Free Access
Volume 556, August 2013
Article Number L3
Number of page(s) 5
Section Letters
Published online 01 August 2013

Online material

Appendix A: SSI variations at 500 nm versus TSI variations

A robust linear regression analysis of monthly snapshot variations of SSI at 500 nm versus TSI confirms the positive correlation reported here. The lowest irradiance values occur during sun spot passages, the largest values during the active mid-term of cycle 23. Thus, data points shown in the lower-left corner of Fig. A.1 (x,y < 1) represent measurements taken during passage of sunspots over the solar disc while those shown in the upper-right corner represent observations during the declining phase of solar cycle 23 and rising phase of cycle 24. All data points are weighted by a bi-square function of their regression residues, and so the leverage of outliers, e.g. transit of large sun spots, on regression is minimized. We note that the regression slope is clearly dominated by the number of data points (green) with positive correlation, specifically by those in the upper-right corner representing higher solar activity.

We also note that all points (magenta) that correspond to anti-correlation between the TSI and SSI-500 are located very close to the (1, 1) point in Fig. A.1. This means that all cases of anti-correlation occur when the amplitude of the cycle is small

thumbnail Fig. A.1

SSI variation at 500 nm versus TSI variation for the period from 2001 to 2012 with robust linear regression. Green dots (n = 104) have positive correlation; magenta dots (m = 21) are anti-correlated with TSI.

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and comparable to the uncertainty of the degradation correction. Therefore, these points are the most sensitive to small deviations in the degradation correction and accordingly the least reliable. When the data are restricted to the central cluster (within a square region of 1 ± 0.0005), the slope is reduced to +0.561 [0.436, 0.686], but remains strictly positive.

The trend of annual-mean data shown in Fig. A.2 is apparent to the eye: the green curve (500 nm) resembles the black (TSI) while the red (862 nm) and blue (402 nm) lines almost correspond to images of TSI mirrored at normalization level 1.0. We note that the temporal position of the extrema of SSI variations may be affected by the choice of the selected period. However the sign of the correlation remains unaffected (see Sect. 3.1).

thumbnail Fig. A.2

The same as Fig. 3 but for annual mean values.

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Table A.1

Slopes of the linear regression (shown in Fig. 4) between relative variations in the annual means of the VIRGO TSI and SSI monthly snapshots.

© ESO, 2013

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