Understanding the long-term evolution of green line coronal emission and its relation to the sunspots

Abstract

Significant association between flux emergence and the complexity of the involved processes in the solar corona could be substantial in estimating magnetic field activities and related driving mechanisms. In this study, we analysed solar magnetic activity in the time period between 1939 and 2022, covering solar cycles 17 to the present cycle 25. Our study was principally based on green coronal intensity, which was calculated using observations collected from a global network of coronal stations. Specifically, we utilized the homogenized Fe XIV 530.3 nm coronal emission line provided by the Astronomical Institute of the Slovak Academy of Sciences, as well as of the International Sunspot number index. The analyses were carried out using the Cross-Correlation and Empirical Mode Decomposition techniques. Firstly, the study found that there are strong and positive correlations between the two indices, with high coefficients specifically during the examined solar cycles. Secondly, the empirical mode decomposition technique reveals unique properties of the intrinsic mode functions (IMFs), highlighting distinctions between the emergence of sunspots and green coronal emissions based on their various modulations. Indeed, these IMFs are most likely closely linked to the magnetic flux rope structure and indirectly connected with the emergence of sunspot events. The observed lag between MCI and the SSN could potentially be linked to the dynamics between coronal response time and the evolutions of active regions. Furthermore, there is a steady decrease observed in the green coronal index from solar cycle 17 to the current cycle 25 that could be attributed to waning behaviour of solar magnetic field strength. This decline can also be regarded as evidence of the Centennial Gleissberg solar activity cycle during the descending phase. Interestingly, the green coronal index exhibits a significant degree of phase synchronization with sunspot numbers, suggesting that the intricate relationship between green coronal intensity and sunspot numbers can be potentially driven by processes such as heating, the formation of active coronal regions, and the emergence of magnetic flux.