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

Auroras occur when charged particles from the Sun, funneled along Earth’s magnetic field lines, collide with atoms and molecules in the upper atmosphere. Oxygen atoms glow green and red at altitudes between roughly 100 and 500 kilometers, while nitrogen contributes blues and purples. These displays are most common inside the auroral ovals that encircle the magnetic poles, but during powerful geomagnetic storms, when coronal mass ejections slam into Earth’s magnetosphere, the ovals expand equatorward and the lights become visible at middle and even low latitudes. Because the frequency and intensity of such storms follow the roughly 11-year solar cycle, historical aurora sightings serve as a proxy record of solar behavior, complementing the sunspot number, the longest-running direct dataset of solar observations.

The Extremadura newspaper, founded in 1923, is one of the oldest and most recognized media outlets in its region, and it was never intended as a scientific instrument. Yet its complete run, digitized in PDF form with help from the newspaper’s staff, allowed the team to run optical character recognition searches using more than a dozen geophysically relevant keywords, including the Spanish term for the phenomenon, aurora, along with related words such as phenomeno and the less common phrase for northern lights. The same systematic approach had previously yielded records of a super-bolide, a meteorite fall, and an exceptional month of thunderstorms, demonstrating that regional newspapers can be mined as surprisingly rich documentary archives.

The recovered articles fall into two categories: 12 items describing direct observations and 19 offering general reports and scientific explanations. The direct sightings cluster around the great geomagnetic storms of the 20th century, with notable events in 1926, 1938, 1950, 1956, 1957, 1958, and 1991. Crucially, when the researchers plotted these dates against the smoothed sunspot number, the auroras lined up with the maxima of solar cycles 16 through 19 and cycle 22. That correlation matters, because no systematic catalog of auroras observed in the Iberian Peninsula during the 20th century previously existed; the newspaper collection now fills that gap.

The earliest event, the geomagnetic storm of January 1926, generated two news items. One briefly reported a brilliant aurora seen for several hours over the United States, while a longer piece published that July described a formidable magnetic storm that disrupted telegraph, telephone, and submarine cable communications for hours. The article recounted how the Norwegian physicist Carl Störmer measured auroral rays reaching 72 degrees of elevation in the Oslo sky, implying altitudes of about 503 kilometers. It described the spectacle unfolding from a yellow-greenish arc in the north into an immense corona of dazzling red rays, visible from Sweden and Denmark to France, and noted that observatories in Meudon and Ebro recorded the accompanying magnetic disturbance. The sunspot group responsible was estimated to span roughly 100,000 kilometers.

Perhaps the most dramatic event in the collection is the great aurora of 25 to 26 January 1938, remembered as the Fatima Storm because some Roman Catholics linked it to the Fatima prophesies. The newspaper reported that the Astronomical Observatory of San Fernando confirmed an aurora of varying intensity recorded from 23:00 until 02:00, with maxima at 21:00 and around 23:30, and that magnetic instruments in the city had shown alterations for days beforehand. Dispatches from Berlin, Rome, and Warsaw described the display stretching from Scandinavia to the Mediterranean, with multicolored lines on a dark red background. Modern estimates place the storm’s intensity at roughly minus 336 nanotesla on the Dcx index, with a maximum Kp index of 9. The storm cut transatlantic radio communications and even delayed trains in England whose signaling apparatus malfunctioned. In war-torn Spain, citizens and soldiers unfamiliar with the phenomenon interpreted the blood-red sky as a presage of greater tragedy.

The 1950s, near the peak of the strongest solar cycle ever recorded, produced a flurry of sightings. In February 1950, the sky turned red over the Extremaduran town of Hervás, and the newspaper noted that some timorous residents considered it a harbinger of war. In January 1957, red glows were reported from Seville, Huelva, Ávila, Lugo, Monforte de Lemos, and Salamanca, lasting about 20 minutes in Seville, while in Lugo firefighters mobilized for hours believing the aurora was a large fire. That storm reached a minimum Dst index of minus 250 nanotesla. In February 1958, the crew of the German ship Beate Bolten sighted an aurora near the Panama Canal, the closest to the Equator recorded during the International Geophysical Year, and the newspaper noted that German captains had spotted about a dozen auroras around Spain during that period. A September 1958 storm, with a Dst minimum near minus 302 nanotesla, produced spectacular rayed bands and pulsating red surfaces across Germany and France.

The most recent major event in the archive is the great red aurora of November 1991, one of the largest geomagnetic storms on record with a Dst peak of minus 354 nanotesla. The newspaper described luminous phenomena in the Huesca Pyrenees, Lleida, and Zaragoza that residents repeatedly reported to the fire brigade as fires, a telltale signature of red auroral light at low latitudes. An astronomer at a local agrometeorological center explained that even trained observers initially mistook the glow for flames. The analysis shows that such misunderstandings are not anecdotal quirks but recurring social responses: in 1957, 1991, and likely 1950, emergency services or frightened citizens mistook auroras for conflagrations or omens. For social scientists, the archive offers a century-long record of how geomagnetic storms rippled through public opinion.

Not every sighting survived scrutiny. A 1952 item described concentric rainbow-colored arcs over Hervás, but the researchers judged it highly improbable to be an aurora, since concentric arcs are uncharacteristic of the dynamic, changing forms auroras display, rainbow hues are physically implausible at Extremadura’s latitude of 38 to 40 degrees where red is the expected color, and no geomagnetic storm is documented around that date. The general reports in the collection also trace the evolution of auroral science itself, covering explanations by Spanish meteorologists in the 1950s and 1970s, Soviet and French attempts to create artificial auroras in 1974, NASA’s THEMIS mission of 2007, and aurora-hunting expeditions to Greenland, Iceland, and Norway in the 2010s.

The broader significance of the work lies in what it says about reconstructing the Sun’s past. Space weather is not merely a curiosity; severe geomagnetic storms can damage power grids, satellites, and communication systems, and understanding their historical frequency is essential for assessing future risk. Because instrumental magnetic records extend back barely a century and a half, historians of the Sun rely on proxies such as aurora sightings, sunspot drawings, and even radiocarbon spikes in tree rings. The Extremadura study demonstrates that local newspapers from low-latitude regions, where auroras are rare precisely because only exceptional storms produce them, are especially valuable: every sighting they record is, by definition, evidence of an extraordinary solar event. A comparison with previous surveys of newspaper space-weather reporting suggests the coverage in Extremadura is entirely typical, meaning similar archives almost certainly await discovery in other regional papers worldwide, each yellowed page a potential data point in the long history of our stormy star.

Subject of Research: Historical aurora records from a Spanish newspaper as proxies for 20th-century solar activity

Article Title: Aurora records in the Spanish newspaper Extremadura for the period 1923–2017

Article References: Sánchez Romero, C., Díaz-Condiño, L., Tovar Hernández, I., Pérez Aparicio, A. J., Sánchez Carrasco, V. M., Gallego Herrezuelo, M. C., & Vaquero, J. M. (2025). Aurora records in the Spanish newspaper Extremadura for the period 1923–2017. History of Geo- and Space Sciences, 16(2), 13-22. https://doi.org/10.5194/hgss-16-13-2025

Image Credits: AI Generated

DOI: 10.5194/hgss-16-13-2025

Keywords: aurora borealis, geomagnetic storms, solar activity, sunspots, space weather, newspaper archives, Extremadura, Iberian Peninsula, solar cycle, history of science, International Geophysical Year, Aurora

The challenge in answering this question lies in the brevity of our observational record. The energy released by a solar flare can be measured directly only from outside Earth’s atmosphere, or determined indirectly from data and images gathered by space probes. Both approaches have been available only since the dawn of the space age roughly seventy years ago. In that entire period, the Sun has produced no superflare. Yet seventy years is a vanishingly small sliver of time for a star that has been burning for 4.6 billion years. As Natalie Krivova, an MPS scientist and lead author of the new study, explained, there is some evidence suggesting that the Sun, too, can produce superflares at great intervals, but there is no direct proof. The new research set out to close that evidentiary gap through an ingenious combination of modern solar observation and historical record-keeping stretching back centuries.

One line of evidence comes from the Sun’s stellar peers. Our galaxy contains many thousands of stars that resemble the Sun in key characteristics such as mass, temperature, and rotation rate. In late 2024, MPS researchers demonstrated that these Sun-like stars produce superflares approximately once every hundred years. If our star behaves like its counterparts, and there is good reason to think it might, then superflares should be a recurring, if rare, feature of solar activity. A second line of evidence comes from natural archives of solar history. Tree trunks preserved for millennia and ice cores extracted from the Arctic’s permafrost contain spikes in the concentrations of radioactive isotopes, isolated, short-lived, and exceptionally strong signals that suggest Earth has repeatedly been exposed to intense bombardment by high-energy solar particles in the distant past. These cosmogenic fingerprints are the only long-term traces such events leave behind, because a flare itself, no matter how powerful, leaves no lasting physical record on the Sun or in its immediate surroundings.

The relationship between these extreme particle eruptions and superflares remains an active area of investigation. According to Valeriy Vasilyev, an MPS scientist and lead author of a companion review article also published in Philosophical Transactions of the Royal Society A, the current state of research indicates that extreme particle eruptions and particularly intense flares often, but not always, occur together. This partial overlap means that isotope spikes in tree rings and ice cores are suggestive but not conclusive evidence of past superflares. What was needed was a way to connect the physics of flare production to measurable quantities that humans have been recording for far longer than the space age. The new study led by Krivova provides exactly that bridge, and it does so by exploiting one of the most carefully maintained observational datasets in all of science.

The first step of the analysis relied on modern instrumentation. The team examined observational data collected by NASA’s Solar Dynamics Observatory between 2010 and 2016, a period of intense solar monitoring during which the spacecraft continuously tracked the Sun’s surface, magnetic field, and atmosphere. From this dataset, the researchers identified the 300 strongest flares of the observation window and correlated the amount of energy each flare released with the size of the corresponding active region on the Sun’s visible surface. Active regions are areas where the magnetic field is particularly strong and structurally complex, and they are intimately associated with sunspots, the dark, cooler patches that dot the solar surface when magnetic fields suppress convection. Active regions are widely considered the potential starting points for eruptions, and quantifying the relationship between their size and flare energy is a cornerstone of modern flare forecasting.

Of course, as Krivova noted, the researchers knew that no superflares had occurred during the observation period. The power of the approach lies in the statistical relationship they uncovered: the link between released energy and active region size should hold true for more powerful events as well, extrapolating the physics observed in ordinary flares into the extreme regime that has never been witnessed on the Sun directly. This empirical scaling allowed the team to convert a measurement of active region area into an estimate of maximum possible flare energy, effectively turning the solar surface into a historical archive of eruptive potential.

The second step reached deep into the observational past. Sunspots have been systematically and regularly recorded for about 400 years, beginning with the telescopic observations of the early seventeenth century. As Theodosios Chatzistergos, an MPS scientist and co-author of the new study, explained, these records offer another way to look into the history of our star. Because sunspots are the visible manifestation of the magnetic complexity in active regions, the size of a sunspot group can be used to infer the size of the associated active region, and from there the strength of a possible eruption. The researchers combed through four centuries of sunspot records with particular interest in the rare outliers, the exceptionally large spots that stood apart from the ordinary run of solar activity and represented the most extreme magnetic configurations the Sun has displayed in the modern era.

One such outlier towers over the historical record: the giant sunspot of April 1947. This behemoth covered approximately 0.6 percent of the visible solar disk, and its diameter was about forty times that of Earth. It remains one of the largest sunspots the Sun has displayed since the beginning of systematic solar observations. Remarkably, no superflare erupted from this region during its transit across the solar disk. The Sun, in other words, loaded the barrel but never fired. Yet the new study demonstrates that a sunspot of this size is statistically capable of triggering a superflare in rare cases. The implication is sobering: the machinery for the most extreme bursts of radiation in the stellar repertoire exists on our own star, and it has assembled itself within living memory.

As Krivova put it, our Sun has superflare potential. It can produce massive sunspots that, in principle, can serve as the starting point for the most extreme bursts of radiation. The finding reframes the question from whether the Sun could ever generate a superflare to whether it has done so, and how often. The statistical frequency suggested by Sun-like stars, roughly once per century, means that a superflare could occur within a human lifetime, and that the absence of one during the space age is not evidence of safety but a matter of timing. Whether a superflare has actually erupted on the Sun in the historical period remains one of our star’s open mysteries, but the new work transforms that mystery from speculation into a quantifiable risk that can be studied, modeled, and prepared for.

The practical stakes are considerable. A Carrington-class event alone would stress twenty-first-century technological infrastructure in ways the nineteenth century never experienced, and a superflare, releasing orders of magnitude more energy, would represent a hazard of a different category entirely. Satellites in orbit, astronauts beyond the protection of Earth’s atmosphere, aviation routes over the poles, and continental power distribution networks would all face exposure. By establishing an empirical link between the largest sunspots in the historical record and the energy scales of superflares, the MPS-led team has given space weather researchers a concrete observational benchmark: when the next giant sunspot rounds the eastern limb of the Sun, scientists will be able to assess its eruptive potential with a rigor that was impossible before this study. The Sun’s most violent capabilities, long hidden in the statistics of distant stars and the chemistry of ancient trees, are now written in its own spotted face, waiting to be read.

Subject of Research: Solar superflare potential inferred from historical sunspot records and flare energy statistics

Article Title: Sun: New evidence of superflares

Article References: Sun: New evidence of superflares. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Sun, superflares, solar flares, sunspots, space weather, Solar Dynamics Observatory, Max Planck Institute for Solar System Research, Carrington Event, active regions, solar activity, stellar flares, Philosophical Transactions of the Royal Society A

The research, published in the journal Space Weather, was co-authored by space weather historian William B. Cade III of Baylor University’s Institute for Aviation Sciences, together with colleagues from Lancaster University in the United Kingdom, the British Geological Survey, Natural Resources Canada, RMIT University in Melbourne, Australia, and RAL Space in the United Kingdom. The team combined scientific observations with railway timetables, archival documents, and nineteenth-century newspaper reports to reconstruct what actually happened on that October night. Their conclusion is elegantly simple: the original account most likely contained a typographical error, with the year 1848 mistakenly printed as 1841. When the correct date is restored, every piece of evidence, from magnetic records to aurora sightings to sunspot observations, falls neatly into place.

The trail begins with an article published by Nature in 1871, which described a very intense magnetic disturbance on October 18, 1841, that interfered with telegraph equipment along the South Devon Railway. In the early days of railway operation, telegraph operators relied on electrical signals to communicate whether sections of track were clear before a train was allowed to proceed. According to the account, the instruments malfunctioned so severely that the superintendent in Exeter initially suspected that someone was playing tricks with the equipment. Unable to determine whether the track near the village of Starcross was safe, railway officials held the train at the station. If the date had been correct, the episode would have stood as the earliest documented case of space weather affecting modern technology, predating even the celebrated Carrington Event of 1859.

Cade, whose research focuses on the history of space weather science, had rediscovered the forgotten Nature article years earlier and brought renewed attention to its possible historical significance. He has spent considerable time examining articles and newspapers from the 1600s to the 1800s to understand how scientific understanding of solar-terrestrial connections progressed, and in 2013 he published a paper citing the Exeter account as the earliest example he could find of space weather impacting technology. The mystery deepened when a research team uncovered a significant discrepancy: the railway line from Exeter through Starcross did not open until 1846, nearly five years after the reported incident. The team also found no evidence of unusual geomagnetic activity over England on October 18, 1841. They invited Cade to join the investigation to help identify possible space weather events that could have inspired the original account and to provide historical context.

The key to solving the puzzle came from an unlikely source: the train’s scheduled departure time. The researchers examined historical railway timetables to determine if and when a 10:05 p.m. train left Exeter for communities along the route. That particular schedule was in place during 1848 and 1849, narrowing the window of possibility considerably. Their attention soon turned to October 18, 1848, the same month and day reported by Nature, but exactly seven years later than the printed date. This time, the evidence lined up with remarkable precision, transforming a frustrating archival contradiction into a satisfying historical detective story.

The corroborating records are striking. Magnetic records from the Greenwich Observatory showed powerful disturbances beginning during the evening of October 18, 1848, exactly the kind of geomagnetic turmoil that would wreak havoc on telegraph circuits. Newspaper reports described brilliant auroras visible across England, while astronomical observations documented a large group of sunspots facing Earth at the time. Together, these independent lines of evidence point to a period of intense solar activity capable of driving electrical currents through the telegraph lines and earth of the era, currents strong enough to render signaling instruments unreliable. The researchers concluded that the original Nature article most likely contained a typographical error, a single mistaken digit that distorted the historical record for generations.

The physics behind the disruption is well understood today. Space weather refers to changing conditions in the near-Earth environment caused by activity on the sun. Solar eruptions can send bursts of charged particles and enhanced magnetic fields toward Earth, and when these structures strike the planet’s magnetosphere, they drive rapidly varying electric currents in the ionosphere. Those currents induce secondary currents in long conductors on the ground, including telegraph wires and, in the modern era, power lines and pipelines. In the 1840s, the electric telegraph was an emerging technology used by railways for communication and signaling, and as the networks expanded, the rise in solar activity exposed a vulnerability that few people knew existed. The Exeter delay captures the precise moment when humanity’s first large-scale electrical infrastructure collided with the sun’s capacity for disruption.

The corrected date also rewrites a small but meaningful chapter of technological history. The Exeter incident was not, in fact, the first recorded technological disruption caused by space weather. That distinction belongs to an event on March 19, 1847, when geomagnetic activity interfered with the Midland Railway’s telegraph system during a vivid auroral display. Still, the Exeter delay remains one of the earliest known examples of space weather affecting infrastructure, occurring more than a decade before the famous Carrington Event of September 1859, when an extreme solar storm caused widespread telegraph disruptions around the world, reportedly shocking operators and allowing messages to be transmitted even after the batteries were disconnected. The 1848 episode now sits in proper chronological context within the early history of solar-terrestrial physics, in the years when scientists were only beginning to suspect that auroras, magnetic compass fluctuations, and sunspots were connected phenomena.

Beyond the correction itself, the study offers a lesson that extends well beyond space science. Historical accounts, even those published in respected scientific journals, can contain errors that persist for generations, and the Exeter story demonstrates how a single typographical slip can reshape the accepted narrative of a field. By comparing magnetic measurements with transportation records, solar observations, and local news reports, the team was able to reconstruct what likely happened on that October night with a confidence that no single source could provide. As Cade observed, the episode is a clear demonstration that as soon as humans developed the first electricity-based technology, it became susceptible to disruption by large space weather events, a vulnerability that has continued and increased to this day with power grids, satellites, communications systems, GPS navigation, and space travel. The result is both a scientific correction and a compelling glimpse into the dawn of our technological dependence on systems that the sun can, and periodically does, disturb.

Subject of Research: Historical correction of the earliest recorded space weather disruption of railway telegraph technology in nineteenth-century England

Article Title: A train, a solar storm and an 185-year-old mystery

Article References: A train, a solar storm and an 185-year-old mystery. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: space weather, geomagnetic storm, telegraph, railway history, solar activity, aurora, sunspots, Carrington Event, Greenwich Observatory, history of science, Baylor University, Space Weather journal

The study, published in the journal Solar Physics, was led by Partha Chowdhury of the University of Calcutta together with Jagdev Singh, V. Muthu Priyal, and Belur Ravindra of the Indian Institute of Astrophysics in Bengaluru. The researchers analyzed digitized images taken in the light of singly ionized calcium, the Ca ii K spectral line, from the Kodaikanal Solar Observatory. These images capture the chromosphere, a thin layer of the solar atmosphere sitting just above the visible photosphere, where magnetic fields heat the gas and make bright regions known as plages blaze in ultraviolet-adjacent wavelengths. Plages are the chromospheric ghosts of sunspots: where magnetic flux crowds the surface, the calcium emission brightens, even when no dark spot is present.

Because plages are visible even when sunspots are not, they offer a more complete census of the Sun’s magnetic activity than sunspot counts alone. The Kodaikanal archive, spanning Solar Cycles 14 through 22, roughly the first nine decades of the twentieth century, is one of the longest continuous records of chromospheric behavior anywhere in the world. After careful digitization and homogenization, a process that corrects for changes in photographic plates, instruments, and observing conditions over the decades, the team measured the total area covered by plages on every available day and compared it with three standard yardsticks of solar activity: the international sunspot number, the total sunspot area, and the 10.7 centimeter radio flux, a microwave emission that tracks coronal magnetic heating.

The correlations the team found are strikingly strong. Across all nine cycles, the chromospheric plage area tracks each of the photospheric and coronal activity indices with correlation coefficients exceeding 0.85, a level of agreement that leaves little doubt that the Sun’s upper and lower atmospheric layers are driven by the same underlying magnetic engine. This matters for more than just bookkeeping. Because plage records extend further back in time than some modern instruments, they can serve as a reliable proxy for reconstructing solar activity across the twentieth century, including the ultraviolet output that subtly influences Earth’s upper atmosphere and climate system.

But the study went beyond simple correlations. The researchers examined whether the plage areas obey two well-known but poorly understood features of the solar cycle. The first is the Gnevyshev-Ohl rule, an empirical oddity noting that in most pairs of consecutive solar cycles, the odd-numbered cycle is stronger than the even-numbered one that precedes it. The team confirmed that plage areas follow this rule, suggesting that the chromosphere inherits the same cycle-to-cycle memory that governs sunspot production. The second feature is the Gnevyshev gap, a temporary dip in activity that often splits the peak of a solar cycle into two humps. The analysis revealed clear Gnevyshev gaps in the plage data during several cycle maxima, confirming that this double-peaked structure is not merely a sunspot quirk but a genuine, whole-Sun phenomenon imprinted on the chromosphere as well.

To dig deeper into the timing of these variations, the team turned to a powerful mathematical technique called Morlet wavelet analysis, which can detect oscillations whose strength and period change over time, something a traditional Fourier analysis cannot do. They supplemented this with wavelet coherence, a method that measures not just whether two signals share a rhythm, but whether those rhythms stay in step with each other, drifting in and out of phase like two musicians who occasionally lose the beat. The technique is widely used in geophysics and climate science, and it is ideally suited to the Sun, whose magnetic activity is famously irregular.

The wavelet analysis uncovered two families of intermediate-term periodicities hiding inside the dominant eleven-year cycle. The first are Rieger-type periods, oscillations of roughly 130 to 190 days. These were first discovered in 1984, when researchers noticed that gamma-ray flares from the Sun seemed to cluster with a period of about 154 days. Since then, similar periodicities have been found in sunspots, flares, and coronal mass ejections, and they are widely interpreted as the signature of magnetic Rossby waves, vast, planet-scale waves of magnetized plasma rolling around the Sun’s interior shear layer known as the tachocline, where the Sun’s differential rotation winds up its magnetic field. The second family comprises quasi-biennial oscillations, or QBOs, with periods of one to four years, which are thought to reflect a secondary magnetic cycle operating in parallel with the main eleven-year one.

Here the story takes an intriguing turn. The Rieger-type periods and QBOs show considerable variability from one solar cycle to the next, and the wavelet coherence analysis reveals that at these intermediate timescales, the chromospheric plage areas and the photospheric activity indices frequently fall out of phase with one another. The rhythms exist, but they do not always march together. This asynchrony suggests that the shorter-period variations may arise from processes that affect the chromosphere and photosphere differently, or from instabilities in the tachocline whose surface manifestations depend on the details of each individual cycle, including the strength of the interior magnetic field at the time.

At the fundamental scale, however, the picture is one of remarkable unity. Within the 9 to 12 year periodicity belt that defines the solar cycle itself, the team found a stable phase synchrony between chromospheric plages and every photospheric and coronal index they examined. In other words, over the long haul, the Sun’s chromosphere and its visible surface rise and fall together like two ends of the same magnetic tide. This tight decadal coupling is exactly what modern solar dynamo models predict: magnetic fields generated by the rotation of plasma at the tachocline buoyantly rise through the convection zone, emerge as sunspots and active regions in the photosphere, and simultaneously light up the overlying chromosphere as plages. The new results provide hard empirical constraints that any credible dynamo model must now reproduce.

The findings also align with theoretical work on tachocline instabilities and magnetic Rossby waves, which have been invoked to explain everything from the double-peaked shape of solar maxima to the so-called seasons of space weather, bursts of intense flaring that come and go over months. By confirming Rieger-type and quasi-biennial signals in a chromospheric dataset spanning nine cycles, the study strengthens the case that these waves are a persistent feature of the solar interior rather than a fluke of a few well-observed cycles. For space weather forecasters, the implications are tantalizing: if the phase relationships between these periodicities and the activity indices can be pinned down well enough, the hidden rhythms of the tachocline might one day help anticipate the timing of the most active and hazardous phases of the solar cycle. For now, the century-old glass plates of Kodaikanal have once again proven that some of the best windows into the Sun’s deepest secrets were captured long before the space age began.

Subject of Research: Long-term chromospheric plage evolution and its relationship to solar activity indices across Solar Cycles 14 to 22

Article Title: Long-term Evolution of Chromospheric Plage Areas from Kodaikanal Observatory Ca ii K Images and Their Relation to Solar Activity Indices

Article References: Chowdhury, P., Singh, J., Priyal, V. M., & Ravindra, B. (2026). Long-term Evolution of Chromospheric Plage Areas from Kodaikanal Observatory Ca ii K Images and Their Relation to Solar Activity Indices. Solar Physics, 301(10), Article 149. https://doi.org/10.1007/s11207-026-02733-y

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02733-y

Keywords: solar physics, chromosphere, Ca ii K, solar plages, solar cycle, sunspots, Rieger periodicity, quasi-biennial oscillations, solar dynamo, tachocline, Rossby waves, Kodaikanal Observatory

The tale at the centre of the investigation is a striking one. The first practical electric telegraph networks were deployed during the 1840s, and as solar activity rose in the years that followed, Victorian telegraph operators began to experience strange and unexplained electrical effects caused by geomagnetic disturbances. According to the original account, the 10:05 p.m. train departing Exeter in Devon was delayed by 16 minutes when a very intense magnetic disturbance interfered with the electric signalling telegraph equipment used to determine whether the railway line ahead was clear. Without a working telegraph link, the signalman could not confirm that the track was unoccupied, and the train was held until the disturbance subsided. It is a vivid illustration of solar activity reaching down into everyday Victorian life through the copper wires of a young technology.

Professor Jim Wild of Lancaster University’s School of Physics and Astronomy, the lead author of the study, worked with scientists from the British Geological Survey, Natural Resources Canada, Baylor University, RMIT University and STFC RAL Space to test whether the account could withstand modern scrutiny. The team encountered a critical problem almost immediately: the railway line referenced in the story did not open until 1846, almost five years after the alleged 1841 incident. A train could not have been delayed on a line that did not yet exist. That single discrepancy opened the door to a broader re-examination of the event, and suggested that the date recorded in the 1871 Nature article might simply have been a typographical error, with 1848 misprinted as 1841.

To establish what really happened, the researchers assembled an unusually rich body of evidence, combining railway timetables, historical newspaper reports, solar observations, auroral accounts and digitised geomagnetic records. Their investigation identified a strong geomagnetic disturbance on 18 October 1848, exactly the kind of event that would have disrupted telegraph signalling. Crucially, the same date is accompanied by reports of sunspots and aurora seen across the United Kingdom and continental Europe, an independent signature of heightened solar and geomagnetic activity. Taken together, the evidence provides compelling support for the conclusion that the Exeter delay occurred in 1848, and that the earlier date was a clerical slip that propagated through the scientific literature for 178 years.

The correction changes the historical record in an important way. While the Exeter incident remains one of the earliest documented examples of space weather disrupting technology, it was not the first. The earliest credible report currently known is interference with telegraph systems on the Midland Railway in March 1847. That distinction shifts the timeline of humanity’s first encounter with the space environment back to the earliest years of the electric telegraph, and it demonstrates how quickly engineers of the 1840s began to notice that their new networks were sensitive to forces they could not yet explain. Geomagnetic disturbances induce currents in long conductors, and telegraph lines stretching across the countryside were effectively enormous antennas that picked up the electrical signature of a restless magnetosphere.

Professor Wild reflected on the significance of the finding for how society understands the hazard. Space weather is often discussed as a modern challenge because of contemporary dependence on satellites, communications systems and electricity networks, yet the study shows that society has been experiencing the effects of space weather on technology for almost as long as electrical technologies have existed. The Exeter train delay, he noted, is a fascinating story because it sits right at the point where emerging technologies first began to encounter the realities of the space environment. By combining historical archives with scientific observations, the team was able to show that the event almost certainly happened in 1848 rather than 1841, and although this means it is not the earliest recorded space weather impact, it remains one of the first clear examples of solar activity disrupting critical infrastructure.

The research also carries a methodological lesson that extends well beyond railway history. Dr Mike Hapgood, Visiting Scientist and space weather expert at STFC’s RAL Space, described the work as having a hint of a detective story, piecing together a wide range of archived records to better understand a historically severe space weather event. He emphasised the importance of preserving older records, which give scientists the evidence base they need to interpret past events and strengthen future predictions. Digitised geomagnetic measurements, newspaper archives and railway timetables proved to be complementary strands of evidence, and their combination allowed the team to date a geomagnetic storm with a precision that no single source could have provided. For researchers reconstructing the history of solar activity, the study is a demonstration of how much information still lies waiting in nineteenth-century archives.

The physical mechanism behind the Exeter delay is well understood today. When the Sun ejects clouds of magnetised plasma toward Earth, the resulting shock compresses the planet’s magnetic field and drives rapidly varying electric currents in the ionosphere. Those variations, in turn, induce voltages in long conductors on the ground, a phenomenon now known as a geomagnetically induced current. In the 1840s these currents manifested as spurious signals, erratic needle deflections and intermittent failures on telegraph circuits, sometimes strong enough to disrupt railway signalling entirely. The same physics operates today, but the conductors are high-voltage transmission lines, pipeline networks and the grounding systems of power transformers, and the consequences of a severe storm can range from GPS errors to regional blackouts.

Nearly two centuries after the Exeter delay, railways and other critical infrastructure remain vulnerable to space weather, although through very different technologies including power systems, signalling equipment, satellite navigation and communications networks. Professor Wild observed that the research highlights space weather as not a new threat but a long-standing natural hazard. The technologies affected have evolved from railway telegraphs to satellites, communications networks and power systems, but the challenge remains the same: understanding the risk and ensuring society is resilient to its impacts. The continuity is striking, since a signalman waiting for a telegraph needle to settle in 1848 and a grid operator monitoring geomagnetic indices today are responding to the same solar-driven disturbance in Earth’s magnetic environment.

Dr Hapgood added a note of caution about the future. While today’s space weather capabilities are far more advanced than anything available in the 1800s, the modern technologies society depends on are also much more vulnerable to solar storms. Deepening the understanding of these events, he argued, is essential for preparing for and mitigating the impacts of space weather, especially as the world looks forward to a decade of ambitious space developments that will face the challenge of a new solar cycle in the 2030s. The Exeter story, corrected at last, thus serves a double purpose: it resets the historical record of humanity’s first technological encounter with the Sun’s outbursts, and it reminds a technology-dependent civilisation that the hazard has never gone away, only changed its address.

Subject of Research: Historical reconstruction of an 1848 geomagnetic disturbance that delayed a train in Exeter, correcting the date of one of the earliest recorded space weather impacts on technology

Article Title: Mystery of one of the earliest recorded space weather impacts solved

Article References: Mystery of one of the earliest recorded space weather impacts solved. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: space weather, geomagnetic storm, telegraph, Exeter, railway history, solar activity, aurora, sunspots, geomagnetically induced currents, Lancaster University, Space Weather journal, Victorian technology

Sunspot oscillations have fascinated solar physicists for more than half a century. Ever since chromospheric inhomogeneities in umbral regions were first reported in 1969, researchers have recognised that the magnetised plasma trapped within a sunspot does not sit still. Instead, it trembles with a superposition of waves, from the famous three-minute umbral oscillations to the five-minute oscillations that dominate the quiet solar photosphere. These waves are not mere curiosities. They are diagnostic tools, carrying information about the temperature, density and magnetic field structure of the solar atmosphere from the photosphere up through the chromosphere. Because the waves are shaped by magneto-acoustic mode conversion, absorption of acoustic p-modes and the geometry of magnetic flux tubes, measuring their periods precisely allows scientists to test models of magneto-convection and wave propagation in conditions that cannot be replicated in any laboratory on Earth.

What makes the new study distinctive is both the observational window and the analytical rigour. The 8 to 10 micrometre band occupies the thermal infrared, a regime in which the solar continuum emission is formed in well-understood layers of the photosphere and lower chromosphere, and in which the contrast between magnetic and non-magnetic plasma behaves differently than in visible light. The AIMS telescope, described in a 2025 overview of its trial observations, was designed to exploit precisely this window, combining infrared imaging with magnetic-field measurements from the high-altitude, thin-air environment of Lenghu in western China. Liu extracted light curves from three distinct regions of each sunspot observation: the dark umbra at the centre, the filamentary penumbra surrounding it, and the magnetically quiet Sun nearby, and then applied wavelet analysis, a mathematical technique well suited to detecting oscillatory signals whose strength varies over time.

The heart of the paper is a methodological comparison. Rather than trusting a single data-processing pipeline, Liu evaluated four distinct approaches for converting two-dimensional image sequences into oscillation measurements. Two of the methods perform wavelet analysis pixel by pixel across the field of view and then aggregate the resulting period information, either by taking the mean or the median across pixels. The other two first collapse the spatial information by computing either the mean or the median intensity across a region at each time step, and only then apply the wavelet transform to the resulting single time series. The distinction may sound technical, but it turns out to matter enormously, because spatial averaging before analysis can distort the very oscillation signal researchers are trying to measure.

The results are striking in their consistency. Across all six sunspot datasets, three of the four methods, namely pixel-wise wavelet analysis with mean aggregation, pixel-wise wavelet analysis with median aggregation, and spatial median aggregation before wavelet analysis, recovered the same period hierarchy with one hundred percent consistency: the umbra always shows the shortest periods, the penumbra intermediate ones, and the quiet Sun the longest. Typical weighted mean periods came out at roughly 260 to 313 seconds in the umbra, 286 to 374 seconds in the penumbra, and 294 to 382 seconds in the quiet Sun. In other words, the magnetically dominated heart of the sunspot rings fastest, while the surrounding plasma oscillates more slowly, a pattern that carries direct implications for how waves are generated and propagate through strongly magnetised regions of the solar surface.

The one method that fell short is revealing. Computing the spatial mean before the wavelet analysis achieved full consistency with the umbral-penumbral-quiet-Sun ordering in only one of the six datasets, partial consistency in three, and outright inconsistency in two, with the failures involving cases where the umbral period no longer came out as the shortest. The reason lies in a well-known pitfall of averaging: when a region contains spatially varying oscillation signals, taking the mean can suppress genuine oscillatory power and blend distinct modes into a misleading composite. The study quantifies this distortion precisely, showing that spatial smoothing generally increases the measured periods, by up to 21 percent, with the umbra showing the highest sensitivity to the effect. For observers planning future campaigns, the message is unambiguous: pixel-wise analysis with careful aggregation is the safer path.

Beyond the hierarchy of periods, the study addresses a deeper question about the physical character of solar oscillations. For each dataset, Liu computed the ratio of the power in secondary oscillation peaks to the power in the dominant peak. Every single ratio fell below 0.3, with a mean of 0.14 plus or minus 0.03. This is a strong quantitative confirmation that sunspot oscillations are inherently multi-mode phenomena rather than clean single-frequency signals. The dominant period, the one that stands tallest in a wavelet power spectrum, therefore tells only part of the story. Liu argues that the weighted mean period, which accounts for the distribution of power across modes, is physically more meaningful than the dominant period alone, better representing the true multi-mode nature of the oscillations that magnetised solar plasma supports.

These findings arrive at a moment when the solar physics community is investing heavily in new ground-based facilities capable of observing the Sun in underexplored spectral bands. The 8 to 10 micrometre window, the study concludes, deserves recognition as a valuable diagnostic for solar physics, complementing the visible and near-infrared channels on which most historical sunspot oscillation measurements have relied. The thermal infrared continuum in this band is sensitive to the temperature structure of the solar atmosphere, and its behaviour during energetic events such as flares has been modelled extensively, making it a natural arena for studying how magnetic structures modulate thermal emission on oscillation timescales. Establishing a robust baseline of oscillation properties in this band, across multiple sunspots and multiple analysis methods, provides the observational foundation that future theoretical work on magneto-convection and wave propagation will need.

The practical stakes extend beyond pure science. Sunspots are the seats of solar flares and coronal mass ejections, the eruptions that drive space weather and can disrupt satellites, radio communications and power grids on Earth. Oscillations within sunspots are thought to be linked to the dynamics of the magnetic fields that ultimately power these events, and some researchers have proposed that changes in oscillation behaviour could serve as precursors of eruptive activity. A reliable, reproducible method for measuring sunspot oscillation periods, validated across six independent datasets and four analytical pipelines, is a step toward making such monitoring quantitative. The study also carries a quiet tribute to observational perseverance: the acknowledgements credit the staff of the Huairou Solar Observing Station, whose engineers and observers worked under the extremely harsh conditions of high altitude and thin air at Lenghu to obtain the high-quality data on which the entire analysis rests.

For now, the immediate contribution is a set of firm numbers and a clear methodological verdict. The umbra of a sunspot oscillates with weighted mean periods of roughly 260 to 313 seconds, the penumbra at 286 to 374 seconds, and the quiet Sun at 294 to 382 seconds, and that ordering holds without exception when the data are handled correctly. Solar oscillations are genuinely multi-mode, with secondary peaks carrying a modest but real fraction of the total power. And the way astronomers average their data can shift the answer by a fifth of the measured period if they are careless. As AIMS and its successors accumulate more observing time in the thermal infrared, the Sun’s magnetic bells will keep ringing, and scientists will keep learning how to hear them with ever greater fidelity.

Subject of Research: Oscillation period properties of sunspots observed in the 8 to 10 micrometre infrared band

Article Title: Oscillation Period Properties of Sunspots in the 8 – 10 (\mu )m Infrared Band: A Multi-Dataset Analysis

Article References: Liu, S. (2026). Oscillation Period Properties of Sunspots in the 8 – 10 $\mu $m Infrared Band: A Multi-Dataset Analysis. Solar Physics, 301(9), Article 131. https://doi.org/10.1007/s11207-026-02707-0

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02707-0

Keywords: sunspots, solar oscillations, infrared observations, wavelet analysis, AIMS telescope, magnetic fields, solar physics, umbra, penumbra, magneto-convection, space weather, Lenghu Observatory

The stakes of this archival detective work are higher than they might appear. French solar observations form the primary data source covering the Maunder Minimum, the period from roughly 1645 to 1715 when sunspots became vanishingly rare and Europe shivered through what climatologists call the Little Ice Age. In the 1990s, the astronomer Elisabeth Nesme-Ribes extensively analyzed the Parisian Observatory archives and published a series of influential reconstructions. Tragically, the solar data she compiled have since been lost; only a single figure of average sunspot latitudes survives. That loss made a fresh, independent reduction of the original documents not merely desirable but essential, and the new study delivers exactly that, digitizing measurements that had previously existed only in ink on paper.

The technical challenge of converting seventeenth- and eighteenth-century observations into modern solar coordinates is formidable. La Hire did not simply note that a spot existed; he measured the times at which spots crossed the solar meridian, recorded angular distances from the disk’s limbs, and drew the spots in engravings that captured their shapes and relative positions. To transform these numbers into latitudes and longitudes, the researchers needed the angular size of the solar disk on every observing day, which changes through the year as Earth moves along its elliptical orbit. They interpolated routine meridian measurements published posthumously by Le Monnier in 1741, recovering disk diameters to within a few arcseconds for dates spanning the entire eight-year window. Combining transit timings with these disk sizes, they traced each sunspot’s path across the visible hemisphere and computed daily heliographic coordinates for every individual spot and group.

The reconstruction demanded careful correction of errors scattered through the historical record. The researchers found typos in dates, mislabeled authorship, and arithmetic slips in both the journals and the printed Mémoires. On 31 May 1703, for example, La Hire made an error in a sunspot transit measurement that could only be resolved by consulting his own journal. On 26 June of that year, an engraving appears to have been accidentally reversed and flipped compared with the other drawings, a conclusion supported by the relative positions of the umbrae and by agreement with a drawing by Giovanni Domenico Cassini. On 2 January 1707, a measurement taken seven minutes after noon required a positional correction of about 3.6 degrees toward the solar Equator. Each such fix, the authors note, matters when the goal is a machine-readable catalogue that modern solar physicists can trust.

The Parisian records also proved decisive in resolving puzzles among contemporary observers across Europe. The reconstructed positions resolved an inconsistency in the observations of Johannes Hoffmann and showed that the observer Müller was using a different type of telescope, a finding that changes how his measurements should be interpreted. The team cross-checked La Hire’s data against records from the Kirch family, Hoffmann, Eimmart, Müller, Becker, Hertel, Sturm, Wideburg, Manfredi, and Derham, weaving a continental network of observations into a coherent whole. In one striking episode from January 1704, Giacomo Filippo Maraldi reported two widely separated sunspot groups on the same day, remarking that it had been a long time since such widely separated spots had been seen together, a vivid reminder of how unusual even modest solar activity had become during those years.

Among the technical fruits of the study is a new estimate of the Sun’s sidereal differential rotation during this quiet epoch. For a synodic rotation period of 27.5 days, the sidereal rotation rate works out to 14.08 degrees per day, slightly slower than the 14.18 degrees per day of the modern Carrington frame that serves as the standard reference for solar rotation today. The researchers derived rotation rates from pairs of measurements separated by hours, days, and full disk passages, tabulating them for individual spots and groups. The persistence of Joy’s law, the well-known tendency of sunspot groups to tilt with their leading spots closer to the Equator than their trailing spots, was also verified in these ancient data, suggesting that the underlying dynamics of magnetic flux emergence were operating much as they do now, even at the tail end of a grand minimum.

The time-latitude diagram constructed from the new catalogue, a butterfly-like plot showing where spots appear as the solar cycle progresses, is consistent with the previously available results from the analyses of the Parisian observations, including Nesme-Ribes’s surviving figure of average latitudes. This agreement is reassuring on two fronts: it validates the lost reconstructions of the 1990s and confirms that the new, independently derived dataset can safely replace them. The researchers also documented the Wilson effect in an unexpected place, noting that on 3 June 1703 La Hire observed a sunspot at the very edge of the disk appearing as a small depression in the limb, a phenomenon now understood to arise because spots are shallow structures rather than flat markings on a perfect sphere.

The day-to-day texture of the observations brings the era to life. La Hire frequently worked with telescopes of 16 feet and longer, switching instruments depending on conditions; on 18 January 1704 he could not discern a spot with a 3-foot tube but saw it clearly with a 16-foot one, likely because of poor contrast near the limb. Clouds, fog, and haze repeatedly interrupted the observing campaigns, and the journals candidly record blank Suns, vanished spots, and rare double appearances. In October 1705, two sunspot groups were seen simultaneously, an event the observers flagged as a rarity, and in November 1707 Cassini marveled that a spot had appeared in the northern hemisphere at a latitude of about 13 degrees, an occurrence he called extraordinary. Such details, once mere color, now feed quantitative reconstructions of the solar cycle’s recovery from its deepest recorded lull.

All of the reconstructed sunspot parameters, including daily latitudes, longitudes, areas, and counts, are provided in electronic supplementary materials accompanying the paper, giving modelers of the solar dynamo a fresh, high-resolution window onto the Sun’s behavior as it emerged from the Maunder Minimum. Understanding how the cycle recovered from that grand minimum is not just an exercise in historical astronomy; it informs how scientists assess the Sun’s capacity for prolonged lulls in activity and what such lulls might mean for climate and space weather on timescales of centuries. La Hire, sketching spots with quill and micrometer in the fog above Paris, could not have imagined that his notebooks would one day anchor twenty-first-century dynamo simulations. Three hundred years on, his patience is paying scientific dividends once again.

Subject of Research: Reconstruction of sunspot positions, areas, and counts from Philippe de La Hire's observations at the Paris Observatory during 1703 to 1710, in the late Maunder Minimum.

Article Title: Sunspot Observations by Philippe de La Hire from 1703 to 1710

Article References: Zolotova, N., & Vokhmyanin, M. (2026). Sunspot Observations by Philippe de La Hire from 1703 to 1710. Solar Physics, 301(9), Article 133. https://doi.org/10.1007/s11207-026-02719-w

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02719-w

Keywords: sunspots, Maunder Minimum, solar cycle, solar physics, Philippe de La Hire, Paris Observatory, solar rotation, Joy's law, historical astronomy, sunspot records, solar dynamo, space climate

The study, conducted by Owen Giersch, analyzed X-ray flares of M class and higher—the moderate to extreme events that matter most for space weather operations. The research took two complementary approaches. First, flares were sorted into logarithmically spaced intensity categories and compared across sunspot cycles. Second, flare rates were tabulated according to the McIntosh classification of the sunspot group that produced each event, allowing a direct test of whether a given type of sunspot region flares at the same rate regardless of which cycle it belongs to. Both tests delivered results that depart from the comforting assumption of cycle-to-cycle consistency.

The McIntosh classification scheme, which underpins the second part of the analysis, has a long pedigree. Sunspot classification began with Alfred Cortie’s scheme of 1901, which is no longer used. Observers in Zurich developed an alternative system in the 1930s, using nine categories to describe the evolution of sunspot groups. In the 1960s, Patrick McIntosh modified this Zurich system, removing two classes and adding parameters describing the penumbra of the largest sunspot in a region and the compactness of spots within the region. The resulting Modified Zurich Classification—now universally known as the McIntosh system—combines the Zurich class, the penumbral character, and the compactness into 60 permitted classes. Because sunspot classification serves as a proxy for the magnetic complexity of an active region, it remains the backbone of most flare forecasting practice.

The data underpinning the analysis came from two archival sources. For flares prior to 1996, records were obtained from the National Centers for Environmental Information, and from 1996 onwards from the Space Weather Prediction Center, both drawing ultimately on the GOES satellite series that has monitored solar X-ray output continuously since cycle 21. Sunspot region reports came from United States Air Force Solar Observing Optical Network sites, whose daily observations include quality flags ranging from 1, indicating very poor seeing conditions, to 5, indicating excellent conditions. Because pre-1996 reports were listed from up to five individual sites without being reduced to a single classification, the author developed a method to consolidate multiple daily reports: retaining the highest-quality report, then adopting the most frequently occurring class, with random selection only in the rare case of irreconcilable ties.

The data cleaning itself illustrates the messy reality of half a century of observational records. Numerous invalid classifications appeared in the archive—classes with missing or implausible parameters—and a set of explicit rules was applied to correct them, drawing on the physical logic of the McIntosh scheme. After all corrections, only 193 regions could not be assigned a valid class, and these were omitted from the analysis. For the flare-rate comparison by sunspot class, only flares with an assigned region were included, while the raw cycle-to-cycle flare counts used all reported events. This careful curation matters, because the study’s conclusions hinge on comparisons between data streams that were assembled by different teams, with different instruments and practices, across 50 years.

The statistical machinery was deliberately conventional, designed to be transparent rather than exotic. To compare flare intensity distributions between two cycles, a chi-squared test was employed, with categories combined wherever either distribution contained fewer than five flares, and p-values computed so that distributions could be deemed similar at the 95 percent confidence level. To compare flare rates by McIntosh class, both chi-squared tests and linear regression were used. Under the regression approach, if two cycles behaved identically, plotting one cycle’s flare rates against the other’s should yield a line with a gradient of one, an intercept of zero, and a correlation coefficient close to one. Departures from that ideal indicate genuine differences in how sunspot classes translate into flares.

The results on intensity are nuanced but troubling. At the 95 percent confidence level, the flare intensity distribution of cycle 21 cannot be considered similar to those of cycles 22 through 25—a discrepancy the author attributes in part to the fact that cycle 21 was the first cycle during which the GOES satellites operated, raising the possibility of calibration or measurement issues. The comparisons between cycles 22 and 25 and between cycles 23 and 25 also failed the similarity test. A broad pattern emerged: the greater the separation between two cycles in time, the more dissimilar their flare intensity distributions tend to be. The striking exception is the pairing of cycles 22 and 24, which produced the highest p-value of any comparison, suggesting those two cycles flared in remarkably similar proportions despite differing in overall amplitude.

The second finding is more fundamental. When flare rates were compared by McIntosh class across cycles, chi-squared tests found essentially no pairs of cycles that could be considered similar. The sole nominal exception—a comparison of cycle 21 as the observed distribution against cycle 22 as the expected distribution—flipped to dissimilar when the roles were reversed, a signature of a statistical false positive. The regression analysis reinforced the picture: while the intercepts of seven out of ten cycle comparisons were close to zero, as expected for the least active sunspot classes, only the comparisons involving cycles 25 and 22 produced gradients close to one. In plain terms, the rate at which a given kind of sunspot group produces M-class or larger flares varies significantly from one solar cycle to another.

What explains this variability? The study is candid that the answer remains unclear. One possibility is observer bias: sunspot classification is a human judgment, and many researchers have documented problems with sunspot group analysis, particularly with sunspot areas, which form one input to the McIntosh parameters. If area measurement techniques—and therefore the resulting values—have drifted over decades, other derived parameters such as extent and compactness may have drifted too, contaminating any cycle-to-cycle comparison. The alternative is more provocative: that the difference reflects a genuine physical change in how the Sun organizes magnetic energy from cycle to cycle. Intriguingly, recent work by other researchers found that as the McIntosh Zurich class increases from A to F, the total magnetic energy of active regions rises, but the free energy—the portion available to power eruptions—does not change significantly, hinting that classification alone may not capture what makes a region flare-ready.

Despite the ambiguities, the analysis closes with something immediately useful for forecasters. Assuming flares follow a Poisson process, the probability of at least one M-class or larger flare occurring in a given region over a specified interval can be computed from its flare rate, and with multiple regions on the Sun—common near solar maximum—the combined probability follows from multiplying the individual exponential terms. The full dataset generated for the study has been released in a public GitHub repository, and the author emphasizes that further analysis of the USAF sunspot classifications is needed to determine whether the cycle-to-cycle variation is intrinsic to the Sun or an artifact of evolving observational practice. Either answer carries weight: one rewrites the physics of flare statistics, the other a cautionary tale about the archives on which space weather science is built.

Subject of Research: Statistical analysis of X-ray solar flare intensity distributions and flare rates by McIntosh sunspot class across solar cycles 21 to 25

Article Title: Analysis of X-ray Solar Flare Rates from 1976 to 2025

Article References: Analysis of X-ray Solar Flare Rates from 1976 to 2025. (n.d.). https://doi.org/10.1007/s11207-026-02743-w

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02743-w

Keywords: solar flares, X-ray flares, sunspots, McIntosh classification, solar cycle, space weather, GOES satellites, flare forecasting, solar physics, statistical analysis, sunspot classification, active regions

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