Solar Spike Suggests a More Active Sun

Radio waves are providing a new way to probe the Sun and suggest that the magnetic field of its corona may be stronger than long thought.


At about the same time the Sun was blowing off the largest flare measured in over a decade, the strength of the magnetic field in its atmosphere hit a record high.

But that may not be abnormal. New research suggests that the Sun’s magnetic field may climb to levels of intensity stronger than currently predicted, a discovery that could have implications for the effects of solar weather on Earth’s technology and infrastructure.

On 6 September 2017, solar astronomers identified a massive X9.3 flare exploding outward from a preexisting sunspot. X describes the most intense class of flare, while the associated number relates to its strength. Most flares are classified between 1 and 9.

The entire area where the X9.3 flare took place had been classified as an active region, where the strongest large-scale magnetic fields are concentrated on the Sun. The Sun also released several other powerful solar flares during the same month, many of them stemming from the same area.

“It was a very interesting active region,” said Gregory Fleishman, a heliophysicist at the New Jersey Institute of Technology in Newark.

Fleishman was part of a research team that captured measurements of the coronal magnetic field, a historically challenging part of the Sun to study. By probing the field in radio wavelengths and comparing their findings to historical observations, the team discovered that the coronal magnetic field may be more powerful than previously expected.

The team’s findings, along with its technique, may allow researchers to improve understanding of what’s happening in the solar atmosphere and how it could affect Earth. Results were published in August in the Astrophysical Journal Letters.

“The magnetic field might be stronger than people have been thinking about,” Fleishman said. “There might be more energy to drive extreme [space] weather.”

More Powerful Than Ever

The Sun’s corona is the outermost layer in the solar atmosphere. Although it is farther away from the solar center than the photosphere, the bubbling layer of plasma from where sunspots arise, it is significantly hotter.

The Sun’s corona is most easily visible during solar eclipses, and the coronal magnetic field is even more elusive. According to Alex Young, associate director for science in NASA’s Heliophysics Science Division at Goddard Space Flight Center in Greenbelt, Md., the infrared lines needed to measure the magnetic field are impeded by Earth’s temperature and atmosphere and are difficult to study even from space. Most infrared observations are taken by aircraft soaring above the terrestrial atmosphere during an eclipse.

“It’s very difficult to measure magnetic fields in the corona,” Young said.

Current models of the coronal magnetic field are based on observations of the photosphere. Young compares the limits of the model to a shag carpet. The photosphere is like the base, where the weave ties into the carpet. That’s the bulk of what scientists can probe. They can only make a guess about what’s happening at the corona, which Young compares to the surface of the carpet. Complicated physics make it difficult to model coronal processes.

“If you don’t know the details about the kind of threads in the carpet, the kind of material, you can never exactly figure out what the shape of that shag carpet’s going to look like,” Young said.

To overcome this problem, Fleishman and his colleagues turned to radio waves. As a local magnetic field increases in strength, it becomes brighter at higher radio frequencies. Using the radio telescope Nobeyama Radioheliograph, the researchers studied the Sun before and after the 6 September flare, a period of high solar activity, and were able to recover the magnetic field strength at the base of the corona. The Japanese instrument captured solar observations at higher frequencies than previously used, allowing for a more in-depth probing of coronal temperatures.

They found that the coronal magnetic field reached 4,000 gauss, twice as strong as previously reported. Because the field appeared at the highest measurable frequency, it’s possible that the field’s magnetic flux density could be even stronger.

In reviewing historical observations, the researchers found previous instances of high radio measurements that were made before the connection between radio waves and strong magnetic field strength was established. The finding suggests that such extreme fields may not be as rare as previously thought.

That could be bad news for Earth.

Occasionally, the Sun blows clouds of plasma known as coronal mass ejections (CMEs) from its surface. As the material moves through space, it can both harm astronauts and damage satellites. When CMEs or flares collide with Earth’s magnetic field, they can dump charged particles that spiral around the planet, not only creating beautiful auroral displays but also overcharging electrical grids.

Understanding and predicting space weather events have become higher priorities in recent years, and the new study offers hope of improving both.

“We can’t even begin to start thinking about predicting [space weather] until we can measure [coronal magnetic fields],” Young said.