Kodaikanal / New Delhi :
Astronomers have developed a method to reconstruct the Sun’s polar magnetic history by analysing more than a century of solar photographs from the Kodaikanal Solar Observatory (KoSO), producing new insights that help forecast future solar activity.
Key takeaway
Researchers have shown that tiny bright features visible in century-old Ca II K solar images — now digitized at KoSO — can act as a reliable proxy for the Sun’s polar magnetic field, extending reliable records of solar magnetism back well before direct measurements began in the 1970s.
Why this matters
The Sun’s polar magnetic fields play a central role in driving the roughly 11-year solar cycle and in controlling space weather that affects satellites, communications and power systems on Earth. Direct polar field observations exist only from the 1970s onward, leaving major gaps in our knowledge of earlier cycles. Extending the record gives scientists a longer baseline to test models and improve forecasts of solar storms and cycle strength.
What the team did
- Data source: The team used digitized KoSO observations in the Ca II K wavelength that began in 1904; these images capture chromospheric features tied to magnetic activity.
- Collaborations: The work was led by researchers at ARIES and included partners from IIST, Southwest Research Institute (USA), Max Planck Institute for Solar System Research (Germany) and INAF Osservatorio Astronomico di Roma (Italy).
- Technique: Using advanced feature-detection algorithms and comparisons with modern Rome-PSPT observations, the team identified small bright elements near the solar poles — the polar network — and quantified them to create a Polar Network Index (PNI).
- Reconstruction: The PNI series was mapped to polar field strength, producing a continuous estimate of the Sun’s polar magnetic behavior across the last century.
Main findings
- The polar network visible in Ca II K images serves as a strong proxy for polar magnetic field strength.
- The reconstructed series provides century-scale context for recent measurements and was used to estimate the amplitude of Solar Cycle 25.
- Automated processing of the KoSO archive demonstrates how historical photographic collections can be repurposed with modern machine learning and image-analysis tools.
Broader impact and data access
This work offers a longer, more consistent baseline for solar dynamo studies and space-weather forecasting, improving the scientific community’s ability to anticipate disruptive solar events. The complete dataset, including the reconstructed polar field and the Polar Network Index, has been made publicly available on GitHub and Zenodo to encourage further research and verification.
