In possibly a first, the Sun’s magnetic field recorded over five decades of the 20th century have been digitally mapped, which will facilitate solar physicists to better understand the Sun’s behaviour in future.

Solar observations recorded between 1915 and 1965 were gathered from Kodaikanal Solar Observatory (KoSO) run by the Indian Institute of Astrophysics (IIA). Among one of the oldest solar observatories in the world, KoSO has been providing continuous and long-term uniform solar observations for over a century now, many of them taken in the form of photographic images.

“Like climate studies, we need information from vast historical datasets in order to better understand the Sun. Using this magnetic field map, it will be possible to make more accurate solar cycle predictions and study the Sun’s pole reversals in future,” Dipankar Banerjee, director, Aryabhatta Research Institute of Observational Sciences (ARIES), told The Indian Express.

Digital data observations of over 15,000 photo images, captured through Ca II-K and H-alpha filters, were used for creating this map. KoSO houses over 4 lakh photo images since 1904, most of which have been preserved in their digital formats during the last three decades.

Like Earth has north and south poles, Sun, too, has similar poles. But, what makes poles of the Sun interesting is that they reverse their polarity at an interval of 11 years, which is the duration of one solar cycle. In December 2019, Solar cycle 25 commenced.

Every solar cycle is different and the Sun’s activities influence the overall space weather. Anomalous solar activities can have a direct impact on Earth’s satellite-based communication systems, power grids and navigation.

Banerjee along with Bidya Karak from IIT-BHU collaborated over the last one year with their Russian counterparts on this mapping project, results of which were published recently in Astrophysical Journal Letters. These digital maps could also help scientists in refining the existing theoretical models. This map is also handy, as regular magnetic field observations of the Sun commenced only in 1967.

Moreover, researchers found two instances when the Sun’s polar reversal was unique within the 50-year study period spanning solar cycles 15 to 19. Scientists observed triple pole reversal in 1927 and 1957.

“Sun’s pole reversal is not realised simultaneously at the end of a solar cycle, but there is a time lag during which a pole can showcase properties of a previously held polarity,” said Karak, who is a Ramanujan Fellow of the Department of Science and Technology.

The mapping has showcased the movement of magnetic fields from lower to higher solar latitudes during the five decades besides pinning the exact time of the pole reversal and its links between two solar cycles