Century-Old Solar Images Help Scientists Reconstruct the Sun’s Magnetic Past

Over the last decade, KoSO’s massive photographic collection — over 1,20,000 plates — has been digitised, converting a century of analog records into high-resolution digital data.

A breakthrough method developed by an international team of astronomers has opened an unprecedented window into the Sun’s magnetic history, using more than a century’s worth of solar images captured at the Kodaikanal Solar Observatory (KoSO). This pioneering work provides the clearest long-term picture yet of the Sun’s polar magnetic behaviour — a crucial element for predicting future solar activity, including potentially dangerous solar storms.

For over 100 years, scientists have sought to unravel the Sun’s magnetic cycles, which influence the appearance of sunspots, the timing of solar flares, and the intensity of coronal mass ejections. These phenomena directly affect Earth, with the potential to disrupt navigation systems, damage satellites, interfere with radio communications and power grids, and pose risks to astronauts. Despite decades of research, the Sun’s polar magnetic fields — the engines that drive the solar cycle — remained poorly understood due to the lack of direct measurements before the 1970s.

Now, researchers from the Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute under India’s Department of Science and Technology (DST), together with collaborators from leading institutions in India, Europe and the United States, have used archival solar data to reconstruct the Sun’s polar magnetic field going back to the early 20th century.

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Unlocking Solar Secrets Hidden in Century-Old Archives

The project, led by scientist Dibya Kirti Mishra, tapped into the treasure trove of historical Ca II K solar images preserved at KoSO. The observatory, first established in 1899 and maintained by the Indian Institute of Astrophysics (IIA), has one of the world’s most continuous solar observation archives.

Since 1904, KoSO astronomers have photographed the Sun in the Ca II K wavelength — a violet light emitted from the chromosphere, the atmospheric layer just above the Sun’s visible surface. This region hosts bright features known as plages and chromospheric networks, which respond directly to changes in the Sun’s magnetic field.

Over the last decade, KoSO’s massive photographic collection — over 1,20,000 plates — has been digitised, converting a century of analog records into high-resolution digital data. The dataset has since been recognised as one of the world’s largest sources of historical solar observations, and a major asset for AI and machine-learning applications.

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The Polar Network: A New Magnetic “Proxy”

The research team combined the KoSO dataset with modern observations from Italy’s Rome Precision Solar Photometric Telescope (Rome-PSPT). Using advanced automated algorithms, they identified tiny bright patches near the Sun’s poles — structures collectively known as the polar network.

These features act as stand-ins for magnetic activity. By analysing how the polar network varied over time, the team successfully reconstructed the Sun’s polar magnetic field from 1904 to 2022, matching it with direct measurements from the Wilcox Solar Observatory (WSO) for the overlapping years.

The results showed remarkable consistency across datasets, confirming that the polar network index (PNI) is a highly reliable proxy for past magnetic field strength.

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A Major Leap in Predicting Solar Cycles

The reconstructed data allowed researchers to make retrospective predictions and forecasts. Notably, the team used their method to estimate the strength of the ongoing Solar Cycle 25, aligned with current solar observations. This is significant because the polar magnetic field at the end of one cycle is a key predictor of the next cycle's intensity.

Long-term reconstructions such as this are extremely valuable in:

• understanding how solar cycles fluctuate over decades

• identifying patterns linked to strong or weak solar activity

• improving forecasting models for geomagnetic storms

• developing early-warning systems for space-weather risks

Given the increasing global reliance on satellite networks, aviation technology, internet-linked infrastructure, and power grids, accurate solar-cycle prediction has never been more essential.

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Why the Sun’s Polar Fields Matter

The Sun’s magnetic field flips every 11 years, switching its north and south poles. This flip marks the peak of the solar cycle, a period characterised by higher numbers of sunspots and solar storms.

But unlike sunspots — which have been regularly recorded for centuries — the polar magnetic field is faint, difficult to observe, and lacked long-term measurements. It acts as the "seed" for the next solar cycle, determining how strong or weak it will be.

Understanding the polar field is therefore central to predicting phenomena such as:

• solar flares that can disrupt radio and GPS signals

• coronal mass ejections (CMEs) that can damage satellites

• geomagnetic storms capable of triggering power-grid failures

• radiation hazards for astronauts and high-altitude flights

The new KoSO-based reconstruction finally fills a 70-year knowledge gap, allowing scientists to study the Sun’s long-term magnetic behaviour with confidence.

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A Public Resource for Global Solar Research

In keeping with the spirit of scientific collaboration, the full dataset — including digitised images, reconstructed polar field values, and the Polar Network Index series — has been made freely available to the world.

The data can be accessed on:

• GitHub

• Zenodo

This ensures that researchers from any country can build upon the findings, refine solar models, or explore new methods to study space weather and solar magnetism.

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A New Era in Solar Astronomy

The work led by ARIES and its international partners represents a milestone in solar research. By reaching into the past, astronomers have gained powerful new tools to understand our star’s future. The technique showcases how a century-old observatory, modern AI algorithms, and global collaboration can come together to solve one of the most enduring mysteries in solar physics.

With solar activity intensifying and Earth becoming more vulnerable to space-weather threats, this reconstruction offers an invaluable long-term perspective — one that could ultimately help safeguard modern technological civilisation.