BOULDER, CO. — For observers on Earth, the Sun appears as a bright, familiar disk—but what we see is only half the story. Like the Moon, one half of the Sun is permanently hidden from our direct view: the far side beyond the visible solar limb. Yet, activity brewing there can eventually turn toward Earth, sometimes unleashing solar flares and eruptions capable of disrupting human technology.
A quarter of a century ago, scientists devised a way to detect these invisible threats before they rotate into view. Using a technique known as helioseismology, scientists analyze sound waves reverberating inside the Sun to locate large active regions forming on its hidden half. These methods can reveal the presence of sunspot groups days before they become visible from Earth.
“Helioseismology has allowed us to detect where active regions exist on the far side of the Sun,” says the lead author of this work, Dr. Amr Hamada of the U.S. National Science Foundation National Solar Observatory (NSF NSO). “However, until recently we could not determine one of their most important properties: the magnetic polarity.”
Magnetic polarity describes how magnetic fields are oriented, with positive polarity pointing outward from the Sun and negative polarity field pointing inward. This structure governs how solar magnetic fields interact with its surroundings, and whether its eruption might produce a powerful geomagnetic storm or merely a weak one.
The breakthrough comes from a new analysis of helioseismic observations collected by the NSF-NOAA Global Oscillation Network Group (NSF-NOAA GONG), built and operated by the NSO with support from the National Oceanic and Atmospheric Administration (NOAA).
“Although magnetic fields have been estimated before, the novelty here lies in the physics‑driven determination of magnetic polarities and tilt angle within the helioseismically identified active regions” explains Dr. Kiran Jain, the Lead Scientist of the NSO Far Side Project and a co-author on this study.
NSF-NOAA GONG is a worldwide network of robotic solar telescopes that continuously monitors the subtle oscillations rippling across the Sun’s surface. These oscillations—caused by waves traveling through the Sun’s interior—carry information about our star’s internal structures and magnetic features. “The Sun is constantly ringing with sound waves,” Hamada says. “By measuring how those waves travel through the solar interior, we can learn about structures both inside the Sun, and on the far side of its surface.”
“For more than two decades, the Sun’s oscillations have been used to produce far-side maps that reveal where large active regions exist. But the new work shows that the waves contain additional clues hidden in their patterns” says Dr. Alexei Pevtsov, the NSO Associate Director for NSO’s Synoptic Program responsible for NSF-NOAA GONG operations.
By examining subtle signatures known as phase shifts in helioseismic maps, Hamada and a team of scientists from the NSO, Instituto de Astrofısica de Andalucıa (Spain), and NorthWest Research Associates (USA) discovered they could infer how magnetic fields are arranged inside those far-side regions. By carefully analyzing the structure of the phase-shift signatures, and applying known dependencies (such as Hale polarity rule), we can infer the magnetic polarity of active regions even though they are not directly visible.
In essence, the Sun’s sound waves are not only showing where active regions exist—they are also revealing information about their magnetic structure.
Using this method, the researchers can reconstruct polarity-resolved magnetograms—maps showing magnetic field orientation—for regions on the Sun’s hidden hemisphere. That capability marks a major step toward a long-standing goal in solar physics: building a complete global map of the Sun’s magnetic field.
Until now, solar magnetic maps have been limited to the side facing Earth. As viewed from the Earth, the Sun takes about 27 days to complete one rotation, meaning active regions forming out of view can become geoeffective long before scientists can measure their magnetic structure directly. Incorporating far-side polarity data into global magnetic field models could dramatically improve predictions of solar activity. Such improvements are increasingly important. Solar eruptions associated with strong magnetic regions can drive space weather events capable of damaging satellites, endangering astronauts in space, and disrupting navigation, communications and the energy transport. A more complete magnetic picture of the Sun could give forecasters earlier warnings of potentially disruptive events.
Even though the far side of the Sun is completely invisible to telescopes near Earth, the Sun’s internal acoustic waves carry information from that hidden hemisphere across the solar interior. “One idea that captures people’s imagination is that we can ‘see’ the far side of the Sun using sound waves,” Hamada says. With the new technique, which combines helioseismology and known properties of magnetic fields, those solar “sounds” are now revealing even more: the invisible architecture of magnetic fields shaping the Sun’s most powerful activity.
If researchers can continue refining the method, scientists may soon achieve something once thought impossible—a continuous magnetic map of the entire Sun, including the hemisphere forever hidden from our direct view of the telescopes near Earth.
The study, “Polarity-resolved far-side magnetograms based on helioseismic measurements,” is in press in the journal Nature Scientific Reports.
