Measurements of solar differential rotation using the century long kodaikanal sunspot data

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±0.004 deg/day and B=−2.72±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.