The magnetic field of the Sun is unique in that, unlike many other celestial bodies, it reverses its polarity roughly every 11 years. Regular observations of the Sun’s magnetic field begin in 1967, and, so far, we have data for the period from 1967 almost to the present, spanning the solar cycles 20-24. Now, an Indo-Russian collaboration has added to this with a plot of the solar magnetic field from 1915 to 1965 (solar cycles 15-19). Using a novel approach, they have calibrated and calculated the field using data from the archives of the Kodaikanal Solar Observatory of the Indian Institute of Astrophysics.
“For about the last 50 years, that is, from around 1970s, regular measurements of magnetic field are available from different observatories. One important one is Wilcox Solar Observatory from the US. Before that Mount Wilson Solar Observatory provided some data, although they are not of high quality and uniform,” says Dipankar Banerjee, Director of Aryabhata Research Institute of Observational Sciences, Nainital, and an author of the paper which has been accepted for publication in The Astrophysical Journal Letters.
Data from Kodaikanal
The present study makes the important contribution of building on this using the Kodaikanal Solar observatory data, and extending the magnetic field calculations for another fifty years approximately. “Understanding the magnetic field evolution in the past improves our understanding of the physics itself. In the work we have also talked about the timing of the polarity reversal, which is also very important,” adds Prof. Banerjee. Solar variations, both of long scale and short, depend on the magnetic field, and these can affect both the climate and space weather. Therefore, understanding the magnetic field variations can contribute to our understanding of climate and space weather.
The Kodaikanal Solar Observatory was established in 1899 and observations of the Sun from this observatory run over a hundred years, providing what is among the longest series of solar data. An important feature of these data is the multi-wavelength observations. The data consist of full disc spectral images of the Sun using the Ca II K and H-alpha lines. Both these lines image the atmospheric layer of the Sun known as the chromosphere.
Extracting information
From the full-disc images of Ca II K features, bright patches above magnetic field regions, known as plages are identified. “Underneath these one can find sunspots, so plages provide information about magnetic field locations and strength,” says Bidya Binay Karak, a Ramanujan Fellow at the Department of Physics, Indian Institute of Technology (BHU), Varanasi, and an author of the paper.
On the other hand, the H-alpha images show filaments, which are formed along magnetic neutral lines. “Their presence also represents the magnetic field distribution beneath. They also allow us to study the evolution of magnetic field with time,” he explains.
Reconstruction
The team first calibrated the Ca IIK images of the Kodaikanal data using magnetic field measurements that were available during that period. Using this, they interpreted the data for the periods when magnetic field measurements were not available, namely, years 1915-1965. “The H-alpha images display the location of filaments, which are believed to be formed along the magnetic neutral lines, so these images provide information on the magnetic polarity,” says Dr Karak. “Combining the two sets, the full magnetic field data was reconstructed.”
