New method gives an accurate analysis of the historical time series of images of the Sun

Understanding the dynamics of the sun, solar cycle variations in the convection zone.


Almost all the variations in the Sun are related to the growth and decay of the Sun’s active regions. Likewise, many regions with weak magnetic fields on the Sun change with time.

Since there is a significant correlation between the magnetic field and the strength of the Ca-K line in the area of the Sun, these can be investigated using magnetograms and Ca-K line images of the Sun. The magneto-grams are only available for a limited time, and the instrument’s properties are evolving. Investigating the long-term Sun variations due to the instrument’s change is challenging. 

Kodaikanal Observatory (KO) has had access to Ca-K line images for over 100 years with no changes to the instrument’s optics. Similar information can be found at other observatories across the globe, including Mount Wilson Observatory (MWO), which has this data for roughly 70 years.

These images can help study the long-term variations in the Sun.

In a new study, scientists used Ca-K spectroheliograms obtained at the Kodaikanal observatory to generate a uniform time series using the equal-contrast technique to study the solar chromosphere’s long- and short-term variations.

In particular, Jagdev Singh, Muthu Priyal, and Ravindra, scientists from the Indian Institute of Astrophysics (IIA), have developed a new method to analyze the Ca-K line images of the Sun in white light. 

The method called Equal Contrast Technique (ECT), works by obviating temporal and latitudinal variations in observations related to instrument and sky conditions. It can aid in properly interpreting historical time series of solar image data and comprehending solar dynamics, variations in the solar cycle, dynamo processes in the convection zone, and the ensuing long-term climatic changes on Earth.

For the first time, scientists have demonstrated using data from the Kodaikanal Observatory that there is a strong association over around 100 years between the derived plage area—a bright region in the Sun’s chromosphere—and sunspot number.

Scientists noted, “The analysis shows that the activity at polar latitude belts is anti-correlated with the sunspot number. This study indicates that a multicell meridional flow pattern could exist in the latitude direction. One type of cell could transport the magnetic elements from mid- to low-latitude belts through meridional flows, and the other cell type could be operating in the polar region.”

Journal Reference:

  1. Muthu Priyal, Jagdev Singh, Belur Ravindra, and G. Sindhuja. Temporal and Latitudinal Variations in Ca-K Plage and Network Area: An Implication for Meridional Flows. The Astrophysical Journal. DOI: 10.3847/1538-4357/acaf60
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