Variation of Chromospheric Features as a Function of Latitude and Time Using Ca-K Spectroheliograms for Solar Cycles 15 – 23: Implications for Meridional Flow

Abstract

We have analyzed the Ca-K images obtained at Kodaikanal Observatory as a function of latitude and time for the period of 1913 -2004 covering Solar Cycles 15 to 23. We have classified the chromospheric activity into plage, Enhanced Network (EN), Active Network (AN), and Quiet Network (QN) areas to differentiate between large strong active and small weak active regions. The strong active regions represent toroidal and weak active regions poloidal component of the magnetic field. We find that plage areas mostly up to 50 • latitude belt vary with about 11-year solar cycle. We also find that a weak activity represented by EN, AN and QN varies with about 11-year with significant amplitude up to about 50 • latitude in both hemispheres. The amplitude of the variation is minimum around 50 • latitude and again increases by a small amount in the polar region. In addition, the plots of plages, EN, AN and QN as functions of time indicate the maximum of activity at different latitude occur at different epoch. To determine the phase difference for the different latitude belts, we have computed the cross-correlation coefficients of other latitude belts with the 35 • latitude belt. We find that the activity shifts from mid-latitude belts towards equatorial belts at high speed at the beginning of a solar cycle and at lower speed as the cycle progresses. The speed of the shift varies between ≈ 19 and 3 m s −1 considering all the data for the observed period. This speed can be linked with the speed of meridional flows, believed to occur between convection zone and the surface of the Sun.

Total Solar Irradiance during the Last Five Centuries

“…where the subscripts n, f, and s indicate network, facular, and sunspot component, respectively. We adopt a linear correlation between α n and α f (e.g., Criscuoli et al 2018;Devil et al 2021):…”

Variation of Small Scale Magnetic Fields Over a Century using Ca-K Images as Proxy

“…Averaging of the measured parameters over several months was required because of the large scatter in their values due to different contrasts of the obtained images. The monthly averaged Ca-K parameters were used by several investigators to study the long-term variations (Priyal et al 2019;Devi et al 2021;Temmer 2021). Bertello et al (2020) rescaled the intensity of Ca-K MWO images such that the full width at half maximum (FWHM) of the intensity distribution of each image was 0.08.…”

Temporal and Latitudinal Variations in Ca-K Plage and Network Area: An Implication for Meridional Flows

“…Following these investigations, Devi et al (2021) selected ∼22960 out of 34453 corrected images to identify Ca-K line features such as plage, enhanced network (EN), and active and quiet network (AN and QN) areas integrated over 10°latitude belts by using the intensity and area threshold values. Due to significant data gaps, the resulting parameters were averaged over a month with a running average over three years.…”

“…Singh et al (2021) have rescaled the Ca-K spectroheliograms (KO) using the equal-contrast technique (ECT) to generate a more uniform time series consisting of all the 34453 images taken over 16981 days during the period 1905-2007. There are fewer gaps in the data than in the time series used by Devi et al (2021). We have analyzed this time series as a function of latitude to study the variation of plage, EN, AN, and QN areas with time.…”