help protect high-tech systems on Earth and in space.
Over the years, routine measurements of the photospheric magnetic field have been made, but the coronal field remains elusive. This limitation has impeded scientists' ability to fully understand the three-dimensional structure of the magnetic field and the dynamic processes that occur in the Sun's atmosphere.
In 2020, Tian Hui's team developed a method called “two-dimensional coronal shocks,” which enabled the first measurements of the global distribution of the coronal magnetic field. This was an important milestone, marking a crucial step towards 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 enhanced multi-channel coronal 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 images of the magnetic field, allowing 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.
With these measurements they were able to create a global map of the intensity of the magnetic field in the solar corona. This map revealed how the magnetic field evolves over time and in different regions of the Sun.
When compared to the predictions of the most advanced global coronal models, such as the one developed by Predictive Science, a US-based company, the team found that their observation data closely matched the model's predictions in regions of latitudes medium and low. However, they observed greater discrepancies in high-latitude regions and active areas of the Sun.
These findings are critical to improving current models of the Sun's magnetic activity and understanding the dynamics of solar flares. As lead author Yang Zihao explains, the team's observations provide a key foundation for refining and optimizing coronal models, which could eventually lead to more accurate predictions of solar flares 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 measurement of 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 tools, but represents a critical goal for the solar physics community in the coming decades.
Through Science