Surface flux transport, or SFT, models are among the most important tools solar physicists use to monitor and forecast the behavior of the Sun. These models track how magnetic flux emerges on the solar surface and how it is subsequently carried around by large-scale flows, including the differential rotation of the Sun and the meridional circulation that transports magnetic material toward the poles. By simulating the evolution of the surface magnetic field over weeks, months, and years, SFT models allow researchers to understand how solar activity builds up and decays across the roughly eleven-year solar cycle. That understanding, in turn, feeds into predictions of sunspots, flares, and coronal mass ejections, the explosive events that can send bursts of radiation and charged particles toward Earth and disrupt communications, navigation systems, and electrical infrastructure.

The central challenge the SwRI-led team set out to solve is a fundamental observational one: at any given moment, half of the Sun is facing away from Earth. Unlike the Moon, which is tidally locked and shows us the same face forever, the Sun rotates completely on its axis roughly every 27 days, so every patch of the solar surface eventually swings into view. But between those appearances, active regions can form, evolve, and decay entirely out of sight. Earlier SFT models, lacking simultaneous data from the hidden hemisphere, were often caught off guard. In some cases, as the Sun rotated, a sunspot would emerge unexpectedly from the far side, forcing researchers into quick recalculations of their magnetic maps and degrading the reliability of forecasts built on them.

To fill this observational gap, scientists have since the 1970s turned to helioseismology, a technique that studies the Sun’s interior and its far side using acoustic waves. Just as geologists probe what lies deep beneath the Earth’s crust by analyzing how seismic waves travel through the planet, solar physicists analyze sound waves reverberating through the Sun’s interior. Active regions on the far side perturb these acoustic signals in characteristic ways, allowing researchers to infer the presence of magnetically active areas before they rotate into view. Helioseismic far-side imaging has led to vast improvements in space weather forecasting, giving forecasters days of advance warning about emerging sunspot groups that would otherwise appear without notice.

Yet even helioseismic models have been hampered by bad information. The acoustic signatures that betray an active region are not always what they seem. Dr. Bibhuti Jha, a research scientist in SwRI’s Solar System Science and Exploration Division in Boulder, Colorado, and the lead author of the new paper, described the problem in vivid terms. Acoustic data can sometimes indicate an active region on the far side, he explained, but when the area finally comes into view, nothing is there. It is almost like a false positive, and the team calls these phantom features ghost active regions. When such false detections are folded into an SFT model, they inject spurious magnetic flux that then propagates through the simulation, contaminating the map of the solar surface and any forecasts derived from it.

The new framework tackles this problem directly by introducing a probabilistic treatment of far-side detections. Rather than accepting every helioseismic detection at face value, the model systematically evaluates the accuracy of each candidate active region before it is incorporated into the surface flux transport simulation. Dr. Lisa Upton, an SwRI lead scientist also based in the Boulder office and a co-author of the paper, explained the approach: before far-side helioseismology data are added to the model, the team associates a probability with each far-side active region based on its properties. Detections with characteristics that closely match confirmed active regions earn high confidence, while ambiguous or weak signals are weighted accordingly or filtered out. The result is a model that weeds out ghost active regions and delivers a more accurate map of the entire solar surface at any given time.

To build and validate this framework, the team studied near-simultaneous maps of the whole solar surface from 2010 to 2014, a period that spans the rise to the maximum of Solar Cycle 24. During this window, observations from Earth-based instruments could be combined with data from space-based assets positioned to view the opposite hemisphere, providing ground truth against which far-side helioseismic detections could be checked. By comparing what the acoustic methods predicted with what was actually observed when each region rotated into view, the researchers could quantify the reliability of far-side detections and calibrate their probabilistic assignments. The resulting SFT models are more complete and accurate than any previous effort, according to the team.

The significance of this work extends well beyond the technical details of magnetic cartography. Solar activity drives space weather, and space weather is increasingly recognized as a practical hazard with economic and security implications. Strong solar flares can degrade high-frequency radio communications used by aviation and maritime operations, while coronal mass ejections can induce currents in long power lines and damage transformers, as demonstrated by historic geomagnetic storms. Satellites in Earth orbit face elevated radiation risks during active periods, and crewed missions beyond low Earth orbit require accurate forecasts to protect astronauts from energetic particle events. A more faithful representation of the Sun’s global magnetic field, including the half that cannot be seen directly, gives forecasters a firmer foundation for anticipating when and where the next eruption might occur.

The probabilistic framework also provides a practical pathway for incorporating helioseismic data into SFT models more broadly, something that has been difficult to do rigorously in the past because of the noise and false positives inherent in far-side imaging. By treating each detection as a probability rather than a certainty, the method allows models to benefit from far-side information without being misled by its errors, a balance that previous approaches struggled to strike. Jha characterized the advance as a step toward a more realistic modeling of the magnetic field of both the visible and invisible hemispheres of the Sun, a goal that has motivated solar physicists for decades.

Looking ahead, the team hopes that better surface maps will translate into better predictions of the solar cycle itself. As Jha noted, modeling the surface of the Sun to see how the magnetic field behaves helps scientists understand how solar activity varies over the cycle, and if the next solar cycle can be plotted out in advance, researchers will have a much better idea of how the Sun is going to behave in the coming years. The study, titled A Probabilistic Framework for Incorporating Helioseismic Far-Side Active Regions into SFT Models, was published on October 8, 2026, in The Astrophysical Journal Supplement Series, and it arrives at a moment of growing public and governmental attention to space weather readiness. As society’s dependence on technology vulnerable to solar storms continues to grow, tools that make the Sun’s hidden half visible, even probabilistically, represent a meaningful stride toward a more predictable star.

Subject of Research: Probabilistic incorporation of helioseismic far-side active region data into solar surface flux transport models

Article Title: SwRI-led team develops improved model for mapping the Sun

Article References: SwRI-led team develops improved model for mapping the Sun. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Sun, solar magnetic field, surface flux transport, helioseismology, far-side imaging, space weather, solar cycle, sunspots, SwRI, solar forecasting, active regions, The Astrophysical Journal Supplement Series

The heliospheric current sheet, or HCS, is one of the largest structures in the solar system, and yet many people have never heard of it. It is an undulating, warped surface that emanates from the Sun and spirals outward past the planets and beyond, effectively dividing the heliosphere into two vast hemispheres. In one hemisphere, the Sun’s magnetic field points away from the Sun; in the other, it points toward it. The HCS is the boundary between these oppositely directed fields, and because the Sun rotates, the sheet is twisted into a shape that scientists have long compared to a ballerina’s spinning skirt, its folds sweeping past Earth and every other planet as they spiral into the far reaches of the solar system. Anchored deep in the solar surface, the current sheet acts, in the words of the SwRI team, like a high-speed pipeline, carrying information about conditions in the innermost corona directly into the space environment around us.

Dr. Keiichi Ogasawara, lead author of the new study, describes the problem the HCS helps to solve as a cosmic paradox. For decades, scientists have known that the Sun blasts a continuous supersonic torrent of charged particles into space, but the precise mechanism by which the corona, the Sun’s outermost atmospheric layer, feeds and accelerates that torrent has remained stubbornly elusive. The key, he argues, lies in magnetic connectivity. By tracing magnetic field lines from a spacecraft back to the solar surface, researchers can link activity in a specific region of the Sun directly to gusts of solar wind measured millions of miles away. The HCS offers a particularly valuable connection point for this kind of tracing, because its magnetic signature is so distinctive: the polarity flip from one hemisphere to the other is unmistakable, allowing scientists to anchor their measurements to a known feature of the Sun’s global magnetic architecture.

The opportunity to study this structure up close arrived when Solar Orbiter, a joint mission of ESA and NASA, passed through a fold of the heliospheric current sheet at a distance of roughly 26 million miles from the Sun. That is closer to our star than the innermost planet, Mercury, and it meant the spacecraft was sampling what researchers describe as the youngest version of the solar wind ever observed. At such distances, the wind has had little time to evolve, mix, or be reshaped by interactions in transit, so its composition still bears the imprint of the coronal processes that created it. Solar Orbiter’s suite of high-quality field, plasma, and composition instruments was able to dissect the particle populations within this mysterious region with a fidelity that no previous mission at such proximity has achieved.

What the SwRI researchers found inside the current sheet was a clear and distinct change in the composition of the plasma, specifically in the relative abundance of ions, atoms that have lost or gained electrons and therefore carry an electric charge. Within the HCS region, the team identified a decrease in the ratio of iron to oxygen ions, and that decrease lined up closely with the magnetic sector boundary itself. According to Ogasawara, while the overall plasma on both sides of the boundary is similar, there is a clear compositional change that is tightly aligned with the point where the magnetic polarity flips. This alignment is significant, because it suggests that the heliospheric current sheet is not merely a geometric feature of the Sun’s magnetic field. Instead, it appears to be linked to the way the Sun sorts ions in the corona and releases them into the solar wind, hinting at physical processes of fractionation and release operating in the Sun’s lower atmosphere.

The implications of that finding extend well beyond the current sheet itself. If the HCS carries a compositional fingerprint of its coronal source regions, then measuring that fingerprint near the Sun provides a way to probe processes that are otherwise hidden from view. The corona is heated to temperatures of millions of degrees, far hotter than the Sun’s visible surface, and understanding exactly how energy and matter flow from the solar interior through the corona and out into the wind remains one of the central unsolved problems in solar physics. Compositional signatures, such as the iron-to-oxygen ratio measured by Solar Orbiter, act as tracers of those flows, recording which material was preferentially accelerated and escape, and under what magnetic conditions. A boundary that flips polarity and simultaneously changes composition is a boundary that models of coronal heating and wind acceleration must take seriously.

Importantly, the SwRI team is careful about what their data can and cannot claim. The new observations do not define the origins of the heliospheric current sheet, and the researchers do not present them as proof of any single theory of how the sheet forms. What the study does provide, they argue, are boundaries, clear constraints that future models and theories must satisfy. Ogasawara emphasizes that the team has delivered a detailed, multi-aspect view of an HCS crossing close to the Sun, and that this view includes organized, measurable variations in the types and amounts of particles within the plasma. The current sheet, in other words, is not just a simple flip of the magnetic field. Any successful future model of current sheet formation, solar wind heating, or magnetic connection between the Sun and interplanetary space will now have to explain these compositional structures alongside the magnetic ones.

This constraint-driven approach reflects a broader shift in how heliophysicists tackle problems that are too complex to solve with any single spacecraft observation or theoretical framework. Rather than betting on one mechanism for how the corona becomes the wind, researchers increasingly map out the space of possibilities, using high-fidelity measurements to eliminate models that cannot reproduce what is actually observed. A close-up crossing of the HCS, with simultaneous measurements of magnetic fields, plasma waves, particle distributions, and ion composition, is exactly the kind of dataset that can prune that space dramatically. The study, titled “Resolving Compositional Features of Solar Wind Source Regions near the Heliospheric Current Sheet at 0.3 au,” was published this month in The Astrophysical Journal, and its observations at 0.3 astronomical units, roughly the distance at which Solar Orbiter encountered the sheet, represent some of the tightest constraints yet available on the young solar wind’s composition.

There is also a practical payoff for life on Earth. Space weather, the collective term for the Sun-driven disturbances that sweep through the heliosphere, can degrade satellite electronics, endanger astronauts, interrupt GPS navigation, and, in extreme cases, overload terrestrial power infrastructure. Forecasters rely on models of the solar wind to predict when fast streams and magnetic disturbances will reach Earth, and those models are only as good as our understanding of how the wind is born and structured near the Sun. By defining the early-stage composition of the heliospheric current sheet, the SwRI study helps scientists construct more accurate models of how the solar wind evolves and how its gusts will affect Earth’s magnetic environment. Every refinement of that predictive chain, from the corona to the current sheet to the forecast of a geomagnetic storm, translates into better protection for the technological systems modern society depends on.

The work also sets the stage for what comes next. Solar Orbiter continues its mission, gradually tilting its orbit to deliver increasingly direct views of the Sun’s polar regions, while NASA’s Parker Solar Probe flies even closer to the solar surface, sampling the corona’s outer reaches directly. Together, these missions are building a three-dimensional, compositionally resolved picture of the inner heliosphere that earlier generations of scientists could scarcely imagine. The SwRI team’s demonstration that the heliospheric current sheet carries an organized, measurable compositional signature near the Sun gives future observers a specific target: repeated crossings of the sheet at varying distances and solar latitudes, each one an opportunity to test whether the iron-to-oxygen depletion they observed is a persistent feature of the boundary or a fleeting product of particular solar conditions. Either answer will sharpen our understanding of the paradox that started it all, how the Sun, day after day, converts its own atmosphere into a million-mile-per-hour wind that shapes the very bubble of space we live in.

Subject of Research: Compositional structure of the heliospheric current sheet and its role in the transformation of the solar corona into the solar wind

Article Title: SwRI study helps unravel how the Sun’s corona transforms into solar wind

Article References: SwRI study helps unravel how the Sun’s corona transforms into solar wind. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: solar wind, heliospheric current sheet, Solar Orbiter, solar corona, space weather, Southwest Research Institute, ion composition, magnetic polarity, heliophysics, The Astrophysical Journal, solar magnetic field, heliosphere

The study, published in the journal Solar Physics, was carried out by V. N. Obridko of the Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN), M. M. Katsova of the Sternberg Astronomical Institute at Lomonosov Moscow State University, and D. D. Sokoloff of the Faculty of Physics at Lomonosov Moscow State University. Their work draws on the full record of soft X-ray flare monitoring by the Geostationary Operational Environmental Satellites, or GOES, which began in 1975 and has provided the standard classification of solar flares ever since. That continuous, homogeneous dataset is what makes a statistically meaningful look at flare behavior across multiple cycles possible.

Solar flares are classified by their peak soft X-ray brightness as measured by the GOES instruments, with X-class flares representing the most powerful events, each releasing energy equivalent to billions of megatons of TNT. The researchers focused on the great flares, from the relatively moderate X4 class up to the extreme X10 class and beyond, and asked a deceptively simple question: where on the solar disk do these events occur, and how does their location and frequency change as the solar cycle progresses? To answer it, they traced the latitudinal distribution of X-class flares throughout four cycles and compared the flare positions directly with the structure of the Sun’s large-scale magnetic field.

The picture that emerges is one of overlapping magnetic waves. Over the course of a solar cycle, the zones where sunspots and active regions appear migrate steadily from mid-latitudes toward the equator, a pattern known since the nineteenth century as Spörer’s law. At the same time, a second wave of activity belonging to the next cycle begins at higher latitudes and propagates poleward. The new analysis shows that the relatively weak members of the great-flare family, those in the X4 to X7 range, tend to occur at relatively high latitudes of roughly 15 to 20 degrees, precisely where these two waves of activity converge: one moving equatorward as the current cycle matures, and the other, the wave of the following cycle, directed poleward. These events are observed almost continuously during the one to two years before the cycle maximum.

The most powerful flares, however, tell a different story. The total number of powerful X-ray flares increases sharply during the maximum phase of the cycle, as expected from the sheer abundance of magnetically complex active regions. But the greatest events, those of X10 class and stronger, are largely absent during the growth phase of the cycle. Instead, they begin to appear one to two years before the maximum number of sunspots is reached, during the overlapping phase when different kinds of activity waves coexist on the Sun at relatively high latitudes. They then continue to appear throughout the declining phase, at the boundary that separates the wave of local fields from the poleward wave of the following cycle.

This timing is significant because it challenges the simplest expectation that flare intensity should peak exactly when sunspot numbers do. The researchers’ conclusion is that the greatest X-ray flares are not merely a byproduct of having many sunspots, but are tied to a specific magnetic configuration that arises when the outgoing and incoming activity waves overlap. In dynamo theory, the solar cycle is understood as a propagating wave of magnetic field generated by the interplay of turbulent plasma motions and the Sun’s rotation, and the interaction zones between successive waves provide conditions in which magnetic fields of opposite polarity and different origins can be forced together, building up the enormous stresses that power the largest eruptions.

The technical foundation of the analysis rests on comparing flare positions with synoptic maps of the photospheric magnetic field, which chart the distribution of magnetic flux across the solar surface over each solar rotation. The magnetic field data were drawn from the synoptic program at Stanford University’s Wilcox Solar Observatory, while sunspot numbers came from the World Data Center SILSO at the Royal Observatory of Belgium. By overlaying the latitudes of great flares on these magnetic structures, the team could show that flare sites are not randomly distributed within the activity belts but are preferentially located at the interfaces between magnetic regimes, where the wave of local active-region fields meets the large-scale poleward-migrating field of the next cycle.

The practical implications of this finding reach well beyond solar physics. Great X-class flares, particularly when accompanied by coronal mass ejections, are the primary drivers of space weather, capable of disrupting satellite operations, radio communications, GPS navigation, and power grids on Earth. The May 2024 G5-level geomagnetic storm, triggered by a barrage of eruptions from a single enormous active region, offered a vivid recent reminder of how much is at stake. If the largest flares preferentially occur in the years around and after sunspot maximum, and especially at the magnetic boundaries identified in this study, then space-weather forecasting could gain a valuable statistical tool: the periods of greatest extreme-flare risk may extend well past the sunspot peak, rather than ending with it.

The results also connect to a broader body of research on the extended solar cycle and the large-scale magnetic field. Earlier work by members of the same team has traced cyclic variations in the main components of the Sun’s large-scale magnetic field and examined the asymmetry introduced by the extended cycle, in which traces of the next cycle appear years before the current one fades. The new flare analysis adds an independent observational thread to this picture, suggesting that the overlap of activity waves is not a subtle theoretical curiosity but a physically consequential configuration that leaves a measurable imprint on the most energetic events in the solar system.

For now, the study’s conclusions rest on four cycles of GOES data, a sample that is large enough to reveal the pattern but still limited in the sense that each solar cycle has its own character. Some cycles are strong and some are weak, and the recalibration of the greatest GOES soft X-ray flare measurements across two Hale cycles remains an active area of research. Nevertheless, the message of the new work is clear and testable: to understand when the Sun will unleash its very worst, astronomers need to look not only at how many sunspots dot the surface, but at where two great waves of solar magnetism meet and mingle. As the current cycle progresses toward and past its maximum, the boundaries identified in this study will be among the most closely watched regions on the Sun.

Subject of Research: Cycle-dependent latitudinal occurrence of great soft X-ray solar flares in relation to overlapping solar magnetic activity waves

Article Title: Cycle Variation in the Occurrence of Great Soft X-ray Solar Flares

Article References: Obridko, V. N., Katsova, M. M., & Sokoloff, D. D. (2026). Cycle Variation in the Occurrence of Great Soft X-ray Solar Flares. Solar Physics, 301(9), Article 140. https://doi.org/10.1007/s11207-026-02732-z

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02732-z

Keywords: solar flares, X-class flares, solar cycle, space weather, solar magnetic field, GOES, solar dynamo, sunspots, Solar Physics, coronal mass ejections, activity waves, solar maximum