100 years with our closest star

Indian Institute of Astrophysics releases digitised images of the sun for researchers and science enthusiasts

Every day, since 1904, staff at the Kodaikanal Solar Observatory in Tamil Nadu have aimed their telescope at the sun, freezing the images of its disc. This data, spanning a hundred years and more, has now been digitised by astrophysicists from the Indian Institute of Astrophysics, Bengaluru, and made available to the public.

Apart from use in academic studies of long term behaviour of the sun, the data can be used to better understand sunspot activity which impacts climate and affects telecommunication systems. It also throws light on major events in the past which had an impact on the earth’s magnetic field. “From that knowledge we may understand the current and future events with greater precision. This also allows us to predict future [sunspot] activity levels with better accuracy,” says Dipankar Banerjee, IIAP, the Principal Investigator.

While ‘spectroheliograms’ were taken at the Kodai observatory since 1902, it was in 1909 that the data was used to discover the Evershed effect – that gases in the sunspots flowed radially outwards. The discovery by John Evershed put the KSO at par with the best observatories in the world. But its importance eventually declined as it was not upgraded or maintained. In a backhanded way, though, this turned out to be beneficial, because “the pictures had all been taken with the same instrument over the years, and this made it much easier to calibrate and digitise,” says Sudip Mandal, a Ph.D student who has worked on the project.

The data is unique not only in that it spans a hundred years, but that there are three sets of images, taken using different filters – White light, H-alpha and Calcium-K. It is known that the sun has a layered structure, and each of these data sets exposes a different layer.

Under white light filtering, the sun’s photosphere and the sunspots are visible, while the Calcium-K light can show layers some 2,000 km above this, in the chromosphere. The H-alpha images show up layers a little above the Calcium-K images. Features called “filaments” which are related to large expulsions of material from the sun’s surface can be viewed in the Calcium-K sets.

Opening up the digitised data has attracted international attention: Max Planck Institute, Gottingen; National Astronomical Observatories of China, Beijing and Big Bear Solar Observatory, US are interested in studying the way the sun’s luminosity changes. Though the sun appears to have a steady brightness, its luminosity actually undergoes changes over time. Some of the groups. The Big Bear Solar Observatory and the Beijing teams are interested in the H-alpha data in order to study the filaments that can be observed in those shots. Within India, groups from IUCAA, Pune; Physical Research Laboratory, Ahmedabad; and IISER, Kolkata, want to make studies.

A movie that the scientists made out of a sequence of hundreds of white light images shows how the sunspots appear and disappear periodically over an eleven-year cycle. Such movies offer immense possibilities for developing educational software, as classes of students can visually experience how the sun and the sunspots behave over the years. Just like CERN offers its data to science hobbyists, for analysis that does not require much training and yet cannot be carried out without human intervention, this data, too, could be used by science fora in India to build citizen science projects.

The data was historically archived in photographic plates and film. After the digitisation, the images are preserved in high-resolution digital format. “We store it in FITS [flexible image transport system] which is the most commonly used digital file format,” clarifies Dr Banerjee.

Digitising this has been a challenging task wthat involves not just reading and displaying the image but also extracting information – for instance differentiating a sunspot from artefacts such as a scratch or a fungal streak. “It can only be done using a lot of sophisticated mathematical tools. Some are available some we have had to develop to handle these challenges,” says Dr Banerjee.

This data can be freely downloaded from https://kso.iiap.res.in and wis also available on request through the contact details given on this website.

The project which was initiated about six years ago by S.S. Hasan, then the director of IIAP, has succeeded in converting to digitised format some sixty-seventy thousand images previously stored in photographic plates. The team includes scientists and the big team of research assistants at the Kodaikanal lab.

At the moment, the group has released the “lowest level” or raw data and plans are on to eventually release the processed ones, too.

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