The Sun isn’t just a steady lamp in the sky. It’s a dynamic machine that hurls out storms, accelerates particles to near-light speeds, and sends signals across the solar system. One of the most surprising signals is gamma radiation that appears on the Sun’s visible disk even when the eruption happens on the far side of the Sun. On 9 September 2024, a backside solar eruption lit up the Sun’s front like a hidden engine suddenly switching on. The glow came from gamma rays detected by the Fermi Large Area Telescope (LAT), a discovery that challenged simple pictures of how solar explosions unleash high-energy particles. A multinational team led by Nat Gopalswamy at NASA Goddard Space Flight Center, with collaborators across UC Berkeley, Kiel University, FHNW, and CAU, used a constellation of spacecraft to trace the chain of events from the blast to the gamma rays on the Sun’s surface.
What makes this study compelling isn’t just the data deluge from a hundred instruments; it’s the story it tells about the Sun’s “shock engine.” The gamma rays are a fingerprint of protons accelerated to GeV energies, a process long associated with shocks driven by coronal mass ejections (CMEs). But the mystery has always been where those protons come from and how they illuminate the visible Sun when the eruption is occulted. The 2024 event provides a clean, multi-instrument case that supports a single, elegant picture: a CME-driven shock around a colossal flux rope accelerates particles enough to bathe the front side of the Sun in gamma rays, even when the eruption itself sits behind the limb.
What behind-the-limb gamma rays reveal about solar shocks
The central question is deceptively simple: how can gamma rays appear on the Sun’s disk when the eruption is hidden behind its edge? Gamma rays at energies above 100 MeV arise when protons collide with solar material and create neutral pions that decay into gamma photons. Those protons must have energies above roughly 300 MeV, which points to acceleration by a powerful, extended shock rather than a fleeting flare site. In a behind-the-limb event, flare-related gamma rays would have trouble lighting up the front side; what we need is a mechanism that can sling energetic protons along the shock front and deposit them onto the solar surface from the outside in the right place and time.
The 2024 September 9 eruption delivered exactly that: an ultrafast CME, with a peak speed around 2,160 km/s and an impulsive early acceleration of about 3.54 km/s², launched a shock that extended well into interplanetary space. The gamma-ray emission started around the soft X-ray peak of the associated flare and persisted for roughly 1.75 hours, with a temporal relationship to a long-duration interplanetary (IP) type II radio burst. This radio signature, produced by the same shock wave, mirrors what one would expect if the shock were efficiently accelerating protons over an extended interval. In short: the shock was not a quick flash but a sustained engine, capable of driving protons to GeV energies long enough to light up the Sun’s surface on the visible disk.
One of the paper’s striking achievements is linking the gamma rays to the 3D structure of the CME. Using a forward-modeling technique called Graduated Cylindrical Shell (GCS) fitting, the team reconstructed the CME as a large flux rope that deflected northward by about 26 degrees and rotated as it expanded. The leading-edge height, deprojected speed, and the geometry of the flux rope all harmonize with a scenario in which the CME’s shock sheath surrounds the flux rope and extends to the front side of the Sun. That’s the geometry you need for electrons and protons accelerated at the shock to illuminate the visible disk, even when the eruption originates far behind the limb.
The Gamma-Ray emission itself, mapped against the particle data from Solar Orbiter’s EPD (Energetic Particle Detector), shows a hard proton spectrum with a fluence index around 2.29, indicating a substantial population of high-energy protons. The SolO detector also captured protons up to about 1 GeV in the associated SEP event, a crucial piece of evidence that the Sun’s shock had the power to supply the protons responsible for the gamma rays. Taken together, the observations paint a coherent picture: a robust, shock-driven acceleration process operating in the CME’s early life and continuing as the CME expands and interacts with the solar wind.
A fleet of observers across the solar system
The breadth of this study rests on a veritable parade of spacecraft and ground instruments. SolO’s STIX provided on-disk X-ray imaging of the flare, even though the eruption was predominantly backside. Fermi’s LAT stood watch for the gamma rays on the visible disk, capturing the sustained emission as the eruption evolved. In parallel, SDO/AIA and STEREO/EUVI traced the eruption’s evolution on the Sun’s surface and in three dimensions, while_SOHO/LASCO_ and STEREO/COR offered white-light views of the CME as it erupted and expanded.
Radio observations formed another critical pillar. Ground-based networks like e-CALLISTO detected the metric type II burst that starts around 05:07 UT and evolves into a DH (decameter-hectometric) burst, echoing the presence of a shock that travels through the corona and into interplanetary space. Wind/WAVES and PSP/FIELDS extended the radio story into even lower frequencies, giving a planetary-scale map of the shock’s reach. The team’s synthesis shows a single driver—the CME-driven shock—producing coherent signatures across gamma rays, X-rays, radio bursts, and energetic particles. It’s a rare win for multi-messenger solar science: different messengers, same engine, telling the same tale from different angles.
The analysis also underscores an important practical point: the event’s timing is intricate. The SGRE on-disk emission began around the time the EUV wave crossed onto the front side, a hint that the tilting and deflection of the flux rope brought the shock into a geometry that could illuminate the visible disk. The authors carefully align the SGRE onset with the flare’s soft X-ray peak and the IP type II burst, acknowledging a small data-gap in the Fermi/LAT coverage but arguing for a consistent, shock-based origin. In other words, even when the eruption is hidden, the Sun’s own surface still bears the mark of its shock-driven particle accelerator.
What’s particularly striking is the consistency of this event with the broader catalog of behind-the-limb (BTL) SGRE eruptions. When researchers compare the 2024 September 9 event with six others, the pattern stands out: BTL events are well explained by energetic shocks driving CMEs, with gamma rays riding along the same shock system that accelerates solar energetic particles. And there’s a curious east-versus-west asymmetry in the source locations, suggesting complex magnetic connectivity and CME geometry may steer where the gamma rays appear on the Sun’s disk. The picture that emerges across events is not of a fluke but of a recurring, shock-dominated mechanism.
Why this changes our view of solar storms and space weather
For decades, solar physicists have debated whether gamma rays in behind-the-limb events were a flare-driven signature or a signature of a fast, efficient particle accelerator in the CME’s wake. The 2024 September 9 SGRE study tips the balance in favor of the latter: the shock surrounding the CME flux rope acts as a persistent engine capable of accelerating protons to GeV energies and delivering them to the front side of the Sun long after the flare’s impulsive phase. This isn’t just a parade of data points; it’s a coherent, physically grounded narrative that ties together gamma rays, SEPs, radio bursts, and the three-dimensional geometry of a CME.
The implications extend beyond academic curiosity. If behind-the-limb gamma rays are the signature of a strong, sustained CME-driven shock, then gamma-ray observations provide a new diagnostic for the most energetic solar events. They become a telling proxy for shock strength, the efficiency of particle acceleration, and the potential for high-energy solar energetic particle (SEP) events that can impact space travelers and spacecraft. In practical terms, this work strengthens the bridge between solar physics and space weather forecasting: a backside eruption with a powerful shock and a hard SEP spectrum isn’t just a distant curiosity; it’s a forewarning that a robust SEP population could ride the CME’s wake and influence near-Earth space.
Another takeaway is methodological. The study demonstrates what you can do when you assemble a planetary-scale observatory: solve a puzzle by fitting a 3D CME model to multi-wavelength images, cross-checking with in-situ particle spectra, and anchoring the timing with X-ray and radio bursts. It’s a template for future investigations—an approach that will become increasingly essential as new missions join the solar fleet. And it’s a reminder that the Sun, for all its familiar grandeur, still has surprises that require teamwork across instruments, agencies, and continents to illuminate.
Finally, the work speaks to a broader scientific culture that values cross-cutting data synthesis. The authors—Nat Gopalswamy and colleagues at NASA Goddard, with partners across The Catholic University of America, UC Berkeley, Kiel University, and FHNW—show how a shared scientific language and coordinated observations can turn a seemingly paradoxical phenomenon into a coherent, testable physical picture. The Sun’s shock engine isn’t an abstract concept; it’s a mechanism that we can now observe, model, and compare across events. It’s a sign that solar physics is entering an era where large collaborations and diverse data streams are not just nice-to-haves but essential tools for understanding the most energetic processes in our solar neighborhood.
At the heart of the study, the 2024 September 9 event is a telling reminder that behind-the-limb gamma rays are not a puzzle about the Sun’s interior. They are a story about the Sun’s outer atmosphere—its shock waves, magnetic geometry, and the way a single eruption can cascade into a hemisphere-spanning dance of particles. It’s a story about the Sun’s capacity to accelerate particles to GeV energies in a way that leaves an indelible mark on the visible disk—an astrophysical parable written in gamma rays, X-rays, radio waves, and energetic protons, all tuned by the shape and motion of a colossal CME.
As the team notes, this event sits in a broader ensemble of BTL (behind-the-limb) gamma-ray eruptions. If you stand back and look at the larger pattern, the Sun’s gamma-ray production appears to be a natural outgrowth of a CME-driven shock that envelops the flux rope, rather than a freak occurrence tied to a single impulsive flare. The 2024 September 9 eruption is a measured but poignant reminder: the Sun’s most extreme particle accelerators reveal themselves not with a single flash, but with a sustained, orchestrated performance that science can now watch in near real time across a planetary orchestra of observatories.
The study was conducted by researchers at NASA Goddard Space Flight Center, with contributions from The Catholic University of America, University of California, Berkeley, Kiel University, and FHNW. The lead author is Nat Gopalswamy, and the paper includes a chorus of co-authors who helped bring together the diverse data streams that made the narrative possible. It’s a collaboration that exemplifies how contemporary space science works: dozens of datasets, cross-validated by physics, stitched together to reveal how the Sun truly operates at its most energetic moments.
