Century-Old Kodaikanal Solar Records Sun’s Mysterious Surface Patterns

The Sun transports energy from its interior through convection, a process similar to the movement of hot water in a boiling pot.

For more than a century, astronomers at the Kodaikanal Solar Observatory have been quietly recording the Sun's changing face. That uninterrupted record, one of the oldest continuous solar datasets in the world, is now helping scientists understand a long-standing mystery about the giant convection patterns that shape the Sun's surface.

Researchers from the Indian Institute of Astrophysics (IIA) have used over 100 years of observations from the observatory to investigate how large cellular structures known as supergranulations behave during different phases of the solar cycle. Their findings offer fresh insights into solar activity and could improve future predictions of the Sun's behaviour.

Giant Solar Cells Show Strong Links With Sunspot Activity

The Sun transports energy from its interior through convection, a process similar to the movement of hot water in a boiling pot. This motion creates patterns on the solar surface, ranging from tiny granules to much larger supergranulations that can stretch across nearly 30,000 kilometres.

These supergranular cells live for about 24 hours and form a network-like structure across the Sun's surface. Scientists have long debated how these structures form, why they have a particular size, and how they interact with the Sun's 11-year activity cycle.

Using around 34,000 Ca II K spectroheliograms recorded between 1907 and the present, the IIA team examined changes in lane widths and brightness within the solar network. Their analysis revealed a strong connection between these properties and sunspot numbers, especially around latitudes between 11° and 22° north and south of the solar equator.

The study found that lane widths respond most strongly near 18° north and 20° south, while brightness variations show the strongest connection near 13° north and 14° south. These results indicate that different solar properties respond differently to the solar cycle rather than following a single common pattern.

Time Delays Offer New Insight Into Solar Dynamics

One of the most intriguing discoveries involved the timing of these changes. Lane widths reached their strongest correlation with solar activity during solar maximum itself. Brightness changes behaved differently and peaked roughly 1.25 to 1.5 years later. The delay was not uniform across the Sun. Near 20° latitude, the lag was almost absent, while it became smaller at higher latitudes and larger closer to the equator. Lane-width variations showed delays of about 0.5 to 0.8 years, whereas brightness changes ranged from 0.3 years to nearly 2.5 years.

According to Prof. K.P. Raju, these patterns suggest that local magnetic fields and overall solar activity levels play a significant role in shaping supergranular properties. Understanding these relationships is important because they influence variations in solar radiation, particularly in ultraviolet wavelengths that affect Earth's upper atmosphere. The findings add new evidence to ongoing efforts to explain the origin of supergranulation and its role in transporting magnetic fields across the Sun. With information covering more than nine solar cycles, the Kodaikanal archive remains a valuable scientific resource. Researchers also expect the National Large Solar Telescope (NLST) to provide even more detailed observations that could help unravel the remaining mysteries of solar dynamics.