Using 100-year-old data to help predict future solar cycle activity

To reconstruct the Sun’s polar magnetic behavior over more than 100 years, an SwRI scientist first corrected anomalies in historical data from Kodaikanal Solar Observatory (KoSO) to sync with direct modern measurements of the Sun’s poles. Comparing observations from KoSO in Calcium-K light with magnetic field measurements from the Solar and Heliospheric Observatory’s Michelson Doppler Imager (left) highlights the association of bright regions in Ca K with magnetic activity in the Sun. The right image shows dark blue and green (north polarity) yellow and orange regions (south polarity) regions, indicating where Ca K light and magnetic data are highly correlated.
Image Credit: KoSO/IIA, SOHO/NASA/ESA

Scientific Frontline: "At a Glance" Summary

• Main Discovery: Reconstruction of the Sun's polar magnetic behavior spanning over a century to enhance the prediction of future solar cycle activity.
• Methodology: The research team analyzed historical Calcium K (Ca II K) observations from the Kodaikanal Solar Observatory (KoSO), dating back to 1904. An automated algorithm processed approximately 50,000 images to identify magnetic field proxies in the Sun's chromosphere, while correcting for data anomalies such as time zone slips and rotation errors.
• Key Data: The study utilized over 100 years of archival data, significantly extending the record beyond direct polar field measurements which only began in the 1970s. Current predictive capabilities are limited to approximately five years, whereas this method aims to facilitate multi-decadal forecasting.
• Significance: Understanding the polar magnetic field is critical for forecasting solar processes, including sunspots, solar flares, and magnetic storms. Improved predictions are essential for safeguarding satellites, power grids, and other Earth-based technologies from adverse space weather events.
• Future Application: The findings will assist NASA and other space agencies in planning long-term missions decades in advance by providing a clearer understanding of expected solar conditions.
• Branch of Science: Heliophysics / Solar Physics
• Additional Detail: Researchers are proposing a future solar polar mission to directly observe these magnetic mechanisms from an ecliptic viewpoint to further validate and refine these models.

An international team of astronomers devised a new way to look at historical solar data and reconstruct the Sun’s polar magnetic behavior across more than 100 years, with the goal of predicting the activity of future solar cycles. By merging historic and modern solar observations, SwRI and KoSO scientists reconstructed the Sun’s polar magnetic field back to the early 20th century, opening a new window into understanding long-term solar behavior to ultimately help future space-weather predictions.
Image Credit: SwRI/KoSO/IIA

Research conducted by an international team of astronomers from Southwest Research Institute, Aryabhatta Research Institute of Observational Sciences in India and the Max Plank Institute in Germany could help predict upcoming solar cycle activity. To enable these predictions, the team devised a new way to look at historical data from the Kodaikanal Solar Observatory (KoSO), a field station of the Indian Institute of Astrophysics (IIA) Bangalore, to reconstruct the Sun’s polar magnetic behavior over more than 100 years. 

“We needed to find the polar magnetic information hidden in the historical data,” said SwRI scientist Dr. Bibhuti Kumar Jha, second author of a paper about these findings in the Astrophysical Journal. “To start, we cleaned up and calibrated early data to today’s standards and then correlated patterns with modern observations. I addressed anomalies like time zone slips and rotation errors to enable this kind of study.”

The rhythms of solar activity across its approximately 11-year cycle have left scientists puzzled for more than a century. Understanding the patterns of sunspots, solar flares and magnetic storms is crucial to protecting satellites and other Earth technology from space weather events. The Sun’s polar magnetic field is pivotal to understanding solar processes to forecast future solar cycles, but the first direct measurements of the polar field only started in the 1970s. Calcium K (Ca II K) observations capture the Sun’s chromosphere, revealing its magnetic fields and activity through bright features using specific narrow-band filters to isolate the Ca II K line.

“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 Aryabhatta Research Institute of Observational Sciences (ARIES) in India.

“Additionally, due to the Earth’s tilt, we only get a really good view of the Sun’s poles a couple of times a year,” said Jha, who helped mentor Mishra during her Ph.D. research, culminating in this paper. “We needed to go back to the past to provide the information we needed to forecast the future. We targeted the Sun’s chromosphere, which has emerged as an indicator of solar magnetic activity.”

Solar astronomers at KoSO began observing the Sun in the Ca II K spectrum as early as 1904. With insufficient modern data to predict activity, scientists were looking for ways to unlock the secrets of solar magnetism hidden in more than a century of KoSO data. The chromosphere just above the Sun’s visible surface forms bright patches and networks associated with magnetic activity. 

Each member of the team contributed their expertise to examine the data and connect the dots. Mishra developed an algorithm to automate the image review from the corrected historical 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. 

“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. “We will have to wait another four to five years to actually tell how Solar Cycle 26 will behave and if the activity will accurately match our predictions.”

SwRI researchers are proposing a solar polar mission from the ecliptic point of view to directly observe these magnetic mechanisms over time. In the meantime, new insights from this study will help advance space weather forecasting.

Published in journal: The Astrophysical Journal

Title: Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field

Authors: Dibya Kirti Mishra, Bibhuti Kumar Jha, Theodosios Chatzistergos, Ilaria Ermolli, Dipankar Banerjee, Lisa A. Upton, and M. Saleem Khan

Previous Research: Butterfly Diagram and Other Properties of Plage Areas from Kodaikanal Ca II K Photographs Covering 1904 – 2007

Source/Credit: Southwest Research Institute

Reference Number: spw012026_01

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