British astronomer John Evershed (left) working on a spectroheliograph in the early 1900s. Credit: IIA Archives
Analysis of a century of solar images from an Indian observatory reveals that different features on the Sun’s chromosphere1, the layer above the visible photosphere, all show the star’s uneven spinning — they rotate faster at the equator than at the poles. This differential rotation, is essential to understanding solar magnetic fields and space weather, say scientists in India.
“Over time, this rotation causes the magnetic field to twist and tangle giving rise to sunspots and solar activity, such as flares and Coronal Mass Ejections (CMEs) tracked by space weather forecasters to help predict solar storms that can impact Earth,” says Venkatachalam Muthu Priyal at the Indian Institute of Astrophysics (IIA), Bengaluru, who is associated with the India’s Aditya L-1 solar mission.
Set up by British scientists in 1899 in the Palani hills of the Western Ghats, the Kodaikanal observatory has been observing the Sun since 1904, clicking at least one image each day across different wavelengths, making it one of the world's longest records of solar data.
Solar observers using an array of instruments meticulously collected images of the photosphere — the Sun's visible surface—and chromosphere. These were preserved on photographic plates and later, on film, until modern telescopes took over in the early 2000s. The historical images were recently digitised.
Muthu Priyal and colleagues looked at images from 1907 to 2007 for the current research. After improving image clarity with innovative techniques, they focused on two key features of the chromosphere that offer a unique window into solar behaviour.
Captured in the Ca-K line (wavelength) at Kodaikanal, they are plages, which are bright areas linked to weaker magnetic fields, and mesh-like network structures (active, enhanced, and quiet), consisting of smaller magnetic elements.
Even the less-active structures (quiet networks) follow the differential rotation pattern — the equatorial region completing a spin in about 25 days, the poles taking around 35 days, says Muthu Priyal.
Concentrating on the network features enabled the team to extend their measurements to higher (polar) latitudes, reaching up to 80–90 degrees. Previous studies that relied on sunspots to estimate solar rotation were confined to mid-latitudes, and limited to short-term observations.
“Sunspots only appear between approximately 40 degree latitudes,” says Sudip Mandal, a post-doctoral fellow at the Max Planck Institute for Solar System Research in Germany, who worked with digitised, white-light sunspot data (1923 – 2011) from Kodaikanal to derive2 the solar rotation profile.
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Also unlike sunspots, plages and networks can be seen on the Sun's surface all the time during its 11-year cycle. This helped the researchers study the rotation rate of plages and networks from the equator to the poles. They used a method called Fourier analysis to find that the Sun rotates at about 13.98 degrees per day at the equator and slower at 10.5 degrees per day near the poles (at 80 degrees latitude). Notably, both plages and network features rotated at similar speeds.
If features of different sizes rotate at similar speeds, it could mean they originate from the same depth inside the Sun —something we still don’t fully understand, says Mandal.
“Further studies using different approaches on data from different observatories are needed to verify the robustness of these results,” he says.
Muthu Priyal hopes that by studying the network features at polar latitudes, one could also zoom in on how intense the Sun’s activity3 will be in the upcoming cycle.
2024 was marked by intense solar activity. A series of powerful solar flares and CMEs sent clouds of charged particles and magnetic fields toward Earth in May, triggering the strongest geomagnetic storm in 20 years and possibly one of the strongest aurora displays on record in 500 years that lit up the night sky in low-latitude regions, including Ladakh.
