Chennai, Dec 21 (PTI) Daily solar observations gathered for years at the century-old Kodaikanal Solar Observatory have led to a breakthrough understanding of how the Sun’s magnetic activity varies across its latitudes — a discovery that could deepen insights into the solar dynamo, impact space-weather forecasting, and even help climate models.
The study, led by researchers from the Indian Institute of Astrophysics (IIA), an autonomous body under the Department of Science and Technology (DST), analysed 11 years (2015–2025) of spectroscopic data captured in the calcium-K line of sunlight — a spectral signature that forms high in the Sun’s chromosphere and serves as a sensitive marker of magnetic activity.
The Kodaikanal Solar Observatory, which recently celebrated 125 years of continuous observation, maintains one of the world’s longest solar datasets, said Apoorva Srinivasa, one of the authors of the study, to PTI.
According to him, the research team used this rich archive to track how the Sun’s magnetic intensity changes across latitudes — revealing consistent zones of heightened activity that correspond with the Sun’s well-known 11-year cycle of sunspot peaks.
“The Sun is not a static ball of fire, but a magnetically active star that follows large-scale cycles of activity,” said K P Raju, lead author of the study.
“By slicing the Sun into latitude bands and analysing integrated light from each, we can reveal patterns invisible when studying isolated features like sunspots,” added the former professor at IIA.
The team found that most magnetic activity clusters between 40 degrees north and south latitude, with pronounced peaks around 15 degrees to 20 degrees, matching zones where sunspots are most frequent.
“These spectral variations directly mirror how the Sun’s magnetic field evolves,” said co-author K Nagarju.
“We verified this correlation using data from NASA’s Solar Dynamics Observatory,” added the associate professor at IIA, whose focus area of research is solar magnetism.
They also observed hemispheric asymmetries — with the southern hemisphere generally showing stronger or faster variations in activity — providing new hints about how the internal magnetic field reorganises itself over each solar cycle.
The findings were published in the journal, ‘Monthly Notices of the Royal Astronomical Society’ and represent a step toward refining solar dynamo models, which explain how magnetic energy is generated and cycles through the Sun’s surface layers.
Srinivasa, who was interning at IIA when he began to study the datasets, and who is currently a PhD scholar at Amrita Vishwa Vidyapeetham in Coimbatore, said the work on Kodaikanal data opened up several new research paths — including the possible use of machine learning in solar studies.
“When we first analysed the spectroscopic data for this study, I wanted to see whether we could apply machine learning to predict solar activity trends,” he said. “But after a lot of discussion and reading, it became clear that we didn’t yet have enough data to train such models meaningfully.” That realisation led him to investigate full-disk images of the Sun — large photographic archives from India’s Kodaikanal and Coronal Solar Observatory, which hold more than a century of observations in the calcium-K and H-alpha wavelengths.
“I wanted a dataset that was much more extensive if I were ever to apply AI-based methods,” Apoorva explained.
However, the shift to image-based research brought its own challenges. “These images were captured on photographic plates over the past century and later digitised. Many of them have streaks, hairs, fingerprints — even issues of poor illumination or contrast,” he said.
“So before analysing them, I had to test and balance different image-cleaning methodologies developed over decades to create one that could work for my specific problem.” Srinivasa’s current focus is on tracking chromospheric features such as filaments, plages, and network regions — and studying how their size, position, and intensity have changed through the decades.
“Once the method works reliably, we should get a broader picture of how the Sun’s chromosphere has evolved, and that will directly support those studying how magnetic fields vary inside the Sun,” he said.
Going forward, the researcher hopes to connect the spectroscopic dataset (1984–2024) used in the latest study with the full-disk image set (1904–2004).
“There’s an overlap of about 20 years,” he noted. “By combining these archives — and possibly adding data from other observatories — we should be able to see how spectral and physical features match and vary together across more than a century,” Srinivasa told PTI.
Researchers said such continuity of data is rare and gives India’s scientific community an exceptional edge in global solar research. The Kodaikanal Solar Observatory, established in 1899, continues to make daily observations that capture the Sun’s changing face — maintaining one of the longest, most consistent solar records in the world.
Apart from Raju, Nagarju, and Srinivasa, other authors of the study are Anu Sreedevi of IIT-BHU, Narayanankutty Karuppath of Amrita Vishwa Vidyapeetham, and P Devendran, T Ramesh Kumar, and P Kumaravel of IIA. PTI JR JR ROH
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