Century of Solar Photographs Reveals Hidden Rhythms in the Sun’s Magnetic Heart

Deep in the hills of Tamil Nadu, India, a modest observatory has been photographing the Sun almost every clear day for more than a century. Now, a team of solar physicists has mined that extraordinary photographic archive to answer one of the most fundamental questions in solar science: how tightly is the Sun’s churning, glowing outer atmosphere locked to the sunspots we can see on its visible surface? The answer, drawn from nine consecutive solar cycles, is a story of deep coupling, hidden rhythms, and a magnetic engine whose fingerprints reach from the Sun’s interior all the way to Earth.

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