Phase relations between total solar irradiance and the Mg II index

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

“…Xiang and Kong [15] investigated the relation between TSI and magnetic fields and found that the weak magnetic activity seen on the solar full disk dominates the solar cycle variation of TSI. This result is the same as the finding in Li et al [22], in which the conclusion is based on the phase relations between TSI and Mg II core-to-wing ratio. Recently, Xu et al [16] studied the phase relations between TSI and sunspot number and indicated that TSI and sunspot number are very coherent, which hints that the solar cycle variation of TSI should be dominated by the solar activity regions.…”

“…Thus, the relation of TSI with Mg II index on timescales of a few days to several months should mainly reflect that of TSI with the short-term part of Mg II index and indicates that the faculae increase TSI and contribute to its variation at short timescales. The combined effects of the sunspots darkening and the intensification of faculae (the short-term part of the Mg II index) to TSI on timescales of one year have been discussed by Li et al [22] The results of the WTC and PWC of TSI with the sunspot area shown in Figures 2 and 3 indicate that the solar cycle variation of TSI is not probably related to the sunspots blocking, which is represented by sunspot area. Xu et al [16] pointed out that TSI is positively correlated with sunspot numbers, the former lags behind the latter by about a solar rotation period, and TSI and sunspot numbers are very coherent on timescales of one Schwabe solar cycle.…”

“…The results of the WTC and PWC of TSI with Mg II index shown in Figures 5 and 6 display that the solar cycle variation of TSI is probably highly related to the intensification contribution, which is represented by Mg II index. Li et al [22] show that the relation of TSI with Mg II index on timescales of one solar cycle should basically reflect that of the long-term part of Mg II index with TSI; thus the relation of TSI with Mg II index shown in low frequency band of Figures 5 and 6 should mainly reflect that of TSI with the long-term part of Mg II index on timescales of one solar cycle. Moreover, the long-term part of the Mg II index shows the small magnetic features contribution to the intensification of TSI [9,12].…”

“…Moreover, it is known that the Mg II index can be separated into a short-term and long-term part. The short-term part indicates that the facular intensifies TSI, and the long-term part shows the intensification contribution by the small magnetic features to TSI [9,12,22]. Consequently, we use sunspot area and Mg II index and combine extension of wavelet transform to revisit the reason for the solar cycle variation of TSI.…”

The dependence of the [FUV-MUV] colour on solar cycle

“…Meunier, 2003), iii) the presence of an out-of-phase variation between spectral solar irradiance and the Mg II index (e.g. Harder et al, 2009;Li et al, 2016), and iv) a center versus limb distance effect, i.e., the solar active regions appear statistically more inclined with respect to the line-of-sight during the SC rising phase compared to the SC descending phase (e.g. Maunder, 1904).…”