Differential Rotation of the Solar Chromosphere: A Century-long Perspective from Kodaikanal Solar Observatory Ca II K Data

TL;DR

This study analyzes a century of Ca II K data from Kodaikanal Observatory to measure the solar chromosphere's differential rotation, revealing it is faster and less latitudinally variable than the photosphere, with consistent results across multiple observatories.

Contribution

It provides the first long-term measurement of chromospheric differential rotation using multidecadal data, confirming its faster rotation rate and smaller latitudinal gradient compared to the photosphere.

Findings

Chromospheric plages rotate 1.59% faster than the photosphere.

The latitudinal gradient of chromospheric rotation is smaller than that of sunspots.

No significant north-south asymmetry or variation over a century.

Abstract

Chromospheric differential rotation is a key component in comprehending the atmospheric coupling between the chromosphere and the photosphere at different phases of the solar cycle. In this study, we therefore utilize the newly calibrated multidecadal Ca II K spectroheliograms (1907-2007) from the Kodaikanal Solar Observatory (KoSO) to investigate the differential rotation of the solar chromosphere using the technique of image cross-correlation. Our analysis yields the chromospheric differential rotation rate $\Omega (\theta) = (14.61\pm 0.04 - 2.18\pm 0.37\sin^2{\theta} - 1.10 \pm 0.61\sin^4{\theta})^\circ{\rm /day}$. These results suggest the chromospheric plages exhibit an equatorial rotation rate 1.59% faster than the photosphere when compared with the differential rotation rate measured using sunspots and also a smaller latitudinal gradient compared to the same. To compare our results to those from other observatories, we have applied our method on a small sample of Ca II K data from Rome, Meudon, and Mt. Wilson observatories, which support our findings from KoSO data. Additionally, we have not found any significant north-south asymmetry or any systematic variation in chromospheric differential rotation over the last century.