Researchers from Southwest Research Institute, the Aryabhatta Research Institute of Observational Sciences and the Max Planck Institute used archival Calcium K (Ca II K) images from Kodaikanal to probe the Sun's chromosphere, where bright features trace magnetic structures. KoSO began systematic Ca II K observations as early as 1904, providing an unusually long record of solar chromospheric activity that predates direct polar magnetic field measurements by many decades.
The team first cleaned and calibrated early photographic data to meet current standards, correcting issues such as time zone slips and solar rotation errors that would otherwise distort long-term trends. By correlating historical patterns with modern observations, the scientists were able to infer the evolution of the polar magnetic field from brightness patterns and network structures seen in the old images.
"The rhythms of solar activity across its approximately 11-year cycle have left scientists puzzled for more than a century," said SwRI scientist Dr. Bibhuti Kumar Jha, second author of the study. Understanding how sunspots, solar flares and magnetic storms vary over time is critical for protecting satellites and other technologies from disruptive space weather events driven by the Sun.
The Sun's polar magnetic field is a key driver of solar cycles, but direct polar field measurements only began in the 1970s, leaving earlier cycles poorly constrained. Calcium K observations capture chromospheric emission that is closely linked to magnetic fields, allowing researchers to use the long Kodaikanal record as a proxy for the unseen polar magnetism during the first half of the twentieth century.
"More than a century of historic Ca II K observations from the Kodaikanal Solar Observatory offer a unique window into the polar magnetic field of the Sun, a key factor governing solar cycles and future solar activity," said lead author Dibya Kirti Mishra of ARIES in India. The team targeted the chromosphere because it has emerged as a sensitive indicator of solar magnetic activity at high latitudes.
Due to Earth's tilt, telescopes near the ecliptic plane obtain clear views of the Sun's poles only a few times each year, leaving significant gaps in the modern record of polar magnetism. Jha, who helped mentor Mishra during her Ph.D. research, explained that the researchers "needed to go back to the past to provide the information we needed to forecast the future," using historical images to fill in these spatial and temporal blind spots.
To handle the vast archive efficiently, Mishra created an algorithm to automate image analysis of the corrected Kodaikanal data. "Going through 50,000 images manually would be staggering, so having an algorithm to automatically identify the proxies for the magnetic field was crucial," Jha said, highlighting the importance of automated pattern recognition in extracting subtle magnetic signatures.
The improved reconstruction of the Sun's polar magnetic history strengthens links between polar field strength and subsequent solar cycle amplitude, a relationship used in forecasting. At present, solar physicists can only make reliable solar cycle predictions about five years in advance, limiting long-range planning for missions that will operate in space for decades.
"With our current abilities, we can only predict up to five years well in advance, but for NASA or any other space missions, we need to prepare decades or multiple decades before the actual launch to understand expected conditions and plan the mission accordingly," Jha said. The team expects that it will take another four to five years before the full behavior of Solar Cycle 26 can be compared with their predictions to test the method.
Southwest Research Institute scientists are also proposing a dedicated solar polar mission that would observe the Sun from a vantage off the ecliptic, providing direct, continuous views of polar magnetic processes over time. Such a mission would complement the historical reconstructions by capturing the polar dynamics driving future cycles as they unfold.
In the meantime, the newly derived insights from century-spanning Ca II K data are already advancing space weather forecasting by extending the effective baseline of polar magnetic information. The research, published in the Astrophysical Journal under DOI 10.3847/1538-4357/adb3a8, builds on earlier work reported in 2024 that laid the groundwork for using Kodaikanal observations as a robust magnetic proxy.
Research Report:Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field
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