Revisiting the Question: The Cause of the Solar Cycle Variation of Total Solar Irradiance

Abstract

The Mg II index and sunspot area are usually used to represent the intensification contribution by solar bright structures to total solar irradiance (TSI) and sunspot darkening, respectively. In order to understand the cause of the solar cycle variation of TSI, we use extension of wavelet transform, wavelet coherence (WTC), and partial wavelet coherence (PWC), to revisit this issue. The WTC of TSI with sunspot area shows that the two time series are very coherent on timescales of one solar cycle, but the PWC of TSI with sunspot area, which can find the results of WTC after eliminating the effect of the Mg II index, indicates that the solar cycle variation of TSI is not related to sunspots on the solar surface. The coherence of two time series at these timescales should be due to a particular phase relation between sunspots and TSI. The WTC and PWC of TSI with Mg II index show that the solar cycle variation of TSI is highly related to Mg II index, which reflects the relation of TSI with the long-term part of Mg II index that shows the intensification contribution by the small magnetic features to TSI. Consequently, the solar cycle variation of TSI is dominated by the small magnetic features on the solar full disk. Additionally, we also show the combined effects of the sunspot darkening and the intensification contribution represented by Mg II index to TSI on timescales of a few days to several months and indicate that the faculae increase TSI and contribute to its variation at these timescales.

Data Fusion of Total Solar Irradiance Composite Time Series Using 41 Years of Satellite Measurements

“…Timescale variations can be classified in subdaily (minutes to hour), daily to weekly, and yearly to one solar cycle. Major mechanisms, such as the evolution of magnetic features on the solar surface, which dominate each timescale are complex and still under investigation within the solar physics community (Xiang, 2019; Yeo et al., 2017). Several studies (Fontenla et al., 2009; Kopp & Lean, 2011; Yeo et al., 2021) have shown that TSI variations on timescales of hours are a combination of sunspot blocking and an intensification due to bright faculae, plages and other elements.…”

Data Fusion of Total Solar Irradiance Composite Time Series Using 41 years of Satellite Measurements

“…Timescale variations can be classified in subdaily (minutes to hour), daily to weekly, and yearly to one solar cycle. Major mechanisms, such as the evolution of magnetic features on the solar surface, which dominate each timescale are complex and still under investigation within the solar physics community (Yeo et al, 2017;Xiang, 2019). Several studies (Fontenla et al, 2009;Kopp & Lean, 2011;Yeo et al, 2021) have shown that TSI variations on timescales of hours are a combination of sunspot blocking and an intensification due to bright faculae, plages and other elements.…”

Data Fusion of Total Solar Irradiance Composite Time Series Using 41 years of Satellite Measurements

“…Timescale variations can be classified in subdaily (minutes to hour), daily to weekly, and yearly to one solar cycle. Major mechanisms, such as the evolution of magnetic features on the solar surface, which dominate each timescale are complex and still under investigation within the solar physics community (Xiang, 2019;Yeo et al, 2017). Several studies (Fontenla et al, 2009;Kopp & Lean, 2011;Yeo et al, 2021) have shown that TSI variations on timescales of hours are a combination of sunspot blocking and an intensification due to bright faculae, plages and other elements.…”