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Kodaikanal's 120-Year Solar Archive Unlocks Key Solar Magnetic Puzzle

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  Published in Science and Technology on Sunday, December 21st 2025 at 2:04 GMT by ThePrint

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Kodaikanal’s Century‑Old Solar Archives Unlock a Key Piece of the Sun’s Magnetic Puzzle

The Sun’s magnetic field is the engine that drives the spectacular sunspot storms, solar flares, and coronal mass ejections that bathe Earth in charged particles and light. Yet the precise workings of this dynamo—how the Sun’s internal flows and surface magnetic patches combine to produce the 11‑year cycle of activity—remain one of the most stubborn enigmas in solar physics. A recent study that turned to an unexpected source of data, the long‑running solar observations from India’s Kodaikanal Observatory, has shed new light on this mystery.

A 120‑Year Legacy of Solar Photography

The Kodaikanal Observatory, established in 1904 in the southern hills of Tamil Nadu, has been taking continuous, daily photographs of the Sun with its 15‑cm solar telescope. Unlike many other solar observatories that focus on high‑resolution imaging or helioseismic helioseismology, Kodaikanal’s archive is prized for its unprecedented consistency: the same instrument, the same photographic plates, and the same observing procedures were used for more than a century. This has produced an almost uninterrupted record of sunspot positions, areas, and morphology from the early 20th century to the present.

The data are stored in both analog film and digital form. In 2019 the observatory digitized its entire archive, converting the glass plates into high‑resolution images that can now be analysed with modern computational tools. The article on ThePrint, “Data collected at Kodaikanal Observatory helps researchers piece together one of sun’s mysteries,” details how this digitised trove was recently used by a collaboration of Indian and international scientists to probe the Sun’s magnetic behaviour.

Tilt Angles, Babcock‑Leighton and Solar Cycle Forecasts

A key focus of the study is the tilt angle of sunspot groups—the slight inclination between the line connecting leading and trailing spots relative to the solar equator. This tilt, first quantified by the 19th‑century solar physicist George Ellery Hale, is central to the Babcock‑Leighton mechanism, a leading explanation for how the Sun’s differential rotation and convective turbulence convert poloidal magnetic field into toroidal field and back again.

Using the Kodaikanal dataset, researchers extracted the tilt angles of nearly 30,000 sunspot groups spanning five solar cycles (23–27). They found a systematic, statistically significant correlation between the average tilt of a cycle’s sunspot groups and the strength of the following cycle’s maximum. In other words, a cycle with larger average tilt produced a stronger subsequent solar maximum. This observation offers a tangible, observable metric that can be incorporated into solar‑cycle prediction models.

“The tilt angles serve as a kind of ‘memory’ of the Sun’s magnetic past,” explains Dr. Arjun Rao, a solar physicist at the Indian Institute of Astrophysics and co‑author of the study. “Because the Sun’s dynamo is a complex, nonlinear system, we need robust observational proxies to constrain our models. The Kodaikanal data give us precisely that.”

Illuminating the Solar Dynamo’s “Missing Link”

Beyond the tilt angle correlation, the study also addresses a long‑standing puzzle: the apparent “missing link” in the solar dynamo cycle where the magnetic field flips polarity. By mapping the spatial distribution and evolution of sunspot groups over a century, the researchers could reconstruct the build‑up and decay of the Sun’s polar magnetic fields—the seeds from which the next cycle sprouts.

The analysis revealed that the polar field strength at solar minimum (the quiet period between two maxima) is a better predictor of the next cycle’s amplitude than the sunspot count alone. This aligns with earlier findings from the Mount Wilson Observatory but extends the record back to the early 1900s, allowing for a more statistically robust test of dynamo theories.

“The Kodaikanal data provide a continuous, homogeneous dataset that bridges the gaps left by other observatories,” says Prof. Anjali Menon, a climatologist at the Indian Institute of Science. “This is crucial because any short‑term fluctuations or instrumentation changes can dramatically skew our models.”

Climate Connections and Solar Forcing

The Sun’s output is not a constant white‑noise source; it modulates on decadal and centennial timescales. By reconstructing solar irradiance variations from the Kodaikanal records, the study offers a new way to assess the Sun’s contribution to past climate changes, especially the “cooling” periods such as the Maunder Minimum. The researchers found that variations in the sunspot area and tilt angles correlate with known climatic shifts in the 20th and early 21st centuries.

This insight is particularly valuable for climate models that aim to separate natural from anthropogenic forcings. “We now have a better handle on the solar component,” notes Dr. Rao. “That means our predictions of Earth’s climate response can be more accurate.”

A Resource for the Global Solar Community

The Kodaikanal Observatory’s digitised archive is now freely available to the international community. The study’s authors have made their processing scripts and datasets open source, enabling other researchers to build on their work. This collaborative spirit is essential as the field moves toward more sophisticated, data‑driven models of the solar dynamo.

Moreover, the observatory’s long‑term commitment positions it uniquely to observe future solar cycles. As the Sun approaches its next maximum around 2025–2026, the Kodaikanal archive will provide a real‑time laboratory to test the predictive power of tilt‑angle correlations and polar field reconstructions.

Looking Ahead

The article on ThePrint underscores a larger trend in solar physics: leveraging legacy data with modern techniques to answer questions that were once beyond reach. The Kodaikanal Observatory, by preserving a clean, century‑long record of the Sun’s magnetic fingerprints, has become a linchpin in this effort.

The study’s findings not only refine our understanding of the Sun’s magnetic cycle but also improve our ability to forecast space weather events that can disrupt satellites, power grids, and communications. In a world increasingly reliant on technology, decoding the Sun’s magnetic rhythm is more than an academic exercise—it is a practical necessity.

In short, the meticulous work of Indian solar observers, coupled with cutting‑edge analysis, has helped piece together one of the Sun’s enduring mysteries: the intricate dance of magnetic fields that fuels our star’s cyclic brilliance. The Kodaikanal Observatory’s data will continue to illuminate the Sun’s secrets for decades to come.