Abstract
The rotational profile of the Sun is considered to be one of the key inputs in a solar dynamo model. Hence, precise and long-term measurements of this quantity is important for our understanding of solar magnetism and its variability. In this study, we use the newly digitised, white-light sunspot data (1923 – 2011) from Kodaikanal Solar Observatory (KoSO) to derive the solar rotation profile. An automated correlation-based sunspot tracking algorithm is implemented to measure the rotation parameters, \(A\), the equatorial rotation rate, and \(B\), the latitudinal gradient. Our measurements of \(A=14.381\pm 0.004\) deg/day and \(B=-2.72\pm 0.04\) deg/day compare well with previous studies. In our analysis, we find that the bigger sunspots (with area > 400 μHem) rotate slower than the smaller ones. At the same time, we do not find any variation in the rotation rates between activity extremes, i.e. solar maxima and minima. Lastly, we employ our tracking algorithm on the Michelson Doppler Imager (MDI) data and compare the MDI results with our KoSO values.
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Notes
Available for download at https://kso.iiap.res.in/new/white_light.
Individual spots are not yet classified into groups in the current KoSO catalogue.
Detail of this function is available at https://hesperia.gsfc.nasa.gov/ssw/gen/idl/maps/drot_map.pro. This routine uses the differential rotation parameter from Howard, Harvey, and Forgach (1990) to differentially rotate an input image.
We have used the mpfit_fun.pro function available in IDL for this purpose.
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Acknowledgements
Kodaikanal Solar Observatory is a facility of Indian Institute of Astrophysics, Bangalore, India. These data are now available for public use at http://kso.iiap.res.in through a service developed at IUCAA under the Data Driven Initiatives project funded by the National Knowledge Network. We would also like to thank Ravindra B. and Manjunath Hegde for their tireless support during the digitisation, calibration, and sunspot detection processes. We also thank Ritesh Patel and Satabdwa Majumdar for fruitful discussions during the work.
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Appendix A: Flow Chart
Appendix A: Flow Chart
Figure 8 in this appendix depicts the flow chart of our spot tracking algorithm.
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Jha, B.K., Priyadarshi, A., Mandal, S. et al. Measurements of Solar Differential Rotation Using the Century Long Kodaikanal Sunspot Data. Sol Phys 296, 25 (2021). https://doi.org/10.1007/s11207-021-01767-8
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DOI: https://doi.org/10.1007/s11207-021-01767-8